HomeMy WebLinkAboutSUB201900058 Calculations 2022-04-04 (3)SCOUR ANALYSIS REPORT
Pleasant Green Connector Road Culvert
Froehling & Robertson, Inc. Project No. 71ZO219
Pleasant Green Subdivision, Albemarle County, VA
Prepared for:
Stanley Martin Homes / Contech Engineered Solutions LLC
Charlottesville, Virginia 22911
by
Endesco,Inc.
15245 Shady Grove Road
Suite 335
Rockville, MD 20850
January 2022
Pleasant Green Connector Road Culvert
Froehling & Robertson, Inc. Project No. 71Z0219
Pleasant Green Subdivision
Albemarle County, Virginia
SCOUR ANALYSIS REPORT
TABLE OF CONTENTS
REPORT
Section 1.
INTRODUCTION...........................................................................................................................2
Section2.
HYDROLOGY.................................................................................................................................4
Section 3.
DESCRIPTION OF PROPOSED STRUCTURE...................................................................................8
Section 4.
GEOTECHNICAL INVESTIGATIONS.............................................................................................10
SectionS.
ANALYSIS...................................................................................................................................13
Section 6.
SUMMARY OF RESULTS.............................................................................................................25
Section 7.
RECOMMENDATIONS................................................................................................................27
TABLES
Table 1 - Flood Discharges and Water Surface Elevations at the Bridge..............................................................4
Table2 - Boring Log Description.........................................................................................................................11
Table 3 - Channel and Bridge Configuration per HEC-RAS Model......................................................................13
Table 4 - Contraction Scour Conditions for 10-, 100- and 500-Year Storm Event..............................................14
Table 5 - Determining k1 factor for the main channel.......................................................................................16
Table 6 - Live -Bed Contraction Scour Results.....................................................................................................18
Table 7 - Clear -Water Contraction Scour Results...............................................................................................19
Table 8 - Local Abutment Scour Results.............................................................................................................20
Table 9 - Summary of Results —10 Year Storm Event.........................................................................................25
Table 10 - Summary of Results —100 Year Storm Event.....................................................................................25
Table 11- Summary of Results — 500 Year Storm Event.....................................................................................25
Table 12 - Riprap Sizing for Abutment Scour Protection....................................................................................26
FIGURES
Figure1- Vicinity Map..........................................................................................................................................2
Figure 2 - FEMA FIRM 51003CO229D....................................................................................................................3
Figure 3 - Upstream of Pleasant Green Connector Road Culvert Looking Upstream...........................................5
Figure 4 - Upstream of Pleasant Green Connector Road Culvert Looking Downstream......................................5
Figure 5 - Downstream of Pleasant Green Connector Road Culvert Looking Upstream......................................6
Figure 6 - Downstream of Pleasant Green Connector Road Culvert Looking Downstream.................................6
Figure 7 - HEC-RAS Bridge Model Results for Pleasant Green Connector Road Culvert ......................................7
Figure 8 - Proposed Pleasant Green Connector Road Culvert Profile..................................................................8
Figure 9 - Proposed Culvert under Pleasant Green Connector Road...................................................................9
Figure10 - Boring Locations...............................................................................................................................10
Figure 11 - Fall Velocities (Figure 6.8, HEC-18 Manual)......................................................................................15
Figure 12 - Scour Profile for 100-Year Discharge................................................................................................22
Figure 13 - Scour Profile for 500-Year Discharge................................................................................................23
ENDESCO, INC. Page No. 0
January 2022
APPENDIX
Appendix A - Geotechnical Report (Extracts)
Appendix B - Sizing Rock Riprap at Abutments
Appendix C - Reference Only Computations
ENDESCO, INC. Page No. 1
January 2022
Pleasant Green Connector Road Culvert
Froehling & Robertson, Inc. Project No. 71Z0219
Pleasant Green Subdivision
Albemarle County, Virginia
SCOUR ANALYSIS REPORT
Section 1. INTRODUCTION
The proposed Pleasant Green Connector Road Culvert is located on the southwest side of the Pleasant Green
Subdivision in Albemarle County, Virginia. The new connector road is planned to connect the roundabout on
Alston Street in the Pleasant Green Subdivision with Orchard Drive to the west. The new road with a ConSpan
Culvert will cross Powells Creek near the midpoint between the two existing roads.
The 267 square -mile South Fork Rivanna River watershed is located in Albemarle County. Powells Creek is
one of the tributary of South Fork Rivanna River watershed, originated from the steep mountainside 1,000
feet above western Albemarle County's Mint Springs Park, and it confluence with Lickinghole Creek.
1. Project:
Pleasant Green Connector Road Culvert
2. County/ City:
Albemarle County
3. Pleasant Green CONSPAN Bridge over Powells Creek
4. Plane Coordinates or Latitude and
Longitude
Lat: 38.0707 N Long: 78.7075 W
5. Location (Nearest Town, etc.):
Crozet, VA
6. Drainage Area:
2,037 acres (3.18 sq. mi.)
7. Station:
13+12, Rambling Brook Lane
Vicinity map and the relevant FEMA Flood Insurance Rate Map are given in Figures 1 and 2 below:
GRAYROCK
/F
if
rneyards
Old Trail Golf Club Q
LAUREL HALLS
CROZET rzw
CROSSING
WAY ANDS
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WAYLAND PARK
' SITE tar ud E3 e
Craze:
REA PARKSIDE
VILLAGE
Figure 1- Vicinity Map
(Not to Scale)
t�
PARK VIEW
BROOKWOO
ENDESCO, INC. Page No. 2
January 2022
FIRM
FLOOD INSURANCE RATE YAP
um acoaroanxxn exsss
OF tsae woeemmver arc
or aauanervaU
MR 220 OF 0
lta sn sue
!NO>blMl
EPSON![ RATE
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rwe fl.epn ��� Mn
Figure 2 - FEMA FIRM 51003CO229D
(Not to Scale)
ENDESCO, INC. Page No. 3
January 2022
Section 2. HYDROLOGY
Methods Used For Discharges: The hydrologic information for the scour analysis was obtained from
HEC-RAS model develop by Kimley-Horn, Virginia Beach, VA. Design peak flow information is
referenced from FEMA Flood Insureance Study #51003CV000C, Albemarle County, VA.
The 10-Year, 100-Year and 500-Year discharges at the connector road culvert are taken directly from
the FEMA Effective model for Powells Creek in the HEC-EAS modeling, and water surface elevations
at the bridge site are given below (Table 1).
Table 1 - Flood Discharges and Water Surface Elevations at the Bridge
Discharge Water Surface
Return Period (Years) River Station 4339 BR U Elevation (ft)
(cfs)
10 790 690.20
100 1420 691.42
500 2060 692.49
Influence and Control of Site:
The contributing drainage area at the culvert site is 2,037 acres. The wetlands in the floodplain are a
possible habitat for varieties of plant and animal species. The impact to these wetlands is kept to the
minimum extent possible. The upstream approach and downstream conditions are given in the
photographs shown in Figures 3 to 6. There are no signs of erosion and degradation showed in the
pictures of the stream near the proposed culvert site.
High Water Elevation Observed: Not available
Debris Potential: None
HEC-RAS modeling for the Pleasant Green Connector Road Culvert on Powells Creek was developed
by Kimley-Horn, Virginia Beach, VA for the FEMA floodplain validation, and the dataset is used in this
scour studies for the road culvert. The HEC-RAS Bridge Model and water surface elevations for the
10-year, 100-year and 500-year storm events at the culvert site are shown in Figure 7.
The roadway above the culvert does not overtops for discharges up to 500-year design storm base on
the HEC-RAS study result.
ENDESCO, INC. Page No. 4
January 2022
AeT 1 Fyn3.AIT QK'�.�' i
�M1b }f! F�'' • I
.ri
Figure 3 - Upstream of Pleasant Green Connector Road Culvert Looking Upstream
Figure 4 - Upstream of Pleasant Green Connector Road Culvert Looking Downstream
ENDESCO, INC. Page No. 5
January 2022
t
_t
,Figure
i.e
5 - Downstream
of Pleasant Green
Connector Road Culvert
Looking Upstream
� Y �=�`3-
pit„� 4 r. _
�°+•'
• l ��
T
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:P a
Pleasant Green Xing
Plan: Proposed Model 1/5/2022
RS=4339 BR
045 .03
.045
730
Legend
WS 500 YR
WS 100 YR
720
-----------
WS 50 YR
WS 10 YR
Grouts
710
Beek Ste
c
c
o
_
A
a
w
700
680
0
200 460
600 800 1000
12f1C
Station (ft)
Pleasant Green Xing
Plan: Proposed Model 1/5/2022
RS = 4339 BR
'Al
—.045 .03
.045
Legend
WS 500 YR
WS 100 YR
720
WS 50 YR
WS 10 YR
•
Ground
710
Bank Sta
�
c
•
w
700
690
680
0
260 460
600
12W
Station (ft)
Figure 7 - HEC-RAS Bridge Model Results for Pleasant Green Connector Road Culvert
ENDESCO, INC. Page No. 7
January 2022
Section 3. DESCRIPTION OF PROPOSED STRUCTURE
Span Length: Single span 43'-0" x 8'-9" rise (Figure 8 and Figure 9) Skew: 00 to C.L.
Abutment Type: Conspan on footing 00 to flood flows
No. of Piers & Type: None.
East Abutment Station: 13+34
West Abutment Station: 12+89
Finish Grade Elevation: 700.50
Finish Grade Elevation: 702.0
Wing walls: Four Precast CON/SPAN Wingwalls with Anchor.
Bridge Scour Analysis Method Used: HEC-RAS
roaa
E
DOWNSTREAM END ELEVATION
Figure 8 - Proposed Pleasant Green Connector Road Culvert Profile
ENDESCO, INC. Page No. 8
January 2022
Figure 9 - Proposed Culvert under Pleasant Green Connector Road
(Not to Scale)
ENDESCO, INC. Page No. 9
January 2022
Section 4. GEOTECHNICAL INVESTIGATIONS
Geotechnical investigations for the Pleasant Green Connector Road Culvert site were conducted by Froehling
& Robertson, Inc, Crozet, Virginia. A total of 7 soil borings were performed and concluded the site to be
suitable for the proposed Conspan® culvert structure as shown in Figure 10.
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Figure 10 - Boring Locations
The predominant soil profiles found were derived from the in -place physical and chemical weathering of the
underlying bedrock materials with as much as 2 to 5 inches of topsoil in each of the boring locations.
Alluvial soils, placed by moving water, were encountered in each boring, below the surficial organics and
extended to depths of 5 feet to 9.5 feet below existing grades. Sampled alluvium consisted of Lean CLAY
(CL), SILT (ML), clayey SAND (SC), silty SAND (SM), and silty GRAVEL (GM) with varying amounts of sand and
gravel.
Residual Soils, formed by the in -place weathering of the parent rock, were encountered below the alluvial
soils in borings TB-01A, TB-02, TB-03, and B-01 and extended to the intermediate geomaterial layer. At
boring TB-03 a layer of residual soils was encountered in the intermediate geomaterial at a depth of 38 to 43
feet below existing grades. The residual soils were generally described as sandy SILT (ML), or silty SAND
(SM).
Intermediate Geomaterial (IGM) is a transitional material between soil and rock which contains the relic
structure of the rock with very hard consistencies or very dense densities. IGM materials were encountered
ENDESCO, INC. Page No. 10
January 2022
in borings TB-01A, TB-02, TB-03, and B-01 below the residual soils at a depth of depths of 18 feet to 38 feet
below existing grades and extending to the auger refusal depth
Auger refusal occurs when materials are encountered that cannot be penetrated by the soil auger and is
normally indicative of a very hard or very dense material, such as boulders, rock lenses, rock pinnacles, or the
upper surface of rock. Auger refusal was encountered in each of the borings at depths ranging from 5.4 to
48.3 feet below existing grades as indicated in the table below.
Table 2 below provides a summary of the geotechnical findings for the four borings.
Table 2 - Boring Log Description
Boring
Sample
Existing
IGM Depth
IGM
Refusal Depth
Refusal
DSO
Location
No.
Type
Elevation
(feet)
Elevation
(feet)
Elevation
(mm)
TB-01
SPT
690.9
9.5*
681.4
West
abutment
TB-OIA
SPT
690.9
18
672.9
27.5
663.4
West
abutment
TB-02
SPT
689.9
38
651.1
48.3
640.8
0.2205
West
(6-8 ft)
abutment
TB-03
SPT
692.6
28**
664.6
43.2
649.4
5.2444
East
(4-6 ft)
abutment
B-01
SPT
693
22
671
22.2
670.8
East
abutment
B-02
SPT
691
5.4
685.6
East
abutment
HA-01
Hand
1.2
2.1726
Upstream
Auger
(0-1 ft)
Channel
HA-02
Hand
1.0
Downstream
Auger
Channel
Notes: * Indicates boring terminated due to skewing augers,
** Indicated that a layer of soil was encountered within the IGM at this boring.
The soil boring information, laboratory testing table and material test reports are shown in the geotechnical
report for reference. The geotechnical report has indicated that Rock Quality Designation (RQD) has been
performed only at one boring location TB-01 and the elevation of RQD greater than 50% was reported at
663.4 feet. It should be noted that contraction scour can occur in erodible rock due to weathering and
abrasion. In addition to hydraulic forces, channels in rock materials may degrade due to wetting and drying,
freeze -thaw, abrasion, and chemical reactions. It is not only necessary to determine the critical shear and
erosion rate information, but also to account for the different potential factors that may create rock erosion.
The geotechnical engineer may take full note for scour in rock while formulating final footing elevations for
the widened sections of the foundations. At other boring locations, it is not reported if the auger refusal
elevation would be non-scourable rock in the geotechnical report.
Particle size distributions at the bridge site are available for several boring locations. Based on the
recommendation in the FHWA Publication HEC No. 18, Fifth Edition, it is a characteristic size of the material
that will be transported by the stream. In this case, a D50 of 2.1726mm at hand auger excavation location
HA-01 was used for the main stream, a D50 of 0.2205mm at boring location TB-02 was used for the west
ENDESCO, INC. Page No. 11
January 2022
abutment location, and a D50 of 5.2444mm at boring location TB-03 was used for the west abutment
location.
ENDESCO, INC. Page No. 12
January 2022
Section S. ANALYSIS
According to VDOT's Drainage Manual, the design for bridge foundation scour should consider the magnitude
of the flood, including the 1 percent event that generates the maximum scour depth. The design should use a
geotechnical design practice factor of safety from 2 to 3. A plot or sketches showing the scoured bed profile
for the design event is required. The following paragraphs outline the procedures adopted for scour analysis
for the 100-year and 500-year flood events.
Abutment Scour: The method used for estimating abutment scour is based on methodology presented in the
FHWA Publication HEC No. 18, Fifth Edition.
1. Contraction Scour:
The contracted section can be represented as the downstream end of the bridge where the flow is
still contracted. The upstream main channel section is considered to be about one bridge length
upstream where the flow is uniform and not influenced by the bridge contraction. The Pleasant
Green Connector Road Culvert over Powells Creek has a length of 43 feet with the center line at
Section 4339. The stream cross -sections at Section 4425 and Section 4339-BR-D are, therefore, taken
as the normal and contracted sections respectively. Table 3 shows the channel and bridge
configuration.
Table 3 - Channel and Bridge Configuration per HEC-RAS Model
Channel
Left Bank
Bridge
Bridge
Right
Channel
Bridge
Section
Elev.
Left Sta.
Right
Bank
Width
Width
Sta.
Elev.
(Ft.)
(Ft.)
4425
691.1
-
691.4
21.3
-
Bridge U/S
694.2
550
593
690.2
188.0
43.0
4339BRU
Bridge D/S
694.7
535
578
689.7
195.6
43.0
4339BRD
4318
1 694.7
1 -
689.7
1 195.6
1 -
The abutments are located at the edge of the main channel indicating a flow condition similar to
Case lain HEC-18 Figure 6.1.
The next step is to determine if the contraction scour is clear -water scour or live -bed scour. To
determine if the flow upstream of the bridge is transporting bed material, calculate the critical
velocity for beginning of motion V, of the D50 size of the bed material being considered for
movement and compare it with the mean velocity V of the flow in the main channel or overbank area
upstream of the bridge opening. If the critical velocity of the bed material is larger than the mean
velocity (V, > V), then clear -water contraction scour will exist. If the critical velocity is less than the
mean velocity (V, < V), then live -bed contraction scour will exist. The following equation is used to
calculate Vc:
VC = KuY1/6D1/3
Where, V, = Critical velocity above which bed material of size D and smaller will be transported,
feet/s
ENDESCO, INC. Page No. 13
January 2022
y = Average depth of flow upstream of the bridge, feet
D = Particle size for V, feet
Dso = Particle size in a mixture of which 50 percent are smaller, feet
Ku = 11.17 English units
The calculated results indicate that the live -bed contraction scour exists in the main channel as well
as the left/right overbanks (Tables 4). Therefore, the modified version of Laursen's equation (1960)
for live -bed scour (Equation 6.2 of HEC-18) is used to predict the depth of scour in the main channel
section.
Table 4 - Contraction Scour Conditions for 10-, 100- and 500-Year Storm Event
10-Year
Left Overbank
Main Channel
Right Overbank
Avg. Velocity M (ft/s)
3.33
7.68
2.89
Ku (English Units)
11.17
11.17
11.17
Avg depth of flow upstream of
1.16
2.18
0.89
bride (y) (ft
Particle size for Vc (ft)
0.01721
0.00713
0.00072
Critical Velocity (Vc) (ft/s) Eq 6.1,
2.96
2.45
0.98
HEC-18
Contraction Scour Condition
Vc<V, Live -Bed
Vc<V, Live-
Vc<V, Live -Bed
Scour
Bed Scour
Scour
100-Year
Left Overbank
Main Channel
Right Overbank
Avg. Velocity ft/s
4.18
8.98
3.58
Ku (English Units
11.17
11.17
11.17
Avg depth of flow upstream of
bride (y) (ft
1.65
2.72
1.22
Particle size for Vc ft
0.01721
0.00713
0.00072
Critical Velocity (Vc) (ft/s) Eq 6.1,
HEC-18
3.13
2.54
1.04
Contraction Scour Condition
Vc<V, Live -Bed
Vc<V, Live-
Vc<V, Live -Bed
Scour
Bed Scour
Scour
500-Year
Left Overbank
Main Channel
Right Overbank
Avg. Velocity ft/s
2.17
7.81
3.55
Ku (English Units)
11.17
11.17
11.17
Avg depth of flow upstream of
bride (y) (ft
0.82
3.55
1.96
Particle size for Vc ft
0.01721
0.00713
0.00072
Critical Velocity (Vc) (ft/s) Eq 6.1,
HEC-18
2.79
2.66
1.12
Contraction Scour Condition
Vc>V, Clear-
Water Scour
Vc<V, Live-
Bed Scour
Vc<V, Live -Bed
Scour
ENDESCO, INC. Page No. 14
January 2022
Based on the results show in Table 4, it has been determined that the majority of contraction scour
for the main channel and the overbanks will be live -bed scour, except for the left over bank under
500-year storm event is clear -water scour. The fall velocity for bed material with a particle size of
2.17 mm is around 0.21 m/sec or 0.689 ft/sec (main channel), the fall velocity for bed material with a
particle size of 0.22 mm is around 0.03 m/sec or 0.098 ft/sec (right bank), and The fall velocity for
bed material with a particle size of 5.24 mm is around 0.33 m/sec or 1.082 ft/sec (left bank) based on
Figure 6.8, HEC-18 (Figure 11).
+o
itmm
5.2cmrr
VAN
............
...........................
0.22 mm . .. T-d'
.�
. A.. ..:
0m1
3
i3
C
v.
.
a.aoo+
o.0000+
0.0+ — r —
0.am 0.0+
Figure 11 - Fall Velocities (Figure 6.8, HEC-18 Manual)
The HEC-RAS analysis carried out for the 10-year, 100-year and 500-year storm events gives average
depths in the upstream main channel and the average energy slopes between the approach section
and bridge section. Table 5 below summarizes the determination of kl.
ENDESCO, INC. Page No. 15
January 2022
Table 5 - Determining k1 factor for the main channel
Average
Shear
Fall
Ratio — Shear
k,
Sediment Movement
Storm
Energy
Location
depth
Velocity
Velocity
Velocity/Fall
Slope
(0) (ft)
(ft/s)
(ft/s)
Velocity
Left Bank
1.16
0.410
1.082
0.379
0.59
Mostly contact bed
material discharge
10-
Main
Some suspended bed
Year
0.0045
Channel
2.18
0.562
0.689
0.816
0.64
material discharge
Mostly suspended bed
Right Bank
0.89
0.359
0.098
3.651
0.69
material discharge
Left Bank
1.65
0.491
1.082
0.453
0.59
Mostly contact bed
material discharge
100-
Main
Some suspended bed
Year
0.0045
Channel
2.72
0.630
0.689
0.915
0.64
material discharge
Right Bank
1.22
0.422
0.098
4.289
0.69
Mostly suspended bed
material discharge
Main
Some suspended bed
500-
Channel
3.55
0.527
0.689
0.764
0.64
material discharge
Year
0.0024
Right Bank
1.96
0.391
0.098
3.976
0.69
Mostly suspended bed
material discharge
The modified version of Laursen's equation for live -bed scour (Equation 6.2 of HEC-18) is used to
predict the depth of scour in a contracted section. Laursen's equation for estimating scour in a
contracted section in a rectangular channel can be expressed as follows:
where,
Y2/Y1 = M2/006" (Wi/W2)kl Equation 6.2 HEC-18 Manual
Y, = Average contraction scour depth = y2 - yo
Y1. Y2 = Average Flow depths in the upstream main channel and the contracted section
ya = Existing depth in the contracted section before scour
Q1, Ct2 = Flows in the upstream main channel and the contracted section
W1, W2= Bottom widths of the upstream main channel and the contracted section
ki = exponent related to sediment transport.
This is a comparative equation, which balances the rates of sediment transport at the un-contracted
and contracted sections. The contraction scour is computed separately for the main channel and the
left and right overbank areas.
The left and right overbank portions of the approach sections show vegetation. This, in conjunction
with the computations shown in Table 4 demonstrates the likelihood for clear -water scour conditions
to occur at the two overbank sections.
Clear -water contraction scour is determined using Laursen Equation (6.4 — HEC-18 Manual) as
follows.
ENDESCO, INC. Page No. 16
January 2022
where,
Z l
KU Q J
Y2 — D2l3 w2 sn
m
y2 = Average equilibrium depth in the contracted section after contraction scour, (ft)
Q = Discharge through the bridge or on the set -back overbank area at the bridge associated
with the width W,(ft3/sec)
Dm = Diameter of the smallest non -transportable particle in the bed material (1.25 Dso) in the
contracted section, (ft)
Dso = Median diameter of bed material, (ft)
W = Bottom width of the contracted section (ft)
K. = 0.0077 English units.
Live -bed scour and clear -water scour computations for the 10-year, 100-year and 500-year storm
events are given in Table 6 and Table 7.
ENDESCO,INC.
January 2022
Page No. 17
Table 6 - Live -Bed Contraction Scour Results
10-Year Storm
Left
Overbank
Main
Channel
Right
Overbank
Average depth in upstream main channel (yi) (ft)
1.16
2.18
0.89
Flow in the upstream channel transporting sediment (01) (ft3/s)
45.98
355.78
388.25
Flow in the contracted channel (%) (ft3/s)
790.00
Width of the upstream main channel that is transporting bed material (Wl) (ft)
48.23
21.30
175.55
Width of the main channel in the contracted section (W2) (ft)
0.00
43.00
0.00
Exponent Kl
0.59
0.64
0.69
Average depth in the contracted section (y2) (ft) - Eq 6.2, HEC-18
0.00
2.76
0.00
WSEL
689.46
689.46
689.46
Average Bed Elevation
689.28
687.56
688.78
Existing depth in the contracted section before scour (yo)(ft)
0.18
1.90
0.68
Average contraction scour depth (yj (ft) = y2 - yo- Eq. 6.3, HEC-18
0.00
0.86
0.00
100-Year Storm
Left
Overbank
Main
Channel
Right
Overbank
Average depth in upstream main channel (yi) (ft)
1.65
2.72
1.22
Flow in the upstream channel transporting sediment (01) (ft3/s)
85.46
520.42
814.12
Flow in the contracted channel (Q2) (ft3/s)
1420.00
Width of the upstream main channel that is transporting bed material (Wl) (ft)
48.23
21.30
234.07
Width of the main channel in the contracted section (W2) (ft)
0.00
43.00
0.00
Exponent Kl
0.59
0.64
0.69
Average depth in the contracted section (y2) (ft) - Eq 6.2, HEC-18
0.00
4.10
0.00
WSEL
689.83
689.83
689.83
Average Bed Elevation
689.28
687.56
688.78
Existing depth in the contracted section before scour (yo)(ft)
0.55
2.27
1.05
Average contraction scour depth (y,) (ft) = y2 - yo- Eq. 6.3, HEC-18
0.00
1.83
0.00
500-Year Storm
Left
Overbank
Main
Channel
Right
Overbank
Average depth in upstream main channel (yi) (ft)
3.55
1.96
Flow in the upstream channel transporting sediment (Q,) (ft3/s)
589.77
1368.19
Flow in the contracted channel (%) (ft3/s)
2060.00
Width of the upstream main channel that is transporting bed material (Wl) (ft)
21.30
234.07
Width of the main channel in the contracted section (W2) (ft)
43.00
0.00
Exponent Kl
0.64
0.69
Average depth in the contracted section (y2) (ft) - Eq 6.2, HEC-18
6.62
0.00
WSEL
690.76
690.76
Average Bed Elevation
687.56
688.78
Existing depth in the contracted section before scour (yo)(ft)
3.20
1.98
Average contraction scour depth (yj (ft) = y2 - yo- Eq. 6.3, HEC-18
3.42
0.00
ENDESCO, INC. Page No. 18
January 2022
Table 7 - Clear -Water Contraction Scour Results
500-Year Storm
Left Overbank
Ku
0.0077
Discharge through the bridge or on the set -back overbank
area at the bridge associated with the width W, Q (cfs)
0.00
Diameter of the smallest non -transportable particle in the bed
material (1.25 x D50) in the contracted section, Dm (ft) (D50 =
0.0164 ft)
0.0215
Bottom width of the contracted section, W (ft)
0.00
Average equilibrium depth in the contracted section after
contraction scour, y2 (ft) — Eq. 6.4, HEC-18
0.00
WSEL
690.76
Average Bed Elevation
689.28
Average existing depth in the contracted section Yo (ft)
1.48
Average contraction scour depth (ys) (ft)Ys = Y2 - YO
0.00
Pressure flow scour will not be considered in this study since it only apply to conditions involving a
submerged bridge superstructure. For the current analysis, pressure flow conditions are not
presented under the 100-yr and 500-yr storm events, so this scour situation is not presented in this
study.
2. Local Abutment Scour Equation - Vertical Wall Abutments with wing walls
As a check on the potential depth of scour to aid in the design of the foundations and placement of
rock riprap, Froehlich's live -bed scour equation (Equation 8.1 HEC-18) or an equation from HIRE
(Highways in the River Environment) (Equation 8.1 HEC-18) is used:
yjy, = 2.27 x ki x k2 x (L'/y,)043 x Fr061 + 1.0 ... Equation 8.1 HEC-18
or
y,/y1= 4 x Fr 0.31 x (ki /0.55) x k2 ... Equation 8.2 HEC-18
Where, k1
= Coefficient for abutment shape (0.82 for Vertical -wall abutment with wing
walls)
k2
= Coefficient for angle of embankment flow (1.0 for 900)
L'
= Length of active flow obstructed by the embankment
Fr
= Froude number of flow upstream of abutment
ya
= Average depth of flow on the floodplain (AQ/L)
A.
= Flow area of the approach cross section obstructed by the embankment
L
= Length of embankment projected normal to flow
yi
= Depth of flow at abutment
ys
= Scour depth
The HIRE equation is used when the ratio of projected abutment length (L) to the flow depth (y1) is
greater than 25. In this study, the ratio is greater than 25 for both abutments. Therefore, HIRE live -
bed scour equation is used to calculate local abutment scour. The above equations are
recommended for both live -bed and clear -water abutment scour conditions.
ENDESCO, INC. Page No. 19
January 2022
Local abutment scour computations have been determined for the 10-year, 100-year and 500-year
storm events as they represent more severe cases and the results are given in Table 8.
Table 8 - Local Abutment Scour Results
10-Year
Left Overbank
Right Overbank
Abutment Shape
Vertical -wall abutment with
wing walls
Vertical -wall abutment
with wing walls
k, (Table 8.1,
HEC-18)
0.82
0.82
9
900
900
k2
1
1
L'
146.40
160.00
L
146.4
160
y1
3.44
3.35
Qe
747.9
531.96
Ae
174.94
153.27
ya - A,/L
1.19
0.96
L/y1
42.56
47.76
Equation Type
HIRE
HIRE
Ve
1.45
1.43
Fr
0.14
0.14
ys (Eq. 8.1,
HEC-18)
ys (Eq. 8.2,
HEC-18)
10.67
10.38
100-Year
Left Overbank
Right Overbank
Abutment Shape
Vertical -wall abutment with
wing walls
Vertical -wall abutment
with wing walls
k, (Table 8.1,
HEC-18)
0.82
0.82
A
900
900
k2
1
1
L'
146.87
194.69
L
146.87
194.69
y1
5.05
4.96
Qe
1292.1
1023.17
Ae
254.91
250.91
ya = A,/L
1.74
1.29
L/y1
29.08
39.25
Equation Type
HIRE
HIRE
Ve
1.49
1.47
Fr
0.12
0.12
ENDESCO, INC. Page No. 20
January 2022
ys (Eq. 8.1,
HEC-18)
ys (tq. ts.z,
HEC-18)
500-Year
Left Overbank
Right Overbank
Abutment Shape
Vertical -wall abutment with
wing walls
Vertical -wall abutment
with wing walls
k, (Table 8.1,
HEC-18)
0.82
0.82
A
900
900
k2
1
1
L'
191.99
205.04
L
191.99
205.04
y1
6.27
6.18
Qe
1730.65
1603.91
Ae
392.54
416.23
ya = A,/L
2.04
2.03
L/y1
30.62
33.18
Equation Type
HIRE
HIRE
Ve
1.58
1.56
Fr
0.11
0.11
ys (Eq. 8.1,
HEC-18)
11
ys (Eq. 8.2,
HEC-18)
18.11
17.82
The above results are compared to the results of scour analysis using HEC-RAS program shown in
Appendix B. Scour analysis performed for 10-year design storms using HEC-RAS model further
indicated that these design storms did not produce greater scour elevations than the 100-year and
500-year storm events. Plots of scour profiles for the 100-year and 500-year storm events are given in
Figures 12 and 13.
In addition to the HIRE and Froehlich methodology, the NCHRP 24-20 methodology has been included
for reference in Appendix C. This methodology is not as conservative as the HIRE and Froehlich
computations, so the NCHRP has been included only as reference and not as a determining factor for
our overall recommendations.
ENDESCO, INC. Page No. 21
January 2022
Bridge Scour RS = 4339
Legend
WS 100 YR
CiMunC
Bank Ste
Abutmtrtt Toe
Contr Scour
!20
.................
Trial Scour
71
c
700
w
690
6B0
670
0 260 400 600 am
1&0 1200
Station (0)
Figure 12 - Scour Profile for 100-Year Discharge
ENDESCO, INC. Page No. 22
January 2022
Bridge Scour RS = 4339
Tao
Legend
WS 500 YR
Ground
•
Bank Sta
Abut ent Toe
.................
720 Total Scour
I
710 r
1
i
r
I
700
i
m
to
690 NO
680
670
660
0 200 400 600 800 1000 1200
Station It
Figure 13 - Scour Profile for 500-Year Discharge
ENDESCO, INC. Page No. 23
January 2022
3. Long-term Aggradation and Degradation
Long-term aggradation or degradation in the form of shoal formations and head -cuts are not evident
at the site. Field observations indicate that the channel is relatively stable vertically, at present. Due
to the short length of the stream upstream of the bridge, it is considered that future aggradation or
degradation of the channel, due to changes in sediment delivery from the watershed, are minimal.
Based on these observations, and due to the lack of other possible impacts to the river reach, it is
determined that the channel will be relatively stable vertically at the bridge crossing and long-term
aggradation or degradation potential is considered to be minimal.
ENDESCO, INC. Page No. 24
January 2022
Section 6. SUMMARY OF RESULTS
The results of the scour analysis for the 10-year, 100-year and 500-year storm events are presented in Tables
9 to 11. The average bed elevations given are those expected at the centerline of bridge alignment lowest
after -scour bed elevations.
Table 9 - Summary of Results -10 Year Storm Event
Design 10-Year Event
East Abut.
Main Channel
West Abut.
Net Contraction Scour Depth, Ft
0.0$
0.9$
0.0$
Abutment Local Scour Depth, Ft
10.7%
-
10.4%
Total Scour Depth, Ft
10.7
0.9
10.4
Average Bed Elevation at Structure
689.3
687.6
688.8
Lowest After -Scour Bed Elevation
678.6 ±
686.7 ±
678.4 ±
"Clearwater Scour $ Live Bed Scour 'Froehlich % HIRE
Table 10 - Summary of Results -100 Year Storm Event
Design 100-Year Event
East Abut.
Main Channel
West Abut.
Net Contraction Scour Depth, Ft
0.0$
1.8$
0.0$
Abutment Local Scour Depth, Ft
14.8%
-
14.5%
Total Scour Depth, Ft
14.8
1.8
14.5
Average Bed Elevation at Structure
689.3
687.6
688.8
Lowest After -Scour Bed Elevation
674.5 ±
685.7 ±
674.2 ±
"Clearwater Scour $ Live Bed Scour 'Froehlich % HIRE
Table 11- Summary of Results - 500 Year Storm Event
Design 500-Year Event
East
Abut.
Main
Channel
West
Abut.
Net Contraction Scour Depth, Ft
0.04
3.4$
0.0$
Abutment Local Scour Depth, Ft
18.1%
-
17.P
Total Scour Depth, Ft
18.1
3.4
17.8
Average Bed Elevation at
Structure
689.3
687.6
688.8
Lowest After -Scour Bed Elevation
671.2±
684.1±
671.0±
"Clearwater Scour $ Live Bed Scour 'Froehlich % HIRE
Due to the nature of soils encountered at site, the proposed Pleasant Green Connector Road Culvert may be
supported on a shallow foundation system. Cutoff wall should be installed along culvert foundations to 1 foot
below the scour elevations (designed for 100-year storm event in Table 10) to support the culvert structure,
and to prevent the scour to the culvert foundations on both sides. The top of footings as well as the lengths
of the abutments will be determined by the structural engineer in consultation with the geotechnical
engineer.
The presence of wing walls flared both on the upstream and downstream sides of the culvert helps in
streamlining flow and reducing local abutment scour.
ENDESCO, INC. Page No. 25
January 2022
Countermeasures
Riprap was sized using HEC-23 methodology for an abutment scour protection countermeasure for 100-yr
storm event. See Table 12 for a summary table and Appendix B for detailed computations.
(K) ryl
Aso = (ss 1) LaY1 Y (Eq. 8.3)
Dso = median stone diameter, (ft)
V = characteristic average velocity in the contracted section (fps)
S, = specific gravity of rock riprap
g = gravitational acceleration (32.2 ft/sZ)
y = depth of flow in the contracted bridge opening, (ft)
K = 1.02 for vertical wall abutment
Table 12 - Riprap Sizing for Abutment Scour Protection
Left Abutment
Right Abutment
Overbank Flow (cfs)
1292.10
1023.17
Bank Section Area (sf)
254.91
250.91
Depth of flow in the contracted
bridge opening (ft)
5.05
4.96
V (ft/s)
5.07
4.08
FR
0.40
0.32
Dso(ft)
0.49
0.32
APPENDIX B provides the design of riprap countermeasures proposed for the abutments. The following gives
a summary:
1. Provide Class Al Riprap D50 = 0.8 feet based on VDOT Standard Sizes.
2. The apron at the toe of the abutment should extend along the entire length of the toe of extended
abutments.
3. The apron should extend from the toe of the abutment into the bridge waterway at a distance equal
to twice the flow depth (2 x flow depth=10.1 feet), Provide 11 feet. On the approach embankment,
provide 25 ft apron.
4. Riprap mattress thickness = 2 x D50 = 2 x 0.8 = 1.6. Provide 20-inches (Appendix 7D-3 VDOT D.M.)
5. Provide Class Al Riprap 20-inch Thick over Geotextile matting and a 4-inch stone cushion layer (VDOT
No. 25 or 26 aggregate).
The apron should extend for a minimum length of 11 feet in the waterway and 25 feet on the approach
embankment.
Standard method of placement of riprap is shown in Appendix B.
ENDESCO, INC. Page No. 26
January 2022
Section 7. RECOMMENDATIONS
1. The geotechnical report for the culvert indicates the presence of alluvial soils at a depth of 5-feet to
9.5-feet below the bed. Based on the lab result, a D50 of 2.1726mm at hand auger excavation
location HA-01 was used for the main stream, a D50 of 0.2205mm at boring location TB-02 was used
for the west abutment location, and a D50 of 5.2444mm at boring location TB-03 was used for the
west abutment location. In addition, the Rock Quality Designation (RQD) has been identified only at
one boring location. The refusal depth of the boring locations varies from 5.4 feet to 48.3 feet below
the existing ground elevation.
2. As indicated in Table 4, live -bed contraction scour occurs for both 100-Year and 500-Year events. In
addition, clear -water contraction scour depth for 500-Year are presented in Table 7 for the left
overbank.
3. The proposed Pleasant Green Connector Road Culvert may be set on deep foundations to 1 foot
below the scour elevations shown in Table 10 as designed for 100-year storm event to support the
culvert structure based on the scour analysis and geotechnical report recommendation.
4. Riprap countermeasures are proposed for the abutments. Class Al riprap D50 was determined to be
0.80 ft. The extent of the riprap is approximately 11 feet extend from the toe of the abutment into
the bridge waterway. The apron should extend for a minimum length of 25 feet beyond the bridge
on either side. Class Al Riprap 20-inch thickness over geotextile matting and a 4-inch stone cushion
layer (VDOT No. 25 or 26 aggregate) should be provided.
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ENDESCO, INC. Page No. 27
January 2022
APPENDIX A
GEOTECHNICAL REPORT (EXTRACTS)
Pleasant Green Connector Road Culvert
Albemarle County, Virginia
ENDESCO, INC.
1/11/2022
Report of Geotechnical Study
Pleasant Green Connector Road Culvert
Albemarle County, Virginia
F&R Project No. 71Z0219
Prepared For:
Stanley Martin Homes
404 People Place, Suite 303
Charlottesville, Virginia 22911
Prepared By:
Froehling & Robertson, Inc.
6185 Rockfish Gap Turnpike
Crozet, Virginia 23932-3330
December 20, 2021
434.823.5154 6185 Rockfish Gap Turnpike A Minority -Owned
Crozet, VA 22932 Business
FROFHLIG
�aR ROB RTSON
Engineering Stability Since 1881
December 20, 2021
Mr. Gregg O'Donnell
Stanley Martin Homes
404 People Place, Suite 303
Charlottesville, Virginia 22911
Reference: Report of Geotechnical Study
Pleasant Green Connector Road Culvert
Albemarle County, Virginia
F&R Project No. 71ZO219
Dear Mr. O'Donnell:
The purpose of this study is to present the results of the subsurface exploration program and geotechnical
engineering evaluation undertaken by Froehling & Robertson, Inc. (F&R) in connection with the referenced
project. Our services were performed in general accordance with F&R Proposal No. 2171-0283G rev1 dated
October 21, 2021. The attached report presents our understanding of the project, reviews our exploration
procedures, describes existing site and general subsurface conditions, and presents our geotechnical
evaluations and recommendations.
We have enjoyed working with you on this project, and we are prepared to assist you with the
recommended quality assurance monitoring and testing services during construction. Please contact us if
you have any questions regarding this report or if we may be of further service.
Sincerely,
FROEHLING & ROBERTSON, INC.
A*0i164
Matthew E. DuBois, P.E.
Senior Engineer
CLYDE A. SIMMONS, III
Lic. No. 037900
ZONAL -
Clyde A. Simmons, III, P.E.
Senior Geotechnical Engineer
1:\Projects 71Z\71Z0219 (Pleasant Green Connector Road Culvertj\GEO Report\Pleasant Green Connector Road Culvert Report.docz
434.823.5154 6185 Rockfish Gap Turnpike A Minority -Owned
Crozet, VA 22932 Business
1.0 PURPOSE & SCOPE OF SERVICES
The purpose of the subsurface exploration and geotechnical engineering evaluation was to
explore the subsurface conditions in the area of the proposed development and provide
geotechnical engineering design and construction recommendations that can be used during the
design and construction of the proposed structures.
F&R's scope of services included the following:
• Visited the site to observe existing surface conditions;
• Coordinated utility clearance with Miss Utility;
• Reviewed readily available geologic and subsurface information relative to the project site;
• Completion of three soil test borings to depths of 9.5 feet to 48.3 feet below the existing
ground surface;
• Preparation of typed Boring Logs and development of a Subsurface Profile;
• Performing geotechnical laboratory testing on representative soil samples;
• Performing a geotechnical engineering evaluation of the subsurface conditions with regard
to their suitability for the proposed construction;
• Provided recommendations regarding lateral earth pressure coefficients for the design of
below grade walls by others.
• Provided recommendations regarding the placement and compaction of fill materials
required to achieve site subgrades, including an assessment of the suitability of the on -site
soil for re -use as structural fill, and recommendations regarding rock excavation;
• Preparation of this geotechnical report by professional engineers.
Our scope of services did not include a survey of the boring locations, rock coring, quantity
estimates, preparation of plans or specifications, or the identification and evaluation of wetland
or other environmental aspects of the project site.
Stanley Martin Homes
F&R File No. 71ZO219
Page -1 -
Pleasant Green Connector Road Culvert
December20, 2021
2.0 PROJECT INFORMATION
2.1 Site Description
The project site is located on the southwest side of the Pleasant Green Subdivision in Albemarle
County, Virginia (See Site Location Plan, Drawing No. 1, Appendix 1). The new connector road is
planned to connect the roundabout on Alston Street in the Pleasant Green Subdivision with
Orchard Drive to the west. The project corridor is mostly wooded, but clearing has been
performed for a sanitary sewer line located to the south of the road. The new road will cross
Powells Creek near the midpoint between the two existing roads. The existing grades range from
approximately El 708 at Alston Street down to El 687 at Powells Creek and back up to El 710 at
Orchard Drive.
2.2 Proposed Construction
Project information was provided by email and included the "Pleasant Green Connector Road
Plan Set," 11 Sheets, by Collins Engineering, dated 12/8/21, and the "Pleasant Green —
Preliminary ConsSpan Drawings-11-23-2021.pdf", which included 3 sheets by Contech
Engineered Solutions, LLC. We understand that the proposed roadway crossing of Powells Creek
is planned to consist of a single span arch culvert. The arch culvert is planned to consist of precast
concrete, with an approximate span of 43 feet, approximate length of 72 feet, and an
approximate clear rise of 8'-9". The soil cover measured at the middle of the arch will be
approximately 5.5 feet or less. The design will also include precast concrete wingwalls with a
maximum height of approximately 12 feet. As indicated in the provided loading information,
vertical loads of 32.4 kips per linear foot and horizontal loads of 23 kips per linear foot are
anticipated at the base of the arch culvert.
3.0 EXPLORATION PROCEDURES
3.1 Subsurface Exploration
The exploration program was performed on November 23 and 24, 2021, and consisted of three
soil test borings designated TB-01 through TB-03 and two hand auger excavations designated HA-
01 through HA-02. An F&R geotechnical engineer was onsite to monitor drilling, log the borings
and perform visual classification of the recovered samples during the exploration program. In
January 2021, F&R performed a previous study in this location for evaluation of the triple box
culvert stream crossing. Two soil borings, designated B-01 and B-02 were performed as part of
that study. The boring logs and laboratory testing performed during that exploration have been
incorporated into this report. The borings were drilled to depths of 5.4 feet to 48.3 feet below
existing grades. The hand auger excavations were extended to depths of 1 to 1.1 feet below the
existing grades before reaching refusal on cobbles. Boring TB-01 encountered auger refusal at
9.5 feet due to auger skewing. An offset boring, TB-01A was drilled approximately 5 feet south
Stanley Martin Homes
F&R File No. 71ZO219
Page-2-
Pleasant Green Connector Road Culvert
December 20, 2021
n
of the original location. Boring TB-01A was extended 10 feet past the auger refusal depth of 27.5
feet with NQ rock coring techniques. The locations of the borings are shown on the attached
Boring Location Plan (Drawing No. 2, Appendix 1). The test boring locations were staked in the
field by the project surveyor. The elevations shown on the boring logs were copied from those
marked on the survey stakes. Given that some minor shifting of pre -staked locations may have
occurred during drilling, we recommend that the test boring locations and elevations shown on
the attached Boring Location Plans and Boring Logs be considered approximate.
The soil test borings were performed in accordance with generally accepted practice using a
track -mounted Diedrich D-50 rotary drill rig equipped with an automatic hammer. Hollow -stem
augers were advanced to pre -selected depths, the center plug was removed, and representative
soil samples were recovered with a standard split -spoon sampler (13/8 in. ID, 2 in. OD) in general
accordance with ASTM D 1586, the Standard Penetration Test. For these tests, a weight of 140
pounds was freely dropped from a height of 30 inches to drive the split -spoon sampler into the
soil. The number of blows required to drive the split -spoon sampler three consecutive 6-inch
increments was recorded, and the blows of the last two increments were summed to obtain the
Standard Penetration Resistance (N-value). The N-value provides a general indication of in -situ
soil conditions and has been correlated with certain engineering properties of soils.
Research has shown that the Standard Penetration Resistance (N-value) determined by
automatic hammer is different than the N-value determined by the safety hammer method.
Most corrections that are published in technical literature are based on the N-value determined
by the safety hammer method. This is commonly termed N60 as the rope and cathead with a
safety hammer delivers about 60 percent of the theoretical energy delivered by a 140-pound
hammer falling 30 inches. Several researchers have proposed correction factors for the use of
hammers other than the safety hammer. The correction is made by the following equation:
N60 = Nfield X CE
where Nfield is the value recorded in the field, and CE is the drill rod energy ratio for the hammer
used. The guidelines provided in the Performance and Use of the Standard Penetration Test in
Geotechnical Engineering Practice manual published by the Center for Geotechnical Practice and
Research at the Virginia Polytechnic Institute and State University recommend that a correction
factor (CE) be used to convert Nfield values to N6o values when using an automatic hammer. We
recommend that a correction factor (CE) of 1.3 be used to convert Nfield to N60 values.
Plotted N-values reported on Boring Logs are the actual, field -derived blow counts (Nfield). Drilling
notes on each Boring Log indicates whether penetration resistances presented on the Boring Log
were determined using automatic hammer or conventional hammer systems. Corrected N60
values were used for all analyses.
Stanley Martin Homes
F&R File No. 71Z0219
Page - 3 -
Pleasant Green Connector Road Culvert
December20, 2021
n
The test borings were advanced through the soil overburden by soil drilling procedures to the
auger refusal materials were encountered. Rock coring was performed at boring TB-01 from the
auger refusal depth of 27.5 feet to the boring termination depth of 37.5 feet below the existing
grades. Rock coring was accomplished in general accordance with ASTM D 2113 using a 2-inch
nominal inside diameter diamond -impregnated drill attached to the end of a double -tube core
barrel. Rock core specimens were measured for recovery immediately upon retrieval, placed in
core boxes for protection, and transported to our laboratory for evaluation by our professional
staff. The rock core specimens were measured for Percent Recovery and Rock Quality
Designation (RQD) by a member of our professional staff. Percent Recovery is the ratio of the
recovered core length to the length of rock drilled, expressed by a percentage. RQD is the ratio
of the cumulative length of all pieces of rock greater than or equal to four inches to the total
amount drilled, expressed as a percent of the total amount drilled. The RQD value is related to
the soundness and quality of the rock mass and has been correlated with engineering properties
of rock. Qualitative descriptions of the rock cored were also developed and are included on the
boring logs. Subsurface water level readings were taken in each of the borings immediately upon
completion of the drilling process. Upon completion of drilling, the boreholes were backfilled
with auger cuttings (soil). Periodic observation of the boreholes should be performed to monitor
subsidence at the ground surface, as the borehole backfill could settle over time. Borings
performed in asphalt or concrete pavement were patched with non -shrink grout or asphalt cold
patch.
Representative portions of the split -spoon soil samples obtained throughout the exploration
program were placed in glass jars and transported to our laboratory. In the laboratory, the soil
samples were evaluated by a member of our engineering staff in general accordance with
techniques outlined in the visual -manual identification procedure (ASTM D 2488). The soil
descriptions and classifications discussed in this report and shown on the attached Boring Logs
are based on visual observation and should be considered approximate. A copy of the boring
logs are provided and classification procedures are further explained in Appendix II.
Split -spoon soil samples recovered on this project will be stored at F&R's office for a period of 60
days. After 60 days, the samples will be discarded unless prior notification is provided to us in
writing.
3.2 Laboratory Testing
Representative soil samples were subjected to Water Content (ASTM D 2216), Atterberg Limits
(ASTM C4318) and #200 Sieve Wash (ASTM D1140) testing to substantiate the visual
classifications and assist with the estimation of the soils' pertinent engineering properties. PH
and resistivity testing were also performed to estimate the soil's corrosive potential. Test results
are provided in Section 4.4 of this report
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4.0 REGIONAL GEOLOGY & SUBSURFACE CONDITIONS
4.1 Regional Geology
The project site is located in the upland area of the Piedmont Plateau, at the western edge of the
Piedmont Physiographic Province, an area underlain by ancient metamorphic rocks. Information
obtained from the Geologic map of Virginia (1993) indicates that the project site is underlain by
Charnockite, a Plutonic Rock of Grenville Age. The virgin soils encountered in this area are the
residual product of in -place chemical and mechanical weathering of the parent bedrock
formation that underlies the site. These materials consist of SILT and CLAY soils near the surface
where soil weathering is more advanced, underlain by silty SAND and clayey SAND.
The boundary between soil and rock is often times not sharply defined. The transitional term
"Intermediate Geomaterial" is normally found overlying the parent bedrock. For engineering
purposes, IGM is described as broken and partially weathered rock with Standard Penetration
Resistance N-values greater than 50 blows per 6 inches.
Weathering is facilitated by fractures, joints and the presence of less resistant rock types.
Consequently, the profile of the IGM is often quite irregular, even over very short horizontal
distances. Also, it is not unusual to find lenses, layers, or zones of less resistant or more resistant
IGM, and boulders of hard rock within the soil mantle well above the general bedrock level.
4.2 Subsurface Conditions
4.2.1 General
The subsurface conditions discussed in the following paragraphs and those shown on the
attached Boring Logs and Subsurface Profile represent an estimate of the subsurface conditions
based on interpretation of the boring data using normally accepted geotechnical engineering
judgments. The transitions between different soil strata are usually less distinct than those
shown on the boring logs. Sometimes the relatively small sample obtained in the field is
insufficient to definitively describe the origin of the subsurface material. In these cases, we
qualify our origin descriptions with "possible" before the word describing the material's origin
(i.e. possible fill, etc.). Although individual soil test borings are representative of the subsurface
conditions at the boring locations on the dates shown, they are not necessarily indicative of
subsurface conditions at other locations or at other times. Data from the specific soil test borings
is shown on the attached Boring Logs in Appendix II.
Below the existing ground surface, the borings generally encountered surficial materials, alluvial
soils, residual soils, intermediate geomaterial, and auger refusal materials. These materials are
generally discussed in the following paragraphs.
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4.2.2 Surficial Materials
Surficial organic soils were encountered in each of the borings to depths of approximately 2 to 5
inches. Surficial organic soil is typically a dark -colored soil material containing roots, fibrous
matter, and/or other organic components, and is generally unsuitable for engineering purposes.
F&R has not performed any laboratory testing to determine the organic content or other
horticultural properties of the observed surficial organic soil materials. Therefore, the term
surficial organic soil is not intended to indicate a suitability for landscaping and/or other
purposes. The surficial organic soil depths provided in this report are based on driller
observations and should be considered approximate. We note that the transition from surficial
organic soil to underlying materials may be gradual, and therefore the observation and
measurement of surficial organic soil depths is subjective. Thicker layers of surficial organics
should be expected in wooded areas to account for the presence of root balls. Actual surficial
organic soil depths should be expected to vary.
4.2.3 Alluvial Soils
Alluvial soils, placed by moving water, were encountered in each boring, below the surficial
organics and extended to depths of 5 feet to 9.5 feet below existing grades. Sampled alluvium
consisted of Lean CLAY (CL), SILT (ML), clayey SAND (SC), silty SAND (SM), and silty GRAVEL (GM)
with varying amounts of sand and gravel. Sampled alluvium was brown, tan brown, light brown
and gray in color, with water contents visually characterized as moist to wet. The Standard
Penetration Test values (N-values) in the alluvium ranged from 3 bpf to 100+ bpf. The higher N-
values obtained in the alluvium layer are attributable to gravel and cobbles deposited within the
alluvial soils.
4.2.4 Residual Soils
Residual Soils, formed by the in -place weathering of the parent rock, were encountered below
the alluvial soils in borings TB-01A, TB-02, TB-03, and B-01 and extended to the intermediate
geomaterial layer. At boring TB-03 a layer of residual soils was encountered in the intermediate
geomaterial at a depth of 38 to 43 feet below existing grades. The residual soils were generally
described as sandy SILT (ML), or silty SAND (SM). The sampled residual soils were brown, light
brown, and tan, in color, with moisture contents visually characterized as wet. N-values in the
residual soils ranged from 2 bpf to 52 bpf.
4.2.5 Intermediate Geomaterial
Intermediate Geomaterial (IGM) is a transitional material between soil and rock which contains
the relic structure of the rock with very hard consistencies or very dense densities. IGM materials
were encountered in borings TB-01A, TB-02, TB-03, and B-01 below the residual soils at a depth
of depths of 18 feet to 38 feet below existing grades and extending to the auger refusal depth as
shown in the table in section 4.2.6. At boring TB-03 a less resistant layer of residual soils was
encountered within the IGM at a depth of 38 to 43 feet below existing grades. When sampled
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IGM was generally described as silty SAND (SM) with varying amounts of gravel. The sampled
IGM was brown gray or gray in color with moisture contents visually characterized as wet. The
N-values in the IGM ranged from 50/6 to 50/2.
4.2.6 Auger Refusal Materials
Auger refusal occurs when materials are encountered that cannot be penetrated bythe soil auger
and is normally indicative of a very hard or very dense material, such as boulders, rock lenses,
rock pinnacles, or the upper surface of rock. Auger refusal was encountered in each of the
borings at depths ranging from 5.4 to 48.3 feet below existing grades as indicated in the table
below. The auger refusal conditions encountered at boring TB-01, B-02, and B-02A are likely due
to alluvial cobbles or boulders and are not expected to be indicative of the bedrock surface.
Auger refusal conditions with a Diedrich D-50 drill rig do not necessarily indicate conditions
impenetrable to other equipment. Auger refusal conditions will likely vary in unexplored areas
of the site.
Notes:
4.3
Boring
No.
Existing
Elevation
IGM Depth
(feet)
IGM
Elevation
Refusal Depth
(feet)
Refusal
Elevation
TB-01
690.9
9.5*
681.4
TB-01A
690.9
18
672.9
27.5
663.4
TB-02
689.9
38
651.1
48.3
640.8
TB-03
692.6
28**
664.6
43.2
649.4
B-01
693
22
671
22.2
670.8
B-02
691
5.4
685.6
B-02A
691
5.4
685.6
* Indicates boring terminated due to skewing augers,
** Indicated that a layer of soil was encountered within the IGM at this boring.
Subsurface Water
The test borings were monitored during and after drilling operations to obtain short-term
subsurface water information. Subsurface water was encountered at depths of 1.8 to 7 feet
below existing grades as shown in the following table.
Boring
No.
Existing
Elevation
Subsurface Water
Depth (feet)
Subsurface Water
Elevation
TB-01
690.9
1.8
689.1
TB-01A
690.9
1.8
689.1
TB-02
689.9
2.0
687.9
TB-03
692.6
7.0
685.6
B-01
693
5
688
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Boring
No.
Existing
Elevation
Subsurface Water
Depth (feet)
Subsurface Water
Elevation
B-02
691
2.5
688.5
It is anticipated that the groundwater elevation should closely match that of the water level in
Powells Creek. It should be noted that the location of the subsurface water table could vary by
several feet because of seasonal fluctuations in precipitation, evaporation, surface water runoff,
local topography, and other factors not immediately apparent at the time of this exploration.
Normally, the highest subsurface water levels occur in the late winter and spring and lowest
levels occur in the late summer and fall.
4.4 Laboratory Test Results
As discussed in Section 3.2, laboratory testing was performed on selected soil samples collected
during our subsurface exploration. The results from the laboratory testing are included in the
following table.
Boring
No.
Sample Depth
(Feet)
Natural Water
Content (%)
Liquid Limit/
Plasticity Index
% Passing
No. 200 Sieve
LISCS
Class.
B-01
0-2
17.1
--
--
B-01
2-4
23.2
27/7
36.1
SM
B-01
4-6
21.1
--
--
B-01
6-8
40.6
B-01
8-10
26.4
B-01
13.5-15
21.1
B-01
18.5-20
30.0
B-02
0-2
19.3
--
--
B-02
2-4
21.7
42/18
52.1
CL
B-02
4-6
13.8
--
--
TB-01
0-2
21.0
TB-01
2-4
13.3
TB-01
6-8
21.3
TB-01
13-15
19.8
TB-01
18-20
16.4
TB-02
0-2
15.7
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Boring
No.
Sample Depth
(Feet)
Natural Water
Content (%)
Liquid Limit/
Plasticity Index
% Passing
No. 200 Sieve
LISCS
Class.
TB-02
2-4
16.9
TB-02
4-6
30.7
TB-02
6-8
26.8
NP/NP
25.0
SM
TB-02
13-15
24.7
TB-02
28-30
36.8
TB-02
38-40
25.6
TB-03
0-2
19.1
TB-03
2-4
15.1
TB-03
4-6
10.4
NP/NP
17.3
GM
TB-03
6-8
32.0
TB-03
8-10
36.3
TB-03
13-15
31.8
TB-03
18-20
30.2
TB-03
23-25
21.7
TB-03
28-30
16.8
TB-03
33-35
18.1
TB-03
38-40
23.2
HA-01
0-1
12.5
NP/NP
1.3
SP
PH and resistivity testing was performed on a composite sample collected at a depth of 4 to 6
feet below existing grades at boring B-01 to evaluate the potential corrosivity of the on -site
materials. The pH of the sample was 5.2 and the resistivity was 8,620 ohm -cm.
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5.0 GEOTECHNICAL DESIGN RECOMMENDATIONS
5.1 General
The following evaluations and recommendations are based on our observations at the site,
interpretation of the field data obtained during this exploration, and our experience with similar
subsurface conditions and projects. Soil penetration data has been used to estimate an allowable
bearing pressure and associated settlement using established correlations. Subsurface
conditions in unexplored locations may vary from those encountered. If the structure locations,
loadings, or elevations are changed, we should be notified and requested to confirm and, if
necessary, re-evaluate our recommendations.
Determination of an appropriate foundation system for a given structure is dependent on the
proposed structural loads, soil conditions, and construction constraints such as proximityto other
structures, etc. The subsurface exploration aids the geotechnical engineer in determining the
soil stratum appropriate for structural support. This determination includes considerations with
regard to both allowable bearing capacity and compressibility of the soil strata. In addition, since
the method of construction greatly affects the soils intended for structural support, consideration
must be given to the implementation of suitable methods of site preparation, fill compaction,
and other aspects of construction, where applicable.
Based on the provided loading, shallow foundations are not feasible for support of the arch
culvert foundations. Therefore we recommend the use of driven steel piles and a pile cap for
support of the arch culvert. The wing walls which are expected to have much lower loads, can
be supported by shallow foundations.
5.2 Arch Culvert Support — Deep Foundations
We understand that the proposed crossing is planned to consist of a precast concrete arch culvert
with a span of approximately 43 feet and a clear rise of approximately 8'-9". The culvert will be
approximately 72 feet long with concrete wing walls at each end. We recommend that the arch
culvert be supported by driven steel piles. The recommended pile designs for support of the
culvert is presented in the following tables.
Factored
Substructure
Estimated
Minimum Tip
pile Type
Resistance
Unit
Tip Elevation
Elevation
(kips)
East
HP 10x42
162.5
651-673
673
Foundation
West
HP 10x42
162.5
665-671
671
Foundation
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Max.
Max
Max Shear
Substructure
Deflection
Fixity
Moment
Max Shear
Unit
(in)
Depth (ft)
Moment
Depth
(kips)
Depth
(ftkip)
(feet)
(feet)
Arch Culvert
Foundation
0.5
17.4
30.7
6.7
6.5
0
The HP 10x42 piles should be ASTM A709 Grade 50 steel piles equipped with pile points. We
recommend that the pile locations be predrilled to an elevation of El 680 to penetrate the alluvial
soils some of which contain gravels and cobbles that could be difficult to drive the piles though.
The piles should have a minimum pile to pile spacing of at least 3 feet.
The piles should be driven to a driving resistance of 250 kips as determined by the pile driving
analyzer testing (PDA). The design factored resistance is based on piles driven to 250 kips and an
applied resistance factor of 0.65 for PDA tested piles. We recommend a minimum of 1 test pile
be performed on either side of Powells Creek.
All of the piles should be driven in accordance with VDOT Road and Bridge Specifications 2020
Section 403. Prior to pile driving, the Contractor should engage a Geotechnical Engineer to
perform wave equation analyses in accordance with VDOT Road and Bridge Specifications 2020
to evaluate the suitability of the contractor selected hammer and to establish the initial driving
criterion. The Contractor shall provide hammer and cushion data for the proposed pile driving
equipment. This analysis should be required prior to mobilizing the hammer and equipment to
the job site. The contractor should also engage with a Geotechnical Engineer to provide PDA
testing during driving.
The following comments are based on the results of the pH and resistivity testing and available
references regarding soil corrosion potential. A soil sample from the on -site borrow area was
tested for pH and resistivity, with results of 5.3 and 8,620 ohm -cm. The soils tested generally
exhibit characteristics associated with low corrosion potential. We note that the project
structural and civil designers and/or other applicable parties should also review the soil pH and
resistivity test results for their determination of whether any corrective or preventative actions
are required to protect foundations and other below -grade materials (such as pipes or other
buried steel) from corrosion.
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5.3 Wing Wall Shallow Foundations
The arch culvert will have wing walls with heights of up to 12 feet on each side of the culvert.
Based on the results of the subsurface exploration, it is anticipated that the concrete wing walls
on each end of the arch culvert will be supported by alluvial soils or rock. We recommend that
shallow foundations be designed for a factored bearing resistance not to exceed 2,000 pounds
per square foot (psf).
We recommend that the wing wall foundation subgrades be observed by the geotechnical
engineer to determine if suitable subgrade conditions are present. Where soft or very loose
consistency soils, such as those encountered in TB-03 from elevation El 686.6 to El 679.6 are
encountered, they should be undercut and replaced with VDOT No. 57 stone down to suitable
firm materials or to a maximum depth of 3 feet. The anticipated undercut depth is shown in the
following table:
Boring
Location
Anticipated Undercut Depth (feet)
No.
TB-03
North End of Culvert
3
Where VDOT No. 57 stone or similar open graded materials are used as backfill, they should be
encased with a geotextile filter fabric. To reduce the possibility of localized shear failures, we
recommend that the foundations have a minimum width of 3 feet. We recommend that all
exterior footings be placed a minimum of 2 feet below finished exterior grades to protect against
the effects of frost or as required based on the scour analysis.
F&R recommends that the wing walls be backfilled with VDOT No. 57 stone. A friction angle of
40 degrees and a moist unit weight of 115 pcf can be used for earth pressure calculations with
the No. 57 Stone. Considering the anticipated subgrade soils, F&R recommends a base friction
coefficient of 0.34 and a sliding resistance factor (0t) of 0.85 be used for sliding calculations.
5.4 Settlement
Based on the boring data, proposed grading, and assumed structural information, we estimate
that foundation settlements will be less than 1 inch with differential settlement of up to one-half
the estimated total settlement. The magnitude of differential settlements will be influenced by
the variation in excavation requirements across the foundation footprint, the distribution of
loads, and the variability of underlying soils.
Our settlement analysis was performed on the basis of the assumed structural loading and
provided grading information discussed above. Actual settlements experienced by the structure
and the time required for these soils to settle will be influenced by undetected variations in
subsurface conditions, final grading plans, and the quality of fill placement and foundation
construction.
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5.5 Lateral Earth Pressures
Earth pressures on walls below grade are influenced by structural design of the walls, conditions
of wall restraint, methods of construction and/or compaction, and the strength of the materials
being restrained. The most common conditions assumed for earth retaining wall design are the
active and at -rest conditions. Active conditions apply to relatively flexible earth retention
structures, such as freestanding walls, where some movement and rotation may occur to
mobilize soil shear strength. Walls that are rigidly restrained, such as basement, pit, pool and
tunnel walls, should be designed for the structure requiring the use of at -rest earth pressures.
A third condition, the passive state, represents the maximum possible pressure when a structure
is pushed against the soil, and is used in wall foundation design to help resist active or at -rest
pressures. Because significant wall movements are required to develop the passive pressure, the
passive earth pressure resistance factor (4)eP) of 0.5 should be used.
F&R recommends that VDOT No. 57 Stone be used as below grade wall backfill. The
recommended lateral earth pressure coefficients and equivalent fluid pressure parameters for
design of below grade walls using these materials are provided in the following table.
Lateral Earth
Equivalent Fluid
Soil Type
Base Friction
Coefficient
Pressure
Coefficient (k)
Unit Weight (yeq, pcf)
At -rest
Active
At -rest
Active
Passive
VDOT No. 57 Stone
0.34
0.36
0.22
41
25
300
A moist unit weight of 115 pcf for No. 57 Stone should be used for design calculations. The backfill
material should be extended a minimum distance of 0.5 times the wall height laterally from the
back face of the wall, or for a cantilevered wall, from the heel of the wall footing.
Our recommendations were given assuming that the ground surface above the wall is level. The
recommended equivalent fluid pressures were provided assuming that constantly functioning
drainage systems, consisting of crushed stone blanket drain and slotted 4 inch diameter PVC pipe,
are installed between walls and backfill to preventthe accidental buildup of hydrostatic pressures
and lateral stresses in excess of those stated. If a functioning drainage system is not installed,
then lateral earth pressures should be determined using the buoyant weight of the soil.
Hydrostatic pressures calculated with the unit weight of water (62.4 pcf) should be added to
these earth pressures to obtain the total stresses for design.
Heavy equipment should not operate within 5 feet of below grade walls to prevent lateral
pressures in excess of those cited. Adjacent footings or other surcharge loads located a short
distance outside below grade walls will also exert appreciable additional lateral pressures.
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Surcharge loads should be evaluated using the appropriate active or at -rest pressure coefficients
provided above. The effect of surcharge loads should be added to the recommended earth
pressures to determine total lateral stresses.
6.0 GEOTECHNICAL CONSTRUCTION RECOMMENDATIONS
6.1 Site Preparation
Before proceeding with construction, existing footings, utilities, concrete and crushed stone, and
other deleterious non -soil materials (if any) should be stripped or removed from the proposed
construction area. Attention should be given to these areas to ensure all unsuitable material is
removed prior to continuing with construction. During the site preparation operations, positive
surface drainage should be maintained to prevent the accumulation of water. Existing
underground utilities should be re-routed to locations a minimum of 10 feet outside of any
proposed structure footings or abandoned in place with flowable fill. Prior to fill placement, the
subgrades to receive backfill should be evaluated by the geotechnical engineer. Additional
requirements for earthwork construction is included in Section 303 of the VDOT Road and Bridge
Specifications 2020.
6.2 Excavation Conditions
Auger refusal conditions were encountered in boring TB-01, B-02, and B-02A at depths of 5.4 to
9.5 feet below existing grades. The shallow auger refusal at these locations are likely attributed
to alluvial gravel, cobbles and boulders encountered well above the general bedrock elevation.
As such, we anticipate that difficult excavations in alluvial materials, could be encountered, but
bedrock is not expected.
6.3 Foundation Construction
All foundation subgrades should be observed, evaluated, and verified for the design bearing
pressure by the geotechnical engineer after excavation and prior to reinforcement steel
placement. If low consistency soils, such as those encountered in TB-03 are encountered during
foundation construction, localized undercutting and/or in -place stabilization of foundation
subgrades will be required. The actual need for, and extent of, undercutting should be based on
field observations made by the geotechnical engineer at the time of construction.
Excavations for footings should be made in such a way as to provide bearing surfaces that are
firm and free of loose, soft, wet, or otherwise disturbed soils. Foundation concrete should not
be placed on frozen or saturated subgrades. If such materials are allowed to remain below
foundations, settlements will increase. Foundation excavations should be concreted as soon as
practical after they are excavated. If an excavation is left open for an extended period, a thin
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F&R
mat of lean concrete should be placed over the bottom to minimize damage to the bearing
surface from weather or construction activities. Water should not be allowed to pond in any
excavation.
6.4 Structural Fill Placement and Compaction
Fill materials for may consist of the non -organic on -site soils, or an off -site borrow having a
classification of CL or more granular. Controlled structural fill should be free of boulders, organic
matter, debris, or other deleterious materials, should have a maximum particle size of no greater
than 4 inches, and should have a maximum dry density, as determined by the standard proctor
test (VTM-1), of at least 90 pcf. As previously mentioned in Section 5.5, additional restrictions
will apply for the backfill materials behind below grade walls. Additional requirements for fill
placement and compaction are included in Section 303 of the VDOT Road and Bridge
Specifications 2020.
Based on our visual classifications and the laboratory test results, we anticipate that the on -site
soils should serve satisfactorily as fill provided that the moisture contents can be maintained
within acceptable limits. The on -site soils are considered moisture sensitive and may be difficult
to work with when they are wet of the optimum moisture content. Based on our visual
examination and the laboratory test results, the soil samples were above their anticipated
moisture content. Therefore, drying of the on -site soils should be anticipated.
Predicated on the boring and laboratory results, and the recommendations provided above, the
best time for construction of the structural fills and compacted subgrades would be during the
warmer, drier months of the year, such as from late April through early October. During this time
frame, on -site soils that are wet of optimum can usually be dried to near optimum levels with
relatively little effort. If grading is performed during the colder, wetter months of the year, such
as late October through early April, and suitable dry materials are not available on -site, then off -
site drier borrow sources will likely be necessary.
Fill materials should be placed in horizontal lifts with a maximum loose lift thickness of 8 inches.
New fill should be adequately keyed into stripped and scarified subgrade soils. The fill should be
compacted to at least 95 percent of the material's maximum dry density as determined by the
standard Proctor method (VTM-1). In confined areas, portable compaction equipment and thin
lifts of 3 to 4 inches may be required to achieve specified degrees of compaction. Excessively wet
or dry soils should not be used as fill materials without proper drying or wetting. We recommend
a moisture content range of plus or minus 3 percentage points of the material's optimum
moisture content. We recommend that the contractor have equipment on site during earthwork
for both drying and wetting of fill soils.
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F&R
Where construction traffic or weather has disturbed the subgrade, the upper 8 inches of soils
intended for structural support should be scarified and re -compacted. Field density tests to
determine the degree of compaction should be performed on each lift of fill, with a minimum of
two tests per lift.
6.5 Surface Water/Groundwater Control
Subsurface water for the purposes of this report is defined as water encountered below the
existing ground surface. Based on the subsurface water readings obtained during our exploration
program, we anticipate that subsurface water will be encountered during excavation for the
foundation of the single span arch culvert and some dewatering should be anticipated. In
addition, the contractor should be prepared to dewater should water levels vary from those
encountered during the drilling program. Fluctuations in subsurface water levels and soil
moisture can be anticipated with changes in precipitation, runoff, and season.
An important aspect to consider during development of this site is surface water control. During
the construction, we recommend that steps be taken to enhance surface flow away from any
excavations and promote rapid clearing of rainfall and runoff water following rain events. It
should be incumbent on the contractor to maintain favorable site drainage during construction
to reduce deterioration of otherwise stable subgrades.
6.6 Temporary Excavation Recommendations
Mass excavations and other excavations required for construction of this project must be
performed in accordance with the United States Department of Labor, Occupational Safety and
Health Administration (OSHA) guidelines (29 CFR 1926, Subpart P, Excavations) or other
applicable jurisdictional codes for permissible temporary side -slope ratios and/or shoring
requirements. The OSHA guidelines require daily inspections of excavations, adjacent areas and
protective systems by a "competent person" for evidence of situations that could result in cave-
ins, indications of failure of a protective system, or other hazardous conditions. All excavated
soils, equipment, building supplies, etc., should be placed away from the edges of the excavation
at a distance equaling or exceeding the depth of the excavation. F&R cautions that the actual
excavation slopes will need to be evaluated frequently each day by the "competent person" and
flatter slopes or the use of shoring may be required to maintain a safe excavation depending
upon excavation specific circumstances. The contractor is responsible for providing the
"competent person" and all aspects of site excavation safety. F&R can evaluate specific
excavation slope situations if we are informed and requested by the owner, designer or
contractor's "competent person".
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APPENDIX B
SIZING ROCK RIPRAP AT ABUTMENTS
ENDESCO, INC.
1/11/2022
APPENDIX B
SIZING ROCK RIPRAP AT ABUTMENTS
NOTE: Design Guideline 14 of HEC-23 Outlines the sizing of rock riprap for abutments. The following are the
additional protection measures taken for this bridge.
Riprap sizing is done for the 101 Design Storm Event.
CASE A
Froude Number> 0.80 (from Abutment Scour Computations, HEC-23). Use the following relationship:
D5o=(K/(S,-1))x(V'/g y)"'xy ... Equation 14.1(DG14.6 HEC23)
CASE B
Froude Number s 0.80 (from Abutment Scour Computations, HEC-23). Use the following relationship:
DSp=(KI I%-1))x(V2/gy)xy ... Equation 14.2(DG14.6 HEC23)
Where. V = characteristic average velocity in the contracted section (feet / second)
%= specific gravity of riprep (2.65)
g = 32.2 feet / second
y = depth of flow in the contracted bridge opening (feel)
K = 1.02 for vertical wall abutment and 0.89 for a spill -through abutment.
Step 1 - If SBR (set -back length / average channel flow depth) > 5, characteristic average velocity
in the contracted section In/A) is com uted for the overbank section flow on! If SBR < 5 com ute
ft (0.6 m) Freeboard
Thickness -1.5D® or Drw
P y I P
characteristic average velocity based on the entire contracted area through the bridge opening. Fli 14.8. Typical crave sedbn fix abuknent dprep (Legavee et W. 2006).
Left Bank Length = 146.87 ft
Depth of Flow = 5.05 ft
SBR = 29.1
Right Bank Length = 194.69 ft
Depth of Flow = 4.96 ft
SBR = 39.3
Therefore, SBR >= 5 for both abutments
Computations below are based on the entire contracted area through the bridge opening. The same stone will be placed at BOTH abutments.
Depth of flow in the contracted bridge Opening =
Top w itch =
Bank section area =
Overbank flow=
1V=
FIR =V I(g xy)s=
Froude Number <0.80. Use the following equation: Froude Number> 0.80. Use the following equation
D5o=(KI(S,-1))x(V2/g y)xy D.=(K/I%-1))x(V2/g Y)o14 xY
LEFTR250.91
5.05It
146.87
254.911292.105.07
0.40
K=1.02 Vertical Abutment Wall D5c=1 0.49 1 0.32 Ifeet I
Provide Class Al Riprap D.= 0.8 feet based on VDOT Standard Sizes.
Step 2 - Determine riprap extent and layout
1. The apron at the toe of the abutment should extend along the entire length of the toe of abutments and wingwalls.
2. The apron should extend from the toe of the abutment into the bridge waterway at a distance equal to twice
the flow depth (Min 6 feet).
2 x flow depth = 10.1 ft Provide 11 feet
3. Riprap mattress thickness = 2 x D. = 2 x 0.8= 1.6'. Provide 20-inches (Appendix 713-3 VDOT D.M.)
4. The apron should extend for a minimum length of 25 feet beyond the bridge on either side.
Provide Class AI Riprap 20-inch Thick over Geolextile matting and 4 inch stone cushion layer (VDOT No. 25 or 26 aggregate).
The apron should extend for a minimum length of 25 feet beyond the bridge on either side, and minimum length of 20 feet in the bridge waterway.
Main Charnel
Charnel Bank
Floodplaln
FLOW
R'aae Berm. ex Jnxn lee
2x flew dxpth>25 a, \
w Icli arb leas
lift ......
2x flow depth ar 25 R,
whicli is greater
251
Figure 14.7. Plan view of the extent of rock riprap apron (Lagasse et al. 2006).
APPENDIX C
REFERENCE ONLY COMPUTATIONS
ENDESCO, INC.
1/11/2022
Results of Scour Analysis using HEC-RAS program
10-Year Storm Event
Hydraulic Design Data
Abutment Scour
Input Data
Results
Station at Toe (ft):
Toe Sta at appr (ft):
Abutment Length (ft):
Depth at Toe (ft):
K1 Shape Coef:
Degree of Skew (degrees):
K2 Skew Coef:
Projected Length L' (ft):
Avg Depth Obstructed Ya (ft):
Flow Obstructed Oe (cfs)-
Area Obstructed Ae (sq ft)
Scour Depth Ys (ft):
Froude 0:
Equation-
ENDESCO, INC.
1/11/2022
Left Right
547.88
592.94
375.64
253.96
146A0
160.00
3.44
3.35
0.82 - Vert. with wing walls
90.00
90.00
1.00
1.00
146.40
160.00
1.19
0.96
747.90
531.96
174.94
153.27
10.67 10.38
0.14 0.14
HIRE HIRE
Results of Scour Analysis using HEC-RAS program
100-Year Storm Event
Hydraulic Design Data
Contraction Scour
Input Data
Results
Abutment Scour
Input Data
Results
Lett
Channel
Right
Average Depth (ft):
1.65
2.72
122
Approach Velocity(ftfs):
4.18
8.98
3.58
Br Average Depth (ft):
3-67
0.61
BR Opening Flow (cfs):
1390.56
29.44
BR Top WD (ft):
37.99
19.62
Grain Size D50 (mm):
0-15
0.15
0.15
Approach Flow (de):
85.46
520.42
814.12
Approach Top WD (ft):
12.38
21.30
186.19
K1 Coefficient:
0-690
0.690
Scour Depth Ys (ft):
0.57
Critical Velocity (f 1s):
1.04
Equation:
Live
Left Right
Station at Toe (ft):
547.88
592.94
Toe Sta at appr (ft):
375.64
253.96
Abutment Length (ft):
146.87
194.69
Depth at Toe (ft):
5.05
4-96
K1 Shape Coef:
0.82 - Vert. with wing walls
Degree of Skew (degrees):
90.00
90.00
K2 Skew Coef:
1.00
1.00
Projected Length U (ft):
146.87
194.69
Avg Depth Obstructed Ya (ft):
1.74
1.29
Flow Obstructed Cle (cfs):
1292.10
1023.17
Area Obstructed Ae (sq ft):
254.91
250.91
Scour Depth Ys (ft):
14.83
14.54
Froude #:
0.12
0.12
Equation:
HIRE
HIRE
Combined Scour Depths
Left abutment scour + contraction scour (ft): 15.40
Right abutment scour + contraction scour (ft): 15.11
ENDESCO, INC.
1/11/2022
Results of Scour Analysis using HEC-RAS program
500-Year Storm Event
Hydraulic Design Data
Contraction Scour
Left
Channel
Right
Input Data
Average Depth (ft):
0.82
3.55
1.96
Approach Velocity (ft/s):
2.17
7.81
3.55
Br Average Depth (ft):
5.06
1.17
BR Opening Flow (cfs):
1909.52
150.48
BR Top WD (ft):
35.33
36.33
Grain Size D50 (mm):
0.15
0.15
0.15
Approach Flow (cfs):
102.04
589.77
1368.19
Approach Top WD (ft):
57.50
21.30
196.55
K1 Coefficient:
0.690
0.690
0.690
Results
Scour Depth Ys (ft):
1.79
0.00
Critical Velocity (11/s):
1.09
0.99
Equation:
Live
Live
Abutment Scour
Left
Right
Input Data
Station at Toe (ft):
547.88
592.94
Toe Sta at appr (ft):
375.64
253.96
Abutment Length (ft):
191.99
205.04
Depth at Toe (ft):
6.27
6.18
K1 Shape Coal:
0.82 - Vert. with wing walls
Degree of Skew (degrees):
90.00
90.00
K2 Skew Coel:
1.00
1.00
Projected Length L' (ft):
191.99
205.04
Avg Depth Obstructed Ya (ft):
2.04
2.03
Flow Obstructed Qe (cfs):
1730.65
1603.91
Area Obstructed Ae (sq ft):
392.54
416.23
Results
Scour Depth Ys (ft):
18.11
17.82
Froudell:
0.11
0.11
Equation:
HIRE
HIRE
Combined Scour Depths
Left abutment scour+ contraction scour ift): 19.91
Right abutment scour + contraction scour (ft): 19.61
ENDESCO, INC.
1/11/2022
Abutment Scour
NCHRP - Equation 8.3, HEC-18
100-Year
Left Overbank
Right Overbank
Abutment Shape
Vertical w/ ww
Vertical w/ ww
Projected length of the embankment (L); ft
125.37
173.19
The width of the floodplain (Bf); ft
146.87
194.69
L/Bf
0.85
0.89
Condition Type
Live -bed Scour
Live -bed Scour
The bridge/abutment unit discharge ; ft2/S
33.02
33.02
Upstream channel unit discharge ; ftZ/s
16.45
10.07
g2dgi
2.01
3.28
From Figure 8.10/8.12, the value of dA or dB
1.23
1.10
Upstream flow depth (yi); ft
1.65
1.01
L: yc(Eq. 8.6, HEC-18) R: yc(Eq. 8.5, HEC-18)
3.00
2.80
ymax (Eq. 8.3, HEC-18)
3.69
3.07
Flow depth prior to scour (yo); ft
0.55
1.05
ys (Eq. 8.4, HEC-18)
3.14
2.02
Water Surface Elevation at Bridge (From HEC-RAS)
689.83
689.83
Lowest after -scour elevation
686.14
686.76
500-Year
Left Overbank
Right Overbank
Abutment Shape
Vertical w/ ww
Vertical w/ ww
Projected length of the embankment (L); ft
170.49
183.54
The width of the floodplain (Bf); ft
191.99
205.04
L/Bf
0.89
0.90
Condition Type
Live -bed Scour
Live -bed Scour
The bridge/abutment unit discharge ; ft2/S
47.91
47.91
Upstream channel unit discharge ; ft2/s
4.84
13.82
g2dgi
9.89
3.47
From Figure 8.10/8.12, the value of dA or dB
1.10
1.10
Upstream flow depth (yi); ft
0.62
1.77
L: yc(Eq. 8.6, HEC-18) R: yc(Eq. 8.5, HEC-18)
4.42
5.14
ymax (Eq. 8.3, HEC-18)
4.86
5.65
Flow depth prior to scour (yo); ft
1.48
1.98
s (Eq. 8.4, HEC-18)
3.38
3.67
Water Surface Elevation at Bridge (From HEC-RAS)
690.76
690.76
Lowest after -scour elevation
685.90
685.11
Flood lain Abutment
yuu
Alain Channel
2.0
1.8
�t
n
�.
1.2
1.0
LO 1.5 2.0 2.5 3.0
9,/9�
1,
eon slant,
�L%l3—> U
1.6
L decreasing,
L,B � 0 as
1.4
• c
Figure 8.10. Scour amplification factor for wingwall abutments and live -bed conditions
(NCHRP 2010b).
Figure 8.10. Scour amplification factor for wingwall abutments and live -bed conditions
(NCHRP 2010b).