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HomeMy WebLinkAboutSUB201500100 Presentation Stormwater Management Plan 2015-12-29 . t Nosy 'gorse PROJECTMANAGEMENT ENGINEERING LAND PLANNING ENGINEERING December 29, 2015 Albemarle County Department of Community Development 401 McIntire Road Charlottesville, VA 22902 Regarding: Earthen Embankment Design—Free State Run—SDP 2015-00014 The contractor shall follow the attached design standards for the stormwater management embankment for Free State Run. Best Regards, Justin Shimp, P.E. Shimp Engineering, P.C. MINIMUM STANDARD 3.01 CHAPTER 3 MINIMUM STANDARD 3.01 EARTHEN EMBANKMENT Definition An earthen embankment is a raised impounding structure made from compacted soil. Purpose The purpose of an earthen embankment is to impound stormwater runoff. 1111 Conditions Where Practice Applies An earthen embankment is appropriate for use with infiltration, detention, extended-detention or retention facilities. The design procedures presented in this section may not apply to small embankments or to storm drainage outfall structures with less than 3 feet of embankment height. The review and approval of such structures should be based on sound engineering practices and supporting calculations that verify a stable outfall for the 10-year storm, at a minimum. Similarly, this section does not apply to embankments with a height of 25 feet or more and a maximum storage capacity of 50 acre-feet or more, as measured from the top of the embankment. Such structures may be regulated under the Virginia Dam Safety Act and the Virginia Dam Safety Regulations (VR 625-01-00). The height of an earthen embankment is the vertical distance from the natural bed of the stream or watercourse, measured at the downstream toe of the embankment, to the top of the embankment. If the embankment does not span a stream or watercourse,the height is the vertical distance between the lowest elevation, measured at the outside limit of the embankment, and the top of the embankment. 3.01 - 1 MINIMUM STANDARD 3.01 CHAPTER 3 Planning Considerations Earthen embankments are complex structures that must be designed and constructed with consideration given to the following: a) specific site and foundation conditions, b) construction material characteristics, c)purpose of the impoundment, and d)hazard potential associated with the particular site and/or impoundment. The hazard potential associated with an impoundment is defined in the Virginia Dam Safety Regulations. It is based on the potential for loss of life and/or economic loss due to facility failure. While stormwater management embankments are typically much smaller than those regulated under the Virginia Dam Safety Program,the potential for significant property damage and loss of life may still be present. The engineer is responsible for analyzing potential downstream impacts and for determining if more stringent analyses are required. Minimum guidelines for those facilities not covered under Virginia's Dam Safety Regulations are provided in this handbook. Embankment Types The type of embankment selected will depend on the purpose of the stormwater facility(detention, extended-detention,retention,etc.)and the available soil material for construction. The two general types are listed below: 1. A homogeneous embankment is composed of one kind of material (excluding slope protection). The material used must be sufficiently impervious to provide an adequate water barrier, and the slopes must be moderately flat for stability and ease of maintenance (see Figure 3.01-1a). 2. A zoned embankment contains a central impervious core,flanked by zones of more pervious material, called shells. These pervious zones or shells enclose, support, and protect the impervious core. Typically, a zoned embankment requires an internal drain, or filter, between the impervious zone and the downstream shell and between the shell and the foundation(see Figure 3.01-1b. Soils Investigation A soils investigation, or geotechnical study, should be completed before designing any earthen embankment covered in this section.The scope of such a study will vary from site to site based upon the size of each project.Recommended minimum guidelines for a geotechnical study are provided below. Refer to U.S. Department of Interior (USDI), Design of Small Dams, latest edition, for additional information. 3.01 -2 MINIMUM STANDARD 3.01 CHAPTER 3 FIGURE 3.01 - lb Zoned Embankment Relatively impervious core Transition filter zone Coarse pervious shell ,`'' No; t?'? �" - " rr''4Coarse pervious shell , ,-;''': •,:,t4'h,°Cr.A y8i.;:; ;,P(')-'80.0'-?:,. 4' min Cut off trench — 4' below suitable material (min.) 14' min, Transition filter zone Relatively impervious zone 110- A ''`° Coarse pervious shell / Coarse pervious shell - ` R ' of drz :,z, , sa{ Coarse pervious shell Relatively impervious core Relief wells and blanket deep trench Transition drain c:\3_01—lb filter 3.01 -4 MINIMUM STANDARD 3.01 CHAPTER 3 Stream Diversions The design of some earthen embankments will require provisions for stream diversions around or through the embankment site during construction. A stream diversion can be accomplished by a variety of acceptable means, including open channels, conduits, coffer dams, and pumping. Occasionally, stream diversions may be required to meet additional requirements and/or to be permitted by agencies such as the U.S. Army Corps of Engineers, the Virginia Department of Environmental Quality, and/or the Virginia Marine Resources Commission. Refer to the Virginia Erosion and Sediment Control Handbook(VESCH), 1992 edition,for additional guidance on stream diversions. Design Criteria To establish design water surface elevations and spillway capacity for earthen embankments, various hydrologic design methods and spillway storm frequencies may be used. Factors that affect their selection include: a) the purpose of the stormwater facility: flood control, water quality enhancement, and/or channel erosion control, b) the contributing watershed size, and c) local regulations. Despite the design method selected or the frequency storm is used,the embankment should always be analyzed to ensure safe passage of the maximum spillway design storm while maintaining its structural integrity and stability. Furthermore, the embankment height should be set such that runoff from the spillway design storm can safely pass through one of the following spillways without overtopping the embankment: C a natural or constructed spillway, C a principal spillway, or C a combination of a principal spillway and an emergency spillway. Hydrologic and hydraulic methods are described in Chapters 4 and 5, respectively. Local ordinances or watershed conditions may require a more stringent analysis of the embankment concerning overtopping or spillway capacity. The Soil Conservation Service's (SCS) National Engineering Handbook and the Virginia Dam Safety Regulations provide a classification of dams based on the potential hazard from failure. A dam failure analysis, or breach analysis, may be required to learn the extent of the potential hazard. Any dam breach analysis should use a method similar to the Army Corps of Engineers, SCS(TR-60),National Weather Service,or that specified by the local authority. 3.01 - 6 MINIMUM STANDARD 3.01 CHAPTER 3 3. Foundation materials. The character and distribution of the foundation material must be considered for its shear strength,compressibility,and permeability.Occasionally,the shear strength of the foundation may govern the choice of embankment slopes. Permeability and stratification of the foundation may dictate the need for a zoned embankment. Quite often, foundations contain compressible soils that settle under the weight of the embankment, although the shear strength of these soils is satisfactory.When such settlement occurs in the foundation,the embankment settles.This settlement is rarely uniform over the basal area of the embankment. Therefore, fill materials used on such sites must be sufficiently plastic to deform without cracking. (Minimum Standard 3.02,Principal Sp illway discusses the use of a concrete cradle to protect the spillway barrel sections from separating due to the forces of differential settlement.) A foundation composed of homogeneous soil is simple to evaluate; however,this condition rarely occurs in natural soil deposits. Most often, a stratified deposit composed of layers of several soil types is encountered. To determine the suitability of such a foundation,the following information becomes very important: 1)the geologic history of the site, 2)the degree of stratification, and 3) the order in which materials occur within the stratification. A complex, stratified foundation containing plastic or compressible soil should be investigated by an experienced engineer or geologist. t Foundation cutoff - A foundation cutoff trench of moderately impervious material should be provided under the embankment. The cutoff trench should be installed at or upstream of the dam's It centerline, and should extend up the abutments to the 10-year water surface elevation. The bottom of the cutoff trench should be wide enough to accommodate excavation, backfill and compaction equipment. The trench's minimum width and depth should be 4 feet and the side slopes should be no steeper than 1 H:1 V(refer to Figures 3.01-1 a,b and 3.01-2). Rock foundations - The presence of rock in the embankment foundation area requires specific design and construction recommendations (provided in the geotechnical engineering analysis) to insure a proper bond between the foundation and the embankment. Generally,no blasting should be permitted within 100 feet of the foundation and abutment area. If blasting is essential, it should be carried out under controlled conditions to reduce adverse effects on the rock foundation, such as over-blasting and opening fractures. This is especially critical in areas of Karst topography. Embankment zoning and seepage-The stability of an embankment slope and the seepage pattern through it are greatly influenced by the zoning of the embankment. (Refer to Embankment Types in the Planning Considerations section of this standard.) The position of the saturation line within a homogeneous embankment is theoretically independent of the type of soil used in it. Although soils vary greatly in regard to permeability, even the tightest clays are porous and cannot prevent 3.01 - 8 MINIMUM STANDARD 3.01 CHAPTER 3 C a downstream drainage blanket, C a downstream toe drain, or C a combination of these measures (see Figure 3.01-1b). Seepage encountered in the cutoff trench during construction may be controlled by foundation drains. These drains must be downstream of the embankment centerline and outside the limits of the proposed cutoff trench. Including a toe drain in the design of most homogeneous embankments may be desirable. Embankments built on pervious foundations or constructed of materials that exhibit susceptibility to piping and cracking should always be protected by adequate toe drainage. Toe drains may be constructed of sand, gravel, or rock, depending on the nature of the embankment fill material. Whenever a rock toe drain is installed,a graded filter should be placed between the fill and the drain. Often,a 12-inch layer of well-graded, stream-run,sandy gravel will satisfy this requirement.Filter and drainage diaphragm design criteria are presented in the references listed as USDA-SCS Soil Mechanics Notes No. 1 and No. 3 at the end of this section,and provided in Chapter 5 Appendix 5B. Piping The contact areas between the embankment soils,foundation material,abutments,and conduits are the most susceptible locations for piping failures.Piping occurs due to the variation in materials at contact points and the difficulty in compacting the soil in these areas. Compaction is especially difficult next to and under conduits and seepage collars. Therefore, it is highly recommended that all utility conduits, except the principal spillway, be installed away from the embankment. When utility conduits through the embankment cannot be avoided,they should meet the requirements for spillways, i.e., water tight joints, no gravel bedding, restrained to prevent joint separation due to settlement, etc. Seepage along pipe conduits that extend through an embankment should be controlled by use of the following: C anti-seep collars, or C filter and drainage diaphragms. Refer to Minimum Standard 3.02, Principal Spillway for additional information on the use of anti-seep collars. Filter and drainage diaphragms are presented in USDA-SCS Soil Mechanics Notes No. 1 and No. 3, available upon request from DCR or USDA-SCS. When filter and drainage diaphragms are used,their design and construction should be supervised by a registered professional engineer. 3.01 - 10 MINIMUM STANDARD 3.01 CHAPTER 3 An embankment without an emergency spillway must provide at least 2 feet of freeboard from the maximum 100-year storm W SE to the lowest point on the top of the embankment. (Note that the spillway design storm WSE, if specified,may be used instead of the 100-year elevation.) 2. Top Width - The top of an earthen embankment should be shaped to provide positive drainage. The top width is based on the following table: TABLE 3.01 - 1 Embankment Top Widths Total Height of Minimum Embankment Top Width (ft.) (ft.) 14 or less 8 15-19 10 20-24 12 25 or more 15 Compacted Fill The soil types,as covered in the geotechnical analysis, should be specified by using the Unified Soil Classification System. The compaction requirements should include the percent of maximum dry density for the specified density standard, allowable range of moisture content, and maximum loose lift thickness. Refer to Construction Specifications for Earthen Embankments later in this standard. In general,the design of an embankment should account for approximately 10% settlement unless otherwise specified by a geotechnical report based on the embankment foundation and fill material. The top of the embankment must be level in order to avoid possible overtopping in one location in cases of extreme storms or spillway failure. Compaction tests should be performed regularly throughout the embankment construction; typically, one test per 5,000 square feet on each layer of fill or as directed by the geotechnical engineer. Generally, one of two compaction tests will be specified for embankment construction: the Standard Proctor Test(ASTM D698) or the Modified Proctor Test(ASTM D1557). For the construction of earth dams, the Modified Proctor Test is likely to be more appropriate(Terzaghi, 3.01 - 12 MINIMUM STANDARD 3.01 CHAPTER 3 4. Access should be provided to all areas of an impoundment that require observation or regular maintenance. These areas include the embankment, emergency spillway, basin shoreline,principal spillway outlet, stilling basin,toe drains, riser structure, extended- drawdown device, and likely sediment accumulation areas. FIGURE 3.01 - 3 Profile Along Centerline of Principal Spillway I---1 Constructed topof embankment / \ Settled top of embankment / \ / \ _ Emergency spillway / Assumed phreatic line 4H:1V Riser-- / structure .. \ e* Ow. Anti–seep collars Oitkhab .1.75 h.� RCP '4C.. barrel 4' min.—�'"� Outlet protection Cut off trench c:\3_01-3 3.01 - 14 MINIMUM STANDARD 3.01 CHAPTER 3 3. Compaction - Fill material should be compacted with appropriate compaction equipment such as a sheepsfoot,rubber-tired or vibratory roller. The number of required passes by the compaction equipment over the fill material may vary with soil conditions. Fill material should contain sufficient moisture such that the required degree of compaction will be obtained with the equipment used. The minimum required density is 95% of maximum dry density with a moisture content within±2%of the optimum,unless otherwise specified by the engineer. Each layer of the fill should be compacted as necessary to obtain minimum density and the engineer should certify, at the time of construction, that each fill layer meets the minimum density requirement. All compaction is to be determined by either Standard Proctor Test(ASTM D698)or the Modified Proctor Test(ASTM D1557)as directed by the geotechnical enginer based on site and soil conditions and the size and type of structure being built. 4. Cutoff Trench - The cutoff trench should be excavated into impervious material along or parallel to the centerline of the embankment as shown on the plans. The bottom width of the trench should be governed by the equipment used for excavation,with the minimum width being 4 feet. The depth should be at least 4 feet below existing grade or as shown on the plans. The side slopes of the trench should be 1H:1V or flatter. The backfill should be compacted with construction equipment,rollers,or hand tampers to assure maximum density and minimum permeability. 5. Top Soil - The surface layer of compacted fill should be scarified prior to placement of at least 6 inches of top soil. The top soil shall be stabilized with in accordance with the Virginia Erosion and Sediement Control Handbook, latest edition. Structure and Conduit Backfill Backfill that is beside pipes or structures should be of the same type and quality as specified for the adjoining fill material. The fill should be placed in horizontal layers not to exceed 4 inches in thickness and compacted by hand tampers or other manually directed compaction equipment. The material should completely fill all spaces under and beside the pipe. During the backfilling operation,equipment should not be driven closer than 4 feet,as measured horizontally,to any part of a structure. Also,equipment should NEVER be driven over any part of a structure or pipe,unless compacted fill has been placed to a depth specified by the structural live load capacity of the structure or pipe in order to adequately distribute the load. Filters and Drainage Layers In order to achieve maximum density of clean sands,filter layers should be flooded with clean water and vibrated just after the water drops below the sand surface. The filter material should be placed in lifts of no more than 12 inches. 3.01 - 16 MINIMUM STANDARD 3.01 CHAPTER 3 opportunity. Similarly, any vine cover and brush should be removed from the embankment to allow for inspections. 7. Any repairs made to the princpal spillway(riser or barrel) should be reviewed by a professional engineer. Vertical trenching to expose the barrel should not be allowed under any circumstances. The trench side slopes should be stepped back at a 2:1 slope, minimum. • 3.01 - 18