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HomeMy WebLinkAboutWPO201600008 Certified Engineer's Report 2017-07-25Frank Pohl County Engineer Albemarle County PROJECT MANAGEMENT SHIMCIVIL ENGINEERING LAND PLANNING ENGINEERINGa July 25, 2017 Delivered via E-mail Regarding: Glenmore Subdivision, Carroll Creek Road Crossing Guardrail Certification Dear Mr. Pohl, I have reviewed your bond reduction comment letter for the Glenmore Carroll Creek Road crossing and have made several additional trips to the site to inspect the guardrail and other conditions brought up in your comment letter. I understand that the guardrail issue must be resolved in order to satisfy a zoning requirement, which will permit the approval of plats in Section K2C-II. This letter specifically addresses the guardrail comment. Additional information will be provided to you via separate correspondence in regard to other questions about the crossing. The principal issues are whether the guardrail height is correct or adequate to serve its purpose as a barrier and whether the post depth is adequate to provide structural support in the event of a collision between a vehicle and the barrier. To determine if the guardrail is acceptable in its current condition, I have reviewed the VDOT GRIT Manual and the AASHTO Roadside Design Manual. The Roadside Design Manual details some of the calculations and considerations for design parameters of standard guardrail and I have relied upon those general principles to form my opinion. The GR -2 guardrail installed is a strong post system, the system typically requires a minimum of 43.25" of post to be buried in the ground. When within 1' of a slope steeper than 4:1, a longer post is typically required. I measured the height of exposed post from the back of the guardrail post and found a general height of 36" and a maximum of 39", leaving a buried depth of between 33" and 36" behind the post with the post on a 2:1 slope. Under most circumstances, I would share your concern on this issue, but after reviewing the details of the standard testing for guardrail I am confident that the structural concern is not an issue. When the tests were conducted, as published in NCHRP Report 350 the height of post was 54", leaving a buried depth of closer to 35". In those tests, the GR -2 type system achieved a passing rating on a TL -3 test condition. That condition was the impact of a 4,400 LB light weight truck traveling at 62 MPH. The speed limit on the Carroll Creek Road crossing is 15mph; a vehicle traveling at twice the speed limit would still be traveling at less than'/2 of the test speed. While the 2:1 back slope will provide slightly less support than designed, I conclude that an adequate margin of safety exists between the design parameters and the as -built condition to alleviate any structural concerns due to the reduced depth of post burial. The second question is the height of guardrail relative to the adjacent path. Since the guardrail is installed on a slope, there is a drop of several inches of drop between the path and the grade at the face of guardrail. I measured the guardrail in many locations and found that the height of the guardrail, as measured from the grade at the face of guardrail, is within the acceptable limits. The guardrail in question is on the trail side of the road which has an offset of 12' from the edge of pavement. Test have shown that at high speeds (62 MPH) vehicles leaving a road cross slope into a shoulder cross slope will be prone to raise up and strike the barrier at a higher height that designed which may adversely impact the performance of the barrier. As noted above, the speed limit for this section of road is 15 MPH. Given the low speed limit, and the large clear zone and recovery area of 12' 1 conclude the chances of a vehicle striking the guardrail with a raised suspension is unlikely. Based on my field observations, It appears that if a passenger vehicle were to strike the guardrail that the front wheels would likely be off the edge of the path pulling the bumper down to a level closer to the guardrail height. Based upon the information I have reviewed, and the nature of the road and the expectation of low speed traffic, I can confirm that the guardrail as installed will be adequate to serve its purpose as a roadside barrier. Sincerely, V 1N M. SHIW LicNo. 5183 -7/�.i SSI0NAL 1 Justin Shimp, P.E. Shimp Engineering, P.C.