Design Technical Memorandum - Geotextile.pdf
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- Attached to
- Great Smoky Mountains National Park
- Solicitation number
- 693C73-20-R-000015
About this file
This memorandum documents the design decisions made for a federal highway project in Great Smoky Mountains National Park. The project involves replacing five bridges along Forge Creek Road, including removal and replacement of abutments, piers, superstructures, approaches, and installation of riprap and streambank restoration. While geotextile fabric is typically required below riprap per design guidelines, it will not be used due to concerns from the National Park Service about debris accumulation. Instead, graded stone filter will be placed under the riprap at the Mill Creek bridge to allow water flow without clogging. The design change was approved by officials from the Highway Design, Technical Services, and Bridge branches.
The related federal contract opportunity involves solicitation number 693C73-20-R-000015 for the Great Smoky Mountains National Park project. Certified small businesses will submit proposals to replace five bridges on Forge Creek Road for an estimated cost between $1 million to $5 million. The project will be split into multiple schedules with construction starting no earlier than November 2, 2020 contingent on permit receipt. Bid documents and specifications will be issued on June 30, 2020 with proposals due on a date specified in the solicitation. Offerors must demonstrate experience with similar bridge projects. The solicitation provides further qualification requirements and instructions to access bid documents through the SAM website.
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Page 1 of 2 EFL-FM-HWY-04(01)
U.S. Department Of Transportation Federal Highway Administration
Subject: Design Technical Memorandum Project NP GRSM 126(1) Removal of Geotextile Filter Fabric for Riprap Placement Great Smoky Mountains National Park
Date: February 10, 2020
From: Alazar Feleke, Highway Design Manager Mark Hoover, Lead Designer
Reply to Attn of:
HFHD-15
To: Marc Carruthers, Highway Design Branch Lead Libby O’Brien, Technical Services Branch Lead Hratch Pakhchanian, Bridge Branch Lead
Memo Purpose - The purpose of this memorandum is to document the design decisions made for not placing geotextile filter fabric layers below proposed riprap placement for abutment and slope scour protection.
Project Improvement - This project consists of the removal and replacement of 5 low water bridges along Forge Creek Road. One bridge crosses Mill Creek and the 4 other bridges cross Forge Creek at various locations. The work includes reconstruction of the approaches, abutments, superstructure and piers. Road improvements include subgrade and asphalt repair, minor grading of shoulder and ditches, and other miscellaneous work. Riprap revetment protection is proposed along the abutments and adjacent stream channel areas for the Mill Creek bridge crossing.
Site Conditions – The existing bridges were rated “fair/poor” for overall structural integrity during the last few inspections. Since Forge Creek Road has no potential for increase in usage or ADT, the proposed condition improvements will continue to convey as a one-lane two-way unpaved local road with very low traffic volume, slow speeds, and located within mountainous terrain.
Design Criteria - Each of the proposed bridge crossings do not adequately convey the 50-yr design flood and are designed as “low water bridges” to convey the 10-year flood (vented) and fortified for the 25-year per FHWA requirements. Thus, the improvements needed to meet the FHWA Hydraulic Engineering Circular (HEC) Publication Nos. 18 (Scour Evaluation) and 23 (Design Guideline 4 -for Revetment Design). The design guidelines require geotextile filter fabric to be placed below proposed riprap placement such that sediments are separated from above and below the Class III riprap proposed for Mill Creek Bridge. Geotextile filter fabric is intended to help adequately pass surface and groundwater flows through the riprap layer without clogging.
Design Restrictions – The National Park Service has requested that all proposed geotextile materials be removed from the waterways. Filter fabric in high velocity streams can uproot, tear, and separate from originally installed location. This can cause problems with debris accumulation and blockage downstream if riprap is not inspected and maintained routinely and after significant rainfall events.
Memorandum
Discussion of Design Mitigation – In lieu of geotextile filter fabric, the proposed Class III riprap can be placed upon layers of ‘graded stone filter’ to allow passage of surface and groundwater flows through the riprap layers without clogging voided areas that will exist between the Class III stones.
Proposed riprap layers will only be provided for the Mill Creek bridge and provide protection around the north and south abutments, but also include the upstream north approach embankment to help protect the cut bank from scouring and sloughing of material over time.
The above design variances are approved.
Approved:
Marc Carruthers, Highway Design Branch Lead _____________________
Libby O’Brien, Technical Services Branch Lead ______________________
Hratch Pakhchanian, Bridge Branch Lead ______________________
| 2020-02-10T14:04:58-0500 | |
| ALAZAR FELEKE |
| 2020-02-10T15:42:56-0500 | |
| MARC ANDREW CARRUTHERS |
| 2020-02-12T10:11:51-0500 | |
| MARY ELIZABETH ROSE O'BRIEN |
| 2020-02-12T11:27:30-0500 | |
| HRATCH H PAKHCHANIAN |
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