TOP DOWN ROAD RECONSTRUCTION Design Methodology
The design/build process is best completed when there are synergies between the designers and construction crews. The flexibility of a system and process are critical to allow for field alteration as required.
The preliminary design was based off basic site measure- ments, observations, experience, and past studies (when avail- able). The assumptions made in the initial design work were verified through the construction process and alterations were made as warranted.
Design methods for the soil anchors, micopiles and GRS followed the following publications:
FHWA0-IF-03-017, “Geotechnical Engineering Circular No.7”
FHWA – NHI-05-039 (Dec 2005) “Micropile Design and Construction”
FHWA-HRT-11-027, “Geosynthetic Reinforced Soil Integrated Bridge System Synthesis Report”
Project Overviews
The following case studies are highlights from the initial design build contract and the additional work completed on Highway 14.
Both Highway 4 and Highway 14 traverse extremely steep and rugged coastal terrain. The routes were initially pioneered in the 1950’s as forestry and mining roads carved out of the slopes and wilderness as single lane dirt tracks. Much of the road length was cut fill construction with frequent rock cuts and log cribbing to cross the steeper ground and gully systems.
The routes were further challenged with high rainfall, up to 4m/year, and limited width due to cliff bands, streams and lakes.
Highway 4, Kennedy Hill West Slide Kennedy Hill West Slide was a shoulder fill area that was
impacting the westbound lane for decades. During wet weather events it was not uncommon for the slide to move several cm’s/day and 100-300mm annually causing frequent need for asphalt patch repairs. The road width as also very narrow constrained by large boulder (5-8m diameter) colluvium on a steep slope above and Kennedy Lake below.
In 2008 the Ministry made some improvements through the area with a road widening project using concrete blocks and geogrid. The project was initially successful at gaining the desired width, however the additional loading only exac- erbated the slope movement (Figure 1).
Geotechnical investigations found the soil to be a boulder colluviam. Bedrock was not intercepted by the drilling but could be found as an outcrop 15m below the road. The pre- liminary engineering suggested complete road reconstruction using retaining walls founded on the bedrock. Unfortunately the preliminary estimates made the project cost prohibitive and maintaining traffic through the work zone could not be guaranteed.
The preliminary engineering by GeoStabilization International (formerly Soil Nail Launcher based in Colorado) was completed and the construction work through their Canadian contractor was completed (Figure 2).
Figure 2. Preliminary modeling to calibrate the soil anchor design work.
The work involved soil anchors varying in length from 6 to 18m through every concrete block and a third row above on an offset pattern. The anchors were all grouted into the boulder colluviam and bedrock was not intercepted (Figure 3). Torsional stiffness and corrosion protection was gained with a shotcrete shell (Figure 4)
Figure 3. Tie back soil anchors through the 2008 concrete blocks and shoulder fill, pre shotcrete.
Figure 4. Kennedy Hill West Slide complete.
Figure 1. Kennedy Hill West Slide.
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