The award is part of the 2008 International Achievement Awards presented by the Industrial Fabrics Association International (IFAI).
This project upgraded an existing, narrow rural road to a British Columbia Ministry of Transportation (BCMoT) standard 8m-wide, gravel surface road. About 85m of the existing road crossed a stream and swamp area located between 2 lakes, and the existing road surface was only 80cm above the surface of the swamp.
Geotechnical investigations determined that the existing road had settled about 1.7m since it was constructed in the mid 1970s. The existing stream-crossing structure consisted of a 600mm-diameter steel culvert.
The developer requested an expedited construction schedule and the environmental monitors preferred winter construction. For fish passage, it was a requirement to install an open bottom structure. Construction of a pile-supported geotextile reinforced soil (GRS) embankment permitted the construction of the road and open stream crossing structure. The structure has also re-established fish passage between the 2 lakes.
The objective was to upgrade the existing road (Airmail Drive) and swamp crossing to a condition that would be acceptable to the BCMoT, so the road could then be maintained by the MoT.
The proposed road was designed to provide legal vehicle access to the existing residential and recreational lots located within an isolated area near Sheridan Lake, B.C., and to permit further subdivision of larger land parcels in this area. The project is located within the Thompson Plateau area of south-central British Columbia, about 29km east-southeast of 100 Mile House, between Sheridan Lake and Staley Lake.
Challenges and solutions
Designing and constructing a road over soft soil conditions, while limiting short- and long-term maintenance associated with embankment settlement and slope instability, is always a challenge. In addition, the existing road was situated in a wetland area that is considered to “offer important biological function” for aquatic life, according to project documents.
The existing road consisted of about 0.8m high embankment overlying 0.5m–2.5m of fill, the upper 0.2m consisting of sand and gravel. Below this, the fill consisted of predominately silt and clay. Beneath this were deposits of fibrous and amorphous peat ranging in depth of 0.3 to 5m. Under the peat, the subsurface conditions consisted of 0.3m soft silty clay then stiff to very stiff silty clay. The estimated settlement of the previous 35-year-old road was 1.7m.
Settlement of this magnitude within the proposed widened portion of the embankment, coupled with much less settlement within the existing portion of the road, presented significant maintenance and service design issues with both the road and the stream crossing structure. Special construction techniques were examined, including:
- excavation and replacement.
- lightweight fill.
- pile-supported embankment (tightly spaced piles).
- pile-supported geosynthetic-reinforced soil (GRS) embankment.
Based on the developer’s requested expedited construction schedule, the environmental monitors’ preference for winter construction to limit impacts on the environment, a comparison of likely costs, and the anticipated short- and long- term performance, a pile-supported GRS embankment was selected as the best alternative.
The pile-supported GRS embankment fill design is similar to conventional piled-supported embankment design, except that the piles are spaced wider apart and the embankment fills are reinforced with tightly spaced layers of geotextile that then form a soil beam over the piles. Piles used for this project consisted of economical, untreated fir logs. Fir logs split in half lengthwise were used as pile caps.
The project was constructed expeditiously with little environmental impact.
The project has exceeded short-term performance expectations and is expected to provide acceptable long-term support for the expected vehicle traffic. The structure has also re-established fish passage between the 2 lakes.
This was the first GRS structure installed in British Columbia in conjunction with BCMoT. Based on this project, the MoT is evaluating other potential sites where this unique technology could be used.
The completed embankment cross section consisted of 184 piles installed on 2-m centers. The piles consisted of green fir logs with an average butt diameter of 330mm and a minimum tip diameter of 250mm. Pile length varied from 1.5m to 10.9m. Pile caps consisted of half-round 400mm-diameter green fir logs.
The set criteria were: 20 1.5m blows per 300mm using an 1800kg drop hammer. The piles and pile caps were installed below the permanent water level to provide additional assurance they would remain in an anaerobic state.
The GRS embankment fill material consisted of alternating layers of well-graded, clean pit-run sand and gravel with layers of sand and crushed gravel, compacted to a minimum 95% SPD. In all, 10 layers of geotextile (935m2), with a vertical spacing of about 190mm, were installed in between these alternating layers of granular fill.
The open bottom stream crossing structure installed consists of a 2.8m clear span, GRS-steel composite beam structure founded on 1.68m-high pile supported GRS abutments. These novel GRS-steel composite structures have been used at a number of stream crossing sites on forest resource access roads throughout British Columbia.
The concept utilizes a similar soil beam concept used in the pile supported GRS embankment design except that the tensile moment capacity of the structure is carried by a number of trapezoidal steel box beams. The compressive moment capacity of the structure is carried by the GRS. Transverse shear keys are installed between the box beams and the GRS.