204th Street Overpass
Location: Langley, British Columbia, Canada
Client: Hatch Mott MacDonald / Langley Road and Bridge Limited
In 2005, Langley Road and Bridge Limited and Hatch Mott MacDonald retained MEG Consulting Limited (M+EG) to provide geotechnical design services for the new overpass structure at 204th Street in Langley, BC. The overpass alignment initiated on the south side of the intersection of 204th Street and 62nd Avenue and carries traffic to the north by means of an elevated structure crossing the Langley Bypass and the BC Hydro right-of-way that is used by CP Rail and Southern Railway. The overpass is a twelve span, 420 meter long bridge structure with lightweight fill (EPS) bridge abutments.
M+EG was responsible for all geotechnical engineering design aspects, including:
- Site investigation (drilling and sampling with in-situ vane testing and shear wave velocity measurements);
- Laboratory testing (including advanced dynamic testing with resonant column and cyclic simple shear tests);
- Site-specific seismic response analyses;
- Pile analyses for single piles and pile groups under both axial and lateral loading;
- Global stability and settlement analyses for the proposed EPS embankment.
- M+EG also provided field testing and inspection services during the construction of the overpass, which included on-site supervision, vibration monitoring for pile installation and dynamic pile testing using Pile Driving Analyzer (PDA) and CAPWAP analyses.
- Originally, the piles were intended to carry load in end bearing, as defined in the contract specifications, based on the presence of a competent founding layer at some depth across the site. However, the additional geotechnical investigation and laboratory testing performed by M+EG indicated that the competent bearing stratum was deeper than indicated on the contract documents and an alternate design considering friction piles was proposed.
- PDA testing and CAPWAP analyses were to be performed as part of the contract requirements to demonstrate the ultimate axial capacity of the piles. However, due to the tight construction schedule, it would not have been possible to re-strike the piles at periods of 30 days or more after initial driving. M+EG proposed a pile testing program that required the early installation of four piles at the location of one of the proposed pile bents in an area where access was readily available. The idea was to test the piles at periods of up to 30 days after initial driving and develop a setup plot for axial capacity. The results of the PDA/CAPWAP testing provided the basis for determining axial capacity set-up of the piles and for finalizing the length of the piles to be used for the remaining piled foundations along the overpass structure.
- The settlement of the pile groups for the various loading conditions was analyzed using the in-house program API-LAX, which performs a 3D equivalent-elastic non-linear analysis of the soil-pile interaction. The elastic analysis was combined with the API recommendations for generating the axial t-z and Q-z curves.