Strength Gain of Driven Steel Piles in Variable Ground Conditions in Calgary, Alberta

ABSTRACT: A large number of driven steel H piles were installed in cohesive soils (clay and clay till) with layers of sand and silt to support bridges for a major transportation project in Calgary, Alberta. The total number of driven steel piles installed were 871. Due to significant variability in subsurface conditions across the site and at each bridge site, the risk for increased piling quantities and splicing existed which would have impacted the project schedule and cost. To manage this risk, it was decided at the onset of the project to rely on the time dependent strength gain of soils (set up). This set up is attributed to increase in effective stresses resulting from dissipation of porewater pressure in cohesive soils after pile driving. For this purpose, Pile Dynamic Analyzer (PDA) testing was performed at End of Initial Drive (EOID) and at restrike followed by Case Pile Wave Analysis Program (CAPWAP) analyses and calibration of analysis using Wave Equation Analysis Program (WEAP). The EOID PDA testing was performed on at least 15 % of the total piles at each foundation element and restrike testing was performed on at least one third of the PDA tested piles at EOID. The set up was found to be variable consistent with variable subsurface conditions. The risk with respect to increased piling quantities and splices was also managed using the set up. This paper presents the strength gain of soils measured between the EOID and restrike PDA testing on large number of piles installed in variable ground conditions.