Bi-Directional Testing Using Super Cells for the Jiaxing-Shaoxing River Crossing Bridge

Super Cells are an innovative, open-shaped bi-directional static load testing device used to promote concrete flow during placement while utilizing greater cross-sectional area of the pile to achieve large applied axial loads at relatively low hydraulic pressures (≤ 250 bar). This paper describes the results of multi-level load testing performed on a sacrificial drilled shaft that was 3.8 m (12.7 ft) in diameter and 110.6 m (363 ft) in length for the Jiaxing-Shaoxing River Crossing Bridge that spans Hangzhou Bay in Shaoxing, China. The test shaft was instrumented with embedded strain gauges and telltale rods along with displacement transducers at the top of the pile to measure the load-induced displacements along the length of the shaft. Load testing was performed twice using a Super Cell placed near the tip of the shaft, prior and subsequent to tip post-grouting, and then again using a Super Cell placed higher in the shaft. The axial resistance for the tested conventional drilled shaft was more than 5 times greater than the estimated axial resistance. In addition, the axial resistance after tip post-grouting (347,100 kN (39,033 ton)) was about 5.3% greater than the ungrouted axial resistance (329,600 kN (37,050 ton)). INTRODUCTION During the past few decades, the magnitude of axial, lateral, and flexural loading has considerably increased due to updated design codes, larger superstructures, and the manner in which extreme event loading (e.g. seismic and scour) is incorporated into design. The diameter and depths to which bored piles or drilled shafts are constructed have increased considerably during the last three decades due to the advancements in equipment technology and capabilities. In addition, monoshaft foundations have become increasingly used, as one large drilled shaft is able to replace a group of smaller diameter piles connected by a cap. Correspondingly, adequate strata, for both side and end resistance, must be discovered/determined as well as achievable to be able to construct the required foundations to support the greater design demands. As design demands have increased, the need to ensure integrity, quality, and performance of a constructed drilled shaft has become ever more crucial. Load tests are essential to validate design assumptions of side and end resistances, to assess the behavior and load transfer characteristics of the constructed drilled shafts, and to evaluate construction methods. Load tests utilizing a bi-directional static load testing (BDSLT) device can achieve considerably large applied axial loads and can be performed more economically, in less time, and in a much safer manner than conventional top-down static load testing methods.