Page 1 Boundary Effects of Pile Cap on the Integrity Testing of Group Piles Jiunnren Lai 1 , Bo-Huan Yang 2 , Chi-Ling Pan 3 and Chiung-Fen Cheng 4 1 Associate Professor, Dept. of Construction Engineering, Chaoyang Univ. of Tech., jrlai@cyut.edu.tw 2 Civil Engineer, Taiwan Power Company, u782146@taipower.com.tw 3 Professor, Dept. of Construction Engineering, Chaoyang Univ. of Tech., clpan@cyut.edu.tw 4 Chair, Dept. of Multimedia and Game Design, Overseas Chinese Univ., cfcheng@ocu.edu.tw ABSTRACT: Low-strain surface reflection pile integrity testing (PIT) methods have been successfully used for the quality assurance of newly-built individual piles. However, for existing piles with pile cap, previous studies have shown that the stress- wave reflected from the boundaries of pile cap may cause difficulties in determining their integrity. In this paper, the boundary effects of pile cap on the signal of PITs were studied by numerical simulation and tests on model piles. Feasibility of minimizing the boundary effects by embedded accelerometers was also investigated. Results from this study indicate that signals obtained from numerical simulation are similar to those obtained from tests performed on model piles. It also shows that boundary effects of pile cap on low strain pile integrity tests can be reduced by using embedded accelerometers, thus making it easier to assess the integrity of piles with a pile cap. INTRODUCTION The ability to assess the structural integrity of piles after a major event such as earthquake or scouring of riverbed plays a important role in evaluating the safety conditions of brides. Several non-destructive testing (NDT) methods based on wave propagation theory have been used to assess the integrity of drilled shafts or cast-in- place piles. These tests are also called pile integrity test (PIT) and can be classified into two groups: direct transmission method and surface reflection method. The most common direct transmission PIT is the Cross-hole Sonic Logging [CSL] (Stain and Williams, 1991) or the Cross-hole Tomography [CT] (Han and Wang, 1992). A CSL/CT test normally requires two pairs of steel or PVC access tubes installed in the shafts and tied to the rebar cage. The cage is then lowered into the borehole and the concrete is placed. A sound source and receiver are lowered into the tubes, maintaining a consistent elevation between source and sensor. A signal generator generates a sonic pulse from the emitter which is recorded by the receiver. Relative energy, waveform and differential time are recorded and logged. This procedure is repeated at regular intervals throughout the pile. By comparing the graphs from the various combinations of access tubes, a qualitative idea of the structural soundness of the concrete throughout the pile can be gleaned. The CSL/CT method is considered to be more accurate than the surface reflection PITS in the determination of structural soundness of concrete within the drilled shaft (Hollema and Olson, 2003). However, for existing piles with pile cap,