Using nondestructive testing techniques to evaluate the length of a pile in a capped pile group is always an important issue for its high value of potential applications. One of the applicable occasions is to decide whether the underneath piles of a capped pile group are broken after the attack of severe earthquakes. There seems no economically feasible way at current stage to achieve this goal for capped piles. In this paper the essence of this difficulty is further illustrated with the results of the sonic echo tests carried out before and after a pile was capped. Then a new method utilizing the traditional sonic echo test incorporated with multiple receivers is proposed to approach the solution of this kind of problems. The idea of this method is to calculate the difference of the response signals obtained in the receivers so that the interferences from the pile cap may be reduced or eliminated and then the differenced result is exponentially amplified for identifying possible reflections from the pile toe. Numerical simulations using finite element models and a field test on a miniature pile group were carried out to study the feasibility of this new method.
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