Improvement and extension of the parallel seismic method for foundation depthmeasurement

The parallel seismic (PS) method is used for determination of the unknown or undocumented depth of foundations, mostly piles. It was developed several decades ago and has not much changed since. PS is based on impulse generation on the pile head and registration of travel times in a borehole parallel to the foundation. The method is applicable to many foundation types, including single piles, pile walls, or sheet piles. While the accuracy is known to be high if the distance between foundation and borehole is small, the foundation length is overestimated with increasing distance.This paper presents the results of a systematic study on the influence of geometric and material parameters on the measurement results and explains the effects by studying the underlying wave phenomena based on numerical studies. It can be shown that several parameters (e.g. borehole inclination) have a strong influence. Foundation flaws and soil layers have also to be taken into account, while the possibilities in the other direction (derivation of soil parameters or detection of flaws from the results) are limited.Based on the simulation results, a new mathematical algorithm for data interpretation has been developed which takes into account the soil layers and the borehole inclination. This novel data interpretation scheme was used in combination with different data inversion methods. The new interpretation method was successfully validated using several sets of simulated data. Not only was it shown to be more accurate than all other available methods, but it also extended the maximum allowable pile–borehole distance to 2–3 m.Today, parallel seismics is the only method applicable on both metallic and non-metallic foundations which can be used without calibration. It has the largest range of all borehole methods. To enhance its efficiency it can be combined with downhole seismic measurements in the very same borehole to retrieve soil parameters.