Abstract: In a dynamic replacement (DR) process, the length and the subsidence of the replacement pier need to meet the design specifications. The times of tampings and the fillings to fulfill a DR process is generally determined by field test. Understanding the forming mechanism of the replacement pier, and quantifying the factors such as energy releasing time, motion characteristics of fillers, stress of pounder bottom, etc, play critical roles for optimizing the DR process. In this paper, a model test method to study the interactions of the fillers and soil under the multiple impacts of the pounder is established. Experimental rusults show that each acceleration-time curve in DR can be divided into four stages, representing different phases of the soil-fillers motions under each pounding. Through analyzing the force-displacement (F-s) curves of the pounder in this process, the characteristics of each F-s curve are distinguished and the intrinsic mechanism is discussed. It was shown that with the increasing of filling times, the energy releasing time during each pounding decreases and the maximal force increases, signifying the gradual forming of the replacement pier in the soft soil foundation.

Key words: dynamic replacement method, model test, replacement pier, impact dynamics, acceleration, high speed photography