Experimental and analytical study on the reinforcement mechanism of in-pipe deep dynamic compaction in loose sandy soil

Considering that conventional dynamic compaction (CDC) method has limitation in the effectiveness of improvement depth because the improved shallow soil layers prevent the impact energy further transmitted to the deep ground, a new technique of in-pipe deep dynamic compaction (IDDC) is proposed in which the tamper can compact soil from the deep to the shallow soil layers. In this paper, the main objective is to illustrate the work mechanism of IDDC. Firstly, main components of equipment and construction process of IDDC are introduced. Then, model tests of CDC and IDDC were conducted on loose sand to obtain the influence depth using the acceleration of soil particles during impact and the distribution of cone resistance and side friction through static cone penetration tests (CPTs) after impact. Finally, the analytical formulae of superimposed stress and settlement due to IDDC was derived based on the Mindlin’s solution and equation of motion, and verified with model test results and a practical case. The results indicate that with the falling height of 1 m in model tests, the further impacts after the 6th impact of CDC could hardly improve ground, resulting in the improvement depth of around 45 cm, whereas the improvement depth of IDDC was over 80 cm. Moreover, at the falling height of 1 m, the average increment in cone resistance after IDDC is 82% greater than that after CDC. Finally, compared with experimental results, the errors of the predicted settlement and the superimposed stress are less than 26 and 14%, respectively, and the proposed formulae succeed to predict the improvement depth of IDDC applied in a coastal area of China.