An Approach Of Dynamic In-situ Liquefaction Test And Characteristics Of Liquefied Soil

Soil liquefaction is one of the most important topics in soil dynamics and geotechnical earthquake engineering. Liquefaction usually is triggered by strong ground motion and can aggravate seismic damage. Machine vibration or wave loading can also trigger liquefaction. Research on soil liquefaction and relevant engineering issues, which is of important significance in theoretical and engineering practice, remains an important and active branch in anti-seismic engineering.Post-earthquake investigation and laboratory tests are two main and important approaches in liquefaction research, and besides in-situ monitoring technique is a direct approach to investigate such phenomena under seismic loading. Many significant achievements have been obtained and applied to engineering practice. However, these approaches possess intrinsic limitations:(1) since strong earthquakes rarely occur that the dynamic response of soil liquefaction can hardly be obtained;(2) Laboratory tests are unable to comprehensively the dynamic response of real liquefied soil;(3) due to rare occurrence of strong events and immaturity in short-term forecast technology quite a few records at liquefied sites have been recorded.With the progress of liquefaction investigation, in-situ test provides a new way for liquefaction research, and provides a new alternative to investigate dynamic responses of liquefied soil. To overcome the disadvantages and limitations existing in current approaches mentioned before, in-situ liquefaction test apparatus under artificially dynamic loading has been developed. Through the approach, it is effective to obtain the dynamic response and characteristics of liquefied soil in a real site. Research on liquefaction by in-situ liquefaction tests has been done at certain extent at abroad. Nevertheless, the relevant research has just started in China mainland and casts a prosperous future.In this thesis, investigation and research on special liquefaction problems have been conducted through in-situ liquefaction tests by improving the current artificially loading liquefaction test technique. Then in-situ investigation on pore water pressure increasing of liquefying soil with acceleration and soil strain have been done. The characteristics of liquefied soil are specifically analyzed. The results obtained in this thesis are compared and mutually verified with the test data available so that the knowledge of liquefaction is advanced, and the strategies to mitigate liquefaction hazard are improved.The main achievements and points of this paper are outlined as follows.(1) Key technical issues as artificial loading, data acquisition, indexes for describing characteristics of liquefied soil, and test layout are studied. Improvement works of in-situ liquefaction test under artificial loading have been conducted. Two types of liquefaction tests are prepared, and the feasibility and credibility of the testing technique are verified by comparing with other similar tests finished abroad.(2) By the means of in-situ tests, the effect of liquefaction on ground motion is studied so that the correlation of soil liquefaction and ground motion is acquainted. The surface ground acceleration with respect to the content of liquefying, and liquefaction influence on ground motion are obtained.(3) The pore-water pressure in field increases gradually with time, which differs from the pattern obtained from laboratory tests. An incremental model for predicting pore pressure of saturated sands is established, accounting for acceleration, sand-buried depth, density of sands, which are commonly used to describe soil characteristic in field.Numerical simulations and comparisons between predictions and test results indicate that the new model is reliable.(4) Based on the in-situ liquefaction tests, a relationship between pore water pressure buildup and strain of saturated sands is studied that a quantitative formula is proposed.