碎石桩处理液化地基抗液化研究现状及存在问题

就目前国内外碎石桩处理液化地基抗液化理论、动力分析以及液化判别等方面的研究作简要的归纳和评述。在加固机理研究方面,主要认为碎石加固砂土有以下几种作用:(1)挤密作用;(2)振密作用;(3)排水减压作用;(4)预震作用;(5)加筋作用。其次,在理论研究方面如排水效应和桩体效应方面的研究也取得了长足进展,已经从总应力法深入到有效应力法,从只对孔压的研究发展到对水土共同作用的研究。并且随着计算机的发展,数值计算为复杂条件下的问题解答开辟了一个新的途径,现在已经可以模拟地震时土体的变形和孔压变化情况。但是在碎石桩处理液化地基的判别标准研究方面发展较慢。最后,对高速公路碎石桩复合地基抗液化研究方面的一些不足之处提出了解决的思路,主要包括:(1)上部荷载对复合地基的影响;(2)碎石桩处理深度设计时应考虑的影响因素;(3)碎石桩加固区与周围环境的相互作用;(4)考虑碎石桩排水、应力集中和附加应力共同影响下的液化判别标准。

Current State and Unsolved Problems in Researches of Liquefaction Resistance by Gravel Columns

Since vibro sinking method was invented in 1937, gravel columns foundation has been widely applied in engineering . Up to now, great achievements have been attained in the research of gravel columns foundation. In this paper the recent advances in theoretical research and dynamic analysis and criteria for liquefaction resistance of gravel columns foundation are reviewed. Firstly, the reinforcement effects of gravel columns are recognized gradually; i.e.(1)densifing the soil by squeezing and vibrationg in the progress of forming columns;(2)draining away water through gravel columns and reducing water pressure;(3)liquefing in advance in the progress of forming columns; (4) reinforcing soil. Secondly, many achievements in the theoretical research have been made, especially in the mechanisms of draining away water columns and focusing stress by gravel. In the past, the research was mostly concentrated on the total stress method or only the water pressure change, while at present the research has already turned to the effective stress method and combined action between soil and water. With the development of computer, numerical simulation is provided to solve complicated problems, thus deformation of soil and change of water pressure during an earthquake can be simulated by computer. Thirdly, the development is very slow in the criteria for liquefaction resistance of gravel columns foundation, due to the slow development in the criteria for liquefaction resistance of sand foundation. Present criteria for liquefaction of sand foundation are based on in situ tests, such as standard penetration test(SPT), cone penetration test(CPT) or shear wave velocity measurements(VS). Corresponding criteria for liquefaction resistance of gravel columns foundation are obtained by that of sand foundation. At last a few deficiencies are pointed out in restraches of liquefaction resistance of gravel columns foundation. To overcome these deficienies, further studies are sugsested which includes:(1)influence of superstructure loads on gravel columns foundation;(2)some effective factors for the design of reinforce depth of gravel columns;(3)interact between reinforced foundation and surrounding;(4)criteria for liquefaction of gravel columns foundation considering influence of draning away water and focusing stress of gravel columns and additional stress for superstructure loads.