基于超固结黏性土CU指标求解函数的“m”法修正

 由于基坑开挖卸载，墙前被动区土体由正常固结状态转化为超固结状态，其固结不排水剪(CU)指标将发生变化，因此弹性地基梁计算理论应予以修正。根据超固结比(OCR)与不排水强度、上覆土压力间关系，引入不排水强度与固结不排水剪(CU)指标间关系，逐步推导出多土层条件下超固结土CU指标的求解函数，将其应用于弹性地基梁“m”法计算理论，提出一种新的考虑分步开挖卸载后被动区黏性土超固结作用影响的m值修正计算法。武汉长江隧道工程有限元算例结果显示，随着超固结比增大，开挖面一定深度下各黏性土层CU指标、m值均有不同程度提高，其中粉土提高幅度尤为大，按该修正法计算出的支护体位移较未修正前更接近实测结果。

"M" METHOD MODIFICATION BASED ON FUNCTION TO SOLVE CU INDEXES OF OVERCONSOLIDATION COHESIVE SOIL

As a result of excavation and unloading of foundation pit,the state of passive zone soil body in front of diaphragm wall was transformed from the normal consolidation to the overconsolidation. At the same time,the indexes of consolidated undrained triaxial shear test(CU) of cohesive soil will be changed,therefore,the calculational theory of elastic foundation beam could be modified. Based on the relationship among over- consolidation ratio(OCR),undrained shear strength and overburden pressure,the relationship between undrained shear strength and CU indexes was introduced and a function used to solve the CU indexes of overconsolidated soil in condition of several layers was derived gradually. Then the function was applied to the "m" method calculation theory of elastic foundation beam. A new modified m value calculational method was set up which considers the effect of the overconsolidated cohesive soil in the passive zone after excavation and unloading step by step. The result of finite element calculation of Wuhan Yangtze River tunnel indicates that CU indexes and m value of each cohesive soil layer in a certain depth below the excavated surface both increase in different degrees with increasing of OCR value. In particular,the indexes and value in silt increase greatly. In addition,the displacement of diaphragm wall calculated by the new modified method was close to the actual measurement results.