冻融对开封城墙土抗剪性能影响

Freeze-Thaw Influence on Shear Performance of Kaifeng City Wall Soil

开封城墙是全国重点保护文物,为探究冻融循环的影响,从城墙维修施工现场取样,分别进行颗粒筛分实验、界限含水率实验、击实实验、冻融循环实验以及直剪实验.实验表明:城墙土的液限、塑限、塑性指数分别为26.1%、11.4%和14.7%,属于低液限黏土;土样的最佳含水率为13.3%,对应的最大干密度为2.09 g/cm^(3);经11次冻融,黏聚力与内摩擦角整体呈下降趋势.-5、-10和-15℃温度下土体的黏聚力分别降低至52.78、22.04和19.15 kPa,与未经冻融的土样相比,分别下降了25.58%、26.62%和30.70%;此外,含水率对于土体经多次冻融后的抗剪强度也有较大影响,在相同工况下最优含水率与天然含水率时内摩擦角分别降低了2.42%、3.98%、6.22%和1.26%、1.81%、3.65%.总之,随着冻结温度的降低土体的黏聚力逐渐降低,含水率与冻融循环次数对土体的抗剪强度有较大影响.

The city wall of Kaifeng is a key national heritage conservation unit.To explore the influences of freeze-thaw cycles,the samples were taken from the maintenance and construction site of the city wall.Then,the particle sieve test,limit moisture content test,compaction test,freeze-thaw cycle test and direct shear test were carried out.The results show that the liquid limit,plastic limit and plastic index are 26.1%,11.4%and 14.7%,respectively.The wall soil belongs to the low-liquid limit clay.The optimum moisture content of soil samples is 13.3%,and the corresponding maximum dry density is 2.09 g/cm^(3).After 11 times of freezing-thawing,both the cohesion and internal friction angle decrease.At-5,-10 and-15℃,the cohesive force of soil samples decreases to 52.78,22.04 and 19.15 kPa,respectively,which falls by 25.58%,26.62%and 30.70%,compared with the specimen without freezing-thawing cycle.In addition,the moisture content also has a great influence on the shear strength of soil after repeated freezing-thawing.Under the same working condition,the internal friction angle of the soil with optimal moisture content and natural water content decreased by 2.42%,3.98%,6.22%and 1.26%,1.81%,3.65%,respectively.In a word,with the drop of freezing temperature,the cohesion of soil decreases gradually,and the moisture content and freeze-thaw cycles have great influences on the shear strength of soil.