基于多功能CPTU测试的基坑开挖扰动深度确定方法

基坑开挖卸荷扰动区深度的确定是基坑设计的重要指标。现有的关于扰动区深度的确定大都采用室内试验方法,无法反映土体的原位应力状态。基于多功能孔压静力触探(CPTU)原位测试,对南京、无锡、常州3个基坑工程进行开挖前、后的原位测试,分析了开挖卸荷对锥尖阻力q_t、侧壁摩阻力f_s、摩阻比R_f、电阻率ρ的影响;进一步结合室内卸荷回弹试验,提出了基于原位测试的基坑开挖扰动深度确定方法。结果表明,对于粉土和粉砂地基,开挖卸荷会使锥尖阻力和侧壁摩阻力降低,且随着土体埋深增加,其影响程度逐渐减小;对摩阻比和电阻率影响则较小;基坑开挖扰动区深度为CPTU锥尖阻力qt衰减20%时对应的深度距坑底开挖面的距离。

Determination of disturbance depth due to excavation using multifunctional CPTU tests

The disturbance depth of excavations is an important parameter for their design.Most of the existing methods for determining the disturbed depth are laboratory tests,which cannot reflect the in-situ stress state of soils.The in-situ multifunctional piezocone penetration tests(CPTU)are carried out before and after the excavation of three projects in Nanjing,Wuxi and Changzhou.The influences of unloading induced by excavation on the cone resistance(q_t),sleeve resistance(f_s),friction ratio(R_f)and resistivity(ρ)are also analyzed.Further combining with the laboratory unloading resilience tests,the disturbance depth of excavation based on in-situ tests is proposed.The results show that the unloading effect may reduce the values of cone resistance and sleeve resistance for silt and silty sand foundations,and the corresponding influences on the amplitudes decrease with the increase of depth.However,the excavation has little effect on the friction ratio and resistivity.The disturbance depth of excavation can be determined by the depth beneath the excavation bottom where the cone resistance(qt)is reduced by 20%as compared to that prior to excavation.