受施工卸载扰动黄土的变形与强度特性研究

各种施工活动都不同程度地对周围土体产生扰动影响,应力状态与应力路径的改变作为施工卸载的关键影响因素,对施工扰动起着主导作用。在分析基坑开挖及隧道与地下洞室掘进卸载作用的基础上,根据不同施工工况的特点,制定了室内非常规三轴卸载试验方案,对不同卸载应力路径下的应力-应变关系曲线、强度与变形特性及破坏特征进行了一系列卸载试验研究。试验结果表明,卸载作用下的强度与变形特性、变形模量及破坏特征与加载路径存在很大差别,挤长破坏的抗剪强度较压缩破坏低,挤长破坏的最大轴向应变仅为压缩破坏的1/3～1/2。

UNLOADING-INDUCED DEFORMATION AND STRENGTH PROPERTIES OF LOESS DURING CONSTRUCTION

During excavating tunnels and subway tunnel with shield,different unloading effects in construction occur to disturb surrounding earth and adjacent buildings. The change of stress state and stress path,as a principal factor,plays an important role in disturbance caused by various construction activities. For example,the states of stress and consolidation at sides and bottom of pit change during excavation. In the process of shielding,earth surrounding the gaps of shield ends takes various unloading effects to alter the stress states. There exist compressive disturbances in the earth right in the front of tunneling. The gaps between the end of shield and lining give rise to moving and loosened earth to form unloading disturbance areas. In current theories of design and calculations,loading models are usually applied without really considering the unloading effect in excavation. Besides,the factors of various construction methods,indefiniteness of underground construction and complexity of surrounding environment have all contributed to make underground construction as risky projects,which have rendered several disastrous accidents in recent years that jeopardize life and property,adjacent buildings,underground pipelines and interrupt intercommunication systems. Based on the analysis of unloading effects in different types of construction and various construction conditions,this paper proposes some laboratory test schemes for loess with methods of unconventional triaxial shear to conduct a series of study on the behaviour of stress-strain relationship,characteristics of strength,deformation and failure on dissimilar unloading stress paths. The test results show that behaviors of strength,deformation and failure under unloading paths are much different from those under loading paths. The shear strength at extension failure is less than that at compression failure. The maximal axial strain at extension failure takes only 1/3-1/2 of compression failure.