Abstract：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.
