Strength Characteristic Experimental Study On Soil Under Different Stress Path And Field Monitoring Analysis

There exists inevitable disturbing influence to the nearby architectures during the underground engineering excavation. For a long time, characteristic alterations of nearby soil caused by construction perturbation, and deformation,unstability and failure of structures and soil during the construction have not been clearly apprehended, a series of collapse and destruction of foundation pit and side slope occurred during the infrastructure construction in city, and casualties remains common occurrence.To study the actual stress and strain state of underground perturbed soil in different places under construction, investigate the deformation and failure characteristic of soil under different stress paths,4 group tests are devised. They are conventional triaxial compressed tests, equal P compressed tests, equal cell pressure decompressed tests and unequal cell pressure decompressed tests, and 26 group tests have been completed.Results indicate that the soil samples show different strength under different shearing paths. The strength under conventional triaxial compressed test is the highest, decompressed test the lowest, equal P test the medium, therefore shearing paths vigorously affect the strength. When normalized by (σ1-σ3)/p′, strength of the soil gets appropriate normalizing characteristic under the same shearing path.Under the different cell pressure but the same shear path condition, when samples get to limit state, they tend to be constant at the same stress ratio. Moreover, different consolidation stress ratio barely affects the stable stress ratio under the same shear paths. On the contrary, underground perturbed soil in different places under construction behaves different internal friction angle. The internal friction angle under decompressed test is the highest, conventional triaxial compressed test the lowest, equal P test the medium. The similar regular has been found in the undisturbed soil. But tests in sand indicates that the shearing strength parameters of sand is barely affected by shearing paths.To simulate the difference of deformation of multi-step excavation and simultaneous excavation in double metro tunnels, a group of remoulded soil and undisturbed soil test has been carried out. Results indicate that the strain in condition of off-load completely once is obviously larger than off-load twice or third, the difference is beyond 8 times. As a result, simultaneously excavation is avoided in parallel tunnel construction, it is preforable to excavate step by step.Tragedies have been caused by collapses and losses been brought about. To inssure the construction proceed safely and to make connection between field monitoring and the laboratory tests, Fangshan railway station and Xisi railway station have been chosen to be monitored. So it makes complement and authentication between the laboratory tests and the field monitoring. Monitoring in the Fangshan railway station indicates that, the deformation with is bigger than without storage in off-loading. It is in accordance with the laboratory tests. Monitoring in the Xisi railway station indicates that the lateral displacement shows different mutative characteristics, and generally doesn't affect the construction.