Abstract
Site investigations of the soils are considered very important for evaluation of the site conditions, as well as the design and construction for the project built in it. Taihu tunnel is thus far the longest tunnel constructed in the lake in China, with an entire length of over 10 km. However, due to the very insufficient site data obtained for the lacustrine clay in the Taihu lake area, a series of self-boring pressuremeter (SBPM) field tests was therefore carried out. Undrained shear strengths were deduced from the SBPM test, with the results showing generally higher than those obtained from the laboratory tests, which may be attributed to the disturbance to the soil mass during the sampling process. Degradation characteristics of the soil shear modulus (Gs) were mainly investigated, via a thorough comparison between different soil layers, and generally, the shear modulus would cease its decreasing trends and become stable when the shear strain reaches over 1%. Meanwhile, it was found that a linear relationship between the plasticity index and the shear modulus, and between the decay rate of the shear modulus and the plasticity index as well, could be developed. Further statistical analysis over the undrained shear strength and shear modulus distribution of the soils shows that the undrained shear strength of the soils follows a normal distribution, while the shear modulus follows a log-normal distribution. More importantly, the spatial correlation length of the shear modulus is found much smaller than that of the undrained strength.
Highlights
Investigations of soil properties are normally carried out by laboratory and in situ tests, with in situ tests considered more reliable [1,2,3,4], as they almost apply no disturbance to the soil
Assumptions of linear/nonlinear elastic behavior of the soil may lead to different solutions of the stresses and strains, but the result undrained shear strength, which equals to the ratio of the incremental total pressure over the incremental logarithmic shear strain of the soil, will not be affected, as the later part of the curve corresponds to the plastic phase and the slope of the curve is the undrained shear strength
The field test results of Self-boring pressuremeter (SBPM) are interpreted in which the in situ horizontal stress, undrained shear strength, and shear modulus of the soils along the depth are analyzed
Summary
Investigations of soil properties are normally carried out by laboratory and in situ tests, with in situ tests considered more reliable [1,2,3,4], as they almost apply no disturbance to the soil. Self-boring pressuremeter (SBPM) is developed by Worth and Hughes in 1973 [5], and since it has been taken as one of the most important in situ testing techniques to investigate soil properties. During each test, both cavity expansion pressure and radial strain can be recorded, which renders an entire pressure-cavity–strain relationship curve, including both loading and unloading stages. Houlsby and Withers [11] analyzed soil properties from unloading and loading portions of pressuremeter curves and showed that the derived values of undrained shear strength and shear modulus are in good accuracy, compared to other tests’ results. Two aspects were mainly focused on this paper—for one, a considerable amount of SBPM tests were carried out in Taihu Lake, China, aiming to obtain a straightforward understanding of the soil parameters variation at the Taihu lake bed with respect to the depth, as well as the loading strain, etc.; for the other, by combining the statistical analysis, the test data were further interpreted from a probabilistic view, which can provide the designer necessary information to assess the safety condition of the geo-structures via probabilistic methods
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