Analysis on Long-term Settlement of Soft Soil under Subway Dynamic Load in Wuhan Area

Abstract

Through conducting the GDS dynamic triaxial test, this paper makes an analysis for the deformation characteristics of soft soil and its settlement mechanism under the subway traffic traveling load in Wuhan area. The test result shows that the cumulative plastic strain and pore water pressure of the soft clay soil samples are all experiencing the rapid growth stage, the slow growth stage and the stationary stage. The sandy soil samples’ stages are similar to the clay, but its strain value is smaller and pore water pressure value is greater. The total settlement under subway traveling load is consist of settlement caused by the undrained cumulative plastic strain and settlement caused by the dissipation of undrained pore water pressure. Also through the test we can find that the main settlement has completed during the first 3 years, later it becomes stable gradually. Introduction At present the subway has become the important means of transport in big cities. But with the increasing of subway operation time, a series of problems are exposed such as the settlement of ground foundation and vibration problems surrounding the environment [1]. Soft soil is widely distributed in Wuhan area, thus subway tunnel will inevitably cross the soft soil layer which has properties of low strength, high compressive strength and high sensitivity. How to accurately predict the long-term settlement and effectively reduce the subsidence has become an urgent problem in the construction of subway system. Because of its complex properties, soft soil has its own specific engineering characteristics, and effects of the strength and deformation are also complicated. This paper takes samples of soft soil in the process of engineering construction in Wuhan area and studies the relationship between cumulative plastic strain and pore water pressure of the clay with its vibration number under different confined pressure and dynamic stress ratio by GDS dynamic triaxial test. Based on this, a long-term settlement under the subway traveling load in the operation period can be calculated and predicted. Deformation Characteristics of Clay under the Dynamic Load The GDS Dynamic Triaxial Test Plan At present it is common that multifunctional dynamic cyclic triaxial test system is used to study dynamic characteristics of soft soil. Before the test, cylindrical samples need to be back pressed until they are saturated. In order to simulate the actual situation, samples should be consolidated under the condition of K0. According to relevant research results and investigation data, K0 is defined to be 0.6. The geotechnical engineering investigation of Wuhan rail transit line No.2 [2] points out that the initial consolidation confining pressure of soft soil around the tunnel generally distributes from 100kpa to 300kpa. Considering the effect of different tunnel depths on test results, the initial consolidation confining pressure is taken 170kpa, 280kpa respectively. According to the research achievements of Tang Yi-qun [3], we take the cyclical stress amplitude of 40kpa as the maximum additional stress in this test. By investigating the frequency response of the holes, the International Conference on Mechanics and Civil Engineering (ICMCE 2014) © 2014. The authors Published by Atlantis Press 413 vibration frequency of 0.5 Hz is adopted in this experiment. Tab. 1 Test Program Numb er Density( g/cm-3) natural water content (%) confining pressure (Kpa) cyclical stress amplitude(Kp a) Freque ncy (Hz) the cyclic number A1 1.67 34.1 170 40 0.5 2000 A2 1.67 34.1 280 40 0.5 2000 B1 1.63 16.7 170 40 0.5 2000 B2 1.63 16.7 280 40 0.5 2000 Growth Characteristics of Pore Water Pressure According to the classic consolidation theory, the external load is borne by the pore water when load imposes, with penetration consolidation process going on, external load gradually transfers from the excess pore water pressure to the effective stress, thus the strength and the deformation of soil mainly depending on effective stress. For analyzing the mechanical properties of soil, change rules of the pore water pressure under external load must be studied firstly. Through the time trend table of pore water pressure in the confining pressure of 170kpa, 280kpa under the same dynamic stress ratio, it can be seen that pore water pressure increasing trend is divided into three distinct phases:the rapid growth stage, the slow growth stage and the stationary stage. At stage one the duration is short, pore water pressure increases sharply and the growth rate decreases gradually, but overall maintain a rapid rate. It has a slightly longer duration at stage two; pore water pressure increases at a slow speed, the growth rate gradually continue to decrease. At stage three the duration is the longest; the growth rate of pore water pressure gradually decreases toward zero and finally becomes stable. 0 2 4 6 8 10 12 14 16 18 20 0 50