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Investigation of Soft Soil Subway Tunnel Subsidence Based on Multiple Factors

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Investigation of Soft Soil Subway Tunnel Subsidence Based on Multiple Factors International Conference on Mechatronics, Control and Electronic Engineering (MCE 2014) Investigation of Soft Soil Subway Tunnel Subsidence Based on Multiple Factors Li Guihua Zhao Bingshuai School of Earth Science and Engineering School of Earth Science and Engineering Hohai University Hohai University Nanjing, China Nanjing, China [email protected] [email protected] Tang Wei Guo Xiantao School of Earth Science and Engineering School of Earth Science and Engineering Hohai University Hohai University Nanjing, China Nanjing, China [email protected] [email protected] Abstract—Objective: the subsidence of soft soil subway key drivers to research the settlement of subway tunnel in tunnel structure is a key threat to the safe operation of soft soil. subway system. In order to detect and respond the problems At present, the subway tunnel settlement monitoring timely, it is necessary to constantly monitor the subsidence of mechanisms during the operation period are not perfect soft soil subway tunnel structure and to analyze its variation enough, with limited capability to meet the demand of over time. Methods: a subway tunnel is buried in the ground practicability, reliability and accuracy of the settlement and is usually in operation all the time, making it difficult to monitoring. Also, it is difficult to meet the long-term use common methods of monitoring the deformation of the demand during the operation of subway tunnel settlement tunnel structure. This difficulty can be overcome by monitoring. As a result, it is urgent to find new research deploying sensors. In this study, based on the characteristics means and methods. This study used a variety of relevant of soft soil subway tunnel subsidence, data of various influencing factors have been monitored. According to the monitoring information as fundamental factors, analyzed principal component analysis and the qualitative analysis of the degree of influence of the factors, and studied the different factors, a mathematical relationship was derived subway tunnel settlement cause during the operation between each influencing factor and soft soil subway tunnel period. subsidence. By applying principal component analysis from both time and space, the degree of influence of each factor II. ANALYSIS OF THE CAUSE OF SOFT SOIL SUBWAY was analyzed both qualitatively and quantitatively. Results: TUNNEL SUBSIDENCE the results accurately showed the influence of different According to the structural characteristics and settling factors in the process of subway tunnel subsidence. characteristics of subway tunnel in soft soil, the factors Conclusion: this study shows that the use of sensors could be influencing the settlement of subway tunnel structure in a practical and effective method to monitor the settlement of service period are divided into four categories as, soft soil subway tunnel. environment factors of soil, tunnel structure factors, train Keywords- subway tunnel; subsidence; multiple factors; load factors and factors of external environment load [4-7]. soft soil; principal component analysis The environment factors of soil includes geological factors and factors about the variation in groundwater level; the I. INTRODUCTION tunnel structure factor is mainly the leakage in tunnels caused by cracking; the train load factors include train The adverse conditions of the subway tunnel in soft traffic and passenger volume; and the factors of external soil, such as stratum geological environment, complex environment load mainly consist of building load, traffic engineering characteristics and significant spatial variation load, regional land subsidence, and regional atmospheric of soil distribution, have posed a serious threat to the precipitation on the ground area above the tunnel. settlement of tunnel structure [1-3]. At the same time, some other factors, such as the external environmental A. Soil Environmental Factor influence, the change of the load and other unforeseen Soft soil is characterized by low strength, variable causes, have increased the complexity of the tunnel thickness and complex characteristics of consolidation and structure settlement. Therefore, identification of the cause creep. Also, a large difference exists between the vertical of the subway tunnel settlement in the soft soil, and and the horizontal permeability. The soil environmental analysis of the qualitative and quantitative extent of the factors can be grouped into two categories, namely, settlement caused by different influencing factors are the geological factors and the underground water level change. The geological factors can be further divided as, the tunnel © 2014. The authors - Published by Atlantis Press 741 under the bottom of the soil distribution, and the aging st =+Ctln d (3) (soil consolidation and creep) characteristics. On the other 2sC In equation (3), v（constant for a cross- hand, the underground water level factor represents the soil C=2 subsidence caused by the variation in groundwater level. He section). Equation(3) proves that there is a linear 1) Geological Factor relationship between the amount of consolidation a) Effect of the substratum soil layer distribution on settlement and ln t at time t. the baseboard of a tunnel 2) Variation in groundwater level When the substratum soil layer distribution on the According to Terzaghi’s effective stress principle, soil baseboard of a tunnel is thick, the tunnel experiences a compression after lowering the groundwater level to any large settlement, and vice versa. Therefore, the thickness depth Hcan be obtained as: of soft soil layer directly affects the settlement stability of the longitudinal subway structure. Based on the layer-wise HHHDe aD-pa()yhg summation method, the foundation settlement for S===dyvdyvwdy (4) òòòhhh unidirectional compression is given by the formula[8]： 1+e011++ee00 nnDp where eis the initial void ratio of soil; Deis the S=DSH=×i (1) 0 ååiiE ii==11Si change in the void ratio; avis the compressibility of soil; g is the pore water bulk density; h is the initial where is the number of thin compressed soil layers w i groundwater level; and His the lowered groundwater from 1 to n; His the thickness of the layer; Dpis the level. average additional stress on the layer; and ESis the For a point on the soft soil substratum on the floor of compression modulus of the soil layer. the subway tunnel, each parameter in equation (4) can be For the case of a thin compressed soil layer, where the considered as constant, except H. Hence, by using the additional stress and the average compression modulus are integral method, we get: fairly constant, equation (1) shows that the foundation settlement is proportional to the thickness of soil. Test data 2 S=a1H++a23Ha (5) showed that the additional stress and the soil compression modulus at the tunnel substratum at a cross-section agaggha remained the same before and after the construction of a vwvwvw 2 where a1=,,a23=-=ah. tunnel in soft soil. Even for different nearby sections, the 2(1+e0)1++ee002(1) change in additional stress of tunnel substratum soil is Equation (5) shows that soil subsidence is a second insignificant because the change in the longitudinal grade degree polynomial of groundwater level. of the tunnel is small. However, the soil compression modulus may exhibit certain differences because of the B. Load Factors of Train Operation geological inhomogeneity along the length of the tunnel. Under the action of train-induced dynamic load, soft Thus, the amount of compression of substratum on soil has elastic deformation and plastic deformation. The different fracture surfaces follows a linear relationship former does not affect the post-construction settlement but between S and HE/Si . the latter contributes to the tunnel settlement. By a preliminary study, the relationship between the soft soil b) The effect of aging (soil consolidation and creep) [9] deformation and the train load is derived as: Deformation of soft soil is actually a process of mn consolidation and creep simultaneously. According to the æqqöæö E=+aNçds÷ç÷1 b (6) soil unidirectional consolidation theory： pç÷ç÷ èqqfføèø ì p2 æö8 - Tv 4 where Eis cumulative plastic deformation. ïst =se×-ç÷1 p ïç÷p2 (2) íèø N,qd,qf,and qsare respectively the number of cyclic ïCtv ïTv = îH2 loading, the dynamic stress of traffic load（ kPa ）, the （ ） In equation (2), final settlement of foundation S is a static destructive stress kPa , and the static deviator theoretically determined value which is independent of stress. Parameters a,b,,mn are constants. time; C is the coefficient of consolidation and is a Equation (6) shows that soil parameters, static deviator v stress and static destructive stress are constant at the same constant; and His the thickness of compressible stratum location. For subway tunnel, the passenger traffic and the and is a constant. The consolidation settlement of the traffic frequency are relatively stable on the section for foundation, St, is a function of time (t). However, the final non-transportation hub. Thus, the dynamic stress of traffic settlement needs to be a theoretical value which is difficult load in certain cases can also be regarded as a constant. to measure in reality. Also, huge complexity and Consequently, equation (6) can be adapted as follows: uncertainty are involved in acquiring the value of mb consolidation coefficient and the thickness of compressible Epd=ABqN (7) stratum. Thus, by simplifying and deducing equation (2), we can obtain: 742 n long-term basic constant, which is a vertical constant and 1 æöq where A=,1Ba=+ç÷s are constants. the other factors of each are different. m ç÷ qqffèø C. Effect of Tunnel Leakage TABLE I. THE CONTRAST TABLE OF VARIATION CHARACTERISTICS Apart from the direct erosion of the loose soil variation surrounding a tunnel, leakage can create soil sediment flow No. Factor Lateral Longitudinal characteristics paths, reduce pore water pressure, and increase soil Substratum effective stress. Consequently, soil gets compacted to soil layer per issue distribution longitudinal cause subsidence, which in turn leads to the settlement of 1 the variable on the variation the subway tunnel.
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