Study On Evolution Characteristics Of Engineering Effects And The Disaster Mechanism For Shallow Gassy Sand

Currently, a type of soil layer stored with high pressure and flammable gas was encountered during the exploration of Hangzhou metro line 1 project under construction. Such the wide distribution and shallow buried gassy ground condition is bound to bring adverse effects on the construction of this subway engineering. However, experiences and related research achievements about building subway under such condition are lacking both at home and abroad. What engineering properties and hazard mechanisms of the layer and which control measures should be adopted to ensure the construction safety and the operation of subway smoothly have become the problems solved urgently in the current metro design and construction.Taking the gassy sand in shallow gas reservoirs of the typical section in Hangzhou metro as an object, based on the theory of unsaturated soil mechanics, and with the laboratory test, model experiment and numerical simulation analysis, the evolution law of its engineering properties and hazard mechanism are studied systematically. Intrinsic mechanisms, models of gassy sand causing disasters and measures taken for controlling disasters in the subway construction are discussed in this thesis. The main contents and the conclusions are as follows:Firstly, geologic origin, type of gas reservoir, gas component, feature of gas pressure distribution and law of basic occurrence of the shallow gas stratum are summarized; the formation mechanism of such shallow gas, the history of gas migration and accumulation and the zone theory of its occurrence are deduced. The shallow gas reservoir existing widely in Hangzhou area is determined by its special geographical position and climate condition. It is buried shallowly and belongs to that of normal pressure reservoirs. The gas composition is main methane. Silt layer with rich organic matter is the main source and sealing layer; and uncemented sand shell bed is the main gas reservoir. The state of its occurrence in gas reservoirs is from dry to wet, and saturation degree of it is from low to high until the sand is saturated along the depth; the gas occurrence has an obvious feature of zonation.Secondly, on account of the different saturation degree of sand, basic mechanical properties such as moisture holding capacity, water permeability, gas permeability, shearing and consolidation features are studied, and their variations with the suction or saturation degree are analyzed. Results show that the gassy sand with high porosity is silty fine sand with a small quantity of clay fraction, and the matric suction of the sand in gas reservoirs varies from 0 to100kPa. Van Genuchten equation can favorably describe the soil water characteristic curves of gassy sand (SWCC), and other engineering indexes of the gassy sand such as gas pressure in reservoirs, shearing strength and coefficient of permeability can be predicted by the curve of SWCC under lacking test conditions. Gas permeability of the gassy sand is studied by using a self-designed experimental device, and results show that influences on gas permeability from the variation of saturation degree is more significant than that of density. VGM and Parker models can be used to describe the water and gas permeability of the gassy sand satisfiedly. The apparent cohesion of gassy sand can be described with the power exponent function and its compressibility and rebound resilience decrease with the increasing of suction.Thirdly, on account of the common stress path in metro construction, strength properties of gassy sand under different stress paths and with different gas pressure levels are studied through lab stress path experiments, and a method to forecast the strength of gassy sand after being released of gas is proposed. Results show that the stress-strain relationships of gassy sand are distinct under the different stress paths, which dramatically affects its features of strength. The shear strength of gassy sand under different stress paths can be described by a unified formula. The strength of gassy sand will be enhanced, and a method is proposed to forecast it after the release of gas in it. It tends to be unsafe if the strength indexes of saturated sand are adopted and it should be considered to be reduced in practical engineering.Fourthly, gas release is one of the most common unloading paths of gassy sand in subway engineering construction. Deformation behaviors of gassy sand under the path of release of gas are studied through the lab tests. Results show that the higher the original reservoir pressure is, the larger the soil deformation caused by gas release is. The way to release of gas under controlling can effectively reduce the deformation of gassy layers. The wetting and drying history of gassy sand has a significant influence on its deformation behaviors. Deformations of gassy sand in the re-wetting process are less than those in drying process. With the increasing of net stress, influences on deformation of gassy sand due to the variation of suction weaken gradually, and the structure of sand gradually becomes more stable. In the path of gas release, both the matric suction and the net stress affect deformation behaviors of gassy sand. But in the original gas reservoirs, the deformation of gassy sand caused by the variation of suction or saturation degree is very small, and the dominating deformation arises from the increasing of net stress because of the decreasing of gas pressure. Re-accumulating of the gas only leads to a small swelling deformation, and has little influence on the deformation of gassy sand in practical engineering.Fifthly, a stress and strain constitutive model describing the solid-phase macro deformation behavior of gassy sand in the path of gas release under controlling is established based on the lab experiments and parameters of the model can be determined through the experiments. A relative permeability curve between the water and gas under different saturation degrees are given, by which a two-phase flow model describing the migration law of water and gas in gassy sand is established. Coupling with the constitutive model, behaviors of deformation influenced by the effects coupling with the three phases such as solid, liquid and gas can be calculated with the numerical analysis for gassy sand in the path of gas release.Sixthly, in order to investigate influences on subway tunnels in the path of gas release, two working conditions which are that the tunnel passes through the gas reservoir bed and that the tunnel passes through the cap bed are carried out respectively with the self-designed experimental device system for model tests. Results show that gas release in gassy sand will cause the additive deformation of tunnel and additive stress of the shield segment, and make important impacts on the global displacement of tunnel but little influences on the stress of shield segments and the relative displacement of tunnel. It is inclined to arouse the differential settlement and cracks on the tunnel so as to cause hazardousness. Re-accumulating of the gas can also produce additional stress and the additive resilience, but it can be ignored in practical engineering because the influence degree is small. The shallow gas in reservoirs is appropriate to be discharged before construction of the subway tunnel.Finally, the disaster mode of the gassy ground at different stages of the subway engineering is analyzed, and some counter measures during construction and operation period of the metro project are proposed. According to the analysis, the gassy ground has different disaster mechanisms and manifestations at the different stage of construction, and the corresponding measures for prevention should be taken with distinguishing different periods such as exploration, design, construction preparation, construction and metro operation. Only thus can it ensure the safety of metro engineering and operation smoothly.