Numerical Method To Study The Physical And Mechanical Characteristics Of Sandy Pebble Soil And The Response Caused By Shield Tunneling

Nowadays, the development of urban railway system in our country is at a bursting age.One of the most important components of urban railway system, subway, is underconstruction in many populated cities. The challenge of complex geological conditions isinevitable with lots of tunneling projects going on across the country. One of the mostcomplex geological conditions is sandy gravel layer and a typical project dealing with suchdifficulty is located at a subway in Chengdu, where water-rich sandy gravel layer exists andconsists of a large quantity of gravels with high compressive strength. Sandy gravel soildemonstrates a very complicated dynamic behavior due to discreteness of gravels and sand,high compressive strength of gravels, and random geometrical distribution of gravels.Therefore the underground shield construction in such soil layer is very difficult. To bettersolve challenges encountered in the construction, a fundamental issue, the static and dynamicbehaviour of sandy gravel soil, should be investigated. Although a lot of research about sandygravel soil already exists, most of them are carried out in an experimental way and few weretackling the problem theoretically, not to mention the amount of theoretical study focusing onunderground shield construction in sandy gravel soil. Thus it is of great engineering value toinvestigate the physical behaviour of sandy gravel soil and its effect on underground shieldconstruction.This thesis uses fractal theory, inclusion theory and discrete element method toinvestigate the geometric and static’s characteristics. By comparing the laws acquired fromprevious theories and the real application, conclusions can be drawn as follows:(1). A mathematical expression for the distribution of cobble radius using fractal theorywith such an assumption that all cobbles have spherical surface. A statistical analysis on graingradation of naturally settled sandy gravel soil is also provided. It shows that the distributionof radius of naturally settled sandy gravel soil is consistence of the fractal structure. Most ofthe fractal dimensions lie between2.3to2.7, and the average value is about2.5. However,part of Chengdu area has a larger proportion of cobbles. For example, the dense sandy gravelsoil has a fractal dimension varies from2.05to2.5.(2). Two numerical methods used to rebuild sandy gravel soil are proposed to overcome thegreat content difference between different soil zones. The first one is Randomly ReplaceMethod and the second one is Float-Settlement Method. The first method is suitable for soilwith small proportion of coarse particles and the second method is good for soil with a large amount of coarse particles. Both methods are described in details.(3) In the numerical sample of particles where the distribution of particle radius meets thefractal structure, particles of diameter between5mm to40mm are chosen to rebuild sandygravel sample based on Float-Settlement Method, and the porosity of the rebuilt model isanalyzed statistically. It is found that the porosity of the model is about50%which decreaseswith the increase of fractal dimension and the radius ratio of particle (the ratio of maximumradius to the minimum radius). The selection criterion of numerical rebuild method is that ifthe sample has a content of coarse particles less than50%, then the Random Replace Methodshould be used, otherwise the second method should be used.(4).According to inclusion theory, sandy gravel soil with a low content of cobbles can besimplified into a two phase mixture by assuming that sand and gravels are substrate andcobbles are inclusion. Using Eshelby’s solution of elliptical inclusion stain problem, amathematical expression for the equivalent young’s modulus of sandy gravel soil can bedeveloped. It shows that the equivalent young’s modulus can be accurately obtained when thevolume percentage of cobble is smaller than50%, which grows with the increases of volumepercentage of cobble.(5). To investigate the mechanical properties of dense sandy gravel soil which contains alarge proportion of cobbles, a cylinder model is built using Float-Settlement Method. ParticleFollow code is used to carry out numerical triaxial test on the model. The numerical resultsshow that there is a relationship between the compressive strength of the model and the radiusdistribution of particles, confining pressure and friction factor between particles.(6). Numerical analysis methods and parameter selection on surface settlement, seepage, andstress of cutterhead are also discussed. This paper focuses on the effects of cutterhead stress,surface settlement and erratic boulder on underground shield construction in dense sandygravel layer. The results show that the cutterhead stress varies greatly in dense sandy gravellayer, where the maxima could be twice or even twice higher of the average thrust. If a largeerratic boulder is encountered and the cutterhead can’t cut the boulder effectively, thecutterhead stress will continuously grow and the excavation is forced to stop. This predictionagrees well with the reality. Tunnel over break is a major cause of stratum settlement. Thesubsidence regularity of sandy pebble soil layer satisfies the Peck formula curve. In theprocess of the excavation，the stratum settlement is decided by many factors，which includethe ground loss ration, tunnel depth and the stratum position in vertical direction.