Numerical Simulation Of Soil-Nailing And Its Forecast System On Excavation

With the development of industry and municipal construction, ground surface is becoming less and less. As a kind of still unexploited resources, underground is attracting more attentions and being widely utilized for underground establishments as sub-carport, sub-marketplace and subway, etc. As a result, the depth of excavation is becoming deeper. Soil-nailing is finding its wider application in foundation engineering for its outstanding profits and favorable working characters, therefore they have been studied theoretically by lots of scholars. Yet the mechanism and genuine character for its partially losing stability is still left unclear, demanding further research.?NMM(Numerical Manifold Method), which is developed on the basis of limited coverage system, is being widely paid attention in geotechnical engineering. It is on the basis of topological manifold, combined with physical grid and mathematic grid, fusing analytics, limited element and DDA(Discontinuous Deformation Analysis) method which is suitable for block analysis into one new uniform numerical method by analysing general coverage function established on region-in physical coverage and total displacement function formed by weight function.This paper mainly adopts following two methods to analyse the characters of the excavation which is being digged: (1) By applying the basic principle of NMM, limited grid is transferred into numerical limited coverage, grid attribute and coverage function is re-defined. With the beginning of non-linear theory system, a theoretical calculation formula for non-linear NMM which is based on variational principle is worked out, and its governing equation is established. Duncan-Chang Model is employed as soil-mass model for its wide application in geotechnical engineering, it reflect soil-mass's stress-strain characteristic preferably, the original model is aimed at plane strain, and is now extended to three-dimensional problems; Yinzong-Ze model is employed for interface character model, because it can more reasonably reflect distort characters of shear destroyed strip of the interface or its surroundings, without quoting no extra parameters; Between the interface of soil-mass and soil-nail, a thin three-dimensional interface element can be setup, By applying pole element or entity element to simulate soil-nail,corresponding calculating program is compiled. Having considered the intricate construct procedures of the excavation, soil-nail and section setup, this program is of powerful back and forth process function, realizing the visualization of calculation results. (2) Combined with uncertain reasoning principle, a kind of on-line study calculation method is employed, by which a self-study real-time network model is established. By training the network with data obtained from the excavation, the trained network can be utilized to predict the character of the anaphase excavation. By applying the principle of the trend surface analysis, soil-mass's element displacement of excavation wall is calculated. Accordingly, some research conclusions are obtained.1 Soil-nailing is of more evident dimensional domino offect, the horizontal displacement curve of support structure exhibits bigger for the upper and smaller for the lower within the section of weaker dimensional domino offect, it exhibits smaller at the tow ends and bigger in the middle in the section of stronger dimensional domino offect, distributing like a drum. This effect is also more obvious in the direction of breadth than in length. It's more obvious above the excavated section than near or underneath the excavated section.2 Under the condition that soil-mass has been distorted, stress surrounding the soil-nail is transferred to it. By means of bond stress and friction between the interface of soil-nail and soil-mass, soil-nail is made to draw passively; acting as transferable and diffusible function, the soil-nail greatly reduce the distributing section of tensile stress in soil-mass, and thereby, reduce the stress concentration area and its degree at the foot of slope. Therefore, the shear stress within soil-nail distributing area is relatively reduced.(1) Bend-resist and shear-resist function of the soil-nail's forepart is evidently larger than that of its tail. In the mean time, there exists a counterpoise point of torsion moment in the middle of soil-nail, which is easy to yield or break off.(2) The soil-nail's tensile stress function contains following two aspects: one function is that it directly reduce the soil-mass's element shear stress; the other one is the increase of soil-mass's shear-resist strength by normal component; Its shear function lies in following two aspects: one function is increasing the shear stress exerted on the soil-mass's element by tangential component; The other one is reducing shear-resist strength by normal component.(3) From soil-nail's whole point of view, its shear-resist function is not quite evident when it is being digged, while bearing tensile stress is its main behavior, according to its shear stress magnitude.3 Soil-nail is of the function to connect and restrict the framework, and of anchor-hold function to some degree. It can fully bring soil-mass into play, self-stabilizing and blocking soil-mass. On one hand, the friction among soil-nails increases its strength; on the other hand, the form of flexible support structure, as a result of soil-nail and soil-mass's co-operation, can bear soil-mass's lateral pressure from outside the soil-nail and sustain its stability, in turn, passive support is transferred to active support. The above function is determined by soil-nail's rigidity, strength and its distributing space, which is manifested in the case that the soil-mass's lateral deformation can be restricted by soil arching effect under the condition of adequate soil-nail distance.4 As an effective approach to deal with non-linear problems, neural network can be applied to study soil-mass's structure. Inspecting soil-nail stress's prophase change and predicting its anaphase change will be helpful to analyse support structure's inner stability; While inspecting the horizontal displacement change of maintaining structure and predicting its anaphase change will be helpful to analyse system's outside stability.5 Combining the measuring data and the forecast data from single observation dot, curve and surface chart of changing trend for different sections and layers at the same time can be formed by introducing trend surface analysis principle. Hence, the three dimensional analysis system, consisting of dot, line and surface, will redound to analyse support structure's dimensional domino offect.