Study Of The Soil Arch Effect And Design Theory Of Soil Nail Supporting System

Based on reading literatures extensivly and lots of enigheering practices of slopes and foundation pits, it was found that theory on soil nail supporting system lagged behind practices largely in spite of its wide use. The design theories are still rough, incomplete and some of them even contradict engineerings.According to analysis on in-situ tests and the existing literatures on soil nail supporting system, by means of theoretical study and numerical simulation, a series of researches was done systematically on the soil arch effect, the shape of sliding plane, the earth pressure acted on face board and the soil arch behind, design of face beam board and soil nail, compound supporting system of soil nail and water-tight purdah. These rusults were used on design of large foundation pit engineering and was tested by in-suit monitoring data, the following achievements were obtained:(1) Forming conditions, types, shapes and influence factors of soil arch are analysed. Expressions of thickness and maximum stress of the arch are derived under the continuously-distributed load increased in the shape of the arch axis and uniformly distributed load. From which the height of soil arch,the horizontal and vertical forces acting on the arch foot could be obtained while the arch foot was on the limit equilibrium state.The load transfer mechanism among soil nail, face board and soil is made clear and the soil arch effect principle on soil nail supporting system is established.(2) The construction effect and sliding plane shape of soil nail supporting system are analysed. The vertical load acted on soil between nails reduces sharply because of soil arch above the nail. Based on this, the shape of sliding plane was developed.(3) Considering the most dangerous working condition and the forming process, the distribution form and calculation formula of the earth pressure acting on the arch behind the face board are developed.(4) The formula of the earth pressure acting on the face board is derived through mechanical analyse by comparing such structure forms as single continuous beam, lattice beam, no beam and flat board, taking soil arch effect behind face into consideration, arch foot force acting on beams and face board bearing only soil pressure before soil arch. The value is much smaller than soil pressure behind arch. Thus the reasonable form of face board is put forward and the defect of the traditional flat board structure is analyzed.(5) Through analysis on the mechanical characteristics of soil nails, the mechanics model is founded and the calculation method are put forward.(6) The analysis method of compound supporting is established by studying the soil arch effect of the compound supporting system formed by soil nail and water-tight purdah. This method can be used in pit with high gound water level and soft soil.The results of theoretical analysis coincide with the in-situ test results.The monitoring data of two engineerings designed according to the theory of this paper verified the theoretical analysis. The types and forms of soil arch with FLAC 3D numerical simulation is identical with the results of theoretical analysis.Research results of this paper can be used to designing and calculating horizontally layered or homogeneous soil or rock slope, foundation pit with level gound surface supported by soil nail system.