Study On The Selection And Application Of M Value In Elastic Resistance Method Of Foundation Pit Engineering

In the design of foundation pit engineering,pile anchor and pile support system are commonly used at present.The linear elastic foundation reaction method is generally used to calculate the horizontal resistance,displacement and effect of pile body.The value of parameter m is determined according to the empirical formula.The value depends on the friction Angle and cohesion of soil layer,but has nothing to do with soil layer deformation parameter.How to avoid problems such as the empirical value of m and the need for correction in the calculation of silty soil and cohesive soil,this paper carried out the following research work and reached the following conclusions:（1）This paper summarizes the research results of how to evaluate the m value of horizontal subgrade coefficient based on soil deformation parameters.From the back analysis method,laboratory and field test research,finite element numerical calculation research,theoretical derivation and comprehensive method,the innovation and rationality of the analysis are summarized.In this paper,the relationship between m value and deformation parameters is discussed from semi-infinite soil and finite soil.（2）For semi-infinite soil mass,the proportional coefficient m of the horizontal subbed coefficient in the elastic resistance method is obtained from the simple shear model and the Mindlin elastic semi-infinite spatial displacement model respectively.Through the derivation of theoretical formula and some reasonable model assumptions,the m value is connected with the compression modulus of soil layer often measured in geological exploration,which avoids the problems of dimensionality mismatch and large randomness caused by the use of soil layer cohesion c and internal friction Angleφto determine the m value in the Technical Regulations of Building Foundation Pit Support.（3）For finite soil mass,k value and m value of reinforced soil mass and general soil mass are deduced by generalized Hooke’s law,and m value formulas of constant section and variable section dark brace are obtained.For the reinforced soil,the K method is more suitable to calculate the horizontal resistance coefficient k value when the width L of the passive zone is low because of its high strength.With the increasing span of the passive area,the k value will be very small,so that the calculation result of deformation is larger than the calculation result of skirt reinforcement with the same thickness,and the effect of soil at the bottom of the dark brace is completely ignored.In the face of the large span concealed bracing k value may be small,it is more practical to use the m method to calculate.For ordinary soil mass,when the width of passive zone is less than 5m,the m value of soil mass should be increased due to the superimposed effect of passive resistance on both sides.It is also safe to appropriately reduce pile length and support measures when designing trench foundation pit.（4）The mechanical properties of soil are complex and there are many kinds of determination parameters.The study extends the application range of m value formula.When the compression modulus is missing,the proportional coefficient m value of the horizontal resistance coefficient can be calculated according to the bearing capacity f_ak,static penetration resistance p_s,pore ratio e,compression index C_C and dynamic penetration hammer number N,etc.The form of table and empirical formula provides reference for practical engineering application.（5）Taking two practical projects as cases,the elastic resistance method and finite element simulation are used to verify the results.The results show that the deduced formula of simple shear model and empirical formula of provincial norm are in good agreement with the measured values under any soil condition.When the passive area is silty soft soil,the displacement of pile wall calculated by the derived formula of elastic semi-infinite spatial displacement solution is small.When this formula is used for auxiliary calculation,it should be multiplied by the corresponding safety factor to ensure that it is close to the calculated actual displacement.Under other soil layers,the formula is also highly consistent with the measured value.The accuracy of the proportional coefficient m formula derived from the compression modulus is verified.