Back Analysis Based On The Monitoring Results Of Shear Strength Parameters Of Pile Anchor Foundation Soil

The soil shear strength parameters obtained from laboratory test are the most important parameters of foundation pit design. Confined to the experiment conditions and test methods and other aspects, there may appear a high dispersion, and great differences from that of the in-situ test real indexes. Concentrated embodied in:(1) the obtained test data is quite limited and discrete limiting to the test conditions and field condition;(2) due to the influence of sampling method and the soil sensitivity, laboratory test results differ from that of the characteristics of in situ soil. Consequently, combining with the existing foundation pit monitoring data and applying back analysis to get the practical shear strength parameters range have played the significant roles in timely updating data、instructing later construction supervisory control and guiding the similar projects design.This article integrated Enumeration Method based on the RS (Response Surface) technique and proposed a process of inversing shear strength parameters range according to the pit deformation observation data. The essence of which is an Exhaustive Search Method built on the base of the RS, and the basic idea is that first to do some calculations with certain times, according to the requirements in Technical specification for retaining and protection of building foundation excavations"(JGJ120-99) then to obtain the RS between the pit deformation and soil parameters through regression analysis. On this basis, directly using the RS to predict the pit deformation can greatly reduce the workload on the pit deformation calculation and analysis. Furthermore, making use of the high efficiency of the RS in pit deformation prediction, to conduct a large number of enumeration in a bigger range of soil shear strength parameters, and input to the RS to figure out deformation value, then get the maximum contrasted to the measured value. Now, filtering between them to find the soil shear strength parameters data corresponding to the predicted and within limited value, which are regarded as approximate of the soil real indexes. Synthesizing the monitoring results under different working conditions, and screening gradually in each working condition through the method above, and by screening the data range gradually to make the shear strength parameters more accurate and reflect to the practical situation.For the specific technical means, this article adopted the united way of UD (Uniform Design) based and ACE nonparametric regression, and take advantage of less times of trial of UD and the high fitting precision of ACE nonparametric regression to build the efficient RS. On this base, Monte Carlo simulation was used to enumerate amount of soil shear strength parameters to submit to sample uniform distribution. Use the RS to predict the homologous pit deformation and contrast the predicted value and the measured value; build constraint conditions (sieve) with the deformation in different depth under different working conditions, then screen the values in a set tolerance range for the homologous sample data, and regard the data sample range as the approximate soil shear strength parameters within the deformation tolerance range. Through increasing the screening working conditions and narrowing down the tolerance range gradually to tighten the sieving conditions, with repeated screening, and gradually narrow down the data sample range to close to real value.According to the techniques above, combined an actual foundation pit and observed deformation data, with the initial calculating data arrays designed by UD, then adopt the related requirements of "Technical specification for retaining and protection of building foundation excavations"(JGJ120-99) to analyzed the pit deformation value with the arrays. Moreover, using the ACE nonparametric regression to build the RS, and inversing and screening the data generated by Monte Carlo simulation to obtain the ranges of shear strength parameters c、φOn this foundation, united the range of c、φ by back analysis, and checked the anchor cable force. Finally comparisons are made between the measured results and the checked force to verify the feasibility and effectiveness of the inversion results.