The Improvement On The Calculation Method Of Internal Force And Displacement Of Pile Retaining Structure

The most common calculation method of internal force and displacement of enclosure structures is incremental method,which is based on the theory of the method of beam on elastic foundation.The paper discussed the shortcomings of the current incremental methods and proposed some improved method.Furthermore,the several calculation methods based on elastic foundation beam were compared.The main contents of this paper are as follows:1.The improvement of load incrementAt present,the load increment of the incremental method mainly consists of earth pressure increment and reaction increment released by the excavated soil elastic resistance,which is called the traditional incremental method in this paper.Combined with excavation construction process,the following conclusions were drawn through the comprehensive analysis.In addition to the above two parts,the load increment also include reaction increment partly released by the soil elastic resistance which is caused by the decrease of the soil excavation face and the reduction of the horizontal resistance coefficient.For the reason,an improved incremental method considered the mentioned reaction increment was proposed in the paper.Taken Shuangliu Airport Station Foundation Pit Engineering as the background,the internal force and displacement of retaining piles were calculated by using the traditional incremental method and the improved incremental method,respectively.Then,the results were compared with the measured displacement.The results show that the lateral displacement of the retaining pile calculated by the improved incremental method is much closer to the measured value compared with the traditional incremental method.The result is more in line with the engineering practice.2.The comparison of selection method of the ratio coefficient m value of horizontal resistance coefficientAccording to the provisions of the current procedure of "Technical specification for retaining and protection of building foundation excavations(JGJ120-2012)",the selection of the ratio coefficient m value of horizontal resistance coefficient by the horizontal load test of the pile and regional experience is better than by empirical formula.According to empirical formula,if the horizontal displacement of the retaining structure in the excavation face is no more than 10mm,10mm is taken to participate in the calculation.There is no local empirical value at Shuangliu Airport Station Foundation Pit Engineering.So,the horizontal load test of the pile and the empirical formula are used to calculate the m value,respectively.Then,the lateral displacement of the retaining pile is calculated by the improved incremental method and compared with the measured displacement.The results of comparative analysis of ten sections showed that:when the horizontal displacement of the excavation face is no more than 10mm,the lateral displacement of the retaining pile is much closer to the measured value which the m value is obtained according to the empirical formula.So,it can be explained that the selection of the ratio coefficient m value of horizontal resistance coefficient by the calculation method of the empirical formula is better by the horizontal load test,which is contrary to the provisions in the current procedure.3.Comparison of several calculation methods based on elastic foundation beamThe calculation methods based on elastic foundation beam include the full volume method,the traditional incremental method,the improved incremental method and the calculation method of the support erection time.The improved incremental method has been verified more reasonable than the traditional method in respect of the improvement of load increment.Thus,the full volume method,the improved incremental method and the calculation method of the support erection time are used to calculate internal force and displacement of Shuangliu Airport Station foundation pit engineering piles retaining structure and compared with the measured displacement.The results show that:the calculation method,which is the most close to the measured displacement,is the calculation method of the support erection time,followed by the improved incremental method,the full volume method.