Abstract: In terms of support form for foundation pit, the most common calculation method of internal force and displacement of retaining piles (or enclosure wall) plus internal bracings is incremental method, which is based on the theory of beam on elastic foundation. Reasonable calculation of load increment is the key to implement this method. At present, the load increment of this method mainly consists of earth pressure increment and reaction increment released by the excavated soil elastic resistance. Based on the basic principle of the method of beam on elastic foundation and incremental method, 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 included reaction increment by soil elastic resistance released, which was partially caused by the decrease of the excavation face and the reduction of the horizontal resistance coefficient. The reaction increment of random point of the retaining piles (or enclosure wall) in this part that below the excavation face was equal to the product of the lateral displacement of this point in the previous working condition and the proportional coefficient of the soil horizontal resistance coefficient (m) , and the excavated soil thickness in the current working condition. The comparative analysis of theoretical calculation results and measured results of engineering examples show that: compared with the traditional incremental method, utilizing the load increment mentioned in this paper to do load increment calculation (the improved incremental method) yields a value of lateral displacement of retaining piles (or enclosure wall) that is much closer to the measured value. The calculation result is more in line with the engineering practice.

Key words: foundation pit, enclosure structure, lateral displacement, incremental method, elastic resistance