Method for Calculating Bearing Capacity of Bridge Pile Foundation in Ningbo Soil with Deep Soft Base

The problems caused by the improper treatment of the transition section of deep-soft-subgrade bridges have become increasingly obvious. From the perspective of design optimization, to solve the differential settlement deformation of roads and bridges in deep soft foundation areas, we analyzed the bearing characteristics of seven bridge pile foundations in the deep soft foundation areas of the Beicang district in the city of Ningbo based on their P-s curves, as measured in static load tests. We constructed a three-dimensional numerical model of a pile foundation with different soft-soil-layer thicknesses, and investigated the bearing capacities of pile foundations that do and do not pass through soft soil layer in a deep soft soil area. Using regression analysis, we derived bearing capacity formulas for pile foundations that do and do not pass through a soft soil layer for 40-mm and 60-mm displacements of the pile top. The results show that the P-s curves of pile foundations in deep soft soil areas are mostly the slow type with no downturn section. In our tests, the settlement of the pile top reached a maximum of 49.6mm, and the settlement of any level did not exceed five times the settlement of the previous level. Based on our results, we conclude that the current safety reserve of the designed bearing capacity of a bridge pile foundation in deep soft areas is too large. When the pile crosses through the soft soil layer, the bearing capacity of the pile increases linearly with the thickness of the soft soil, but the thickness of the soft soil layer exhibits less effect on the bearing capacity when the pile does not go through the soft soil layer. The bearing capacity of pile foundations with a pile length of 30m appears to have an obvious cut-off point when the soft soil thickness is 30m. Based on the results of a static load test and numerical simulations, we used regression analysis to establish a pile bearing-capacity formula for pile top displacements that are more that 40mm and 60mm, and we verified the feasibility of this formula using engineering examples. As such, we propose an optimization formula for pile length and its applicable conditions, and provide the calculation formula for pile-length reduction coefficient α, which can be used for reference. © 2019, Editorial Board of Journal of Tianjin University(Science and Technology). All right reserved.