The influences of karst cave on load transfer law and bearing characteristics of pile passing through karst caves are investigated. The variation of load-settlement law, vertical bearing capacity, load sharing ratio, axial force and lateral resistance of pile passed through karst caves of 4 different heights, 4 different spans, and 3 caves layers are studied by centrifugal model test. The sensitivity analysis model was introduced to study the sensitivity of vertical bearing capacity of pile to two factors. The load-settlement calculating formulas of pile passing through karst caves were established by load transfer method. The results reveal that increasing the height, span, and number of caves all have detrimental effects on the vertical bearing capacity of pile, and the influence of height on bearing capacity is greater than that of span. The pile axial force almost keeps unattenuated and unit lateral resistance decrease suddenly to zero under the change of two factors. Based on the improved load transfer method and hyperbolic model, the calculation formula of pile bearing capacity is established. Compared calculation results with test results, the maximum error s only 5.96%, which has a high coincidence degree. The study can provide references for calculating vertical bearing capacity and settlement of pile in karst area.

With the purpose of researching the distinctions between the anti liquefaction capability of large-diameter variable cross-section group piles foundation and single pile foundation under earthquake condition, relying on the Second East Passage Project, the dynamic response difference of large-diameter variable cross-section single pile and group piles foundation was studied by vibration table model testing, and the difference of pore pressure ratio, pile shaft acceleration, pile top horizontal displacement and pile shaft bending moment between large-diameter variable cross-section single pile and group of piles in saturated sandy soil under earthquake load was analyzed. The research results show that under the seismic load of 0.30 g, the foundation of large diameter and variable cross-section single pile and pile group are liquefied, and the complete liquefaction time of large diameter and variable cross-section group piles foundation is 1.58~1.61 s later than that of single pile foundation; The maximum acceleration of large diameter variable cross-section group piles is reduced compared to single piles, and the occurrence time is more delayed than single piles; The maximum horizontal displacement of the top of a large diameter variable cross-section pile group is reduced by 0.68 mm compared to a single pile, with a time lag of 1.44 seconds; The maximum bending moment of large diameter variable cross-section group piles foundation is 11.99% less than that of single pile foundation, and the maximum bending moment of great diameter variable cross-section group piles foundation lags 1.88 s behind that of single pile foundation; It reveals that the anti liquefaction features of large diameter and variable cross-section group piles foundation are significantly higher than that of single pile foundation, and the dynamic response is more delayed. In the design of pile foundation in earthquake area, the anti liquefaction features of large diameter and variable cross-section group piles foundation can be increased by using the form of large diameter and varying cross-section group piles foundation.

To study the dynamic response rules of pile foundations of mega-bridges over faults in strong seismic areas, a finite element model of the pile foundation-soil-fault interaction of the Haiwen Bridge is established. The 0.2–0.6 g peak acceleration of the 5010 seismic waves is input to study the effect of the seismic wave of different intensities and the distance changes between the fault and the pile foundation on the dynamic response of the pile body. The results show that the soil layer covering the bedrock amplifies the peak pile acceleration, and the amplifying effect decreases with increasing seismic wave intensity. However, bedrock has less of an effect on peak acceleration. The relative pile displacement shows the mechanical properties of elastic long piles. The pile foundation generates a large bending moment at the bedrock face and the upper soil layer interface, and a large shear force at the pile top and the soft-hard soil body interface. The relative displacement, bending, and shear bearing characteristics of the pile foundations on the upper and lower plates of the fault are significantly different. The deformation characteristics are affected by faults in a region ten times the pile diameter. Analysis of the dynamic p-y curves shows that the soil resistance on the pile side of the lower plate at the same depth is greater than that of the upper plate. Sensitivity of the dynamic response of pile foundations on either side of the fault to the effects of seismic intensity and distance between the pile foundation and the fault: distance l > seismic intensity q.

High-fill culverts placed in valley topography have a complex earth pressure distribution law due to their locations around culverts in various valley topography conditions. In order to investigate the influence of valley topography on the earth pressure around the culvert of high-fill arch culverts, an interaction model of "topography-culvert-fill" was established by using centrifugal model test and numerical simulation method. Additionally, the distribution laws of earth pressure around the arch culverts and earth pressure concentration coefficient K_s at the top of the culvert under different valley widths B and valley slopes α were analyzed and were compared with the latest Chinese culvert design code. Furthermore, the mechanism of earth pressure formation of high-fill arch culverts under valley topography was presented. The research findings are as follows: (1) The influence of the valley width B on the earth pressure concentration coefficient K_s at the top of the culvert is significant, and the increment of the earth pressure concentration coefficient K_s at the top of the culvert is larger when B is 4D−6D（D is the clear span of arch culvert）. (2) When B is less than 4D, the topography would play the role of load reduction to the culvert. (3) With valley slopes ranging from 45° to 60°, the earth pressure at the tops and K_s would be affected dramatically. (4) When fill height is 20 m with α >70°, K_s≤1; and when 40 m fill height with α >50°, K_s≤1. (5) The K_s recommended by the latest Chinese culvert design code differs to some extent from those by centrifugal model test and numerical simulation. In the case of α =45° with a small B value, the earth pressure concentration coefficient K_s at the top of a culvert of the code is more conservative. (6) K_s of high-fill arch culvert in valley is related to the formation of arch top compaction area and isobaric surface. The arch top compaction area can cause the earth pressure concentration on the top of the arch culvert, and cause the vertical earth pressure of the soil around the compaction area to be arched. At a certain depth approaching the surface of the fill, the arch distribution gradually transits to the horizontal distribution, thus forming an isobaric surface. The upper load of the isobaric surface will be dispersed on the valley slope, so the unloading effect of the valley topography can be exerted.