摘要:
国内多数工地试桩未达到极限荷载,承载力估算偏低,桩基潜力得不到合理发挥。为正确计算嵌岩桩的承载能力,采用桩端极限平衡经典理论计算桩端极限阻力,再将桩端平面以上的滑动面摩擦力作为承载力增量的方法计算,与11根试验桩静载成果的对比表面,对于桩端嵌入中风化花岗岩的桩端极限承载力比按GB 50007—2011《建筑地基基础设计规范》的计算值高30%。对10根工程桩的检测发现:在最大试验荷载下,五根桩端岩芯呈碎柱状,未达到极限端阻力,具有较大潜力;两根桩端岩芯呈碎块状,已接近极限端阻力,但仍比GB 50007—2011计算值高22.8%。经对11根试验桩和10根工程桩的试验成果分析表明:离地面25倍桩径以下的桩周土摩阻力与嵌岩段摩阻力及端阻力均未得到充分发挥;桩体入岩长度以1.0~2.0倍桩径为最佳;孔底沉渣厚度在50 mm以内对承载力的影响并不明显,超过50 mm后的影响较大;桩端岩层构造差异对承载力的影响在30%以上。
Abstract:
Most of tested piles in domestic construction sites are not loaded to the ultimate loads, and the estimation on the bearing capacity of piles is often underestimated, so that the potential of pile foundations cannot be reasonably brought into play. To correctly calculate the bearing capacity of rock-socketed piles, the classical theory of the limit equilibrium for pile tips was used to calculate the ultimate resistance of pile tips, the friction force on sliding surfaces above the plane of pile tips was taken as the increment of the bearing capacity. By the verification of static load tests for 11 test piles, the ultimate bearing capacity of pile tips embedded in moderately weathered granite was 30% higher than that one calculated by Code for Design of Building Foundation(GB 50007-2011). Further, 10 engineering piles were screened out to be tested. The results showed that: on the same maximum load level, the rock foundation under 5 pile tips damaged to column-block shapes but the bearing capacity of pile tips did not reach the ultimate, and still had greater bearing potential. The rock foundation under 2 pile tips damaged to crushed stones, the bearing capacity of pile tips was close to the ultimate but 22.8% higher than that one calculated by GB 50007-2011. By comparing and analysis of test data from 11 test piles and 10 engineering piles, it showed that If the pile length was more than 25 times of pile diameters, the friction resistance of stratum around piles including the rock-socketed length and the resistance of pile tips all didn't fully bring into play. The optimum rock-socketed depth was 1.0 to 2.0 times the pile diameter. The influence of the sediment thickness under the pile tips on the bearing capacity was not obvious when the thickness of the sediment was more than 50 mm, the influence of structure difference for the rock stratum on the bearing capacity was more than 30%.