风化岩地基大直径长螺旋钻孔灌注桩承载性状试验研究

Experimental study on bearing capacity of large diameter long auger bored pile in weathered rock foundation

基于风化岩基中3根大直径长螺旋钻孔灌注桩的单桩竖向抗压静载荷试验以及桩身应力测试结果,探讨大直径嵌岩灌注桩承载机制与变形特性。研究结果表明:3根试桩的荷载(Q)-位移(s)曲线呈缓变-陡降型,桩顶沉降较大,均超过80 mm,单桩竖向极限承载力均未达到设计要求;桩顶残余沉降量达92.0%～95.1%,桩顶回弹率达4.9%～8.0%,桩的弹性工作性状不明显,表现出端承摩擦桩的特性;在本试验条件下,桩侧极限摩阻力与JGJ94—2008"建筑桩基技术规范"推荐值相比,桩侧极限摩阻力在松散的粉细砂、稍密的中粗砂、可塑—硬塑的黏性土、全风化硬岩以及强风化硬岩中有所降低,仅在稍密—中密、中密状态的中粗砂有所提高,且提高幅度较大,因此,在类似岩土层中要慎用长螺旋钻孔灌注桩;3根桩的桩侧尺寸效应系数与JGJ 94—2008"建筑桩基技术规范"推荐值相比,在嵌岩段降低17.0%～74.0%,在非嵌岩段中,砂土、碎石类土中提高20.0%～126.0%,而在黏性土中降低7.0%～43.0%。

Based on results of the vertical compressive static load test conducted on three large-diameter long auger bored piles socket into weathered rock,the bearing capacity and deformation characteristics were studied.The results show that the load?displacement(Q-s)curves of the three test piles initially change slowly and then decrease steeply.The settlement of the pile top is larger than80mm and the vertical ultimate bearing capacity of single pile cannot meet the design requirements.The residual settlement of the pile top reaches92.0%-95.1%,with the rebound rate being between4.9%and8.0%,and the elastic working behavior of the piles is not obvious,showing the characteristics of the end bearing frictional pile.Under the test conditions,the ultimate friction resistance of the test pile reduces compared with the value recommended by JGJ94-2008“Technical code for building pile foundation”in the loose silty-fine sand,the slightly dense medium-coarse sand,the hard plastic clay,the whole weathered hard rock and the strong weathered hard rock,while increases greatly in the slightly dense-dense coarse sand and the coarse sand.Therefore,the long auger bored pile should be carefully used in the rock and soil layers.Compared with the recommended value in JGJ94-2008“Technical code for building pile foundation”,the pile side size effect coefficient of three piles reduces by17.0%-74.0%in the embedded rock section.In the non-embedded section,sand and gravel soil increase by20.0%-126.0%,while in clay decrease by7.0%-43.0%.