戈壁碎石土地基原状土掏挖基础抗拔试验研究
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摘 要：在甘肃和新疆的7个试验场地完成了40个戈壁碎石土地基原状土扩底掏挖基础抗拔试验，得到了基础抗拔荷载位移特性。结果表明，上拔荷载作用下，戈壁碎石土地基掏挖基础具有良好的抗拔承载性能，荷载位移曲线呈大致相同变化规律，可分为扩大端土体被压密的弹性阶段、土体剪切变形至塑性区贯通的弹塑性阶段、滑动面形成至破坏的3个特征阶段。根据基础荷载位移曲线，采用双切线交法确定了试验基础极限上拔承载力。基于土体滑移线场理论和Mohr-Coulomb强度准则，引入抗拔土体圆弧滑动面假设，推导了戈壁掏挖扩底基础的抗拔承载力理论计算公式，并将试验结果与理论计算结果进行了对比分析。
关键词：戈壁；碎石土；掏挖扩底基础；抗拔；输电线路
中图分类号：TU411
文献标志码：A
文章编号：1674-4764（2012）04-0024-07
Field Test Analysis on Belled Pier Foundations under Tensile Load in Gravel Gobi
LU Xianlong1，2, QIAN Zengzhen2, TONG Ruiming1, ZHENG Weifeng1
( 1. Transmission and Transformation Engineering Research Department, China Electric Power Research Institute，
Beijing 100192, P.R.China; 2. School of Engineering and Technology, China University of Geosciences, Beijing 100083, P.R.China)
Abstract:Forty full-scale field tests were conducted on belled pier foundations at seven gravel Gobi sites in Gansu province and Xinjiang Uygur Autonomous Region in China. The uplift load-displacement response of belled pier foundations in gravel Gobi almost followed the same pattern and presented three-phase behaviors. Under tensile load, the onset of the curve usually started from the elastic linear part resulting from the compression and compaction of gravel Gobi above the enlarged base. And consequently, with the occurrence and further development of a plastic zone around the foundation, the uplift load-displacement curve turned into an elasto-plastic stage. Finally, the formation of whole rupture surface brought to the entire shearing damage of Gobi soil. The ultimate uplift bearing capacities were obtained by using the slope-tangent method for all test foundations. Based on the theory of limiting equilibrium, the Mohr-Coulomb yielding criterion and slip-line field method, the circle arc rupture surface boundary condition was introduced. And the theoretical calculation equation to determine the ultimate uplift bearing capacity of the belled pier foundation in gravel Gobi was obtained. The theoretical results were compared with those of the tests and they turned out in good agreement.
Key words:Gobi; gravel soil; belled pier foundation; uplift; transmission line
戈壁是粗砂、砾石覆盖在硬土层上的荒漠地形，在中国西北地区广泛分布。随着电网建设的发展，越来越多的输电线路需经过戈壁滩地区。杆塔基础作为输电线路的重要组成部分，其造价、工期和劳动消耗量在整个工程中占很大比重。杆塔基础具有明显的行业特点［1］，通常情况下，抗上拔能力是其设计控制条件，选择合适的基础方案可有效降低工程造价，提高环境保护水平。
戈壁碎石土地基一般以圆砾、角砾、卵石等碎石为主，并常有砂类及黄土类堆积填充，或呈交互层状及透镜体产出，土质盐份含量大、粒间胶结效应明显、咬合作用强烈，天然状态下具有较好的力学性质［2］。但中国过去戈壁地区的输电线路杆塔基础多采用钢筋混凝土柔性板式基础，这类基础因通过开挖回填方法施工，未能充分利用原状戈壁碎石土地基良好的承载性能，既增加了材料消耗量，也不利于环境保护。随着输电电压等级的提高，杆塔基础所承受的荷载越来越大，采用这类基础则需要更大的基础截面尺寸和埋深，经济性和环境保护都难以满足建设需求。鉴于此，原状土戈壁地基掏挖扩底基础已在该地区输电线路工程中日益得到推广应用［3-4］。掏挖基础是以混凝土和钢筋骨架灌注于以机械或人工掏挖形成的土胎内的基础，它以天然原状戈壁地基作为抗拔土体以保持基础上拔稳定，其充分发挥了地基承载性能，具有良好的抗拔承载性能。国内外通过模型试验、现场真型试验［5-17］等方法研究了掏挖基础的承载性能，并给出了其极限抗拔承载力的计算方法，但对戈壁地基掏挖扩底基础抗拔承载特性和计算理论都还缺乏系统研究。笔者通过甘肃、新疆2地区7个场地共40个戈壁掏挖扩底基础的抗拔承载力试验，得到了其抗拔荷载位移特性，提出了戈壁掏挖扩底基础抗拔极限承载力计算方法，并与试验结果进行对比。