青藏输电工程回填细粒冻土性质和铁塔基础载荷试验研究

Experimental Study on Fine-grained Soil Properties and Tower Foundation Loading for the Oinghai-Tibet DC Transmission Line Engineering

为分析青藏直流输电工程回填冻土地基和铁塔基础承载性能,在浅部冻土处于冻结和融化两种状态时,分别进行回填细粒冻土性质试验和在三向荷载共同作用下锥柱式扩展基础的载荷试验。基于土工试验结果分析了冻融过程中回填地基的工程性质,得到一个冻融周期后地表沉降量为0.23m,与实际情况一致。通过对基础承载特性及荷载与位移双曲线关系模型的分析,得到了基于地基基础相互作用极限状态条件下基础的极限承载力。针对季节活动层融化和冻结状态,应用试验结果验证了上拔工况下铁塔扩展基础稳定性分析模型和方法,获得了冻结状态时细粒冻土的上拔角。试验研究表明:当浅部活动层处于冻结状态时,地基强度及基础承载性能优于融化状态;冻土融化可缩小细粒土的孔隙比、提高密实度,并增加含水率和饱和度;土体冻结与否对地基的抗剪、上拔角等力学指标有显著影响,因此,保持深部回填地基的冻结状态对基础安全承载至关重要。

To analyze properties of frozen ground and bearing capacity of tower foundation for the Qinghai-Tibet DC Engineering,tests of fine-grained soil properties and frustum-column spread foundation loading under three-directions loading condition were carried out in the Wudaoliang area of Qinghai-Tibet Plateau,while the shallow backfilled ground was in the freezing and thawing state.According to results of loading tests,bearing characteristics of frozen ground and tower foundation was analyzed,and ultimate bearing capacity was obtained in the interaction limit state between foundation and ground through establishment and fitting analysis of the load-displacement hyperbolic model.Based on measured values of soil tests,engineering properties of the ground were studied,the ground surface settlement of thawing period relative to freezing period was calculated and was 0.23m,and the calculated value was consistent with actual situation.For thawing or freezing state of seasonally thawed layer,the stability analysis model and method of tower spread foundation under uplift conditions were proposed respectively,and uplift angle of frozen backfilled soil was calculated and was 20.5°.The results show that shear strength of ground and bearing performance of foundation are larger in the freezing state than in the thawing state of seasonally thawed layer,and the bearing capacities of foundations can meet the design requirements.Void ratio of fine-grained soil is reduced,density is improved,water content and degree of saturation are increased while frozen soil thawing.The freezing or thawing state of soil has great influence on mechanical parameters of ground including the shear strength,uplift angle,etc.So keeping the freezing state of deep backfilled ground is very important to bearing safety of the engineering foundations.