锚杆静压桩技术在既有建筑物增设地下空间中的应用

运用锚杆静压桩技术在既有建筑物中增设地下空间是可行的，一方面可利用既有建筑物的自重荷载作为压桩的反力，另一方面将压好的桩作为临时托换构件支撑的既有建筑物，建筑物下方的土体才可以开挖。对于黏性土，采用逐根压桩的方案，把所有桩压至设计标高后，待超孔隙水压力消散，才开挖承台下的土体。对于砂性土，可以边开挖土方边压桩，通过控制压桩和挖土速度，很好地控制建筑物的沉降。桩身四周的土体开挖后，其稳定性按照自由长度较大的高承台桩的模型进行分析。锚杆静压桩的压桩阻力是根据土层动力触探的指标确定的，锚杆静压桩的数量根据使用阶段、施工阶段的荷载以及桩身自由段的屈曲稳定性综合确定。按照新旧混凝土界面初始滑移承载力，进行承台的抗冲剪设计是有足够的安全储备的，此时冲剪承载力由几个斜向混凝土柱承担，而承台的下部受拉。锚杆静压桩之间的沉降差会在上部结构中产生附加内力，影响到结构安全。沉降差的计算采用分层总和法，计算时应考虑桩身的重力荷载、桩顶集中荷载等产生的附加应力以及土体开挖附加应力的减少。

Construction of basement in existing buildings by static bolt-pile

Based on the following two reasons, applying static bolt-pile for construction of basement in existing buildings is practicable. (1) The gravity of the existing building can be used as reaction-force to drive piles; (2) As the temporary underpinning components, the driven piles can be used to support the existing building, thus the soils under the building can be excavated. In clay, when all piles are driven to the design elevation and the excess pore pressure is dissipated, the soil under pile caps can be excavated. In sandy soil, settlements of the existing building can be controlled by controlling the speed of driving piles and excavating soil, thus driving piles and excavating soil can be constructed at the same time. After the soils around the pile is excavated, stability analysis of the pile is done according to a high–caps pile model with a major unsupported length. Resistance in driving pile process is determined by using the dynamic penetration results of soil. The number of the static bolt-piles is calculated synthetically by the loads of service and construction processes and the results of pile stability analysis. According to punching shear bearing capacity before interface slip of new-to-old concrete, the pile cap design has sufficient safety. Punching shear strength is held by some slanting concrete columns; while the tensile force is created in bottom of the pile cap. The differential settlements of piles cause structure additional internal forces which influence safety of the existing building. Calculating the differential settlements by layer-wise summation method should consider factors as following: additional stress increase induced by gravity of piles and concentrated loads on the top of piles, as well as additional stress decrease because of excavating soils.