THE ACTIVE EARTH PRESSURE CALCULATION FOR RETAINING STRUCTURE OF DEEP FOUNDATION PIT ADJACENT TO RIVER BASED ON UPPER-BOUND ANALYSIS
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摘要: 沿江临河修建的深基坑中，临水岸坡与基坑围护结构构成了有限宽度土体，其土压力计算方法与常规模式有较大不同。基于极上限分析法，运用改进多线段滑裂面生成技术，推导非均质临水深基坑围护结构主动土压力的上限解；并在均质土条件下，与传统直线或对数螺线滑裂面假定的计算结果进行对比验证。将上述计算方法运用于福州地铁5号线农林大学站基坑工程实例，认为围护结构主动土压力随水位的上升近似呈线性增大，随坡坑宽高比增大呈先快速增长后趋于稳定的趋势；当坡坑宽高比大于0.7时，主动土压力开始趋于稳定，说明滑裂面由模式Ⅰ(相交于坡面)过渡到模式Ⅱ(相交于坡顶)。进一步地，基于支撑刚度折减的数值模拟结果表明，临江侧土体滑裂面形态、围护结构主动土压力及合力作用点位置与理论计算结果较为吻合。上述研究成果可为临水深基坑围护结构的科学设计提供参考借鉴。Abstract: In the deep foundation pit adjacent to river, the ground between bank slope and retaining structure is limited, as a result, the conventional models can not be used for the calculation of earth pressure. Based on the modified generation technology for multiple-segment slip surface, the active earth pressure of waterfront foundation pit in non-homogeneous ground was deduced through upper-bound analysis. On the homogeneous ground condition, the proposed method were validated throught a comparison with their counterparts of conventional linear or logarithmic fracture models. The proposed method was applied to an engineering instance (i.e. deep foundation pit of Agriculture and Forestry University Station in Fuzhou Metro Line 5), it is concluded that the active earth pressure of retaining structure increases approximately linearly with the rising of water level. On the other hand, the active earth pressure increases rapidly and then stabilize with the increase of width-height ratio of foundation pit. When the width-height ratio exceeds 0.7, the active earth pressure tends to stabilize, which implies that the slip surface transferred from mode Ⅰ (intersecting with slope) to mode ⅠI (intersecting with top). Furthermore, the numerical simulation results with bracing stiffness reduction show that the shape of sliding surface, the active earth pressure of retaining structure and the action point of resultant force are consistent with their counterparts of upper-bound analysis. These researches could provide some reference for the scientific design of retaining structures in the deep foundation pit adjacent to river.
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图 1 非均质地层有限宽度土体中的多线段滑裂面
Figure 1. The multiple-segment slip surface of finite width soil in non-homogeneous ground
图 2 滑裂面生成示意图(以OCP1为例)
Figure 2. The schematic diagram for the generation of slip surface
图 3 临水坡体分块示意图
Figure 3. The schematic diagram for the block division of waterfront slope
图 4 超载、水压力及主动土压力功率计算示意图
Figure 4. The schematic diagram for the power calculation of overload, water pressure and active earth pressure
图 5 农林大学站基坑计算简图
Figure 5. The calculation diagram for the foundation pit of Agriculture and Forestry University Station
图 6 主动土压力Ea随水位高差h0的变化
Figure 6. The variation of active earth pressure Ea with different water level h0
图 7 主动土压力Ea随坡坑宽高比L/H的变化
Figure 7. The variation of active earth pressure Ea with different width-depth ratio L/H
图 8 支撑刚度折减后的塑性剪应变云图
Figure 8. The contour of plastic shear strain after reduction of brace stiffness
图 9 基坑围护结构土压力分布(数值模拟)
Figure 9. The earth pressure distribution along the retaining structure of foundation pit (from numerical simulation)
表 1 主动土压力计算结果对比
Table 1. The comparison of active earth pressure calculated by deifferent methods
抗剪强度参数 主动土压力/kN 黏聚力 c/kPa 摩擦角 φ/(°) 直线 对数螺旋 本文 6 24 247.17 248.32 243.77 10 20 253.13 252.00 247.58 10 24 212.08 211.05 208.25 10 28 176.32 175.74 174.01 14 24 177.15 174.39 172.87 
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表 2 地层物理力学参数表
Table 2. The physical and mechanical parameters of ground
土层名称 厚度
/m重度
/(kN/m3)黏聚力
/kPa摩擦角
/(°)粉质黏土 4.1 19.9/18.5 23.6/27.3 14.5/14.0 淤泥质土 6.5 17.1/15.9 14.2/16.1 9.5/11.9 残积砂质黏性土 12.5 17.9/17.1 23.9/32.1 19.9/26.3 全风化花岗岩 12.1 21.05 24.0 24.0
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