Extension Evaluation on Excavation Safety of Deep Foundation Pit in Sandy Cobble Stratum Based on Entropy Method
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摘要: 为客观评价富水砂卵石地层中深基坑开挖的安全稳定性,根据结构变形、受力、地下水以及周边环境等因素选取地面沉降、建筑物沉降、地下水位等8个评价指标进行开挖安全性评价,以成都某地铁车站深基坑工程为例,根据基坑开挖4个月内的实测数据,采用熵值法对所选评价指标进行赋权,基于物元理论与可拓集合的关联函数,建立深基坑开挖安全可拓评价模型,并将评价结果与模糊综合评判结果进行比较. 研究结果表明:熵值赋权计算中,混凝土支撑轴力、桩顶沉降和支护结构水平位移是本案例中最重要的三项指标,对安全评价影响最大;对地面沉降与管线沉降的可拓评价结果比模糊综合评判结果高一个等级,与实际监测数据的评判结果相符,有利于施工过程中对潜在风险的防控;基于熵值赋权的可拓评价模型能够对基坑开挖安全性进行单因素分析与综合分析,所构建的可拓评价模型可以在成都地区富水砂卵石地层基坑工程安全评价中推广使用.Abstract: In order to objectively evaluate the safety and stability of deep foundation pit excavation in watery sandy pebble stratum, eight evaluation indexes, such as ground settlement, building settlement and groundwater level, are selected to evaluate the excavation safety according to the structural deformation, stress, groundwater and surrounding environment. A deep foundation pit project of a metro station in Chengdu is used as an example, and according to the measured data within the four months of foundation pit excavation, the entropy method is employed to assign the selected evaluation indexes. Based on the correlation function between the matterelement theory and extension set, the extension evaluation model of excavation safety for the deep foundation pitis established. Comparing the evaluation results with those of fuzzy comprehensive evaluation, it is concludedthat the axial force of concrete support, pile top settlement and the horizontal displacement of support structure are the most important three indexes in the entropy weight calculation of this case, which have the greatest impact on the safety evaluation. For the safety evaluation of ground settlement and pipeline settlement, the extension evaluation result is one grade higher than the fuzzy comprehensive evaluation result, which is consistent with actual monitoring data and is conducive to the prevention and control of potential risks in the construction process. The extension evaluation model based on entropy weight can accurately improve the safety of the foundation pit excavation through single factor analysis and comprehensive analysis, and the extension evaluation model can be applied in the safety evaluation of foundation pits in watery sandy cobble strata in Chengdu.
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表 1 凤溪站深基坑监测数据
Table 1. Monitoring data of deep foundation pit at Fengxi station
序号 开挖深度/m c1/mm c2/mm c3/m c4/mm c5/mm c6/kN c7/mm c8/mm 1 1 −2.69 −2.62 13.00 3.9 −1.03 29.84 −4.12 −0.62 2 6 −4.02 −2.31 15.00 3.0 −0.81 48.06 −7.24 −6.98 3 9 −6.65 −2.43 19.40 3.8 −1.79 152.90 −10.33 −7.62 4 14 −8.27 −3.17 21.00 5.8 −2.88 181.39 −13.10 −10.34 5 18 −6.81 −6.39 23.10 6.9 −3.68 646.69 −12.28 −12.10 6 22 −8.31 −8.04 25.40 7.6 −4.73 200.69 −12.46 −12.58 7 26 −10.51 −5.40 27.78 8.9 −5.75 212.55 −14.08 −12.94 8 30 −13.15 −7.29 32.00 8.4 −5.70 222.25 −13.65 −14.50 9 32 −12.37 −6.91 33.55 4.3 −6.24 264.09 −12.84 −14.61 表 2 熵值赋权计算结果
Table 2. Results of the entropy weighting method
项目 ej gj wj c1 0.958 873 0.041 127 0.100 230 c2 0.953 535 0.046 465 0.113 240 c3 0.980 939 0.019 061 0.046 453 c4 0.970 981 0.029 019 0.070 722 c5 0.923 029 0.076 971 0.187 586 c6 0.879 137 0.120 863 0.294 555 c7 0.978 304 0.021 696 0.052 875 c8 0.944 878 0.055 122 0.134 338 表 3 凤溪站深基坑开挖安全性判别
Table 3. Excavating safety evaluation for deep foundation pit at Fengxi station
监测项目 安全性判别标准 判别内容 Ⅰ级 Ⅱ级 Ⅲ级 结构变形 ${F_1} = \dfrac{ { {\text{实测变形值} } } }{ { {\text{基坑开挖深度} } } }$ F1 > 0.012
F1 > 0.0070.004 ≤ F1 ≤ 0.012
0.002 ≤ F1 ≤ 0.007F1 < 0.004
F1 < 0.002支撑轴力 ${F_2} = \dfrac{ { {\text{容许轴力} } } }{ { {\text{实测轴力} } } }$ F2 < 0.8 0.8 ≤ F2 ≤ 1.0 F2 > 1.0 基底隆起 ${F_3} = \dfrac{ { {\text{实测变形值} } } }{ { {\text{基坑开挖深度} } } }$ F3 > 1.0 × 10−2
F3 > 0.5 × 10−2
F3 > 0.2 × 10−20.4 × 10−2 ≤ F3 ≤ 1.0 × 10−2
0.2 × 10−2 ≤ F3 ≤ 0.5 × 10−2
0.4 × 10−3 ≤ F3 ≤ 0.2 × 10−2F3 < 0.4 × 10−2
F3 < 0.2 × 10−2
F3 < 0.4 × 10−3沉降值 ${F_4} = \dfrac{ { {\text{实测沉降值} } } }{ { {\text{基坑开挖深度} } } }$ F4 > 1.2 × 10−2
F4 > 0.7 × 10−2
F4 > 0.2 × 10−20.4 × 10−2 ≤ F4 ≤ 1.2 × 10−2
0.2 × 10−2 ≤ F4 ≤ 0.7 × 10−2
0.4 × 10−3 ≤ F4 ≤ 0.2 × 10−2F4 < 0.4 × 10−2
F4 < 0.2 × 10−2
F4 < 0.4 × 10−3地下水位 ${F_5} = \dfrac{ { {\text{降水设计值} } } }{ { {\text{实测水位} } } }$ F5 < 0.8 0.8 ≤ F5 ≤ 1.2 F5 > 1.2 注:1. F1上行适用于基坑附近无建筑物或地下管线的情况,下行适用于基坑附近有建筑物或地下管线的情况;
2. F3、F4的上、中行与F1同,下行适用于对变形有特别严格要求的情况.表 4 各监测指标关联函数值
Table 4. Correlation function values of monitoring indexes
Ⅰ级 Ⅱ级 Ⅲ级 K11 = −0.7945 K21 = 0.0137 K31 = −0.0259 K12 = −0.8744 K22 = −0.3719 K32 = 0.7438 K13 = −0.1740 K23 = 0.9329 K33 = −0.1555 K14 = −0.9603 K24 = −0.8609 K34 = 0.2781 K15 = −0.9025 K25 = −0.5125 K35 = 0.9750 K16 = −0.3958 K26 = −0.3626 K36 = 0.6598 K17 = −0.7800 K27 = 0.0500 K37 = −0.0833 K18 = −0.9348 K28 = −0.7717 K38 = 0.4566 表 5 各特征指标安全等级
Table 5. Safety level of characteristic indexes
评价方法 c1 c2 c3 c4 c5 c6 c7 c8 基于熵值法的
可拓评价Ⅱ级 Ⅲ级 Ⅱ级 Ⅲ级 Ⅲ级 Ⅲ级 Ⅱ级 Ⅲ级 模糊综合
评判方法Ⅲ级 Ⅲ级 Ⅱ级 Ⅲ级 Ⅲ级 Ⅲ级 Ⅲ级 Ⅲ级 表 6 监测指标预警值与监测值
Table 6. Warning values and monitoring values of monitoring indexes
项目 预警要求 最大监测值 预警值/
mm变化速率/
(mm•d−1)监测值/
mm变化速率/
(mm•d−1)c1 22.40 ≥ 3.00 13.15 2.55 c2 16.00 ≥ 3.00 8.04 1.67 c3 1600 ≥ 500 1850 1052 c4 22.40 ≥ 3.00 8.90 1.60 c5 22.40 ≥ 3.00 6.24 1.35 c7 20.00 ≥ 4.00 14.08 2.43 c8 22.40 ≥ 3.00 14.61 1.39 注:变化速率通常取工况前3 d内的最大值. -
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