Characteristics of mud cake in bored cast-in-place piles within granite weathered strata and application of pile side post-grouting
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摘要: 花岗岩风化层具有含砂量高、黏粒胶结紧密、遇水易软化崩解等特性，灌注桩多采用泥浆护壁施工，该工艺下桩周会形成泥皮。通过收集两个花岗岩风化层灌注桩不满足承载力要求的工程实例，结合测试数据及现场开挖验证，总结了花岗岩风化层中灌注桩桩侧泥皮的特性。研究表明花岗岩风化层中灌注桩桩侧泥皮厚度较大，性质接近软塑黏土状，桩侧阻力表现为桩身混凝土与泥皮之间的阻力，对单桩承载力影响很大。工程实践表明，采用桩侧后注浆处理泥皮，可以全部消除泥皮对桩侧阻力的不利影响，但对桩周土体的加固作用不明显，处理后桩侧阻力可按花岗岩风化层本身性质取值。Abstract: Granite weathered strata are characterized by high sand content, tight clay cementation, and easy softening and disintegration upon contact with water. Bored cast-in-place piles are generally constructed with slurry-supported walls, under which a mud cake will form around the pile shaft. By collecting two engineering cases where bored cast-in-place piles failed to meet bearing capacity requirements, combined with test data and field excavation verification, the characteristics of the mud cake of bored cast-in-place piles in granite weathered strata were summarized. The study shows that the shaft mud cake of bored cast-in-place piles in granite weathered strata has a relativety large thickness and properties similar to soft plastic clay. The shaft resistance is manifested as the resistance between the pile concrete and the mud cake,which exerts a significant impact on the bearing capacity of single piles. Engineering practice demonstrates that pile side post-grouting treatment effectively eliminates the adverse effects of mud cake on side resistance. However, this treatment shows limited reinforcement effect on surrounding soils. After treatment, the pile side resistance can be determined based on the inherent properties of the granite weathered strara itself.
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图 4 桩侧后注浆加固泥皮示意图
Figure 4. Schematic diagram of mud cake reinforcement by pile side post-grouting
图 5 桩侧后注浆桩结构大样图（单位：mm）
Figure 5. Detail drawing of structure with post-grouting along pile shaft (Unit: mm)
图 6 工程桩静载试验Q–s曲线
Figure 6. Q–s curve of static pile load test on the engineering piles
表 1 梧州某项目1工程实例桩岩土层情况
Table 1. Stratigraphic conditions of example piles for Project 1 in Wuzhou
岩土层名称 桩侧土层厚度/m 承载力
特征值
fak /kPa极限侧阻力
标准值
qsk /kPa极限端阻力
标准值
qpk /kN桩编号 SY1 SY2 素填土 2.0 8.0 80 13 粉质黏土 4.90 3.2 200 55 花岗岩残积土 12.1 7.9 250 75 全风化花岗岩 9.0 8.1 310 100 1500 强风化花岗岩 460 130 2000 
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表 2 梧州某项目2工程实例桩岩土层情况
Table 2. Stratigraphic conditions of example piles for Project 2 in Wuzhou
岩土层名称 桩侧土层厚度/m 承载力
特征值
fak /kPa极限侧阻力
标准值
qsk /kPa极限端阻力
标准值
qpk /kN桩编号 SY3 SY4 SY5 素填土 0 3.3 3.3 70 10 花岗岩残积土 13.1 11.1 11.1 240 70 全风化花岗岩 6.3 10.1 10.5 330 85 1700 强风化花岗岩 8.1 450 120 2000
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表 3 收集工程实例桩情况汇总表
Table 3. Summary of collected engineering case piles
项目名称 桩号 桩径/mm 桩长/m 桩端持力层 花岗岩风化层
总厚度/m计算风化层
平均侧阻力
qsk /kPa计算极限
承载力
Quk /kN实际极限
承载力
Quk1 /kN反推风化层
平均侧阻力
qsk1 /kPaQuk1 /Quk qsk1 /qsk 梧州某项目1 SY1 800 28.00 全风化花岗岩 21.10 85.7 5985 2998 29.3 0.50 0.34 SY2 800 27.20 全风化花岗岩 16.00 87.6 4929 2500 27.2 0.51 0.31 梧州某项目2 SY3 800 19.40 全风化花岗岩 19.40 74.9 4289 2460 33.0 0.55 0.44 SY4 800 24.50 全风化花岗岩 21.20 77.1 5045 2700 33.1 0.53 0.43 SY5 800 31.00 强风化花岗岩 27.70 95.3 7723 3780 38.0 0.49 0.40
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表 4 场地岩土层主要物理力学指标
Table 4. Main physical and mechanical indices of site stratigraphy
地层编号 岩土层名称 层厚/m 承载力
特征值fak/kPa岩石饱和抗压
强度Rc/MPa黏聚力
c/kPa内摩擦角
φ/(°)钻孔灌注桩极限
侧阻力标准值
qsk /kPa钻孔灌注桩极限
端阻力标准值
qpk /kPa① 素填土 0～6.20 ② 花岗岩残积土 5.50～13.70 240 26.1 25.5 75 1500 ③1 全风化花岗岩 5.90～13.80 320 32.0 26.0 100 1800 ③2 强风化花岗岩 9.20～29.00 450 39.0 27.0 120 2200 ③3 中等风化花岗岩 0～29.90 1300 7.0 400 4200 ③4 微风化花岗岩 22500 89.0
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表 5 检测工程桩情况汇总
Table 5. Summary of inspection for tested engineering piles
桩号 桩径
/mm有效
桩长/m桩端
持力层花岗岩
风化层
总厚度/m按建议参数
计算平均
侧阻力qsk /kPa按建议参数
计算极限
承载力Quk /kN按灌注桩计算
极限承载力
Quk1 /kN试验最大
加载值
Quk2 /kN沉降量
/mm注浆后
平均侧阻力
qsk2 /kPaQuk2 /Quk1 qsk2 /qsk qsk2 /qsk1 37# 1000 28.40 微风化孤石 28.40 99.8 10137 4370 11000 14.50 109.9 2.52 1.10 3.42 43# 1000 32.40 微风化孤石 32.40 107.2 12059 4756 11000 15.44 96.3 2.31 0.90 3.00 60# 1000 28.30 微风化花岗岩 28.30 102.3 11000 10.70 15# 1000 30.70 强风化花岗岩 30.70 101.5 10989 4591 11000 43.26 101.6 2.40 1.00 3.16 18# 1000 30.30 强风化花岗岩 30.30 100.0 10731 4553 11000 40.17 103.0 2.42 1.03 3.21 24# 1000 33.50 强风化花岗岩 33.50 103.5 12048 4862 11000 29.20 93.1 2.26 0.90 2.90 63# 1000 30.40 强风化花岗岩 30.40 103.7 11098 4563 11000 25.60 102.7 2.41 0.99 3.20 40# 1200 28.40 强风化花岗岩 28.40 97.9 11910 5415 12000 26.88 98.8 2.22 1.01 3.08
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