Model tests on penetration and extration of modified suction caissons in clay
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摘要: 开展模型试验研究海洋黏性土中裙式吸力基础沉贯和注水上拔特性,并研究了土体强度、基础尺寸和安装方式等影响因素。研究表明：主桶长径比为1.0和2.0的裙式吸力基础最终沉贯深度较相同高度传统吸力基础仅降低2%和6%,证实了裙式吸力基础在黏性土中具有良好沉贯性。主桶和裙结构在吸力沉贯时对土体造成扰动,导致吸力沉贯阻力小于压力贯入时的阻力。基于极限平衡方法,提出了裙式吸力基础在黏性土中的沉贯阻力与所需吸力的计算公式,并验证其准确性。得到传统和裙式吸力基础在注水拔出过程中基础内部水压力、上拔阻力与基础上拔位移之间的关系,发现基础内部水压力随上拔位移先迅速增加至最大值,然后逐渐降低,裙式吸力基础最终上拔位移小于相同基础高度的传统吸力基础。得到了裙式吸力基础注水拔出阻力计算公式。Abstract: A series of model tests are conducted to investigate the installation and extraction behavior of the modified suction casson(MSC) embedded in marine clay by taking the MSC dimensions, shear strength of clay and installation method into account. It is found that the discrepancies between the final penetration depths of the MSC with the aspect ratio of 1.0 and 2.0 and the corresponding regular suction caissons (RSCs) are 2% and 6%, indicating that the MSC can penetrates into the clay to a desired depth. During suction-assisted installation, the internal compartment and the external skirt are found to disturb the soils around the caisson, leading to the decrease of the penetration resistance compared with the penetration resistance during jacking installation. Based on the limit equilibrium method, the expressions for estimating the required suction to penetrate the MSC into clay and the penetration resistance were proposed. In addition, the variations of the water pressure in the suction caisson and uplift resistance during suction caisson extraction are also obtained. The results show that the water pressure in suction caisson firstly increases sharply to the maximum value with the extraction displacement, and then decreases to a certain value. It is also found that the MSCs and RSCs can not be fully extracted by injecting water into the caisson. The expression obtaining the extraction resistance of the MSC is proposed to guide the foundation design.
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图 4 传统吸力基础沉贯阻力、吸力变化
Figure 4. Relationships among installation resistance, suction and penetration depth for RSCs
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图 5 裙式吸力基础沉贯阻力、所需吸力变化
Figure 5. Relationships among installation resistance, suction and penetration depth for MSCs
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图 6 吸力基础拔出位移随时间变化
Figure 6. Relationships between extraction displacement and elapased time
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图 8 注水拔出试验水压-上拔位移曲线
Figure 8. Water pressure-displacement curves during extraction by water injection
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图 9 注水拔出试验上拔阻力-上拔位移曲线
Figure 9. Penetration resistance-displacement curves during extraction by water injection
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图 10 注水上拔试验上拔位移-桶内压力曲线
Figure 10. Penetration resistance-displacement curves during extraction by water injection
表 1 试验土体参数
Table 1 Parameters of clay
含水率/% 渗透系数/(m·s-1) 饱和重度/(kN·m-3) 塑性指数 液性指数 39 5.46×10-9 19.1 18.4 0.6 
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表 2 试验模型尺寸
Table 2 Dimensions of test caisson models
试验工况 安装方式 土体强度Su 重量G/N di/mm l/mm Di/mm 主桶长泾比(l/Di) L/mm t/mm RSC1 吸力沉贯 S u=12+0.8z 17.3 120 120 — 1.0 — 2 MSC1 28.8 120 120 180 60 RSC2 吸力沉贯 S u=3.3+0.1z 23.1 120 240 — 2.0 — 2 MSC2 34.8 120 240 180 90 RSC3 压力贯入 S u=3.3+0.1z 23.1 120 240 — 2.0 — 2 MSC3 34.8 120 240 180 90 说明：z为土体中计算点的深度。
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表 3 各工况吸力沉贯试验结果
Table 3 Test results of installation for various test cases
试验工况 沉贯时间/s 沉贯量/mm 最大吸力/kPa 沉贯阻力/kN RSC1 138 95.8 (0.8l) 33.1 0.38 MSC1 236 96.9 (0.81l) 40.0 0.45 RSC2 113 201.4 (0.84l) 23.0 0.28 MSC2 124 190.0 (0.8l) 25.5 0.33
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表 4 注水上拔试验数据
Table 4 Test results for suction caissons during extraction
试验工况 上拔时间/s 上拔位移/mm 桶内最大水压力 最大上拔阻力 值/kPa 时间/s 值/kPa 时间/s RSC1 63 21.8（18%） 19.5 23 0.22 23 MSC1 60 20.7（17%） 22.7 5 0.25 6 RSC2 67 65.4（27%） 8.4 5 0.10 6 MSC2 127 43.8（18%） 8.1 31 0.11 31
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