Theory and application of free fall penetration testing technique
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摘要: 离岸工程的不断发展对准确获取水下岩土工程参数提出了更高的要求，自落式动力触探技术（FFP）可较准确地获得水下浅层土体的原位特性参数和土体扰动后的参数，有效缩短勘察时间、降低勘察成本。以自落式动力触探测试技术为研究对象，自主设计了自落式动力触探仪，以牛顿运动第二定律为基础，建立了FFP贯入过程中的力学模型与理论体系，建立了测试数据的处理流程，利用室内模型试验，开展了自落式动力触探FFP相关贯入机理研究，以苏锡常南部高速公路太湖隧道项目的典型湖相淤泥质软土为研究对象，开展了自落式动力触探FFP现场试验研究，建立了基于FFP测试技术的软土不排水抗剪强度评价方法。结果表明：研发的FFP触探仪在测试过程中具有良好的准确性和可靠性，释放高度、触探仪质量、端头形状均会影响贯入深度；FFP测试技术可以在评价浅层超软土时作为CPTU试验或全流触探试验的有效补充。Abstract: The development of marine engineering highly requires the accurate acquisition of parameters for marine geotechnical engineering. The free fall penetration (FFP) testing technique can accurately obtain the in-situ characteristic parameters of shallow seabed soils and the parameters after soil disturbance, which can effectively shorten the survey time and reduce the survey costs. In this study, the FFP equipment is firstly designed. Then, based on the Newton's second law, the mechanical model and theoretical system for penetration process of FFP are established, and the processing process of test data is established. The penetration mechanism of FFP is also studied using the laboratory model tests. Finally, focusing on the typical lacustrine mucky soft soil in the Taihu Lake Tunnel Project of Suzhou-Wuxi-Changzhou Southern Expressway, the evaluation method for undrained shear strength of soft soils is established based on FFP. The results show that the developed FFP equipment has good accuracy and reliability. The release height, the quality of the penetration probe and the shape of the tip can all affect the penetration depth. The FFP testing technique can be used as an effective supplement to the CPTU tests or the full flow penetration tests in the evaluation of shallow super soft soils.
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Keywords:
- free fall penetration /
- in-situ testing /
- soft clay /
- undrained shear strength
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图 4 不同释放高度下的贯入速度随深度变化曲线
Figure 4. Curves of penetration velocity changing with depth at different release heights
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图 5 不同质量下的贯入速度随深度变化曲线
Figure 5. Curves of penetration velocity changing with depth under different masses
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图 6 不同端头下的贯入速度随深度变化曲线
Figure 6. Curves of penetration velocity changing with depth at different tips
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图 7 FFP试验获得的土体不排水抗剪强度
Figure 7. Undrained shear strengths of soils obtained from FFP tests
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图 8 基于3种现场试验的土体不排水抗剪强度
Figure 8. Undrained shear strengths of soils based on three field tests
表 1 室内模型试验方案
Table 1 Laboratory model test schemes
土样箱号 固结时间/d 端头类型 质量/kg 释放高度/cm 试验组数 1 1 A, B, C 6.70, 6.85 20, 40, 60 9 2, 3 2 A 6.70 20, 40, 60, 100 14 8.20 40, 70, 100 B 8.20 40, 60, 70, 100 C 8.35 40, 70, 100 3 10 A 6.70 40, 70, 80, 100 10 8.20 80, 100 9.70 80, 100 B 9.70 100 C 9.85 100 1 30 A 8.20 40, 70, 100 9 B 8.20 40, 70, 100 C 8.35 40, 70, 100 总计 42 注：端头类型A-锥形、B-半球形、C-圆柱形。 
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表 2 FFP室内模型与现场试验的基本参数取值表
Table 2 Values of basic parameter of FFP tests
基本数 质量m/kg 重力加速度g/(m·s-2) 土体饱和密度ρs/(g·cm-3) 探头直径d/mm 参考速度vref/(m·s-1) 端头截面积At/cm2 承载力系数 界面摩擦比α 曳力系数CD 应变率参数β 锥/半球形 圆柱形 Nc-kt Nc-b Nc-t 锥形 半球/圆柱形 室内模型试验 6.70~9.85 9.81 1.80 35.7 0.02 10 12.75 15 12.5 21 0.41 0.22 0.35 0.06 现场试验 8.20, 8.35 9.81 1.70 35.7 0.02 10 12.75 15 12.5 21 0.29 0.22 0.35 0.06
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