An unfunctional case of vertical drains-reinforced soft foundation
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摘要: 台湾高速公路匝道路堤（10 m高）工程,由于基础为软弱土层（N63.5<1）,特选择塑料排水板加固工法。该路堤底层共使用3层土工布为分隔层。由于导管无法穿过第一层上方厚1.5 m的碎石层,因此务必先行引孔（直径200 mm）始能插入排水板,本案共打设900余孔。监测系统包括测斜管、分层沉降仪、水位计、孔压计等。路堤填筑过程中侧向位移（垂直于填筑路堤）持续扩大（于填筑至7.0 m时已超过200 mm）,相关监测数据分析比对以及现地坡趾侧向排水沟陆续产生裂缝的现象,推论因排水板未能有效发挥功能,填筑路堤造成软弱基础土层向外（临农路侧）推挤。因此,研究的阶段性结论要求立刻停止施工,并对现况的稳定性进行分析时亦应纳入土工布的短期加筋效果并提供有效应急处置对策,以顺利完成后续填筑工作。Abstract: The vertical drains (VD) of ground reinforcing method is used for the very soft foundation of highway new ramp projects in Taiwan. The total lift of embankment is up to 10 m, and three layers of woven fabrics are used in the base of embankment. It is found that the mandrel can not insert through the 1.5 m-gravel layer above the first layer of woven fabric. Therefore, pre-bored holes are used to install VD, and a total of 900 pre-bored holes (20 cm in diameter) are made. These pre-bored holes may cause the loss of confining effect, and the excess pore pressure is found to be not built up with each lift of embankment. Based on the series of monitoring data and the cracks found in the side drainage trench adjacent to the dike toe, it is considered that probably the installed VD behaves with poor performance. Consequently, it may result in the lateral squeezing of the mud waves and pushing outward. Through the rigorous analysis at the stage of 7 m-lift constructed, the conclusion is strongly called to stop the lift works. The further analysis is strongly requested for the short-term reinforcement function provided from woven grotextile at this stage of embankment. The emergency action needs to be engaged for on-going 3.0 m-embankment, plus 2.0 m-preloading surcharge.
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Keywords:
- ground improvement /
- reinforcement /
- consolidation /
- soft soil /
- vertical drain /
- woven geotextile
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图 6 施工期沉降测读与填土高度
Figure 6. Measured settlements and embankment heights during construction period
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图 7 测斜管I4在桩号4+360 m处测得的地基土体水平位移
Figure 7. Measured horizontal displacements from inclinometer “I4” at section of 4 + 360 m
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图 8 分层沉降仪D3在4+360 m处各磁环埋置高程（深度）
Figure 8. Elevations (depth) of each cell along continuous settlement rod D3 at section of 4 + 360 m
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图 9 分层沉降仪(D3)在4+360 m处各深度测值与路堤填筑高度的关系
Figure 9. Relationship between reading（D3）of each settlement cell and embankment height at section of 4+360 m
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图 10 孔隙水压力计(W1)在桩号4+360 m处测值与路堤填筑高度的关系
Figure 10. Relationship between reading (W1) of pore pressure and embankment height at section of 4+360 m
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图 14 裂缝扩张造成的玻璃片张力破坏现象
Figure 14. Observed tension failure of the slide glass cause by crack Opening
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图 15 泥波影响测斜管示意图
Figure 15. Schematic diagram of inclinometer displacement affected by mud waves
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图 16 泥波影响分层沉降仪示意图
Figure 16. Schematic diagram of movement of continuous settlement cells affected by mud waves
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图 17 综合泥波影响测斜管与分层沉降仪示意图
Figure 17. Schematic diagram of movement of inclinometer and continuous settlement cells with integrated mud wave effect
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图 18 路堤塌陷侧向（临农路侧）挤出破坏
Figure 18. Mud waves caused by lateral squeezing effect of embankment movement (at side drainage ditch)
表 1 各土层物理力学性质
Table 1 Physical and mechanical properties of foundation soils
土层深度/m 干密度ρd/(g·cm-3) 黏聚力c/kPa 内摩擦角φ/(°) 含水率w/% 孔隙比e GL.0.0~-3.0 1.503 0.15 0 180~75 2.2~1.2 GL.-3.0~-4.5 1.522 0.23 0 75~45 2.2~1.2 GL.-4.5~-6.0 1.779 0.23 0 45~30 2.2~1.2 GL.-6.0~-20.0 2.300 0.10 0 <30 1.3~0.7 
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表 2 土工布规格
Table 2 Specifications of woven geotextile
项目 材料性质 试验规范 材料种类 聚丙烯 — 极限张力强度/(kN·m-1) >70 ASTM D 4595 延伸率/% <17 ASTM D 4595 透水系数/(cm·s-1) >1×10-2 ASTM D 4491
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表 3 排水板规格
Table 3 Specification of VD
项目 材料性质 试验规范 材料厚度/mm 5 ASTM D 5199(P=2 kPa) 整体抗拉降服强度/N >200 ASTM D 4595 整体抗拉伸长率/% 达降服强度前伸长率>20 ASTM D 4595 纵向排水量/(cm3·s-1) >20 ASTM D 4716（i=1） 外围滤层透水系数/(cm·s-1) >1×10-2 ASTM D 4491 外围滤层抗拉降服强度/N >60 ASTM D 4595 外围滤层抗拉伸长率/% 40~100 ASTM D 4595 外围滤层撕裂强度 25(kg·f) ASTM D 4533 注： 1 (kg·f)=9.8 N。
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表 4 安全监测系统配置
Table 4 Configuration of monitoring system
编号 监测仪器 监测频次 S1~S3 沉降板 每周一次 I1~I5 测斜管 每周两次 W1~W4 孔隙水压力计 每周两次 D1~D4 分层沉降仪 每周两次 l1~l4/L1~L6 裂缝观测玻璃片 两周一次
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表 5 玻璃片编号与设置点位
Table 5 Marked No. and location of slide glass
玻片编号 设置点位 L1 4+287 m L2 4+288 m L3 4+303 m L4 4+310 m L5 4+346 m L6 4+350 m l1 4+289 m l2 4+291 m l3 4+292 m l4 4+293 m
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[1] FHWA . Prefabricated Vertical Drains, “Vol. I: Engineering Guidelines”[M]. Washington D C. 1986.
[2] JAMIOLKOWSKI M B, LANCELLOTTA R, PEDRONI R, et al. Prefabricated Vertical Drains: Design and Performance[C]//CIRIA Ground Engineering Report: Ground Impro Vement, 1991, London.
[3] CHANG DAVE T, LIAO T J C, LAI S P. Laboratory study of vertical drains for a ground improvement project in Taipei[C]//Fifth International Conference on Geotextiles, Geomenbranes and Related Products, 1994, Singapore.
[4] DAS Braja M. Advanced Soil Mechanic Fifth Edition[M]. Boca Raton: CRC Press, 2019.
[5] FHWA . Geosynthetic Design &Construction Guidelines Reference Manual[M]. Mclean: Turner-Fairbank Highway Research, 2009.
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