塑料排水板加固软土地基失效案例分析
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.