An unfunctional case of vertical drains-reinforced soft foundation

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.