Base grouting against uplifting water for a deep excavation in Taipei basin

An deep excavation up to 33 m in depth and retained by 53 m deep diaphragm wall was carried out in the silty sand and gravel layers of Taipei basin with high groundwater level. A base grouting work was conducted to cut off the uplifting groundwater coming from gravel layers. Tube-a-manchette (TAM) grouting method was adopted in this project. Based on the recorded pressures response and the spatial coordinates of each grout port on the Manchette tube (M-tube), 2D grouting pressure response contours at given depths can be established using commercial software and then used to pin-point the areas of low grouting pressure. If necessary, TAM grouting might be repeated in the low pressure spots/areas. On-site permeability tests were run on the base grouting zone. The measured coefficients of permeability were all below the required value of 5 x10(-5) cm/sec. In addition, the in-situ transmissivity (T) and storage coefficient (S) of the conglomerate gravel layer were low. Therefore, only six 90 m-deep wells were needed to lower the groundwater level of gravel layer to keep a good base stability (FS > 1.2) against uplifting water pressure for a 160 m x 26 m excavation site with a depth of 33 m.