EPB SHIELD TUNNELING IN MIXED FACE CONDITIONS

The earth pressure balance (EPB) shield has proven to be successful for tunneling in soft ground in homogeneous soils. On three recent soft ground tunneling projects in Washington, D.C., EPB shields were used in mixed ground conditions with a high water table. Many areas were found with sands in the crown, and clays in the invert. In a limited number of cases, silicate grouting was used in the sands as a measure to provide extra control for ground movements. Ground conditions were found to play a major role in the observed response. Sticky, stiff clays caused problems with movement of muck through the bulkhead and conveyor system. Movement data showed that the tunneling consistently caused large ground settlements in those areas with sands over clays where no chemical grouting was performed. Movements were well controlled in instances with clays in the crown or where the sands were chemically grouted.