Mixed Face Conditions Research Articles

EPB shield tunneling in mixed face conditions : G. W. Clough & E. Leca, Journal of Geotechnical Engineering - ASCE, 119(10), 1993, pp 1640–1656

EPB shield tunneling in mixed face conditions : G. W. Clough & E. Leca, Journal of Geotechnical Engineering - ASCE, 119(10), 1993, pp 1640–1656

Some modes of deformation and failure of lined tunnels

Lining behavior of three tunnels and two mining drifts is described. One tunnel served a railroad in Colorado and was constructed in sedimentary rock with plain concrete and timber as a lining. The lining buckled in response to vertical and sidewall rock loosening loads. A transit tunnel on the west coast was constructed in mixed face conditions of sand over stiff clay with initial timber support and a lightly reinforced concrete lining. The lining failed in the mixed face segment in shear due to eccentric loading from the differing sand and clay materials having a sloping boundary at the tunnel elevation. Brick and mortar lining was used for storage tunnels in Wisconsin constructed in weak soil and fill. The 150+ years of seeping water exposure and creep, with limited maintenance, led to the collapse of severely-deformed out-of-round tunnels whose linings were nearly strengthless. In a caving-method mine in granitic rock, plain or lightly-reinforced thick concrete lining of modern rail haulage mine tunnels are situated such as to receive very high compressive and extension strains during various phases of the mining activity. Initially the tunnel linings are torn apart by vertical extension, then crushed by vertical compression. The roof remains intact, while the walls crush. In the same mine, blasting energy and vibration destruction of thick shotcrete linings was observed under controlled conditions; it was found that hairline cracks evolved into shears of the roof closest to the blast, and finally, cratering of the roof into the tunnel occurred.

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.

Mixed Face Tunneling on Melbourne Underground

Adequate preliminary investigation, continued monitoring of ground conditions and construction procedure by the engineer, and a resourceful contractor combined to result in the economical design and construction of a four-track (double-under-double-over) rapid transit railway project in Melbourne, Australia. One tunnel contract required a variety of heading-and-benching-type operations to overcome mixed face conditions, with the soft ground being either on top, underneath, or in the middle of the face. Close monitoring of the tunnel construction by the engineer, skillful execution of the work by the contractor, and a good specification, permitted several variations of the original design resulting in economies.