Numerical analysis of tunneling influences progress on soft clay bearing a raft foundation

This paper provides a finite element Plaxis analysis concerning the construction of a TBM tunnel and the behavior of tunnel and its surrounding soil. A Circular shaped seven meters' diameter TBM tunnel is built 15 m below the natural ground level down to its center. The length of the proposed tunnel is 100 m. Soil profile is simple, down its center there exists a soft (cu = 40 kPa) clay and sand extend down below that clay layer. A square raft foundation having lateral dimension of 30 m is placed over the tunnel and bearing a surface distributed load of 70 kPa. Numerical results of study show an upward movement of soil in zones located at sides and under tunnel, while soil in the zone directly above tunnel heads downwards. As a result, there is a slight upward movement of tunnel which is attributed to the weakening and disturbance of the soft upper clay due to the exaction action of the TBM besides the higher strength properties of the sandy underlying layer. The change in the pore water pressure confirms this thought. In terms of length of tunnel, there is a differential settlement in the area under the raft compared to that at the extreme ends of tunnel. Maximum numerical displacement is 0.65 m at the center of raft foundation before tunneling excavation to begins. Two plastic zones are located in the sandy layer in which no engineering facilities are recommended to pass into.