Thrust Pit – Cambridgeshire

Description:  The project involves a 20m tunnel bore under the new A14 where two pits are required to allow access for the tunnel boring works.  NTS have being asked to design and supply the shoring for the thrust pit which requires an excavation at 8.5m long x 4.2m wide x 7.0m deep.

Solution: NTS provided a slide rail pit which consisted of an excavation requiring a clear open dimension of 4.8m x 4m.  Due to the thrust wall and opening required for the boring process the solution involved standard slide rail plates combined with unique pile chambers which allow KD6/8 trench sheets to be installed within a special slide rail plate.  The required sheets can then be lifted to allow the opening required and also form a bearing surface for the RC thrust wall to be cast against.  The pile chamber solution was chosen for the thrust wall element as after the works this would be removed and pipe work would then need to be connected as this end of the excavation.  Due to the depth of excavation the solution incorporated standard 5.5m rails and corner posts which then required extension members at 3.5m to allow the full excavation depth of 7.0m to be achieved.  The slide rail solution was preferred over a traditional sheet and frame solution which would have required a specialist sheet piling company whereas the slide rail system was installed with a 40t excavator based on a dig and push installation method.