Retaining and protection of foundation excavation is a supporting, reinforcing and protecting method to guarantee safety of underground structure and surrounding environment.

As a structural system, retaining and protection of foundation excavation should meet demand of stability and deformation, that’s also the ultimate limit state and servicing limit state.

Ultimate limit state refers to larger scale unstability when supporting structure is damaged, toppled, slid and collapse. While servicing limit state refers to that soil bodies’ deformation influences normal use, but it doesn’t create unstability.

So supporting design should have enough safety factors to avoid unstability and then control displacement.

As for rigid supporting structures, like soil retaining pile, continuous wall support system, their displacement is small and should be controlled in 30mm; As for soil nails for slope stability in good geological conditions, advance support and pre-stressed bolt are adopted to control to smaller displacement and it will be greater than 30mm.