What is the direct cause of ground deformation during shield construction?

The direct cause is caused by shield tunneling, which mainly includes instability of excavation surface, decrease of groundwater level, excessive thrust, frequent rectification, instability of soil layer in tunnel body, friction between shield and tunnel body, gap behind lining, grouting pressure, lining deformation and lining leakage, etc. Although there are many direct causes of formation deformation, they can be roughly divided into the following four categories:

(1) Elastic-plastic deformation caused by stress release in formation leads to reduced formation response;

(2) The compression deformation caused by the increase of earth pressure leads to the increase of vertical earth pressure or the decrease of stratum reaction force;

(3) Elastic-plastic deformation caused by additional earth pressure leads to the increase of acting earth pressure;

(4) The elastic-plastic deformation and creep deformation accompanied with the change of soil physical properties lead to the decrease of stratum bearing capacity.

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