Grout pressures around a tunnel lining, influence of grout consolidation and loading on lining

Abstract

The influence of the grout properties on the grout pressure distribution around a tunnel lining is investigated. Initial yield stress of the grout and consolidation properties appear of importance. The consolidation properties are determined by means of element tests and appeared also to be influenced by the properties of the soil surrounding the tunnel. Field measurements have shown that grout pressures vary during the bore-cycle. This is attributed to consolidation of the grout. The vertical pressure gradient over the lining decreases with increasing distance from the TBM. It is shown that at some distance from the TBM the grout pressure is in most cases comparable to the pore pressure for a tunnel drilled in sand. The grout applied influences the vertical loading on the lining. For this loading it is of importance that the unsupported part of the lining (where buoyancy forces dominate) is as short as possible to reduce the momentum in the lining and high vertical forces at the TBM and there where the grout is hardened. This can be achieved in three different ways: The grout has a relatively high initial shear stress. In a situation with a high initial shear stress of the applied grout, the shear strength in the not yet hardened grout is already sufficient to prevent upward movement of the tunnel lining. Subsoil and grout allow for a rapid consolidation of the grout, resulting in an increase of allowable shear stress in the grout and as a result in only a limited unsupported length of the lining. The grout used hardens quickly. This also leads to a limited unsupported length of the lining. These three methods can be used for a tunnel made in sand. For a tunnel in clay the second option is not possible, because the low permeability of the clay prevents consolidation of the grout. Results of field measurements on grout pressure decay during standstill of the TBM have been analysed in combination with the results of element tests. Grouts normally consolidate after injection into the tail void. This leads to a reduction of grout volume and to a decrease in grain stress. It was found for several tunnel projects, where a tunnel was bored in sand, that the final pressure distribution around a tunnel was comparable to the pore water pressure and was more or less independent from the injection strategy. Only the initial pressure distribution directly behind the TBM can be influenced by the injection strategy. Grout properties in combination with the soil properties influence the loading on the lining directly behind the TBM. It is therefore necessary to select a grout taking in into consideration the soil properties at the location and desired grout properties (yield stress, bleeding and hardening parameters). (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.