Numerical modelling of compensation grouting above shallow tunnels

Abstract

SUMMARYThis paper describes the development of a numerical model for compensation grouting which is a useful technique for the protection of surface structures from the potentially damaging movements arising from tunnel construction. Pipes are inserted into the ground between the tunnel and the overlaying structure from an access shaft. Buildings on the surface are instrumented and movements are carefully monitored. Once the deformations exceed a certain Trigger Level, grout is injected into the ground to prevent damage. In the finite element model described here, compensation grouting is modelled by applying an internal pressure to zero-thickness interface elements embedded in the mesh. An 'observational algorithm' is used, where the deformations of the surface are monitored and used to control the injection process. Example analyses of compensation grouting are given for three-dimensional tunnel construction underneath a greenfield site. Different strategies are used to control the injection process and their effectiveness in preventing surface movement is assessed. The numerical model is shown to replicate general behaviour expected in the field and is capable of modelling the control of ground surface movements at a greenfield site.

Mechanism of grout bulb expansion and its effect on ground uplifting

“…Some numerical schemes adopting this type of approach were described in Refs. [6][7]15]. However, in order to reach the assigned volume strain for grout bulb, a series of trial calculations are necessary through adjusting the magnitude of pressure, which would cost much time and affect the efficiency especially for complicated mesh.…”

“…And the impacts of grout column length and inclination as well as the depth of grout injection were analyzed simultaneously. By applying an internal pressure to zero-thickness interface elements embedded in mesh, WISSER et al [6] simulated the compaction grouting to arrest the foundation settlement caused by adjacent tunnel excavation. This approach was used by TANG et al [7], who proposed to increase the volume strain of grouting elements by applying a "fictitious" expanding pressure to achieve the heaving effect, and employed software FLAC 3D to study an existing tunnel heaving by grouting during the construction of the Chongwenmen subway station of Beijing Metro Line 5.…”

Laboratory investigation of shaft grouting

“…Au et al, 2003;Contini et al, 2007;El-Kelesh et al, 2001;Wisser et al, 2005). Like numerical analyses, laboratory studies offer the advantage that the environment, such as the soil properties, drainage and stresses, can be controlled, allowing insights into the mechanisms underlying the soil response.…”

Clay Grouting Mechanisms and Applications

“…Additionally, the intrusion of grouts may swell the cohesive soil and generate the positive excess porewater pressure. As long as the porewater pressure dissipates along with time, the associated settlement could override the heave generated during grout intrusion and thus result in a negative final compensation efficiency which is defined as a ratio of the total heaved volume to the injected volume of grouts [25][26][27][28][29]. Since the fracturing grouting due to easy travelling of the low viscosity grouts could generate higher porewater pressure than compaction grouting, regrouting at the same injection point is deemed to be necessary in order to change the stress state of cohesive soil to the overconsolidated state from its ordinary state [24,30].…”