Abstract:

Early support implementation can lead to an excessive loadbearing burden if the surrounding rock lacks apparent rheology during the construction of a tunnel in elasticplastic rocks. This underscores the importance of timely support. In instances where the stiffness and strength of the support structure are inadequate, large deformation failure induced by insufficient support reaction force becomes a distinct possibility, even with reserved deformation in place. To ascertain optimal support stiffness and determine the appropriate supporting duration, the ground reaction curve(GRC) for soft rock tunnels experiencing high geostress is modified. The modification is based on the generalized ZhangZhu strength criterion and large strain analysis theory, accounting for the effects of tunnel expansion. The modified and original GRCs share a common starting point but gradually diverge due to increasing deformation. The modified GRC exhibits an upward trend as the excavation radius expands, resulting in a higher reaction force. Consequently, the modified GRC serves as a valuable guide for designing support structures that minimize the substantial deformations arising from inadequate support stiffness. Following the analysis of the impact of initial geostress and rock strength on the modified GRC, a method for designing optimal reserved deformation in the surrounding rock for soft rock tunnels subject to high geostress is proposed. In cases of low geostress, the optimal support timing aligns with the "lowest point" on the GRC, corresponding to the smallest support reaction force. Conversely, under high geostress conditions, the GRC fails to reach the "lowest point", even with significant deformation, attributed to predominantly small loose pressure than deformation pressure. Therefore, stress release becomes imperative under high geostress conditions, and the optimal reserved deformation can be determined from the curvature of the GRC.

Key words: high geostress, soft rock tunnel, large deformation; reserved deformation, modified ground reaction curve