In-situ monitoring and 3D numerical analysis of reinforced face behaviour of tunnel in composite strata by considering randomly-distributed rigid cobble granules

Abstract

Full-face excavation can be difficult when tunnels are constructed in composite soil containing oversized cobbles. To mitigate construction risks, a section of the mine-by experimental tunnel was excavated as part of the Kokhav Hayerden Pumping Storage Project in Israel. The response on the face was monitored by using the Modular Reverse-Head Extensometer technique and the extrusion mechanism was studied via FLAC3D software. To successfully model randomly-distributed granules mixed in the host clay matrix, a heterogeneous method was developed to nominate individual mesh zones by assigning rigid material properties through a random Numpy list. The behaviour of the response of the face was closely depicted by the proposed model, as compared to in-situ monitoring data. For stability analysis, a strength reduction method was used and demonstrated similar factors of safety to semi-analytical solutions, which suggested the anchor design at the mine-by section may be optimized for subsequent tunnel chainages. Despite the cobble granules limiting face extrusion in the mixed strata, it brought additional concern when granules constantly spall from the working face. This could be attributed to the loss of shear strength in the host material caused by a combination of excessive shear strain and stress relaxation during tunnel excavation.