Investigation of prestress loss in anchor cables and adaptive construction scheme for soft rock tunnelling

The construction and stabilisation of soft-rock tunnels pose significant challenges due to rock’s inherent properties, like low strength and high deformability. Traditional passive support systems often fail to effectively control tunnel deformation, leading to increased construction costs and delays. Alternatively, the adoption of prestressed anchor cables as an active support has proven effective in controlling soft-rock tunnel deformation. However, the issue of prestress loss in anchor cables has received scarce attention, hindering the widespread application of this active support system. This study investigates prestress loss in anchor cables and evaluates how their composition design and construction affect anchoring performance in the soft-rock Muzhailing tunnel (MZT) in China. Through comprehensive field and laboratory tests, the research identifies the original design of prestressed anchor cable system led to a significant prestress loss rate of 30%-55%. To mitigate this, an optimised construction scheme was proposed, adjusting specifications, configurations, and construction techniques according to geological conditions. Implementation of this scheme reduced the prestress loss to 25-35%. Based on these results, a generalised adaptive construction scheme for using prestressed anchor cables to support soft-rock tunnelling is proposed, aiming to provide some practical guidelines for enhancing soft-rock tunnel structural integrity and construction safety.