Mechanics of tunnelling machine screw conveyors: model tests

Control of the excavation process of an earth pressure balance (EPB) tunnelling machine depends on control of the soil flow through the screw conveyor. An instrumented 1:10 scale model screw conveyor was designed and commissioned, and tests were performed with consolidated kaolin and compacted London Clay and Lambeth Group samples pre-conditioned with soil conditioning agents commonly used in EPB tunnelling machines. During steady-state operation, the measured shear stress acting on the conveyor casing was found to be constant along the conveyor, leading to a linear total pressure gradient. The casing shear stress was approximately equal to the un-drained shear strength of the soil, and the screw torque increased linearly with the casing shear stress. Effects of varying sample strengths, screw speeds, screw geometry and the discharge outlet restriction on the total pressure gradient, casing shear stress, and screw torque are illustrated by the test results. The tests provide new insight into the behaviour of soil flow through screw conveyors.