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.

A new theoretical model describing the total pressure gradient and torque in a tunnelling machine screw conveyor is proposed. The theory develops new equations relating the pressure gradient and torque to the screw geometry, the shear stresses acting in the screw conveyor, and the material flow. The equations are expressed in dimensionless form, allowing application to screw conveyors of different scales. The theory is validated against measurements from laboratory model screw conveyor tests with clay soils with varying properties and operating conditions. The theory successfully describes key features of the observed mechanics of the model conveyor operation, and accurately predicts the test measurements. The proposed theory provides insight into the fundamental mechanics of screw conveyor operations, and can be used in the design of tunnelling machine screw conveyors. The theory can also be applied to other forms of screw conveyors and extruders operating with plastic materials.

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