Effect of trenchless technologies on existing iron pipelines

Abstract

Impact moles and directional drilling units form tunnels of up to approximately 300 mm in diameter at a depth of 1–2 m. They are typically used for the installation of electricity cables or small-diameter pipelines. This type of tunnelling differs from conventional tunnelling owing to the lack of soil excavation. The tunnel is formed by dynamically expelling soil outwards in a predominantly radial direction. This creates a stress and displacement field that has the potential to damage existing services and buildings. To reduce the risk of damage to existing structures installation guidelines detailing acceptable proximity distances need to be provided. This paper details a methodology that has been used to calculate acceptable proximity distances to existing grey iron pipelines that have a transverse geometric configuration to the new pipeline. The study utilises finite difference techniques in conjunction with analytical cavity expansion solutions to determine the stress–strain field induced by tunnelling operations in idealised cohesive and frictional soils. Results from the finite difference study have been used in conjunction with a knowledge of acceptable pipeline strain and joint rotation levels to form acceptable proximity guidelines for this type of tunnelling operation.