Maximum allowable pressures during horizontal directional drillings focused on sand

Abstract

Horizontal directional drilling (HOD) is a rather simple installation technique for pipes for public infrastructure and it has therefore become increasingly popular. When HOD is started, a pilot drilling is made first; a small borehole is made in the soil along the trajectory. After the pilot drilling the borehole is enlarged with a reamer until the designed diameter is obtained and the product pipe is pulled in. Drilling fluid or mud is used for the stability of the borehole and to transport the excavated soil. A certain minimum mud pressure is needed to ensure a return flow and to ensure that the borehole does not collapse. In order to prevent soil failure, it is necessary that the pressure will not exceed a certain maximum. At present in the Netherlands and also abroad, in order to compute the maximum allowable mud pressures, it is assumed that the cavity expansion theory applies in the borehole at greater depth. The equations to calculate this pressure were first presented by Luger and Hergarden (L&H, 19881. This theory assumes plastic deformations, which lead to failure. However, recent research has shown that soil failure due to hydraulic fracturing is an important failure mechanism also. In the framework of BTL (Boren Tunnels & Leidingen) research was done to look at whether hydraulic fracturing was observed before in the context of liquid injection in softer soils like clay. The process of hydraulic fracturing in soil with cohesion (clay) was recognised in literature indeed and models were developed. However no literature was found on hydraulic fracturing in soils without cohesion (sand). This thesis research was done to improve the knowledge of the behaviour of sand and bentonite around a borehole in order to better determine the soil failure process under maximum pressures. New soil models to describe soil behaviour were developed for a cylindrical and a spherical cavity under fluid pressure. The soil model for a cylindrical cavity is comparable to the soil model at the basis of the equations derived by L&H. The soil models assume; • the medium to be homogeneous and isotropic, • to have infinite dimensions (gravity neglected) and therefore prior to the application of the load the entire soil mass has an isotropic effective stress, • the soil behaves in the plastic zone as a compressible plastic solid, defined by Mohr-Coulomb shear strength parameters Ie,