Maximum allowable pressures during horizontal directional drillings focused on sand

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 (LH • 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, <pl. Beyond the plastic zone the soil is assumed to behave as a linearly elastic, isotropic solid defined by a modulus of deformation (E) and a Poisson's ratio (v). Next to that, in the derived models for this thesis it is possible to take dilatancy into account. It was tried to introduce failure and especially hydraulic fracturing into these 'new' models. The introduction of the criterion of a maximum strain instead of a maximum radius of plasticity (LH • the assumptions made for the soil model, • the parameters that are used for calculation are difficult to determine, showing wide ranges of variation, • stratified soil can not be introduced. It is therefore recommended to use the equations with a lot of precaution.