Study of ground collapse induced by large-diameter horizontal directional drilling in a sand layer using numerical modeling

With large-diameter horizontal directional drilling (HDD) becoming the preferred method to construct oil and gas pipelines and utility pipelines beneath rivers, the issue of potential ground collapse arises when drilling in loose geological layers such as sand. Ground collapse is a result of borehole collapse and may cause significant damage to the topography and nearby facilities. The present investigation considered the potential causes of ground collapse induced by an actual 1.219 m diameter HDD river-crossing project, using the FLAC3D numerical modeling tool. The analysis showed that the failure zone first developed at the crown of the borehole, resulting in subsequent borehole collapse due to instability of the sand above, and eventually leading to ground collapse. Sequential reaming cycles have been simulated and the results indicate very little effect in comparison with a single reaming cycle. The risk of borehole collapse, and consequent ground collapse, increases with borehole diameter. Parametric numerical modelling has also been conducted to study the influence of soil parameters and drilling mud pressure on the stability of the ground surface above the borehole. The results show that soil cohesion and friction angle have a large influence on stability of the borehole and ground surface, while elastic modulus and Poisson’s ratio have relatively little effect. It was also determined that mud pressure is a very important factor in maintaining stability of the borehole, and therefore the ground surface as well.