Analysis of Stress-Change Disturbance Caused by Ideal Drilling in Clay

Abstract

Soil sampling and pressuremeter testing involve drilling boreholes. As a result of the removal of overburden pressure, the stresses in the soil surrounding a borehole are altered from their initial states, resulting in certain disturbance to material properties. This paper presents two different numerical approaches in evaluating the stress and strain changes associated with an idealized drilling. In this evaluation two finite-element procedures incorporating two different elastoplastic soil models were used. In the first approach, the stresses on the boundaries of a preformed borehole were gradually withdrawn from their in-situ values, and in the second approach, the borehole is formed on-line by remeshing. The results of the analyses show that for a typical soft clay, the ultimate extent of the zone influenced by the idealized drilling extends to depths approximately three borehole diameters below the terminus of the borehole, and the region immediately adjacent to the borehole wall undergoes a significant increase in the deviatoric stress ratio, indicating that this region is highly disturbed.