ANALYSIS OF STRESS-CHANGE DISTURBANCE CAUSED BY IDEAL DRILLING IN CLAY

Soil sampling and pressuremeter testing involve drilling boreholes. As a result of the removal of overburden pressure, the stresses in the s oil surrounding a borehole are altered from their initial states, resu lting in certain disturbance to material properties. This paper presen ts two different numerical approaches in evaluating the stress and str ain changes associated with an idealized drilling. In this evaluation two finite-element procedures incorporating two different elastoplasti c soil models were used. In the first approach, the stresses on the bo undaries of a preformed borehole were gradually withdrawn from their i n-situ values, and in the second approach, the borehole is formed on-l ine by remeshing, The results of the analyses show that for a typical soft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 bel ow the terminus of the borehole, and the region immediately adjacent t o the borehole wall undergoes a significant increase in the deviatoric stress ratio, indicating that this region is highly disturbed.