An experimental investigation into the stress-dependent mechanical behavior of cohesive soil with application to wellbore instability

Abstract

This thesis investigates the mechanical behavior of cohesive soils with reference to the applications of wellbore instabilities through an extensive program of laboratory element and model borehole tests. The laboratory tests use Resedimented Boston Blue Clay (RBBC) as an analog test material. Undrained triaxial shear tests have been performed on specimens that were K0-consolidated to stress levels ranging from 0.15 to 10.0 MPa in both compression and extension shear modes. Compression tests were also performed on overconsolidated specimens. Model borehole tests make use of two new automated, high pressure Thick-Walled Cylinder (TWC) devices to study the effects of the following parameters on the borehole response: mode of loading, specimen geometry, preshear lateral stress ratio, drainage conditions, consolidation stress level, stress history, and cavity volumetric strain rate. This testing program has been performed using small and large TWC devices with outer diameter, Do=7.6cm and 15.2cm, respectively. Both devices allow for independent control of the vertical stress and the radial pressures acting on the inner and outer walls of the cylinder, as well as pore pressure. The triaxial compression and extension test results demonstrate remarkable reductions in the undrained strength ratio (su/[sigma]'vc) with consolidation stress level, notable reduction in the stiffness ratio (Eu/[sigma]'vc), increase in the strain to mobilize the peak resistance (ef), and a significant decrease in the large strain friction angle (f'). The model borehole data indicate that most of the reduction in cavity pressure occurs at volume strains less than 5% before the borehole becomes unstable.

(cont.) Increases in outer diameter and strain rate lead to a reduction in the minimum borehole pressure. The initial cavity stiffness ratio decreases as consolidation stress level increases. Drained tests have larger cavity strain at a given cavity pressure and lower minimum pressure than the undrained tests. The borehole closure curves were analyzed using a framework originally developed for interpreting undrained shear properties in model pressuremeter tests (Silvestri, 1998). Backfigured undrained strength ratios from these analyses range from su/[sigma]'vc=0.19-0.21 corresponding to an average between the measured triaxial compression and extension strength ratios. The relationship between su/[sigma]'vc and overconsolidation ratio is consistent with element tests.