Induced pore pressure response during undrained deformation of tuff and sandstone

Abstract

Deformation under undrained conditions, in which excess fluid mass is prevented from leaving the rock mass, occurs in several situations of interest. For loading under such conditions, knowledge of the induced pore pressure response is clearly necessary for understanding and quantifying material behavior in terms of effective stress. A study was performed of the induced pore pressure response, described by the Skempton ratio B, of tuff and sandstone during undrained loading. The tuff samples, from a vertical borehole, are in varying stages of devitrification and alteration to various authigenic phases, including clay, zeolite, feldspar, and quartz. Two experiments were performed on synthetic (glass) sandstones with porosities of approximately 20 and 30%. This study differs from previous work in two significant ways. First, a highly accurate transducer was designed which allowed us to explore the range in behavior up to applied confining pressures of 400 MPa. Second, we performed simultaneous measurement of axial and radial strain, thereby allowing us to correlate the induced pore pressure response with the measured bulk compressibility. Fully saturated tuff and sandstone are characterized by a B value close to one at near-zero effective pressures. For partially to fully saturated vitric and zeolitic non-welded air-fall tuffs with high matrix compressibility, B is close to one for “effective” (i.e., confining minus pore) pressures from about 25 to 50 MPa (corresponding to applied pressures of 400 MPa). For more rigid silicified zeolotic and feldspathic tuff, B is reduced to 0.7–0.8 at intermediate effective pressures of about 10–50 MPa. For the well-indurated sandstones, B is highly pressure sensitive at low effective pressures due to the closure of low aspect ratio pores and microcracks. At higher effective pressures, B attains an approximately constant value substantially less than one, which increases with the measured drained bulk compressibility.