Dassanayake, A. and Fujii, Y. (2013), Evaluation of Biot's Effective Stress Constant for Peak and Residual Strengths forShikotsu Welded Tuff and Kimachi Sandstone by using Modified Failure Envelope Method, Proc. MMIJ Fall Meeting 2013
Sapporo, PY-6, p. 570, Hokkaido University, Sapporo, Sep. 3

Evaluation of Biot's Effective Stress Constant for Peak and ResidualStrengths for Shikotsu Welded Tuff and Kimachi Sandstone by using ModifiedFailure Envelope Method
Dassanayake, A.B.N. and Fujii, Y. , Hokkaido Univ.
The Biot’s effective stress coefficient, α, is usually

For Kimachi sandstone, α value for peak strength

calculated based on experiments within elastic region using

decreases with effective confining pressure (Fig. 2). α values

poroelasticity theory. However α, for peak and residualfor residual strength however are almost constant and higher

strengths is important when using it to evaluate rock failure.

than the case of peak strength.

Failure envelope method proposed by Franquet and Abass(1999) evaluate α based on peak strength. They however1

showed just two data for a rock and a tedious and not precise

0.8

trial and error method was used for the evaluation. The authors

0.6
value

Residual strength

propose a modified failure envelope method and use it for peak

0.4

and residual strengths of Shikotsu welded tuff and Kimachi

Peak strength

0.2

sandstone. The Modified failure envelope method requires

0strength data on the effective confining pressure-differential

0

stress plane on samples with zero pore pressure first, and then

5

1015

Effective confining pressure (MPa)strength data for samples with specific pore pressures areplotted.

Fig. 2 Biot’s constant variation with effective confining

Data with pore pressure can move to the left by increasingpressure for Kimachi sandstone

α from 0 to 1 (Fig.1). The crossing point can be easily
calculated and the α can be obtained from effective confiningpressure at the point PC′ as

For Shikotsu welded tuff, only two data points wereobtained for peak strength (Fig. 3), because strength values

PC 'PC

with pore pressures were larger than those with zero porePp

pressure in many cases. α value for residual strength decreases
The strange behavior of Shikotsu welded tuff should be

Crossing point(0 ≤ α ≤ 1)

investigated further. The method will be applied to other rock

PP = 0types. Comparison with conventional methods will also be

α=1carried out.
PP ≠ 0
α= 01
Two data points overlapped

0.8

P C′

value

Differential stress (MPa)with effective confining pressure.

PC

0.60.4

Peak strength

0.2

Fig. 1 Evaluation of α by the modified failure envelope method

Residual strength

0Triaxial compression tests were carried out under PC and051015

Effective confining pressure (MPa)PP as show in Table 1 for Kimachi sandstone and Shikotsu
welded tuff.

Fig. 3Biot’s constant variation with effective confiningpressure for Shikotsu welded tuff
Table 1. Target values of confining and pore pressures
Confining pressure

Pore pressure

Reference(MPa)Franquet JA, Abass HH (1999) Experimental evaluation of(MPa)251015Biot's poroelastic parameter - Three different methods, in001
1

4001
1

4
4

37th U.S. Symposium on Rock Mechanics (USRMS), June
997-9 Vail, CO, Balkema, Rotterdam, 349–355.
14