Page 294 - Managing Global Warming

254 Managing Global Warming

7.4.2 Depressurization method

The variations of back pressure, MH saturation, volumetric and axial strains during the
depressurization and repressurization stages are illustrated in Figs. 7.16 and 7.17,
respectively. In each case there was an initial compressive volumetric strain because
of the increase in effective stress. Further volumetric strain occurred as the MH dis-
sipated and bonds dissolved. Dissociation occurred when the back pressure dropped to
4.3 MPa. Dissociation continued for approximately 2–3 h from initiation, with axial
strains still increasing up to 3 h after the start of the test. After 5 h the back pressure
was ramped back up to 10 MPa over 1 h to represent a restoration of postdissociation
equilibrium conditions being restored in situ. In Cases 1 and 2 there was an elastic
recovery of axial and volumetric strains due to a decrease in effective stress. However,
in Case 3 collapses occurred as the current nonbonded soil moved outside the failure
envelope.
Fig. 7.18 shows the stress ratio, axial strain, and volumetric strain relations during
the tests. It can be seen that the axial strain increased dramatically when the stress
strain curve reached the strength of pure sand as the water pressure recovered. In con-
trast to the thermal method no deformations were observed despite the increase in
effective stresses. After dissociation during back pressure recovery large deformations
may occur in the metastable zone.

Fig. 7.16 Depressurization method [18].