Variation of Mechanical Properties of Soft Marine Clay With Methane Gas Content

To quantify the effects of methane gas on mechanical properties of soft marine clay, an exhaustive laboratory testing program was developed using zeolite to uniformly disseminate gas bubbles inside the clay matrix. Results from controlled rate-of-strain (CRS) tests indicated that as the gas content increases, there is a reduction in the interpreted preconsolidation pressure, although the rigidity of the clay with more gas increased throughout the test. Minivane test results indicated that the undisturbed shear strength decreases as the amount of methane gas increases, while the residual and remolded strengths remain practically unchanged, i.e., are independent of the gas content. Similarly results from triaxial tests indicated that the undisturbed shear strength is reduced as the gas content increases, but there was no change in the failure mode. Interestingly, the normalized shear strength increased for the clay with gas, when the samples were tested at 100 percent of deformation per hour. It is theorized that the methane gas bubbles interact with both the clay platelets and the pore water, and, to certain point, bear part of the load, thus modifying the distribution of the load in the soil structure; that is to say, there is a partial load transfer from the gas bubbles to the soil structure, as the clay particles confine the methane gas.