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

Abstract

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.

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“…This compressibility degradation was interpreted as being the result of sediment damaging during free gas generation. Previous experiments also highlighted a decay of compressibility with the presence of gas bubbles in soft marine clays (Sills, et al, 1991;Nava Castro, et al, 2013;Liu, et al, 2016). They concluded that gas bubbles bear part of the load applied on the sample and that soil compressibility measured on gassy sediment is primarily due to bubble compression and secondarily due to normal sediment compression, implying water expulsion.…”

“…The presence of free gas in sediments was also proved to be responsible for a delay in consolidation. As gas escapes or dissolves, consolidation happens faster (Sills, et al, 1991;Nava Castro, et al, 2013).…”

“…The main difficulty when studying gassy fine-grained sediments in laboratory is to obtain a discrete distribution of gas bubbles. Several techniques exist, including the generation of free gas through the zeolite molecular sieve technique (Nageswaran, 1983;Thomas, 1987;Wheeler, 1988;Sills, et al, 1991;Nava Castro, et al, 2013), the generation of biogenic gas through bacteria (Sills & Gonzalez, 2001;Rebata-Landa, et al, 2012) and finally the generation of gas bubbles through the circulation of overpressured carbonated water followed by decompression (Grozic, et al, 2000;Sultan, et al, 2012).…”

“…The mechanical behavior of gassy sediments has been extensively studied over the last decades (Thomas, 1987;Wheeler, 1988;Sills, et al, 1991;Helgerud, et al, 1999;Grozic, et al, 2005;Sultan, et al, 2012;Nava Castro, et al, 2013;Jang & Santamarina, 2014). P-wave velocity attenuation is a typical feature of all gassy sediments (Sills, et al, 1991;Helgerud, et al, 1999;Sultan, et al, 2012).…”

“…Thomas (1987) shows that fine-grained gassy sediments subjected to consolidation behave like a water-saturated soil with discrete compressible solid inclusions modeling the gas bubble compressibility. Moreover, several studies show that the compressible gas bubbles deform under applied stresses causing a delay in the consolidation process and therefore a change in sediment compressibility (Sills, et al, 1991;Grozic, et al, 2005;Nava Castro, et al, 2013;Liu, et al, 2016). Additionally, numerous experiments and numerical simulations show that the presence of discrete gas bubbles that nucleated directly in the sediments decrease its permeability (Egermann & Vizika, 2000;Naylor, et al, 2000;Jang & Santamarina, 2014).…”