INFLUENCIA DEL GAS METANO EN LAS PROPIEDADES MECÁNICAS DE LA ARCILLA MARINA

Abstract

To understand the behavior of clay soil containing gas, it is necessary to know the compressibility and strength parameters derived from field and laboratory tests, as done for clay sediments without gas. For that, undisturbed clay samples containing methane gas are required, however, the sampling techniques are not adequate, because it is not possible to avoid sample disturbance during sampling, which is higher for a sample containing gas, due the gas expansion as the confining pressure is reduced. An alternative method for obtaining mechanical parameters of clay containing methane gas is to perform laboratory test on reconstituted samples of clay containing gas; for this reason, soft and medium clay samples without gas and with 7 and 15 percent by weight of zeolite saturated with methane gas have been reconstituted. The following tests were performed: Results from the incremental consolidation tests indicated that higher loads were required to consolidate the clay samples containing methane gas to obtain the same degree of consolidation as for the clay without gas and, also, it was found from the compressibility curves (controlled rate of strain tests) that the clay with higher gas content generate certain reaction when a load is applied, as indicated by the fact that the compressibility curve is slightly displaced to the right of the curve for the sample with less gas. Regarding the tests to determine the shear strength characteristics of the clay, it was found that the minivane shear strength for the soft clay is reduced as the gas content increases. However, for the firm clay samples, the undisturbed shear strength becomes higher as the gas content increases. Concerning to the triaxial tests, it was found that the undisturbed shear strength of the soft clay is reduced as the gas content increases; however, in the firm clay samples, the undisturbed shear strength becomes higher as the gas content increases. However, there is no change in the failure mode for both clay consistencies. As regards the normalized shear strength (direct simple shear tests) of the soft clay, a slight increase in normalized shear strength was noticed for the clay with 7 percent of methane gas, independent of the rate of deformation. However, for the clay with higher gas content, a slight decrease in the normalized shear strength was noted when a rate of 100 and 1,000 percent of deformation per hour is applied, although the same normalized static shear strength was recorded. The results for the firm clay without and with methane gas indicated that the normalized shear strength is slightly reduced with increased the gas content, when the clay is tested statically and at a rate of 1,000 percent of deformation per hour. It is theorized that the behavior of the clay with methane gas is related to the confinement provided by the clay platelets to the methane gas, thus generating a partial load transfer from the gas and pore water to the clay structure and, thus, forming a new type of clay micro-structure composed by the pore water, methane gas and clay particles, whose behavior can be explained and predicted. It is assumed that, in this “new structure”, the gas remains confined by the clay platelets and behaves, up to a certain level of load, as a shock absorber, thus, increasing the rigidity of the gas, at the same time as it increases the rigidity of the structure, as long as the load increases, being higher for a higher gas content in the clay, and until the applied load exceeds the confining stress provided by the intermolecular forces developed by the clay particles.