Phenomenology of the Consolidation of Marine Clays With Methane Gas

Castro, Raul Nava (Fugro-Chance de Mexico) | Audibert, Jean M.E. (Geotechnical Consultant) | DeGroff, Willard (Fugro Consultants, Inc) | Ruckman, Paul (Fugro-Chance de Mexico)



The results from 1D consolidation tests on gassy clay samples, containing uniformly distributed methane gas bubbles, indicate that their compressibility is ‘delayed’ compared to samples of the same clay without gas. This behaviour is believed to be due to the interaction of the gas bubbles with both the clay platelets and the pore water, which generates menisci representing zones of high pressure and partial load sharing between the gas bubbles to the soil structure as the clay particles confine the methane gas. As time passes, the gas becomes part of the structure, and a new structure is formed consisting of a combination of the pore water, gas and clay particles. In this new structure, the gas is confined and behaves as a spring, which temporarily increases the rigidity of the structure until the applied load exceeds the confining stress provided by the intermolecular forces developed by the clay particles, at which time conventional consolidation occurs.

1. Introduction

In soil mechanics, it is customary to consider that marine soils are completely saturated. However, several studies have identified that numerous types of gas exist in marine sediments, with methane gas being the most common (Claypool and Kaplan, 1974; Schubel, 1974; Christian and Cranston, 1977; Esrig and Kirby, 1977; Bhasin and Leland, 1978; Whelan et al., 1978). In the case of clays, gas is prevented from escaping from the soil into the water column because of the low permeability of clay soils. Over time a gaseous soil or a soil forms containing thin layers of methane gas, which can lead to the formation of gas hydrates under appropriate temperature and pressure conditions. Trying to measure and understand the effects of methane gas on the compressibility behaviour of clay soils using natural samples from the field is confronted by various challenges.