In 1998, a gas emanation occurred in the Gulf of Mexico in an area with shallow gas accumulations, so soil properties were evaluated through two site investigations carried out in 1998 and 2002. The 2002 study was a site investigation that explored both the geophysical and geotechnical aspects of the effects of the gas emanation on the foundation soil properties for four boreholes. Around the site, geotechnical studies in 1978, 2008 and 2011 were conducted for foundation design of platforms. First, it was found that the water content and unit weight of the clay and sand decreased with the presence of gas. Second, the shear strength of the clay could decrease or increase with the presence of gas, similar to what was discovered in Nava (2010), while in sand a decrease could occur.

1. Introduction

One of the most complex phenomena that occur in the marine sediments is the sudden or gradual expulsion of gas from shallow accumulations. It becomes problematic when it occurs next to a platform and alters the properties of the foundation soils, as it has been shown to occur by several researchers (Whelan et al., 1977; Nageswaran, 1983; Wheeler, 1986; Rad et al., 1994; Nava, 2010). This results in the bearing capacity of the foundation soils being affected. In the investigation area, a gas emanation occurred in 1998 next to Platform A. As a result, special marine investigations were performed in 1998 and 2002 to evaluate how the foundation soil properties changed after the gas emanation (Nava and Barrera, 2000; Koh, 2005; Nava et al., 2011). This paper compares the results from the 1998 and 2002 studies, with the results from geotechnical investigations performed in the investigation area before the gas emanation occurred and after the 2002 study.

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