Predictive Equations of Shear Wave Velocity for Bay of Campeche Clay

Abstract

Abstract One of the most important soil parameters required in the analyses of the piles of oil platforms subjected to lateral earthquake loading is the shear wave velocity (VS) of the soil. Since in-situ measurements of VS is part of the scope of work of a limited amount of platform sites, there is the need to develop site specific correlations to estimate VS based on basic soil properties. To cover this need a database with in-situ measurements of Vs and basic soil properties of clay has been established. Data were collected from eleven offshore geotechnical investigations performed on behalf of PEMEX for the design and installation of fixed offshore platforms in the Bay of Campeche. The database was tailored to developed empirical correlations between the shear wave velocity and undrained shear strength, effective vertical stress, water content, void ratio, overconsolidation ratio and net cone resistance, using simple regression analyses and multiple regression analyses. Three of the most prominent empirical correlations developed using multiple regression analyses are recommended to provide a mean of determining the best estimate VS in clay, along with a three step procedure to perform site response analyses in the Bay of Campeche when in-situ measurements of VS at a site are not available. Introduction Background The Bay of Campeche is located in the large bay comprising the southern portion of the Gulf of Mexico between the Yucatan Peninsula to the east, the Isthmus of Tehuantepec to the south, and the coast of Mexico at Veracruz to the west. The Bay of Campeche is limited to that section enclosed approximately by 91o W. Longitude on the east to 95o W. Longitude to the west and 20o N. Latitude on the north to the Mexican coast on the south, as indicated by the rectangular area in Figure 1. The Bay of Campeche covers an area of about 15,540 square kilometers, and is the largest oil field in Mexico which has prompted an extensive installation of shallow water fixed-pile platforms. The oil platforms installed in the Bay of Campeche must be designed against earthquake loading since the piles can be subjected to large seismic lateral loadings. The dynamic structural analyses of the oil platforms require acceleration time histories or acceleration spectra that already include the soil amplification of the earthquake motions. One of the most important dynamic soil property input of the seismic response analyses performed to evaluate the soil amplification is the shear wave velocity or the small-strain shear modulus Gmax. Although, there has been important advances in laboratory testing to obtain the small-strain shear modulus (less than 0.0001 percent) Gmax, the adverse effect of unavoidable sample disturbance and reconsolidation, as well as difficulty in quantifying the aging effect on Gmax, renders in-situ shear wave velocity measurements the currently preferred method for evaluating Gmax in-situ. Unfortunately, it is not always possible to make in-situ measurements of shear wave velocity, VS, at all investigated locations, and the geotechnical engineer is left to use empirical correlations developed mainly for onshore soils with different geological settings than those of the Bay of Campeche clay. Thus, to take advantage of the abundant basic soil properties and in-situ measurements of shear wave velocity, correlations between in-situ shear wave velocity and basic soil properties are needed for the Bay of Campeche clay soils. Regional correlation studies of in-situ measurements of shear wave velocity with other index test or engineering properties of the soils have been shown to be useful for estimating shear wave velocity profiles at sites lacking direct in-situ measurements of VS. In an effort to establish a database for the evaluation and modeling of shear wave velocities of clay soil units in the Bay of Campeche, data have been collected from offshore soil investigation performed between 2004 and 2009. The in-situ shear wave velocity data were collected from measurements made with downhole P-S suspension seismic velocity logger. Data from a total of 22 boreholes at eleven sites in the Bay of Campeche are incorporated in this study.