Effect of Strain Rate, Sampling Technique and Aging on the Soil Properties Used in the Reassessment of Pile Foundations Performance in the Bay of Campeche
Authors: Valle C. , Barrera P. , Taboada V. 
Published: October 2008

After the Hurricane Roxanne passage over the Bay of Campeche in 1995, a reassessment and requalification of the offshore infrastructure was required. In order to represent better the real behavior of the platforms under current, wind and wave forces for the reassessment, effects such as soil sampling, strain rate and aging on the soil properties were incorporated in the analysis. The use of static parameters of the soil is an assumption 'very conservative for the analysis of reassessment of existent structures and for better geotechnical characterization the effect of strain rate, sampling technique and aging are considered. The consideration of these effects on the properties of soil were evaluated in IS sites. The laboratory results for soils with strain rate of 10–4 % hour, which corresponds to the wave forces, show a mean value of increment of 35% in the shear resistance of the soil. The push sampling technique showed an increase of 20% in Su versus the wire line technique. For including aging effect, triaxial and direct shear testing is being carry out on samples of different sites of the Bay of Campeche. The reassessment of the offshore structures is optimum if the analysis includes the mentioned effects since the assumptions of the geotechnical characteristics are more realistic.

Introduction

Offshore facilities in the Bay of Campeche were subjected to hurricane storm loadings during the passage of hurricane Roxanne in 1995. The oceanographic conditions developed were similar to those predicted for a 100 year return period. Due to that event, a reassessment program of the offshore platforms in the Bay of Campeche was initiated. In some cases, the reassessment of the marine platforms involved analytical methods for evaluating the ultimate limit state of the structures. Analyses were carried out using an ultimate limit-state limit- equilibrium analytical model, which is based on plastic-hinge theory and limit equilibrium (Bea et al, 1997). The yield strength of structural elements, the elastic modulus of steel and the soil characteristics have to be defined by the user. The depth variation of the undrained shear strength (for cohesive soils) or the angle of internal friction (for cohesionless soils) are considered, along with the bias and coefficient of variation of the pile capacity subjected to axial and lateral loads. Bias is defined as the ratio between true and computed values. The uncertainties in the parameters considered in the geotechnical problems are thus identified and assessed. Sources of bias in pile capacities may be introduced by the procedure used to (Bea, 1983; Tang, 1988; Tang, Gilbert, 1990):perform the soil boring;sample;perform the testing (in situ, laboratory);define the pile -loads;analyze the individual pile and pile system performance; anddefine the age of the pile at the time that the capacity is to be evaluated.

In this paper, only points 2, 3 and 7 are addressed for the Bay of Campeche conditions. It is important to notice that if each of the above sources introduced a bias of 10%, the total bias in the pile capacity would be equal to 2.