Lucefaccion

1723

On site characterization and air content evaluation of coastal soils by image analysis to estimate liquefaction risk
` P. Breul, Y. Haddani, and R. Gourves

Abstract: Coastal structures are often submitted to intense wave forcing. In some cases, structures may have stability disorders due to the constant weakening of their foundations and to momentary liquefaction of the sea bed.Studies have shown that if classical geotechnical characterization is a necessity, air content in the soil is also a key parameter for liquefaction evaluation. That is why on site air content measurement and its time variation during a tide period may provide information and help to determine a better understanding of this problem. Unfortunately, this parameter is difficult to measure duringinvestigations. This article presents a technique based on the use of geoendoscopy and automatic image analysis, which makes it possible to characterize coastal soils and to estimate their air content. After a description of the technique, the results obtained on laboratory tests and on a real site are presented. Key words: geoendoscopy, image analysis, liquefaction, air content, on site soilcharacterization. ˆ ` ` ˆ ´ ´ Resume : Les structures cotieres sont soumises a la sollicitation de la houle. Ces sollicitations cycliques peuvent etre inˆ ´ ` ´ tenses et entraıner parfois des instabilites dues a des pertes de portance du sol sur lequel reposent les structures par la crea´ ` ´ ´ ´ ´ ` ´ tion de phenomenes de liquefaction locaux. Des etudes ont montre que l’apparition de ces phenomenes etaitfonction ´ notamment de la teneur en air occlus dans le sol. C’est pourquoi, la mesure de la teneur en air in situ et de son evolution ´ ´ ´ ´ ` durant un episode de maree peut permettre de comprendre ou d’apporter des pistes de reflexion pour resoudre le probleme. ´ ` ˆ Or cette caracteristique est a l’heure actuelle difficilement mesurable in situ ou en laboratoire et difficilement maıtrisable ´ ´lors de la realisation d’experimentation. C’est pourquoi la recherche de techniques permettant de mesurer ou d’estimer la ´ ´ teneur en air occlus d’un sable est importante. La technique proposee ici s’appuie sur l’utilisation de la geoendoscopie et ´ ´ ´ ` de l’analyse d’images. En effet, les essais realises avec cette technique ont montre qu’il est possible d’identifier tres claire´ ´ ` ment lapresence d’air au sein du materiau et de venir effectuer une mesure a partir des images acquises. Cette communi´ ´ ´ ´ ´ ´ cation presente le contexte de l’etude, la technique utilisee puis les essais realises aussi bien in situ en conditions reelles ´ qu’en laboratoire et les resultats obtenus. ´ ´ ´ ´ Mots-cles : geoendoscopie, analyse d’images, liquefaction, teneur en air, caracterisation dessols in situ.

Introduction
Coastal structures often experience wave forcing. In some cases, these cyclic stresses may be intense and involve instabilities by the creation of the local phenomenon of liquefaction. Theoretical models have highlighted this phenomenon (Mei and Foda 1981; Sakai et al. 1992). According to many authors (Seed and De Alba 1986; Rad and Lunne 1994; Rad et al. 1994;Robertson and Wride 1998; Grozic et al. 1999, 2000), the apparition of the liquefaction phenomenon is conditioned by the characteristics of the stress and also by the nature and the state of the soil. A parameter called the potential of liquefaction has been deReceived 20 September 2006. Accepted 22 August 2008. Published on the NRC Research Press Web site at cgj.nrc.ca on 10 December 2008. ´ ` P.Breul1 and R. Gourves. LGC – C/U/S/T Universite Blaise ´ Pascal – Clermont II. Campus Cezeaux. 24, av. des landais, BP ` 206, 63,174 Aubiere CEDEX France. Y. Haddani. Sol Solution ZA des portes de RIOM, 63200 Riom, France.
1Corresponding

author (e-mail: breul@lgc.univ-bpclermont.fr).

fined that expresses the ability of a ground to reach this state. It is based on the knowledge of the cyclic...