A new approach for loose silty deposits of the Trois-Rivieres area

B. Rousseau, S. Graveline, A. Philibert

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Soil Mechanics

ABSTRACT: on of soil and geotechnical properties A new approach for loose silty deposits of the Trois-Rivieres area B. Rousseau, S. Graveline, A. Philibert Labos S.M. inc,, Sherbrooke, Quebec, Canada ABSTRACT The Trois Rivieres area presents silty soils that may liquefy under seismic loading. In addition, these soils are very sensitive to sampling. Therefore, conventional sampling techniques (SPT) minimize the Nspt index values. That is why, a new methodology was introduced. On the one hand, it consists in using Pressuremeter data to determine the values of geotechnical parameters. On the other hand, it consists in measuring the downhole shear wave velocities, Vs, to assess the Cyclic Resistance Ratio (CRR). The results of this method are encouraging because they can provide the most accurate picture of the areas studied, without threaten the safety of users. RÉSUMÉ La région de Trois-Rivières présente des sols susceptibles de se liquéfier sous une sollicitation sismique. De plus, ces sols sont très sensibles au remaniement. C'est pourquoi, les techniques conventionnelles de prélèvement (SPT), minimisent les valeurs des indices « Nspt ». Ainsi, une nouvelle méthodologie a été mise en place. Elle consiste à, d'une part, utiliser des données pressiométriques afin de déterminer les valeurs de résistances géotechniques et d'autre part, des données de mesures des vitesses des ondes de cisaillement, Vs, afin d'évaluer le Cyclic Resistance Ratio (CRR). Les résultats de cette méthode sont encourageants car ils permettent de dresser le portrait le plus juste des terrains étudiés, sans toutefois, nuire à la sécurité des usagers. 1 INTRODUCTION The seismic characterization of soils of a site is a very important factor in the evaluation of the seismic response of a building: the properties of soils could dramatically change the ground seismic movements by amplifying or reducing them, producing soil liquefaction. Soil liquefaction is a phenomenon which occurs when solid soils under pressure take on a liquefied state causing the ground to move and settle. These problems are very important in today's geotechnical field of expertise, with the adoption of more restrictive laws, specifically in Quebec, since there are a lot of loose potentially liquefiable soils in very populated areas. In addition, the soils under some older buildings (hospitals, schools, governmental buildings) must be studied to ensure they could bear a seismic acceleration of a seismic hazard of 2% in 50 years as the CNB 2010 recommends. In the area of Trois-Rivieres (Quebec), soils are very variable, but there are some areas with silty deposit. This type of soils is very loose and presents very low values of Standard Penetration test (SPT) indexes. In addition, the estimation of the velocity of shear waves in soils, Vs, and ultimate state loads, using SPT would not be relevant. That is the reason why we developed a new approach for both the determination of the mechanical properties of the deposit and the dynamic analysis of soils. To estimate the velocity of shear waves in soils, we used the median of the Vs, given by a downhole measurement of the shear waves velocity, Vs, performed in three boreholes along the studied site. This way to measure Vs is very accurate and quick to apply on field. Thus, we used pressiometer test to characterize precisely the mechanical behavior of the loose silty sand soils and to assess, as precisely as we can the modulus of these soils and the settlements. In this paper, the studied area will be presented, as well as the used methodology and measuring devices. The results of all the technics will be the center point of the potential of liquefaction of soil, which would be compared to the SPT based method. 2 SITE PRESENTATION AND PROBLEMATICS The area studied in this article is located west of the city of Trois-Rivieres (Quebec) and south-west of the 55 and 40 highways crossing (figure 1). The field approximately measures 1 300 m wide by 900 m width. A new building project is plan on, with both commercial and residential units. The site was used for agriculture and there are some trees on it. Figure 1. Localisation of the studied area TF-02-13 TF-03-13 TF-01-13

RÉSUMÉ: faction of soil and geotechnical properties A new approach for loose silty deposits of the Trois-Rivieres area B. Rousseau, S. Graveline, A. Philibert Labos S.M. inc,, Sherbrooke, Quebec, Canada ABSTRACT The Trois Rivieres area presents silty soils that may liquefy under seismic loading. In addition, these soils are very sensitive to sampling. Therefore, conventional sampling techniques (SPT) minimize the Nspt index values. That is why, a new methodology was introduced. On the one hand, it consists in using Pressuremeter data to determine the values of geotechnical parameters. On the other hand, it consists in measuring the downhole shear wave velocities, Vs, to assess the Cyclic Resistance Ratio (CRR). The results of this method are encouraging because they can provide the most accurate picture of the areas studied, without threaten the safety of users. RÉSUMÉ La région de Trois-Rivières présente des sols susceptibles de se liquéfier sous une sollicitation sismique. De plus, ces sols sont très sensibles au remaniement. C'est pourquoi, les techniques conventionnelles de prélèvement (SPT), minimisent les valeurs des indices « Nspt ». Ainsi, une nouvelle méthodologie a été mise en place. Elle consiste à, d'une part, utiliser des données pressiométriques afin de déterminer les valeurs de résistances géotechniques et d'autre part, des données de mesures des vitesses des ondes de cisaillement, Vs, afin d'évaluer le Cyclic Resistance Ratio (CRR). Les résultats de cette méthode sont encourageants car ils permettent de dresser le portrait le plus juste des terrains étudiés, sans toutefois, nuire à la sécurité des usagers. 1 INTRODUCTION The seismic characterization of soils of a site is a very important factor in the evaluation of the seismic response

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Cite this article:
B. Rousseau; S. Graveline; A. Philibert (2014) A new approach for loose silty deposits of the Trois-Rivieres area in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper453,author = B. Rousseau; S. Graveline; A. Philibert,title = A new approach for loose silty deposits of the Trois-Rivieres area ,year = 2014}