Evaluation of preconsolidation pressure and the overconsolidation ratio from pi

Evaluation of preconsolidation pressure and the overconsolidation ratio from piezocone tests of clay deposits in Quebec Denis Demers and Serge Leroueil Abstract: The preconsolidation pressure and overconsolidation ratio profiles are the most important factors related to the mechanical behaviour of clay deposits. They are interpreted on the basis of a limited number of laboratory tests, but the near-continuous information provided by the piezocone can allow local data to be extrapolated to an entire site. Numerous methods have been proposed for relating piezocone data to preconsolidation pressure and the overconsolidation ratio, but their validity is still uncertain. In this paper, nine methods are compared on the basis of data collected on 31 sensitive clay sites in Quebec whose preconsolidation pressure profiles are well known. Each method is briefly presented and the most promising are identified. The simplest method, which directly relates preconsolidation pressure to net tip resistance, also appears to be the most effective. Key words: clay, preconsolidation pressure, overconsolidation ratio, piezocone, correlations. Résumé : Les profils de pression de préconsolidation et du rapport de surconsolidation constituent les éléments les plus importants en ce qui concerne le comportement mécanique des dépôts argileux. Leur évaluation se fait par un nombre li- mité d’essais de laboratoire, mais les données fournies par le piézocône de manière presque continue peuvent permettre une extrapolation à l’ensemble du site de ces données locales. De nombreuses méthodes ont été proposées pour corréler les données du piézocône et la pression de préconsolidation ou le rapport de surconsolidation mais leur validité est encore incertaine. Dans le présent article, neuf méthodes sont examinées à partir de données provenant de 31 sites d’argile sen- sible de la province du Québec pour lesquels les profils de pression de préconsolidation sont bien connus. Chaque mé- thode est brièvement présentée et les méthodes les plus intéressantes sont identifiées. Il semble que la relation entre la pression de préconsolidation et la résistance en pointe nette, qui est la plus simple, est aussi la plus efficace. Mots clés : argile, pression de préconsolidation, rapport de surconsolidation, piézocône, corrélations. Demers and Leroueil 192 Introduction The preconsolidation pressure ( ′ σp) and the over- consolidation ratio (OCR) are fundamental characteristics of clays used for geotechnical design and for determining the behaviour of structures on clay deposits. They are generally obtained by laboratory oedometer tests on “undisturbed” soil samples. A variety of constraints are associated with the ex- act determination of ′ σp. Firstly, various laboratory tests give different results (Leroueil et al. 1983), which are not always representative of in situ soil behaviour. Secondly, the major problem is soil disturbance during sampling (LaRochelle et al. 1981; Lefebvre and Poulin 1979), which causes results to be scattered, tending to make them less valuable. In addi- tion, the number of tests carried out for a given project is al- ways limited, because performing them is a long, costly process. As a result, it is often difficult to accurately define a complete ′ σp profile for an entire deposit. Considering these difficulties, the committee of experts set up by the Société d’Énergie de la Baie James (SEBJ, Quebec) recommended using the field vane as a reference for normalizing preconsolidation pressure profiles (Paré et al. 1983). The piezocone, which provides near-continuous measure- ments of tip resistance (qc), sleeve friction (fs), and pore pressure (u), sometimes at different positions simulta- neously, appears to be an extremely promising tool for inter- polating or extrapolating local ′ σp values or for providing approximate ′ σp and OCR values for preliminary design (Lunne et al. 1997). Numerous correlations, mostly empiri- cal, have been proposed for determining these two parame- ters from separate or combined piezocone measurements, or by using information such as total vertical stress (σvo), effec- tive vertical stress ( ′ σvo), and initial pore pressure (uo). The results of these studies, several of which will be reviewed Can. Geotech. J. 39: 174–192 (2002) DOI: 10.1139/T01-071 © 2002 NRC Canada 174 Received 21 December 2000. Accepted 1 August 2001. Published on the NRC Research Press Web site at http://cgj.nrc.ca on 20 February 2002. D. Demers.1 Ministère des Transports du Québec, Service géotechnique et géologie, 930, chemin Sainte-Foy, 5ème étage, Québec, QC G1S 4X9, Canada. S. Leroueil. Département de génie civil, Université Laval, Québec, QC G1K 7P4, Canada. 1Corresponding author (ddemers@mtq.gouv.qc.ca). here, tend to show that no unique relationship exists and that only regional correlations are valid. The goal for the authors is to determine the best approach for eastern Canadian clays. This study is based on a database of soundings from 31 different clay sites and 153 sounding locations in Quebec, and seeks to assess the suitability of the main correlations proposed in the literature for interpreting preconsolidation pressures and overconsolidation ratios. Contrary to the situa- tion in several other studies, all tests, except those at Broadback B6 (Paré 1983) and Saint-Jean-Vianney (Zebdi 1987), were supervised by the same person and conducted by a very small group of operators, consistently using the same methodology, in clays under very similar geological conditions. This eliminates several sources of error and vari- ation in the results, which usually creates uncertainty about the validity of the proposed correlations. Database The 31 sites considered in this study are all located in sen- sitive clay deposits in Quebec (Fig. 1), but in different sedi- mentary basins, all of which are postglacial (12 000 years or less BP). The clays in southern Quebec are marine deposits formed between 12 000 and 8500 years BP. Those in north- ern Quebec are either glacial lake varved deposits about 8500 years old (Matagami site), or marine deposits formed between 8000 and 5000 years BP (Broadback and Grande-Baleine sites). At the Matagami site, the grain size of the clay and silt interlayers shows light contrast, hence the presence of varves does not affect the correlations with qt or u, where qt is the corrected total tip resistance. The geographical distribution of the sites makes the resulting da- tabase representative of most sedimentary environments in Quebec and probably in eastern Canada (Quigley 1980). At 3 of the 31 sites studied, there were embankments beneath which detailed investigations were also carried out. The laboratory tests for this study were done, for the vast majority of sites, either by the Ministère des Transports du Québec and its mandataries or by the Groupe de géotechnique of Université Laval, Quebec. In a few well-documented cases, data from Université Laval M.Sc. or Ph.D. theses were used. The samples were taken using a stationary piston sampler 76 mm in diameter or with the large-diameter (200 mm) sam- pler developed at Université Laval (LaRochelle et al. 1981). The main geotechnical characteristics are summarized in Table 1. The plasticity index (IP) ranges from 5 to 48%, the li- quidity index (IL) is generally greater than 1.0 but can attain values as high as 4.7, and the clay fraction (particles less than 2 µm) varies between 20 and 90%. Sensitivity (St) ranges vary widely, with values between 10 and 1000. The determination of the parameter Nkt = (qt – σ v )/Suv, where Suv is the shear strength measured with the field vane test; was based on cor- relations with field vane tests. Its values range between 9 and 19 overall, but fall between 11 and 16 in most cases and aver- age 13.7 independent of the plasticity index. The preconsolidation pressure ( ′ σp) was interpreted using the Casagrande method by conventional oedometer tests using a daily load (every 24 h) increment of 0.5. The values obtained range from 38 to 940 kPa. At each site, piezometric readings were used to calculate the effective vertical stress ( ′ σvo) and then to deduce the overconsolidation ratio (OCR), which ranged from 1.0 to 28. The friction angle of clay in the normally consolidated range ( ′ φnc) was obtained from CIU (consolidated isotropically undrained) or CAU (consolidated anisotropically undrained) triaxial tests or from direct shear box tests for 18 of the 31 sites. The values of ′ φnc range from 25 to 35°, having a slight tendency to decrease as the plasticity index increases, as Leroueil et al. (1983) have previously observed. In the other cases, a mean value of 30° was assumed. Considering the laboratory data, the sites used cover almost the entire range of geotechnical properties met with in eastern Canadian clays (Leroueil et al. 1983). Given the geographical distribution of the sites, the database used can be considered representative of all conditions encountered in this region. Piezocone tests were done in intact clay at all locations. At the Matagami and Saint-Hyacinthe sites, soundings were also performed through embankments to reach the normally consolidated clay underneath. At Saint-Alban, additional tests were carried out where an old embankment had been levelled eight years before (Saint-Alban (under embankment C), site no. 10, in Fig. 1). The three types of penetrometer used all have an apex an- gle of 60° and a tip area of 10 cm2 (Fig. 2). On the first type (Fig. 2a), i.e., standard Roctest uploads/Religion/ 2002-evaluation-of-preconsolidation-pressure-and-the-overconsolidation-ratio-from-piezocone-tests-of-clay-deposits-in-quebec.pdf

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