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Compressibility Parameters of Cohesive Soils From Piezocone
Paramètres de compressibilité de sols cohésifs au piézocone
Hamza M.
Faculty of Engineering, Suez Canal University & Chairman of Hamza Associates, Egypt
Shahien M.
Faculty of Engineering, Tanta University, Egypt
ABSTRACT: Drained compressibilty parameters for cohesive soils can be determined by carrying out one dimensional consolidation
tests on “undisturbed” samples. The compressibility parameters include the compression and recompression indices,
overconsolidation ratio and coefficient of consolidation. Some of these parameters or in other forms have been already correlated in
the literature to results of piezocone. The aim of this paper is to provide additional data on drained compressibility parameters,
focusing on constrained modulus and overconsolidation ratio, for cohesive soils from geotechnical investigations in seven major sites
of river Nile Delta deposits in Egypt where piezocone CPTU data are also available. The results of consolidation tests are used to
evaluate and modify the available correlations(s) with CPTU data. It is believed that the data and analysis in this paper shall be a
valuable contribution to the literature by providing a better ground for improving the current state of the art of estimating the
compressibility parameters from the CPTU data.
RÉSUMÉ : Les paramètres de compressibilité drainée pour les sols cohérents peuvent être déterminés en exécutant un test de
consolidation unidimensionelle sur les échantillons « intacts ». Ces paramètres incluent les indices de compression et de
recompression, le taux de surconsolidation et le coefficient de consolidation. Certains de ces paramètres ont déjà été corrélés dans la
bibliographie aux résultats du piézocone. L'objectif de cet article est de fournir des données supplémentaires sur les paramètres de
compressibilité drainée en se concentrant sur le module contraint et sur le taux de surconsolidation pour des sols cohérents étudiés
dans sept sites majeurs des dépôts du Delta de Nil en Egypte, où des données de CPTU sont aussi disponibles. Les résultats d’essais
de consolidation sont utilisés pour évaluer et modifier les corrélations disponibles avec les données de CPTU. On estime que les
données et l'analyse présentées ici seront une contribution valable à la bibliographie en fournissant de meilleurs fondements pour
améliorer l’état de l’art actuel concernant l'estimation des paramètres de compressibilité à partir de données de CPTU.
KEYWORDS:
constrained modulus, overconsolidation ratio, sample quality designation, piezocone, clay
1 INTRODUCTION
Drained compressibilty parameters for cohesive soils are useful
in; a) carrying out long term settlement analysis, b) providing
key parameters for analysis and design of ground improvement,
and c) profiling undrained shear strength parameters with the
aid of other insitu field investigation equipments such as field
vane and piezocone.
Drained compressibility parameters for cohesive soils can be
determined from End of Primary (EOP) void ratio versus
effective stress relationship that results from carrying out
incremental load one dimensional consolidation tests on
“undisturbed” samples. The drained compressibility parameters
include the compression and recompression indices,
overconsolidation ratio and coefficient of consolidation. These
parameters can be influenced with variable degrees by quality
of samples used in the tests. (Jamiolkowski et al., 1985 and
Terzaghi et al., 1996). Empirical correlations to estimate these
parameters or equivalent in other forms, from insitu tests such
as piezocone are available in the literature (e.g. Jamiolkowski et
al, 1985, Lunne et al., 1997 and Mayne, 2009). Availability of
such correlations provides a great aid for geotechnical engineers
to estimate such parameters in continuous profiles for a site in
relatively short period of time and perform fewer consolidations
tests for confirmation. However, estimating drained parameters
from undrained piezocone test results could be complicated and
sometimes may have various degrees of uncertainties (Lunne
etl. 1997). Therefore, there is a need for continuous feed of data
from local experiences to confirm, validate, and even modify
the existing correlations.
The aim of this paper is to provide additional data on both
constrained modulus and overconsolidation ratio as determined
from oedometer consolidation tests on “undisturbed” samples of
cohesive soils and CPTU data from seven sites from the Nile
Ddelta deposits. The authors believe that the addition of the data
presented in this paper to the literature provides a better ground
for improving the current state of the art of estimating drained
compressibility parameters from the CPTU data. With such
belief, the data are used to evaluate and modify the available
correlations.
2 INVESTIGATED SITES
Comprehensive geotechnical investigation campaigns were
carried out in seven sites of major projects along the north coast
and within the Delta of the Nile River of Egypt. The seven sites
provide full coverage of the Nile Delta deposits starting from
Idku at west of the Nile Delta, to Metobus within the Nile Delta,
to Damietta, to El-Gamil and Port Said further east of the Delta.
Three of these sites were reported in Hight et al. (2000), Hamza
et al. (2002), (2003) and (2005). The seven sites were used by
Hamza and Shahien (2009) to investigate the correlations of
estimating the efective stress friction angle from piezocone data.
The stratifications of the sites are shown in Fig. (1).
The statification of the sites consists of silty sand top layer
over very soft to medium stiff clay layer over sand over stiff to
hard clay. The thickness of the soft clay layer tends to thicken
as moving from west to east of the Delta (Hamza et al., 2005).
