591
Analytical approach for determining soil shear strength parameters from CPT
and CPTu data
Approche analytique pour déterminer la résistance au cisaillement d’un sol à partir d’essai CPT et
CPTu
Motaghedi H., Eslami A., Shakeran M.
Dept. of Civil Engineering, Amirkabir University of Technology, Tehran, Iran
ABSTRACT: The common approaches for soil strength parameters determination from CPT data are on the basis of bearing capacity
and cavity expansion theories. A new method is proposed for C,
ϕ
prediction using all quantities, q
c
, u and f
s
from CPTu considering
bearing capacity mechanism of failure at cone tip and direct shear failure along penetrometer sleeve. One advantage of this method is
improvement the accuracy in the case of erroneous data by using all three output of CPTu. Laboratory test results, the two sets of
nonlinear equations by the proposed approach and existing correlations of C and
ϕ
angle parameters have been compared applying on
a data base compiled from four sources. It has been considered that the internal friction angle which is obtained by current methods is
almost relatively higher than the measured values. Also, the comparison indicates good consistency and low scatter for the proposed
method.
RÉSUMÉ: Les approches communes pour les paramètres de résistance des sols, déterminés par CPT, sont basées sur la capacité de
cisaillement et les théories d’expansion des cavités. Une nouvelle méthode a été proposée pour C,
ϕ
et utilise toutes les quantités de
prévision, q
c
, u et f
s
de CPTu, en considérant la capacité de cisaillement et le mécanisme de failure dans type paramide et failure
cisaillement direct, le long du pénétromètre manchon (sleeve). Une des avantages de cette méthode est d’améliorer exactitude dans le
cas des donnés fausse utilisation, tout les trois sortie de CPTu. Les résultats des essais du laboratoire, les deux combinaisons des
équations non linéaires, l’approche proposée et les corrélations existantes de c et l’angle de
ϕ
est comparée appliquée sur quatre bases
de données. On considère que la friction interne obtenue par la méthode courante est toujours relativement plus grande que la valeur
mesurée, aussi la comparaison montre la bonne consistance et le bas scatter pour la méthode proposée.
KEYWORDS: Soil shear strength, Cohesion and friction parameters, CPT and CPTu data, Bearing capacity theory
1 INTRODUCTION
Geotechnical investigation by CPTu provide continuous vertical
profile of cone tip resistance (qc), sleeve friction (f
s
) and pore
water pressure (u
2
) in every inch of the subsoil depth (Lunne et.
al, 1997). The CPTu test is used in soft to medium deposits, and
not applicable in cemented sand, hard clay and gravelly strata.
The penetrometer is a useful tool to identify of thin layers where
the traditional sampling procedures cannot be employed. Also,
using the CPTu test may distinct the liquefiable or collapsible
soil layers around 50 mm thickness in depth (Tavenas and
Leroueil, 1987), (Eslami and Fellenius, 2004).
In alluvial soils containing gas, determining undrained shear
strength by traditional sampling procedures and using UU
triaxial tests may lead to conservative results. In granular soils,
determining the friction angle (
ϕ
) as one of the major soil
strength parameters by using direct shear or triaxial tests
involves uncertainties due to sampling difficulties, confining
pressure simulation and limitations of size effects (Mitchell and
Durgunoglu, 1983). The main advantage of CPTu versus other
in situ test procedures is the relatively elimination of
undisturbed sampling, performance in real condition regarding
stress level and geological aspects. Furthermore, by using the
continuous data in one inch interval of depth, shear strength
parameters (C,
ϕ
), can be obtained which have significant role in
geotechnical designs.
2 SHEAR STRENGTH PARAMETERS BY CPTU DATA
Two main theories have been implemented for the estimation of
shear strength parameters by using CPT and CPTu results;
bearing capacity (Janbu and Senneset, 1974), (Durgunoglu,
1975) and cavity expansion (Vesic, 1972) approaches. The
methods which are based on bearing capacity theories; for
penetrometer penetration mechanism, it is assumed that cone tip
resistance (q
c
) is equivalent with ultimate load of a deep circular
foundation in subsoil and leads the soil mass to be failed.
Whereas, failure assumption in cavity expansion theory is based
on required pressure for forming of deep hole in an elastic-
plastic environment which is fitted with the pressure needed for
creation and cavity expansion in the same volume under
identical conditions. So far, Muromachi, 1972, Schmertmann,
1978, Mitchell and durgunoglu, 1983, Robertson and
Campanella, 1988, Kulhawy and Mayne, 1990 have studied on
determination of shear strength parameters from CPT and CPTu
data which solely have presented S
u
in fine grained or
ϕ
in
granular soils.
3 ANALYTICAL MODEL FOR C AND
ϕ
BY CPTu DATA
By applying two basic equations on determination of the deep
foundation bearing capacity, one for tip and other for
penetrometer sleeve, using the effective bearing capacity instead
of total stress approach and extension of the relationships, a
dual equation system with two unknowns, can be achieved as
below under static loading conditions.
(1)
CN
+ qN
+ 0.5
γ
BN
γ
= q
= q
− u
C +
σ
′
tan 2 3
ϕ
= f
Considering deep bearing capacity factors proposed by
(Junbu, 1974 base failure model) and applying the analytical
Eslami and Fellenius, (1997) model based on CPTu results, the
relations can be summarized as follows:
