Determination of the shear strength parameters of an unsaturated soil using the direct
J. K. M. GAN
Minconsult Sdn. Bhcl., 14 Jalan 20/16A, Paratnount Gc~rdetz, 46300 Peralitlg Jnyn, Selrngor, Malaysin
D. G. FREDLUND
Depclrtmetzt of Civil Etzgitzeeritzg, Utziversity of Sc~skelrcke~~an,
Saskatoon, Sr~sk., Carlacla S7N OW0
Accepted March 17, 1988
Multistage direct shear tests have been performed on saturated and unsaturated specimens of a compacted glacial till.
ventional direct shear apparatus was modified in order to use the axis-translation technique for direct shear tests on unsaturated
soils. The soil can be subjected to a wide range of matric suctions. The testing procedure and some typical results are pre-
sented. Nonlinearity in the failure envelope with respect to matric suction was observed. Suggestions are made as to how best
to handle the nonlinearity from a practical engineering standpoint.
shear strength, unsaturated soils, negative pore-water pressures, soil suction, direct shear.
Des essais en cisaillement direct
Ctapes multiples ont CtC rCalisCs sur des spkcimens saturCs et non saturCs de till glaciaire
compactC. Un appareil conventionnel de cisaillement direct a CtC modif6 de fa~on
utiliser la technique de translation d'axe
pour des essais de cisaillement direct sur des sols non saturCs. Le sol peut &tre assujetti
un large domaine de succion de
matrice. La procCdure de I'essai et quelques rksultats typiques sont prCsentCs. Une non-IinCaritC de l'enveloppe de rupture en
fonction de la succion de matrice a CtC observCe. Des suggestions sont faites quant
la meilleure fa~on
de tenir compte de cette
non-IinCaritC du point de vue de I'ingCnieur.
rksistance au cisaillement, sols non saturCs, pressions interstitielles nkgatives, succion du sol, cisaillement direct.
[Traduit par la revue]
Can. Geotech. J.
Most man-made earth structures involve the use of com-
pacted soils. The compaction process produces a soil with a
degree of saturation usually in the range of 75 -90%. Earthfill
dams, embankments, and highways are typical examples of
earth structures made of compacted, unsaturated soils. The
theory and measurement of shear strength of unsaturated soils
have gained increasing attention during the past three decades.
A brief review on the development of our understanding of the
shear strength behavior of unsaturated soils is presented in this
This paper presents the results of a series of direct shear tests
on an unsaturated soil. Direct shear testing of unsaturated soils
is desirable since less time is required to fail the soil specimen
than when using the triaxial test. The time to failure in the
direct shear test is greatly reduced because the specimen is
relatively thin. The lengthy testing period for unsaturated soils
is due to the low coefficient of permeability of the soil and the
high air entry disc in contact with the specimen.