3005
Determination of shear strength of MSW. Field tests vs. laboratory tests.
Détermination de la résistance au cisaillement des déchets urbains (MSW). Essais in situ
vs
essais
de laboratoire.
Cañizal J., Lapeña P., Castro J., Costa da A., Sagaseta C.
University of Cantabria. Santander, Spain
ABSTRACT: The knowledge of the mechanical properties of MSW has many implications on landfill design. The shear strength of
the wastes determines the inclination to be given to the landfill slopes, which in turn governs the landfill capacity.
The measurement of these properties is not an easy task. Different approaches are possible. Conventional geotechnical laboratory tests
face two main problems: the lack of representativeness of the samples, and the environmental difficulties associated to testing these
materials in laboratories usually placed in general use buildings (odour problems). Due to these limitations, the number of published
results is relatively short. The use of in situ tests has become an attractive alternative, because it eliminates the above two problems.
However, the interpretation of these tests is based on the experience with conventional geotechnical materials (soils, rocks, compacted
fills), but MSW have a different behaviour, with a seemingly unlimited strain, and no definite “failure” in most cases. A third
approach is the back-analysis of the monitored behaviour of actual landfills. Some of these cases have been published, and even some
compilations are available.
RÉSUMÉ : La connaissance des propriétés mécaniques des déchets urbains (MSW) a de nombreuses implications quant à la
conception d’une décharge. La résistance au cisaillement des déchets détermine l’inclination des talus, qui se répercute sur la capacité
de la décharge.
La détermination de ces propriétés n’est pas facile, différentes approches sont possibles. Les essais conventionnels de laboratoire
présentent deux problèmes : le manque de représentativité des échantillons et les difficultés associées aux aspects environnementaux
du fait que les laboratoires sont généralement situés dans des bâtiments à usage général (problèmes olfactifs). À cause de ces
limitations, le nombre de résultats publiés dans la littérature est relativement faible. La pratique d’essais in situ est devenue une
alternative intéressante, car elle élimine les deux problèmes ci-dessus. Cependant, l’interprétation de ces essais est basée sur
l’expérience tirée des matériaux géotechniques conventionnels (sols, roches, remblais compactés), mais les déchets ont un
comportement bien différent, avec une déformation apparemment illimitée et sans indice précis de “rupture”. Une troisième
possibilité repose sur l’analyse inverse du suivi de cas réels instrumentés. Quelques cas ont été publiés, et certaines compilations sont
disponibles.
KEYWORDS: MSW, municipal solid wastes, landfill, site exploration, mechanical properties, shear strength.
1 INTRODUCTION
The stability of a landfill relies on the shear strength of its
elements. It depends on the characteristics of the waste
materials disposed in it, as well as in the characteristics of the
materials that form the protection, isolation and sealing layers.
As far as the waste material is concerned there are several
factors influencing the strength characteristics such as
composition, age, confining pressure, details of landfill
operation, existence of soil layers as waste cell coverage, etc. In
any case the shear strength of the wastes determines the
inclination to be given to the landfill slopes, which in turn
governs the landfill capacity. The necessity of establishing
berms at mid-height of the slopes has also an important role in
the capacity.
Municipal Solid Wastes (MSW) have some special
characteristics making a clear distinction with soils in terms of
behaviour. However, in landfill stability analyses the behaviour
of MSW is usually based on models derived from soils, mainly
the Mohr-Coulomb failure criterion, defined by two parameters:
cohesion (c) and friction angle (
).
Strength parameters of MSW can be obtained testing
samples in the laboratory or conducting in-situ tests. In addition
to these methods there is also a third way to obtain parameters
using back-analysis of real scenarios, most of the times from
landfill failure cases.
Both laboratory and in-situ tests, in their different variations,
have several advantages and disadvantages, due to that, a pros-
and-cons analysis of the different methods using the existing
bibliography was conducted, considering their methodology,
operative issues, reliability and repeatability of the results to
determine the test procedure that fits best to our purposes.
2 OVERALL STRENGTH CHARACTERISTICS OF MSW
MSW show some overall strength characteristics that are
reflected in almost all the existing bibliography. They can be
summarized as follows (Bray et al., 2009; Stark et al., 2009):
As a general trend, MSW shear strength increases with the
average confining pressure in a nonlinear way, and the slope
of the shear strength envelope decreases with the level of
normal stress. For very low confining pressure, there is
some strength provided by the fibrous material contained in
the waste, giving rise to an equivalent cohesion.
Fitting this non-linear strength envelope with a linear Mohr-
Coulomb criterion line must be done for the range of
interest of normal stress, and the values of frictional angle
and cohesion have to be defined accordingly.
Test results are influenced by test conditions and sample
preparation.
Within the usual ranges, variations in density do not
produce large changes in MSW strength.
Degradation and aging seem to have an important effect on
the shear strength, decreasing the cohesive term and
increasing the friction.
