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Technical Committee 215 /
Comité technique 215
3.2 In situ tests
3.2.1 Comparison with laboratory tests
In-situ tests are an alternative to the execution of laboratory
tests on landfill samples. With in-situ tests there is no need to
take and manipulate samples, with the subsequent alteration,
very high when dealing with MSW. In-situ tests are made over
the material in real conditions, not in a simulated laboratory
scenario.
Besides, scale is larger in field tests, affecting more
material. This bigger scale reduces the influence of MSW
heterogeneity, making possible to take into consideration
medium to large fibrous elements. However, these advantages
over laboratory tests bring some additional problems:
Although the alteration produced by taking the sample is
removed, effects produced by the installation of the testing
elements appear.
Field tests control (stress state, displacements, drainage) is
lesser than in laboratory tests.
Even though the area affected by field tests is larger than the
regular specimen size, scale problems are still present.
Results obtained from some in-situ tests cannot be analysed
using theoretical models to obtain strength parameters, the
only way to obtain them are using empirical correlations.
Interpretation complexity is higher for in-situ tests in
comparison with those conducted in a laboratory. If the
theoretical model depends on two or more parameters, like
Mohr-Coulomb failure criterion, it is only possible to obtain
the relationship between them. This implies that only a
curve for different possible values for cohesion and
frictional angle can be obtained.
In any case, most of the in-situ test procedures are quite fast
and economical, making possible to execute multiple tests in a
reasonable period of time and covering a large volume of
material, which is a clear advantage over laboratory tests.
3.2.2 Test types
The in-situ tests commonly used in MSW are: penetration test,
plate loading tests, pressuremeter tests and in-situ shear tests.
Penetration tests, both dynamic (DPSH, DPH, SPT) and
static (CPT, CPTU) provide an index value for MSW strength,
and from these indexes it is possible to empirically obtain
strength parameters and other characteristics. Their main
advantages are their easy usage and their low time and means
consumption, as well as the possibility to check different
penetrations in time and space to establish tendencies for the
variation of the resistance to penetration.
Use of penetration test for landfill characterization is
frequent, being one of the pioneers Sowers (1968) who used
dynamic tests. The University of Cantabria (UC) Geotechnical
Group has researched about the strength characteristics of
landfills using dynamic and static penetration tests (Palma,
1995; Sánchez et al., 1993). In a recent research, Zhan et al.
(2008) used, among others, static penetrometers.
Furthermore, the interpretation of plate loading tests is not
as straightforward as in soils, due to the heterogeneity of the
landfill. It is advisable to use large diameter plates (>600 mm),
which is feasible because there is no need to apply large loads in
order to produce the needed deformations or even to reach
failure due to the soft nature of the MSW.
Several researchers have used this method for deformability
and strength characterization of MSW. The UC Geotechnical
Group (Palma, 1995; Sánchez et al, 1993) used load plates,
interpreting the results using a multi-layer model for MSW and
covering layers. In some occasions the rigid plate has been
replaced by a container full of material, achieving larger size
but lower pressure.
Pressuremeter tests, both with previous borehole execution
and using self-boring systems have been recently used in
landfills (Dixon et al., 2006).
There are several experiences with in-situ shear tests using
parallelepipedic and cylindrical samples with sizes of 500 mm
and even 1m in landfills (Withiam et al., 1995; Caicedo et al.,
2002).
3.3 Back-analysis of real failures
Failure back-analysis is a widely used method in geotechnical
activity and can be easily extrapolated to the study of MSW
shear strength (Huvaj-Sarihan and Stark, 2008).
However, this method faces also some uncertainties. First,
generalized failure cases are not frequent, and in the few cases
occurred it is difficult to detect the failure surface. Besides,
failure generally affects to the bottom sealing layers and the
foundation ground as well as to the waste mass itself. In other
cases the situation analysed is far from failure so a safety factor
greater than the unit has to be assumed with no precise
justification. Furthermore, the values for material density and
phreatic level position are not known exactly and must be
estimated. In any case, if the Mohr-Coulomb failure criterion is
used, the result of the analysis is only a relationship between
cohesion and frictional angle as in some in-situ tests. Only in
very few cases, the precise knowledge of the sliding surface
position can provide some guidance about the relative ranges
for the two parameters. Otherwise, the result is a line plotted in
a c-
diagram. This diagram must be used with care, because it
does not mean that all the points on the line are valid, but
instead, only one point is the correct result, but it is not possible
to identify it within the whole line (Figures 3 and 4).
av
'
av
tan
=
av
/
'
av
c = 0
= 0
c =
av
Normal effective stress,
'
Shear stress,
Figure 3. Mohr’s plane of the results of a back-analysis
?
Friction, tan
Cohesion,
c
Figure 4. c-
diagram of the results of a back-analysis
4 MOHR-COULOMB STRENGTH PARAMETERS
PROPOSAL
Although the research in strength parameters dates back more
than two decades, the special characteristics of MSW limit the
obtained results. In several publications a compilation of
parameter values is shown, but they do not only refer to test
results, it also does to representative values deduced by the
authors of other previous compilations and to values
successfully used in particular cases of landfill design. Besides,
the available results belong to different test type and
methodology, carried out on MSW of different composition,
age, density, etc. Furthermore, due to strain hardening
behaviour, different values can be established for the same test
according to the deformation level considered as critical.
