Reference:
Zekkos (Zeccos), D. P. 2005. “Evaluation of static and dynamic properties
of municipal solid-waste.” Dissertation, Doctor of Philosophy, Department
of Civil and Environmental Engineering, University of California, Berkeley,
California, USA.
Contact us for a free copy.
Abstract:

EVALUATION OF STATIC AND DYNAMIC PROPERTIES OF MUNICIPAL
SOLID-WASTE
by
Dimitrios Pavlos Zekkos (Zeccos)
Doctor of Philosophy in Civil and Environmental Engineering
University of California, Berkeley
Professor Jonathan D. Bray, Chair

As part of a collaborative NSF-funded research program a comprehensive
investigation of the static and dynamic properties of Municipal Solid-
Waste (MSW) was performed.  The investigation included drilling activities
at the Tri-Cities landfill located in Fremont California, in-situ testing,
sample collection, waste characterization and an extensive, primarily
large-scale, laboratory testing program of municipal solid-waste.  This
thesis summarizes the in-situ testing and drilling operations that were
performed at the Tri-Cities landfill, as well as the waste characterization
of bulk samples collected from the Tri-Cities landfill.  It also presents the
results of the laboratory testing program performed at the University of
California at Berkeley and the University of Patras, Greece.

A comprehensive primarily large-scale triaxial laboratory investigation
was performed at the University of California at Berkeley.  Twenty-nine
large-scale (30 cm diameter, 60-63 cm nominal height) triaxial samples of
MSW were prepared using three waste samples from the Tri-Cities
landfill, varying in age from 2 to 15 years old and sampled from different
depths.  A total of 26 large-scale monotonic triaxial tests have been
performed to evaluate the monotonic stress-strain response of MSW in
triaxial compression, triaxial extension, and triaxial compression-
unloading.  Additionally, more than 80 large-scale cyclic triaxial tests have
been performed to evaluate the dynamic properties (small-strain shear
modulus, normalized shear modulus reduction, and material damping) of
MSW.  The effects of waste composition, unit weight, confining stress,
time under confinement, loading frequency and strain-rate were studied
to evaluate the static and dynamic properties of MSW.  In addition to the
tests performed at UC Berkeley, an extensive large-scale (30 cm x 30 cm
x 18 cm) direct shear testing program was performed at the University of
Patras, using the same waste material, the same specimen preparation
techniques, and with some direction from the UC Berkeley research
team.  

Based upon analysis of available field data and large-scale laboratory
data, most of which have been generated as part of this research study,
a characteristic MSW unit weight profile represented by a hyperbolic
equation was found to exist for individual landfills.  The available data
indicate that MSW unit weight is governed by the waste composition and
compaction effort applied when first placed, and the effective confining
stress currently acting on it, which is largely a function of its current
depth in the landfill.  A hyperbolic model that captures these key factors
in estimating the unit weight of MSW at a specific landfill has been
developed and calibrated with the available field data.  Guidelines for
developing a reliable MSW unit weight profile for a specific landfill under
three likely design scenarios have been provided.  

The MSW Poisson’s ratio was also evaluated during the performance of
cyclic triaxial and monotonic triaxial tests.  The results indicate a Poisson’
s ratio of about 0.3-0.35 for specimens with no fibrous materials.  As the
amount of fibrous materials increases, the total unit weight of the waste
specimen reduces, and Poisson’s ratio reduces to significantly lower
values ranging from 0.3 to possibly as low as zero.  These results are
consistent with findings available in the literature.

The monotonic stress-strain-strength response has also been
investigated from large-scale direct shear and large-scale triaxial tests.  
For the direct shear tests performed in collaboration with Professor
Athanasopoulos of the University of Patras, it was found that the
orientation of the particles is roughly parallel to the horizontal shear
failure surface and for that reason the fibrous materials do not
participate significantly in its shear response.  Additional tests were
performed with the fibrous waste particles oriented perpendicular to the
horizontal failure surface in a direct shear test.  The results suggest a
change in the stress-displacement response, which is attributed to fiber
mobilization.  Thus, the shear strength of MSW is highly anisotropic, and
the angle between the fibrous material orientation and the shear surface
is important.  This observation is consistent with information available in
the literature on direct shear tests performed on reinforced soils, which
have some similarities to MSW and which suggest that the strongest
response is observed in specimens that include fibers in angles of
approximately 60 degrees to the horizontal failure surface.  The
consequences of these observations in practice are significant.  Open
trenches in landfills have shown that the waste material is also
structured in approximately horizontal layers during waste placement.  
Thus, the stress-strain response and the shear strength of MSW in the
field should also be different depending on the orientation of the
potential shear surface and the orientation of the fibers in the waste in-
situ.

An examination of large-scale direct shear test results available in the
literature suggests that the mechanism of shearing parallel to the fibrous
materials that was observed in the direct shear tests of this investigation
on waste from the Tri-Cities landfill is probably also valid for the
previously published test results.  A total of 103 large-scale in-situ and
laboratory direct shear test results from a wide variety of landfills
worldwide exhibited surprisingly small scatter, particularly when the
variations in composition and waste origin are considered.  Regressions
performed at different normal stresses and taking into account the
observed reduction of the friction angle with increasing confining stress
resulted in a non-linear shear strength envelope.  The non-linear shear
strength envelope defined by c = 15 kPa, fo = 36o at σv = 1 atm, and Δf
= - 5o is recommended for use in design practice. It has significant
advantages over previous recommendations.  Limited results from large-
scale simple shear tests yield comparable shear strengths with the
results from direct shear tests, suggesting that large-scale direct shear
tests can be performed for developing a conservative estimate of MSW
shear strength.  

An extensive large-scale triaxial testing program has been performed to
evaluate the effects of unit weight, confining stress, strain-rate, and
composition on the stress-strain-strength response of MSW in triaxial
compression.  The composition of MSW affects significantly the stress-
strain response of MSW.  As the waste composition in fibrous materials
increases the stress-strain response changes, and this change is
attributed to progressive stretching of the fibrous materials.  Because of
the effects of strain incompatibility and to avoid excessive deformations
within a landfill, the use of the peak shear stress measured in very large
strains in triaxial testing is not warranted.  The strain-dependent failure
criterion that has often been used in the practice to define the shear
strength of MSW in triaxial testing was investigated.  The shear strength
of MSW in triaxial compression was defined to be the mobilized shear
stress at K = 0.3 + 5% strain, and the monotonic triaxial compression
tests performed on waste from Tri-Cities landfill indicate a mobilized
friction angle of about 42 degrees.  Similar analyses were performed for
large-scale triaxial data reported in the literature.  The scatter in the data
appears to reduce significantly at larger strains, and for the shear
strength criterion defined previously, the data yield friction angles
between 34 and 44 degrees with a value of 39 degrees being
representative of the data.  The larger friction angles observed in triaxial
compression are reasonable when one considers that shearing occurs at
an angle of approximately 45 + φ/2 = 65 degrees to the horizontal plane,
which is also the plane of particle orientation.  By examining the strain
rate effects on the stress-strain response of MSW it was found that the
dynamic shear strength of MSW is higher than the static shear strength
by a factor of 1.2.

An extensive laboratory cyclic triaxial testing program that included more
than 80 cyclic triaxial test series was performed to evaluate the effects of
different factors on the dynamic properties of MSW.  The effects of
specimen composition, unit weight, confining stress, loading frequency,
and time under confinement on the small-strain shear modulus, the
normalized shear modulus reduction relationship and the strain-
dependent material damping were studied.  The small-strain shear
modulus was found to be affected by all the above parameters, but the
effect of waste composition and confining stress were found to be the
most important.  In terms of the strain-dependent shear modulus
reduction and material damping curves the most important factor is
waste composition.  Confining pressure was also found to have an
important effect.  Normalized shear modulus reduction and material
damping curves were recommended for MSW based on waste
composition and confining stress.

Additional monotonic compression tests accompanied with cyclic triaxial
tests were performed to develop the dynamic back-bone curve, and then,
the shear modulus reduction curve.  In addition, using the Masing criteria,
estimates of material damping were made which were, generally, similar
to the estimates of material damping from the cyclic triaxial tests.

Results of the large triaxial specimens were in agreement with triaxial
tests performed on small (71 mm diameter) triaxial specimens that
included particle sizes that were 100% smaller than 20 mm in size.  
Additional tests were performed in small triaxial specimens using
processed waste and the results indicate that the dynamic properties of
processed waste, are qualitatively similar, but not the same in many
aspects.  
"EVALUATION OF STATIC AND DYNAMIC
PROPERTIES OF MUNICIPAL
SOLID-WASTE"