HYDRAULIC AND CONTAMINANT TRANSPORT PERFORMANCE OF COMPACTED LATERITIC SOIL– BENTONITE MIXTURES PROPOSED AS WASTE CONTAINMENT BARRIER
HYDRAULIC AND CONTAMINANT TRANSPORT PERFORMANCE OF COMPACTED LATERITIC SOIL– BENTONITE MIXTURES PROPOSED AS WASTE CONTAINMENT BARRIER
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Date
2008-09
Authors
AMADI, AGAPITUS AHAMEFULE
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Abstract
Contamination of the geoenvironment due to unsafe waste disposal practices and leakage
from waste repositories is a well-known and widespread problem. Emphasis has therefore
been put on the hydraulic properties and contaminant transport capability of materials
proposed for use as barriers in such facilities. Consistent with the current approaches for
evaluating clayey soil liners, data on geotechnical, hydraulic and contaminant transport
performance in addition to other relevant engineering properties of lateritic soil associated
with bentonite treatment were developed and analyzed. Evaluation of lateritic soil - bentonite
mixtures with bentonite content up to 10% has been carried out with respect to hydraulic
conductivity, unconfined compression strength, volumetric shrinkage, unsaturated flow
behaviour, compatibility with the municipal solid waste MSW leachate at the various
compaction states (dry, wet and optimum water content) and compactive efforts (reduced
British Standard Light, British Standard Light, West African Standard or ‘intermediate’ and
British Standard Heavy). The diffusion properties of inorganic chemical species in MSW
leachate through lateritic soil – bentonite mixtures has been characterized in terms of
effective diffusion coefficients. Results indicate that unconfined compressive strength
together with hydraulic conductivity of compacted samples of the mixtures decreased with
increase in the amount of bentonite while volumetric shrinkage strain increased with increase
in bentonite content for all the efforts. Acceptable ranges of compaction water content with
the corresponding dry unit weight that will provide best performance with respect to the
design parameters were delineated in an envelope on the moisture – density plane for the
various soil - bentonite mixtures. At the various dosage levels of bentonite, the unsaturated
hydraulic conductivity computed using the close form solutions of van Genuchten – Mualem
(VGM) and Brooks and Corey - Burdine (BCB) models was essentially the same for
specimens prepared either dry or wet of optimum water content. Hydraulic conductivity tests
carried out to evaluate compatibility of specimens with MSW leachate showed increase in
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values by a factor of 1.2 to 1.7 at the end of test duration. These increases from a practical
standpoint did not jeopardize the use of the mixtures since hydraulic conductivity at the end
of the test was still below 1.0 x 10-9 m/s for mixtures that hitherto met this threshold value.
Adsorption of metals followed the expected trend of increased metal adsorption
characteristics with higher bentonite content. For the range of conditions examined, K cations
were found to diffuse faster than the other two cations. The effective diffusion coefficients of
the cations were found to be in the order of K+ > Na+ >Pb2+ and diffusion was seen to
decrease as the bentonite content increased for all the cations.
Evaluation of the influence of model parameters on the reliability associated with a laboratory
based hydraulic conductivity model (a dominant failure mode for clay liner materials) for
lateritic soil bentonite mixtures showed that reliability index decreased with increasing
coefficient of variation of initial saturation, plasticity index and clay content. Both normal and
lognormal probability distribution functions of hydraulic conductivity yielded approximately
similar trends in reliability – coefficient of variation relationships. The change in reliability
levels for variability in these parameters suggests that effective quality control measures must
be put in place during construction in order to achieve the specified values for these variables.
The result of this study highlights the capability of compacted lateritic soil – bentonite mixtures
to function effectively both as hydraulic and contaminant barrier when used alone or in a
composite construction in waste containment applications. It is expected to serve as a valuable
tool for designers, researchers and regulators as information provided in this work may be
helpful for a better understanding of soil materials of similar physical and engineering
characteristics to be used as hydraulic barriers.
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Description
A DISSERTATION SUBMITTED TO THE POST GRADUATE SCHOOL
AHMADU BELLO UNIVERSITY, ZARIA
IN PARTIAL FULFILMENT OF THE
REQUIREMENTS FOR THE AWARD OF THE DEGREE OF DOCTOR OF
PHILOSOPHY (Ph.D) IN CIVIL ENGINEERING
DEPARTMENT OF CIVIL ENGINEERING,
FACULTY OF ENGINEERING
AHMADU BELLO UNIVERSITY, ZARIA
NIGERIA.
SEPTEMBER, 2008
Keywords
HYDRAULIC,, CONTAMINANT TRANSPORT,, PERFORMANCE,, COMPACTED LATERITIC,, SOIL– BENTONITE MIXTURES PROPOSED,, WASTE CONTAINMENT BARRIER.