Professor Jeffrey C Evans Lecture Abstracts
Southeast University
June 2014
Introduction to Slurry Trench Cutoff Walls
This lecture, the first in a series, describes the various techniques to construct vertical
barriers in the subsurface. Applications for these technologies include containment of
contaminants and rehabilitation of dikes and levees. Techniques described include soilbentonite slurry trench cutoff walls (SB), cement-bentonite slurry trench cutoff walls
(CB), and in situ mixed walls (SMW). Methods of design and construction are described
along with material properties.
The TRD Method for In Situ Mixed Vertical Barriers in Dams and Levees
Conventional slurry trench methods of constructing low-permeability vertical barriers
have been widely used in the past but more recently in situ mixing using the Trench
Remixing and Deep wall method (TRD) has been introduced to the US. The TRD
method is a one-phase process that involves the simultaneous, full-depth cutting and
mixing of in situ soils with additives to create, in place, a continuous soil mixed wall. A
blend of cementitious materials (granulated ground blast furnace slag and Portland
cement) and slurry are injected as the milling/cutting proceeds horizontally resulting in
continuous vertical mixing of in situ soils with the injected materials. Details of the
technique will be presented as well as an evaluation of the wall performance at the
Herbert Hoover Dike at Lake Okeechobee, Florida.
Cement Bentonite Cutoff Walls for Polluted Sites
Cement-bentonite (CB) cutoff walls have long been used to control ground water flow
and contaminant migration at polluted sites. Hydraulic conductivity and unconfined
compressive strength are two short-term properties often used by industry and owners in
CB specification and are important parameters discussed in this paper. For polluted sites,
long-term compatibility is also an important issue. These properties are coupled to a
number of external factors including the mix design, construction sequence,
presence/absence of contaminants at the site. Additional short-term properties for
engineering assessment include the stress-strain characteristics in both drained and
undrained shear in both with and without confinement as well as one-dimensional
consolidation properties. Long-term CB properties are affected by aging, reaction
chemistry, drying, in situ stress state, and interaction with the polluted environment.
Evans’ Abstracts
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Improved Modeling of Stresses in Soil-Bentonite Slurry Trench Backfill
Soil-bentonite slurry trench cutoff walls have been widely employed as vertical barriers
as a means to control subsurface contaminant transport and groundwater flow primarily
through the low hydraulic conductivity of the backfill. Research has shown the hydraulic
conductivity of soil-bentonite backfill decreases as the effective consolidating stress
increases. Early research (Evans and Fang, 1985) noted that the state-of-stress within a
soil-bentonite wall is significantly less than would be predicted by a geostatic pressure
distribution. The first quantitative model to predict the state-of-stress (Evans et. al 1995)
applied arching principles, conventionally used to predict stresses above buried pipelines,
to predict the state-of-stress in a vertical cutoff wall. A revised model, term lateral
squeezing (Filz 1996), accounts for the movement of the sidewalls. The latest model
(Ruffing et al. 2010) provides a method to account for the non-linear relationship
between stress and strain and then uses this non-linear relationship in a lateral squeezing
model to equilibrate the horizontal earth pressures inside and outside the trench. The
presentation describes the revised model, a methodology to compute the state-of-stress
and results from the revised model compared with those of the previous models.
Thermal and hydraulic conductivity properties of geothermal well seals
subjected to cyclic heating and cooling
This lecture presents the results of a study of the effects of cyclic heating and cooling on
the thermal and hydraulic conductivity properties of a geothermal well seal. In the US,
bentonite is widely used in the annulus between the fluid circulating pipes and the
formation for geothermal well systems and is subjected to many cycles of heating and
cooling throughout the design life. In particular, the seal is expected to maintain contact
with the circulating pipes and the adjacent formation in order to effect efficient transfer of
heat into and out of the earth. Secondly, since many of these wells are 100 m deep and
more, the seal must maintain a low hydraulic conductivity in order to isolate various
aquifers over the well depth from cross-contamination. In the study, after 23 cycles of
heating and cooling, the thermal conductivity remained constant but was different for
cycles of heating and cooling. After 12 cycles of hydraulic conductivity testing, there
appears to be some degradation in the hydraulic conductivity and intrinsic permeability.
The Role of Humanities and Social Sciences in an Engineering Education
Engineering is the application of natural science and mathematics to design, building, and
use of engines, machines, and structures. Engineering educators universally recognize
the need to underpin an engineering education with solid grounding in mathematics and
natural sciences, particularly chemistry and physics. But what is the role of humanities
and social sciences in the education of an engineer? It is readily agreed that engineers
serve humanity with the context of social structure. What then is the importance of
humanities subjects such as art, philosophy and literature? Similarly, are social sciences
subjects such as political science, economics, sociology and psychology important to
engineers? This talk explores these questions.
Evans’ Abstracts
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