52
Geotechnical News June 2011
THESIS ABSTRACTS
provided new insight into the behavior of trapped LNAPL below the
water table when subjected to cyclic freezing.
Supervisor: D. Sego, Dept. of Civil & Environmental Engineering,
University of Alberta
Earth Pressures and Loads on Induced
Trench Culverts
Benjamin L McGuigan
Benjamin L McGuigan, GEMTEC Limited, 191 Doak Road,
Fredericton, New Brunswick E3C 2E6, Tel: 506-453-1025,
Fax: 506-453-9470, email:
Centrifuge tests were performed to measure earth pressures on a
single box culvert installed in several induced trench configurations.
A parametric study performed with numerical modelling identified
a preferred compressible zone geometry having a width of 1.2 times
the culvert width (Bc) and a thickness of half the culvert height. The
earth pressure on top was 0.28 times the overburden. The induced
trench base contact pressures were 50% higher than the top pres-
sures plus dead load due to drag forces mobilized along the side-
walls; however, they were 35% less than the base contact pressures
for the positive projecting condition.
A numerical model calibrated with centrifuge test results was
used to evaluate the effects of culvert spacing and compressible
zone geometry on twin induced trench box culvert pressures. One
compressible zone spanning both culverts was preferred for culverts
spaced at 0.5-1.0Bc, while two zones, 1.2Bc wide, were preferred
for 1.5Bc spacing. The base contact pressures were 41-47% lower
than for the corresponding positive projecting cases.
A field instrumentation of twin 3660-mm diameter culverts in-
stalled in an induced trench under 21.7 m of fill was also performed.
Earth pressures were measured at the crown, shoulder, springline,
haunch, and invert locations. Average crown and springline pres-
sures were 0.67 and 0.35 times the overburden, respectively. Poor
compaction in the haunch regions and stiff bedding conditions led to
stress concentrations at the inverts. Numerical modelling was used
to determine a vertical earth load of 0.87 times the soil prism load,
which was 30% lower than for the positive projecting condition.
Induced trench construction therefore appears to be viable for
single and twin box culverts, provided that drag forces along the
sidewalls are accounted for in the design, as well as for large diam-
eter twin pipes, provided that bedding and constructability issues
are addressed.
Supervisor: Dr. Arun J. Valsangkar, Professor Emeritius, Dept. of
Civil Engineering, University of New Brunswick
Effect of Reinforcement and Soil
Viscosity on the Behaviour of
Embankments over Soft Soil
Chalermpol Taechakumthorn
Chalermpol Taechakumthorn
)
A verified elasto-viscoplastic finite element model is used to
develop a better understanding of the performance of geosynthetic
reinforced embankments over rate-sensitive soil. For rate-sensitive
soils, the generation of creep-induced pore pressures following the
end of construction is evident along the potential slip surface. As
a result, the minimum safety factor with respect to embankment
stability occurs after the end of construction. The combined use of
reinforcement and PVDs are shown to provide an effective means of
minimizing creep-induced excess pore pressure, increasing overall
stability, and decreasing deformation of the embankments.
The combined effects of the viscoelastic properties of geosyn-
thetic reinforcement and the rate-sensitive nature of foundation soils
on the performance of embankments are examined. The effect of
various factors, including reinforcement type, soil viscosity, con-
struction rate and allowable long-term reinforcement strain, on the
time-dependent behaviour of reinforced embankments are consid-
ered. From a series of finite element analyses, the ideal allowable
reinforcement strains to minimize embankment deformation while
providing optimum long-term service height of the embankment,
considering the effect of soil and reinforcement viscosity, are pro-
posed for soils similar to those examined in this study.
The currently proposed design methods for embankments with
creep-susceptible reinforcement constructed over rate-sensitive
soils appears to be overly conservative. This study proposes a re-
fined approach for establishing the allowable long-term reinforce-
ment strains that are expected to provide adequate performance
while reducing the level of conservativeness of reinforced embank-
ment design.
Finally, a previously developed elasto-viscoplastic constitutive
model is modified to incorporate the effect of soil structure using a
state-dependent fluidity parameter and damage law. The model was
evaluated against data from a well-documented case study of a rein-
forced test embankment constructed on a sensitive Champlain clay
deposit in Saint Alban, Quebec. The benefit of basal reinforcement
and the effect of reinforcement viscosity are then discussed for these
types of soil deposits.
Advisor: Dr. R. Kerry Rowe, Professor and Canada Research
Chair in Geotechnical and Geoenvironmental Engineering, Queen’s
University
Diffusive Transport of Volatile Organic
Compounds through Geomembranes
Rebecca S. McWatters
Rebecca S. McWatters, Postdoctoral Fellow, GeoEngineer-
ing Centre at Queen’s-RMC, Queen’s University, Kingston, ON,
Canada, and Monash University, Melbourne, VIC, Australia,
and the Australian Antarctic Division, Kingston, TAS, Australia,
email:
The diffusive transport of volatile organic compounds (VOCs)
through geomembranes is examined. The key diffusive parameters:
diffusion (Dg), partitioning (Sgf) and permeation (Pg) coefficients,
for transport from both vapour and aqueous phases are evaluated.
Consideration is given to different types of geomembrane, expo-
sure to cold climatic conditions, and aged geomembranes exhumed
after 3 and 25 years. Laboratory sorption and diffusion tests are
performed and modeling is used to infer diffusive parameters from
experimental data. The transport of VOCs through polyvinyl chlo-
ride (PVC) and linear low-density polyethylene (LLDPE) geomem-
branes from both aqueous and vapour phases is evaluated by Purge
& Trap-GC/MS. Results indicate that VOC transport through geo-
membranes in a simulated landfill environment is identical despite
the phase they originate from. Subsequently, this finding is con-
firmed by examining diffusion of vapour-phase VOCs using Solid
Phase Microextraction-GC/FID.
Diffusive transport of VOCs through traditional PVC, LLDPE
and high-density polyethylene (HDPE) geomembranes is compared
with that through two novel co-extruded geomembranes, one with a
polyamide inner core, the other an ethylene vinyl alcohol (EVOH)
inner core. Both co-extruded geomembranes show a 10-200-fold
decrease in Pg values and therefore improved diffusive resistance
to VOCs compared to the traditional geomembranes. EVOH also
