Sobhan, Khaled

A numerical investigation was conducted to evaluate the geotechnical safety and slope
stability of Municipal Solid Waste (MSW) landfills, considering the effects of
geosynthetic reinforcements, biodegradation of the waste, and associated changes in
material properties, and extreme wind force simulating hurricane conditions. Three
different landfill slopes, 1:1, 1:2, and 1:3 having the height of 122m and width of 2134m,
were analyzed using Limit Equilibrium Method (SLOPE/W) and Finite Element
Modeling (ANSYS). Techniques developed in this study were used to analyze a case
history involving a geogrid reinforced mixed landfill expansion located in Austria. It was
found that few years after construction of the landfill, there is a significant decrease in the
FS due to biodegradation. Extreme wind loading was also found to cause a substantial
loss in the FS. The geosynthetic reinforcement increased the slope stability and
approximately compensated for the damaging effects of biodegradation and wind
loading.

The characterization of silty soils is usually designated by the percentage of silt
contained within the soil matrix, along with the soil’s void ratio, which is used to
describe the soil’s current state. The use of these parameters to assess a soil’s strength
and undrained behavior is limited when finer material is contained within the soil.
Therefore, additional parameters must be considered in order to correctly assess the
strength and liquefaction potential of silty soils. These additional parameters include the
skeleton void ratio, equivalent void ratio and granulometric factors. The current research
investigates the influence of granulometric parameters, specifically the Median Grain
Size Ratio (D50/d50), denoted as μDR (or MDR within graphs and charts), on the strength
and liquefaction potential of loose silty sands. A series of undrained monotonic triaxial
compression tests (σ3’= 69, 83, and 103 kPa) are performed on reconstituted soil samples,
using three different base sand samples and a constant silt material. As a result, three distinct median grain size ratios (μDR = 4.2, 6.75, and 9) were tested with fines content
ranging from 0-30% for each μDR. The undrained shear strength at all confining pressures
tends to increase with in μDR; beyond 10% fines content there was no noticeable influence
of μDR. At any μDR the excess PWP is higher than that of clean sand, when fines content is
larger than 5% fines content. The slope of the instability line and phase transformation
line are directly affected by the μDR and fines content, with an increase in the instability
line and decrease in the phase transformation line with a growing μDR. The results
indicate loose granular fills can be designed to be stronger and more resilient under
extreme conditions by careful choice of materials in which the μDR>6.75 and the fines
content does not exceed 10%.

Effect of cementitious stabilization on the stress-compressibility characteristics of
three different South Florida organic soils were evaluated in this study. The
objectives of the research were to (l) determine if the secondary compression
characteristics of organic soils and peats can be stabilized with (a) cement only,
(b) binary blends of cement/slag (C-S), cement/gypsum (C-G), and cement/cement-kiln-dust (C-CKD) and (c) ternary blend of cement-slag-gypsum in equal proportions; (ll) quantify the effectiveness of cementitious stabilization by evaluating the time-stress-compressibility (t-log σ'v - e) relationship in terms of the Cα / Cc ratio; and (lll) provide some guidelines for selecting optimum dosage of cementitious materials in deep mixing methods when organic soils and peats are encountered. It was concluded that cementitious mixes containing various waste materials is effective in controlling the secondary compression behavior of organic soils, and therefore should be considered in deep mixing methods as a sustainable practice.

Long term durability is a major concern for wide-scale use of recycled
aggregate materials in civil engineering construction. The purpose of this study is
to provide an insight into the damaging effects of combined wet-dry cycles and
repeated mechanical loading in a recycled aggregate concrete (RAC) base
course material made from recycled crushed concrete aggregate and cement. A
coordinated experimental program followed by a mechanistic pavement modeling
and life cycle analysis was conducted as part of this research study. This
laboratory investigation was divided into three phases each consisting of both
wet-dry exposed specimens (WD), and control or non wet-dry exposed
specimens (NWD). Phase I experiments involved monotonic loading tests under
compression and flexure to evaluate the strength properties. Phase II involved
testing a total of 108 cylindrical specimens in cyclic compressive loading at three different stress ratios. After each regime of cyclic loading, residual compressive
strengths were determined. In addition, the load-deformation hysteresis loops
and the accumulated plastic deformation were continuously monitored through all
loading cycles. Phase III included a flexural fatigue test program on 39 beam
specimens, and fracture testing program on 6 notched beam specimens, each
one having 19-mm initial notch. Traditional SR-N curves, relating the Stress Ratio
(SR) with the number of cycles to failure (N or Nf), were developed. Fatigue crack
growth rate and changes in Stress Intensity Factors were obtained to determine
Paris Law constants and fracture toughness. A mechanistic analysis of a typical
highway pavement incorporating RAC base was performed with KENPAVE
program, followed by a Life Cycle Analysis (LCA) using the GaBi software. It was
found that the specimens subjected to wet-dry cycles suffered significantly higher
damage expressed in terms of accumulated plastic deformation, and loss of
residual compressive strength, modulus, fatigue endurance limit, and design life,
compared to specimens not exposed to wet-dry cycles. Although such
degradation in material properties are important considerations in pavement
design, a concurrent Life Cycle Analysis demonstrated that recycled aggregate
concrete base course still holds promise as an alternative construction material
from environmental stand point.

This research is aimed at investigating the corrosion durability of polyolefin fiber-reinforced
fly ash-based geopolymer structural concrete (hereafter referred to as GPC, in
contradistinction to unreinforced geopolymer concrete referred to as simply geopolymer
concrete), where cement is completely replaced by fly ash, that is activated by alkalis,
sodium hydroxide and sodium silicate. The durability in a marine environment is tested
through an electrochemical method for accelerated corrosion. The GPC achieved
compressive strengths in excess of 6,000 psi. Fiber reinforced beams contained
polyolefin fibers in the amounts of 0.1%, 0.3%, and 0.5% by volume. After being
subjected to corrosion damage, the GPC beams were analyzed through a method of crack
scoring, steel mass loss, and residual flexural strength testing. Fiber reinforced GPC
beams showed greater resistance to corrosion damage with higher residual flexural
strength. This makes GPC an attractive material for use in submerged marine structures.

A comprehensive laboratory investigation was conducted to evaluate the primary and secondary compression behavior of undisturbed organic silts and peats obtained from 11 locations along SR 15/US 98 in Palm Beach County, Florida. A total of 43 consolidation tests were performed. The primary objectives were as follows: (i) to conduct a series of standard consolidation tests to determine the Compression Index, Cc, and the Coefficient of Consolidation, cnu; (ii) to determine the Secondary Compression Index, Calpha, at stress levels (sigmanu'/sigmap') ranging from 0.30 to 1.15; and (iii) to employ the well-known Time-Stress-Compressibility concept to establish a unique relationship between C alpha and Cc. It was found that for all practical purposes, the Calpha/C c ratio at any stress level is 0.03, which is consistent with the values reported in the literature for similar soils. A constant Calpha/Cc ratio provides an approximation of Calpha once C c is determined from a standard consolidation test, and without the need of ongoing laboratory testing to predict long-term settlement.

Organic soils generally are characterized by low strength and high compressibility. Visual observations of State Road 15/US 98 in western Palm Beach County, Florida indicate numerous cracking and significant rutting and raveling along the roadway caused by the consolidation and long-term secondary compression of the organic soils due to soil and pavement dead load. Since sampling of undisturbed soft organic soils is difficult, and subsequent laboratory tests are expensive and time-consuming, there is a need for rapid in-situ characterization of these unstable foundation soils. This study evaluates the capabilities of Piezocone Penetration tests (CPTu), coupled with pore pressure dissipation tests, for estimating the strength, modulus, compressibility, and time rate of consolidation characteristics of organic soils and peat in Florida. The compression index (Cc) and coefficient of consolidation (cv), predicted from CPTu, showed reasonable correlation with laboratory-derived properties.