KSCE Journal of Civil Engineering (2012) 16(6):943-949 Geotechnical Engineering

DOI 10.1007/s12205-012-1388-9
www.springer.com/12205

Unconfined Compressive Strength of Clayey Soils Stabilized with

Waterborne Polymer
Seyed Abolhassan Naeini*, Bahman Naderinia**, and Ehsan Izadi***
Received November 14, 2010/Revised 1st: May 21, 2011, 2nd: October 12, 2011/Accepted November 27, 2011

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Abstract

Improvement and stabilization of soils are widely used as an alternative to substitute the lacking of suitable material on site. Soils
may be stabilized to increase strength and durability or to prevent erosion and dust generation. The use of nontraditional chemical
stabilizers in soil improvement is growing daily. A new stabilizing agent was developed to improve the mechanical performance and
applicability of clayey soils. In this study a laboratory experiment is conducted to evaluate the effects of plasticity index and
waterborne polymer on the Unconfined Compression Strength (UCS) of clayey soils. The laboratory tests include sieve analysis,
hydrometer, Atterberg limits, modified compaction and unconfined compression tests. Three clayey soils with different plasticity
indexes were mixed with various amounts of polymer (2, 3, 4 and 5%) and compacted at the optimum water content and maximum
dry density. The unstabilized and stabilized samples were subjected to unconfined compression tests to determine their strength at
different curing times. The results of the tests indicated that the waterborne polymer significantly improved the strength behavior of
unsaturated clayey soils. Also, an increase in plasticity index causes a reduction in unconfined compression strength.
Keywords: unconfined compressive strength, stabilization, waterborne polymer, clayey soils, curing time, plasticity index
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1. Introduction documented, and the effectiveness of these traditional stabilizers

has been demonstrated in many applications. Traditional stabili-
Soil stabilization is a collective term for any physical, chemical, zation techniques often require lengthy cure times and relatively
or biological method, or any combination of such methods, large quantities of additives for significant strength improvement.
which is employed to improve certain properties of a natural soil Unfortunately, there is no step by step procedure for selecting
to make it adequately serve an intended engineering purpose nontraditional stabilizers (including polymers, enzymatic emul-
over the service life of an engineering facility (Fang, 1991). The sions, lignosulfonates, tree-resin emulsions and etc.) and little
different uses of soil pose different requirements of mechanical research has been completed on these materials.
strength and of resistance to environmental forces. Stabilization The use of soil stabilization has shown a major increase in geo-
may be achieved by mechanically mixing the natural soil and a technical engineering applications such as construction of build-
stabilizing material in order to form a homogeneous mixture or ings, roads, railways, embankments, stabilization of slopes and
by adding a stabilizing material to an undisturbed soil deposit improvement of soft ground. Improvement of certain desired
and letting it permeate through soil voids (Perloff, 1976). Chemical properties like bearing capacity, shear strength and permeability
stabilization is the mixing of soil with one of, or a combination of characteristics of soil can be undertaken by a variety of improve-
admixtures of powder, slurry, or liquid for the general objectives ment techniques. One of the techniques is to improve the soft soil
of improving or controlling its volume stability, strength and stress- with polymeric stabilizers. The major soil stabilization applications
strain behavior, permeability, and durability. Traditional stabilization beneting from the advances in polymeric materials are chemical
methods include the application of various combinations of lime, grouts, geotextiles, and geomembrane liners for ponds, waterways,
cement, fly ash, and bituminous materials. Extensive researches and landlls. A synthesis of potential stabilizers identified by the
have been conducted on the use of traditional stabilizers (Alexander. US Army Corps of Engineers and contract researchers from
et al., 1972; Eades and Grim, 1960; Ferguson, Levorson, 1999; 1946 to 1977 were developed (Oldham et al. 1977). The report
Ferguson, 1989; Little et al., 2000; Prusinski et al., 1999). The identified acids, asphalt, cement, lime, resins, salts, silicates, and
stabilization mechanisms for these traditional stabilizers are well other products as potential stabilizers demonstrating varying

*Associate Professor, Civil Engineering Department, Imam Khomeini International University, Qazvin 34149-16818, Iran (Corresponding Author, E-
mail: Naeini_h@ikiu.ac.ir)
**Research Assistant, Civil Engineering Department Imam Khomeini International University, Qazvin 34149-16818, Iran (E-mail: Naderinia_bahman
@yahoo.com)
***Research Assistant, Civil Engineering Department, Imam Khomeini International University, Qazvin 34149-16818, Iran (E-mail: Ehsanizadi@ikiu.ac.ir)

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 Seyed Abolhassan Naeini, Bahman Naderinia, and Ehsan Izadi

degrees of success. The results of the investigation divided per- The aim of this research was to evaluate the influence of
formance by soil type and demonstrated that product perfor- plasticity index and waterborne polymer on the engineering
mance differed for varying soil types. It was also noted that the properties of clayey soils. The main benet of using waterborne
stabilization mechanisms for particular stabilizing agents, such polymer is that the dispersing medium is water, thus eliminating
as salts, were particularly suited for specific climates and envir- the negative environmental impact associated with chemicals
onmental conditions. A polymer resin provided the greatest found in some grouts and solutions used in stabilization.
increase in the unconfined compression strength for the sand
materials. Gopal et al. (1983) used Urea-Formaldehyde (UF) and 2. Material
its copolymers to stabilize dune sand. Specimens were prepared
at different combinations of UF ratios, pH levels, and acid 2.1 Soils
catalysts. The optimum UF ratio (ureas to formaldehyde) for their Soft clay specimens were collected locally from Abyek (located
experiment was 1:2.25 by weight. They recommended using 9 in Qazvin-Iran). Three fine-grained clayey samples (named: soil
percent resin and 0.3 percent acid catalyst for stabilizing dune A, soil B and soil C) with different plasticity indices were used in
sands. Also, an experimental study was conducted to determine this experimental study. Soil A is the as-sampled soft clay from
the effects of stabilizing clay-silt soils with the combination of an Abyek, Soil B is the same soil with the addition of 10% bentonite
epoxy resin (bisphenol A/epichlorohydrin) and a polyamide by weight, and Soil C is the soft clay with the addition of 20%
hardener (Ajayi-Majebi et al. 1995). It was observed that admixing bentonite by weight. The grain-size distribution of the unaltered
up to 4 percent stabilizer into a clay-silt material produced large soil (sample A) is presented in Fig. 1. According to the properties,
increases in the load-bearing capacity of the material in terms of the soils lie above the A-line in the plasticity chart (Fig. 2), thus
its unsoaked California Bearing Ratio (CBR) for curing times as they are classified as high and low plasticity clay soil according
low as 3 hours. Palmer et al. (1995) investigated the strength and to the Unified Soil Classification System (ASTM D422-63). The
density modification of unpaved roads using lignin sulfonate properties of the tested soils in term of atterberg limits, and
(lignin), calcium chloride (CaCl2), and magnesium chloride particle size and compaction parameters are given in Table 1, and
(MgCl2). Laboratory results indicated that lignin was the only the chemical properties of used bentonite is given in Table 2.
product that increased specimen density and the unconfined com-
pression strength of specimens subjected to 4 wet-dry cycles.
Also several researchers (Green et al., 2000; Moustafa et al.,
2003; Santoni et al., 2003; Naeini and Mahdavi, 2009) have
discussed aqueous polymer applications while others (Daniels,
2003; Daniels and Inyang, 2004; Al-Khanbashi and Abdalla, 2006;
Al-Khanbashi and El-Gamal, 2003) have provided useful data on
polymer-soil interactions that determine the effectiveness of
polymer solution in various applications. Al-Khanbashi and Abdalla
(2006) studied the performance of three different emulsion
systems as sand stabilizers through measurement of the hydraulic
conductivity and compressive strength. Two methods of mixing
and spraying for preparing specimens were chosen. The sprayed
specimens of the three emulsions showed better hydraulic con- Fig. 1. Grain Size Distribution of Unaltered Soil (Sample A)
ductivity reduction compared with their mixing counterparts.
However, the strength and the stiffness of the sprayed specimens
were lower than those of the mixed specimens. Amu (2008)
investigated the possibility of complementing poor lateritic soils
with Palm Kernel Shells (PKS) and subsequent stabilization of
the resulting composite mix with asphalt. The major finding
revealed that the stabilized soils obtained inadequate strength for
subgrade, sub-base and base courses in road construction.
In general, polymer emulsions can be used on most soils; how-
ever, certain products are more effective on specific soil types.
When synthetic polymer emulsions applied at low application
rates (sprayed-on or mixed-in) to the surface of the unbound
roads, they perform well for dust suppression. They bond soil
particles together and so reduce dust generation. At higher
application rates (mixed-in), synthetic polymer emulsions can be
used to stabilize soils (Kestler, 2009). Fig. 2. Plasticity Chart

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 Unconfined Compressive Strength of Clayey Soils Stabilized with Waterborne Polymer

Table 1. Engineering Properties of Collected Soils Table 3. Important Physicochemical Properties of the Acrylic Poly-
Property Soil A Soil B Soil C mer
Specific gravity 2.77 2.75 2.73 Physicochemical properties
Mixed bentonite percentage 0% 10% 20% Name Acrylic Polymer
Grain size
Gravel (2-75 mm), (%) 0 0 0
Sand (0/075-2 mm), (%) 6 8 10 Chemical formula
Silt (2-75 µm), (%) 58 52 47
Clay (<2 µm), (%) 36 40 43
Atterberg limits Phase Liquid
Liquid limits, (%) 31 41 52 Polymer type Cationic
Plastic limits, (%) 19 22 26 Solvability in Water Solution
Plasticity index, (%) 12 19 26 Viscosity (cP) 280
Compaction parameters Density(g/cm3) 1.11
Optimum Moisture Content, (%) 12.0 13.3 14.4 Colour White
Maximum dry unit weight, (g/cm3) 1.92 1.87 1.84
Soil classification, (USCS) CL CL CH eering projects; it is inexpensive and available in Iran, the curing
time for stabilized samples is relatively low, it has long-term
Table 2. Chemical Composition of Bentonite Used in this Study
durability, and it can be simply applied to soil.
Compound Magnitude (%)
SiO2 70.85
3. Laboratory Studies
Al2O3 3.26
Fe2O3 0.07
This experimental work has been done to investigate the in-
TiO2 0.05
fluence of plasticity index, curing time and percentage of aqueous
CaO 2.81
polymer on the unconfined compression strength of unsaturated
MgO 7.00
clayey soils. Use of the unconfined compressive strength test for
Na2O 0.07
soil stabilizers is a quick and simple test and provides a conveni-
K2O 0.95
CO2 0.44
ent basis for comparison between stabilizer types. For this
P2O5 0.66
purpose three clayey soils with different plasticity indices were
LiO 15.25 used and various percentages of aqueous polymers were added to
soils in order to investigate the strength parameters of stabilized
samples. The clayey soils were dried before using in the mixtures.
2.2 Acrylic Polymer First, the required amounts of polymer as a percentage of
Liquid admixtures in clay have the advantage of easier access optimum water content were blended and then added to dry soils.
to clay particle surfaces than granular admixtures, particularly, if The pure amounts of aqueous polymer were chosen as 2%, 3%,
the liquids are water-soluble and do not produce excessively 4% and 5% by total weight of the amount of water needed to
viscous solutions. Some aqueous polymer solutions meet this achieve the optimum water content. These amounts of polymer
requirement. Synthetic polymer emulsions primarily consist of were then diluted in water and mixed with the soil samples. As
acrylic or acetate polymers that are specifically produced for soil the soils tended to lump together, considerable care and time
stabilization, or are by-products from the adhesive or paint were spent to get a homogeneous distribution of the mixture.
industries. In this study, a commercial product of Acrylic Polymer Then all the specimens were compacted at their respective maxi-
(product ID: AP225 – produced by Pars Polymer Khavaran mum dry density in their optimum moisture content, correspond-
(P.K.K.) Co. in Iran) was used, which is an emulsion synthetic ing to the values obtained in the modified proctor compaction
elastic chemical substance that increases the bond with the Tests (ASTM D-1557). The cylindrical specimens (diameter=50
substrate as an additive in optimum moisture, as well as the mm, height=100 mm) were stored in the curing room at the
cohesion and the strength. This material consists of a tackifier/ temperature of ranging 21o to 25o and then tested at 2, 4, 6, 8 and
sealer that is a liquid polymer of methacrylates and acrylates. It is 14 days of curing times. In order to determine the unconfined
an aqueous acrylic emulsion which is blend of 40% solids by compression strength parameters of unstabilized and stabilized
volume that is free from styrene, acetate, vinyl, ethoxylated sur- samples, a series of unconfined compression tests at the rate of
factants or silicates. For soil stabilization applications, it is diluted 1.2 mm/min were carried out according to ASTM D2166. The
with water in accordance with manufacturer’s recommendations. axial strain and axial normal compressive stress are given by the
Some important properties of the Acrylic Polymer used in this following relations:
study are given in Table 3. The polymer used in this study has
some advantages which justify its usage in geotechnical engin- σ = P/A (1)

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 Seyed Abolhassan Naeini, Bahman Naderinia, and Ehsan Izadi

ε = ∆L/L0 (2) Table 4. The Results of (UCS) on Polymer Stabilized Soils (kPa)
A = A0 /(1 − ε) (3) Soilpolymer Curing time
type (%) 0 day 2 day 4 day 6 day 8 day 14 day
where A is the corresponding cross-sectional area [mm2], A0 is 0% 9.6 9.8 9.8 9.8 9.8
the initial cross-sectional area of the specimen [mm2], L0 is the 2% 10.3 11.6 12.4 12.8 12.9
initial length of the test specimen [mm], P is the corresponding Soil A 3% 9.2 10.6 12.8 13.9 14.4 14.5
4% 11.0 13.6 14.5 14.9 14.9
force [kN], ∆L is the length change of specimen [mm], σ is the 5% 10.8 13.1 14.2 14.7 14.7
compressive stress [kPa], ε is the axial strain for the given load. 0% 8.1 8.2 8.2 8.2 8.2
2% 8.9 10.1 10.7 10.8 10.9
4. Results and Discussion Soil B 3% 7.9 9.4 10.7 11.8 12.0 12.1
4% 10.0 11.7 12.6 12.9 12.9
4.1 Effect of Curing Time on the UCS 5% 9.7 11.1 12.1 12.4 12.4
To observe the effect of curing time on the UCS, three clayey 0% 7.3 7.5 7.5 7.5 7.5
2% 7.6 8.5 9.1 9.3 9.4
soils with different plasticity indexes were mixed with different Soil C 3% 7.0 8.0 9.1 9.6 9.8 9.8
percentages of acrylic polymer and tested at 2, 4, 6, 8 and 14 days 4% 8.6 9.9 10.5 10.8 10.8
of curing time. The results of unconfined compressive strength on 5% 8.3 9.6 10.1 10.5 10.6
stabilized and unstabilized soils at different curing times are
presented in Fig. 3 and Table 4. Note that qu is the UCS of the
samples and q0 is the UCS of untreated and uncured soil samples As Fig. 3 shows, the unconfined compressive strength of
which were acquired separately for samples A, B and C. stabilized soils in relation to curing time for all of the polymer
contents increases more rapidly within the first 8 days and then
becomes almost constant up to 14 days. Therefore, optimum
curing time of 8 days has been recommended for all three soil
samples. The polymer-stabilized soil properties improved with
curing time. Curing for the polymer emulsions occurs by break-
ing of the emulsion and subsequent water loss by evaporation.
The breaking of the emulsion occurs when the individual emul-
sion droplets suspended in the water phase coalesce. This occurs
as the emulsion particles wet the surface of the soil particle and
the polymer is deposited on the surface. The amount of polymer
deposited on the surface of the soil particle depends on the
concentration of the polymer added and the degree of mixing
with the soil (Santoni et al. 2003).

4.2 Effect of Polymer Content on the UCS and Stress-strain

Behaviour
As shown in Fig. 4, the addition of polymer played an impor-
tant role in development of the UCS of polymer mixed soils. It is
observed that the value of the UCS increased, when the amount
of polymer percentage increased. Similar trend has been ob-
served in all of the soil samples. Also, it is observed that as the
percentage of polymer increased up to 4%, the value of UCS
increased in soil A, soil B and soil C but with a further increase
in percentage of polymer up to 5% the value of the UCS slightly
decreased. This is because of the adsorption mechanism of
polymer emulsion that more likely to bond to the clay content
(fine particle size) than to the sand or silt particles. As it is
mentioned in Table 3, the used polymer categorized as cationic
polymer. Thus the molecules of polymer easily can form an elec-
trostatic bond with clay particles. Their retention on clay which
is mostly through sorption of polymer molecules can occur on
both internal and external pore surfaces or microscopic interlayer
Fig. 3. Effect of Curing Time on Normalized UCS of: (a) Soil A, (b) spaces that cause more cohesion of particles of clay and an in-
Soil B, (c) Soil C crease in value of UCS. Adsorption leads to change in conforma-

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 Unconfined Compressive Strength of Clayey Soils Stabilized with Waterborne Polymer

Fig. 5. Effect of Polymer Content on the Stiffness of the Soil A

polymer content. This means with an increase in polymer content,

the treated soils show a greater strength at the same strain level.
In other words, increasing polymer content leads to an increase
in the stiffness of soil samples. This is shown in Fig. 5. Also,
similar trends were observed for the soil B and soil C.

4.3 Effect of Plasticity Index on the UCS

Figure 6 shows the effect of plasticity index of soils on uncon-
fined compressive strength for different contents of polymer
after 8-days of curing time. As shown in Fig. 6 the unconfined
compressive strength of soils decreases from soil A to C which is
due to the increasing plasticity index of soils from soil A to C. An
increase in plasticity index causes a reduction in unconfined com-
pressive strength. Again, referring to the fact that the polymer
emulsion tends to bond to the clay mineral phase (bentonite), an
increase in the bentonite content (i.e., plasticity index) results in a
decrease in the availability of the polymer for the rest of the soil
matrix. It should be noted that a similar trend has been observed
Fig. 4. Effect of Polymer Percentage on UCS of: (a) Soil A, (b) Soil for all other curing times.
B, (c) Soil C Plasticity index plays an important role in the stress-strain
behaviour of the soil samples. The stress-strain curves for soil
tion in the polymer, which in solution tends to exist as a random samples mixed with 2% polymer are shown in Fig. 7. Observing
coil, uncoils and spreads out at the solid/solution interface. stress-strain plots of the tests showed that as the plasticity index
The size of polymer molecule in an aqueous solution is highly increases, it makes the soil yield at a higher strain as shown in
dependent on its conformation which in turn, depends largely on
solution concentration, ionic concentration, pH, etc. Conformation
is the extent to which a polymer molecule coils up or stretches
out in solution. If it coils tightly, it will assume a smaller size; and
if it stretches out, it will occupy a larger space of variable shape.
With increasing polymer up to 4% the polymer molecules are
separated from one another and behave like free coils but with
increasing concentration, 5%, the polymer coils began to overlap
and segments of molecules begin to entangle and their bonding
effects on clay particles diminishes. In addition, with regard to
the adsorption mechanism of the polymer emulsion, there is an
important relation between the polymer content and the clay
content. In other words, the increase in clay content dilutes the
polymer phase.
By observing stress-strain plots of tests, one can conclude that,
the gradient of stress-strain curves increased with an increase in Fig. 6. Effect of Plasticity Index on UCS of Soils

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 Seyed Abolhassan Naeini, Bahman Naderinia, and Ehsan Izadi

Fig. 7. Effect of Plasticity Index on Stress-strain Behaviour of Soil

Samples: (a) Samples with 0% Polymer, (b) Samples with
2% Polymer Content

Fig. 7. Also, samples with higher plasticity indexes show softer

stress-strain behaviour, reduced stiffness and smaller peak strength.
Therefore the higher plasticity index could change the stress-
strain behaviour of soil samples from a brittle to relatively ductile
manner.

4.4 Statistical Evaluation of Test Results

To quantitatively evaluate the effect of polymer content and
curing time on the unconfined compressive strength of the
studied soil, a multiple regression analysis was applied to obtain
the following relationships among UCS, polymer content (P) and
time (t) values, with Eqs. (4), (5) and (6) for soils A, B and C,
respectively:

UCS(kN/m2) = 8.7911 + 0.8078 × P(%) + 0.2078 × t(days)

(4) Fig. 8. Measured and Predicted Values of UCS Soils
UCS(kN/m2) = 7.55 + 0.7381 × P(%) + 0.1503 × t(days) (5)
UCS(kN/m2) = 6.8248 + 0.5195 × P(%) + 0.1190 × t(days)(6) 5. Conclusions

The relationships between measured and predicted strength This study was undertaken to investigate the influence of plas-
parameters (obtained from Eqs. (4) to (6)) as well as the 95% con- ticity index and aqueous polymer percentage and curing time on
fidence intervals are shown in Fig. 8. The correlations between the unconfined compressive strength of stabilized and unstabilized
measured and predicted values are 85%, 87% and 88% for soil unsaturated clayey soils. The results of the study are presented in
A, B and C respectively. As it was mentioned before, the UCS of following conclusions:
the treated samples decreases when the polymer content is over 1. The unconfined compressive strength significantly increases
4%. Therefore, the Eqs. (4), (5) and (6) are assigned for polymer with curing time. The UCS values increase more rapidly within
contents up to 4%. the 8 days and then become almost constant up to 14 days.

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 Unconfined Compressive Strength of Clayey Soils Stabilized with Waterborne Polymer

2. The maximum UCS increases with increasing polymer content. of soils, American Society for Testing and Materials, Philadelphia.
Soils stabilized with 4% polymer have higher unconfined com- Daniels, J. L. and Inyang, H. L. (2004). “Contaminant barrier material
pressive strength than other percentages. Stabilized samples textural response to interaction with aqueous polymers.” Journal of
Material and Civil Engineering, Vol. 16, No. 3, pp. 265-275.
with 5% polymer have lower strength than 4% polymer. Also
Daniels, J. L., Inyang, H. I., and Iskandar, I. K. (2003). “Durability of
it should be noted that increasing polymer content lead to Boston blue clay in waste containment applications.” Journal of
higher stiffness of soil samples. Material and Civil Engineering, Vol. 15, No. 2, pp. 144-155.
3. Plasticity index has an important effect on the UCS. An Eades, J. L. and Grim, R. E. (1960). Reaction of hydrated lime with pure
increase in plasticity index causes a reduction in unconfined clay minerals in soil stabilization, Bulletin 262, Highway Research
compressive strength. It is due to decrease in dry density and Board. Washington, D.C.
increase in optimum water content. Also, stress-strain plots of Fang H. Y. (1991). Foundation engineering handbook, Chapman &Hall,
the tests showed that as the plasticity index increases, the soil New York, NY10119.
Ferguson, G. (1989). “Stabilizing with fly ash. Replacement for portland
yields at a higher strain. Therefore the higher plasticity index
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brittle to ductile manner. Ferguson, G. and Levorson, S. M. (1999.) Soil and pavement base
4. Soil stabilization with aqueous polymer improves the strength stabilization with self-cementing coal fly ash, American Coal Ash
behavior of unsaturated clayey soils and can be potentially Association, Alexandria, Virginia.
useful in time and cost savings because of its low curing time. Gopal, R. J., Singh, S., and Das, G. (1983). “Chemical stabilization of
5. From the point of view in mechanics and applicability in sand comparative studies on urea-formaldehyde resins as dune sand
roadways, adding 4% polymer with an 8 day curing time gives stabilizer and effect of compaction on strength (Part IV).” Indian
Society of Desert Technology, Vol. 8, No. 2, pp. 13-19.
an economic and reasonable scheme.
Green, V. S., Stott, D. E., and Norton, L. D., and Graveel, J. G. (2000).
“Polyacrylamide molecular weight and charge effects on infiltration
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Kestler, M. A. (2009). Stabilization selection guide for aggregate and
Authors are so thankful for the support of Imam Khomeini native-surfaced low-volume roads, Civil Engineer, Forest Service,
International University for this research. The laboratory study of San Dimas Technology & Development Center.
this research was carried out in the Soil Mechanics Laboratory of Little, D. N., Males, E. H., Prusinski, J. R., and Stewart, B. (2000).
Civil Engineering Department. Cementations stabilization. Transportation in the new millennium:
Perspectives from TRB standing committees, Committee A2J01,
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