27-09-2014, 11:57 AM
SOIL STABILIZATION WITH FLYASH AND RICE HUSK ASH
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ABSTRACT
The objective of this paper is to upgrade expansive soil as a construction material using rice husk ash
(RHA) and flyash, which are waste materials. Remolded expansive clay was blended with RHA and flyash
and strength tests were conducted. The potential of RHA-flyash blend as a swell reduction layer between
the footing of a foundation and subgrade was studied. In order to examine the importance of the study, a
cost comparison was made for the preparation of the sub-base of a highway project with and without the
admixture stabilizations.
Stress strain behavior of unconfined compressive strength showed that failure stress and strains increased
by 106% and 50% respectively when the flyash content was increased from 0 to 25%. When the RHA
content was increased from 0 to 12%, Unconfined Compressive Stress increased by 97% while CBR
improved by 47%.
Therefore, an RHA content of 12% and a flyash content of 25% are recommended for strengthening the
expansive subgrade soil. A flyash content of 15% is recommended for blending into RHA for forming a
swell reduction layer because of its satisfactory performance in the laboratory tests.
INTRODUCTION:
Clays exhibit generally undesirable engineering properties. They tend to have low shear strengths and to
lose shear strength further upon wetting or other physical disturbances1
. They can be plastic and
compressible and they expand when wetted and shrink when dried. Some types expand and shrink greatly
upon wetting and drying – a very undesirable feature. Cohesive soils can creep over time under constant
load, especially when the shear stress is approaching its shear strength, making them prone to sliding. They
develop large lateral pressures. They tend to have low resilient modulus values2
. For these reasons, clays
are generally poor materials for foundations3
. The annual cost of damage done to non-military engineering
structures constructed on expansive soils is estimated at $220 million in the United Kingdom and many
billions of dollars worldwide4
.
Flyash5-7 was successfully used for stabilizing expansive clays. The strength characteristics of flyash
stabilized clays are measured by means of unconfined compressive strength (UCS) or California Bearing
Ratio (CBR) values. Depending upon the soil type, the effective flyash content for improving the
engineering properties of the soil varies between 15 to 30% 8-10
. Rice Husk Ash (RHA) is obtained from the
burning of rice husk. The husk is a by-product of the rice milling industry. By weight, 10% of the rice grain
is rice husk. On burning the rice husk, about 20% becomes RHA11
.
The objective of this paper is to upgrade expansive soil as a construction material using RHA and flyash,
which are waste materials. The objective of this paper is to upgrade expansive soil as a construction
material using RHA and flyash, which are waste materials. The soils used in this study are found in and
around the Tri-state area (parts of Pennsylvania, New Jersey, and Delaware) of Philadelphia. No research
has been done on these soils with the aforementioned additives. Therefore, the results will be of immense
benefit to the design and field engineers of various infrastructure facilities in the Tri-state area near
Philadelphia.
MATERIALS:
SOILS
The properties of the expansive clay used in this investigation are given in Table 1. As per the USCS
classification system, the soil is a CH soil. A plasticity chart showing the location of the soil is shown in
Fig. 1.
FLYASH
Class C flyash was used. Its constituents are listed in Table 2.
RICE HUSK ASH
In this investigation, RHA passing through No. 100 sieve (150 micrometers) was used. The chemical
composition of RHA is listed in Table 3. The RHA had 90.2% silica content. This high amount provides
good pozzolanic action.
EXPERIMENTS
UCS, CBR, compaction and swell-shrinkage tests were conducted.
TEST METHODS
Compaction
The tests were performed in accordance with ASTM D 1557. The specimens were of 102mm diameter and
116mm height. The degree of compaction of soil influences several of its engineering properties such as
CBR value, compressibility, stiffness, compressive strength, permeability, shrink, and swell potential. It is,
therefore, important to achieve the desired degree of relative compaction necessary to meet the required
soil characteristics.
UCS
The UCS tests were performed in accordance with ASTM D 2166. The sample sizes were of 40mm
diameter and 80mm length. At the Optimum Moisture Content (OMC) and maximum dry unit weight
values of the natural soil, the tests were performed.
CBR
The CBR tests were conducted in accordance with ASTM D 1883. The sample sizes were of 152mm
diameter and 126mm length. At the OMC and maximum dry unit weight values of the natural soil, the tests
were performed.
Swelling
Consolidation test (ASTM D 2435) setup was used for determining the cyclic swell-shrink behavior of the
soil. The sample sizes were 76mm and 50mm in diameter and height respectively. The samples were
prepared at Proctor’s dry densities. The RHA was mixed with 15% flyash and compacted to dry unit weight
of 5.5kN/m3
at a moulding water content of 120%. The compacted admixture was cured for 14 days and
placed over the expansive soil. The efficacy of RHA as a cushioning layer between the foundation and
subgrade was also tested using the consolidation test
TEST RESULTS AND DISCUSSION:
The optimum moisture content and the maximum dry unit weight of the untreated natural soil were 20%
and 15.5 kN/m3
respectively.
The effect of flyash and RHA on Unconfined Compressive Strength for a curing period of 28 days of the
soil is presented in Fig. 2. When the RHA content was increased from 0 to 12%, Unconfined Compressive
Stress increased from 660 to 1300 kPa. Further increase in flyash decreased UCS, indicating that 25% is the
optimum value of flyash. Conversely, at any flyash content, increase in RHA up to 12% increases UCS.
Further increase in RHA decreases UCS, indicating that 12% is the optimum value for RHA. The following
mechanism explains the obtained improvements. The chemical reactions that occur when flyash is mixed
with clay include pozzolanic reactions, cation exchange12, carbonation and cementation. These result in
agglomeration in large size particles. This causes the increase in compressive strength13
. Influence of flyash
content on the UCS of RHA is presented in Table 4.
The influence of flyash on the stress strain behavior of the clay specimens in UCS test is shown in Fig. 3.
The flyash content varied from 0 to 30%. When flyash was increased to 25%, failure stress increased from
330 to 680 kPa and failure strain increased from 6 to 9%
PRACTICAL IMPLEMENTATIONS:
In order to examine the importance of this study, a cost comparison was made for the preparation of the
sub-base of a highway project with and without the admixture stabilizations. For this purpose, an eight lane,
heavy duty highway for a design period of 20 years was considered as per the AASHTO design
procedures16-20
. The highway is to be constructed with the following materials: pavement-a 6 inch high
stability plant mix; base-a 6 inch bituminous treated base; and subbase-crushed stone. The subgrade is
treated with 25% flyash and 12% RHA. A transportation cost of 66 cents per mile and a distance of 50
miles were considered21. A subbase of 13 inch thickness can be eliminated by treating the subgrade with
RHA and flyash. The savings in cost per mile over control group (with natural subgrade) is $ 1.4 million as
shown in Table 5.
There are field implementation hurdles to be overcome for the successful utilization of admixtures in road
construction22. For example, achieving uniform mixing of RHA mixture is important in order to achieve
the laboratory strength values in the field. Moreover, dust issues will be significant in the areas where wind
velocity is high. Hydration/pozzolanic reactions may be significantly disturbed in extreme weather
conditions such as extreme low temperatures, snow and rain. These will be the focus of future research on
the use of RHA as admixtures for stabilization of weak soils for road bases.
CONCLUSIONS:
1. Stress strain behavior of unconfined compressive strength showed that failure stress and strains
increased by 106% and 50% respectively when the flyash content was increased from 0 to 25%.
2. When the RHA content was increased from 0 to 12%, Unconfined Compressive Stress increased
by 97%.
3. When the RHA content was increased from 0 to 12%, CBR improved by 47%.
4. The optimum RHA content was found at 12% for both UCS and CBR tests.
5. The swelling potential of expansive soil decreases with increasing swell reduction layer thickness
ratio.
6. The vertical movement of clay soils with cushioning material stabilizes after 3 cycles of swelling
and shrinkage.
7. An RHA content of 12% and a flyash content of 25% are recommended for strengthening the
expansive subgrade soil while a flyash content of 15% is recommended for blending into RHA to
form a swell reduction layer.