LABORATORY STUDY ON BC SOIL STABILIZATION USING FLYASH
Arash Hosseini Gourabpasi
M.Tech, Working as asst. professor in SRSIT, (India) ABSTRACT
The degree and level of soil engineering problems run high and become multifold in case of expansive soils which exhibit the typical character of swelling and shrinkage. The wide spread of the black cotton soil in Chitradurga region has posed challenges and problems to the construction activities. A task was therefore undertaken to investigate and improve the engineering properties of the black cotton soils of Chitradurga so that, a better understanding is facilitated for the civil engineering practitioners, while dealing with these soils.
The Bellary Thermal Power Station (BTPS) at Kudutini (Bellary Dist, Karnataka), located at about 150 km.
from Chitradurga generates huge quantity of fly ash and the fly ash management is posing serious problem.
Considering the proximity and availability aspects, the fly ash was chosen to be used for the task, as a stabilizer of black cotton soil. The paper investigates the effect of Bellary fly ash treatment to the black cotton soils of Chitradurga on their index, compaction and strength properties in an effort to improve their behavior.
Keywords: Black Cotton Soil, Compaction, Flyash, Shrinkage, Stabilization.
I. INTRODUCTION
Soil stabilization is a technique aimed at increasing or maintaining the stability of soil mass and chemical alteration of soils to enhance their engineering properties. Stabilization can be used to treat a wide range of sub- grade materials from expansive clays to granular materials. This allows for the establishment of design criteria as well as the determination of the proper chemical additive and admixture rate to be used in order to achieve the desired engineering properties.
II. MATERIALS AND METHODOLOGY 2.1 Materilas
The materials used for the present study are Black cotton soil and Flyash. They have been tested for the basic physical properties and details are explained below.
2.2 Black cotton soil
Natural black cotton soil was obtained from Chitradurga district in Karnataka State. The soil was excavated from a depth of 1.5 m from the natural ground level. The soil is dark grey to black in color with light clay content. The obtained soil was air dried, pulverized manually and soil passing through 4.75 mm IS sieve was used.The physical properties of soil used in this investigation are given in the table below. Sieve analysis, Atterberg’s limit were performed to study the index properties, compaction characteristics and tests were carried out for both fine and coarse soil mixtures.
Fig.1 Gradation Curve of BC Soil Table 1. Properties of BC Soil
Properties Values
Liquid Limit, LL (%) 59.50
Plastic Limit, PL (%) 28.09
Shrinkage Limit, SL (%) 10.34
Specific gravity, G 2.62
Optimum Moisture Content, (%) 23.0
Maximum dry unit weight, (KN/m³) 14.47
Plasticity index, PI (%) 30.8
Silt and clay content (%) 42.0
Sand, (%) 58.0
2.3 Flyash
Flyash is a fine residue collected from the burning of the pulverized coal in the thermal power plant. It is a silty non-cohesive material having a relatively smaller specific gravity than normal soils. The disposal of flyash is a serious hazard to the environment that consumes millions of rupees towards the cost of its diposal. Now a day flyash has been used in a variety of construction applications, such as compacted fills, concrete, bricks, construction of embankments in many countries including India. Flyash by itself has a little cementation value but in presence of moisture it reacts chemically and forms cementitious compounds and attributes to the improvements of strength and compressibility characteristics of soils.In the present study, Flyash of class C – was collected from Bellary Thermal Power Station (BTPS), Bellary in Karnataka State and was used for the stabilization. The flyash used was grey in color and the physical properties and chemicals compounds of flyash are given below.
Fig.2 Grain Size Analysis Curve For Flyash 2.4 Methodology
The laboratory tests carried out on the raw BC soil, Flyash, and Flyash Stabilized BC soil. Laboratory tests were carried out for determining the effect of Flyash on BC soil include Atterberg limits test, Specific gravity test, Proctor compaction test, CBR, UCS test. The samples were mixed with FA in varying percentage of 10%, 20%, 30%, 40% and 50%. The corresponding mixes were designated as FBC-10, FBC-20, FBC-30, FBC-40 and FBC-50 respectively.
2.5 Plastic Limit
The addition of fly ash shows a steady incline in the plastic limit as shown in the fig. The addition of 10% fly ash slightly increases the plastic limit, which is due to flocculation owing to the presence of free lime in the fly ash. Further increase in the addition of fly ash results in the marginal increase of plastic limit. This is because, with increased addition of fly ash, the amount of soil to be flocculated decreases and the finer particles of fly ash may be incorporated in the voids of flocculated soil; thereby decreasing the water held in the pores leading to the flatter variation in the plastic limit.
Table 2. Variation of pl with percentage flyash added
% Fly ash Added Plastic limit (%)
0 23.14
10 25.33
20 26.29
30 27.00
40 27.48
50 27.80
60 28.12
Fig.3 Variation of Pl With Percenatge Flyash Added
2.6 Shrinkage Limit
As can be seen from fig., the addition of fly ash increases the shrinkage limit of black cotton soil. It increases with increasing percentages of fly ash. The gradual increase is observed up to 30% of fly ash added and is considerable on further addition of fly ash. The increase in the shrinkage limit is mainly due to flocculation of clay particles by free lime present in the fly ash.
Table.3 Variation of Sl With Percentage Flyash Added
% Flyash Added
Shrinkage limit (%)
0 8.02
10 11.32
20 14.16
30 16.50
40 18.86
50 22.00
60 26.69
2.7 Optimum Moisture Content (OMC) and Maximum dry Density (MDD)
The compaction characteristics for standard Proctor compactive effort for the black cotton soil- fly ash mixes reveal that the MDD increases and the OMC decreases with increasing fly ash content. However, beyond 30%
fly ash, only marginal increase in MDD is observed, with gradual increase in the OMC.
The behaviour of black cotton soil is controlled by diffused double layer. The addition of fly ash in small percentage results in the decrease of repulsive pressure of soil particles. This in turn reduces the resistance to compactive effort and the mix gets compacted to relatively higher densities. Though there will be flocculation due to free lime of fly ash, this effect is dominated when the fly ash percentage is low. Hence a marginal increase in dry density is observed. Further addition of fly ash beyond 30% results in increased flocculation due to increased availability
of free lime content of fly ash. This would increase the repulsive forces of soil particles, thereby increasing the resistance to compactive effort and hence the density of mix starts decreasing. The compaction curve becomes flatter with increasing quantity of fly ash.
The variation of MDD and OMC with flyash content is shown below;
Table 4. Variation of Mdd and Omc with Flyash Content
% Flyash
Added MDD (g/cc) OMC (%)
10 1.72 15
20 1.81 8.7
30 2.07 5
40 1.95 12.3
50 2.37 19.5
Fig. 5 Variation of Maximum Dry Density With Percentage Flyash Added
Fig. 6 Variation of Optimum Moisture Content with Percentage Fly Ash Added
2.8 Unconfined Compressive Strength (UCS
)
The variation of unconfined compressive strength with addition of the different percentages of fly ash for 0 and 7 days curing period is shown in fig.8. It is seen that the strength increases on addition of small percentage of 10% fly ash, but with further increase in fly ash percentage, no considerable increase in strength is observed.
This is due to the probable disturbance of soil skeleton and consequent reduction in cohesion. The strength of soil is observed to improve considerably with curing which is due to the pozzolanic reactivity of the free lime content of the fly ash. For both the samples without curing and with curing, maximum unconfined compressive strength is found to be at 30 % of flyash.
Fig.7 Variation of Unconfined Compressive Strength with Curing Period
III. CONCUSIONS
The following conclusions are drawn from the present investigation;
(i) The black cotton soils of Chitradurga region have high degree of expansion and possess high swelling potential and require stabilization for their better performance.
(ii) The index parameters of the study soils improve with the addition of flyash. The liquid limit decreases from
the corresponding increase in the addition of flyash from 10% to 60% respectively. The shrinkage limit increases from 11% to 27% for increasing in the addition of flyash from 10% to 60% respectively.
(iii) In respect of the compaction parameters, the maximum dry density is found to increase from 1.72 for FBC- 10 mix to 2.07 g/cc for FBC-30 mix ; and further increase in the addition of DFA is observed to decrease it. The optimum moisture content decreases from 15% for FBC-10 mix to 5% for FBC-30 mix.
(iv)The unconfined compressive strength of these soils increases from 0.00016 N/mm2 for M-10 mix to 0.00023 N/mm2 for M-50 mix, with no curing. For a curing period of 7 days, the trend of increase in the strength is found more pronounced and the unconfined compressive strength increases from 0.00032 N/mm2 for M-10 mix to 0.00037 N/mm2 for M-50 mix. For both the cases of curing periods maximum unconfined compressive strength is found to be at 30 % of flyash (FBC- 30 mix).
(v) The geotechnical properties of Chitradurga black cotton soils can be favorably changed with the addition of flyash at an optimum quantity of 30 %.
REFERENCE
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