black cotton soil

EFFECT OF LIME STABILIZED FLYASH LAYER ON CBR VALUES OFBLACK COTTON SOILS

IGC-2005, 17-19 December 2005, Ahmedabad, INDIA

Sridevi GSenior Lecturer, Department of Civil Engg, R.V.R.& J.C.College of Engineering,Guntur-522 [email protected]

Rama Rao MAssistant Professor, Department of Civil Engg, R.V.R.& J.C.College of Engineering,Guntur-522 [email protected]

J Padmini , B Priyanka, M Swapna U.G.Students, Department of Civil Engineering, R.V.R.& J.C.College of Engineering,Guntur-522 019

ABSTRACT: Black cotton soils which are highly sensitive to seasonal moisture content variations are responsible for substantial distress to the structures that are built over these soils. Civil Engineering Structures especially pavements in these areas suffer from premature failures. Stabilizing black cotton soil with chemicals, industrial wastes, geotextiles etc. have been found to be effective in improving their engineering properties, strength characteristics and CBR value. Use of flyash as stabilizing agent is one of the additives that can improve the strength of black cotton soil apart from solving disposal of flyash partly since geotechnical applications require bulk quantities of flyash material. But thorough mixing of soil and flyash is very difficult in the field. In this context an attempt is made to study California bearing ratio of soil and flyash when placed in layers. In the present paper experiments are conducted by placing flyash and soil in layers. Study has also been made with lime stabilized flyash layer by varying lime content from 0 to 10%. The study reveals that providing lime stabilized flyash layer can improve CBR of black soil.

1. INTRODUCTION

Black cotton soils are problematic for Civil Engineers, because of their unconventional behaviour. These soils show large volume changes with respect to variation of seasonal moisture content. These soils when subjected vehicular traffic, road pavement gets heaved and cracked due to swelling and shrinkage. Hence, these soils are to be stabilized before constructing the roads in order to have efficient and long lasting roads. Considerable research has been taken place using different stabilizing materials such as lime, flyash, cement, rice husk ash, industrial wastes and geosynthetics etc. and proved to be useful in stabilization of black cotton soils.

In India thermal power plants produce abundant quantities of flyash. Thermal power plants produce about 100 million tonnes of flyash per year. The annual production of flyash from thermal plants in the world is about 1000 million tones this has resulted in several problems such as land use, health hazards and environmental dangers. Disposal of these huge quantities of flyash is becoming very difficult as the

particles are fine which are easily blown off by air and possess toxic elements and can cause environmental hazards. But the pozzolanic reactivity of flyash plays an important role in improving geotechnical properties of soil. More over bulk utilization is possible only in geotechnical applications like embankments, dams, fills and in road construction. Flyash has several advantages such as low unit weight, high shear strength, low compressibility, insensitivity to moisture variations and pozzolanic properties. In the present paper efforts are being made to study the effect of lime stabilized flyash layer on CBR value of BC soil.

2. PROBLEMS IN EXPANSIVE SOILS

Expansive clay subgrades have low strength in wet conditions and lead to subgrade intrusion into overlying layer and penetration of subbase material into it (Holz.1959; Stevens et al, 1986; Steinberg, 1985). Pavements over expansive soil subgrades exhibit cracks resulting from alternate heave and settlement that lead to ultimate failure of pavements (Sen and Chakraborthy, 1977).

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3. MATERIALS

Materials Used

Soil: The soil used in this investigation is an expansive clay, collected from Chuttugunta, Guntur. The physical properties of the soil are presented in Table 1.

The liquid limit of soil is high. This is indicative of the volume change potential of the soil. A free swell index of 150% shows that the soil is of high degree of expansiveness.

Flyash: The flyash is obtained from the Vijayawada Thermal Power Station, about 15 km away from the Vijayawada City. The flyash used in this study is collected from the electrostatic precipitator (ESP) hoppers. The chemical composition of the flyash is given in Table 2.

4. CBR TESTS

California bearing ratio test is performed on soil sample as per BIS specifications. CBR samples were prepared and tested as per IS:2720 part 16 after four days of soaking. In the experimental study a layer of soil of 42 mm thick is placed and over it lime stabilized flyash layer to a thickness of 42 mm is placed and is superimposed by another soil layer of 42 mm thick. The lime content in the flyash layer is varied from 0% to 10% with an increment of 2%. In another study the lime stabilized flyash layer thickness was reduced to 21 mm and is placed between two soil layers. 63 mm thick soil layer was placed above the flyash layer and 42 mm thick soil layer was placed below the flyash layer. Flyash / lime stablised flyash layers in soil specimens are shown in Fig. 1.

The tests were performed for specimens of soil and flyash in layers with and without lime. Surcharge weight of 5 Kg., sufficient to produce an intensity of load equal to the weight of the base material and pavement was used during soaking and penetration. A metal penetration plunger of diameter 50 mm was used to penetrate the samples at the rate of 1.25 mm/min. Three CBR tests were conducted on each of specimen type and average of the three samples has been used.

5. RESULTS AND DISCUSSION

In the present study soil layers, flyash / lime stabilized flyash are used in layers. Significant increase in CBR of soil can be observed from the test results. The soaked CBR of untreated black cotton soil is 1.69% and the low CBR of the black cotton soil is because of inherent low strength due to the dominance of clay fraction. There is considerable increase in CBR when flyash layer is introduced and the improvement in CBR is 9.49% for 21 mm flyash layer and 12.45% for 42 mm flyash layer thickness. The higher CBR is due to its better strength characteristics, primarily due to friction.

Table 1 : Physical Properties of Soil

Table 2 : Chemical Composition of Flyash from VTPS, Vijayawada

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The effect of lime stabilized flyash layer indicated in Figure 1. The plot is made between % lime content in flyash as abscissa and % CBR as ordinate. The plot reveals that with the increase of lime content in flyash, the CBR value has increased. The lime reacts with flyash and results in cementitious compounds thereby increasing the CBR. The increase in CBR was as much as 28.37% when 21 mm thick flyash layer stabilized by 10% lime is used and the CBR was 39.48% when 42 mm thick flyash layer stabilized by 10% lime is used. The study has shown that CBR of black cotton soils can be improved without blending soil and flyash materials, but can be achieved by placing these materials in layers.

6. SUMMARY AND CONCLUSIONS

Thermal power plants are producing huge amounts of flyash causing environmental hazards apart from occupying precious land. Efforts should be made to utilize these bulk wuantities of flyash rather than dumping i t . Geotechnical appl icat ions l ike embankments, fills, road construction proved to be one of the effective ways of utilizing flyash. The present study focuses on the use of flyash in road construction.

1. Lime stazbilized flyash and soil placed in layers are effective in improving the CBR of soil, which also solves blending problem. This increase is due to pozzolanic reaction between reactive silica present in the soil and flyash and lime.

2. Increase in layer thickness of lime stabilized flyash layer has improved CBR value.

3. The increase of lime content in flyash has resulted in increase in CBR of black cotton soil.

REFERENCES

Holtz, W.G. (1959), "Expansive Clays Properties and Problems", Quarterly of the Colorado School of Mines, Vol.54, No.4,pp.89-125.

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