YEAR 2006

By A.U.Ravi Shankar*, S.N.Suresha1 , M.V.S. Phanikumar2


Stabilising the locally available weak soils is of paramount importance in contemporary practices in construction industry. This paper presents results of laboratory investigation made on strength behaviour of a weak soil blended with pond ash and lime. Shedi soil is a problematic soil and is abundantly available in the Konkan belt. It is blended with pond ash, a solid-waste from the thermal power stations. Lime is added to soil and pond ash mix as binding material. Influence of pond ash content on optimum Moisture content (OMC) and maximum dry density (MDD), and on strength parameters such as CBR values and Modulus of Subgrade Reaction (K) of stabilised soil mixes and soaking effect on mixes are studied and discussed.


Effective and bulk utilisation of both locally available weak construction material and large quantity of disposed industrial waste is of global concern. Two major subjects of serious concern are availability of good construction material and safe disposal of huge amount of solid waste from industries. The very first decade of second millennium India saw sudden boost in infrastructure development and it is still in boom state, which might continue for long. Mega road projects like, Golden Quadrilateral, North-South and East- West Corridor, Pradhan Mantri Gram Sadak Yojana (PMGSY), and many other highway improvement projects are creating heavy demand on the earth resources. Availability of good construction materials to meet the high specifications and standards for these projects is of serious concern. Solely depending on crushed aggregates and on granular material for construction activities will endorse severe threat to ecological balance. Another issue is for safe disposal of huge amount of coal ash generated at a rate of more than 110 Metric tonne per annum as a waste by-product from the large number of thermal power stations. Indiscriminate disposal of fly ash will cause hazards and environmental problems. As the disposal of coal ash requires large area, therefore it is important to have its bulk utilisation in base and sub base courses of the pavement, embankments, backfills etc., to increase ash utilisation in the country.

1.1. Objectives of the Study

In the present investigation, an attempt is made to examine strength behaviours of Shedi soil, which is quite problematic, abundatantly available in the region of South Canara District of Karnataka State, India. Shedi soil is characterised by its poor strength in compression and shear particularly in wet condition, but at unsoaked condition exhibits appreciable strength. Roads constructed on this type of soil-subgrade become problematic during monsoon and exhibit spongy nature and poses difficulties for design, construction, and maintenance of most of the rural roads in these regions.

The main objectives of the present study are as follows:

  • To stabilise the locally available weak subgrade, (i.e. Shedi soil) by using 3 per cent of lime and with varying the pond ash as the main stabilising material.
  • To study the influences of pond ash content on the OMC and MDD, strength parameters, and the soaking effect.
  • To arrive at the optimum pond ash content.
  • To study the load settlement behaviour of soil with different pond ash mixes.

1.2. Scope of the Study

The scope of the work includes characterisation of the Shedi soil and pond ash, determining the moisture- density relationship with respect to IS-light compaction for the soil blended with 3 per cent lime and with varying pond ash content and determine the laboratory CBR and Modulus of Subgrade reaction values for the same mixes. The main purpose of adding pond ash to soil is to alter the gradation and make the soil coarser, and lime is for binding action.


Soil stabilization is the process of improving the engineering properties of in-situ soil and thus making it more stable. In broad sense, stabilisation includes compaction, reconsolidation, blending, drainage and many other such processes. However, the term stabilisation in this paper is restricted to the improvement of strength properties only. Of recent soil-stabilisation techniques, using lime and fly ash in construction of pavements are successful. Since then quite a number of investigations were undertaken to study the behaviour of lime-fly ash with different types of soils available locally. Guruvittal and Murthy 2,3 studied engineering properties of fly ash, such as moisture-density relationship, consolidation and compressibility, strength, permeability and drainage of the fly ash, and suggested for its bulk utilisation in road construction. Seehara et al.8 worked on the lime-fly ash- soil stabilisation, lime-fly ash-concrete and lime rice-husk ash concrete as alternate binder materials for semi –rigid pavement construction. They found that these materials have better load dispersion characteristics due to their semi-rigid character as compared to conventional materials. Promod et al.7 have worked on use of pond ash blend as subgrade material for roads and parking areas of LPG bottling plant complex. Narendra et al.5 studied the effect of different mix proportions of lime and fly ash on Atterberg limits, compaction, CBR, and unconfined compressive strength properties of the local soil of Varanasi. The improvement in CBR and unconfined compressive strength were promising. Swamy et al.9 suggested that, wherever subgrade soils are either clay/silty clay, the soil could be mixed with bottom ash or pond ash to improve its properties. Archan1, Nanda6 suggested pond ash could be very effectively used as stabilising material for sub base layer. Satander et al.10 worked on mixes with different combinations of pond ash, black cotton soil, moorum and lime, and concluded that fly ash could be a good construction material for both flexible and rigid pavements. Venkata11 conducted studies on stabilisation of Shedi soil with lime and arrived at optimum lime content in the range of 3 and 4 per cent.


3.1 Materials

The materials used for the study are Shedi soil, pond ash and hydrated lime. The Shedi soil is procured from the nearby road construction site at Surathkal. Pond ash, a solid waste is procured from the Raichur Thermal Power Station (RTPS),Raichur. The physical and strength properties of the Shedi soil and pond ash are tabulated in Table 1. The chemical composition of the pond ash is shown in the Table 2.

3.2 Methodology

Shedi soil is stabilised with the hydrated lime and pond ash. The quantity of hydrated lime used was 3 per cent by weight of total mix, and the pond ash content varied gradually by 15, 20, 30, and 50 per cent by weight of total mix. The blending of the soil, lime and pond ash are carried out manually. The properties of the different mixes with varying pond ash content were studied for laboratory prepared specimens after curing for 7 days. The MDD and OMC values were determined for IS light compaction, and CBR and K-values for both soaked and unsoaked condition.

3.3 Laboratory Plate Load Test

The plate load tests were conducted on stabilised soil compacted at OMC and MDD. In this investigation, plate load tests were conducted on Shedi soil/soil mixes in a specially fabricated metal sheet tank measuring 500 x 500 x 500 mm. This metal tank has perforations of 10 mm diameter at 90 mm C/C, on all faces. It is placed in a ferro-cement tank of size 600x600x600 mm, so that the entire compacted material gets saturated during soaking. For saturating the soil/soil mix, the space between the tanks is filled with water. Due to capillary action and entry of water through perforations, the soil gets completely soaked. The inner sides of metal tank are also lined with gunny bags to avoid blockage of perforated holes by fine soil particles and thereby to get better results in saturation. The loading frame had a capacity of 20 tonne. Mild steel plate of 100 mm diameter was used to transfer the load from jack to Shedi soil/stabilised soil. The width of box is kept 5 times the width of plate to avoid end effects. The tank is positioned centrally below the hydraulic jack, fixed to the loading frame. Plate load tests are also conducted on soaked soil/stabilised soil since this represents the worst condition of the pavement during monsoon.

Loading: Indian Standard (IS: 1888 – 1962) recommends a seating load of 0.007 MPa, which was applied and released before the actual test was started. Load was applied with the help of hydraulic jack, in convenient increments, say one fifth of expected safe bearing capacity or one tenth of ultimate bearing capacity. Three dial gauges with a sensitivity of 0.001 mm were used, and settlement were observed for each increment of load after an interval of 1, 4, 10, 20, 40 and 60 minutes until the rate of settlement becomes less than 0.001 mm per hour. Loading was continued until failure. Values of subgrade modulus K1 were obtained with smaller plate corrected for standard diameter of 750 mm (equation 1), assuming that Ka is constant4. Typical laboratory set up of plate load test is shown in Figure1.


4.1. Grain Size Distribution

Physical properties of Shedi soil are shown in Table 1. Based on the higher fines content (more than 50 per cent), higher liquid limit (LL) and plasticity index (PI), it is classified under the group of MH as per IS -1498-1970. Where as pond ash contains more than 50 per cent of material retained on 75 microns and due to its non-plastic behaviour it has been classified under the group of SP-SM. Lime is used as additive and it imparts pozzolanic action to Shedi soil and pond ash mix. Replacing Shedi soil by pond ash by more than 15 per cent and with 3 per cent of lime, particle distribution changes from fine grained to coarse grained. Reduction in fine-grained fraction is found to be nearly 15 per cent with 15 per cent pond ash content, and 40 per cent with 50 per cent pond ash content.

4.2. Maximum Dry Density and Optimum Moisture Content

The maximum dry density and optimum moisture content for both Shedi soil and pond ash are shown in Table1. Low specific gravity of pond ash resulted in higher optimum moisture content (OMC) and lesser maximum dry density (MDD) when compared to the values of Shedi soil. With increase in pond ash content in soil, MDD and OMC are gradually changed. Values of MDD and OMC for varying pond ash content are tabulated in Table 3. Decrease in MDD was recorded in the range of 2.78 to 11.33 per cent, and increase in OMC in the range of 3.94 to 15.78 per cent respectively for increase in pond ash content from 15 to 50 per cent and with 3 per cent lime content. Based on these test results, equation (2) and (3) were developed which gives variation in OMC and MDD with pond ash content in the mix of Shedi soil with 3 per cent of lime. Figure 2 shows the variation in OMC and MDD for test results and values obtained by equations 2 and 3

4.3. Effect of Pond ash content on the Strength values

The CBR and K-values are tabulated in Table 1. Shedi soil exhibits high strength at unsoaked condition but its strength drastically reduces when it is soaked. The reduction in CBR is 66.66 per cent and K-value is 60 per cent. The CBR and K-values significantly improved for the Shedi soil with 3 per cent lime and 15 per cent of pond ash at both unsoaked and soaked condition. At unsoaked condition, the CBR values were 1.33 to 2.33 times higher for an increase in pond ash content from 15 to 50 per cent, and in soaked it is 3.5 to 7.75 times. Similar behaviour with K-values were observed, and increase in is in the range of 1.15 to 1.475 times higher in unsoaked condition and 2.63 to 3.88 times higher in soaked condition. The increase in the strength is expressed in terms of Strength Gain Factor (SF). The relationship between the pond ash content and SF is shown in Figure 3. The CBR values for the Shedi soil blended with 3 per cent lime and with change in pond ash content can be calculated by using the equation (4) and (5) respectively for unsoaked and soaked condition. The correlation between the SF and pond ash content is graphically shown in Figure 4.

4.4. Effect of Soaking on Strength Values

Strength of Shedi soil is disgraced by water i.e. CBR and K-values are drastically reduced due to soaking of soil. Stabilizing shedi soil by adding 3 per cent lime and varying percentage of pond ash, negative effect of soaking on strength is considerably reduced. Due to soaking, the percentage change in strength is negative (strength decreases after soaking) up to 18 percent of pond ash mix. Thereafter the percentage change is positive. Coarser and non-plastic fractions of pond ash, and pozzolanic action between lime and pond ash are the main factors, which contribute significantly to increase the strength. Gain in strength with increase in pond ash content is certainly graced by water; thence in soaked condition strength values are comparatively higher to that of unsoaked condition. Cementing action and its acceleration due to soaking are major factors for increase in soaked strength values in Shedi soil when blended with 3 per cent lime and pond ash content of 20 per cent and beyond. Figure 5 shows effect of soaking on Shedi soil when stabilised with 3 per cent lime and varying per cent of pond ash.

4.5. Modulus of Subgrade Reaction and CBR- Values

Variation in the strength values i.e. CBR and Modulus of Subgrade Reaction for the soil mixes at different pond ash content for different test condition

showed similar behaviour as indicated in Figure 5. The correlation between CBR and K is graphical represented in Figure 6.


Based on the test results, the following conclusions have been drawn:

1. The shedi soil properties have been changed by adding coarser material i.e. pond ash.

2. The low specific gravity of pond ash resulted in lower MDD and higher OMC for the soil blend, which improves the drainage property of the mix.

3. The increase in unsoaked strength values of the soil mix is mainly due to the presence of coarser grained particles present in pond ash. The higher soaked strength is due to the cementing action between the lime and pond ash.

4. When the shedi soil is blended with 50 per cent of pond ash and with 3 per cent lime, the maximum increase in CBR value is 2.33 times for unsoaked condition and 7.75 times for soaked condition when compared with Shedi soil. Similarly, the maximum increase in K-value is 1.45 times at unsoaked and 3.88 times at soaked condition.

5. Minimum pond ash content of 20 per cent is required for modifying grain size distribution and strength properties of the shedi soil along with 3 per cent of lime.

6. The Shedi soil stabilised with 3 per cent lime and 20 per cent pond ash satisfies most of the sub base properties and also the present subgrade strength will be increased from 4 to 21 per cent.

7. The coefficients of density and optimum moisture content used in the MDD and OMC equations need to be studied for the soils of different specific gravity and grain size.


The authors are thankful to the National Rural Road Authority (NRRDA) for encouraging to carryout the research activities on utilisation of waste and locally available materials in road construction. The help rendered by the Dr. R.K. Yaji, Dr. R. Shivshaknakr and Prof. Sambatur Sridhar is thankfully acknowledged.


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