RESULTS AND DISCUSSION
4.1 EFFECT OF SAND CONTENT AND PARTICLE SIZE ON THE BEHAVIOR OF SAND-BENTONITE MIXTURES
4.1.1 ATTERBERG LIMITS
Atterberg limits are the oldest and most commonly used tests in the field of soil engineering (Sridharan and Rao, 1975). Liquid limit test is essentially a measure of viscous resistance or shear strength of a soil, which becomes soft as it approaches the liquid state (Sowers et al., 1959). Liquid limit of a sand-clay mixture primarily depends upon its clay content and the type of clay mineral present in the sand-clay mixture.
Sivapullaiah and Sridharan (1985) observed that plasticity depends not only on the type and amount of the clay mineral but also on the interaction between clay and soil particles, and this needs to be considered keeping in view that the sand doesn’t contribute to water holding capacity. Liquid limit of a soil mixture is influenced by the size of soil particles and no influence of particle shape has been found. In the present study, liquid limit and plastic limit were determined in accordance with ASTM D 4318 (2000). Fine sand-bentonite and medium sand-bentonite mixtures for both bentonite-1 and bentonite-2 were prepared with bentonite content ranging from 10 to 50% in the mixture by dry weight. As the mixes were having high sand content, cone penetration test was carried out to determine the liquid limit of sand-bentonite mixes (Sivapullaiah and Sridharan, 1985).
Atterberg limits of fine sand-bentonite mixtures and medium sand-bentonite mixtures are shown in the Table 4.1. For the sake of simplicity, the medium sand, fine sand, bentonite-1 and bentonite-2 have been referred to as MS, FS, B1 and B2, respectively in
the further discussion. Data in the table shows that irrespective of the sand type, the liquid limit increases with increase in the bentonite content in the mixture. As the liquid limit depends upon the net attractive forces between the bentonite particles, an increase in the bentonite content increases the liquid limit of mixture. Similarly, as the liquid limit of a soil is its water holding capacity, the surface area plays a significant role in defining the liquid limit. As the surface area of bentonite is much higher in comparison to sand, increase in the bentonite content in the mixture also increases the specific surface area of the mixture and consequently increases the liquid limit. However, it was observed that the variation in liquid limit was not linear even though the clay content in the mixes varied linearly. In addition to this, the data in Table 4.1 also shows that the mixes containing FS exhibits higher liquid limits as compared to those mix with MS, irrespective of the quality and percentage of bentonite used. However, the difference in the liquid limit was much prominent for the mixes containing B2 in comparison to mixes containing B1.
Table 4.1 Summary of Atterberg limits of fine sand-bentonite and medium sand-bentonite mixtures
Mix.
Proportions (Sand:Bentonite)
Bentonite-1 (B1) Bentonite-2 (B2) LL
(%) PL (%)
PI (%)
SL (%)
LL (%)
PL (%)
PI (%)
SL (%)
Medium sand (MS)
0:100 346.1 33.0 313.1 23 609.2 41.5 567.7 10.4 50:50 69.0 25.1 43.9 24.5 145.2 18.4 126.8 25.3 60:40 54.8 20.6 34.2 29.3 125 17.0 108 29.6 70:30 49.4 19.3 30.1 31.3 97.5 20.3 77.2 37.0
80:20 33.4 * * 28.3 73.8 18.8 55 38.1
90:10 32.4 * * 30.4 47.3 * * 34.3
Fine sand (FS)
0:100 346.1 33.0 313.1 23 609.2 41.5 567.7 10.4 50:50 79.7 19.5 60.2 22.1 171.0 21.7 149.3 23.5 60:40 61.4 17.0 44.4 29.4 139.2 23.9 115.3 32.3 70:30 48.1 18.8 29.3 33 102.3 25.1 77.2 37.3 80:20 41.7 * * 37.2 74.3 33.3 41.0 40.4
90:10 32.8 * * 35.6 48.1 33 15.1 39.7
* Test could not be performed
Similarly, the data in Table 4.1 also shows that plasticity index of sand-bentonite mixtures increased with an increase in the bentonite content. However, the plastic limit
was affected marginally due to increase in the bentonite content in the mixture. The data for the shrinkage limit shows that a higher value of shrinkage limit was observed for the fine sand mixes in comparison to those with medium sand of same bentonite content. At the shrinkage limit, the capillary water present in the pore space starts to evaporate (Sridharan and Prakash, 2000). Further, since the individual pore spaces between the soil grains is large for medium sand in comparison to the fine sand, water starts to evaporate at higher water content for medium sand mixes resulting in a lower value of shrinkage limit. The data also shows that with increase in the bentonite content in the mixture the shrinkage limit decreases. As the bentonite content in the mixture increases, it starts filling the void space between the individual sand grains as a result the size of individual pore size gets decreased resulting in a lower value of shrinkage limit.
Figure 4.1 Variation of liquid limit with bentonite content
Data also shows that the type of bentonite present in the mixtures also influences the shrinkage limit of the soil. When the pore spaces are filled with bentonite of higher quality, marked by a higher liquid limit, swelling capacity and specific surface area, the shrinkage limit increases. When a bentonite with a high specific surface area is present in the mixtures, it will absorb more water and thereby prevent evaporation leading to a higher value of shrinkage limit. Shrinkage limit of a soil is a physical phenomenon
0 20 40 60 80 100 120 140 160 180
0 10 20 30 40 50 60
Liquid limit (%)
Bentonite content (%)
FS-B1 MS-B1
FS-B2 MS-B2
mostly dependent on the relative packing of particle as opposed to other Atterberg limits.
Shrinkage limit results of FS-B2 and MS-B2 mixes were seen to exhibit an increasing trend in shrinkage limit with increasing sand content in the mixture and especially mixes with fine sand exhibited relatively higher shrinkage limits.
Upon comparing the plasticity characteristics exhibited by bentonite and sand-bentonite mixtures, it can be seen that mixing sand with bentonite resulted in reduced plasticity as is evident from the reduction in liquid limit and plasticity index values. Sand-bentonite mixtures exhibited a higher shrinkage limit values compared to pure bentonite, indicating a reduced susceptibility to volume changes with changes in water content.