Symbols Used
CHAPTER 3 MATERIALS AND METHODS
4.3. SUMMARY
This chapter includes study on the effect of various concentration of NaCl and CaCl2
solution on the behaviour of two bentonites with different mineralogical composition. The result concluded that the liquid limit and free swelling of the bentonites decreases significantly, whereas, the plastic limit increases marginally due to the inclusion of salt in the pore liquid. The liquid limit and free swelling of the Bentonite-B, which has a higher cation exchange capacity (CEC), exchangeable sodium percentage (ESP) and specific surface area and marked as high quality bentonite, decreased considerably due to the increase in the salt concentration. The data also shows that the bentonite with NaCl solution exhibited a higher value of liquid limit and free swelling in comparison to CaCl2
solution due to the formation of a relatively large diffuse double layer thickness in the presence of NaCl solution in comparison with the same concentration of CaCl2 solution.
Cc = 0.0009LL + 0.5225 R² = 0.9522
0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
0 100 200 300 400 500 600
Compression index,Cc
Liquid limit, LL (%)
salts decreased with the increase in the salt concentration and reached to an almost identical value at 1 N.
A comparison of the time-swelling plot for the two bentonites showed that at the same time elapsed, the percentage of swelling was higher for Bentonite-B in comparison to Bentonite- A. Irrespective of the bentonite quality, swelling was least in presence of high concentration solution. The results also showed that the bentonites compacted at dry of optimum moisture content (OMC) exhibited higher percentage of swelling in comparison to the samples compacted at OMC indicating the influence of the initial compaction on the swelling behaviour.
A comparison of the values of swelling potential for the two bentonites showed that the swelling potential of Bentonite-B was affected significantly due to an increase in the salt concentration. The decrease in the swelling potential with increase in salt concentration was found to follow different trends for NaCl and CaCl2 solution. An increase in the concentration from 0 to 0.01 N of NaCl solution decreased the swelling potential of both the bentonites marginallly, whereas, same increase in the concentration of CaCl2 solution decreased the swelling potential significantly. Irrespective of the salt concentration, sample compacted on the dry side of the OMC possessed a higher value of swelling potential in comparison to the samples compacted at OMC. The results also showed that the swelling potential of the bentonites increased with an increase in the liquid limit and free swelling.
The results showed that the swelling pressure of the bentonite decreased with the increase in the salt concentration. Similar to the liquid limit, free swelling and swelling potential, the decrease in the swelling pressure with increase in the concentration was different for different range of concentration. In comparison to the CaCl2 solution a higher value of swelling pressure was observed for NaCl solution for any given concentration, however, the difference in swelling pressure value for the two bentonites decreased with the increase
in the concentration. A comparison of the swelling pressure for the two bentonites showed that the swelling pressure of Bentonite-B decreased significantly due to increase in the salt concentration in comparison to Bentonite-A. A comparison of the initial compaction condition on the swelling pressure shows initial water content has a marginal effect on the swelling pressure irrespective of the permeating liquid.
The results showed that irrespective of the initial compaction condition, the hydraulic conductivity of the bentonite increased with an increase in the salt concentration. The increase in the hydraulic conductivity with salt concentration was more prominent for higher salt concentration. The effect of salt concentration was found to be more prominent for the concentration of 0.1 N and above of both NaCl and CaCl2 solution. It was observed that the ksalt/kwater ratio decreased with the increase in the FSsalt/FSwater ratio. The decrease was higher for Bentonite-B compared to Bentonite-A.
A comparison between the two bentonites for a given concentration and compaction condition shows that salt had a significant impact on Bentonite-B in comparison to Bentonite-A and the effect was more prominent at higher concentration.
The results for the coefficient of volume change (mv) showed that irrespective of the permeating fluid and type of bentonite, the mv initially increased and then decreased with an increase in the pressure. The plots also show that the Bentonite-B exhibited higher mv
values in comparison with that of Bentonite-A.
The data for the coefficient of consolidation (cv) showed that it decreases with increasing consolidating pressure indicating a slower rate of consolidation at higher consolidation pressure and this trend was significantly affected by the presence of salt solution. A comparison between the values of cv for the two bentonites at the same concentration and consolidation pressure indicates that Bentonite-B exhibited a lower cv in comparison to Bentonite-A. However, the difference between the cv values for the two bentonites
decreased with the increase in the salt concentration. Samples compacted at dry of OMC exhibited higher cv and mv values.
The plots indicate that with an increase in the consolidation pressure the t90 for the sample increases. The plots also show that the t90 for the samples depends not only on the type of bentonite but also on the saturating fluid as well. For any given concentration and pressure, a higher value of t90 was observed for the Bentonite-B in comparison to Bentonite-A. The result showed that the increase in t90 with pressure decreased significantly with increase in the salt concentration of the saturating fluid.
A higher value of theoretical void ratio obtained for both the bentonites from the diffuse double layer theory at lower consolidation pressure; however, with an increase in the consolidation pressure the value of theoretically obtained void ratio exceeds the experimentally obtained value. However, with the salt as a pore fluid a lower value of theoretical void ratio was obtained for both the bentonites. The results for the compression index (Cc) showed that the Cc decreased with an increase in the salt concentration.
CHAPTER 5