Symbols Used
CHAPTER 6 COMBINED EFFECT OF INORGANIC S ALTS AND HEAVY METALS ON
6.1. INTRODUCTION
6.2.6. Consolidation parameters
6.2.6.3. Coefficient of consolidation (c v )
Figures 6.19 and 6.20 show the relationship between coefficient of consolidation and consolidation pressures of the bentonites in the presence of the inorganic salts and heavy metals solutions mixed in different combination. From the plots it is observed that cv
values decreased with increase in the consolidation pressure. The plot also shows that the decrease in cv with increase in the pressure was significantly affected by the presence of mixed salts. In presence of DI water, with an increase in the pressure from 19.6 kPa to 784.5 kPa, the cv decreased from 1.60x10-4 cm2/sec to 2.28x10-5 cm2/sec (7.0 times) for Bentonite-A. However, for the similar range of increase in the pressure the cv decreased from 2.7x10-3 cm2/sec to 6.7x10-5 cm2/sec (40.3 times) when Bentonite-A was permeated with the high concentration combination solution. Similarly for Bentonite-B when permeated with DI water, the cv decreased from 5.48x10-5 cm2/sec to 1.03x10-5 cm2/sec (5.3 times) due to an increase in the pressure from 19.6 kPa to 784.5 kPa. However, when permeated with the mixed salts, the cv decreased from 2.90x10-3 cm2/sec to 4.14x10-5 cm2/sec (70.0 times).
From Figs. 6.19 and 6.20 and, it is also observed that cv values increased with increase in concentration of the salts. With the increase in the salt concentration the diffuse double layer thickness reduces resulting in higher value of hydraulic conductivity which in turn
increases the cv of the sample. For Bentonite-A, at a consolidation pressure of 196 kPa, cv
increased from 0.48x10-4 cm2/sec in presence of DI water to 0.63x10-3 cm2/sec (13.1 times) in presence of 1 N CaCl2+0.1 N NaCl solution, whereas, it increased to 0.83x10-3 cm2/sec (17.3 times) in presence of 1 N CaCl2+0.1 N NaCl+1000 ppm. When one of the salts was of 1 N concentration, the cv value of the bentonites were found to be higher than with combinations of 0.1 N solution. The cv increased from 0.48x10-4 cm2/sec in presence of DI water to 0.98x10-4 cm2/sec (2.0 times) in presence of 0.1 N NaCl+0.1 N CaCl2, it increased to 0.83x10-3 cm2/sec (17.3 times) in presence of 1 N CaCl2+0.1 N NaCl and it increased to 0.54x10-3 cm2/sec (11.3 times) in presence of 1 N NaCl+0.1 N CaCl2. The cv increases marginally in presence of combination of heavy metals. The cv increased from 0.48x10-4 cm2/sec to 0.86x10-4 cm2/sec (1.7 times) in presence of 1000 ppm combination of heavy metals. Similarly for Bentonite-B, at a consolidation pressure of 196 kPa, the cv increased from 0.22x10-4 cm2/sec in presence of DI water to 0.67x10-3 cm2/sec (30 times) in presence of 1 N CaCl2+0.1 N NaCl, whereas, it increased to 0.47x10-3 cm2/sec (21.3 times) in presence of 1 N CaCl2+0.1 N NaCl+1000 ppm solution. When one of the salts was of 1 N concentration, the cv value of the bentonites were found to be higher than that with a combinations of 0.1 N. The cv value increased from 0.21x10-4 cm2/sec in presence of DI water to 0.12x10-3 cm2/sec (5.7 times) in presence of 0.1 N NaCl+0.1 N CaCl2, it increased to 0.67x10-3 cm2/sec (31.9 times) in presence of 1 N CaCl2+0.1 N NaCl and it increased to 0.86x10-3 cm2/sec (40.9 times) in presence of 1 N NaCl+0.1 N CaCl2. The cv increases marginally in presence of combination of heavy metals. The cv increased from 0.214x10-4 cm2/sec to 0.53x10-4 cm2/sec (2.5 times) in presence of 1000 ppm combination of heavy metals. A comparison between the values of the cv for the two bentonites at the same concentration and consolidation pressure indicates that the Bentonite-B, which has a higher
swelling and liquid limit and lower hydraulic conductivity values, exhibited a lower cv in comparison to Bentonite-A.
Figure 6.19 Plots between coefficient of consolidation and consolidation pressures of Bentonite-A in presence of the combined solutions
1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01
1 10 100 1000
Coefficient of consolidation,cv (cm2/sec)
Pressure (kPa) Water
100 ppm all 1000 ppm all
0.1 N NaCl + 0.1 N CaCl2 1 N CaCl2 + 0.1 N NaCl 1 N NaCl + 0.1 N CaCl2
Open symbol with dotted lines are for salt solution with100 ppm of all heavy metals Open symbol with solid lines are for salt solution with1000 ppm of all heavy metals
Figure 6.20 Plots between coefficient of consolidation and consolidation pressures of Bentonite-B in presence of the combined solutions
Figures 6.21 and 6.22 shows the relationship between the coefficient of consolidation and liquid limit of the bentonites. The plots show that irrespective of the consolidation pressure, the cv decreased with the increase in the liquid limit of the bentonite. The decrease in cv
with increase in the liquid limit was prominent at low consolidation pressure compared to at high consolidation pressure. For Bentonite-A, in presence of the permeating liquids and under a consolidation pressure of 784.5 kPa, cv decreased from 3.54x10-5 cm2/sec to 2.28x10-5 cm2/sec when the liquid limit increased from 85.8 % to 218.0 %; whereas, under a consolidation pressure of 49.1 kPa, the cv decreased from 1.87x10-3 cm2/sec to 3.5x10-4 cm2/sec. For Bentonite-B, in presence of the salt as permeating liquids and under a consolidation pressure of 784.5 kPa, the cv decreased from 9.03 x10-5 cm2/sec to 1.03x10-5 cm2/sec when the liquid limit increased from 98.5 % to 560.0 %; whereas, under a consolidation pressure of 49.1 kPa, cv decreased from 2.53x10-3 cm2/sec to 1.56x10-4 cm2/sec.
1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01
1 10 100 1000
Coefficient of consolidation,cv (cm2/sec)
Pressure (kPa) Water
100 ppm all 1000 ppm all
0.1 N CaCl2 + 0.1 N NaCl 1 N NaCl + 0.1 N CaCl2 1 N CaCl2 + 0.1 N NaCl
Figure 6.21 Plots between the coefficient of consolidation and the liquid limit of Bentonite-A in presence of the combined solutions
Figure 6.22 Plots between the coefficient of consolidation and the liquid limit of Bentonite-B in presence of the combined solutions
6.2.6.4. Time for 90% of consolidation (t90) 1.E-06
1.E-05 1.E-04 1.E-03 1.E-02
0 50 100 150 200 250
Coefficient of consolidation,cv (cm2/sec)
Liquid limit (%) 19.6 kPa
196.1 kPa 784.5 kPa
1.E-06 1.E-05 1.E-04 1.E-03 1.E-02
0 100 200 300 400 500 600
Coefficient of consolidation,cv (cm2/sec)
Liquid limit (%)
19.6 kPa 196.1 kPa 784.5 kPa
Figures 6.23 and 6.24 show the relationship between consolidating pressure and time to complete 90% of consolidation (t90). The plots indicate that with an increase in the consolidation pressure the t90 for the sample increases in the presence of the salt solutions.
For any given concentration and pressure, a higher value of t90 was observed for the Bentonite-B in comparison to Bentonite-A. The plots also show that the increase in t90 with the consolidation pressure was gradual in the beginning and significant at higher pressures.
However, the increase in t90 with the consolidation pressure was less in presence of the salt solutions in comparison to that of DI water. With an increase in the consolidation pressure from 9.8 kPa to 784.5 kPa the t90 for the Bentonite-B permeated with DI water increased from 24.0 minutes to 918.1 minutes; whereas, it increased only from 9.1 minutes to 74.0 minutes for combination solutions of high concentrations of heavy metals and inorganic salts. Similarly, for Bentonite-A the t90 increased from 13.6 minutes to 445.1 minutes when
Figure 6.23 Plots between the time for 90% of consolidation and consolidation pressures of Bentonite-A in presence of the combined solutions
0 200 400 600
1 10 100 1000
Time for 90% consolidation,t90 (minutes)
Pressure (kPa) Water
100 ppm all 1000 ppm all
0.1 N NaCl + 0.1 N CaCl2 1 N CaCl2 + 0.1 N NaCl 1 N NaCl + 0.1 N CaCl2
Open symbol with dotted lines are for salt solution with 100ppm of all heavy metals Open symbol with solid lines are for salt solution with 1000ppm of all heavy metals