of kaolin to change up to four orders of magnitude (Michaels and Lin 1954), ...... 2.08. 1.55. 0.68. Hydraulic conductivity ratio from flexi-walt permeameter. (3).
S H O R T - T E R M AND L O N G - T E R M P E R M E A B I L I T I E S OF CONTAMINATED CLAYS By Namunu J. Meegoda, ~ Member, ASCE and Ruwan A. Rajapakse 2 ABSTRACT: The change in hydraulicconductivityof saturated clays due to shortterm and long-termexposure of organicchemicalsto claysis investigatedto explain the change in hydraulicconductivityof contaminatedsoils. The long-termexposure of soils to chemicalsis simulated in a laboratory environmentby prcmixingchemicals with soils. Under short-term and simulated long-term exposures of water soluble chemicals to soils, the hydraulicconductivitiesof laboratory consolidated clays are measured and reported. The permeabilitytests are performed using the both flexi-walland modified fixed-wallpermeameters with soil samples subjected to confiningpressures. The fixed-walland flcxi-wallpermeametcrsproduced similar results. The short-termpermeabilitytests showed a change in hydraulicconductivity valuesbut not the intrinsicpermeabilities'ofsoils. The simulatedlong-termexposure of organic chemicalsto soils showed a change in the soil structure and an increase in the intrinsicpermeabilityof soils. The increasingtrend in the intrinsicpermeability of a given soil was weakly correlated to the decreasing dielectric constant of the pore fluid. INTRODUCTION
The influence of chemicals on the hydraulic conductivity of clayey soils is a major concern in determining the long-term performance of clay liners for waste impoundments. It has been reported by Michaels and Lin (1954) that extremely large values of hydraulic conductivity were observed when liquid hydrocarbons dominated the fluid phase. The hydraulic conductivities of contaminated clays approached values of the order 10 .4 cm/s, which were more characteristic of fine sand than clay. If the hydraulic conductivity values are increased during the seepage of chemicals, the contaminants can move with greater ease and reach critical ground-water resources in a much shorter time than that predicted from ground-water models assuming no change in hydraulic conductivity. When soils are contaminated with chemicals, physico-chemical interactions may occur between the soils and the chemicals. Due to these interactions, the physical properties of soil may change drastically. This drastic change occurs during the passage of chemical contaminants. Yet, it is controversial whether there is an improvement or deterioration of the hydraulic conductivity of soils due to chemical contamination. For a conservative estimate of the extent of ground-water contamination, adverse values of hydraulic conductivity have to be used in the hydrogeological computations. If the physico-chemical interactions in a soil-chemical medium are quantified and an approach to estimate the change in hydraulic conductivity is proposed, then the adverse hydraulic conductivity values can be estimated. The change in hydraulic conductivity due to the permeation of organic ~Assoc. Prof., Dept. of Civ. and Envir. Engrg., New Jersey Inst. of Tech., Newark, NJ 07102. 2Envir. Engr., SESI Consulting Engrs., Whippany, NJ 07981; formerly, Grad. Student, New Jersey Inst. of Tech., Newark, N.J. Note. Discussion open until January 1, 1994. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on January 29, 1991. This paper is part of the Journal of Environmental Engineering, Vol. 119, No. 4, July/August, 1993. 9 ISSN 0733-9372/93/0004-0725/$1.00 + $.15 per page. Paper No. 1343. 725
chemicals is attributed to the change in soil structure due to the collapse of the electrical double layer. The collapse of the double layer results in a flocculated clay structure. The flocculated clay structure is usually accompanied by a higher hydraulic conductivity due to the larger flow channels between soil flocs or soil clusters. The electrical double layer theory (Babcock 1963) states that a completely flocculated structure may be obtained from a clay-electrolyte system having a pore fluid with high salt concentration or a pore fluid with low dielectric constant while keeping all the other variables that contribute to the formation of soil structure constant. A permeating fluid with a low dielectric constant caused hydraulic conductivity of kaolin to change up to four orders of magnitude (Michaels and Lin 1954), whereas a permeating fluid with high salt concentration only caused the hydraulic conductivity of kaolin to change by only one to two orders of magnitude (Olsen 1961). If the two different chemical environments produced completely flocculated clay structures then those two clay structures should be the same. Therefore, the hydraulic conductivities of the same flocculated clay structure formed from two different chemical environments, cannot differ by two orders of magnitude. Further, the double layer theory is developed for polar pore fluids. Therefore, the collapse of the double layer cannot be used as the only explanation for the change in hydraulic conductivity of soils due to the permeation of organic fluids, especially the nonpolar fluids. The intrinsic permeability or the coefficient of permeability (K in length 2) is a better parameter than the hydraulic conductivity (k in length/time) of a soil when studying the change in hydraulic conductivity of that soil during the passage of different fluids. The intrinsic permeability is independent of physical properties of the permeating fluids as well as the physical factors such as temperature, pressure, etc. The relationship between the intrinsic permeability and the hydraulic conductivity is given by (1) K
=
k
x
v
.................................................
(1)
where v = the coefficient of kinematic viscosity. LITERATURE REVIEW
There is a broad agreement among the environmental/geotechnical engineers that the changes in hydraulic conductivities of clays are due to the collapse of double layer. Furthermore, for the water-soluble organic compounds with chemical concentrations of up to 90%, the observed small changes in hydraulic conductivities of sequential permeability tests are due to the changes in the pore fluid viscosity (Mitchell and Madsen 1987; Bowders and Daniel 1987; Fernandez and Quigley 1988; Storey and Peirce 1989; Acar and Olivieri 1990). Yet, when the chemical concentrations in sequential permeability tests increased beyond 90%, there appears to be a sudden change in clay structure due to the collapse of electrical double layer. However, confinement of clay specimen prevented the collapse of electrical double layer with the permeation of chemicals with concentration greater than 90% (Fernandez and Quigley 1991). Also, the insoluble organic chemicals do not flow through saturated clayey soil (Foreman and Daniel 1986; Uppot and Stephenson 1989). The following are some of the difficulties associated with the aforementioned findings: 1. Can there be a change in clay structure (dispersed to flocculated) under 726
in situ conditions that is associated with a volume change over a geologically short time when pure organic chemicals are permeated through clays? 2. Why is there a sudden change in clay structure when the concentrations of chemicals exceed 90%? 3. If there is a decrease in volume due to change in clay structure (or shrinkage), confinement of the sample should facilitate such a soil structure change. Why doesn't that soil structure change occur in a sequential hydraulic conductivity tests with flexi-wall permeameters or fixed-well permeameters with confining pressures? Therefore, to further understand the mechanism behind the change in hydraulic conductivity due to permeation of organic chemicals, and to propose an approach to estimate the change in hydraulic conductivity due to the short-term and long-term exposures of soils to organic chemicals, the following hypothesis is proposed and tested. Since the soils and chemicals are products of the nature, in this paper the word short-term is used to describe time periods shorter than two years. The word long-term is used to describe geologically long time periods, say longer than 10 years. However, the time span depends on the reactive nature of soils and chemicals, and other factors such as quantity of chemical flow through soil. HYPOTHESIS
Under short-term conditions (even when several pore volumes were permeated) with the presence of confining pressure the pore fluid in the interconnected voids is replaced by the permeating chemicals. However, due to the long-term chemical contamination of a soil, there is a change in soil structure causing a change in mechanical properties of the soil. It is difficult to measure hydraulic conductivity values in the laboratory for durations more than two years. Brutsaert (1987), performed several hydraulic conductivity tests, which lasted nearly 100 days. He also performed extensive chemical analysis of the pore fluids. Based on the test results, he concluded that there were some chemical reactions between the soil and the permeating fluids, but durations of experiments were too short to be conclusive. If the permeating fluid is quite different from the equilibrium pore fluid in the soil-water mixture, as in the case of leachate from landfills seeping through uncontaminated soils, there will be gradual replacement of pore fluids in the soil pores that do not contribute to fluid flow by the permeating fluid due to the chemical process of diffusion to achieve a chemical equilibrium. This process can continue for a long period to achieve a thermodynamic equilibrium, because the driving force (or the chemical gradient) may be comparatively weak compared to the intermolecular forces. The gradual replacement of pore fluid molecules by the molecules of the permeating fluid may lead to a modification of net intermolecular forces. But the confined pressure and the locked-in stresses that were developed during the formation of soil fabric will restrain any change in soil structure. However, a sudden input of energy can cause an immediate change in clay structure as in the case of sensitive soils. Therefore, the worst-case scenario, for the computation of the fate and transport of chemicals through soils, should consider the hydraulic conductivity of the soil having the long-term clay structure produced by leaching and replacing all the pore water by the desired chemical. One way to simulate this long-term condition is to premix the soil with the permeating fluid and to measure the hydraulic conductivity 727
of the resulting soil-chemical mixture. The structure created from the aforementioned process will simulate a worst-case scenario. In this paper longterm test results are obtained from soils premixed with chemicals and permeated with the same chemicals, and the short-term test results are obtained from soils mixed with water subjected to confinement and sequentially permeated with chemicals. In this investigation three soils were exposed to varying concentrations of three chemicals and to pure hexane. To test the above hypothesis several hydraulic conductivity tests were conducted using flexi- and fixed-wall permeameters. The precontaminated and consolidated soil samples were permeated with fluids having the same chemical composition as the pore fluid. Then the permeating fluids were switched to perform the sequential hydraulic conductivity tests. EXPERIMENTAL PROCEDURE
Three different soils were used in this investigation, two artificial soils and one natural soil. These soils were selected such that they represent a high plastic or a fat clay, a low plastic or a lean clay and a silty clay. The two artificial soils were hydrite PX from Georgia Kaolin (soil 1 and also referred to as Kaolin), 15% white bentonite (GK129 from Georgia Kaolin) and 85% hydrite PX (soil 2 and also referred to as Mixed soil). The natural soil was New Brunswick clay from Brunswick formation in northern New Jersey (soil 3 and also referred to as New Brunswick Soil). Some of the engineering properties of these three soils are listed in Table 1. In this research, instead of compacting the soil, a soil slurry was made and consolidated to a desired stress level. Then the hydraulic conductivity tests were performed on consolidated soil samples. There were several reasons to adopt this procedure. Through the application of back pressure in a flexi-wall permeameter, compacted soils can be saturated and the maximum hydraulic conductivity can be determined. However, since pore air in the compacted soil sample is dissolved in pore fluid, it may be difficult to estimate the dielectric constant o f the pore fluid for quantitative analysis. The procedure adopted in this research also eliminates the influence of air on the formation of soil structure and simulates in situ soil structure. It produces a soil structure only due to the interaction of soil and pore fluid, and it eliminates the problems associated with interactions of pore air with the soil structure and the permeating fluid. Since saturated and consolidated soil samples were used in this study, there is no contribution to the measured hydraulic conductivity values from the capillary forces, which were found in compacted soils. The four chemicals used in this investigation were glycerol, 1-propanol, acetone, and hexane. The first three fluids were selected as they are water TABLE 1. Basic Soil Properties of Clays Tested
Soil (1)
Liquid limit (%) (2)
Plastic limit (%) (3)
Specific surface area (m2/g) (4)
Percent finer than 2 #,m (5)
Kaolin Mixed soil New Brunswick soil
48 73 37
36 35 34
55 128 98
84 88 18
728
TABLE 2. 25~
Measured Physical and Chemical Properties of Pore Fluids Tested at
Fluid (1) Water 12.5% Propanol + water 25% Propanol + water 50% Propanol + water Propanol 16.54% Glycerol + water 28.39% Glycerol + water 44.22% Glycerol + water Glycerol 12.5% Acetone + water 25% Acetone + water 37.5% Acetone + water Acetone Hexane
Absolute viscosity
Mass density (kg/m 3) {2}
(cp) (3)
998 973 949 900 803 1,040
0.89
1,071
1.63
1.09 1.38 1.87
1.90 1.18
1,113
3.83
1,261
1,490.0 1.03 1.03 1.04
972 946 920 789 660
0.316 0.294
Dielectric constant (4) 78 72 64 50 20.1 71 66 60 42.5 72 64 57 20.7 1.9
Conductivity (mohs/crn) (5} 5xlO
-4
5•
-4
5X10
-4
5xlO
-4
5X10
-4
5 x l O -4 5 x l O -~ 5xlO
-4
5X10
-4
5xIO
-4
soluble and represent a high, m e d i u m , and low dielectric constant organic fluids, respectively. The low water soluble hexane was used in a limited number of tests for the purpose of comparison of simulated long-term conditions. The proportions of three fluids were selected such that they cover a full range of possible mixtures as explained below. NaC1 was a d d e d to all the solutions to maintain a similar ionic concentration. The propanol percentage was limited to 50% due to a limitation in electrical conductivity (it was difficult to dissolve the desired a m o u n t of NaC1); glycerol to 44% due to a limitation in viscosity (it was difficult to mix and p e r m e a t e with higher percentages of glycerol); and acetone to 37.5% due to a limitation in evaporation and to limit health and safety problems. M e a s u r e d physical and chemical properties of the fluids and the mixtures of fluids considered in this investigation are listed in Table 2. All three soils were sieved through a 200 U.S. standard sieve in o r d e r to eliminate the coarse particles (if any). The chemicals were mixed with water to obtain a desired solution and a sufficient amount of NaC1 was a d d e d to bring the solution to a conductivity value of 1.2 x 10 3 mohs/cm. A sufficient amount of the p r e p a r e d solution (about four times the liquid limit of the soil) was a d d e d to the soil and was mixed for about 4 hours using a mechanical mixer, to obtain a viscous slurry. A t this point the slurry was passed through the 200 U.S. standard sieve. The aforementioned procedure ensured the soil to behave as a sedimented normally consolidated soil from a diluted suspension of soil (Olson and Mitronovas 1962). The slurry was kept for 5 days in o r d e r for it to attain an ionic equilibrium with the pore fluid. The slurry was then p o u r e d into a specially designed 63.5-mm-diameter and 203-ram-high teflon consolidometer. Each soil specimen was consolidated by applying vertical loads. The initial vertical pressure was limited to 0.9 kPa (0.13 psi) as higher pressures resulted in oozing the soils from the sides in the form of a viscous slurry. W h e n the specimen was fully consolidated ( M e e g o d a and A r u l a n a n d a n 1986) the load was doubled. The above p r o c e d u r e was r e p e a t e d until the m a x i m u m total
729
vertical pressure was 198 kPa (28.7 psi). A small portion of soil was then removed from the specimens for water-content measurements. The specimen was then installed in the appropriate permeameter. The consolidated soil specimen was extruded from the consolidometer and was placed on top of a flexible-wall permeameter with 2.5-in. sample diameter. Two porous stones were placed between the specimen and, top and bottom plates. The two plates were lightly coated with vacuum grease. A rubber membrane was fitted to the sample with two " O " rings. The membrane was thoroughly checked for possible leakages, by immersing it in water. A cell pressure of 124 kPa (18 psi) was applied and the specimen was TABLE 3. Hydraulic Conductivity and Water Content Results from Flexi-Wall and Fixed-Wall Permeameters Flexi-wall Permeameter
Soil type and permeant type
Hydraulic conductivity at 25~ x 10 8 cm/s
(1)
(2)
(%) (3)
10.0 6.0
Soil 3--Water ---Propanol Soil 3--12.5% Acetone --Propanol Soil 3--25.0% Acetone --Propanol Soil 3--37.5 % Acetone --Propanol Soil 2--Water --Propanol Soil 2--12.5% Propanol --Propanol Soil 2--25.0% Propanol --Propanol Soil 2--50.0% Propanol --Propanol Soil 1--Water --Propanol Soil 1--16.5% Glycerol --Propanol Soil 1--28.4% Glycerol --Propanol Soil 2 4 4 . 2 % Glycerol --Propanol
Water content
Fixed-wall Permeameter Hydraulic conductivity at 25~ x 10 - 8 crn/s
Water content
(4)
(%) (5)
36.0 31.3
15.0 5.7
35.7 31.1
9.0 6.0
38.0 32.0
8.0 2.8
38.3 31.0
5.6 3.0
33.9 27.1
7.6 6.0
31.1 28.0
10.6 4.0 1.0 0.22
41.6 30.3 57.1 48.1
9.5 3.8 1.1 0.30
39.6 30.9 55.6 46.9
1.1 0.65
52.7 46.3
2.5 1.2
52.3 45.0
3.5 1.4
53.5 46.7
3.0 1.9
54.4 44.5
3.9 3.5 18.0 5.6
50.1 45.8 56.0 52.0
3.7 2.7 18.0 7.2
51.1 42.6 57.1 49.4
9.0 5.0
49.8 47.4
5.0 2.5
48.1 44.0
5.0 3.0
37.9 47.6
5.0 4.8
37.7 41.0
2.4 3.9
57.4 40.3
3.0 2.0
56.1 38.8
730
allowed to further consolidate. The cell pressure of 124 kPa (18 psi) corresponded to almost the same hydrostatic pressure as that when the specimen was subjected to the highest vertical pressure in the consolidometer. The coefficient of lateral earth pressure at rest (or K0) was assumed as 0.44 in the aforementioned calculations (Meegoda and Arulanandan 1986). When the sample was completely consolidated, inflow and outflow valves were closed and the cell pressure was increased to 23 psi 159 kPa (23 ksi) to apply a back-pressure of 41 kPa (6 psi). Then the sample was permeated under a desired pressure difference (mainly 35 kPa [5 psi]). At this stage the cell pressure value was 159 kPa (23 psi) and the outlet and inlet pressures were 41 kPa (6 psi) and 76 kPa (11 psi), respectively, if the pressure difference was 35 kPa (5 psi). In the flexi-wall permeameter, the valve connected to the bottom of the sample was connected to the inflow port of the pressure panel. This eliminated the consolidation of the soil sample due to seepage forces, as seepage forces acted in the opposite direction to consolidation pressure. The same pore fluid that was used to mix the soil was sent through the sample as the permeant and permeability measurements were taken until the volume changes at the inlet and outlet were about the same. The hydraulic conductivity of the specimen when subjected to an effective cell pressure of 124 kPa in a flexi-wall permeameter, corresponded to almost that when the specimen was subjected to a vertical pressure of 198 kPa inside the consolidometer as both had almost the same average confining pressure and hence almost the same soil structure. (The water content values shown in Table 3 indicate similar void ratio values for specimens in both fixed-wall and flexi-wall permeameters.) To keep the same soil structure during the permeability test, the pore fluid used to prepare the soil was used as the permeant. Twenty four hours after the inflow became equal to the outflow, the permeant was changed to propanol and the above measurements were repeated. A bladder accumulator was connected between the permeameter and the pressure panel to separate the permeant from the distilled water used in the pressure panel. This procedure eliminated the contamination of pressure panel. Each time the permeant was changed, the bladder accumulator was thoroughly washed and all the air bubbles in the bladder accumulator and the connecting tubes were flushed out. The following measurements were. made continuously while recording the duration of the test: 1. Outlet, inlet and chamber pressures in psi. 2. Outlet, inlet, and chamber volume changes in ml. 3. Temperature. At the end of the permeability test, the specimen height was measured and the whole specimen was used for the determination of the water content. The aforementioned measurements were used to calculate the change in hydraulic conductivity with time. The fixed-wall permeameter used in this investigation was similar to a consolidometer type permeameter shown in Fig. 1, and the connection details are illustrated in Fig. 2. The bottom drain was used as the inlet and the top was used as the outlet. This eliminated the consolidation of the specimen due to seepage forces as in the case of tests using flexi-wall permeameter. The sample was installed in the permeameter and a 198 kPa (28.7 psi) vertical pressure was applied. The specimen was then allowed to consolidate and stabilize under a vertical pressure of 198 kPa (28.7 psi). At 731
Load
Steel Rod
Out-let "L.r
Top Cap "O" Rings ~
------1 IP L
Teflon Cylinder Pern~ating Fluid
|
-
I
l
Perforated R i g i d Disk
q '" II"tl" II
~orOUs Stone
3ample
In-let ~
~
~
- ~ Bottcm Cap
FIG. 1. Schematics of Fixed-Wall Consolidation Type Permeameter
the end of the consolidation, the inlet pressure was increased to 52 kPa (7.5 psi). The outlet pressure was increased to 17 kPa (2.5 psi) in order to apply a pressure difference of 35 kPa (5 psi) and a back pressure of 2.5 psi. As in the flexi-wall permeameter, the same pore fluid used for the specimen preparation was used as the permeant. The permeant was separated from the pressure panel by a bladder accumulator. The permeability test was continued until the inflow was equal to the outflow. Twenty four hours after the inflow became equal to the outflow, the permeant was replaced with propanol and the permeability test was continued until the fluid from the outlet was about 100% propanol. At the end of the permeability test, the 732
Pressure Supply-.....~
ressureRegulator PressurePannel J~ol~neBurettes
:
.road
,j ~-Rigid-Wall Permeameter BladderAcurmal~,
FIG. 2.
Setup for Fixed-Wall Permeameter
soil specimen was removed and the height and the water content of the specimen were determined. The measured inflow and outflow rates were reduced to calculate the hydraulic conductivity. Based on the aforementioned procedure a total of 12 flexi-wall and twelve fixed-wall permeability tests were performed. On average each specimen took 20 days to complete the consolidation. At the end of the consolidation average height of the specimen was around 3 in. The specimens were then mounted in respective permeameters and were allowed to consolidate for one more day to reach their past stress state in the consolidometer. Then the specimens were permeated with the same fluid as their pore fluid for about 7 days until a steady state was reached. Then the permeating fluid was switched to propanol and the test was continued for about another 10 days until the outlet fluid was predominantly propanol. The mixed soil (soil 2) with water as pore fluid and tested in the flexi-wall permeameter was continued for nearly six months to further demonstrate the hypothesis that under short-term conditions (six months was assumed as a geologically short period) there is no change in soil structure when a soil under confinement subjected to a different chemical environment. For the permeability tests with 100% hexane, all three soils were mixed with hexane similar to other tests and consolidated to a vertical pressure of 198 kPa (28.7 psi) and hydraulic conductivities were measured using fixedwall permeameters. For this test series NaCI was not added. Then the 733
2-
1,8-
E o
1.61.4-
I--
o 121 z O o _J
< rr 121 >T
1.
0.80.60.40.20
o
lO'OO 2O'oo aO'oo
40'00 50'00 TIME (min)
-m-kin
~
kout
60'00
70'00
8000
]
FIG. 3. Hydraulic Conductivity with Time for New Brunswick Soil in 12.5% Acetone Solution from Sequential Permeability Test with 12.5% Acetone and 100% Propanol as Permeating Fluids in Flexi-Wall Permeameter
consolidometer was converted to a fixed-wall permeameter by filling the cell with hexane and attaching the top cap (see Fig. 1). Then the hydraulic conductivity was measured using the procedure described under fixed-wall permeameter. In this test series the permeating fluid was not changed or sequential permeability test was not performed. EXPERIMENTAL RESULTS
Permeability tests were performed in flexi-wall and fixed-wall permeameters for all three soils with either pore fluid or propanol as the permeant. Figs. 3 - 8 show the variation of hydraulic conductivity with time for five selected cases. The variation of hydraulic conductivity at 25~ calculated based on the inflow and outflow rates, and corrected for the temperature fluctuations, were plotted against the time. Table 3 summarizes the hydraulic conductivity results. It can be observed from Figs. 3 - 8 that the measured hydraulic conductivity values fluctuated with time. If the sampling duration was small, these fluctuations may be due to the precision of the volume measurement of flow rate. The smallest volume measurement was 0.1 ml. Therefore, each volume measurement had a +0.05 ml error. For long sampling durations the fluctuation was mostly due to temperature. Even though temperature corrections were applied calculations were not accurate, as corrections were based on the average temperature of the two consecutive readings. This average may not be the true average especially in the winter and summer months. Since the laboratory was not temperature controlled, during the middle of the night the temperatures may reach extreme values and may come back to normal temperatures in the morning. This error can 734
1
9
o~
0.9t
0.8 0.7 0.6
g 0.s
i
0.3 0.2
"i-
o
;
TIME (rain) (Thousands)
l--l--kin
8
I'o
2
, koutJ
FIG. 4. Hydraulic Conductivity with Time for Mixed Soil in 25.0% Propanol Solution from Sequential Permeability Test with 25.0% Propanol and 100% Propanol as Permeating Fluids in Flexi-Wall Permeameter
only be eliminated by performing tests in a temperature-controlled environment. The hydraulic conductivity based on the outflow rate was mostly smaller than that based on inflow rate. This was found to be due to the difference in chemical compositions of the pore fluid and the permeating fluid. In a separate electrical conductivity measurements, the pore fluid and the permeating fluid showed slightly different electrical conductivities. This may be due to the salts present in the soils and due to the preferential adsorption of chemicals. There were a few isolated cases where the hydraulic conductivity, based on outflow rate, was higher than that based on inflow rate. This was found to be due to membrane leakage. Those tests were repeated. For the fixed-wall permeameter the hydraulic conductivity based on outflow was always equal or smaller than that based on inflow rate. The initial segments (up to the vertical arrow shown in each figure) of Figs. 3, 4, and 5 show the selected test results from flexi-wall permeameters where the fluid used to prepare the soil was used as the permeating fluid. As stated above, though there were minor fluctuation in the hydraulic conductivity values, they remained almost constant. Also, it is clear from Figs. 3, 4, and 5 that after about six to seven days the inflow becomes almost equal to the outflow. The final segment (beyond the vertical arrow in each figure) of Figs. 3, 4, and 5 show flexi-wall permeameter test results with propanol as the permeating fluid. These figures indicate that there is either an increasing or a decreasing trend in the hydraulic conductivity values. The increasing trend occurred when the kinematic viscosity of propanol was lower than that of the pore fluid. The decreasing trend occurred when the kinematic viscosity of propanol was higher than that of the pore fluid. Changes in
735
1
"6" | E
0"9 0.8t 0.7 0.6-
I-o D c3 cn 0.5z 0 o F_.. 0.4._1
D
0.30.2-
121 I
0.1 0
+
0
I+
8
TIME (min) (Thousands)
kin
1'o
1'2
16
,- kout I
FIG. 5. Hydraulic Conductivity with Time for Kaolin in 44.2% Glycerol Solution from Sequential Permeability Test with 44.2% Glycerol and 100% Propanol as Permeating Fluids in Flexi-Wall Permeameter
hydraulic conductivity ceased after about 7-10 days; thereafter, the hydraulic conductivity value remained almost a constant. At this stage, the fluid from the outlet was essentially propanol. The initial segment of Figs. 6 and 7 show the permeability test results from the fixed-waU permeameter with the fluid used to prepare the soil as the permeating fluid. The final segments of Figs. 6 and 7 show the permeability test results from the fixed-wall permeameter with propanol as the permeating fluid. Observed trends were similar to those observed for the flexi-wall permeameter. During the permeation of propanol using the flexi-wall permeameter, there was no change in chamber volume. Fig. 8 shows the results of nearly six-months long permeability test for mixed soil, with water as pore fluid and propanol as permeating fluid. During this test the volume measuring device (which could accurately measure up to 0.1 ml or approximately 0.15 % of the pore volume of the specimen) attached to the cell pressure remained virtually constant. The hydraulic conductivity value decreased during the first two weeks and remained almost constant during the rest of the test except for minor fluctuations. TEST RESULTS
Twelve flexi-wall and twelve rigid-wall permeability tests were performed and the results are shown in Table 3. Both permeameters produced similar results (Table 3), suggesting that the version of the rigid-wall permeameter used in this study is an acceptable device to measure hydraulic conductivity
736
~"
0.9
oE
0.8-
i~
0.7-
~o D a
~ 0.6,~a 0.5
Z
00 --~ 0.40"i 0.3r162 < a >I
0.2
0.10
o
2
8
1'o
1'2
14
TIME (min) (Thousands) l--~-kin
,
kout ]
FIG. 6. Hydraulic Conductivity with Time for Mixed Soil in 25.0% Propanol Solution from Sequential Permeability Test with 25,0% Propanol and 100% Propanol as Permeating Fluids in Fixed-Wall Permeameter
of soils. The fixed-wall permeameter used in this investigation simulates the Ko condition, and hence the in situ stress state. Therefore, the version of fixed-wall permeameter used in this study may be considered as a better choice over the flexi-wall permeameter due to its ability to simulate the in situ stress and flow conditions. During the sequential permeation of propanol using the flexi-wall permeameter, there was no change in chamber volume. Also during the nearly six-month permeability test the level of volume measuring device that was attached to the cell pressure remained virtually constant. The aforementioned results were consistent with observations made by Uppot and Stephenson (1989). The value of hydraulic conductivity, from the six-months long permeability test (see Fig. 8), decreased during the first two weeks and remained almost constant during the rest of the test. During permeation of propanol in the sequential permeability test, since there was no detectable volume change in the sample, the clay structure should remain the same. Therefore, the change in hydraulic conductivity was due to change in kinematic viscosity of the permeating fluid. The original and final fluid content values shown in Table 3 further confirms the observation that, where the variation in water (or fluid) content is due to change in density of the pore fluid. The fact that there was no change in intrinsic permeability during shortterm tests, may be explained using (1). Intrinsic permeability can be calculated using (1) if the hydraulic conductivity of the specimen and the kinematic viscosity value are known. Now, if a sequential permeability test was performed on the same specimen and the hydraulic conductivity was 737
oE I-o D 121 z 0 o F_. 2o ..../
n-121 I
10
m
0
2
4
6
8 1'0 TIME (min) (Thousands)
1'2
1'4
16
]
k in
~ k out l
FIG. 7. Hydraulic Conductivity with Time for Kaolin in 16.5% Glycerol Solution from Sequential Permeability Test with 16.5% Glycerol and 100% Propanol as Permeating Fluids in Fixed-Wall Permeameter
evaluated when the chemical compositions of inflow and outflow fluids were the same, then it is possible to use (1) again to calculate the intrinsic permeability. If the ratio of the two hydraulic conductivities (one with pore fluid as permeating fluid and the other from sequential permeability test) was the same as inverse ratio of the kinematic viscosities of the two permeating fluids, then the intrinsic permeability should be a constant during shortterm tests. Table 4 shows the hydraulic conductivity ratios from sequential permeability tests using the flexi-wall and fixed-wall permeameters along with the inverse kinematic viscosity ratios. The correlation coefficient (r) for the two data columns was equal to 0.71 for all the data points and was equal to 0.85 for fixed-wall permeameter results. Therefore, it may be concluded that for short-term permeability tests with confinement, if there is a change in hydraulic conductivity value, it is due to the change in kinematic viscosity of the permeating fluid. For long-term permeation (more than 10 years), as stated earlier, it is difficult to measure hydraulic conductivity values in the laboratory. If the permeating fluid is quite different from the equilibrium pore fluid in the soil-water mixture, the soil structure may change slowly, which may take a long time (longer than the duration of laboratory permeability tests for the complete reaction). One way to simulate the long-term condition is to premix the soil with the permeating fluid and measure the hydraulic conductivity of the resulting soil water-chemical mixture. Test results in Tables 5, 6, and 7 show how the simulated long-term hydraulic conductivities and the intrinsic permeabilities vary with different chemical environments for the three soils. Tables 5, 6, and 7 also show selected geotechnical properties for the three
738
Ill
E
0.9 -3 5t-
I-o Izl
4-
z
0 0 0
--I-
~Z o.5-53 ID
..
++ + + ~a~-
< n.a >-r"
+ . ,~- ~,..T~-+~
....+
_J
4+ 0.1~ 0
, 20
, 40
, 60
~
-b
8'0 1O0 TIME (days)
kin
[]
kout
120
140
160
]
FIG. 8. The Nearly Six Months Long Permeability Test, Hydraulic Conductivity with Time for Mixed Soil in Water with 100% Propanol as Permeating Fluid in FlexiWall Permeameter
TABLE 4. Comparison of Measured Hydraulic Conductivity Results from FlexiWall and Fixed-Wall Permeameters
Soil type and permeant type
Inverse of kinematic viscosity ratio
Hydraulic conductivity ratio from flexi-walt permeameter
Hydraulic conductivity ratio from fixed-wall permeameter
(1)
(2)
(3)
(4)
Soil 3--Water/Propanol Soil 3---12.5% Acetone/Propanol Soil 3---25.0% Acetone/Propanol Soil 3--37.5% Acetone/Propanol Soil 2--Water/Propanol Soil 2--12.5% Propanol/Propanol Soil 2--25.0% Propanol/Propanol Soil 2--50.0% Propanol/Propanol Soil 1--Water/Propanol Soil 1--16.5% Glycerol/Propanol Soil 1--28.4% Glycerol/Propanol ;oil 1--M4.2% Glycerol/Propanol
2.66 2.73 2.18 2.10 2.66 2.10 1.62 1.14 2.66 2.08 1.55 0.68
1.66 1.50 1.87 2.50 4.54 1.69 2.50 1.11 3.21 1.80 1.66 0.62
2.63 2.85 1.27 2.50 3.67 2.08 1.58 1.37 2.50 2.00 1.04 1.50
Note: inverse of kinematic viscosity ratio = kinematic viscosity of propanol/kinematic viscosity of pore fluid; and hydraulic conductivity ratio = k with pore fluid as permeating fluid/k with propanol as permeating fluid.
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TABLE 5. icals
Geotechnical Properties of New Brunswick Soil with Different Chem-
Chemical type and concentration
Intrinsic Liquid limit
(1)
(%) (2)
Water 12,5% Acetone 25,0% Acetone 37.5% Acetone 100% Hexane
37.0 38.0 38.0 39.0 34.0
TABLE 6.
Plastic limit (%) (3)
Fluid content (%) (4)
34.0 28.0 29.0 28.0
36.0 38.0 34.0 42.0 33.0
Liquid limit
Plastic limit
Fluid content
concentration (1)
(%) (2)
(%) (3)
(%) (4)
Water 12.5% Propanol 25.0% Propanol 50% Propanol 100% Hexane
73.0 73.0 71.0 70.0 66.0
35.0 28.0 28.0 29.0
57.0 52.0 53.0 53.0 56.0
1.0 1.1 7.6 1.3 8.8
x x x x x
10 -16 10 -16 10 -17 10 -16 10 -15
Intrinsic permeability (m 2)
(5) 1.0 1.4 5.9 9.4 3.4
X • X X X
10 -17 10 -17 10 -17 10 -17 10 -15
Geotechnical Properties of Kaolin with Different Chemicals
Chemical type and concentration
(m 2) (5)
|eotechnical Pro )erties of Mixed Soil with Different Chemicals
Chemical type and
TABLE 7.
permeability
Liquid limit
Plastic limit
(1)
(%) (2)
(%) (3)
Water 16% Glycerol 28% Glycerol 44% Glycerol 100% Hexane
48.0 56.0 64.0 68.0 40.0
36.0 28.0 27.0 28.0
Fluid content (%) (4) 56.0 50.0 38.0 57.0 62.0
Intrinsic permeability (m 2) (5) 1.9 1.7 1.2 1.3 3.4
x • x x x
10 -16 10 -16 10 -16 10 -16 10 -is
soils tested under the simulated long-term conditions. Th e data suggests that the introduction of organic chemicals causes a consistent increase in the intrinsic permeability of mixed soil under a given stress level. T h e other geotechnical properties do not indicate the same strong trend. Th e o t h er two soils did not show a m a r k e d change. The explanation for this behavior will be provided in another technical paper that will also attempt to predict the hydraulic conductivity using soil parameters such as total void ratio, anisotropy, and specific surface area. It can also be seen that the ratio of the permeability of a contaminated soil to that of the same soil with water as a pore fluid was weakly correlated to the decrease in the dielectric constant of the pore fluid. It is interesting to note that the m a x i m u m increase in the intrinsic permeability for all three soils is about three orders of magnitude. H o w e v e r , increase of up to five orders magnitude of hydraulic conductivity has been reported for compacted clays p e r m e a t e d with organic chemicals (Mitchell and Madsen 1987).
740
The results of the fixed-wall permeameter test without confined pressure, which showed a large increase in the hydraulic conductivity during a sequential permeability tests with concentrated organic fluids (Mitchell and Madsen 1987), may not be only due to the change in the clay structure. The lack of confinement may have accelerated the replacement of water molecules by chemicals. This causes the soil sample to shrink and crack. Flow through those cracks may have caused the hydraulic conductivity to increase by several orders of magnitude. The confining pressure retards the osmotic and the diffusive processes and provide identical results from the flexi-wail permeameter and the fixed-waU permeameter with confined stresses. Peirce et al. (1986), discussed the importance of overburden pressure and showed that overburden pressure should be considered in all hydraulic conductivity tests involving landfills. The writers believe that soils beneath landfills will have some overburden pressure, and hence, the hydraulic conductivity tests with zero confined pressure do not represent the worst conditions but a nonrepresentative test condition. It is a nonrepresentative condition because the permeating fluids flow through the shrinkage cracks. The high hydraulic conductivity values in fixed-wall permeameters during permeation of organic chemicals exceeding 90% should be due to this shrinkage cracks and not a change in soil structure. Since there should not be any change in soil structure, flexi-wall permeameters did not show large changes in hydraulic conductivity during permeation of pure organic chemicals. Fernandez and Quigley (1988) showed that there was no appreciable change in intrinsic permeability even with concentrated organic chemicals under a confined stress. Fernandez and Quigley (1991) showed that only a vertical pressure value less than 160 kPa (23 psi) showed an increase in hydraulic conductivity of soil from Sarnia. Brutsaert (1987), made similar conclusions based on fixed-wall permeameter without applying confining pressure but his testing arrangement produced some confinement. If one wants to apply the test results in this paper to field conditions, then he should: (1) Determine the short-term hydraulic conductivity of the soil with the leachate or with the chemical as permeating fluid using either a flexi-wall permeameter or a fixed-wall permeameter with confining pressure simulating the field conditions; and (2) the long-term hydraulic conductivity of the same soil premixed with the leachate or the chemical and permeated with the same leachate in either flexi-wall permeameter or fixed-wall permeameter with confining pressure simulating the field conditions. SUMMARY AND CONCLUSIONS
It may be concluded from the test results obtained in this study that there is no change in intrinsic permeability of a soil in a sequential permeability test if the second chemical is water-soluble and the soil sample is subjected to a confining pressure between 100-200 kPa. The hydraulic conductivity will be adjusted according to the kinematic viscosity of the nonpolar permeating fluid. In this situation there is mere replacement of pore fluids in the interconnected voids by the permeating fluid. When chemicals are allowed to permeate through a soil for an extended time period, there will be a gradual replacement of pore fluids in the soil pores that do not contribute to fluid flow by the permeating chemicals. The gradual replacement of pore fluid molecules by the permeating chemicals leads to a modification of intermolecular forces. Hence due to the longterm chemical contamination of soils, the soil structure may change causing a change in mechanical properties of the soil. However, confining pressures 741
and locked-in stresses during the initial formation of soil structure tend to restrain change in soil structure. One way to simulate the final long-term and worst-case clay structure is to premix the soil with the permeating fluid. Associated with the change in the soil structure there is an increase in hydraulic conductivity and intrinsic permeability of soils. Therefore, the structure created from the aforementioned process simulates a worst-case scenario with respect to hydraulic conductivity. The increasing trend in the change of permeability of a given soil is weakly correlated to the decreasing dielectric constant of the pore fluid in the soil. ACKNOWLEDGMENTS
The work described in this paper was supported by a grant from the National Science Foundation (grant CES-8708834). The continued research support from Dr. Edward Bryan of National Science Foundation is gratefully acknowledged. The constructive suggestions from the three anonymous reviewers greatly enhanced the presentation. APPENDIX.
REFERENCES
Acar, Y. B., and Olivieri, I. (1990). "Pore fluid effects on the fabric and hydraulic conductivity of laboratory-compacted clay." Transp. Res. Rec., 1219, 144-159. Babcock, K. L. (1963). "Chemical properties of soil colloids-section ]II." Hilgardia, 31(II), 471-480. Bowders, J. J., and Daniel, D. E. (1987). "Hydraulic conductivity of compacted clay to dilute organic chemicals." J. Geotech. Engrg., ASCE, 113(12), 1432-1448. Brutsaert, W. F. (1987). "Suitability of marine clays as hazardous waste site liners." J. Envir. Engrg., ASCE, 113(5), 1141-1148. Fernandez, F., and Quigley, R. M. (1985). "Hydraulic conductivity of natural clays permeated with liquid hydrocarbons." Can. Geotech. J., 22, 205-214. Fernandez, F., and Quigley, R. M. (1988). "Viscosity and dielectric constant controls on the hydraulic conductivity of clayey soils permeated with water-soluble organics." Can. Geotech. J., 25,582-589. Fernandez, F., and Quigley, R. M. (1991). "Controlling the destructive effects of clay-organic liquid interactions by application of effective stresses." Can. Geotech. J., 28, 388-398. Foreman, D. E., and Daniel, D. E. (1986). "Permeation of compacted clay with organic chemicals." J. Geotech. Engrg., ASCE, 112(7), 669-681. Meegoda, N. J., and Arulanandan, K. (1986). "Electrical method of predicting insitu stress state of normally consolidated soils." ASCE Specialty Conf. on Use of In-situ Tests in Geotech. Engrg., Blacksburg, Virginia, June 23-25,794-808. Meegoda, N. J., Rajapakse, R., Gunasekera, S. D., Chang, K. G., Hyjack, R., and Perez-Cobo, A. (1990). "The influence of chemical contaminants on the permeability and effective porosity of clays." Final Report to the National Science Foundation, New Jersey Inst. of Tech., Newark, N.J., Sep. Michaels, A. S., and Lin, C. S. (1954). "The permeability of kaolinite." Industrial and Engrg. Chemistry, 46, 1239-1246. Mitchell, J. K., and Madsen, F. T. (1987). "Chemical effects on clay hydraulic conductivity." Proc. Specialty Conf. on Geoteeh. Practice for Waste Disposal. R. D. Woods, ed., ASCE Geotechnical Special Publication No. 13, ASCE, New York, N.Y., 87-1t6. Olsen, H. W. (1961). "Hydraulic flow through saturated clays," DS thesis, Massachusetts Institute of Technology, Cambridge, Mass. Olson, R. E,, and Miltronovas, F. (1962). "Shear strength and consolidation characteristics of calcium and magnesium illite." Proc. 9th Nat. Conf. on Clays and Clay Minerals, 185-209.
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Peirce, J. J., Sallfors, G., and Murry, L. (1986). "Overburden pressure exerted on clay liners." J. Envir. Engrg., ASCE, 112(2), 280-291. Storey, J. M. E., and Peirce, J. J. (1989). "Influence of change in methanol concentration on clay particle interactions." Can. Geotech. J., 26(1), 57-63. Uppot, J. D., and Stephenson, R. W. (1989). "Permeability of clays under organic permeants." J. Geotech. Engrg., ASCE, 115(1), 115-131.
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