A field trial of a bentonite landfill liner

Waste Management & Research (1991) 9, 27 7-291

A FIELD TRIAL OF A BENTONITE LANDFILL LINER J. W . Cowland and B. N. Leung Civil Engineering Services Department, 9th Floor Empire Centre, 68 Mody Road, Kowloon, Hong Kong

(Received 5 December 1989 and accepted in revised form 15 March 1990)

Field and laboratory trials were carried out for a bentonite amended weathered granite landfill liner . The trials were undertaken to gain experience in the installation method, and to establish the appropriate mixing techniques and mix design . It was established that a bentonite-amended weathered granite liner could be constructed on steep slopes . This paper presents the results of the trials, which included both field and laboratory permeability testing . It was found that the success of a soil bentonite liner is critically dependent on the mixture having the right moisture content at the time of compaction. Key Words-Bentonite landfill liner, field trial, steep slopes, permeability testing

1 . Introduction Following the decision to install a bentonite-amended weathered granite liner in a new landfill in Hong Kong, a trial was carried out . As the technique had not been used before in Hong Kong, the aim of the trial was to gain experience in the installation method, and to establish the appropriate mixing techniques and mix design . The design specification for the liner was that it should be 150 mm thick, and have a coefficient of permeability 9 less than 10_ m s - ' under a 1 m head of leachate . A diagrammatic section through the intended liner and drainage system is shown in Fig . 1 . Field trials were carried out on both flat and sloping ground . The trials investigated mix design, methods of mixing, compaction, strength, protection and drainage layers, and the difficulties of working in wet weather . A series of in situ and laboratory permeability tests were carried out with both water and leachate . A review of the literature on bentonite-amended soil liners indicated that the technique had not been very widely used elsewhere (Stevens, 1984) ; although it had also been used for heap leach pads (Van Zyl 1982) . The literature review indicated some aspects that merited further consideration . 1 .1 Quality of bentonite

The most important property of bentonite for a landfill liner is its ability to swell, thereby sealing cracks in the liner . Bentonite is a naturally occurring material, consisting of the clay mineral montmorillonite . It has been formed as a deposit of volcanic ashes at shallow wet sites in various locations around the world (Grim & Guven 1978) . These deposits are variable, depending on the nature of the volcanic ashes and the salinity of the water into which they were deposited . The swelling characteristics of bentonite are essentially related to whether it is formed of sodium montmorillonite (highly swelling) or calcium montmorillonite (less highly 0734-242X191/040277 + 15 $03 .00/0

©1991 ISWA



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J. W. Cowland & B . N. Leung

~`

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Fig . 1 . Diagrammatic section through the liner and drainage system .

swelling) . A lot of bentonites, especially European ones, are naturally occurring calcium montmorillonites which are then sodium enriched in a processing plant . Wyoming bentonite from the U .S .A, occurs naturally with a high sodium content and has a very high swelling capacity . Some bentonites are amended with polymers (Liao 1988) . Hoeks et al . (1987) reported the swelling capacity of various bentonites to range from 2 to 12 ml of water absorbed per gram of dry bentonite . There are some sources of bentonite in Asia, although they are not as well researched or reported upon as American or European bentonite . Diaphragm walling contractors in Hong Kong usually import their bentonite from Europe and the U .S .A ., even though it is more expensive . 1 .2 . Slope angle Bentonite amended soil liners have rarely been installed on slopes steeper than 1 in 3 (18 ° ) (Pywell 1986) ; although Hoeks et al. (1987) reported a successful bentonite capping liner trial carried out on a 1 in 1 .75 (30 ° ) slope . About two-thirds of the area that needs to be lined for the first stage of the new landfill will be formed at a slope of 1 in 2 (27 ° ) . Therefore, it was recognized that the field trials would need to consider performance on an inclined surface . 1 .3 Method of Installation In the early construction of bentonite liners and heap leach pads, dry bentonite powder was sprinkled on the ground, and then mixed in with an agricultural rotavator . Moisture was either added naturally by rainfall, or sprinkled on (Van Zyl 1982, Tewes 1985, Pywell 1986, Garlanger et al . 1987) . This technique did not always produce a homogeneous mix, and was often found to produce unsatisfactory results with regard to impermeability (Garlanger et al. 1987) . More recently, the soil has been excavated and then thoroughly mixed with the bentonite and the water in a pugmill (a mixing box containing twin counter rotating screw feed paddle augers) with a computerized control system . The liner mix is then transported back, and thoroughly compacted . This technique has produced much more satisfactory results with regard to impermeability (Wallace 1987, Hoeks et al. 1987) . 1 .4 Permeability Garlanger et al. (1987) reported successfully achieving a coefficient of permeability of 10 -9 ms - ' for a bentonite liner tested with water . Grantham & Robinson (1988)



Field trial of a bentonite landfill liner

27 9

measured a very low infiltration rate of leachate through a bentonite landfill liner . However Hoeks et al . (1987) measured an increase in permeability from 10 -9 m s - ' to 10 - ' m s - ' for one type of leachate placed in contact with a bentonite liner . The hydraulic conductivity of clayey landfill liners to leachate is a complex subject . For instance, the speed with which a particular chemical can pass through a clay liner can far exceed the speed with which water passes through ; although the quantities involved may be unimportant (Quigley et al . 1987) . 2 . Materials It was decided to carry out the trials with a European bentonite, "Bentonil GTC 4", as it was readily available in bags with a consistent quality . Bentonil GTC 4 is a sodium enriched calcium montmorillonite . Its swelling capacity was found to be 11 ml of water absorbed per gram . A comparison was made in the laboratory between this bentonite and two others . "Premium Gel" from the U .S .A . and "bentonite" from Dalian in China . A series of chemical tests were carried out, and specific gravity and liquid and plastic limits were determined . A summary of the results is presented in Tables I and 2. From these results it can be seen that the bentonites from Europe and the U .S .A . have similar properties, but the bentonite from China is quite different . The proposed landfill is located in a valley where the underlying rock is a coarse grained granite . This granite has weathered in situ from the ground surface to depths of 10-25 m . During excavation the remaining cementation is destroyed, and the material forms a gravelly, silty sand . As the land at the proposed landfill was not yet available, the field trial was carried out in an adjacent valley with the same rock type and topography .

TABLE 1 Chemical analysis of three bentonites Bentonite from Dalian (China)

Type of test Cation exchange capacity, meq 100g - ' Exchangeable Cal', meq 100g - ' Mg", meq 100g - ' K', meq 100g - ' Na', meq 100g - '

243 45 13 2

1

Bentonil GTC 4

Premium Gel

(Europe)

(U .S .A .)

268 61 10 1 54

252 47 8 1 61

TABLE 2 Liquid and plastic limits and specific gravity of three bentonites

Type of test Liquid limit Plastic limit Specific gravity

Bentonite from Dalian (China)

Bentonil GTC 4

Premium Gel

(Europe)

(U .S .A .)

123 36 2 .47

244 30 2 .70

278 28 2 .69



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J. W. Cowland & B. N. Leung

A representative grading curve for the completely weathered granite (CWG) is shown in Fig . 2 . Standard Proctor density tests (British Standard 1377, 1975) were then carried out at different moisture contents, and different bentonite contents, to determine the optimum moisture content for the maximum dry density (Fig . 3) . The water used for the laboratory tests was tap water, and for the field trial there was a ready supply of clean spring water . A chemical analysis of the leachate that was supplied for the trials is shown in Table 3 . 3. Field trials The field trials commenced with the formation of a few panels of liner simply to observe the nature of the material, and progressed through an examination of the mix design, formation of the liner on a slope, to the formation of protection and drainage layers . The trials were started in the dry season and carried through to the middle of the wet season . A series of field permeability tests were carried out with both water and leachate . 3 .1 Mixing technique For the field trials we were provided with a small drum concrete mixer . Using the concrete mixer it was found that there was a tendency for the mixture to form balls, which made the mixture more difficult to compact . During one of the initial mixing trials it was found that the size and quantity of the balls was much larger if the water was added before the bentonite, and also if the weathered granite contained lumps . For the rest of the trials, the stockpile of completely weathered granite was spread out and allowed to dry, coarsely sieved through a 15 mm mesh, and then mixed with the bentonite first before addition of water . This procedure reduced the size and quantity of the balls, and allowed the formation of satisfactory liner panels . However, it was concluded that for the landfill liner itself the contractor should use a variable speed pugmill with a computerized control and feed system . 3 .2 Compaction technique Compaction was carried out with a I t vibrating roller . The 150 mm thickness was built up in two layers with 10 passes of the roller on each . Where necessary lateral confinement was provided by wooden boards, in order to simulate the field laying of adjacent strips or panels . 3 .3 Mix design It was initially thought that the permeability of a bentonite-amended soil would be reduced if compacted at high moisture contents, because of the swelling properties of the bentonite . However, it was found that the strength of the material reduced rapidly with a relatively small increase in moisture content . Therefore, it was decided to follow the standard practice for the compaction of soils . A series of panels, with varying moisture contents, were constructed on flat ground . To facilitate permeability testing a 75 mm thick sand drainage layer was placed under the liner panels . In situ dry density measurements were made, and some results are shown in Table 4 . Figure 4 shows the field compaction data for 5% bentonite content contrasted with the laboratory curve .



Field trial of a bentonite landfill liner

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282

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1 .8

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1 .6

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12

14

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20

22

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TABLE 3 Chemical analysis of the leachate pH Value Biological oxygen demand Chemical oxygen demand Suspended solids Ammoniacal nitrogen Total kjeldahl nitrogen Phosphate Chloride Fluoride Sulphide Sulphate Chromium Zinc Nickel Lead Cadmium Copper

(mg 1 - ') (mg 1 - ') (mg 1 - ') (mg 1 - ') (mg 1 - ') (mg 1 - ') (mg 1 - ') (mg 1') (mg 1 - ') (mg V 1 ) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)

8 .7 75 2900 35 3000 3500 15 3600 3 .9 0 .10 54 1 .4 0 .58 0 .26 <0 .01 <0 .01 0 .08



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TABLE 4 Field permeability tests

Bentonite content (%)

Average moisture content (%)

Average dry density (mg m - ')

Average degree of compaction (%)

Permeability coefficient k (m s - ')

12 .4 15 .0 17 .3 16 .8 17 .0 15 .7 13 .2 17 .9

1 .74 1 .85 1 .79 1 .75 1 .75 1 .74 1 .79 1 .70

96.8 102 .8 99 .5 98 .2 98 .3 96 .7 99 .9 95 .4

1 x 10 -9 (C)0 4 x 10 -10 (C) 3X10-"(C) 2 x 10 -10 (C) 2x10-'O(C) 3 x 10 -10 (C) 3x10-"(C) 5x10-'O(C) 8 x 10 -10 (L)b 1 x 10 -9 (L) 4 x 10 -10 (L) 8 x 10 - ' 0 (L) 3x10-'O(L) 2 x 10 -10 (L)

5 5 5 7 10 5 7 10

b

(C) denotes permeability test with water (L) denotes permeability test with leachate

2 .0

1 .8

1 .7

1 .6 10

12

14

16

18

20

Moisture content (%)

Fig . 4 . Results of field compaction on flat ground compacted with laboratory compaction curve for CWG with 5% bentonite . Key: 0 =field compaction on flat ground .



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J. W. Cowland & B. N. Leung

The main conclusion of these field compaction trials with varying moisture content is that achieving the maximum degree of compaction was very dependant on the mixture being placed with the right moisture content . It was observed that the mixture needed to be placed between optimum moisture content (OMC) and OMC + 4% (where OMC was the laboratory value) . In addition to the degree of compaction, it was found that below OMC + 2% the liner panels were brittle and were susceptible to cracking, and above OMC +4% the liner panels became too soft . It should be noted that the moisture content of the mixture changed rapidly with evaporation, and between mixing and compaction . During the course of the field trials, some liner panels with 7% and 10% bentonite content were formed on flat ground . Some details of these panels are also shown on Table 4 . It was thought that the liner would become more flexible, and therefore less susceptible to cracking, with higher bentonite contents . However, the small benefit in this regard was negated by the increased stickiness of the mixture-which made it more difficult to handle-the increased softness of the liner panels, and an uneven distribution of water content within the mix . In addition, a 7% bentonite content provided adequate impermeability . The effect of changing the bentonite content on the permeability of the liner is discussed in greater detail later .

3 .4 Formation of the liner on a slope A slope adjacent to the flat platform was selected which had an inclination of 28 ° (1 :1 .9), which is similar to the 1 :2 slopes which will be formed for the landfill . Various attempts were made to form a sloping panel of a final thickness of 150 mm in the plane of the slope surface (Fig . 5), but a number of practical problems were encountered . Instead, it was decided to build narrow horizontal layers up the slope, compacting them with the vibratory roller (Fig . 5) . Sixteen horizontal layers were built up the slope, using the mixture with 5% bentonite content . To provide adequate working space, the width of the compacted horizontal layer had to be increased to 1 .4 m, which was about twice the width of the roller . (This resulted in a liner thickness of 0 .7 m .) The outer edges of each layer were compacted first, before the area close to the slope . The trial was then repeated using 7% bentonite content . The main aim of this part of the trials was to determine whether the liner could be formed on a slope . It was found that using horizontal layers was a feasible method, and a satisfactory degree of compaction could be achieved . However, that should hopefully not preclude a resourceful contractor, properly set up with winching equipment and wire ropes, from forming the liner in the plane of the slope . Alternatively, the extra material placed on the slope using horizontal layers could possibly be removed and re-used .

3 .5 Protection layers During the dry season one of the trial panels was covered up with 400 mm of wet sand, to see if that would stop the liner from drying out and cracking . After a few days the panel was uncovered again, and it was found that the sand blanket had been effective in keeping the liner from drying out . As a comparison, at the same time another panel was left uncovered, and it was found that considerable desiccation cracking occurred .



Field trial of a bentonite landfill liner

SECTION

28 5

PLAN

2

Density test

2 3

4

14 13 12

Fig . 5 . Diagrammatic representation of liner trials on slope .

3 .6 Permeability measurements Two series of falling head permeability tests were carried out in the field on panels of the liner; one series using water as the permeant and the other series using leachate as the permeant . For these field tests, open ended oil drums were hammered 50 mm into the 150 mm thick liner, which had an underlying 75 mm thick compacted sand drainage layer . A thin seal of bentonite putty was placed in the few small cracks that developed between the



286

J. W. Cowland & B. N. Leung

panels and the oil drums during placing, and the panel outside the oil drum was covered with a blanket of wet sand . Water or leachate was placed inside to a depth of about 700 mm . It was found that the drop in water (or leachate) level due to evaporation, if the surface was left uncovered, was significantly greater than the drop due to flow through the liner . Hence, in all the permeability tests the surface was carefully covered . Eight field permeability tests were carried out using water as the permeant on panels with 5, 7 and 10% bentonite content . Details of compaction and moisture contents, and the monitoring data, are summarized in Table 4 . These tests were continued until the measurements had been taken for a period of 70-80 days . The permeability of the liner panels was found to be between 10 -9 and 10 -10 ms - ' . No noticeable differences were found for different bentonite contents . At the conclusion of these tests the portion of the bentonite liner that had been submerged was examined . A thin layer on the surface of the liner was found to have changed to a gel-like consistency . Six field permeability tests with leachate as the permeant were carried out on panels with 5, 7 and 10% bentonite content (Table 4) . These tests were continued until the measurements had been taken for a period of 105-120 days . The coefficient of permeability of the liner panels to the leachate was found to be between 10 -9 and 10 -10 m s - ' . There was a slight tendency for the permeability to decrease with increasing bentonite content, and also with time . At the conclusion of these tests the portion of the bentonite liner that had been submerged was examined . A thin layer on the surface of the liner was found to have discoloured and also changed to a gel-like consistency .

4. Laboratory trials To complement the field trials, a series of laboratory tests were carried out to examine the compaction characteristics, permeability and strength of the bentonite/CWG mixture .

4 .1 Bentonite content A series of methylene blue tests (Alther 1983, Higgs 1988) were carried out on CWG/ bentonite samples containing different bentonite contents . This test was developed in the oil drilling industry for checking the bentonite content of soil/bentonite mixtures . The methylene blue solution (C 16 H 18 N3 SC1 .3H 20) was added to prepared soil/bentonite samples on filter paper until a blue ring was observed, indicating the end of a chemical reaction . The test procedure was in accordance with the American Petroleum Institute's RP 131 . The results of the tests are shown in Fig . 6 . As can be seen from this figure, there is a very good linear correlation between bentonite content and the volume of methylene blue solution . For construction control purposes, this test is recommended for checking the bentonite content of the bentonite/CWG mixtures . (It should be noted that the correlation in Fig . 6 is for a particular bentonite type . A new series of tests will be required for the specific bentonite used in construction .)

4.2 Permeability Before commencing the testing, a review was carried out of the most suitable methods of measuring the permeability of low permeability materials in the laboratory .



Field trial of a bentonite landfill liner

287

10

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I 20

I 30 Volume of methyhlene blue (ml)

I 40

I 50

Fig . 6 . Percentage of bentonite vs . volume of methylene blue solution .

4 .2.1 Advantages and disadvantages offixed wall and flexible wall permeameters Over the last few years a controversy has developed over the use of fixed wall vs . flexible wall permeameters for measuring the hydraulic conductivity (k), or coefficient of permeability, of low permeability soils (Daniel et al . 1985) . Fixed wall apparati have the advantage that soils may be compacted inside them (Day & Daniel 1985) . However, there may be imperfect contact between the soil and the inside of the fixed wall cell, which can lead to sidewall leakage and to large, erroneous values of k . Sidewall leakage may be particularly important when the soil is permeated with a leachate which may cause the soil to shrink and to pull away from the walls of the permeameter . Flexible wall apparati allow sidewall leakage to be minimized (Daniel et al . 1984) . They also permit control over vertical and horizontal stresses . However, if the effective stress applied to the soil in a flexible wall cell exceeds the effective stress in the field, the measured k values may be too low . Also, shrinkage in the presence of leachate may remain undetected . In view of these advantages and disadvantages, it was decided to carry out tests concurrently in both fixed wall and flexible wall test cells .

4 .2.2 Hydraulic gradient The hydraulic gradient that will be experienced by the liner in the field is expected to be in the range of 1-10 . However, to carry out a permeability test on a low permeability material within a reasonable time frame requires the adoption of a much higher hydraulic gradient . The American Society for the Testing of Materials recommend a hydraulic gradient of 20 for a material with a coefficient of permeability of 10 -8 m s ', and a hydraulic gradient of 50 for a material with a coefficient of permeability of 10 - 'm s - ' . So hydraulic gradients of 20 and 60 were selected . The use of high hydraulic gradients, and leachate as the permeant, does have the advantage of better simulating the longer term performance of the liner as a barrier to leachate .



J. W . Cowland & B. N. Leung

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4.2 .3 Fixed wall permeability testing with water as the permeant 100 mm high samples were compacted inside a constant head permeameter, consisting of a perspex tube, of internal diameter 88 .5 mm, and a constant head was applied to the top of the tube . Three tests were carried out on samples containing 0, 5 and 7% bentonite content, using water as the permeant. The results, Table 5, indicated that the addition of bentonite decreased the coefficient of permeability of the CWG to water to below 10 -9 m s - ' . 20 mm high samples were compacted in the 75 mm diameter Rowe Cell . The cell is more sophisticated than the permeameter as it allows the samples to be saturated by back pressure and then consolidated before the permeability test is carried out . Three tests were performed on samples with 0, 5 and 7% bentonite content, with an effective consolidation pressure of 50 kPa, and a hydraulic gradient of 20 . The tests were then repeated on the same samples with a hydraulic gradient of 60 . All the tests were carried out with water as the permeant . The results of these tests are also shown in Table 5 . Despite the use of silicone grease to seal the gap between the sample and the side wall of the cell to minimize leakage, and the consolidation of the sample, the measured coefficient of permeability was higher than that obtained in the constant head permeameter . This may be because the cell does not allow a big enough sample to be tested . The 20 mm high sample is only twice the height of the larger (10 mm size) particles of the CWG . Therefore, preferential flow paths could easily form within and around the sample . Nevertheless, the results, Table 5, indicated that the addition of bentonite decreased the coefficient of permeability of CWG to water to about 10 - 'm s - ' .

4 .2 .4 Fixed wall permeability testing with leachate as the permeant A rigid wall permeameter, modified from a compaction mould, was used for these tests . To avoid the leachate contaminating the air pressure system, the pressure gradient was applied via an air/water cell and a water/leachate cell . Four permeability tests were carried out on samples with 5 and 7% bentonite content, compacted to 95% of maximum dry density at 2% wet of optimum moisture content . The results of the permeability tests are shown in Table 6 . For the two tests carried out at higher hydraulic pressure gradients (i = 60 & 200), the specimens became hydraulically

TABLE 5 Laboratory permeability tests with water as the permeant Sample

Coefficient of permeability (m/s) By Rowe cell

Type

CWG CWG with 5% bentonite CWG with 7% bentonite

Density (mg m - ')

1 .74 1 .71 1 .70

Moisture content (%)

15 .3 16 .0 16 .2

By permeameter

Hydraulic gradient i=20

i=60

i=19

-0 1 x 10 5 x 10_' 8 X 10 - °

1 x 10 -6 1 X 10 - ' 8 x 10 -1

2 x 10 -6 3 x 10 -10 < 10 -10



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TABLE 6 Laboratory permeability tests with leachate as the permeant Sample

Type

CWG with 5% Bentonite

CWG with 7% Bentonite

Density (Mg m - ')

1 .71

1 .70

Moisture content

Hydraulic gradient

(%)

i

16 .0

Duration of the test (days)

Coefficient of permeability (m s - ') (by modified compaction mould)

20

34

2 x 10 - e

60

< 1

Specimen was fractured by the high hydraulic gradient

20

53

6 x 10 -10

200

< 1

Specimen was fractured by the high hydraulic gradient

16 .2

fractured . The coefficients of permeability for the two samples with 5% and 7% bentonite content tested at the lower pressure gradient were 2 x 10 -8 and 6 x 10 -10 m s - ' respectively .

4 .2.5 Flexible wall permeability testing with water as the permeant One flexible wall permeability test was performed, using a triaxial cell . It had been intended to carry out more, and also tests using leachate as the permeant . Eventually, only one was carried out because of the time constraints associated with swelling problems . The test was carried out on a specimen, 76 mm in diameter and 152 mm high, of CWG mixed with 5% bentonite . The specimen was compacted to 95% of maximum dry density at 2% wet of the optimum moisture content . It was first saturated by back pressure, and then allowed to swell and consolidate to an effective pressure of 700 kPa . The test was carried out at a hydraulic gradient of 60, and water was used as the permeant . The coefficient of permeability was calculated to be 1 .7 x 10 -10 m s - ' .

5. Conclusions The field and laboratory trials have led to the following conclusions :

• There is a large difference in the properties of different bentonites . Three bentonites readily available in Hong Kong were tested in the laboratory . Two were found to have similar chemical and engineering properties, but the third was found to be quite different. • Assuming a high quality bentonite is used, then a bentonite content of around 5-7% should be adequate to reduce the hydraulic conductivity of the completely weathered granite (a gravelly, silty sand) to 10 -9 m s - ' . • A series of methylene blue tests were carried out on CWG/bentonite samples



J. W. Cowland & B . N . Leung

290

containing different bentonite contents . A very good linear correlation was found

•

between bentonite content and the volume of methylene blue solution . A satisfactorily high degree of compaction was achieved in the field trials . However, the degree of compaction was found to be critically dependent on the mixture having the right moisture content at the time of compaction . In addition to the degree of compaction, it was found that below OMC +2% the liner panels were brittle and were susceptible to cracking, and above OMC +4% the liner panels became too soft .

•

Two series of falling head permeability tests were carried out in the field on panels of the liner ; one series using water as the permeant and the other series using leachate as the permeant . The coefficient of permeability of the liner panels to both water and 9 leachate was found to be between 10_ and 10 -10 m s - ' .

•

A variety of different techniques were used in the laboratory to test the permeability of the bentonite amended CWG . The coefficient of permeability of CWG mixed with 5% bentonite was found to be around 5 x 10 -$ m s - ' . When mixed with 7% bentonite it was found to be consistently lower than 10 -8 m s - ' . The two test results obtained using leachate as the permeant showed no significant differences from those obtained

•

using water as the permeant. It was concluded that the thickness of the liner should be increased to 300 mm, placed in two layers with lapping of strips or panels, so that imperfections would be unlikely to exist throughout the total thickness of the liner .

6. Recommendations

•

During construction of a bentonite liner, the optimum moisture content should be carefully and frequently determined . Allowance should be made for loss or gain of moisture due to evaporation or rainfall during mixing, transportation and placing . The moisture content of the mixture should be frequently measured, to ensure that the mixture is compacted at the right moisture content .

•

The base soil should be dried and coarsely sieved, and thoroughly mixed with the bentonite, before addition of the water . The mixing should be carried out in a variable speed pugmill with a computerized feed and control system .

•

Consideration should be given to placing the liner in two layers, with lapping of strips

or

panels, so that imperfections would be unlikely to exist throughout the total

thickness of the liner . It is also suggested that consideration be given to forming the liner with a road paving machine .

Acknowledgments A large number of people contributed to the trials, especially S . Y . Cheong, K . S . Yuen, A . M . Carbray, H . M . Chu, and W . H . Choi . This paper is published with the permission of the Director of Civil Engineering Services of the Hong Kong Government .

References Alther, G . R . (1983), The methylene blue test for bentonite liner quality control, Geotechnical Testing Journal, 6, 128-132 . British Standards Institution (1975), Methods of Test for Soils for Civil Engineering Purposes, London: BSI 143 pp . Daniel, D . E ., Trautwein, S . J ., Boynton, S . S . & Foreman, D . E . (1984), Permeability testing with flexible wall permeameters, Geotechnical Testing Journal, 7, 113-122 .



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