Mechanical behaviour of landfill barriersystemsL. Edelmann, PhD, DGGT, M. Hertweck, PhD, SIA, USIC and P. Amann, PhD,COST, DGGT, DIA, SIA

& Land®lls may need to be surrounded by

engineered liner systems to prevent dan-

gerous leakage to the environment.

Depending on the location of the liner, that

is, at the bottom, surface or sides of the

land®ll, the system may be highly stressed

in bending or shear. Full-scale tests and

additional ®eld measurements were

carried out to de®ne the ultimate state of

the permeability of horizontal and vertical

compacted clay liners. The results of these

tests can be used in the overall design of

such land®ll liners. The testing equipment

developed may be used to improve new

system designs or to test alternative com-

binations of materials.

Keywords: geotechnical engineering; land-

®ll; research & development

Notationcc cohesion

h height

K hydraulic conductivity

Ko initial hydraulic conductivity

k lateral stress ratio

R radius of curvature

g unit weight

ee strain

een volumetric strain

eeg shear strain

y volumetric water content

f internal friction angle

IntroductionTo protect the environment against leakage

from land®lls, multi-barrier systems have been

developed. One component of such systems

involves technical barriers, which surround the

deposits. Within a land®ll, it is possible to

distinguish between bottom, surface and side

liners.

2. As a ®rst requirement, the arti®cial

barrier must be technically impermeable, which

means low conductivity (e.g. k � 1079 m/s)

and stressed by a `low' hydraulic gradient

(®eld: i � 2; but often in laboratory tests,

i � 30).

3. The sealing component of most liner

systems consists of, or is combined with, clayey

materials. The conductivity will increase and

the containment capacity will be lost due to

deformation or cracking. Bending occurs with

non-uniform settlements of the subsoil (bottom

liner) or of the waste material (capping) (Fig. 1).

The latter will also be in¯uenced by the

sequence of ®lling. Steep wall liners are mainly

strained by shear deformation due to `silo'

conditions, with di�erential settlements

between the deposited material and the side

walls. Additionally, kinematic failure mechan-

isms need to be analysed (Fig. 2).

4. Sloped liners are strained under com-

bined conditions. While the tendency for

bulging is predominant, it may occur in

conjunction with shear failure. The sealing

system can become unserviceable because of

large lateral deformations or collapse under

shear failure. Large lateral displacements can

cause increasing volumetric strain and there-

fore an increase in hydraulic conductivity.

Considering the inclined drainage layer as a

solid body, overturning around the toe of the

wall is also possible. Gravity and external

loads decrease safety with respect to bearing

capacity failure.

5. Few laboratory or ®eld investigations of

the stress±strain behaviour of barrier systems

exist. In this paper, the authors report results of

1:1 scale tests performed with horizontal and

vertical barrier systems in combination with

®eld measurements, and give suggestions for

practical applications. The idea for these inves-

tigations came from two land®lls in Germany

where the last author was called in for expert

consultations. The experimental investigations

were performed at the Technical Universities at

Darmstadt (horizontal barrier) and Zurich (ver-

tical barrier).

Horizontal barrier system

Principle of investigations

6. The experimental investigations con-

centrated on the bending e�ects of a thin

(0´6 m) mineral liner.1 First, the damage

criterion had to be de®ned. It was decided to

use an increase in water content in the

drainage layer underneath the mineral liner

(Fig. 3) as a sign of the beginning of leakage.

With this increase in water content it was

possible to detect leakage due to cracking.

Therefore, the liner was covered with a 0´6 m

layer of water during the test. After some

initial tests, it was found that the best way to

induce bending was by lowering the founda-

Lorenz Edelmann,

Consulting Engineer,

Amann Infutec

Consult Ltd, Muhltal

Michael Hertweck,

Consulting Engineer,

SKS Engineers Ltd,

Zurich

Peter Amann,

Professor of Soil

Mechanics and

Foundation

Engineering,

Institute of

Geotechnical

Engineering, Swiss

Federal Institute of

Technology, Zurich

215

Proc. Instn

Civ. Engrs

Geotech. Engng,

1999, 137, Oct.,

215±224

Paper 11855

Written discussion

closes 31 March 2000

Manuscript received

26 October 1998;

revised manuscript

accepted 27 May

1999

tion bed of the liner to simulate conditions for

local settlement.

Testing equipment

7. The simulator is shown in Figs 3 and 4. The

support construction consists of two parts: the

peripheral support and the central support with

a lowering capacity. The central support was

made of 19 cylindrical rubber cushions, con-

centrically arranged and ®lled with water. The

arrangement of the rubber cushions formed three

independently operated ring circles with cushion

No. 1 as the ®rst circle in the middle (Fig. 3).

8. The support construction could be

lowered by the hydraulically operated cushions

in a highly controlled way to simulate sub-

sidence and deformation. By removing di�erent

quantities of water from the rings of cushions, a

vertical displacement and a bending deforma-

tion of the barrier model could be imposed.

9. The 0´6 m thick barrier was constructed

by compacting approximately 16 t of soil

material placed on the top of the support

elements and the 0´25 m thick drainage layer

made of sand. The leakage was measured by

time domain re¯ectometry (TDR) probes and

tensiometers in the drainage layer immediately

beneath the barrier. TDR is a method in which

volumetric water content is determined from

dielectric properties of wet soil.2 Thirty TDR

probes and four tensiometer measuring lines

were installed in a grid which enabled the

measurement of changes in water content and

the precise localization of percolation and

leakage spots. The deformation subsidence was

measured by displacement transducers both on

top and at the bottom of the barrier. A

subsidence velocity of Dh=4 mm per day in

the centre of the barrier was selected.

Material

10. Two natural soil types judged to be

characteristic of the range of materials used in

soil liner construction were selected for the

tests, namely silt and clay (Table 1). The silt

had a relatively low plasticity. The material

was susceptible to imposed deformations and,

even though it represented the lower limit of

the quality range of materials, is frequently

used in land®ll construction. The clay exhibited

higher plasticity and more or less represented

the upper limit of liner materials quality used

in construction.

Test results and analysis

11. Three large-scale tests were carried out,

two with silt and one with clay, lasting up to

three months each. With the tests using the silt,

the ®rst leakage was measured at a maximum

settlement of 3´15 cm in the bottom centre (Fig. 5).

Initial stateDeformed state

Soil liner

R

Fig. 1. Deformation of horizontal barriers

2

1

(a) (b) (c) (d)

Fig. 2. Failure mechanism of steep-slope barrier systems: (a) bulging;

(b) shear failure; (c) overturning; and (d) bearing capacity failure.

Fig. 3. Cross-section of testing equipment (horizontal barrier)

216

EDELMANN ET AL.

Additional leaks were observed in the range of

maximum subsidence of 3´15±5´53 cm. The

deformation of the barrier corresponded to an

arc of a circle. The radius of curvature for the

maximum subsidence in the centre at the onset

of leakage can be calculated as the limiting

value for acceptable deformation. For a sub-

sidence of 3´15 cm the radius of curvature was

calculated to be R=70 m for the silt (Fig. 5).

The radius of curvature corresponding to a

subsidence of 5´53 cm was calculated to be

R=40 m. During dismantling of the models, no

cracks were observed visually.

12. In the deformation test with clay, the

testing apparatus support was lowered to its

maximum extent of h=38 cm without any

failure occurring. The corresponding radius of

curvature was calculated to be R=6 m. The

clay barrier was still intact at that subsidence

condition. Permeability tests (i=30) were

carried out on extracted undisturbed samples

of the deformed clay barrier, yielding an

average coe�cient of permeability of

k=2´86 10710m/s. A comparison with the

results of permeability measurements made

prior to the test (Table 2) indicates that no

signi®cant change in permeability occurred for

the clay in spite of a calculated average volume

expansion of the samples in the deformation

test of eev =+5´6%.

13. A ®nite element analysis was carried

out to investigate the deformed barrier

numerically. The stress±strain behaviour was

modelled using an elastic Mohr±Coulomb

plastic relationship. Therefore, the attention

was focused on the strain distribution in the

barrier. For the silt, the maximum horizontal

strain at the outer edge area of the barrier,

where the ®rst leaks were identi®ed, was

calculated to be eeh =+0´2% (Fig. 6). For the

clay, the maximum horizontal strain at the

outer edge area of the barrier was about

eeh =+1´3%.

Results of in situ measurements

14. As a part of an industrial and municipal

land®ll, a 30 m thick waste body of sludge from

industrial waste water treatment was deposited

in Hessia, Germany, and covered by municipal

waste (Fig. 7). The two di�erent waste bodies

are separated by a thin intermediate liner

system composed of a 0´75 m thick clay

composite liner. The material properties of the

sludge have been reported by Amann et al.3 The

liner system was subject to imposed deforma-

tions due to the consolidation settlements of the

sludge itself, its strongly pronounced creep

behaviour and also from settlements caused by

the weight of the municipal waste (Fig. 7).

15. A measuring programme was developed

for in situ investigations of the deformation

behaviour of the intermediate liner system and

the industrial sludge.4 As a part of the complete

Table 1. Geotechnical parameters of tested materials

Soil parameters Units Horizontal tests Vertical tests

Silt Clay Barrier Weak

suppport

Sti�

supportField Test

Classi®cation Ð Ð Ð CL CL Peat GP

Grading C, S, S, G Ð % 18/71/11/0 48/44/8/0 17/60/23/0 23/36/23/19 Ð 0/10/22/58

Permeability K m/s 66 10710 2´96 10710 0´56 10711 56 10711 Ð Ð

Water content w % 17´7 17´5 19´8 21 180 3

Liquid limit wL % 31´4 42´8 39´5 47 Ð Ð

Plastic limit WP % 20´1 20´6 15´2 26 Ð Ð

Plasticity index IP % 11´3 22´2 24´3 22 Ð Ð

Shrinkage limit wS % 16´5 17´2 15´3 16´6 Ð Ð

Proctor density rpr t/m3 1´81 1´78 Ð 17´2 0´5 21´7

Opt. water content w % 15´2 16´7 Ð 21 90 6´3

Uniax. compr. qqu kN/m2 230 294 Ð Ð Ð Ð

Cohesion cc ' kN/m2 23 20 20 22 Ð Ð

Friction angle f' Degrees 27 27 18 24 30 >35

CC-modulus ME MN/m2 Ð Ð 5´2±12 6´5±23 1´4±7´7 28±58

Fig. 4. Testing

equipment for

horizontal barrier

systems (D= 4´2 m)

217

MECHANICAL BEHAVIOUR

OF LANDFILL BARRIER

SYSTEMS

programme, an approximately 170 m long

¯exible tube placed in the upper drainage layer

was installed for hydrostatic pro®le gauge

measurements to investigate the settlements

of the intermediate liner along the tube. The

results of the hydrostatic pro®le gauge meas-

urements show comparatively continuous

settlement curves (Fig. 8).

16. The measurements were carried out in

1 m long sections. The radii of curvature were

back-calculated from the data after carrying out

a regression analysis to smooth out the settle-

ment curves. Thus, the minimum radius of

curvature back-calculated with the measure-

ments from August 1996 were found to be in

the order of R=20±30 m. In comparison with

the large-scale test results with clay, it is

concluded that the current deformation state

was within the serviceability limit of the

intermediate liner system. This is in accordance

with the ®eld leakage measurements.

Vertical barrier system

Principle of investigation

17. Vertical or steep-sloped barriers are

used in pits, for example deep quarries. The

mineral component of the sealing must be

constructed simultaneously with the ®lling of

the waste, which acts as a retaining material.

The most stressed component of these barrier

systems is the footing, especially the connection

between the vertical and the horizontal parts. It

was decided to concentrate the research on this

area by 1:1 scale loading tests.5 Higher con-

struction stages up to a height of 35 m were

simulated. The various weights of the compo-

site construction were simulated by stepwise

and di�erent additional loading with loading

beams (see Fig. 9). The investigations included

50

45

40

35

30

25

20

15

10

0

5

7 14 21 28 35 42 492

4

6

8

10

12

14

16

18

20

22100 90 80 70 60 50 40 30 20 10 0

Radius of curve, R: m

Testing time, t: days

Displacement against testing tube

Wat

er c

onte

nt, θ

: vol

-%

Dis

plac

emen

t, h:

cm

TDR12 TDR13TDR25 TDR24 TDR3

Fig. 5. Water content measurements of the ®rst leakage (silt)

Table 2. Summary of strains and permeability of horizontal and vertical barriers

Horizontal barrier: Notation Units Model tests Laboratory tests Field

measurements

Silt Clay Silt Clay Clay

eeh % 0´2 >1´3 Ð Ð Ð

R m 70 6 Ð Ð 20±30

eev % Ð Ð Ð +5´6 Ð

[K0] m/s Ð Ð [66 10710] [2´96 10710] Ð

K m/s Ð Ð Ð 2´86 10710 Ð

Vertical barrier:

Waste (support):

Notation Units Silt Silt Silt

Weak Sti� Weak Sti� Weak

eeh % 8 1 Ð Ð 5±9

eez % 710 76 Ð Ð 74

eev % 76 76 Ð Ð 75

eeg % 10±16 eev* eeg Ð Ð 17

[K0] m/s Ð Ð 76 10711 3610711 Ð

K m/s Ð Ð 26 10711 16 10711 Ð

Fig. 6. Isograph of horizontal strains (silt, R= 70 m)

218

EDELMANN ET AL.

varying the relative sti�nesses of the barrier

and the waste material.

Testing equipment

18. The installed model and the deforma-

tion measuring device are shown in Figs 9 and

10. A typical wall sealing system with a width

of 1´1 m made of a silty clay was reproduced.

A ®nal height of the composite construction of

37 m was simulated by the application of

surcharge loads. The inclined natural wall

(e.g. rock wall) was evened with a bituminous

layer. Two tests with the same sealing material

but with di�erent waste material were carried

out. The ®rst test used a weak material,

modelling MSW (municipal solid waste); the

second used a sti� material, modelling MSW

slag.

19. The load was increased on the sealing

system and on the waste deposit alternately.

Generally, the wall sealing system was loaded

®rst, to simulate preconstruction in the ®eld.

Up to a height of 12 m, 1 m loading increments

were used with 3 m increments above 12 m.

Both tests were executed with the same loading

stages. Each test required one year.

20. The deformations, stresses, pore water

pressures and the volumetric water content in

the liner were measured during loading. Prior-

ity was given to the deformation measurements

in the sealing, which were measured by dis-

placement transducers (LVDTs). The arrange-

ment of the transducers gave a framework of

eight rectangular grid elements (Fig. 10). The

volumetric strain could be calculated in each

one. The stress state could also be recorded at

di�erent levels using pressure cells (GloÈ tzl

type). The volumetric water content was

observed by measuring the electrical resistance.

In the waste, no measurements were made. A

detailed description of the measuring device is

found in reference 5.

Material

21. The parameters of the material used are

listed in Table 1. For the weak support a peat±

compost mixture was used, having similar

deformation characteristics to MSW.5

Test results and analysis

22. In both full-scale tests the proposed

height of 37 m was reached without any failure

or loss of serviceability of the wall sealing

system. After the ®rst test (using weak waste

material), the wall sealing system showed a

total surface settlement of 10% of the actual

model height (4´0 m). The deformation of the

body of waste near the gabions was 28% of the

actual height of the waste material. Up to a

simulated height of 17 m, the waste material

slid along the gabions, producing an average

shear stress of 30 kN/m2 over the height of the

model, after which the waste material adhered

to the gabions. The average shear stress then

increased with height up to a maximum of

90 kN/m2.

23. The deformations in the second test

were smaller; at the top of the wall sealing a

total settlement of 6% of the actual model

height was measured. The settlement of

the body of waste was of the same order

of magnitude as the liner. The mobilized

shear stress along the gabions was constant

(20 kN/m2). In the ®rst full-scale test, a large

lateral strain of the sealing of ee h = 8% was

measured at the maximum simulated height. In

360

350

340

330

320

310

300

Leve

l: m

+N

M

240 220 200 180 160 140 120 100 80Horizontal tube length: m

Municipal waste

Intermediate liner

Industrial sludge

Subsoil

12/95

10/9612/95

11/93

4/96

7/94

3/92 3/92

Fig. 7. Cross-section

of Asslar land®ll

336

334

332

330

328

326

324

322

320

Leve

l: m

+N

M

160 140 120 100 80 60 40 20 0Length: m

–0·6

–0·4

–0·2

0·2

0·4

0·6

0·8

1·0

1·2

1·4

1·6

0

Set

tlem

ent,

s: m

Zero measurement 11/93Height 6/95Height 8/96

Settlements11/93–6/9511/93–8/96

Fig. 8. Results of

hydrostatic settlement

measurements (Asslar

land®ll)

Fig. 9. Testing

equipment for vertical

barrier systems (6´0 m

long6 5´2 m

wide6 5´0 m high).

On top can be seen the

loading beams

219

MECHANICAL BEHAVIOUR

OF LANDFILL BARRIER

SYSTEMS

the second full-scale test, a horizontal strain of

only ee h = 1% was measured in the sealing. The

horizontal strain of the gabions was measured

to be ee h = 19% in the ®rst test and approxi-

mately zero in the second test. The large

deformation during the ®rst test was required

to mobilize su�cient support in the weak waste

body to prevent failure.

24. The sealing and the gabions acted like a

composite, interlocked construction. The settle-

ment of the wall sealing was not a�ected by the

interface between the wall sealing and the

gabions. No relative movements took place at

this interface and deformations in the wall

sealing and the gabions were of the same order

of magnitude. Unhindered vertical deformation

could take place, resulting in a volume

decrease. The volumetric strain of the ®rst test

is shown in Fig. 11(a). An overall volume

decrease was registered. In both tests, identical

volumetric strains of eev =76% were measured

in the wall sealing but with di�erent distortions

eeg. In the ®rst test (Fig. 11(a)), the grid elements

of the wall sealing (Nos. 4±6) were in e�ect

under triaxial compression with a signi®cant

distortion of eeg=10±16%.

25. The observed plastic deformation of the

upper element (No. 6) during the ®rst test was

accompanied by high excess pore pressures. A

small volume increase occurred at large dis-

tortions but the wall sealing did not fail due

to the bearing capacity reserve. At higher

stress levels, the volumetric deformation came

close to con®ned compression behaviour. In

the second test, the wall sealing was under

con®ned compression during the whole test

(Fig. 11(b)). The observed behaviour during the

®rst test can also be seen in central cores in

earth dams.7 In both tests, the measured stress

ratio k= sH/sV in the wall sealing corre-

sponded to the active state of plastic equili-

brium (Fig. 12(a)). Therefore, the mobilized

coe�cient of earth pressure for the waste body

can be expressed by the active stress ratio of

the sealing, the relationship between the di�er-

ent unit weights and heights assuming that the

strength of the waste is not fully mobilized.

kd � gb

gdhb

hdkb �with hb > hd; hd > 0� �1�

kd � gb

gdkb �with hb � hd� �2�

Fig. 10. Cross-section of testing equipment

(vertical barrier) (dimensions in metres)

20

15

10

5

0

Dis

tort

ion

ε y =

εho

r + ε

ver:

%

20

15

10

5

0

Dis

tort

ion

ε y =

εho

r + ε

ver:

%

0 –2 –4 –6 –8 –10Volumetric strain εv: %

Isotropic compression

Isotropic compression

Triaxial compression

Triaxial compression

Confinedcompression

εv = εy

Confinedcompression

εv = εy

(b)

(a)

el. 4

el. 4

el. 1–3 andel. 7–8

el. 6

el. 5

Constant load atmaximumheight (62 days)

el. 1–3 andel. 7–8

Fig. 11. Volumetric strains plotted against

distortions during the tests with: (a) weak

support; and (b) sti� support

220

EDELMANN ET AL.

where `d' is the deposit and `b' is the barrier.

26. Equation (1) is valid for a parallel

construction and ®lling sequence, and equation

(2) where the steep slope sealing system is

constructed prior to ®lling.

27. A higher stress ratio was obtained

when the liner was constructed prior to ®lling

(Fig. 12(a)). In the earlier construction stages,

higher deformations occurred, but for higher

walls this e�ect became less pronounced.

28. The stress ratios required to prevent

failure were calculated for the proposed failure

mechanisms. The results where construction

takes place before the placement of a 3 m depth

of waste are presented in Fig. 12(b). Further

results are presented in Reference 6. For

inclinations of 708 and greater, bulging of the

wall is also a critical failure mechanism. For

the analysed parameters, the safety reserve for

shear failure will be su�cient. Independent of

wall inclination, the shear failure mechanism

with plastic ¯ow at the toe of the wall (Fig. 2(b),

No. 1) will be more critical than a failure

mechanism across the wall liner (Fig. 2(b),

No. 2).

29. For bearing capacity failure, the width

of the sealing system (wall liner and gabions) is

important with regard to the ®nal height. The

®nal height in the large-scale test for a factor of

safety of 2´0 was 30 m. The height for the MSW

deposit presented, with a width of 3 m, was

56 m. The factor of safety for bearing capacity

failure strongly depends on the inclination of

the wall and the thickness. The ultimate height

decreases signi®cantly for less inclined walls

and thinner liners.

Results of in situ measurements

30. The in situ measurements were carried

out in the wall sealing of a waste deposit for

MSW (Wirmsthal, Germany). The inclination of

the rock wall was 808, evened by concrete

plates covered with a bituminous layer. The

mineral liner consisted of a silty clay (CL)

(Table 1). The thickness of the retaining

gabions which were used to collect leachate was

1 m. They were ®lled with coarse crushed rock.

The wall sealing system was built to a height of

between 3 and 5 m before placement of any

waste. At the time of evaluation the wall

sealing was installed to a maximum height of

20 m, with the total depth of the quarry being

60 m. Two measuring cross-sections were

installed to analyse the deformations and the

stress state of the wall sealing during construc-

tion and after construction (Fig. 13).

31. The lateral extension of the wall

sealing could be obtained from the inclinometer

measurements and is shown in Fig. 14(a). In

cross-section I, the wall sealing widened from a

height of 15 m upwards, causing an overturning

of the gabions towards the waste body as a mode

of movement. In contrast, in cross-section II,

1·2

1·0

0·8

0·6

0·4

0·2

0S

tres

s ra

tio k

= σ

h/σ

v

1·2

1·0

0·8

0·6

0·4

0·2

0

Str

ess

ratio

k =

σh/σ

v

0 20 40 60 80Height of the wall liner: m

(b)

(a)

k d = (γ b/γ d)(h b/h d)k b

(with h b > h d; h d > 0)γ b = 20 kN/m3

γ d = 11 kN/m3

k d = (γ b/γ d)k b (with h b = h d)

k b (calculated φ = 20˚; c′ = 20 kPa)

k b (measured from first large-scale test)

h d = h b – 3 m

σhd

= σhb

d = depositb = barrier

Existing stress ratio

Shear failure (1)

Shear failure (2)

Bulging (overturning)

Bulging (buckling)

Inclination 80˚

Fig. 12. Existing and required stress ratios in the waste body:

(a) comparison of measured and calculated results; and (b) requirements

for di�erent failure mechanism

2·0 m 1·0 m

1·0 m

2·0 m

1·0 m

0·5 m

3·5 m

Leachate collection systemgabions; mesh wire filled withcoarse crushed rock

Rock wall

Sliding layer(bituminous)Ground waterdrainage

Wall sealing

Inclinometersliding micrometer

Earth pressurecells (Glötzl type)

Drainage layer

Displacementtransducer (LVDT)

Fig. 13. Barrier system and measuring device, Wirmsthal land®ll

221

MECHANICAL BEHAVIOUR

OF LANDFILL BARRIER

SYSTEMS

the maximum lateral extension was recorded at

a height of 12 m. This led to a bulging of the

gabions. It can be seen from the in situ

measurements that during the construction of

the sealing system large horizontal strain

occurred in the wall sealing (eeh = 17% at

maximum). The horizontal strain in the wall

sealing increased linearly with increasing

height. The low support of the gabions during

compaction resulted in these large horizontal

strains. Additionally, this construction tech-

nique led to an exponential increase in the

horizontal strain in the base sealing under the

gabions.

32. The vertical deformation in the wall

sealing was constant across its width due to the

smooth surface along the side walls and the

large ductility of the gabions. The di�erential

vertical settlements at maximum were

ee Z =78% (Fig. 14(b)), the horizontal strain at

maximum ee h = 5±9% (Fig. 14(a)). The

maximum extension produced a tilt of about

1´28. The horizontal strain during construction

produced an additional rotation of 28. In cross-

section II the vertical and horizontal deforma-

tions resulted in a distortion of ee g=17% at a

height of 6 m above the foundation of the

gabions. The volumetric deformation can be

calculated approximately from the inclinometer

and the sliding micrometer measurements. A

volume decrease of 5% (without volume

decrease during compaction) at maximum was

obtained in the wall sealing, implying a

decrease in hydraulic conductivity.

Comparison of the 1:1 scale tests33. With the two model tests, horizontal and

vertical, the stress±strain behaviour of possible

barrier systems has been investigated.

However, there exist di�erent conditions which

possibly lead to failure and loss of service-

ability. Generally, there are three main points

which increase the hydraulic conductivity

(a) horizontal stretching

(b) increase of volume

(c) increase of shear strain.

While (a) is the most likely e�ect for horizontal

barriers, vertical or inclined barriers might fail

by (b) and/or (c).

34. The maximum horizontal strains and

volume changes of the models and laboratory

tests performed are summarized in Table 2

together with the permeability measured on

samples both compacted before and taken after

the model tests. Without overburden (horizontal

tests), the stretching strains at which damage

occurs are much less than in an overall stress

state (0´2% (silt) to 1´3% (clay) instead of

1´0% (silt) to 5´0% (clay)). On the other hand,

laboratory tests showed that for clayey material

under volume extension up to 5´6% no sig-

ni®cant change of permeability (k=2´9 to

2´86 10710 m/s) was measured.

35. The vertical model tests show that

distortions up to 16% also do not cause an

increase in permeability for silty material. But

it should be considered that the strain condi-

tions produced a ®nal volume decrease of 6%

within the same loading procedure. So, in the

investigated system, the material can also

withstand temporary volume increase (up to

6%) without reducing the hydraulic conductiv-

ity signi®cantly.

Conclusions36. There is only scant information avail-

able in the literature and in regulations con-

cerning the deformation behaviour of soil liners

for practical applications. For example, in

German regulations, a minimum radius of

curvature of R=200 m is given for which no

deformation improvement is necessary for hor-

izontal soil liners of at least medium plasticity.8

From the large-scale tests, the limiting value of

deformation of silt is about R=70 m, corre-

sponding to the maximum radius of curvature

(b)

(a)

20

15

10

5

0

Hei

ght:

m20

15

10

5

0

Hei

ght:

m

0 2 4 6 8 10

–5 0 5 10 15

Vertical strain: %

Foundation ofthe gabions

Foundation of the gabions

Vertical line near the side wallVertical line near the body of waste

Horizontal strain: %

Side wall(rock)

Cross–section Ι

Cross–section ΙΙ

Body of waste

Fig. 14. Measured

strains versus height

for Wirmsthal

land®ll, showing:

(a) horizontal (cross

sections I + II); and

(b) vertical (cross-

section II)

222

EDELMANN ET AL.

which caused the ®rst leak. Silt of low plasti-

city already meets the requirements concerning

deformability of barrier material of medium

plasticity according to the LWA Instructions.

The barrier model with clay sustained a radius

of curvature of R=6 m without reaching a

limit state. The important di�erence in the

amount of deformation is attributed to the

di�erent plasticity of the materials. The limits

of R were measured in the model test without

overburden. These conditions are similar to

caps. The ®eld test showed values of R� 30 m

without damage under an overburden. These

conditions are similar to conditions of inter-

mediate liners.

37. The stress±strain conditions of vertical

or steep-sloped liners are characterized by the

construction detail of the barrier system. It is

obvious that if the inclined wall is rough and if

large displacements occur along the wall, the

slender wall sealing will be subject to uncon-

trolled shear stress, producing vertical cracks.

Beside the roughness of the wall, the low

normal stress on the wall also contributes to the

uncontrolled shear stress. Therefore the shear

forces along this interface must be reduced.

However, it is not necessary to have a smooth

contact surface between the wall sealing and

the leachate collection system, if a transition

zone (drainage layer) is installed.

38. The magnitudes of the total vertical and

horizontal strains in the vertical liner depend

on the sti�ness of the waste body. In the case of

a sanitary land®ll in a quarry, the principal

load case is the so-called self-weight loading.

The load transfer from the weak waste material

causes smaller additional vertical deformations.

The requirements concerning the strength of

the sealing material depend upon the height.

39. The ®lling and construction sequences

have an important in¯uence on the stability and

the deformation of the sealing system. Increas-

ing displacements can be avoided by adapting

the ®lling sequence accordingly. High compac-

tion of the MSW is recommended near the

sealing system, to provide su�cient support

and smaller lateral deformation in the wall

sealing.

40. Both vertical model tests and the ®eld

measurements con®rmed that the chosen con-

struction of the Wirmsthal barrier system with

a sealing material of high plasticity, ful®lled

the requirements. Nevertheless, a sealing

material should be compacted and installed at a

moulding water content on the wet side of

Proctor optimum, yielding densi®cation at low

stress levels and insurance against volume

change due to drying during the long-term life

of a waste deposit.

41. The investigations presented in this

paper give the possibility to perform an overall

design for horizontal, vertical and sloped dis-

posal barrier systems. The given design criteria

are stretching, bending, buckling and also

bulging. The testing equipment developed also

allows tests for the quality control of new

system designs or a combination of materials.

Nevertheless, 1:1 scale tests are very costly and

time-consuming. Therefore the results will be

evaluated by further ®nite element computa-

tions and more sophisticated element tests with

the construction materials used.

Acknowledgements42. The horizontal model test was per-

formed at the Technical University in Darm-

stadt and ®nanced by the German Research

Institution. The vertical model test was

installed at the Swiss Federal Institute of

Technology in Zurich and ®nanced by the

Research Commission and fund of the school.

The ®eld measurements in both cases were

funded by the owner of the land®lls in Asslar,

Germany and Wirmsthal, Germany. Thanks

are given to all who have supported this

extensive work, especially to the people of the

university workshops who constructed, oper-

ated and repaired the equipment with a con-

sistently high level of commitment to the

research work.

References1. EDELMANNDELMANN L. Contribution to the Limiting Defor-

mation Behaviour and to the Serviceability of

Horizontal Soil Liners for Land®lls. Phd thesis,

Mitteilungen des Institutes und der Versuchsan-

stalt fuÈ r Geotechnik der Technischen UniversitaÈ t

Darmstadt, 1998 (in German with English

summary).

2. ROTHOTH K., SCHULINCHULIN R., FLUEHLERLUEHLER H. and ATTINGERTTINGER

W. Calibration of time domain re¯ectometry for

water content measurements using a composite

dielectric approach. Water Resources Research,

1990, 26, No. 10, Oct., 2267±2273.

3. AMANNMANN P., RAMHOLTAMHOLT T. and WEISSEISS J. Geotechnical

problems related to depositing of industrial

waste. The Interplay between Geotechnical Engin-

eering and Engineering Geology, Proceedings

of the 11th ECSMFE, Copenhagen, Denmark,

28 May±1 June 1995, vol. 2, 7±11.

4. EDELMANNDELMANN L., KATZENBACHATZENBACH R. and AMANNMANN P. New

experimental investigations for the determination

of serviceability of soil liners for land®lls.

Proceedings of the 14th ICSMFE, Hamburg,

Germany, 6±12 September 1997. Balkema, Rotter-

dam, 1997, 1899±1902.

5. HERTWECKERTWECK M. Research about the Behaviour of

Steep Slope Sealing Systems with Large-scale

Tests. Phd thesis, Mitteilungen des Institutes fuÈ r

Geotechnik der ETH ZuÈ rich, Nr. 211, 1998 (in

German with English summary).

6. HERTWECKERTWECK M. Slope stability of clay liners at

waste deposit rock walls. Proceedings of the 14th

ICSMFE, Hamburg, Germany, 6±12 September

1997. Balkema, Rotterdam, 1997, 1899±1902.

7. MARSALARSAL R. J. and DEDE ARELLANORELLANO L. R. Performance

of El In®ernillo Dam 1963±1966. Journal of the

Soil Mechanics and Foundations Division, ASCE,

1967, 93, No. SM4, paper 5318, 265±298.

223

MECHANICAL BEHAVIOUR

OF LANDFILL BARRIER

SYSTEMS

8. LWA Instructions; Mineral Soil Liners for Land-

®lls. Landesamt fuÈ r Wasser und Abfall (LWA),

North Rhine Westfalia (NRW). Abfallwirtschaft

NRW, Richtlinie Nr. 18, DuÈ sseldorf: Schriftenreihe

des Landesumweltamtes NRW, 1993, 59pp. (in

German).

Please email, fax or post your discussion contributions to the Secretary:

email: Wilson_l@ice.org.uk; fax: 0171 799 1325; or post to Lesley Wilson,

Journals Department, Institution of Civil Engineers, 1±7 Great George Street,

London SW1P 3AA.

224

EDELMANN ET AL.