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Laboratory evaluation of the load-deflection behaviour of clay beams reinforced with galvanised wire netting
Geotextiles and Geomembranes 14 (1996) 555-573 O 1997 Elsevier Science Limited
All rights reserved. Printed in Ireland 0266-1144/96 $15.00
ELSEVIER
S0266-1144(96)00020-9
Laboratory Evaluation of the Load-Deflection Behaviour of Clay Beams Reinforced with Galvanised Wire Netting
B. Indraratna a & G. Lasekb "Department of Civil and Mining Engineering, University of Wollongong, NSW 2522, Australia bGeo-Environmental Engineering Division, Cleary Bros. (Bombo), Pty Ltd, NSW, Australia
ABSTRACT In Australia the utilisation of clay in the construction of landfill liners has recently come under criticism either due to the difficulty in reproducing the laboratory results on site, or because of the potential for cracking of the compacted liner after construction. Realising that clay is weak in tension and given the possibility of certain areas of the liner being subjected to flexure, a testing program was undertaken to determine the effects of placement (compaction), moisture content and internal reinforcement on the potential failure of clay liners. In particular, this paper discusses the load carrying capacity and deformation sustained by compacted clay beams in flexure, and the improvement gained through the introduction of an economical reinforcement in the form of galvanised steel wire netting. Results obtained by flexural bending tests are discussed in detail, and the extent of improvement provided by galvanised wire netting is evaluated. © 1997 Elsevier Science Ltd. All rights reserved.
INTRODUCTION The main role of a landfill clay liner is to prevent the migration of contaminated fluid or leachate from the containment area by providing a hydraulic barrier, thereby minimising the risk of pollution of adjacent and underlying land and water resources. The performance of a clay liner is directly related to the leachate characteristics which can be extremely variable from one location to another. The high levels of organic solvents resulting from hazardous solid wastes have often dictated stringent criteria 555
556
B. Indraratna, G. Lasek
with regard to the liner thickness and its hydraulic conductivity (permeability). Irrespective of the achieved degree of compaction and permeability in the field, subsequent cracking of clay liners due to differential settlement of the sub-grade or due to excessive surcharge load can have catastrophic consequences in terms of pollution control. Particularly in the Illawarra region of NSW, ground subsidence due to underground mining is not uncommon. Considerable research has been undertaken to assess the desirable strength and conductivity properties of clay liners (Daniel, 1993); the effect of moisture content at compaction (Hilf, 1975; Lambe, 1960); and the effect of compaction on permeability (Boynton & Daniel, 1985; Mitchell et al., 1965) among other design and construction aspects. Studies have also been conducted on the effectiveness of laboratory tests in relation to field practice (Holtz & Broms, 1977; Ingold, 1983; Boutwell & Hedges, 1989); the maintenance and defects of liners (Giroud et al., 1992); the potential for desiccation cracking (Kleppe & Olsen, 1985) and numerous methods of clay stabilisation by internal reinforcement (Koerner, 1994; Yamanouchi, 1986; Giroud, 1984). In order to verify the suitability of a silty clay for the construction of a landfill liner, basic laboratory tests were conducted on a selected clay from Menangle, NSW, including Atterberg limits, compactability, permeability, particle size distribution, etc. However, knowing that the effective functioning of a liner depends upon the movement of the sub-grade, the evaluation of (a) behaviour of this compacted clay in tension (flexure) at varying moisture contents, and (b) the degree of improvement that can be achieved by internal reinforcement, seems imperative. The thickness of typical clay liners in New South Wales ranges from 0.2 m to 1.5 m depending upon the type of solid waste to be contained and the type of sub-grade. The conservative design of thick liners involves a substantial loss in space that would otherwise be occupied by the waste itself. Naturally, the thickness of liners can be reduced substantially if they are placed above rock foundation, but where the foundation consists of weak or soft compressible soils, the thickness of lining should be adequate to provide sufficient bending rigidity. It is therefore logical that if the cracking potential and the associated deflection behaviour can be properly evaluated, the optimum thickness of the liner can be reduced significantly. For this purpose, laboratory clay beam models were cast and tested in flexure, with and without reinforcement. Reinforcing the clay with galvanised wire netting was considered to be an economical alternative to the use of geomembranes. The effect of this wire netting on the load-deflection response is discussed in the paper for clay beams compacted at different moisture contents. The durability of galva-
Load-deflection behaviour of clay beams
557
nised wire netting can be improved by plastic coating. For instance, such wire netting used for a variety of purposes in acid-sulphate soil terrains (e.g. South Coast of New South Wales) is retailed with appropriate protective coating. As in the case of acidic soils or where the acidic leachate generated from the waste has a high corrosion potential, such protective coating will be useful for enhancing long-term durability.
LABORATORY SAMPLE PREPARATION
Soil properties Approximately 200 kg of clay sourced from the Menangle region of south-west Sydney was utilised in the formation of clay beams later subjected to flexural loading. This material can be categorised as a well-graded, light-brown silty clay with traces of gravel. Its basic properties are summarised in Table 1, and its plasticity group is CH (clay of high plasticity) in the unified soil classification system. The optimum moisture content and the dry density of the compacted clay were determined on the basis of the standard Proctor compaction method. The falling head permeability estimated for compacted Menangle clay (2.8 × 10 -9 c m / s ) is sufficiently low to prevent infiltration of leachate, unless cracking takes place. Kalteziotis et al. (1994) suggest that the permeability of effective landfill liners should not exceed 1 x 10 -7 c m / s under most circumstances. As the Menangle clay is a highly plastic clay (liquid limit > 50%), it is particularly advantageous in providing sufficient flexibility to accommodate consolidation settlements and lateral movements after construction.
Casting of beams After drying the clay, the particles greater than 19 mm in size were removed, and water was added to cast test specimens at different moisture contents
TABLE 1 Properties of Menangle clay Liquid limit, % Plastic limit, % Plasticity index, % Optimum moisture content, % Maximum dry density, kN/m 3 Hydraulic conductivity, cm/s (close to OMC) Particle size distribution % finer than 0.075 mm
51.8 21.25 30-55 19.0 16.9 2.8 x 10 9 68%
558
B. Indraratna, G. Lasek
mainly greater than the optimum moisture content (OMC) of 19%. In practice, the compaction of clayey fills of moisture contents less than OMC has been found to have adverse effects such as encouraging surface cracking or desiccation due to increased brittleness (Ajaz & Parry, 1975). In the following text, moisture contents on either side of the OMC are referred to as 'wet of optimum' or 'dry of optimum'. Beams were made according to the standard Proctor compactive effort (594kJ/m 3) using a mould of internal dimensions: 500ram x 100mm x 100ram. Compaction was carried out using a rammer of 5 0 m m in diameter and 2.5 kg in weight, falling freely from a height of 300mm above the surface. This standard compaction advocated the beams being constructed in four layers of 25 m m thickness, each layer imparted with 100 evenly distributed blows, with the reinforcement placed over the initially compacted layer, i.e. 25 mm from the base of the beam. Out of the many beams tested, the behaviour of 22 selected samples is considered in this paper including both unreinforced and reinforced beams.
Type of reinforcement Although limited testing has been undertaken to study the behaviour of reinforced clay in flexure, much emphasis has been placed on improving the shear strength of soils using geotextiles and geonets (Fluet, 1988; Jewell, 1982; Ingold, 1982). Koerner and Hwu (1991) give a comprehensive account of the tensile considerations of a reinforced slope. In this study, an economical alternative was adopted in the form of galvanised steel wire netting with apertures of 12.5mm and a wire thickness of 0 . 5 6 m m (Fig. 1) that could provide sufficient tensile strength to the soil, as well as being easily installed and moulded to an uneven subgrade. Although not used as reinforcement in the past, galvanised wire netting was expected to provide the necessary tensile strength and exhibit good pull-out characteristics, because of the clay material on either side of the net providing a strong interface by 'bonding' through the apertures. Beams were cast at similar moisture contents with and without wire netting to evaluate the advantages achieved through the introduction of this reinforcement. In contrast to the wire netting, two additional beams were constructed at moisture contents significantly higher than the OMC, but reinforced with a nonwoven geotextile and tested in the same manner. The main difference is that the wire netting acts purely as a reinforcement with no drainage function, whereas needle-punched nonwoven geotextiles are characterised by a predominant drainage function apart from also providing some form of reinforcement.
Load-deflection behaviour of clay beams
559
( 23mm
( 12.5mm
I
I
WIRE THICKNESS= 0.56mm
(a) Dimension of aperture
(b) Layout of wire netting
in mm.
Fig. 1. Type of galvanised wire netting used as beam reinforcement.
FLEXURAL TESTING Testing was undertaken using the Instron 4302 machine with a modified loading head, enabling four point flexural loading (i.e. every third-point loading, see Fig. 2), thereby providing a constant maximum bending m o m e n t over the central third of the beam. U p o n placement of each beam on the end supports, deformation due to the self-weight of the beams was monitored. Bending due to self-weight occurred to a slight extent (approximately 0.51-0mm) only in the case of the unreinforced beams cast wet of optimum, whereas this deflection was negligible in reinforced beams. The beams were then loaded at a constant rate of 1.5ram per min with the deflection and corresponding load recorded by the automated plotter. U p o n completion of each test, the cracking pattern of the beams was traced with minimum disturbance. Unreinforced beams
Six selected clay beams tested without reinforcement at moisture contents on either side of the o p t i m u m are listed in Table 2. C o m m o n practice involves the construction of clay liners at moisture contents generally 2% or greater than the O M C for silty clays such as the Menangle clay, in order to ensure the desirable very low hydraulic conductivity. Observation of these unrein-
560
B. Indraratna, G. Lasek
Fig. 2. Failure of unreinforced beam using four point flexural loading.
TABLE 2
Moisture contents of beams cast without reinforcement Beam
Moisture content
u1 u2 u3 U4 U5 U6
17.6% 18.2% 19-0% 20-1% 22.6% 24.3%
+/
O.M.C.
-1-4% 0.8% OMC +1.1% +3.6% +5.3%
forced beams during testing indicated the inception of a tensile crack at the base of the beam (tensile zone) and its subsequent propagation to the top with the increased loading until failure (Fig. 2). The load~leflection responses of the unreinforced beams are shown in Fig. 3(a), and from this plot it can be concluded that the m a x i m u m load of 0-21 kN was sustained by the beam U3 compacted at the optimum moisture content of 19%. This corresponds to a m a x i m u m bending m o m e n t of 15.8 x 10 -3 k N m and a m a x i m u m flexural strength of 94.5 kPa at its crosssection. After reaching this peak load at a deflection of 1-4 mm, the pre-peak,
Load-deflection behaviour of clay beams
561
0,25
MOISTURE CONTENT
0.2
K / ~ Elasto-plastJc
~
A~
T ~k
Brittle failure
~
[
u1 U2
17,6% 18.2%
190%
-A-U3
:
/
• ÷
Leo_, 2,.3°,°
O, 0,1 i
allure
0.05
0
1.0
DEFLECTION
(ram)
2.0
(a) 120
!
100 L
Z uJ Z
8o~
e~
40
20
~_
t
~ t 18
I
i
i 19
[
~
i
1 i I i i i i i i 20 21 22 M O I S T U R E C O N T E N T (%)
i
I
t
~ 23
t
i
L 24
I
L
(b) Fig. 3.
Behaviour of unreinforced beams: (a) load~leflection response and (b) variation of flexural stiffness against moisture content.
562
B. Indraratna, G. Lasek
elasto-plastic behaviour was followed by rapid (brittle) failure involving pronounced tensile cracking at the mid-span with an ultimate deflection of 2.2 mm. A similar type of failure was observed for beams near the OMC, for instance, in the case of beams U1 (1.4% dry of OMC), U2 (0.8% dry of OMC) and U4 (1.1% wet of OMC) with their maximum sustained loads being 0-137kN (O-t = 6 1 . 6 5 k P a ) , 0.176kN ( a t --79.2kPa) and 0.174kN (at = 78.3 kPa), respectively. The flexural tensile strength is given in brackets. Out of the unreinforced beams, U1 (1.4% dry of optimum) sustained the greatest deformation of 2.1 m m before failure, whereby its initial stiffness (deformation modulus) is significantly less than the beams constructed at higher moisture contents. These results indicate a loss of strength of the clay as its moisture content departs from the optimum. Both beams U5 and U6 cast at moisture contents significantly greater than the optimum failed at considerably smaller loads of 0.037kN (at = 16.65kPa) and 0-03kN (at = 13.5 kPa), respectively, with the corresponding deflections of approximately 0 . 7 m m at the peak stress. The load-deflection behaviour of these 'wet' beams is relatively more ductile (lower stiffness) in comparison with the beams cast at or near the OMC. Figure 3(b) illustrates the corresponding variation of flexural (bending) stiffness plotted against the moisture content. The flexural stiffness is determined at 50% of the peak load, where the loaddeflection response can be linearly approximated. The above results carry definite implications on the flexural behaviour of unreinforced clay liners installed on a soft subgrade. It may be deduced that the maximum tensile strength occurs at or close to the optimum moisture content with a significant loss in strength occurring as the moisture content departs substantially from the optimum. According to these results, when a clay liner is constructed dry of optimum it seems to undergo a greater deflection before brittle failure occurs. In general, all tests irrespective of the moisture content indicate that little deflection is required to initiate failure of unreinforced beams, unless the tensile zone is properly reinforced as discussed below. Reinforced beams
Testing was undertaken to determine whether through the introduction of a wire net type of reinforcement a clay liner could withstand enhanced load deformation characteristics before failure. An increase in tensile (flexural) strength is attributed to the stability of the soil-reinforcement interface (Juran et al., 1988; Indraratna et al., 1991; Ali, 1993), the moulded moisture content and the degree of compaction, and the frictional (pull-out) characteristics of the selected geotextile (Farrag & Griffin, 1993; Hausmann, 1990).
Load-deflection behaviour of clay beams
563
TABLE 3 Moisture contents of beams cast with reinforcement
Beam
Moisture content
+ / - O.M.C.
RI R2 R3 R4 R5 R6 R7 R8 R9
17.3% 18.8% 19.5% 20.4% 21.0% 22-1% 23.7% 23-9% 24.5%
-1,7% -0,2% +0,5% + 1,4% +2,0% +3.1% +4.7% +4.9% +5.5%
Table 3 summarises nine selected clay beams constructed at different moisture contents. These beams were tested in flexure in exactly the same manner as that for the unreinforced beams discussed previously. As mentioned earlier, the form of reinforcement was a galvanised steel wire netting with apertures of 12.5 mm and a wire thickness of 0.56mm (Fig. 1). Due to its flexibility, this type of reinforcement may be easily moulded to an uneven subgrade and, if successful, may provide an economic alternative to the more conventional geonets and geomembranes. Due to the large number of reinforced beams tested at differing moisture contents, results and interpretations have been categorised into the following sections for clarity of discussion: (a) (b) (c) (d)
beams beams beams beams
cast dry of optimum moisture content, cast wet of optimum moisture content, subjected to extended deformation, and reinforced with a nonwoven geotextile material.
Beams tested dry of optimum moisture content Figure 4 illustrates the load-deflection relationship of three reinforced beams (R1-R3) compacted dry of optimum, and two selected unreinforced beams (U1 and U3) to enable comparison at similar moisture contents. It is observed that the wire net reinforcement increases the load capacity of the beams by at least 50-100%, and at the same time almost doubling the deflection at failure (peak stress). Although the initial loading (quasi-linear elastic) is similar in both cases, the post-peak behaviour indicates that the unreinforced beams exhibit a rapid decline to zero load (brittle failure), whereas the reinforced beams sustain a more gradual
564
B. Indraratna, G. Lasek 0.4
MOISTURE CONTENT
! Elasto-plas~c/plastic
u1
17,6%
÷ u3
19.0%
R1
17.3%
-e. R2
18.5%
-~ R3
18.8%
0.3 -~,~,,,, Quasi-linear
~ o2 ~
elastic Brittle-plastic
i
Brittle failure
I
I
0
- t
0
Fig. 4.
1
2
3
.
4 5 DEFLECTION (mm)
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
6
Load-deflection behaviour of clay beams (dry of optimum) in comparison with selected unreinforced beams.
(brittle-plastic) deformation response. Even at central deflections exceeding 5mm, the reinforced beams continued to sustain at least 50% of the peak load. The behaviour of beams at much larger deflections is discussed later. Beams tested wet of optimum moisture content
Figure 5 shows the load deflection relationships of reinforced compacted clay beams (R4-R9) cast wet of the optimum moisture content. The responses of three selected unreinforced beams (U4, U5, U6) cast at similar moisture contents are also plotted for comparison. In contrast to the previous Fig. 4, it is immediately apparent that the reinforced beams exhibit a ductile-plastic, post-peak behaviour in comparison with the beams compacted dry of optimum. In general, the reinforced clay beams compacted wet of optimum (Fig. 5) exhibit slightly smaller load carrying capacities than their dry of optimum counterparts. Increase in moisture content causes a reduction in the load carrying capacity and the initial stiffness of the beams. For instance, while the beam with 20-4% moisture content (R4) shows a peak load of 0.33kN, beam R9
Load-deflection behaviour of clay beams
565
0.4 MOISTURECONTENT
~ _ ~
/
0.3
~
Eiasto-plastic/plastic f
~
! U4 20.1% ,.e- u5
Ductile-plastic
22.6%
~ U6 24.3% ÷ R5 21.0% R6 22.1%
D 0.2 <: ;
S
/
4. R7 23.7% . R8 23.9%
Ductile-plastic
Quasi-linear
-~ R9 24.5%
o.1
o
0
Fig. 5.
1
2
3
4 5 DEFLECTION (ram)
6
Load~teflection behaviour of clay beams (wet of optimum) in comparison with
selected unreinforcedbeams. at a moisture content of 24.5% carries a much smaller peak load of 0.07kN. Also, it seems that increasing the moisture content enhances the degree of plastic straining at constant load, with the load-deflection response of beams R8 and R9 illustrating an almost 'perfectly plastic' deformation. It is also apparent that all beams of similar moisture content (with and without reinforcement) initially follow a quasi-linear elastic load-deflection response, until considerable disparity is evident as the peak load of the unreinforced beams is approached. This is because initial cracking of the base of the beam is not prevented by the reinforcement. However, after cracking of the predominantly tensile zone, the beam with reinforcement continues to carry more load, whereas the unreinforced beam rapidly sheds its load to zero in a brittle manner. At the base of the mid-span, inception of a vertical crack followed by its propagation along the reinforcement interface may be noticed (Fig. 6), but the upper clay seam remains relatively unaffected as the wire netting becomes activated in tension. The compression load is resisted by the upper portion of uncracked clay above the wire netting.
566
B. Indraratna, G. Lasek
Fig. 6. Initial failure of the clay beam with cracking extending to the reinforcement layer. Beams subjected to extended deformation Practically all the tests described earlier were conducted until the peak load started to drop, with the extended post-peak large deflection behaviour not studied. In order to investigate the full contribution of the wire net at large deflections, five specimens were subjected to extended deformation up to 30 mm, and the corresponding load~leflection response was monitored. As in the previous tests, these samples were subjected to a constant loading rate of 1.5 m m per min, with the extended load~teflection behaviour illustrated in Fig. 7. F r o m Fig. 7, it is apparent that at say 3 m m deflection, initial failure occurs due to the cracking of the clay from the base of the beam towards the layer of wire net. This initial peak load is directly related to the flexural tensile strength of the clay below the wire netting, the region in which pronounced cracking occurs within the middle third of the beam. The postpeak behaviour is characterised by a considerable loss in load carrying capacity (brittle-plastic), until the load is subsequently taken up by the reinforcement. With further loading, the wire net embedded in the tension zone becomes fully activated, and the load continues to rise again even at large deflections (up to 30 mm), without any signs of brittle failure. This observation implies that as long as the galvanised wire netting can be laid as close as practically possible to the base of the beam, the effects of initial cracking can be minimised. At large deflections, the clay beneath the reinforced layer
Load-deflection behaviour of clay beams
567
0.5 MOISTURECONTENT ! Full-activation 0.4
of reinforcement
-
/II-B-II
E1
18.8%
4- E2
19.5%
~ E3
20.4%
0.3
I/~
I ~-'k JIl(-
i1~/ in#
"
J
~/
.
7
of reinforcement
_.J
0.2
0.1 -
~
A
Qoasi-ll~ar e l a ~ c
0 0
10
20
30
DEFLECTION (mm)
Fig. 7.
Load-deflection testing of reinforced clay beam subject to extended deformation.
continued to suffer from localised failure, where 'slabbing' and debonding of clay was evident (Fig. 8), with this causing some drop in load carrying capacity as indicated by beam E4 in Fig. 7. Considering the fact that the galvanised wire netting embedded in the tension zone of a compacted clay liner can sustain deformations of greater than 30 mm without complete failure (i.e. 30% of thickness), their utilisation in practice should be given serious consideration, particularly where the liner is to be installed on a soft sub-grade material. The introduction of this form of reinforcement should allow relatively large deflections to be tolerated by the liner without cracking. In this respect, the contribution of galvanised wire netting is significant both technically and economically. The variation of the computed flexural (tensile) strength with moisture content is given in Fig. 9, based on the bending moment-stress relationship ( M / I = a/y) for the beam, where M = maximum bending moment, I = moment of inertia, tr = flexural tensile strength and y = maximum distance to neutral axis. The maximum tensile strength is achieved at or close to the optimum moisture content with a significant decline in strength on either side of the OMC. With the introduction of galvanised
568
B. Indraratna, G. Lasek
Fig. 8.
Compacted clay beam after undergoing 30mm defection.
200
OMC
-ff
~ UNREINFORCEDI ,G REINFORCED I
0-
--~ 150
(,9
~W" 100 Ziii D tw X
50
0
~k-t
III
FI
18
I III
IPfll
J I J II
20
I1
lit
il~l[I
22
'P
Iql
I
24
MOISTURE CONTENT %
Fig. 9. Variation of flexural (tensile) strength with moisture content.
Load-deflection behaviour of clay beams
569
wire netting, an increase in tensile strength in the order of 60 kPa (65%) was achieved at the optimum moisture content. At increased moisture content, the difference between the reinforced and unreinforced samples tends to decrease, which implies that the role of wire netting is limited at water contents much greater than the OMC. Effect of geotextile layer on flexurai behaviour Wire netting provides only a reinforcement effect, and has no drainage function with regard to the compaction of clays with moisture contents significantly exceeding the OMC. In contrast, nonwoven geotextiles have been popular in the Asia-Pacific region for efficient compaction of 'wet' borrowed fills in low-lying land development. Even though the gain in load carrying capacity and flexural stiffness and strength due to wire netting was appreciated, it was decided to cast a few additional beams stabilised with a conventional geotextile to assess its infuence on the flexural behaviour of clay. The geotextile used was a needle-punched nonwoven (Bidim), having a thickness of 1.4 mm and a specific weight of 120 g/m 2. The drainage function of needle-punched nonwoven geotextiles is generally more prominent than their reinforcement effect (Indraratna et al., 1991). Considering the excellent pore-pressure dissipation characteristics of these geotextiles, the additional clay beams were cast at moisture contents significantly greater than the OMC of 19%. The behaviour of two such beams (G1 and G2) is illustrated in Fig. 10, where their 'as compacted' moisture contents were 22.0% and 23.1%, respectively. The geotextile was placed above the first layer of compacted clay (25 mm above base), and the subsequent clay layers were compacted above the geotextile. At similar moisture contents, the load-deflection behaviour of beams G1 and G2 is very different to the beams reinforced with wire netting (R6) and the unreinforced beam (U5) as shown in Fig. 10. As expected, it is evident that galvanised wire netting provides a greater flexural strength to the clay beam than the conventional geotextile. Nevertheless, the geotextile also enables a significantly increased deflection at peak load of more than twice that sustained by the relatively brittle unreinforced beam. In comparison with wire netting, premature failure of the clay beneath the layer of geotextile occurred as a result of the relatively poor bonding between the soil and geotextile. At similar moisture contents (22-23%), the wire netting provides a flexural strength at least three times that of the nonwoven geotextile mainly because of the well-bonded interface. Moreover, while the geotextile reinforced samples indicate plastic strain-softening behaviour, the wire netting once fully activated seems to introduce a stress-hardening character to the clay beam. This implies that the large
B. Indraratna, G. Lasek
570 0.2
0.15
-
GALVANISED
WIRE
NET-\
~nforcement
z
O
<
0.1
O
J
F
NON-WOVEN
GEOTEXTILE
~
U5
22.6'/o
0.05
il--geotextJle bond
e
J' 0
Fig. 10.
i
~
r
p 1~ 1
t
t
I 2
J
I
i
P
I
i
3 DEFLECTION
I
I
t
4
I
I
P
r
P
P
~
i
I
I
i
I
5 (mm)
Load~deflection testing of clay beams reinforcedwith a non-wovengeotextile.
deformation behaviour of the two reinforced systems is quite different, hence, their ultimate failure modes cannot be directly compared. In comparison with the geotextile-reinforced specimens, the better interface stability provided by wire netting is mainly responsible for the enhanced flexural strength at large deflections. Even though wire netting has no 'drainage function', its contribution at moisture contents significantly exceeding the OMC is still marked. However, this does not imply that utilisation of wire netting in 'wet' clays would always work. On the contrary, compaction of excessively wet clayey fills would benefit with some form of pore pressure dissipation, and under such adverse conditions, a combination of wire netting with 'drainage type' geotextiles would be an attractive solution during compaction.
CONCLUSIONS The flexural testing of clay beams using four point loading indicates that only a small deflection is required to cause cracking in the tension zone. The
Load-deflection behaviour of clay beams
571
maximum load carrying capacity and the corresponding flexural tensile strength are obtained at or very close to the optimum moisture content. Unreinforced clay beams show an initial elasto-plastic deformation upon loading, followed by brittle failure in the post-peak region. An increase in moisture content much in excess of the OMC tends to transform the postpeak behaviour to a more ductile mode, but at the expense of substantially decreasing the load carrying capacity. The use of galvanised steel wire netting as reinforcement in the tensile zone of the beams provides a substantial increase in flexural tensile strength, and enables the beams to sustain much larger deformations (exceeding 30% of the depth of the beam) without causing complete failure. Reinforced clay beams cast at either side of the OMC continue to support the applied load even after the initial cracking of the beam base has occurred. In this case, the load carrying function is transferred to the clay portion above the interface, and the tensile load is taken by the wire netting. Subsequent slabbing and localised failure of the clay below the interface does not lead to total failure of the beam, once the wire netting is fully activated, whereby the interface 'pull-out' resistance is fully mobilised. At or close to the OMC, the increase in flexural tensile strength provided by the galvanised wire netting is generally greater than 90%. Moreover, the reinforced clay beams demonstrate a definite ductile-plastic (post-peak) behaviour following the pre-peak elasto-plastic response, whereas the unreinforced beams are generally characterised by a post-peak brittle failure mode. The findings of this study have important implications on the design and construction of clay liners on a soft sub-grade where ground settlements (subsidence) cannot be neglected, or in situations where the surcharge loads (landfill overburden) are excessive to cause significant liner deflections. Considering the excellent ability of galvanised wire netting in improving the flexural tensile strength of compacted clay and their 'low cost' in relation to conventional geomembranes, their potential use in landfill clay liners should be further encouraged and investigated, preferably through a field trial.
REFERENCES Ajaz, A. & Parry, R. H. G. (1975). Stress-strain behaviour of two compacted clays in tension and compression. Geotechnique, 25, 45-51. Ali, F. H. (1993). Field behaviour of geogrid reinforced slopes. Geotextiles and Geomembranes, 12, 53-72. Boutwell, G. P. & Hedges, C. S. (1989). Evaluation of waste retention liners by multivariate statistics. Proc. 12th Int. Conf. on Soil Mechanics and Foundation Engineering, Rio de Janiero, Vol. 2, pp. 815-818.
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Boynton, S. S. & Daniel, D. E. (1985). Hydraulic conductivity tests on compacted clay. J. Geotechnical Engineering, ASCE, 111, (April) 465-478. Daniel, D. E. (1993). Clay liners. In Geotechnical Practice .for Waste Disposal, Chapman and Hall, London 683 pp. Farrag, K. & Griffin, P. (1993). Pull-out testing of geogrids in cohesive soils. Geosynthetie Soil Reinforcement Testing Procedures (STP 1190), ed. S. C. Cheng. American Society for Testing and Materials, Philadelphia, USA, pp. 7 ~ 89. Fluet, J. E. (1988). Geosynthetic for soil improvement. Geosynthetics Jor Soil Improvement, Proc. of ASCE National Convention, Nashville, Tenessee, USA, pp. 1-21. Giroud, J. P. (1984). Geotextiles and geomembranes. Geotextiles and Geomembranes, 1, 5-44. Giroud, J. P., Badu-Tweneboah, K. & Bonaparte, R. (1992). Rate of leakage through a composite liner due to geomembrane defects. Geotextiles and Geomembranes, 11, 1-28. Hausmann, M. R. (1990). Engineering Principles of Ground Modification, McGrawHill, New York, 632 pp. Hilf, J. W. (1975). Compacted fill, Chapter 7, Foundation Engineering Handbook, eds H. F. Winterkorn & H. Fang. Van Nostrand Reinhold, New York, pp. 244~304. Holtz, R. D. & Broms, B. B. (1977). Walls reinforced by fabrics - - results of model tests. Proc. of Int. Conf in the Use of Fabrics in Geotechnics, Paris, Vol. 1, pp. 113 117. Indraratna, B., Satkunaseelan, K. S. & Rasul, M. G. (1991). Laboratory properties of a soft marine clay reinforced with woven and nonwoven geotextiles. Geotechnical Testing Journal, ASTM, 14, 288-295. Ingold, T. S. (1982). An analytical study of geotextile reinforced embankments. Proc. of the Second Int. Conf. on Geotextiles, Las Vegas, USA, Vol. 3, pp. 683 688. Ingold, T. S. (1983). A laboratory investigation of grid reinforcements in clay. Geotechnical Testing Journal, ASTM, 6, 112-119. Jewell, R. A. (1982). A limit equilibrium design method for reinforced embankments on soft foundations. Proc. of the Second Int. Conf on Geotextiles, Las Vegas, USA, Vol. 3, pp. 671-676. Juran, I., Knochenmus, G., Acar, Y. B. & Arman, A. (1988). Pull-out response of geotextiles and geogrids. Geosynthetics for Soil Improvement, Proc. of ASCE National Convention, Nashville, Tenessee, pp. 92-126. Kalteziotis, N., Sarsby, R. W. & Zervogiannis, H. (1994). Compacted clay soils for landfill liners. 13th Int. Conf. Soil Mechanics and Foundation Engineering, New Delhi, India, Vol. 4, pp. 1591-1594. Kleppe, J. H. & Olsen, R. E. (1985). Desiccation cracking of soil barriers, ASTM Special Technical Publication No. 874, pp. 263-275. Koerner, R. M. (1994). Designing with Geosynthetics, 3rd edition, Prentice Hall, New York, 783 pp. Koerner, R. M. & Hwu, B. (1991). Stability and tension considerations regarding cover soils in geomembrane lined slopes. Geotextiles and Geomembrantes, 10, 335-355. Lambe, T. W. (1960). The structure of compacted clay. Transactions ASCE, 125, 682-705.
Load-deflection behaviour of clay beams
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Mitchell, J. K., Hooper, D. R. & Campanella, R. G. (1965). Permeability of compacted clay. Soil and Foundation Design, ASCE, 92, 41-66. Yamanouchi, T. (1986). Historical review of geotextiles in Japan. Geotextiles and Geomembranes, 4, 165-178.
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ELSEVIER
S0266-1144(96)00020-9
Laboratory Evaluation of the Load-Deflection Behaviour of Clay Beams Reinforced with Galvanised Wire Netting
B. Indraratna a & G. Lasekb "Department of Civil and Mining Engineering, University of Wollongong, NSW 2522, Australia bGeo-Environmental Engineering Division, Cleary Bros. (Bombo), Pty Ltd, NSW, Australia
ABSTRACT In Australia the utilisation of clay in the construction of landfill liners has recently come under criticism either due to the difficulty in reproducing the laboratory results on site, or because of the potential for cracking of the compacted liner after construction. Realising that clay is weak in tension and given the possibility of certain areas of the liner being subjected to flexure, a testing program was undertaken to determine the effects of placement (compaction), moisture content and internal reinforcement on the potential failure of clay liners. In particular, this paper discusses the load carrying capacity and deformation sustained by compacted clay beams in flexure, and the improvement gained through the introduction of an economical reinforcement in the form of galvanised steel wire netting. Results obtained by flexural bending tests are discussed in detail, and the extent of improvement provided by galvanised wire netting is evaluated. © 1997 Elsevier Science Ltd. All rights reserved.
INTRODUCTION The main role of a landfill clay liner is to prevent the migration of contaminated fluid or leachate from the containment area by providing a hydraulic barrier, thereby minimising the risk of pollution of adjacent and underlying land and water resources. The performance of a clay liner is directly related to the leachate characteristics which can be extremely variable from one location to another. The high levels of organic solvents resulting from hazardous solid wastes have often dictated stringent criteria 555
556
B. Indraratna, G. Lasek
with regard to the liner thickness and its hydraulic conductivity (permeability). Irrespective of the achieved degree of compaction and permeability in the field, subsequent cracking of clay liners due to differential settlement of the sub-grade or due to excessive surcharge load can have catastrophic consequences in terms of pollution control. Particularly in the Illawarra region of NSW, ground subsidence due to underground mining is not uncommon. Considerable research has been undertaken to assess the desirable strength and conductivity properties of clay liners (Daniel, 1993); the effect of moisture content at compaction (Hilf, 1975; Lambe, 1960); and the effect of compaction on permeability (Boynton & Daniel, 1985; Mitchell et al., 1965) among other design and construction aspects. Studies have also been conducted on the effectiveness of laboratory tests in relation to field practice (Holtz & Broms, 1977; Ingold, 1983; Boutwell & Hedges, 1989); the maintenance and defects of liners (Giroud et al., 1992); the potential for desiccation cracking (Kleppe & Olsen, 1985) and numerous methods of clay stabilisation by internal reinforcement (Koerner, 1994; Yamanouchi, 1986; Giroud, 1984). In order to verify the suitability of a silty clay for the construction of a landfill liner, basic laboratory tests were conducted on a selected clay from Menangle, NSW, including Atterberg limits, compactability, permeability, particle size distribution, etc. However, knowing that the effective functioning of a liner depends upon the movement of the sub-grade, the evaluation of (a) behaviour of this compacted clay in tension (flexure) at varying moisture contents, and (b) the degree of improvement that can be achieved by internal reinforcement, seems imperative. The thickness of typical clay liners in New South Wales ranges from 0.2 m to 1.5 m depending upon the type of solid waste to be contained and the type of sub-grade. The conservative design of thick liners involves a substantial loss in space that would otherwise be occupied by the waste itself. Naturally, the thickness of liners can be reduced substantially if they are placed above rock foundation, but where the foundation consists of weak or soft compressible soils, the thickness of lining should be adequate to provide sufficient bending rigidity. It is therefore logical that if the cracking potential and the associated deflection behaviour can be properly evaluated, the optimum thickness of the liner can be reduced significantly. For this purpose, laboratory clay beam models were cast and tested in flexure, with and without reinforcement. Reinforcing the clay with galvanised wire netting was considered to be an economical alternative to the use of geomembranes. The effect of this wire netting on the load-deflection response is discussed in the paper for clay beams compacted at different moisture contents. The durability of galva-
Load-deflection behaviour of clay beams
557
nised wire netting can be improved by plastic coating. For instance, such wire netting used for a variety of purposes in acid-sulphate soil terrains (e.g. South Coast of New South Wales) is retailed with appropriate protective coating. As in the case of acidic soils or where the acidic leachate generated from the waste has a high corrosion potential, such protective coating will be useful for enhancing long-term durability.
LABORATORY SAMPLE PREPARATION
Soil properties Approximately 200 kg of clay sourced from the Menangle region of south-west Sydney was utilised in the formation of clay beams later subjected to flexural loading. This material can be categorised as a well-graded, light-brown silty clay with traces of gravel. Its basic properties are summarised in Table 1, and its plasticity group is CH (clay of high plasticity) in the unified soil classification system. The optimum moisture content and the dry density of the compacted clay were determined on the basis of the standard Proctor compaction method. The falling head permeability estimated for compacted Menangle clay (2.8 × 10 -9 c m / s ) is sufficiently low to prevent infiltration of leachate, unless cracking takes place. Kalteziotis et al. (1994) suggest that the permeability of effective landfill liners should not exceed 1 x 10 -7 c m / s under most circumstances. As the Menangle clay is a highly plastic clay (liquid limit > 50%), it is particularly advantageous in providing sufficient flexibility to accommodate consolidation settlements and lateral movements after construction.
Casting of beams After drying the clay, the particles greater than 19 mm in size were removed, and water was added to cast test specimens at different moisture contents
TABLE 1 Properties of Menangle clay Liquid limit, % Plastic limit, % Plasticity index, % Optimum moisture content, % Maximum dry density, kN/m 3 Hydraulic conductivity, cm/s (close to OMC) Particle size distribution % finer than 0.075 mm
51.8 21.25 30-55 19.0 16.9 2.8 x 10 9 68%
558
B. Indraratna, G. Lasek
mainly greater than the optimum moisture content (OMC) of 19%. In practice, the compaction of clayey fills of moisture contents less than OMC has been found to have adverse effects such as encouraging surface cracking or desiccation due to increased brittleness (Ajaz & Parry, 1975). In the following text, moisture contents on either side of the OMC are referred to as 'wet of optimum' or 'dry of optimum'. Beams were made according to the standard Proctor compactive effort (594kJ/m 3) using a mould of internal dimensions: 500ram x 100mm x 100ram. Compaction was carried out using a rammer of 5 0 m m in diameter and 2.5 kg in weight, falling freely from a height of 300mm above the surface. This standard compaction advocated the beams being constructed in four layers of 25 m m thickness, each layer imparted with 100 evenly distributed blows, with the reinforcement placed over the initially compacted layer, i.e. 25 mm from the base of the beam. Out of the many beams tested, the behaviour of 22 selected samples is considered in this paper including both unreinforced and reinforced beams.
Type of reinforcement Although limited testing has been undertaken to study the behaviour of reinforced clay in flexure, much emphasis has been placed on improving the shear strength of soils using geotextiles and geonets (Fluet, 1988; Jewell, 1982; Ingold, 1982). Koerner and Hwu (1991) give a comprehensive account of the tensile considerations of a reinforced slope. In this study, an economical alternative was adopted in the form of galvanised steel wire netting with apertures of 12.5mm and a wire thickness of 0 . 5 6 m m (Fig. 1) that could provide sufficient tensile strength to the soil, as well as being easily installed and moulded to an uneven subgrade. Although not used as reinforcement in the past, galvanised wire netting was expected to provide the necessary tensile strength and exhibit good pull-out characteristics, because of the clay material on either side of the net providing a strong interface by 'bonding' through the apertures. Beams were cast at similar moisture contents with and without wire netting to evaluate the advantages achieved through the introduction of this reinforcement. In contrast to the wire netting, two additional beams were constructed at moisture contents significantly higher than the OMC, but reinforced with a nonwoven geotextile and tested in the same manner. The main difference is that the wire netting acts purely as a reinforcement with no drainage function, whereas needle-punched nonwoven geotextiles are characterised by a predominant drainage function apart from also providing some form of reinforcement.
Load-deflection behaviour of clay beams
559
( 23mm
( 12.5mm
I
I
WIRE THICKNESS= 0.56mm
(a) Dimension of aperture
(b) Layout of wire netting
in mm.
Fig. 1. Type of galvanised wire netting used as beam reinforcement.
FLEXURAL TESTING Testing was undertaken using the Instron 4302 machine with a modified loading head, enabling four point flexural loading (i.e. every third-point loading, see Fig. 2), thereby providing a constant maximum bending m o m e n t over the central third of the beam. U p o n placement of each beam on the end supports, deformation due to the self-weight of the beams was monitored. Bending due to self-weight occurred to a slight extent (approximately 0.51-0mm) only in the case of the unreinforced beams cast wet of optimum, whereas this deflection was negligible in reinforced beams. The beams were then loaded at a constant rate of 1.5ram per min with the deflection and corresponding load recorded by the automated plotter. U p o n completion of each test, the cracking pattern of the beams was traced with minimum disturbance. Unreinforced beams
Six selected clay beams tested without reinforcement at moisture contents on either side of the o p t i m u m are listed in Table 2. C o m m o n practice involves the construction of clay liners at moisture contents generally 2% or greater than the O M C for silty clays such as the Menangle clay, in order to ensure the desirable very low hydraulic conductivity. Observation of these unrein-
560
B. Indraratna, G. Lasek
Fig. 2. Failure of unreinforced beam using four point flexural loading.
TABLE 2
Moisture contents of beams cast without reinforcement Beam
Moisture content
u1 u2 u3 U4 U5 U6
17.6% 18.2% 19-0% 20-1% 22.6% 24.3%
+/
O.M.C.
-1-4% 0.8% OMC +1.1% +3.6% +5.3%
forced beams during testing indicated the inception of a tensile crack at the base of the beam (tensile zone) and its subsequent propagation to the top with the increased loading until failure (Fig. 2). The load~leflection responses of the unreinforced beams are shown in Fig. 3(a), and from this plot it can be concluded that the m a x i m u m load of 0-21 kN was sustained by the beam U3 compacted at the optimum moisture content of 19%. This corresponds to a m a x i m u m bending m o m e n t of 15.8 x 10 -3 k N m and a m a x i m u m flexural strength of 94.5 kPa at its crosssection. After reaching this peak load at a deflection of 1-4 mm, the pre-peak,
Load-deflection behaviour of clay beams
561
0,25
MOISTURE CONTENT
0.2
K / ~ Elasto-plastJc
~
A~
T ~k
Brittle failure
~
[
u1 U2
17,6% 18.2%
190%
-A-U3
:
/
• ÷
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O, 0,1 i
allure
0.05
0
1.0
DEFLECTION
(ram)
2.0
(a) 120
!
100 L
Z uJ Z
8o~
e~
40
20
~_
t
~ t 18
I
i
i 19
[
~
i
1 i I i i i i i i 20 21 22 M O I S T U R E C O N T E N T (%)
i
I
t
~ 23
t
i
L 24
I
L
(b) Fig. 3.
Behaviour of unreinforced beams: (a) load~leflection response and (b) variation of flexural stiffness against moisture content.
562
B. Indraratna, G. Lasek
elasto-plastic behaviour was followed by rapid (brittle) failure involving pronounced tensile cracking at the mid-span with an ultimate deflection of 2.2 mm. A similar type of failure was observed for beams near the OMC, for instance, in the case of beams U1 (1.4% dry of OMC), U2 (0.8% dry of OMC) and U4 (1.1% wet of OMC) with their maximum sustained loads being 0-137kN (O-t = 6 1 . 6 5 k P a ) , 0.176kN ( a t --79.2kPa) and 0.174kN (at = 78.3 kPa), respectively. The flexural tensile strength is given in brackets. Out of the unreinforced beams, U1 (1.4% dry of optimum) sustained the greatest deformation of 2.1 m m before failure, whereby its initial stiffness (deformation modulus) is significantly less than the beams constructed at higher moisture contents. These results indicate a loss of strength of the clay as its moisture content departs from the optimum. Both beams U5 and U6 cast at moisture contents significantly greater than the optimum failed at considerably smaller loads of 0.037kN (at = 16.65kPa) and 0-03kN (at = 13.5 kPa), respectively, with the corresponding deflections of approximately 0 . 7 m m at the peak stress. The load-deflection behaviour of these 'wet' beams is relatively more ductile (lower stiffness) in comparison with the beams cast at or near the OMC. Figure 3(b) illustrates the corresponding variation of flexural (bending) stiffness plotted against the moisture content. The flexural stiffness is determined at 50% of the peak load, where the loaddeflection response can be linearly approximated. The above results carry definite implications on the flexural behaviour of unreinforced clay liners installed on a soft subgrade. It may be deduced that the maximum tensile strength occurs at or close to the optimum moisture content with a significant loss in strength occurring as the moisture content departs substantially from the optimum. According to these results, when a clay liner is constructed dry of optimum it seems to undergo a greater deflection before brittle failure occurs. In general, all tests irrespective of the moisture content indicate that little deflection is required to initiate failure of unreinforced beams, unless the tensile zone is properly reinforced as discussed below. Reinforced beams
Testing was undertaken to determine whether through the introduction of a wire net type of reinforcement a clay liner could withstand enhanced load deformation characteristics before failure. An increase in tensile (flexural) strength is attributed to the stability of the soil-reinforcement interface (Juran et al., 1988; Indraratna et al., 1991; Ali, 1993), the moulded moisture content and the degree of compaction, and the frictional (pull-out) characteristics of the selected geotextile (Farrag & Griffin, 1993; Hausmann, 1990).
Load-deflection behaviour of clay beams
563
TABLE 3 Moisture contents of beams cast with reinforcement
Beam
Moisture content
+ / - O.M.C.
RI R2 R3 R4 R5 R6 R7 R8 R9
17.3% 18.8% 19.5% 20.4% 21.0% 22-1% 23.7% 23-9% 24.5%
-1,7% -0,2% +0,5% + 1,4% +2,0% +3.1% +4.7% +4.9% +5.5%
Table 3 summarises nine selected clay beams constructed at different moisture contents. These beams were tested in flexure in exactly the same manner as that for the unreinforced beams discussed previously. As mentioned earlier, the form of reinforcement was a galvanised steel wire netting with apertures of 12.5 mm and a wire thickness of 0.56mm (Fig. 1). Due to its flexibility, this type of reinforcement may be easily moulded to an uneven subgrade and, if successful, may provide an economic alternative to the more conventional geonets and geomembranes. Due to the large number of reinforced beams tested at differing moisture contents, results and interpretations have been categorised into the following sections for clarity of discussion: (a) (b) (c) (d)
beams beams beams beams
cast dry of optimum moisture content, cast wet of optimum moisture content, subjected to extended deformation, and reinforced with a nonwoven geotextile material.
Beams tested dry of optimum moisture content Figure 4 illustrates the load-deflection relationship of three reinforced beams (R1-R3) compacted dry of optimum, and two selected unreinforced beams (U1 and U3) to enable comparison at similar moisture contents. It is observed that the wire net reinforcement increases the load capacity of the beams by at least 50-100%, and at the same time almost doubling the deflection at failure (peak stress). Although the initial loading (quasi-linear elastic) is similar in both cases, the post-peak behaviour indicates that the unreinforced beams exhibit a rapid decline to zero load (brittle failure), whereas the reinforced beams sustain a more gradual
564
B. Indraratna, G. Lasek 0.4
MOISTURE CONTENT
! Elasto-plas~c/plastic
u1
17,6%
÷ u3
19.0%
R1
17.3%
-e. R2
18.5%
-~ R3
18.8%
0.3 -~,~,,,, Quasi-linear
~ o2 ~
elastic Brittle-plastic
i
Brittle failure
I
I
0
- t
0
Fig. 4.
1
2
3
.
4 5 DEFLECTION (mm)
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
6
Load-deflection behaviour of clay beams (dry of optimum) in comparison with selected unreinforced beams.
(brittle-plastic) deformation response. Even at central deflections exceeding 5mm, the reinforced beams continued to sustain at least 50% of the peak load. The behaviour of beams at much larger deflections is discussed later. Beams tested wet of optimum moisture content
Figure 5 shows the load deflection relationships of reinforced compacted clay beams (R4-R9) cast wet of the optimum moisture content. The responses of three selected unreinforced beams (U4, U5, U6) cast at similar moisture contents are also plotted for comparison. In contrast to the previous Fig. 4, it is immediately apparent that the reinforced beams exhibit a ductile-plastic, post-peak behaviour in comparison with the beams compacted dry of optimum. In general, the reinforced clay beams compacted wet of optimum (Fig. 5) exhibit slightly smaller load carrying capacities than their dry of optimum counterparts. Increase in moisture content causes a reduction in the load carrying capacity and the initial stiffness of the beams. For instance, while the beam with 20-4% moisture content (R4) shows a peak load of 0.33kN, beam R9
Load-deflection behaviour of clay beams
565
0.4 MOISTURECONTENT
~ _ ~
/
0.3
~
Eiasto-plastic/plastic f
~
! U4 20.1% ,.e- u5
Ductile-plastic
22.6%
~ U6 24.3% ÷ R5 21.0% R6 22.1%
D 0.2 <: ;
S
/
4. R7 23.7% . R8 23.9%
Ductile-plastic
Quasi-linear
-~ R9 24.5%
o.1
o
0
Fig. 5.
1
2
3
4 5 DEFLECTION (ram)
6
Load~teflection behaviour of clay beams (wet of optimum) in comparison with
selected unreinforcedbeams. at a moisture content of 24.5% carries a much smaller peak load of 0.07kN. Also, it seems that increasing the moisture content enhances the degree of plastic straining at constant load, with the load-deflection response of beams R8 and R9 illustrating an almost 'perfectly plastic' deformation. It is also apparent that all beams of similar moisture content (with and without reinforcement) initially follow a quasi-linear elastic load-deflection response, until considerable disparity is evident as the peak load of the unreinforced beams is approached. This is because initial cracking of the base of the beam is not prevented by the reinforcement. However, after cracking of the predominantly tensile zone, the beam with reinforcement continues to carry more load, whereas the unreinforced beam rapidly sheds its load to zero in a brittle manner. At the base of the mid-span, inception of a vertical crack followed by its propagation along the reinforcement interface may be noticed (Fig. 6), but the upper clay seam remains relatively unaffected as the wire netting becomes activated in tension. The compression load is resisted by the upper portion of uncracked clay above the wire netting.
566
B. Indraratna, G. Lasek
Fig. 6. Initial failure of the clay beam with cracking extending to the reinforcement layer. Beams subjected to extended deformation Practically all the tests described earlier were conducted until the peak load started to drop, with the extended post-peak large deflection behaviour not studied. In order to investigate the full contribution of the wire net at large deflections, five specimens were subjected to extended deformation up to 30 mm, and the corresponding load~leflection response was monitored. As in the previous tests, these samples were subjected to a constant loading rate of 1.5 m m per min, with the extended load~teflection behaviour illustrated in Fig. 7. F r o m Fig. 7, it is apparent that at say 3 m m deflection, initial failure occurs due to the cracking of the clay from the base of the beam towards the layer of wire net. This initial peak load is directly related to the flexural tensile strength of the clay below the wire netting, the region in which pronounced cracking occurs within the middle third of the beam. The postpeak behaviour is characterised by a considerable loss in load carrying capacity (brittle-plastic), until the load is subsequently taken up by the reinforcement. With further loading, the wire net embedded in the tension zone becomes fully activated, and the load continues to rise again even at large deflections (up to 30 mm), without any signs of brittle failure. This observation implies that as long as the galvanised wire netting can be laid as close as practically possible to the base of the beam, the effects of initial cracking can be minimised. At large deflections, the clay beneath the reinforced layer
Load-deflection behaviour of clay beams
567
0.5 MOISTURECONTENT ! Full-activation 0.4
of reinforcement
-
/II-B-II
E1
18.8%
4- E2
19.5%
~ E3
20.4%
0.3
I/~
I ~-'k JIl(-
i1~/ in#
"
J
~/
.
7
of reinforcement
_.J
0.2
0.1 -
~
A
Qoasi-ll~ar e l a ~ c
0 0
10
20
30
DEFLECTION (mm)
Fig. 7.
Load-deflection testing of reinforced clay beam subject to extended deformation.
continued to suffer from localised failure, where 'slabbing' and debonding of clay was evident (Fig. 8), with this causing some drop in load carrying capacity as indicated by beam E4 in Fig. 7. Considering the fact that the galvanised wire netting embedded in the tension zone of a compacted clay liner can sustain deformations of greater than 30 mm without complete failure (i.e. 30% of thickness), their utilisation in practice should be given serious consideration, particularly where the liner is to be installed on a soft sub-grade material. The introduction of this form of reinforcement should allow relatively large deflections to be tolerated by the liner without cracking. In this respect, the contribution of galvanised wire netting is significant both technically and economically. The variation of the computed flexural (tensile) strength with moisture content is given in Fig. 9, based on the bending moment-stress relationship ( M / I = a/y) for the beam, where M = maximum bending moment, I = moment of inertia, tr = flexural tensile strength and y = maximum distance to neutral axis. The maximum tensile strength is achieved at or close to the optimum moisture content with a significant decline in strength on either side of the OMC. With the introduction of galvanised
568
B. Indraratna, G. Lasek
Fig. 8.
Compacted clay beam after undergoing 30mm defection.
200
OMC
-ff
~ UNREINFORCEDI ,G REINFORCED I
0-
--~ 150
(,9
~W" 100 Ziii D tw X
50
0
~k-t
III
FI
18
I III
IPfll
J I J II
20
I1
lit
il~l[I
22
'P
Iql
I
24
MOISTURE CONTENT %
Fig. 9. Variation of flexural (tensile) strength with moisture content.
Load-deflection behaviour of clay beams
569
wire netting, an increase in tensile strength in the order of 60 kPa (65%) was achieved at the optimum moisture content. At increased moisture content, the difference between the reinforced and unreinforced samples tends to decrease, which implies that the role of wire netting is limited at water contents much greater than the OMC. Effect of geotextile layer on flexurai behaviour Wire netting provides only a reinforcement effect, and has no drainage function with regard to the compaction of clays with moisture contents significantly exceeding the OMC. In contrast, nonwoven geotextiles have been popular in the Asia-Pacific region for efficient compaction of 'wet' borrowed fills in low-lying land development. Even though the gain in load carrying capacity and flexural stiffness and strength due to wire netting was appreciated, it was decided to cast a few additional beams stabilised with a conventional geotextile to assess its infuence on the flexural behaviour of clay. The geotextile used was a needle-punched nonwoven (Bidim), having a thickness of 1.4 mm and a specific weight of 120 g/m 2. The drainage function of needle-punched nonwoven geotextiles is generally more prominent than their reinforcement effect (Indraratna et al., 1991). Considering the excellent pore-pressure dissipation characteristics of these geotextiles, the additional clay beams were cast at moisture contents significantly greater than the OMC of 19%. The behaviour of two such beams (G1 and G2) is illustrated in Fig. 10, where their 'as compacted' moisture contents were 22.0% and 23.1%, respectively. The geotextile was placed above the first layer of compacted clay (25 mm above base), and the subsequent clay layers were compacted above the geotextile. At similar moisture contents, the load-deflection behaviour of beams G1 and G2 is very different to the beams reinforced with wire netting (R6) and the unreinforced beam (U5) as shown in Fig. 10. As expected, it is evident that galvanised wire netting provides a greater flexural strength to the clay beam than the conventional geotextile. Nevertheless, the geotextile also enables a significantly increased deflection at peak load of more than twice that sustained by the relatively brittle unreinforced beam. In comparison with wire netting, premature failure of the clay beneath the layer of geotextile occurred as a result of the relatively poor bonding between the soil and geotextile. At similar moisture contents (22-23%), the wire netting provides a flexural strength at least three times that of the nonwoven geotextile mainly because of the well-bonded interface. Moreover, while the geotextile reinforced samples indicate plastic strain-softening behaviour, the wire netting once fully activated seems to introduce a stress-hardening character to the clay beam. This implies that the large
B. Indraratna, G. Lasek
570 0.2
0.15
-
GALVANISED
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NET-\
~nforcement
z
O
<
0.1
O
J
F
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GEOTEXTILE
~
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il--geotextJle bond
e
J' 0
Fig. 10.
i
~
r
p 1~ 1
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I
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I
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P
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5 (mm)
Load~deflection testing of clay beams reinforcedwith a non-wovengeotextile.
deformation behaviour of the two reinforced systems is quite different, hence, their ultimate failure modes cannot be directly compared. In comparison with the geotextile-reinforced specimens, the better interface stability provided by wire netting is mainly responsible for the enhanced flexural strength at large deflections. Even though wire netting has no 'drainage function', its contribution at moisture contents significantly exceeding the OMC is still marked. However, this does not imply that utilisation of wire netting in 'wet' clays would always work. On the contrary, compaction of excessively wet clayey fills would benefit with some form of pore pressure dissipation, and under such adverse conditions, a combination of wire netting with 'drainage type' geotextiles would be an attractive solution during compaction.
CONCLUSIONS The flexural testing of clay beams using four point loading indicates that only a small deflection is required to cause cracking in the tension zone. The
Load-deflection behaviour of clay beams
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maximum load carrying capacity and the corresponding flexural tensile strength are obtained at or very close to the optimum moisture content. Unreinforced clay beams show an initial elasto-plastic deformation upon loading, followed by brittle failure in the post-peak region. An increase in moisture content much in excess of the OMC tends to transform the postpeak behaviour to a more ductile mode, but at the expense of substantially decreasing the load carrying capacity. The use of galvanised steel wire netting as reinforcement in the tensile zone of the beams provides a substantial increase in flexural tensile strength, and enables the beams to sustain much larger deformations (exceeding 30% of the depth of the beam) without causing complete failure. Reinforced clay beams cast at either side of the OMC continue to support the applied load even after the initial cracking of the beam base has occurred. In this case, the load carrying function is transferred to the clay portion above the interface, and the tensile load is taken by the wire netting. Subsequent slabbing and localised failure of the clay below the interface does not lead to total failure of the beam, once the wire netting is fully activated, whereby the interface 'pull-out' resistance is fully mobilised. At or close to the OMC, the increase in flexural tensile strength provided by the galvanised wire netting is generally greater than 90%. Moreover, the reinforced clay beams demonstrate a definite ductile-plastic (post-peak) behaviour following the pre-peak elasto-plastic response, whereas the unreinforced beams are generally characterised by a post-peak brittle failure mode. The findings of this study have important implications on the design and construction of clay liners on a soft sub-grade where ground settlements (subsidence) cannot be neglected, or in situations where the surcharge loads (landfill overburden) are excessive to cause significant liner deflections. Considering the excellent ability of galvanised wire netting in improving the flexural tensile strength of compacted clay and their 'low cost' in relation to conventional geomembranes, their potential use in landfill clay liners should be further encouraged and investigated, preferably through a field trial.
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