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EAST MIDLANDS GEOTECHNICAL GROUPTHE INSTiTUTION OF CIVIL ENGINEERS

Geotechnical Engineering of Landfills

Proceedings of the syl.lposiumheld at The Nottingham Trent University

Department of Civil and Structura: engineeringon 24 September 1998

.I Thomas Telford

Organizers: The East Midlands Geotechnical Group of theInstitution of Civil Engineers

Organizing Committee: Dr N. Dixon, Dept. Civil and StructuralEngineering, The Nottingham Trent University; Dr EJ. Murray,Murray Rix Geotechnical; D.R.V. Jones, Golder Associates (UK)1,1:1 Dr l. JcfTn\on, Dept. Civil and Structural Engineering, The

'r', . r""jr'nllI1WIlI

The symposium on Geotechnical Engineering of l.andfills and thi, .1Ssociatedpublication aim to provide an opponunity for the pr",,'ntation and Ji-:u:;sion ofrcccnt developments in the d,,:;i~n, construction ,md operation ,.' landfill

"';1;'" Th,' 'ipecifir nbi,'c,;' ",. 'highlight:Ilic IIlJponant ruk pl,l)l~ .dC mechanic.,: propenies ,); ',lsle inoptimising barrier design ,md iJndfll1 operation:

issues related to the design and testing of Plineralliners. includin~ ','ntoniteenriched soils and colliery spoil: and -

recent developments in the assessment of geos\ nrhetics, includin~ barrierstability, assessment of protection materials f~r liners and pro!:'~nies ofgeosynthetic clay liners.

A Ithough there have been a number of conferences c.nd meetings both :n the UKand throughout the world covering issues of la:ldfiIJ desin, mate;'ialperformance and landfill viJeration, it was felt by the organising c;mmirtee thatmany of these are aimed at specific sub groups of practitioners and researchers.Therefore it was considered timely to hold a symposium w'1ich co\erd a rangeof issues relevant to geotechnical engineers and associated disciplines, ~ohighlight new areas i)f research and practice, and to provide a forum fordiscussion.

It was for these reasons that the East 0.11dlands Geot~chnical Group(EMGG) decided to organise a symposium on the subject in 1998, following thesucce:;sful seminars on Groundwater Pollution in 19q4 and Lime Stabiiisation in1996. The EMGG was formc,-j in 1992 with the aim of pro\iding theenvironment of a learned society on geotechnical subjects, for the benefit ofcivil engineers and engineering geolog:sts living ak,ng the hinterhnd of the M Ifrom North.amptonshire to South Yorkshire. Most of tre ~ctivity concerns theevelling meetings j::rogramme, ",Ithough site visib and sy:-:1pos;a are 0'ga~lise,-jto complement tpis wair, r01e. The Nottingham (rent U:~iversity was consideredan ap;,roprialt: ~ost for the symposium since it has beell active in r.esnrch i;1 ~hearea cf landfill engineering for several y:::ars.

The editors wisi1 te acknowledge the considerable s~PlJon c' both theorganisin; committee and the full EMGG ,:ommirtee. They also wi3r. to thankthe contributors for their excellent papers, particularly since the ti

Contents

COll1pression of waste allli Il11plications for pr:1LticeW !'OIvrie, f)J Richards (1/1(//

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I IMechanical Properties ofLandfill Waste

While significant 3ltcntion is givcn to thc design, construction and long termperformancc of landfill side slope lining and capping systems, relativel: littlework has been directed towards gaining an understanding of the mechanicalproperties of the Ilaste and assessing its interaction with the surrounding barrier.1\ knowledge of the mechanical properties of waste is required to 'enable costeffective, environmentally safe landfill facilities to be constructed and operated.A number of issues related to landfill design require consideration of the wastebody behaviour. Shear strength of the waste mass are reCjuired for use in slope stability

stt.:dies. Compressibility uf waste has to be quantified to enable predictions of total

and differential settlements, and of the distribution of these with time duringthe life of the landfill facility (e.g. this information is needed to JptimiseCapf1i'lg dp.sign)

An understanding of the permeability of waste, and of the influence ofoverbun.icn pressure and degradation, is .equired to improve operation ofleachate collection wells and the jevelopment of leachate re-circulationsystems (e.g. for potential use ir. flushing bio-reactors).

The in situ stress within, and stiff:1ess of emplaced waste are needed toenable assessment of the support pruvided to vertical and near vertical sideslope lining systems by the wa~te (i.e. leading to an understanding of the inservice d'.:'formations o~the lining system).

The pap.;:rs in this section address aspects of housel.old waste compres5'bility,iJcimeabi:ity and stiffness.

The paper by Powrie e: ul. covers issues related tv the compression ofland filled waste 'shich occurs both during placement (due to machinecOlllpaction and olerburden effects) and in tne long-term (as a result ofprocesses such as degradation ane ra,elling). The changes in waste deilsitythat result from compressioil during land filling are considered including theaffect on the mass of waste that a site can accept, and on the permeability of thewaste. The authors emph:::.si~e that knowledge of the permeability of the wasteis important becau"e of its influence on leachate production and management.In additio'l, they stress that long-term settlement must be taken into account inthe riesign of the landfill site closure systems, and comment that provision

should be made for the continuing care and maintenance of restoration works ..The paper summarises the various mechanisms of waste cumpression anddiscusses the implications for landfill operation, closure ;"ld aftercare.

Dixon & Jones highlight the important role 1I1at waste plays in thestability and structural integrity of vertical or near vertical lining systems. It iscontended that although the landfill industry is developing and constructingnovel barrier systems for steep side slopes, there is still a dearth of information,and hence limited understanding, of the factors which control both short-termand long-term deformations of the lining; these are intluenced by constructiontechniques and waste degradation processes respectively. The paper describes anovel method for nbtaining the relevant mech~nical properties of waste for usein assessment of designs, based an the pressuremeter, and presents the results ofinitial field trials. Results from in situ tests, to measure stiffness and stresses inboth fresh and partially degradcG household waste, are discussed.

The work presented in this section provides a timely and significantcontribution to the study of waste mechanics. Whi:e the study of wastebehaviour is fraught with difficulties (i.e. studying a heterogeneous materialwhose engineering properties change with time), t'1C papers demonstrate thatuseful results can be obtained. This area of research requires continued attentionfrom the landfill community.

Compression of waste andimplications for practice

w. Powrie, DJ. Ric"art!.\ a"d R.P. Beave"Dep"rtment ol( 'i1'd& !''"''l'ironmen{(l! Engineering, Ul1lvasity olSouthampton, ,)'017 113.J,UK

IntroductionCompression of landfilled waste occurs both during placement \.d1Je to machinecompaction and overburden effects), and in the long-term (as a result ofprocesses such as degradation and ravelling). The changes in wa~te densityresulting from compression during landfilling will affect the as-received volumeof 'Naste that a site can accept, rind also the permeability of the waste. Thepermeability of the waste is import::::1t because of its influense on leachatelevelS, production and maI~:tgement.

Long-ten.1 settlements must be taken into account in the dC3ign oflandfill site closure systems, and may require provision to be made for thecontinuing care anJ Plainte:lance of res:Olation works. In this paper, the varicusmechanisms of waste compressiJn ai1d the f::ctors influencing them aresummarized. Tile implications for landfill operation, closure, and aftercare arethen d:scussed.

Mechanisrr;s of compressionThe follov;ing lY'echanisms uf compression of waste '.vere identified by 2dil etat. (1990).

i.1echanic:t! compression, due to the crushing, distortion, reorientatiop.bending and/or breaking of waste partie ies as vertical stresses are increased,either during compaction or due to the self weight of the fill as furthermaterial is deposited on top. In the absence of pie-compaction, the degreeof mechanical compression depends (other factors being equal) on thedepth of the waste.

Degradation, .llIc to biological decomposition and physico-chemicalprocesses such as corrosion and oxidation oflhe waste in the longer term.

Ravelling, which is the f;radualmigration of finer particles into,the largervoids and which can occur during both mechanical compression anddegradation.

The cylinder is suspended vel1ically within a steel support frame. Thefeet of the support frame are mounted on load cells, enabling the weight of thecontento of the cell to be monitored continuously. The base of the cylinder issealed oy a 2m diameter platen which is seated on an '0' ring, Refuse in thecylinder is compressed by an upper platen, just under 2m in diameter, which canbe moved vertically up and down inside the testing cylinder. The upper platen isconnected to, and moved by, two 200mm diameter hydraulically operatedpistons. I\t the start of a test the upper platen is lowered onto the refuse: aconstant vertical load can then be applied by means or the hydraulic pistons.The maximum vertical load is 1900kN, giving a stress of 600kPa distributeduniformly over the plan area of the refuse sample.

Water can be allowed to flow dpward through the sample, from two450 litrc water header tanks mounted on a scaffold tower :Ip to 3 metres abovethe top of the testing cylinder. The tanks are connected to 12 evenly spaced25mm diameter ports on the lower platen, Water flows out of the upper platenthrough similar ports, and through a 2mm annular clearance gap between theouter edge of the platen and the inner surface of the testing cylinder. In-lineelectromagnetic flow meters are used to record the total volumes and flow ratesof water entering and leaving the sample,

At the start of each test, refuse was loaded into the cell and compactedin layers to the desired initial density, until the overall refuse depth wasapproximately 2.5m. Following placement of the refuse, 18 piezometers wereillstalled horizontally through ports in the side of the cylinder, located at verticalspacings :)f between 150mm ane I100mm, Lengths of string anchored at known~oints within the refuse and running out of the cell throug:l the piezometer portswere used to measure the total compression at various depths, The uppf'r pla~enwas then lowered onto the sample and an initial load was applied u~ing thehydraulic system. The compression of the refuse was monitored as a funstion oftime by measuring the downv .ard movement cf the upper platen. Any leachatesqueezed out of the refuse W'lScollected and its volume recorded,

When compression had substantially ceased (in pmctice, when the rateG f settlement had hllen to less than I% of the sample Ilf'ight per rlay~, the wastewas saUrated by introducillg water through tne lower platen and then ai:owed ,0drain to leach its field capacity ~i e, the moisture cor tent of the refuse underconditions f)f free down~ard drainage conditions). The saturated hyoraulicc'1nductivity ofth~ refuse was drterrllined (ilt constallt applied vertical stress) bycarrying Ollt a constant head flow test. '.Vater from the header tanks was aiiowedtv flow upward through the refuse, Gverflowing at the tC;J of the oample. Thehydraulic gradient was measured by means of the piezometer ports in the side ofthe colunlI~ and the flow rate using the electmmagnetic flowmeters. At highvertical stresses and low refuse permeabilities the flow rate of water into thesample was low, and direct measurement of the (small) fall in water level in theheade' tanks with time was found to be more reliable than the flowmeterreading.

Compression may also occur on wetting of thc waste, due to the loss of strengthor structurc of ccrtain componcnts on contact with moisture, e.g. paper andcardboard. Waste settlemenl is convcntionally classified as either primary orsec()nd~II")'.depcnding on the lill1escak over which it occurs.

Primary compressionPrimary compression of thc waste in the landfill will probably occur within aperiod of days following the deposition of further material on top (Bleiker ef al.,1995; Beaven & PO\lTie, 1995).

Beaven and Powrie (1995) carried out a series of tests on a number ofsamples of domestic refuse to investigate the variation of density and hydraulicconductivity with vertical stress. The tests were carried out in a large purpose-built compression cell, located at Cleanaway LId's Pitsea landfill site in Essex,England. The cell consists of a sieel cylinder, 2m In diameter and 3m high, intowhich refuse is placed for testing (Figure I).

Figure 1 The Pitsea compression cell during a recent overhaul andrefurbishment

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The refuse was then drained, the applied stress increased and the cycl.':of operation and measurement repeated (i.e. the moisture content of the refuseunder conditions of free downward drainage conditions). Tests were carried outon a number of samples of crude and pulverized domestic wastes. In this paper.the results from one test on crude waste (OM3) and one test on pulverized waste(PV I) arc discussed.

DM3 was a crude domestic refuse obtai:1ed from the iipping face of alandfill. The water content at field capacity at the start of compression was-102% (by dry mass). I'V I was a processed refuse, obtained by pulverizingcrude dOl11estic refuse, passing it through a 150111111filter aild removing densefines (including some putrescibles). The water cu,:tent at field capacity at thestart of compression was -141 % (by dry mass). The composition of thesewastes is summarized in Iable I.

measured density p and the initial woter content II' obtained from a sub-sampleof the refuse, using the standard soillllechanics relationship Pd = p/(I+w). Theinitial dry density of the processed refuse PV I was 0.25Mg/~), while for thecrude waste (OM3) the initial dry density was 0.33Mg/IllJ The correspondinginitial wet densities were 0.62 Mg/mJ (PV I) and 0.50 Mg/mJ (OM3). .

Figure 2 shows the variation in dry density and wet density at fieldcapacity with vertical dfective stress. (It has been assumed that the appliedvatical stresses at equilibrium stages of the tests under discussion are effectivestresses. lransmitted through the refuse matrix by interpanicle contact. This isbecause alihough the refuse is nOI dry, it is free to drain verticallv dl)\\'n\\,ardand the sample is of substantiall)' uniform hydraulic conductivity: llllls the fluidin the \'oids is at zero gaugt: prt:ssure). [3oth axes arc plotted 10 logarithmicscales, :lIId it has been assumed that the dell,il" and stress are uniformthroughout the sample, i.e. the cffects of self weigh; alld sidcwall li'iction havebeen ignored.

. The final dry density of OM3 at an applit:d stress of 600kPa was 0.71Mg/m~. while the tinal dry density of the processed refuse PV I was 0.60Mg/mJ. The coiTesponding final wet densities at field capacity were 1.18 Mg/mJ(OM3) and 0.97 Mg/IllJ (PVI). These may be compared with wel densities of0.62 - 0.67 Mg/mJ and 0.81 - 1.11 Mg/mJ reported by Caterpillar (1995)following compaction of crude domestic wastes in layers using Caterpillar 8168and 826 compaction plant. Assuming a water content in the range of :;0 - 40%,these ~'anges of wet density correspond to dry density ranges of 0.37 - 047M~/mJ and 1).49 - 078 Mg/mJ resp::~ti'!~ly. The implication is that, in terms 01'tile waste density achieved, compaction at the tipping face can have a similareffect to the uurial of the waste by several tens of metres of overbu,den.

OM3 PV IRefuse Ory density of Saturated %of %01'component component, density of total total

Mg/m ] component, mas~ massMg/m ]

Paper/card 0.41 1.21 39.8 49Plastic fi 1m 1.01 1.01 4.4 8.3Oense plastics I. II 1.11 6.4 7.8Textiles 0..,1 0.01 5.::; 5.7.JMisc. 1.02

combustiblesMisc. non- :'81 2.01 24 1.1comhustiblesGI

The hydraulic; conductivities of PV I and DM3 at different applied stresses areshown in Table 2. Between low and high stress states,~he hY~iauhc conductivityreduced by over 2 orders of magnitude (fro~ 3.5x10 t,~ 10 m/s) for DM3 andby nearly 5 orders of magnitude (from 10 to 3.5x10 m/s) for the processedrefuse PV I.

Applied stress, kPa Saturated hydraulic conductivity, mlsPVI DM3

Initial 2x 10,4 ND

40 3.6x 10'5 3.5x1O,5

87 7x 10'6 2x 10'5

165 2x1O,6 3x 10'6

322 9x1O,88xlO,7

600 3.5xlO,91X 10,7

Table 2 Hydraulic conductivity at different applied stresses

Hydraul ic conductivity i:; plotted as a function of dry density for both wastes (toa douhle logarithmic scale) in Figure 3.

- \~--~---l@J

,-~-,-

1E-8 - ---- ~-~--\~~---

1E-9L--'--------~-~-~-.0.2 0.:::; 0.5 07

Dry density (tlm3)

Secondary compressionQuantitative investigation into secondary compression of waste due todegradation is rare. Although case records are given in papers such as those byGasparini et at. (1995), Hilde & Reginster (1995) and Kostantinos et at. (1997),there are few data concerning the likely ultimate settlements of municipal solidand industrial wastes taking the effects of degradation into account.

The UK guidance manual on landfill design and operational practice,Waste Management Paper 26B (1995), suggests a secondary settlement f~gureof 15-20% of the initial refuse depth on the basis of research at a limitednumber of UK household waste sites (Coulston 8: Wye College. iQ(3). Otherresearch has sliggested that settlements in excess of 25% of the depth of thelandfill may occur (3jarngard 8: Edgers. 1990; Di Stefano, 1(93).

The f:rst draft of the DoE Waste Management Paper (WMP) 26D(landfill monitoring) suggested that long-term settlements of betll'een 25 and50% of the original fill thickness are typical of the allowance that may need tobe made in determining the final fill levels of a domestic waste landfill. Thelatest draft: for public consultation of WMP 26D (January 19(6) suggests long-term settlements in excess of 20%. The paper also notes that acceleratedstabilisation will cause more of the waste to degrade during the filling phase,reducing the magnitude of settlement following r::losure.

The considerable uncertainty regarding the secondary compression ofwaste is understandable in view cf the various factors that are known toinfluence its eventual magnitude. Tllf~se include:

the composition of the waste: wastes with (l higher proportion ofdegradable (principally organic) materia is willlJenore susceptible to long-term settlemPiit due to decol11oosition than wastes containingpredom inantly inert materials:

the as-plilced dry density of :he waste: for a given degradable fraction, agiven mass of compacted (i.e. de:1ser) waste would be expected to have lesspotential tor mechanical volume loss and hence long-term settlelllent -lyp;c"l dry rJcasities of wastes can be found III Beaven & Powrie (1995);

the dep~h of the fill mateia!: for a givell waste type dno density, (hepotential lo:;s of volume or ll1aos on riegrariat~C'n is roughly rr,opc:1ioEal to~he original volume or mil'SSof waste, so ,hat in absolute terms cleeperlarylfiils would bexpecteJ to e,hibit greater settlements

The rak of waste degraaation iT' the JOng-ter:n depends primarily on theco.nposition of the waste, its water content and the rate at which \\'at~r passesthrough it. Forced gas extrJction may also influence the rate or pattern ofdegradation, e.g. larger settlements tend to occur around gas extraction wells.

Increasing the water content of the fill incn~ases its rate ofdecomposition (Chen & Chynoweth, 1995), while increasing the rate of waterniOv~ment through waste without changing the water content has been found to

increase methane generation rales by approximately 25-50%, (Klink & Ham,1982). Although current UK policy is directed towards the s:abilizatio!l of!,illdfills within a 30-50 year time period (Grono,,!, 1996), this cani,ot beachieved with "dry" containment techniques, for which stabilization times ofhundreds or even thousands of years are predicted (Knox, 1996).

Measures currently beil~; taken to reduce stabilisation times at landfillsites in the UK principally involve the recirculation.of leachate. Treated and/oruntreated leachate collected at the boltom of a containment cell is pumped up tothe top of the landfill and is allowed to percolate down through the wask mass.This has been shown in laboratory scale experiments (l3arlaz c'/ (d.. In9;Reynolds & Blakey, 1995), landtill Iysimeters (Buivid el al. 1981: Kinman c/aI, 1987) and controlled landfill cells (ilaivadakis ef al., 1988; Townsend ef aI,1996) to enhance microbial activity within the waste and is suggested in WMP2613 (1995) as a method of achieving accelerated ~tabilization of 'l landfill. Asthe leachate is itself a product of the waste degradation process, it is likely tocontain the micro-organisms and nutrients necessary for the degradation of theputrescible fraction of the waste mass. However, prolonged recirculation ofuntreatf'd leachate could ('ead to high concentrations of soluble inorganic ionsdue to leaching and decorrposition processes within the waste mass. These ions,principally sodium and chlorid

conductivity of 10:7 m/s for a 30 m deep landfill ,md 5 x 10-7 m/s for a 60 mdeep landfill. According to the data shown in Figure 3, acceptable flushing ratescould be 'achieved for crude domestic wastes under a hydraulic gradient of unityfor dry waste densities not exceeding about 0.6 - 0.7 Mg/m3 Excessivecompaction of the waste at the tipping face would lead to waste densities inexcess of this range. and waste permeabilities below the required m::limum.

At effective stresses in excess of 300 kPa, the pel1l1eability of thepulvcrized wilste flYI is signilicantl\ less than that of the crude waste DM3.Although plJiVl'riz,ltion may be desirable. in that it promotes a more even flowof liquid 111l'ough thl' waste, till' reduced permeability will limit the depth ofl:lIldlili that C

because settlements have to be monitored over a period of years if not decadesfor mean ingful trends to emerge. Settlement data must also be correlated withthe waste type and placement histOly to enable the most important controllingvariables to be identified.

In the absence of any proven theoretical or empirical method ofestimating rates and magnitudes of secondary compression, it must be acceptedthat the attainment of a desired finished profile will involve a consideraJleamount of uncertainty Realistically, operators and regulators must accept thatthere will probably be a need for a long-term management plan, perhapsinvolving stripping back the cap !"mlll time to time and placing further materialbelow it.

Cap performanceThe restorative cap geomet ..y should be designed so as to minimize thelikelihood of cap failure by cracking or rupture as secondary settlement occurs.This is particularly important in the case of containment sites ~"here an intactcap forms part of the leachate management strategy, as failure of the cap willtend to lead to increas~d infiltration and a larger volume of leachate beinggenerated.

Magnitudes of secondary settlement are difficult to estimate withconfidence, but the pattern of waste settlement can have a significant effect onwheti,er nr not a resistive capping layer ruptures. Damage is more likely 0 becaused by differential settlements than uniform settlements. In addition, thegauge leilgth 'lver which t!~edifferential sett!~ment 0CCLrswill ~IS0 be ;:-:lpurtantin determining the severity of its effect on the cap;:>ing layer.

Recent experiment?1 studies carried out using a geotechnical centrifugeby Richards and Powrie (in preparation) have ShOWI~that the long-term integrityof 'I low permeability capping system can depend on the pattern of subsidencein the waste below it. The imposition of a displacement discontinuity (i.e. a stepsubside:1ce pattern) below a low l~ermeability cap is much more likely to resultin the rupture of the car t'.an the imposition of a discontinuity in slope (i.e. aiamp subsidence IJdttem) .. '\Iso, (".IpS\A'ith convex upward slopifl~ eriges wer::generally foune to out-:'erform caps with ~at edges, in terms of the degree ofdifferential movement acros~ the discon~ir.uity requirerJ to cause through-ruplLlre (Figure 4). Where possible, tile depth of the !andfill shou:d be sjJecifiedso that sudden changes in the depth of the v.''lste fill (e.g. at the edg>' or a steep-sided pit), "Ihich could result in the in'position of a displacemen, disconti.llIity(stej-J) below the cap as the waste degrades, are avoided. The caiJ geometryshould be specified so that settlement of the underlying waste ..,ill tend to causecompression, rather than exteilsion, in any resistive or low permeability layerincorporated into the cap.

capping layer displacementfollowingwaste settlement

t ,-'---_L ~e S;,I:I~~,~nl -r

step displacement

capping :ayerdisplacementf==:~Figure.t Step and slope patterns of waste settlement (displacement and slope

d iscont inu ities)

The potential vulnerability of Ihe cap edge is confirmed by data presented byMorris & Woods (1990), who suggest that severe differential settlements mayoccur in this zone owing to the difficulty in achieving the same degree ofcompaction in the waste neC.~to the edge of the site as in the remainder of thelandfill.

ConclusionsTypical domestic or municipal solid W2.sion affects the waste densitj and pern~eability may dp.pend tosome extent on the degrep. of cOl:lpaction during placement. .

StructlIres such as gas and leachate wP.lIs and lanclfill l!I1ers must bedesig.led to rc.;ist or accommodate he sheer stresses imposed by settling waste.final fili levels and the geometry cf a ,estorative cap must take account of thelikely long-term settlement cf the waste as it degrades. As this is likely to belarge but difficult to I)redict, there may well be a need for a long-termmanagement pian that inccr;::orates periodic maintenance of, and placement offurther material belo\\. the cap.

I IReferencesBarlaz, M.A., Ham, R.K. & Schaefer, D.M. (1989). Mass balance of

anaerobically decomposed refuse in laboratory scale Iysimeters. J.Environmen/al Engineering, ASCE, I 15(6), pp 1088-1102.

Beaven, R.P. & Powrie, W. (1995). Determination of hydrogeological andgeotechnical propertics of refuse using a large compression cell.Proceedings uJ/he 5/11In/erna/innal conj'erence on landfills. Sardinia95 (ed. T H Christensen. R Cossu and R Stegmann) 2, pp 745-760.Cagliari: CISI\ Environlllcntal Sanitary Engineering Centre.

Bj

Landva, A.O. & Clark, 1.I. (1990). Geotechnics of Waste Fill. In Geoiechnicsoj' Waste FiUs - TheulJI and Practice. (eds. A Landva & G l)Knowles). ASTM STP; 1070.

Morris, O.V. & Woods, CEo (1990). Settlement and engineering considerationsin landfill and final cover design. Geotechnic.l' oj' waste fills - theory &practice. ASTM STP 1070 (ed. A Landva and G 0 Knowles).Philadelphia, USA: American Society for Testing and Materials

l'owrie, W. & Beaven, R.P. (in preparation). The hydraulic properties of waslesand their implications for sustainable landfill practice.

Reynolds, P. 8:-Blakey. N.C (1995). Lond/ill 2(}(}(}. Fin,11report for the D\)I ~Report No. CWM 03 1/91. 192pp.

Richards, OJ. & Powric, W. (in preparation). Geometric factors af!\;cting lhedurability of landfill C

Issues of barrier stabilityNumerical mudellin~ of typic;i1lv used harrier contigurations which rely on the

urprisingly.that the waste properties conrrol pcrlilrlnance (Reddy eI (/1.. Il)()6, Jones & Dixon,1l.)l)8b). J'his leads to invesligate the interar:tion between aspecific design of a steep side slope barrier system and waste (th~ barrier designinvestigated was a compacted clay liner supported uy a gabon wall installed ona slope ot 80), and compared the observed trial behaviour with the performanceof the actual barrier system obtained by in situ i110nitoring. Findings from thisdetailed study included:

a) the barrier experienced significant vertical and horizontal strains,with the magnitude dependent on the stiffness of the waste body;

b) the method of construction, including the phasing of barrierconstruction dnd supporting waste lifts,. had an influence on the magnilllde anddistribution of barrier deformations;

c) differential stl .ins were found in the barrier components; andd) a number of IJ\ltenti,,1 failure mechanisms were predicted resulting

from the magnitude of deformations required for equilibrium between thebarrier ane! waste body to be reached (see Figure I).

method for obtain ing these parameters bCisedon the pressuremeter, and presentsthe results of initial field trials. Results from in situ tests undertaken in both recent(I to 3 year old) and partially degraded (! I year old) household waste at depths of1.7 to 12 metres below ground level arc discussed, and proposed further workdescribed.

bearing capacity shear failurefailure

Hertweck (1997) concluded that ultimate limit state :otndserviceability limit statemust be examined 1'0. eacl, barrier design separately and should be checked byap;:>ropriate in situ measurements. The landfill barrier system investigated isone of only a very small number which have been instrumented to check thestress/strain behaviour during construction and operation.

Waste propertiesThe majority of research on the mechanical properties of household waste has todate concentrated on shear strength and coml='re~sibility (e.g. Beaven & Powrie,199:5; Jcssberger, 1994; Kolsch, 19C;S;Landva et of., 198,1;Van Imfle & Bouaz.zti,i996; aIld Watts & Charles 1990), As with illl part:culate materials obtainingundisturbed samples for use in hboratory tests is prJblem:otti". The heterc.seneousnature uf waste also dictales th'lt large saIllple sizes should be used in order thatthey be representative. In most cases if is not possible for large undist,-,rb~dsamples to be obtained, and this has lead to the majority of labor"tory studiesusing processed (e.g. milled) ~nd fe-compacted s~.mples. While these can provideuseful information related to the general mechaIlisms of waste behaviour, thesemeasured mechanical properties are of limited use and can not be applied to fieldproblems with any confidence.

The deficiencies of using relatively small laboratory samples hJS lead tethe development of a limited number of large scale test facilities for assessingunprocessed wastes, although there are still problems associated with sampledisturbance due to the waste having to be re-compact~d in the test apparatus. Testsdeveloped include a large shear box (Kolsch, 1995) and a compression cell(Beaven & Powrie, 1995). Studies of certain properties have been undel1akenusing in situ waste. These include trial failures of artificially steepened slopes toobtain shear strength parameters, and compression experiments to obtain stiffnessparameters for use in settlement calculations. Ilowcvcr, to date therc is noevidence in the literature of any investigations to mcasurc horizontal stresscs in, orlateral stiffncss of, in situ household waste. Thc small sample sizcs which can bcscnsiuly obtaincd, and the inevitablc 'Iisturbance which will be caused, has lead tothe development of an in situ testing techniqil" bascd on the pressuremeter tomeasure these important parameters.

Pressuremeter testingJustificationAn approach based on in situ testing has been devised, due to the difficulties ofobtaining representative laboratory test samples described above. In situ testinghas been carried out using commercially available Cambridge typepressuremeters. A pressuremeter is a cylindrical device designed to apply auniform pressure to the walls of a borehole by me:lIls of a flexible membrane. Thismembrane is expanded against the wrrounding mat~rial by means of gas underpressure supplied from the ground surface. Outward radial deformation of thewaste occurs as the membrane expailds. The object of the test is to obtain therelationsh!p be ':ween the applied pressure and the deformation of the soil, andfrom this information the in situ stress condit;ons and de~oril1ation properties cfthe m1teriai surrounding the pressurcmeter can be obtained. Pressuremeters wereused for this study for thefollowirg reasons: The device measures the average stress acti:~g on the mcn,brane and hence any

large variations [:1stress d;.;e to the heten'gf''lee~:s :lature of the waste will beaveraged. While th:s may be urdesirabte when using the s~'stC!-:lfor

SB? test methodThe SBP is about 83 millimetres in diameter and 1.2 metres long, and is aminiature tunnellin' machine, the central part of which is covered with an elasticmembrane. This nv'mbrane is in two parts. The inner layer, which is sealed, ismade of polyurethane and is about 1.25 mm thick. This inner skin is covered byan outer layer, known as a 'Chinese Lantern' (CHL), which is formed fromstainless steel strips bonded to a thin rubber skin. The CHL is used to take thefrictional forces that occur when the instrument is being bored into the ground,and to provide some protection from inclusions that might otherwise puncture theinncr tllClllblclllC. fhc lattcr function is particularly important when using theinstrumcnt in wastc. The foot of the instrument is fitted with a sharp edge. Whenboring, the instrument is jacked into the ground, and material is removed either bysiicing it into small pieces using a rotary cutting device (soft materials) or bygrinding the material using a rock roller bit (hard materials). The instrument isconnected to tl',e jacking system by a drill string. This is in two parts, an outercasing to transmit the jacking force and an inner rod which is rotaterl to drive theboring device. The cutting material is flushed back to the surface through theannulus bet" ee:~ the rods and outer casing. Water is the most common flush fluidused. Disturbance is :llillimised by optimising the boring method, flush fluidpressure and jacking force. A schematic of a SBP is shown in Figure 2.

expressions for the expansion e,f a cyli:ldrical cavity. The solution isconventionally. quoted in terms of strength and stiffiless parameters for thematerial tested; specitically shear modulus, shear strength and in situ lateral stress.The above description of the SBP is based on that of Cambridge Insitu (1998).

Field trialsSite detailsSclection of all appropriatc tcst sitc was an important consideration. as Ihe aim wasl'l devclop a tcst procedure at the samc time as obtaining pr'climinary d:lla. /\ sitcwith a well known construction history and 1't~lali\'cl\ uniform W,ISle typc wasI'cquircd in order to minimi,,: the nutllber of factors affecting thc results. Thel'ollowing criteria were used f0r site selection: Only sites which were formed {i'om household \\aste were considered. Sites

where co-disposal with commercial or industrial waste had OCCUrlcd wereavoided due to possible difficulties drilling through such mixed material (e.g.building rubble).

The construction history needs to be well documented including informationon the history of phased filling, placement and compaction methods used,thickness of waste layers, material used for daily cover and distance betweencover layers.

/\ minimum thickness in the order of20 metres of wastes was required in orderto assess the performance of the pressuremeter over a range of stress levels.

. The ,ite should have areas of waste of different age~ to enable :he effects ofdegradation to be investigated.

Tile site selected was Calvert, which is locakd equidistant fromBuckingham and Aylesbury, and operated by Shanks & McEwan (Southern WasteS"rvlces) Limited. LanJfill 0peratio'ls, which sta;-~ed in 1980, are bJck-filling anold brick pi~ fonned in the Oxford Clay. W?ste from collection rounds isdelivered to site by train from the Bristol and London areas, aild this results in itbeiiis almost entirely househ01d household material. The maximum depth ofwaste is in excess cf 20 metres, with each cell taking approxima,ely one tv twoyears te fi;l. Waste is comracted using a Jead-'veight rlJlIer in c:pproximately cnemE'tre thic;';' layers, with typically two such layer, p!:Ic~d before USl: or' a, 0.3 ~o0,5metre deep clayey ~oil cover layer. Tne final capping system is a 1 to 2 metre t:lickcompacted clay layer.

Testing scheduleIn September 1997 the self boring technique was used to insert the pressuremeterfrom the base of boreholes formed using the barrel auger drilling technique(method ii). Tests were conducted in both recent (1 to :3 year old) waste at depthsof 3.5 to 10.7 metres below ground level, and one test in partially degraded (IIyear old) houc,ehold waste at a depth of 11.7 metres below ground level. A second

Once inserted to the test depth the pressuremeter membrane is expandedand readings of d'Jp!(!cement against applied pressure are logged, and plotted as aloading curve. TI-Jis loading curve can be solved directly using mathematical

trial was cO~lducted in May ; 998 using a SBP which was operated from thcground surface (method i), with tests carried out in 2 year old waste at depths of1.7 to 3.5 metres.

The composition of the fresh waste (1994 to 1996) as retrieved duringborehole formation was approximately 40% plastics; 40% paper and organicmaterial; 10% textiles; and 10% timber, metals and brick. The material obtainedfrom the test depth in thc partially degraded material (1987) consisted ofapproximately 20% plastics: 10% paper; 60% degraded material: and theremaining IO'~" \\'as textiles. brick and metals.

in situ horizontal stressIn ordci' to carry out analyses of the data it is necessary to determine the originfor l' pansion of the cavity (i.e. to take into account any disturbance). For theSBP I( is assumed that some stress reiiefwill occur and the origin is taken as thepoint where the in situ conditions are restored to the cavity. It is possible torecngnise the in situ lateral stress by inspection (the lift-ofT method). as bein!.!thl' prCSSllrc required to cause movement of the membrane. Where disturbanc~is more p['()Jlounced it is taken as the pressurc to cause significant movcment. ;\sindicatcd by an abrupt change in the slope of the PI'cssure/radial strain curvc/\pplicdtinn or this technique to thc waste test data has becn pwhknl;ltic as dl'cslIlt nr the significant degree or disturbance caused by inserting thc Slli'I'hcl'clllrL', a second method developed by Marsland & Randolph (1977) hdsalso bcr:n used. This method employs an iterative approach b;lsL'c! nilidentif\ing the onset of plastic behaviour. Although this mcthod can bcconsidered more robust than the lift-off approach i: is still influenced b\'dlsturuance, An introduction to both these methoc1s is provided by Mail' 8:.Wood (1987).

The estimated values of in situ horizontal stress (JIJ) Ciln be used tocalcu latE' ccefficients of earth pressure at rest (Ko) where:

Analysis methods and preliminary resultsResults from a pressuremeter test are presented as plots of pressure againstradiill displacement (i.e. pressure vs radial strain) for each 0:' three separatesensors used to measure expansion of the cavity. However, the values ofdisplacement obtained do not necessarily give the correct deformation of theexpanding borehole Willi ilt the sensor location as the ilxis of the instrument canmove. Therefore, it is standard practice to use plots of pressure vs averagestrain. This is also required due to tl'e analyses method assuming isotropicmaterial surrounding the cavity. Figure 3 shows a typical pressure/displacementplot, which is for a test in one year old waste at a depth of 5.5 metres belowground level. It should be remembered that to date pressuremeters, and hencethe associated analyses methods, have been developed for use in soils and rocks,c;;1d therefc;'e ~~e apiJlication of tl-je standard analyses techniques to the r~sultsof tests in waste material is ope;l to debate.

Using an assumed bulk unit weight of 10 kN/m' lor the waste and a bulk Ullitweight c:' 20 kN/mJ for the compacted clay cap, values of Ko have beencalculated and plotted against depth in Figure 4. It can be seen that there is n0clear relationship between lateral stress and vertical stress. It is considered(Cambridge lnsitu, 1998) that the heterogenec~ls nature of the waste testedresulted in the SSP b2ing ove:' drilled. This was probably caused by items 01'waste such as pieces of wr-,od, metal cr briCk being pushed ilhead of the SBP(i.e. into tIle underlying compressible waste) hence resulting in a -:avity with alarger diameter than the instrument. This has resulted in the calculate0 values oiKo showing consider::b!e scatter, ~nd has lead to conilJence ill tile values heille(low. However ')f some interest is the olle test in the partially degr"ded wasl~whicl1 indicates a higher vaiue lhan the fresh waste. AI, f'xplaniltion could bethat ilS the was,c degrades and settles it Jecomes den,er With an .associated:ncrease in horizontal :tress, al,hough

I:;.

L ~_~_~~_~ __

II:;. II:;. I I I

-~I~------t I I. I

: II

I1:;.. 1 to 3 year old

11 year old

Whether this value is due to a general trend with depth, an isolated area ofcompressible materia! VI' a function of the tesi method (I~.g.the introduction ofwater from the boring which has caused 10cal softening), is IInclear. The test inpartially degraded waste indicates a relatively high shear nh,du!us at a depth ofII metres.

Coefficiei1t of Earth Pressure at Rest (Ko)

0.0 0.2 0.4 0.6 08 1.0 1.2o

Iill 4>(l)-l

Ucis 6(5.(l)

I:;.(l)-l

U I:;.C:J 60(5 I:;. -+1~0ill

to Interpret the plot as showing this general trend (i.t:. higher Ko values in theupper layer of waste). .

In laboratory unconfined compression tests on milled waste it has beenobserved that the stiffness increased during application of stress as a result ofthe waste compressing (Jessberger & Kockel, 1993). This is consistent with thestiffness increasing with depth (Figure 5), but in addition means that ar-elationship would be expected between the mean pressure during aunload/reload loop and the measured shear modulus (i.e. it' the waste behavessimil,lr to a compressible fully drained soil) The inlluence of incre;lsingstiffness with depth is represented by the test pl'essure because the deeper' wastewili in general require larger pressures to delc)["Ill the material. Figure 6 is a pl"lt"or the fresh waste of shear modulus against average pressure during theunloadheload loop used to calculate the G value. Despite there being SOlll"scatter of the data a clear relationship of increasing shear modulus withpressure, and hence lateral stress, is indicated. However, this trend may not besu pronounced for more degraded waste with higher density and moisturecontent, as behaviour may be closer to that of a low comfJressibility partiallyd(ained soil 9uring application of stress.

Shear Modulus (MPa)co 0 3 5 9 12 15Q..

0~(f) :::,.Q.

Ll:.00-l

U 30~C1l0u;

Aer:: ~-- :::,.u 60C1l0C :::,. :::,.:J :::,.OJ r

:::,.c

I --t- L'C --- l::l 90r--- -Iw ~ I I f:::,. t~ :::,.6-::l(f)(f)OJ 120 -cl: :::,.c:OJC1l(jj> 150:

Comparison with values from the literatureThe results obtained from the small number of SBP tests indicate relationshipsbetwep.11 stiffness and depth, including effects of construction processes, andstiffness with stress level. However, it must be appreciated that ihe results relateto mainly fresh household waste at one site. It is important to compate theresults from this investigation with measurements of waste stiftness described inIhe literature. The problem in undertaking such a compar'ison is thai a numberof c1ilTerent forms of stiffiless arc reported. The main rlJrJmetel's hcmgY (lung's modulus (E) from laboratol"\ triax ial comlxe" ion tests. andcOllstrained modulus (I) (ie. one dimensional compression) "!'".rincd I'rom hnthsl11

In order to compare t:le n;sllits from the various sources the stillness valuesInve been converted into shear modulus values. Based on an assumedPoisson's ratio for househo"1 waste of 0.1 (Jessberger & Kockel, 1993) it can beshown that constrained Illouulus (D) is approximately equal to 2.2.G. Van lmpe& Bouazza (1996) sUllllllarised stiffness values available in the literature andproduced lower and upper bounds for the relationship between stiffnessmodulus (E) and vertical stress. It should be noted that the valul'~ from theliterature cover a range ofwaqc composition, age and incilldes reSII', uhlainedIIsing in situ and laboratory leSl ICClllllqll';~. I h

Clarke. B.G. (1995). !'resswcllleters i/1Geotechnical Design. Pub. BlackieAcademic & Professional.

Gilbert, R.B. & Byrne, R.J. (1996). Strain-softening behaviour of wastecontainment system interfaces. Geo.lynthe/ics In/emational, 3, 2, 181-203

Heltweck, M. (1997). Structural analysis of steep slope sealing systemssupported by waste. !'roc. 6/h In/. LandjiIIS)III1p., Sardinia, October, 3.507-516

Jessberger, ILL. (1994). Geotechnical aspects of landfill design and construction.Part 2: material parameter,; and test methods. I'mr.:. Imtll ('il'. ElIgrsGeotechnical Engineering. 107. 105-113.

Jessberger, H.L. & Kockel, R. (1993). Determination and assessment of themechanical propel1ies oh\'aste material. Proc. Geotechnics l

The paper by HiI'd, Smith &- Cripps continues the theme of thedifficulties associated with natural mineral liners to the use of colliery spoil, andexemplifies the above points. The properties of spoil from different mini~gprocesses are described and factors influencing permeability are addressed, torelation to both field permeability testing using ring infiltrometers andlaboratory testing using flexible wall permeameters. In particular, the. size oftest sample and the need to repliC1te in laboratory tests the likely con.dltlons 10.situ are highlighted, and issues relating to the specification and validatIOn otsuch materials are outlined.

.lelTeris covers the engineering of bentonite enriched soils. Bentoniteexhibits a vcry low permeability and for this reason is an 'attractive' materialfor enhancing the properties of other'vise llOsuitable materials. However, theswellinlJ characteristics and the influence on such materials of the prevailing

b .

environment has cast doubts on the long-term performan-:e of bentoll1teenriched soils. In this paper a simple two phase model is used to represent thebentonite enriched soil, to highlight important parameters for control of thematerial on site, and to identify situations which may lead to significant increasein its permeability on exposure to aggressive chemical environments.

A major conclusion from the papers presented is that, althoughadvances are being made in our understanding of the efficacy of mineral linersto prevent contaminant escape, there are still many uncertainties which are thesubject of current research. SUC') research must not only aim to provide a betterunderstandino of the behaviour of mineral liners and to improve on the

b

protectio'l afforded !:Jut shodd also, where possible, aim tv reduce the ccst tothose c'eveloping Ie b?lanced 'lgains:the potentially prohibitive cost of attempting perfect cOI'tainment. For thisreaSOli DOE (1996) advo;;ates the cO:lcept of 'environinentali) safe' :andfillsbased on riSK assessment of individual sites and the determiliation of safeenginperir.g If':akage crit:::ria.

fhe potential of a clay to form a landfill illling is defined nerein interllls of 'material suitability'. However, suitable materials may not be capableaf achieving the desired permeability without conuitioi,ing and a clay is Gnlydefined as 'acceptable' following any pre-treatment and CQA testing necessaryto establish and prove its compliance with the design requirements (Murray etal. 1996). To this end the testing undertaken may be divided into the following:

Matcrial Suit3bility Testing-Material Acceptability Testing- Physical Design Testing

Chemical Design TestingCQA Testing

f= -c.k. dbe1z

These distinctions have been found useful and are introduced tocomply with the generalised staged testing and reporting procedures in theinvestigation. design, specification and construction of landfill linings and alsoto facilitilte an appreciation of the role and significance of the dilTerent testsundeltakcn. Table A I appended details thc testing both in common ust: andother testing sometimes deemed necessary.

The CQA Plan forms a vital component in the construction of a liningand detai Is the checking procedures, testing and means of ensuring that theemplaced lining achieves the desired standard. At an early stage 01development it is also necessary to evaluate the risk and potential impact ofpollution migration on the surrounding environment (NRA, 1992) andRegulation 15 of the Waste Management Licensing Regulations (DOE, 1994)require: th:tt a risk assessment is undertaken detailing the potential influenceson groundwaters "rum the discharge of List I and List" substances. NWWRO(1996) addresses this regulation and suggest that it is likely that the WasteRegulation Authc;'ity will require the risk assessment report to deal with thewider issues of cor.taminant escape in assessing the influence of the proposeddesign on the environment. This will necessitate an appraisal of the pot.entialr~ceptors, their se'lsitivity, pcllution patilways and the likelihood of pollutantsimpactlllg the receptors.

In order to appreciatc the significance of the testing undertaken on cluylining material in developing a landfill site, it is first r.~cessary to outline themcci,anisms of contaminant rr,igration 3'1d the roles played by a soil's physic

(vi) Dilution (di .ect re'ilill p, ':iipersion ofsoilltes within pore water andintluences all contaminants)

Murray el al. (i 992), incorporates the NRA criteria and may be used todistinguish between sllitable, unsuitable and marginal materials; the latterclassification including the Gault Clay and, in their more weathered states, theLondon Clay and Fuller's Earth where they exceed the NRA limits.

Table I also presents limits for the minimum plasticity index of asuitable clay. This is a direct consequence of a marked increase in permeabilityat low PI. As shown in Figure I, this might influence the selection or clays suchas those derived frolll the Mercia Mudstone and Coal Measures.

The efficacy of a clay to attenuate and retard the migration ofcontaminants is influenced by the pollution pathways and residence times andthus by the soil micro- and macro-structure. The ability ofa clay liner to retardthe physical passage of the leachate not only reduces the advective movement ofcontaminants but increases the residenCt' time of the permeant within the liner.This can enhance ,\". , I" , tillle dependent. Although themajority of attenuation i, likely [0 be iUIl 7-10%NRA (1992) LL < 90%

PI < 65%Murray 1'1 (I/. (1992) PI> 12%Gordon (1987) PI> 15%Williams (1987) 1'1> 15%

Percentage Danicl (1993) clay and silt ..>20-30%fines NRA (1992) clay partic'es :..-10%

Gordon (1987) clay and silt> 50%Activity DOE (1995a) > 0.3(P[fclay content)Percentage Daniel (1993) gravel (>4.76ml11)< 30%gravelMaximum j'>JWWRO(1991) size must not affecl liner integrityparticle size Daniel (1993) < 25-30mm

(Notc: there 2re SO;'.C difference.; between the ASTM :,nd DS test melhJds used '0determin~ the suitability critc:-ia in Table 1 bpt these do not precludc comparison of theproposed criteria)

The variability of a deposit alsC' iflfluer.:es its suitabiLty. Fc:- example,gl~cial till whilsttJredomin:mtly clay may exhibit ~ignificaw 'variations ii.plasticity over short distances and contain pockets of sand, silt or otherunsuitable materials whicn may not be easily segregatec during excavati0n.Care ['Hlst be taken when collating laboratory test results on a oeposit to ensurethat preferential sampling and testing is taken into account and the influence ofmaterial variability is fully assessed. The influence of gravel content onpermeability has been examined by Shakoor afld Cook (1990) and Shelley andDaniel (1993) and both report a rapid increase in permeability where the gravelcontent exceeds a critical value which appears to be around 50% to 60%. Asshown in Table I, Daniel (1993) suggests that the percentage gravel should be

The requiremen' of ensuring a thoroughly compacted, uniform.hOillogeneous liiling of low pClIlleability will necessitate detailed testing. sill:monitoring and compaction gcncrall\ in excess of normal earthworks levels.Acceptability testing encompasses t Jse tests listed in Table A I appended andwhich are deemed necessary for desi""l purp;Jses and CQA control.

The following discusscs thc significance and some important aspects ofspecific soil tests. and the links betll'cen measured properties, which need to belInderstood by those designing

unnecessarily strict requirement. For the higher plasticity clays the 2.5kgmethod is likely to be adequate but for low plasticity clays a greater degree ofcompaction may be required in order to achieve the permeability criterionconsistently. If: this latter case, compaction in accordance with the 4.5kgrammer method may be necessary. Densities obtained using the MCV testprovide an intermediate level of compaction which should be achievable formost clays.

~0NI-ZW

Z0()

W0::::>l-(/)

5 kg methodwhere a 'clean' clay is being tested. IlowevC!'. the test for PI, precludesmaterial retained on the 425~m sieve and for a clay with mudstone I'I'aglllenls orgravel, or for greater compactive effort, the plastic limit can he signilic- 1.95f-;/) 1.9z,~Q 1.85>-0::: 1.8Q

~N

'\.

I'\. ~5%AIR VOIDS', "-:"

lower limit for ~he moisture content should be dictated by the pcrmeabilityrequircment. However, the upper limit to the moisture content may be dictatedby the shear sl 'ngth of the clay because although the permeability requirementmay be met, hilndling. compaction and trafficking become more difficult. This,in conjunction with stability considerations, dictates the requirement for aminimum shear strength. Typically an undrained shear strength (cu) of no lessthan 40 to 50 kN/m2 is required in earthworks.

Disturbance due to mechan ical plantPunching ofwastemateriab into the liningErosion/suffusion due to movement of free water

Tests to assess permeability and advection propertiesOf particular interest in permeability testing are the tindings such as those ofBrulH.:lk: c'I (//. (1987). Their results indicate no notable differcnce betweenpcrmeabilities using water or leachate (from an active landfill), but do indicatesignificant differences between the permeabilities from the three differentpermeameters used. It may be concluded that, in general, fixed wallpermeamelers (such as the falling head method of Head, 1986) give higherpenneabilities than flexible wall permeameters (such as tbe constant headtriaxial test of B.S.1377: 1990) because of side wall leakage and the influence ofconfining j.Jressure. Amongst other factors, the size of the sample tested alsoinfluences the measured permeability, and Rarden et al. (1969) report on theinfluence of unsaturated conditions and the significant increase in permeab;lityas saturation is approached.

In practice ~he leachate le"els and thus hydraulic gradients in landfillsare kept low by pumping from wells. In laboratory permeability tests thehJ'draulic gradients are usually significaptly higher ~han in practice for reasonsof practical flow measurement. There is an argument as to whether Darcy's lawis applicable at low hydraulic gradients and Mitch~1l and Younger (1967)present results which suggest a deviation from Darcy's law at a thresholdhyuraulic gradient of about 6, whereas in landfIll cells the hydraulic gradient istypically around I. This deviation may in a la,ge part be a result of the highviscosity of the ad:,orbed water attached to clay particles. Other factors beingequal, some comfort may be gained from the inferred greater laborato"ypermeabilities under elevated ,",ydraul;c gradients compared to thosepermeabiliries which rr.3Ypertain in the field unde~ Ic'ver hydraulic :leads.

These potentially disruptive influences have to be addressed at thedesign stage and where deemed necessary investigation and testing should becarried out to facilitate a ouantitative appraisal and to evallJ(~tc the inherent risksassociated with the various factors .. This might inclllo

presents a favourable environment. Conversely, at low pH, soil particlesurl~lces can revert to a 'net positive charge and may not in this evel~t attenuateposi:ively charged heavy metal species.. The pH of till' system not onlyinfluences cation exchange but also the precipitation from solution of manymetals as hydroxides and carbonates onto soil partiele surfaces and into porewater (Bright el aI., 1996).

NWWRO (1996) suggests tests should be carried out to provide anundcrstanding of how leachate will interact with the lining and proposedeterminations of Cation Exchange Capacity (CEC), Aninn Exchange Capacity(ACC) and Partition Coefficient (Kd) as appropriate. The CEC and AEC aremeasures of the abundance of exchangeable ions required to be adsorbed ontothe clay platelets to render them neutral, while Kd may be viewed as a globalmeasure of [he sorption and precipitation attenuation potential of a clay. Kdmay be incorporated within a linear model to predict pollutant attenuation,where,

For organic contaminants, the organic carbon (or octanol-carbon).partition coefficient (Koc) is used as a measure of a soils adsorption potentia~.There is a close relationship bel\veen the adsorption of organic pollutants 'to sot!organic matter and the amount measured by partitioning and almost all of theadsorption of organic chemicals by soil is governed by the organic carboncontent of the soil (eg. Hines and Failey, 1997). Koc is the ratio of the amountof chemical adsorbed per unit weight of organic carbon to the cl1emicalconcentration in solution at equilibrium and is given by.

It is far easier to calculate the partition coefficient for organics than thepartition coeftlcient for metals as the former is largely independelll of soilproperties. Kd and Koc for a range of organic contaminants are presented byHattemer-Frey and Lau (1996) and Rowe el at. (1997). Clays derived :rom theCoal Measures, Oxford Clay ami similar strata are likely to have relatively highorganic carbon concentrations which would be conducive to the attenuation oforganic contaminants in leachate. In a complete formulation of the risksassociated with contaminant migration the influence of biological degracationof organic pollutants should also be taken into account (German GeotechnicalSociety, 1993). This process results in analytical difficulties as it variesthrouahout the life of the landfill and is depndent on the presence and survival

b

of suitable rr.icro-organisms.Though ciay barriers are capable of sorption of chemical species frem

le:.:chate over considerable periods of time (Davies el a/., 1996), the ir.teractionof leachate and clays pre~ents a complex p~oLlem and even wit!~ det3.iled sitespecific testing the situation will be far :rom determinc'e. Yet the protection ofthe groundwater is ef prime environmental concern and aquifer vulnerability isa key facto; in any assessment cf landfill dev~lopment and design (eg Foster,1998).

where, S = mass of solute removed from solution per unit mass of liquidc = equilibrium concentration of solute in pore fluid

Other more complex partitioning relationships have been proposed butseem unwarranted for general design purposes. Kd m

le;jchates with high chemical concentrations, well in excess of those normallyencounten::d in landfill. Although there appears to be no unaniinity of view 'atthis time on the influence of organic contaminants on the permeability of clays,there seems to be a consensus that the influence is small at concentrations in theleachate from normal domestic waste (DOE, 1996; Brunelle el af.. 1987; Danieland Liljestrad, 1984). Nevertheless, the influence or the leachate chemistrypresents uncertainties which cannot be fully addressed without further research.

Of some com fort is the suggestion by farquhar (1994) that linerpermeability often decreases with time as a result or scaling due to precipitateformation, sui ids accumulation and biomas growth along the upper surfacc of aliner and within cracks and fissures. Dakin el 01. ( 1997) appear to support thiscontention but also highlight the uncertain innuences on permeability of aerobicand anaerobic ~nvironments.

which should be designed to simulate as close (1S possible proposedconstruction.

(i) Conditioning or preparation of the clay (eh, addition of water,screening)

(ii) Compliance with the design parameters(iii) Destruction of clods and elimination or associated fissuring(iv) Interlift bonding(v) Liti thickncss(vi) Type and weight or roller, number ur passes and coverage(vii) Degree of compaction and saturation(viii) !'ossiolc construction, sampling, testillg ;llld validation difficulties(i:,) Adequacy of the CQI\ Plan

Correlatioris between physical and chemical testsIn soil mechal,ics terms, the greater the plasticity of a clay (the greater the LLand PI) the greater the quantity of clay particles and the higher their surfaceactivity (defined as PI/clay content after Skempton, 1953). These mineralogicalproperties are closely related to the physio-chemical prope,ties of cationexchange capacitj (CEC) and specific surface area (SSA) (Yong el aI., 1997)where SSA is a measure of the surface area of soil particles per unit mass ofsolids (or sometimes defined as ~he snrfar;e area per unit volume of solids).Generalised relationships can thus be exr~cted between rlasticity, ~he chemistryof the clay particles and permeability. The more highly plastic the clay thegreater the CEC 31,::1 SSA and the less t"~ pel meability and tile advectionpotential. Such relationships have been shown experimentally by a number ofresearchc~s including Lai.lbe (195d), Mesri and Olsr,n (197 I) ar.d Benson el af.(1994). Thus, clays comprising mainly kaolinite (as determineC: by X-raydefraction) are of relatively low plasticity, CEC and SSA, and generally have agreater perme~~ility than clavs comprisir.g illite, which in turn have a .::,reaterperm~ability than clays c0l1lprising smtctite ('..:h'ch includ~s mvntmorilloniteand the gen~1 ic minerai species bentun;te) whir:h exhlLit the hig'lest piastir;ityand a high CEC and SSA (Yong anrl Warkenin, 1

provide, immediate results as oppo~ed [0 core sampling and sand replacementdensity determinations. However, bec;;use of inaccuracies in NOM moisturecontent determinations (and thus in the conversion from bulk density to drydensity) normal practice is to recover soil samples to check on fieldmeasurements. NevertheiL:ss, it is essent ial to undertake very careful instrumentcalibration for individual soil types. The instrument should preferably becalibrated against accurately measured densities in the laboratory containermethod (B.S.1377:1990 Part l), Clause 2.5.5.3.1) but more frequently it iscalibrated against in-situ cores or salld replacement density determinationswhich ill themselves Illay he suhject to nrms

laboratory permeability tests on e~re samples recovered from a clay lining(compacted in accordance with I:igure 3b) confirming the acceptability of theconstruction.

Ide1i1ly: in-situ permeability testing as detailed in Table A I should alsobe carried out but such testing suffers from the disadvantages that it is timeconsuming, does not reflect the true confining stress conditions imposed by thelandfill and can result in prolonged expose of the lining to the elements.Consideration might be given to the use of carefully prepared trial areas,isolated from lining construction, where in-siw permeability tests could becarried out prior to or at an eady stage of development. Such testing wouldprovide valuable information on the larger scale performance of liningmaterial.

ConclusionsThe design and construction of landfills requires a detailed appraisal' of site andenvironmental conditions encompassing a broad appreciation of a number ofengineering and scientific disciplines. In assessing the risks to the aquaticenvironment it is necssary to have an understanding of the mechanisms bywhich pollution associated with leachate can escape through clay linings and anal:;preeiation of the Iimitations of Fning system~ and the construction difficultiesencountered in practice. The physical migration of contaminants due topermeation through a clay lining is generally accepted as greater than predictedfrom laboratory tests and advecticn is usually deemed the main mechanism ofC'1;ltan~:nant escape. i-!owuer, fOi low permeability bJrriers, or in the case ofnet inflow into a b:ldfill, diffusion cannot be ignored in assessing the influenceof landfill on the aquatic environment. The current state of knowledge allowsanalysis of the mechanism of cuntaminant movement cat the significance ofsuch analj'sis is gre"tl~' restricted hy the uncertainties inherent in testing andactual field performance. Analysis at tltis time is probably Dest limited tosensitivity studies, in particular, to determining the significance of variability intest parameters anri design features.

In selecting ane approving clays f0r land:ill sites, the aUthar f.nds thedefinitions 'material suitability' and 'acceptability of mater;als' liseful in thediStinction betw

allows validation of a clay lining to the sati~:factjon of the regulatory authorityand provides a check that the de,ign parameters are achieved

With the inherent de:;ire of thc EI1\'i-"'nl11ent Agency to improvestandards and provide pressure on the industry to uevelop a better understandingof the potential influence of landfill on the environment, more detailed andincreasingly intricate testing and analysis will be Iequired. This must, however,be tempered with the recognition that a landfill presents a complexthermodynamic system not readily amenabk to analysis, though the principlesof thermodynamics are appropriate as they apph Gqually to solids, liquids andgases. Such principlcs as the c0l1servaliol1 llf energy apply to the chemicalreactions within the landfill, the heat and ~a,e, generated, the stresses andvolume changes, permeation and the advecti"n ,1Ild diffusion of contaminants.However, detailed analysis of il landt! II as a the! l1lodynamic system, where theenergy levels within are balanced with IhG Gnergy exchanges with thesurrounding environment, are beyond current capabilities.

lngincering (jclliogy Special Publicatiun NO.1 i, Ed. Bentley. S. P.,159-16'L

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Brunelle, T.M., Dell. LR. and Meyer, c.J. (1987). Effect of permeameter andleachate on a clay liner. Geotechnical Practice/or Waste Disposal '87,(iSr 13, I:d Woods. R., i\SCE. 347-359.

Dakin. M.(~., Wright. S.I', Williams, l8., Langdon, N.J., Sangha. C.M. andWalden. P J. (Il)l)?). I'hysio-chcmical and microbial factors effectingthe passage "f kachate thruugh clay liner~. GcoeJlI'iro/lllleJltal/~I,...;i}:..T' ~. ,1.,'tdJliJlo!l'11 t.;rIJlti,',/. rdf. (~; r.l/,aftllil I ,flill

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Gordon, M.E. (1987). Design and performance monitoring of clay-linedlandfills. Geotechnical Pmctir:e for Waste Disposal '87. SC:. Woods,R.D., Geotechnical Special Publication No. 13, ASCE, 500-514.

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head, I(.:! C 1981). Manual of soil la!Jol"([liJ/Y testing, Vol. 2, permea':Jility,shear strcngtn and compressibility tests. Pentech Press.

Head, K.H. (1986). Manual of soil labomtoj} testing, Vo1.3, effective stresstests. Pentech Press.

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Je~sberge(, H.L. (19941. Geotechnical design ai-,d quality cOiltrul of m;neralliner sysrems. LC'ndjiliing 0/ rVaste: Barriers, Ed Christensen, T.H.,Cossu, R. and Stegmann, R.E hibi. F.N.Spon. 55-68.

Jones, G..M., Murray, E.J. and Rix, D.W. (1993). Selec~ion of cl"ys for USe aslandfii; liners. /1'aste disposal by Landjill, I'roc Symp. Gre"n '93,Bolton Institute of Higher Educ3Li0n. Ed. Sarsby, R.W., 433-438

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Mesri, G. and Olson, R.E. (1971). Mechanisms controlling the permeability ofclays. Clays and Clay Minerals, Pergamon Press, J 9, 151-158.

Mitchell, J.K., Hooper, D.R. and Campanella, R.G. (1965). Permeabilit .. ofcompacted clays. Jnl. Geotech. Engrg, ASCE, 91 (SM4),41-65.

Mitchell, J.K. and Younger, J.S. (1967) Abnoll~lalities in hydraulic tlowthrough fine grained soils. AST.\fSrp 417.106-139.

Mohamed, A.M.O and Yong, R.N. (1996). Diffusion of contaminants through aclay barrier under acidic conditions. Engineering Geology of WasteDisposal, Geological Society Engineering Geology Special PublicationNo.ll, Ed. Bentley, S.P., 279-289.

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~v~lIrray. [..I., Rix, D.W and Humphre\'. RD (1992) Clay linings to landfillsites Q.JEG, 25, 371-376.

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Parkinson, C.D. (1991). The permeability of landfill liners. The Planning andEngineering of Landfills, Conf. Proc. University of Birmingham. 147151.

Parsons, A.W. and Boden, J.B. (1979). The moisture condition test and ibpot-.:ntial arplication in earth ,yorks. Transport and Road ResearchLaborntory Supplem..;ntary Reporl 522

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Skempton, A. (1953). The colloidal activity of clay. Proc 3rd Int. Conf SoilMech and FOllnd. Engng., London, England: Butterworths ScientificPublications, Vo1.1, 57-61.

Shakoor, A. and Cook, B. (1990). The effect of stolle content, size and shapeon the engineering properties of a compacted silty clay. BII/I. Assoc.Engrg. Geologists, XXVII(2),245-253.

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Williams, C.E. (1987). Containment applications for earthen liners. InProceedings of the 1987 Speciality Conference. on Environmenta.'Engineering. Ed. Dietz, J.D., ASCE, 122-128.

Yong, R.N., Tan, B.K., Bentley, S.P., Thomas, H.R., WIIlI,'". ." '., Pooley,F.D. and Zuhairi, W. (1997). Attenuation characteristics or naturalclay materials in South Wales and their use as landtill liner~.Geoenvironmental Engineering. Contaminuted Ground: Fale oj'Pollutanls and Remediation, Proc. Con f., University of Wales, Cardiff,Ed. Yong, R.N. and Thomas, H.R., !61-169.

Yong, R.N. and Warkenin, B.P. (1975). Soil properties and behaviour. 2nded., Elsevier Scientific.

Triaxial Constant Ikad (:ISI,7i:1990 Pan61vlclhod 6)'Ilydraolic Cflns"lidation Cdl Constanl 1kau (BS 1377: 11) Metho:l4)TI iaxial Constant and r:tlling I kad (Head 1986 Tests 20.'1.1 to 20.4.4)Falling Head I'erm~ameter (Head 1981 Test 10.7.2)'railing Head Test in Sample Tube (Head 1981 Test 10.- 3)Falling Head Test in Oedometer Cell (Head 1981 Test I" 7.4)Falling Ilead Test in Rowe Consolidation Cell (lillfizontal and Venieal Pe~meability)

(Head 1986 Test 24.7.2 and 277.3)

Table A 1 Testing of clay liners

Tests to mitigate physical damageShe:ir Strength (nn rewmpaeted samples):

Il;lI1d Shear Vanl' (lIS 11,77: I 'N() I'art 7 \lctlmd .1)lJndrained Tria"ial Strength (1ISI377:II)'I() I':irt 7 ~lcIIH,d ,:)Shear nn\' SII1,dl (I\S 1;77: Il)

Tests to check on material acceptabilityMev (BS 1377: 1990 I'art ~ Method 5.5)Shear Strength (in-situ or on undisturbed Sal'lpIeS)

Hand Shear Vane (nS 1377: 1990 Part 7 MethQd 3)Undrained Triaxial Strength (I3S 1377: 1990 Part 7 Method 8)Shear Box: Small (BSI377:1990 Part 7 Method 4)

Largc (nS 1377: 1990 Part 7 Method 5)I'ermeability by water on uudisturbed samples (' indicates those tests in more common usage)

Triaxial Constaul I lead (BS 1377: 1990 Part 6 Method 6)'Il\'draulie COllsulidalion Cell Cnllstant Ilead (BS 1377: 1990 Part 6 Method 4)T;iaxial Constallt and Falling I lead (I lead 19X(, Tests 20A.I 1020.4A)'"allingllead l'ermeamcter(llead 1981 Test 10.7.2)'I'allins I lead Test in Sample Tube (I lead 19X I Test 10,7.3)1 ,ill ille: I lead Te,t in (kdollleler Cell (I lead 1981 Te~;( IO.7A)Fallill~~ I lead TeSI in Rowe Consolidation Cell (ilori ~ont::: and Vertic

Engineering characteristics of colliery spoilColliery spoil heaps may incluuc both coarse rock discard, arising from theconstruction of undcrground :ccess tunnels and galleries or opcncast workings,and finer slurries and taili.ngs separated from the coal in washery plants.Materials such as furnacc ashes and a grcat variety of other solid and liquidwastes may also have bcen disposcd "I' \\'ilhin d spoil ':l. All the lithologieswithin the Coal Measures sequencc are usually present within tips. Taylor andSpears (1970) report that in the East Midlands, for exam pic, thc sequcncecomprises the following lithologies: siltstones 40%, mudstone and shale 30%,seat earth 10-20%, sandstonc 5-10% and coal 2-7%. In tips from undergroundworkings there is li:lble to be a predom inance of shales and seatearths and oldertips usually contain a sign ificant proportion of coal. Taylor (1984) indicatesthat the average organic carbon content (mostly coal) for English and Welsh tipsis 13.3%, although in some old tips it may be as high as 47%. The variation incoal content, and also the variable presence of ironstone,' explains Jconsiderable variation of specific gravity (see Table I below).

The geotechnical properties of colliery spoils are very variabledepending upon the l:th0logies present, the extent of weathering and, moreparticularly, the grading. The gnGings of spoils span a large range as shown onFigure I. Taylor (1984) notes :hat coarse discards range from silty sand tocoarse gravel and cobbles, whc;eas fine discards range from clay to sandymedium gravel. The properties pre~ented in Table I also show large ranges.

Because weathering action is a significant cause of variation in thegeotechnical properties of colliery spoil, mainly through the resulting reductionsin particle size, it is heipful to understand the processes involved. This isespecially true in the present context, since permeability is strongly linked toparticle size.

Type or .Liquid Plastic Specific Moisture Optimum Maximum dryspnil limit:? limit1 gravity content moisture: dCl1sit~

(",,) ('Yo) ('Yo) conlclH I'J/)) (Mg/m',( 'oars...: .;('.X 21.4 231 120 13.2 !I/! I .7-l '!

(6.lJ) (4.0) (020) (60j (4.2) 13 _"', (0.16) It !(,)Fine ~R.X 23.5 1.94 34.R No data No da"

(7.6) (4.5) (0.24) (12.9)Noll: I standard devlallons arc given in brackets.Note 2: or Illateria' passing a 0.425mm sieve.Note 3: It)r standard Proctor (2.5kg rammer) comp~ction in lirst column and modilied Proctor(4.5kg rammer) compaction in second column, in italics.

Table 1 Mean values ur sume geotechnical properties of UK spoils(from Taylor, 1984)

0.10.063

Although weathering effects in a tip might be expected to be mostpronounced near its surface, tips may contain material that has been exposed toweathering for protracted periods of timp before being subsequently tipped overor capped. Also, the upper parts of the sequence stripped in the course ofopencasting o;;erations are liable to ha','e suffered some effects of weathering,de;;radation in situ. The more vulnerable lithologies, such as the mudrocks, areliable to show evidence of sllrfac~ derived weal;lering action down to depths of7m ar so (Taylor & Spears, 1972).

A reg;oual variation of tho::geutechnical -:haraccer of co!liery spoiioccurs in r~spor,se to diff~rences in the materials originally ceposited and alsobecause of differences in the depth of subsequent burial. Geothermal heatingand ::1Creased p;'essure lead to increasts in the der.:ity of the aepositedm2te"ials, [he precipitation of !~'ineral ce'1lents in pore s::,aces and thecc:wersion of swell;i,g clay min('rais, il1cluding :n mudrocks anymontmorillonite and mixed l~.yer iliite-smet::tite, to more staole illite '1nr mica~.Hence ,he rOCKSbecome strong.,r and more resistant to degradation. In parallelwith these changes the vegetable mJterial bec'lmes trar,sfor::1ed, a; volatilecomponents are driven off, into so?1 of successively higher rank.

Tay;or (1988) notes that, alihough some COals in Scotland have lowrank, the mudrocks contain fTlore kaolinite and less unstable mixed layer claythan is found in the Yorkshire and Midlands coalfields. \;Iudrocks originatingfrom North Derbyshire, Nottingham and the Western Region, which includesthe South Staffordshire coalfield, dre of lower strength than those fromYorkshire, Scotland, South Wales and North-East England.

100

90

80

Oli'0

c'iIl

60(/)roQ.

Q)

SO -1-g.cQ) 4001;a..

30

20

10 -

0

0.01

Coal Measures rocks may be subject ta degradation due to strcss n:liefand other physical weathering processes but, .as most of the mineral componentswere de~ived by weathering processp.s, they are quite stable chemicallv inpresent day weathering environments. However, certain minerals, of whichpyrite (FeS2) is the most important, are unstable in such environments. Pyriteoccurs most comPlonly in coals and dark coloured shales and constitutcs about2% of fine discard (Taylor, 1984); traces, at least, are usually prescnt in coarsespoil. Slow chemical oxidation can be considerably accelerated by the activitiesof bacteria, leading to the rapid removal of pyrite and the production of acid.The laller may att,:ck carbonates, clay minerals and other components, and giverise to sulphate rich solutions. In engineering operations the possible generationof aggressive sulphate bearing solutions due to weathering of the material needslU be borne in mind. On the other hand, ."w permeahility is achieved, theprocesses will be limited by the ra:es at which reactants can be transported toand from reaction sites. Furthermore, it is reported by Taylor (1984) that 71%of the unburnt spoils he studied possessed water and acid solublc suiphatecontents of less than 2.0g/l and 1% respectively.

It may be possible to encourage degradation of an unweathered spoilprior to placement in a liner in order to improve its performance. The rapidphysical oreakdown of Coal Measures rocks is favoured by the presence ofswelling clay minerals, including mixed layer illite-smectite, and small-scalecompositional laminations. The latter may include slight changes in thecomposition or texture of the rock. However, CzereV/ko (1997) has snown thatthese factors aJe not the sole controls on the heakdm"n of such rock. In SOi

Comparison of testing methodsSome research into the effect of testing 11Iethodon the measured permeability ofcolliery spoil has been carried out at the Univcrsity of Sheffield. The spoiltested was from a deep anthracite mine in South Wales and had been stored forover 10 years iil a tip. However, the spoil was relatively resistant to weatheringand the fines (silt plus clay) content had remained very low at 4-11 %. The meangrading is indicated by the curve labelled "Spoil A- ungradcd" in Figurc I.

Two test pads were constructed, one f!"Omungradcd material (withsome particles in cxcess of 50mm) and one from material which had beencrushed and graded to less than Gmm, so Ihat its lines content was incrcased toI 1-20%. The mean grading curve of Ihis material is labelled "Spoil A -processed" in Figure I. Compac;ion data for each test pad are suml11arised inTable 2. Test Pad 1 was fcrmed by compacting the ungraded material at itsnatural water content in six 150mm lins using a smooth vibrating roller. Thisproduced fairly good results. Test Pad 2 was formed by spreading the crushedand graded material, mixing it in situ with II'ater to bring the moisture can lent toslightly wet of optimum, and compacting it in a single 150-200111mlift on top ofa base of coarser compacted spoil. A smooth vibrating roller was 3sain usedbut in this case the compaction was relatively poor and also more variable.

In addition to thc these test~. small scale permeability lests were carriedout b~' commercial laboratories on 100mm diameter specimens in flexible-waileD pcrmeameters. Disturbed samples werc iaken from Tcst p. I I, graded to20mm. recompacted and tested using a hydraulic gradient of about 90 and aneffective confining pressure of about 60kPa. It is likely that the fines content ofthese test specimens was increased as a result of the recompaction. Tubesamples were taken from Test Pad 2 and tested using a hydraulic gradient ofabout I I() and an e1fective conlining prcssure of about 190kPa Back pressuresII'ere cmployed to ensure saturation of all the specimcns

The results of thcse various tests \Vere Ilrst prescntcd hy Ilird ('1 01.(1997a) and are summariscd here in Figure 2. Thc tcs' data \vCI'C rcprocessedand rcinterpretcd by '-Jorton (19()8) but. although some (If Ihc permeabilityvaluc's dillcr from those given prcviously, tht' nUl11crieal difrcrences do notaffect the overall research findings. Ff'r the SDRls thc permcability wasdetcrmined using the virtually constant rate of infiltration achieved after aperi.,.j uf54 or 14 da:, for Tcst Pads I anu 2 respectil'cly It 1\';15dssumcJ thatthe tlow had thcn penetrated the whole thickness of Test Pad I or ihc singlelinal lift of Test Pad 2. This was consistent with the traT"sit times of flow inlaboratory infiltration tests which were also carried out (Hird et 01., 1997a); onTest Pad 2 flow was actually observed emerging from the basc of the pad.When calculating hydraulic gradients in ordcr to apply Darcy's law, theinfluence of suction wa