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Conservation and Management of Archaeological Sites

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The use of geosynthetics for archaeological sitereburial

Edward Kavazanjian

To cite this article: Edward Kavazanjian (2004) The use of geosynthetics for archaeologicalsite reburial, Conservation and Management of Archaeological Sites, 6:3-4, 377-393, DOI:10.1179/135050304793137847

To link to this article: https://doi.org/10.1179/135050304793137847

Published online: 18 Jul 2013.

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CONSERVATION AND MANAGEMENT OF ARCHAEOLOGICAL SITES (2004) volume 6 pages 377-393

The use of geosynthetics for archaeologicalsite reburialEDWARD I<AVAZAN]IAN,]R

ABSTRACT

Applications of geosynthetic materials in reburial practice include geotextiles for separation, filtration andprotection (cushioning); geomembranes and geosynthetic clay liners for infiltration control; geonets andgeocomposites for sub-surface drainage; and geocells for erosion control. Mechanically stabilized earthreinforcement using geocells, geogrids and geotextiles can also provide substantial benefits for reburial projectsby reducing lateral earth pressure against backfilled structures. Other aspects of modern geotechnology thatmay be useful for reburial projects include micro-piles and soil nailing for foundation and excavation supportand evapotranspirative capping technology to establish the depth of soil cover required to isolate a structureor artefact from moisture and temperature fluctuations. Optimal application of these geotechnologies requiresan understanding of the basic engineering principles associated with their implementation, as well as knowledgeof the factors influencing archaeological site preservation.

INTRODUCTION

Use of ge0synthetic materials can contribute substantiallyto the optimal design of an archaeological reburial system,from both technical (performance) and cost perspectives.Geosynthetic materials, originally developed for isola-tion of solid and hazardous waste from the environmentand for soil stabilization and erosion control, can beuseful for many different aspects of archaeological sitereburial. Other modern geotechnology solutions, devel-oped for waste isolation, infrastructure development andsupport of excavations and foundations in sensitiveurban settings, may also be valuable for archaeologicalreburial projects.

The use of geosynthetic materials for separation, filtra-tion, protection (cushioning) and infiltration control hasbecome fairly common on reburial projects. Other usesof geosynthetic materials, such asearth reinforcement and

ISSN 1350-5033 ©2004]AMES&]AMES (SCIENCE PUBLISHERS) LTD

erosion control, have also been applied in reburial prac-tice, although somewhatless frequently. However, manyof these applications have been ad hoc solutions ratherthan engineered applications, sometimes resulting inineffective or less than optimal performance, unnecessarycost and, at times, even counter-productive (damaging)field performance. Design and construction techniquesdeveloped for traditional geotechnical projects (e.g.infra-structure development, waste management), combinedwith an understanding of the particular issues associatedwith archaeological sitereburial, can mitigate the potentialfor inadequate performance and reduce unnecessary ex-penditure when applying geosynthetic materials toreburial projects. Other aspects of modern geotechnologythat may be applicable to reburial practice include soilnailing and micro-piles, which were originally developedfor infrastructure proj ects and for protection and preser-vation of historic structures in urban environments, and

378 EDWARD KA VAZAN] IAN ,]R

evapotranspirative capping technology, developed forthe design of waste containment systems.

GEOSYNTHETIC MATERIALS

Overview of material types

The term 'geosynthetic' generallyrefers to amanufacturedplanar material employed for geotechnical engineeringpurposes. Geosynthetic materials aregenerally fabricatedin panels, sheets and/ or rolls, and are typicallycomposedprimarily of polymeric materials, although natural fibresand soils are sometimes used. Geosynthetic materialsinclude geotextiles, geomembranes, geosynthetic claylin-ers, drainage cores and drainage composites, erosioncontrol nets, geocells and geogrids. In engineering prac-tice today, uses for these materials include separation,protection (cushioning), filtration, drainage, infiltrationresistance, reinforcement and stabilization (erosion con-trol). Table 1,drawn from Bouazza [1],lists their primaryfunctions.

Geotextiles include woven and non-woven fabricsand are used for separation, protection, filtration, rein-forcement and, sometimes, drainage. Geomembranesare polymeric sheets with a very high resistance to flowperpendicular to the sheet. Geomembranes are usedprimarily asabarrier to flow, although they have alsobeenused for separation and protection. A geosynthetic clayliner is composed ofa relatively thin (6mm) layer of verylow-permeability soil, typically bentonite (sodiummontmorillonite), either bonded to acarriergeomembraneor encased between two carriergeotextiles.A geosyntheticclay liner is also primarily used as an infiltration barrierbecause of its high resistanceto flowperpendicular to theplane of the material.

Drainage cores are either nets composed of strands ofpolymeric materials or membrane-like polymeric panelsand sheets with raised nodules or pedestals. When com-bined with a filter geotextile( s), a drainage core provides

a relatively open channel for flow parallel to the plane ofthe core. When the drainage core and surroundinggeotextile material are delivered bonded together as asingleproduct, itis referred to asadrainage geocomposite.

Geogrids are nets orwebs of high-strength polymericmaterialused in earth reinforcement applications. Geocellsare diamond-shaped cells fabricated into a sheet byweld-ing together relatively stiff, rectangular panels of poly-mericmaterialat regularintervals.Geocellsprovide erosionresistance by retaining soilwithin the cells.They can alsobe used for earth reinforcement (e.g. to build retainingwalls)·or filled with concrete to form erosion-resistantchannel linings.Erosion control nets are open planar netsof polymeric threads and strands that hold soil in place,typicallywith the aid of vegetation that grows through thestrands and secures the underlying soil.

Geotextiles

Geotextiles are fabrics made from polymeric fibres. Ac-cording to I<:'oerner[2],over 950/0 of geotextiles are madeof polypropylene or polyester, with the remainder madeprimarily from polyethylene or nylon. Individual fibresare sometimes twisted or spun together to form larger(thicker) strands known as yarn. The fibres or yarns areformed into geotextiles by either woven or non-wovenmethods.

Woven geotextiles, illustrated in Figure 1, are manu-factured using traditional weaving methods and avarietyofweave types. Non-woven geotextiles are manufacturedbyplacing and orienting the fibres or yarns on a conveyorbelt and bonding them by needle punching ('needle-punched') or heat bonding (sometimes referred to as'spun-bond'). The needle-punching process consists ofpushing numerous barbed needles through the fibreweb, thereby mechanically interlocking the fibre into astable configuration with a felt-like texture. Figure 2shows a familyof needle-punched non-woven geotextilesofvarious weights (mass per unit area). In heat bonding,

Table I. Functions of common geosynthetic materials (from [I ]).

Function Product

GTX GM Geo-grid Gel geo-composite Geocell Erosion control product

Separation X X XReinforcement X X XFiltration XDrainage X XInfiltration barrier X X XProtection X X

GTX, geotextile; GM, geomembrane; GeL, geosynthetic clay liner.

USE OF GEOSYNTHETICS FORARCHAEOLOGICAL SITE REBURIAL 379

Figure I. Woven geotextile.

the fibre web ispressed together and heated to its meltingpoint, creating a geotextile with a thinner, more 'paper-like' texture than that produced by needle- punching.

Geotextiles are typically provided in rolls approxi-mately 4m wide and from 30m to 60m in length.Geotextiles maybe seamed in the fieldby sewing,lystering(heat bonding) or simple overlapping. For many appli-cations, seam strength is not important and overlappingis sufficient. Often, seaming is used simply to maintainthe geotextile overlap until it is secured by the soiloverburden. However, there are applications where theseam is required to have some strength. The strength ofa properly made seam will be close to that of the fabric.Lystering is a relatively rapid and expedient seamingprocess; however, if too much heat is applied it canweaken the geotextile, adverselyaffecting itsperformance.Therefore, most field seams are sewn. Recently, a newgeneration of advanced micro-processor-controlledlystering devices has been developed that significantlyreduce the risk of overheating a seam. To eliminate fieldseaming concerns, large geotextile panels, tens of metres

Figure 2. Non-woven, needle- punched geotextiles(with increasing mass per unit area from top tobottom).

in dimension, can be fabricated in the factory (or off-site)and delivered to the field folded in a manner that expe-dites deployment.

Geomembranes

Geomembranes are flexible planar sheets of polymericmaterial. They may be smooth or textured (for enhancedfrictional resistance), can be composed of a variety ofdifferent polymers and come in a range of thicknesses.Geomembranes used in geotechnical practice for liquidand waste containment are typicallybetween 0.7Smm and2.Smm thick and are provided in rolls. Geomembranesare most often employed as liquid or vapour barriersbecause of their very low permeability, but are also usedas separators and physical barriers. Very thingeomembranes, of the order of O.lmm thick (e.g.'Visqueen'), are sometimes used for temporary protec-tion of soil stockpiles or earthwork in progress. The mostcommon types of geomembrane polymers are high-density polyethylene (HDPE), low-density or very flex-ible polyethylene (VFPE), polypropylene (PP), polyvinylchloride (PVC) and reinforced chlorosulfonatedpolyethylene (CSPE). However, there are awide variety ofother types of specialty geomembranes commerciallyavailable.Some specialtygeomembranes are reinforced bya grid of embedded high-strength fibres (i.e. scrim-reinforced) for applications that require a high tensilestrength.

Thin, semi-permeable membranes (e.g. Goretex®)have been suggested for use on archaeological reburialprojects because of their ability to retain liquids whileremaining pervious to vapour transport [3,4]. This typeof product is not generally used in geotechnical applica-tions, therefore there is little information on theirgeotechnical engineering properties.

Geomembrane rolls are typically approximately 4mwide and from 30m to 60m in length. Roll width isgenerally controlled by the manufacturing process, whileroll length is limited by handling considerations.Geomembranes can also be supplied to the field in panelsfabricated in the factory or off-site from geomembranesheets that are the same width as geomembrane rolls.Owing to their high ductility, VFPE and PVC can befabricated off-site into panels tens of metres wide. Fab-ricated panels are folded for handling and delivery to thesite. HDPE panels, on the other hand, are rarely morethan the width of one roll because of the limited ductilityofHDPE.

380 EDWARDKAVAZAN]IAN,]R

Double-tract field seaming techniques forgeomembrane rolls and panels include fusion welding,extrusion welding and taping or gluing. In fusion weld-ing, two pieces ofHDPE are joined together by heatingunder pressure. The most common fusion weld is a'double track weld' that creates two parallel weld lines.This type ofweld allows for non-destructive air-pressuretesting of the seam integrity along its entire length.Extrusion welding involves placing a molten bead ofmaterial (extrudate) along the seam. The extrudate bondsto both pieces of material as it cools, joining themtogether. In a glued seam, the adjacent pieces are joinedby an adhesive. Geomembranes can also simply be over-lapped.

Fusion welding is the preferred method of seamingfor environmental applications because it provides ahigh-strength, redundant seam that facilitates non-de-structive integrity testing. In areas where fusion weldingis not possible (e.g. at corners and connections), extru-sion welding is generally used. Both fusion welding andextrusion welding are carried out by skilled technicians.Taping or gluing is not generally used with HDPE or PPgeomembranes because of their low ductility (whichmakes it difficult to glueor tape pieces that arenot perfectlyflat), concerns over seam longevity and, on environmen-talprojects, concern over the volatile organic compoundspresent in most adhesives. Overlapping is used only innon-critical situations, as it does not create agood barrierto moisture or vapour migration unless both pieces areperfectly flat and arelatively high normal stress is appliedat the overlap.

Geo!Jnthetic clqy liners

A geosynthetic clayliner (GCL) typicallyincorporates dry,granulated sodium montmorillonite clay, commonlyreferred to as bentonite, which is either glued to a carriergeomembrane or secured between two carrier geotextilesby needle punching or stitching. Figure 3illustrates thesebasic types of GCL. The bentonite layer is typically 6mmthick. GCLs are the primary geosynthetic alternative togeomembranes for creating ahydraulic barrier: because ofthe ultra-low permeability of bentonite, a 6mm-thickGCL is roughly hydraulically equivalent to 1-2m of thetype oflow-permeability claysoil typicallyused in landfillliners and covers. GCLs are usually supplied in rolls withdimensions similar to those for geomembranes. How-ever, GCL rolls more than 30m in length are difficult tohandle because of their weight (a 30m-long roll of GCL

(A)GeQtextUe~EnQasedGCL(~ento~iteSandwiehed .bs(weenTWoGeotextiles)

(B) Bentonf1frSupported GeL(Bentonite Glued to ,Geomembrane)

Figure 3. Fabric-encased and membrane-supportedGeLs.

weighs approximately O.75t).GCLs are seamed by simply overlapping adjacent

sheets; powdered bentonite is applied to the seam forsome of the needle-punched products. Laboratory test-ing indicates that overlapping in this manner produces aseam highly resistant to fluid flow, providing there are nowrinkles and sufficient normal stress (typicallythe equiva-lent of O.5m of soil) is placed on the seam.

Drainage cores and composites

Drainage cores include drainage nets and panels. Drainagenets are generally composed of HDPE strands of theorder ofO.5-2mm thick. In a typical drainage net, two or,for high-flow capacity, three sets of parallel strands areoverlaid to create a diamond-shaped pattern and weldedtogether, asillustrated in Figure 4.Either two non-wovenfilter geotextiles or a non-woven filter geotextile and ageomembrane are placed either side of the core to protectit from clogging by soil infiltration. Drainage panels aregeomembrane-like sheets ofHDPE with raised nodulesor pedestals at regular intervals. The nodules or pedestalsare formed by applying heat and pressure to a smoothHDPE panel. A non-woven filter geotextile is placed ontop of the pedestals or nodules to protect the core fromsoil infiltration. When the drainage core is supplied withthe ge0textile (s) already attached, as illustrated in Figure5, it is referred to as a drainage geocomposite. Drainagenets and geocomposites are seamed by overlapping ad-

USE OF GEOSYNTHETICS FOR ARCHAEOLOGICAL SITE REBURIAL 381

Figure 4. Drainage net with two layers of HDPEstrands.

jacentsheets and securing them with plastic or Teflon tiesalong the seam at 15-30cmintervals. Drainage panels areseamed by overlapping and either pressing the pedestalsor nodules together or securing them with an adhesive.

Geogrids

Geogrids, illustrated in Figure 6, are relatively open two-dimensional nets or frameworks. Geogrids are usuallycomposed of high-strength HDPE or other relativelystrong and stiff material (e.g. plastic-coated wire mesh).A geogrid may have a preferred orientation (uniaxialgeogrid) or may have the same strength both longitudi-nallyand across its breadth (biaxialgeogrid). Geogrids areused primarily for earth reinforcement applications andare generally supplied in rolls of similar dimension tothose of geotextiles and geomembranes. Geogrids aretypicallylaid flaton ascarifiedsoilsurface and then coveredwith additional soil. They engage the surrounding soil

Figure 5. Drainage geocomposite.

Figure 6. Uniaxial (left) and biaxial (right)HDPEgeogrids.

through both friction between soil and the geogrid andinterlocking of soil through the relatively open net orframework of the grid. If seaming is necessary, specialconnector clips and bars, provided by the manufacturer,are used to link the edges of adjacent geogrid sheets orpanels.

Geocells

Geocells, illustrated in Figure 7, are three-dimensionalframeworks of HDPE strips connected (welded) to-gether at regular intervals to form a pattern of diamond-shaped vertical cells.Soil-filledgeocells are typicallyusedfor erosion control and shallow burial/low overburdenreinforcement applications; they can also be filled withconcrete to create surface \-vater control channels andshoreline protection features. Geocells are supplied in'stacks' of strips that are spread open by hand, staked tothe sub-grade to hold them open and then filledwith soil

Figure 7. Geocell deployment for erosion control.

382 EDWARDKAVAZAN]IAN,]R

Figure 8. Temporary erosion control netting.

or concrete. The geocell strips are typically 15-30cm inheight and are welded together to form diamond-shapedcells with dimensions similar to their height. One stackmay create a panel on the order of several metres to tenmetres in length and width when spread open. Adjacentpanels can be connected using clips provided by themanufacturer. Geocells filled with soil can be stackedvertically to create retaining walls several metres in height.

Erosion control materials

TEMPORARY EROSION CONTROL :MATERIALS

Temporary erosion control materials, illustrated in Fig-ure 8, are nets of geosynthetic or natural fibres. They areapplied to the ground surface to help hold soil in placeuntil vegetation becomes established. They may be sup-plied as rolled products or as individual fibres that arespray-applied to the soil surface. The spray-applied fibresare referred to as 'fibre roving material'. Temporary ero-sion control material is degradable and is generally notrelied upon to last more than one growing season.

PERMANENT EROSION CONTROL MATERIAL

Permanent erosion control materials, illustrated in Figure9,are three-dimensional cellsof geosynthetic fibres. Theystabilize surficial soils against erosion by trapping soilwithin the cells.Vegetation isgenerally also established toretain the soilwithin the cell.Permanent erosion controlproducts are usually provided in rolls. They can be de-ployed by manual labour and either be left unfilled on thesoil surface to 'trap' eroding soil, filled loosely with soilor buried under a very shallow soil cover. To provide'permanence', the fibres are stabilized against degrada-

Figure 9. Permanent erosion control netting.

tion induced by ultra -violet radiation. Cable-linked matsof articulated concrete blocks are a specialized type ofpermanent erosion control material, employed primarilyin high-flow earthen surface water channels to provideerosion resistance. However, they are expensive, andrequire specialized manufacturing facilities and heavyconstruction equipment for installation.

APPLICATIONS

Overoiew of applications

Table 1 summarizes common engineering applicationsof geosynthetic materials, including separation, ftltration,drainage, reinforcement and protection. Separation,wherein a geotextile is used to prevent co-mingling ofdisparate soils and/ or to facilitate future removal (exca-vation) of alayerof soil,isperhaps the simplestgeotechnicalapplication. For protection applications, a geotextile isused both to physically separate a granular soil from asubstrate and to protect the substrate (e.g. a geo-mem-brane) from abrasion and puncture resulting from thegranular soilparticles. In ftltration, the geotextile not onlyphysically separates two disparate soils or a soil and adrainage conduit but also allows groundwater to flowfreely across the material boundary while preventing thetransport of soil particles.

When used in drainage applications, geosyntheticmaterials provide a 'preferred pathway' for fluid flowalong the plane of the geosynthetic material, i.e. a pathalong which the resistance to fluid flow islower comparedwith other paths. Conversely, when used as an inftltrationbarrier, a geosynthetic material impedes groundwater

USE OF GEOSYNTHETICS FOR ARCHAEOLOGICAL SITE REBURIAL 383

flow perpendicular to the plane of the material.Geosynthetic drainage and barrier materials can be usedtogether to create lined channels for surfacewater control.

For reinforcement applications, the geosynthetic ma-terialis employed as aplanar tensile element within a soilmass, providing confinement that helps to mobilize theinternal shear strength of the soilwhen placed under load.When used to stabilize soil for erosion control purposes,geosynthetic materials typically physically restrain soilparticles against the traction forces applied by flowingwater or wind, as well as from mass transport by gravity.

Geofexfiles

GEOTECHNICAL APPLICATIONS

In geotechnical practice, needle-punched non-wovengeotextiles are used for separation, filtration, protection(cushioning) and sometimes drainage (if the requiredflow capacity is not too great). Heat-bonded, non-wovengeotextiles are also used for separation and filtration, butare not commonly used for cushioning. Although non-woven geotextiles can provide some limited reinforce-ment, their capacity to do this is limited because of theirlow tensile strength; consequently they are generally notemployed in cases where reinforcement would be theirprimary function. Most non-woven geotextiles are madefrom PP and thus degrade rapidly when exposed to ultra-violet radiation (e.g. sunlight). Most manufacturers willnot guarantee non-woven PP geotextile that is exposedto sunlight for longer than 30 days. However, whenburied and isolated from ultra-violet radiation, the lon-gevity ofPP geotextiles is believed to be on the order ofhundreds ofyears [2].The resistanceofpolyestergeotextilesto ultra-violet radiation is much greater than that ofPP.Polyester fibres are also significantly stronger. However,polyester is more expensive and thus is used primarily inwoven geotextileswhere its superior strength givesgreaterreinforcement capability.

Owing to their superior tensile strength, wovengeotextiles are used most often in reinforcement applica-tions. They are somewhat less frequently employed forseparation and filtration, generally with coarser-grainedgranular soils.They have also been employed forprotec-tion of non-woven geotextiles against ultra-violet radia-tion when operational considerations require thenon-woven geotextile to be exposed to sunlight for anextended period of time.

Typical reinforcement applications for wovengeotextiles include improving foundation-bearing capac-

ity, enhancing sub-grade stability when placing fill oversoft soils and construction of mechanically stabilizedearth walls. Placing a high-strength geotextile over a softsoil sub-grade before placing fill can be a rapid, cost-effective means of improving the sub-grade with a mini-mum of skilled labour and heavy equipment. When usedto enhance sub-grade stability, the woven geotextile mayalsohave a separation function, preventing coarse-grainedroadway aggregate from penetrating into the underlyingsoft, generally finer-grained soil.

High-strength woven geotextiles can be used to con-struct mechanically stabilized earth walls and embank-ments with a minimum of skilled labour and heavyequipment. By interspersing horizontal layers of high-strength geotextile with earth fill, the fill can be con-structed with an almost vertical face. By wrapping thegeotextile around the fill layer at the face of the wall, asillustrated in Figure 10,vertical walls can be constructedto heights of tens of metres. Geotextile reinforcement(alongwith geocells and geogrids) can be used to stabilizebackfill inside a structure such that the load (lateral earthpressure) applied to the walles) of the structure is mini-mal.It can alsocreate aphysicalbarrier to protect sitesfromwind, erosion and surface water.

ARCHAEOLOGICAL APPLICATIONS

Geotextiles are the most commonly used geosyntheticmaterials on archaeological reburial projects [4]; theirapplication is primarily for separation and protection.The geotextile is eitherdeployed asa sheet product or sewnto createsacksto retain selectbackfill(e.g.'clinkered' clay)[3,4].When appliedasasheet,alayerof finesandisoften placedon top of the geotextile before backfillingwith on-site soil,

Figure 10. 'Wrap-around' geotextile retaining wall [16].

384 EDWARDKAVAZAN]lAN,]R

minimizing the potential for voids and so preventingmoisture accumulationandbiologicalactivityadjacentto theartefact surface [5].Forthis samereason, geotextilehas longbeen. recognized as superior to plastic sheeting (i.e.geomembrane) for.this application· [4, 6].

While spun-bonded, non-woven geotextiles appearto have been relatively commonly employed directly ontop of artefact surfaces in backfill projects [4,7,8], theirrelative stiffness suggests that a needle-punched, non-woven geotextile may be superior for this applicationbecause of its··superior ability to conform to unevensurfaces and its enhanced cushioning ability. Some wo-ven geotextiles are also softer and more flexible thanspun-bonded products and have superior strength tonon-woven geotextiles, suggesting that woven productsmay be most suitable for creating sewn sacks for retainingselectbackfill.

Demas [4]cites accumulation of moisture, resulting inroot penetration and enhanced microbial activity, andadherence of the geotextile to the artefact as the twoprimary disadvantages of geotextiles for reburialprojects.Experience has shown that, while spun-bondedgeotextiles may be somewhat more resistant to· rootpenetration than needle-punched geotextiles,they arenot entirely successful at deflecting root penetration [5].A needle-punched non-woven product will also have agreater moisture-retention capacity than will a spun-bonded product, making it more susceptible to microbialactivity that can result in precipitation of minerals fromthe sub-surface water: a likely cause of adherence of ageotextile to artefacts. Geotextiles can be impregnatedwith both biocides and herbicides to mitigate root pen-etration and biological activity. Demas et al. [9]report onthe use of a geotextile impregnated with a herbicide tomitigate root penetration as part of the preservation ofthe Laetoli trackway inTanzania. Calarco [10]also reportson the use of geotextiles as a barrier to root penetration.Even with the use of a herbicide, regular maintenance toremove deep-:-rootedvegetation is probably warranted if ageotextileis to be reliedupon asabarrier to root penetration.(Geomembranes arerecognized in geotechnical practice aseffective barriers to root penetration, as discussed below.However, Mora [6]recommends that they never be placeddirectly in contact with an artefact.)

Geomembranes and geo.rynthetic clqy liners

GEOTECHNICAL APPLICATIONS

Geomembranes and GCLs are used primarily as barriersagainst groundwater infiltration and vapour migration.

Thin geomembranes have occasionally been used toprotect exposed geotextiles from ultra-violet degrada-tion and for erosion control on· unvegetated surfaces.GCLs have occasionally been used as protective cushionlayerswhen the particle size and/ or overburden pressureis too great for even the thickest non-woven geotextile.The primary advantages of a GCL as a hydraulic barriercompared with a geomembrane are ease of installation(ease of seaming) and cost. Although, in general, not aseffective as aproperly installed geomembrane as an infil-tration barrier layer,for non -criticalapplications GCLs canprovide a highly effective barrier at a lower cost than forageomembrane and can be easilyinstalled without skilledseaming technicians. Furthermore, GCLs are relativelyrugged compared with many geomembranes, requiringless carewith respect to backfllling, compacting overlyingsoil layers and damage from construction traffic.

Geomembranes and GCLs are often used in combi-nation to create 'composite liners' for critical infiltrationcontrol applications such as landfill liner systems. Com-posite liner systems, composed of a geomembrane witheither low-permeability soil or a GCL underlying, aregenerally considered to be the most effective type ofengineered infiltration barrier layer. The geomembraneprovides avery high general overall resistance to infiltra-tion; the underlying low-permeability soil or GCL pro-vides a redundant layer that minimizes fluid transmissionthrough holes or defects in the geomembrane. Field dataindicate the flow through a composite liner will be anorder of magnitude less than that through a single liner,even when the single liner is constructed in accordancewith stringent quality assurance procedures. While theflow through the single liner may be acceptable for manycommon applications (e.g. recreational water impound-ments), most modern landfill liner systems employcomposite liners to minimize leakage. Furthermore, forcritical applications (e.g.hazardous waste landfills) dou-ble composite liner systems may be employed.

ARCHAEOLOGICAL APPLICATIONS

In the 1980s, thin plastic sheeting (i.e. geomembranes)was used as separators over artefacts in a number ofreburial projects [4].However, it was soon recognizedthat this was a bad practice, as the inability of the plasticsheeting to conform to an uneven surface created voidsbetween the sheeting and the surface, resulting in amicro-environment conducive to moisture accumulation andbiological activity [6]. Geomembrane (often 'off-the-shelf' plastic sheeting) has been successfully used at thetop of archaeological backfills, both to keep moisture in

USE OF GEOSYNTHETICS FOR ARCHAEOLOGICAL SITE REBURIAL 385

and infl1tration out. Ashurst et al. [7]report on the use of'visqueen' polyethylene sheeting at the top of the backfillfor the Rose Theatre. The geomembrane was used inconjunction with 'leaky pipes' beneath it to maintain awater-logged environment. Silver et al. [11] report on theuse of geomembranes at the top of a backfilled room inthe Aztec Ruins National Monument in New Mexico topreventinfl1tration. Geomembrane has also been used inEngland to line the sides of trenches excavated in water-logged environments to prevent them from drying out[8].Another potential application of a geomembrane atthe top of a backfill is as a very effective barrier to rootpenetration.

It has been suggested that semi-permeablegeomembranes that retain liquids but transmit watervapour may be effective in archaeological backf111s.Thistype of application isnot common in geotechnical practiceand no information is available on the geotechnicalproperties or behaviour of such membranes.

The use of GCL on archaeological reburial projects hasnot been reported. However, Mora [6]recommends theuse ofhorizontallayers of bentonite at the top of abackfillto minimize infl1tration: a GCL would provide an expe-dient and cost-effective means of creating such a layer. Ifnot saturated, GCLs also have limited vapour transmis-sion and a significant vapour absorption capacity, whichmay be beneficial in some backfill environments, e.g. inan arid environment to help control relative humidity.

Drainage cores and composites

GEOTECHNICAL APPLICATIONS

Drainage cores and composites are generally employed aspart of an infl1tration control strategy. These elementsprovide a preferred pathway for infl1trating fluid, con-ducting it away from the protected area. Drainage layersaregenerallyplacedon top ofbarrier layers(geomembranes,GCLs and composite liners) to maximize the effective-ness of the barrier layer by minimizing the hydraulic headabove it. As drainage layersrelyon gravityto conduct fluidthrough thedrainagecore,itis essentialthat the drainagelayerhas an outlet through which the fluid can flowunimpeded.Drainage layers can also be used as capillary breaks to cutoff the suction from surrounding fine-grained soil andthus inhibit sub-surface transport of groundwater anddissolved minerals in the unsaturated zone.

ARCHAEOLOGICAL APPLICATIONS

Silver et al. [11] report on the use of'geodrains' as part ofthe engineered backfill system employed at the Aztec

Ruins National Monument in New Mexico. The drainswere placed vertically and horiz ontally within the backfillbelow a geomembrane. Demas [4] cites a report on theuse of a 'geocomposite' as a root barrier, although thenature of the geocomposite is not discussed. However,geocomposite drains are known to be penetrated andclogged by roots in geotechnical applications. Althoughthe roots tend to follow the plane of the geocomposite(as it is generally the most abundant source of water),unless one side of the drainage geocomposite is a poly-meric panel, it should not be counted upon as a barrierto root penetration.

Geogrids, geocells and erosion control materials

GEOTECHNICAL APPLICATIONS

Geogrids, geocellsand erosion control materialsareused toreinforce and mechanically stabilize soil. Geogrids can beused to buildvertical and nearlyverticalwallstens of metresin height and to reinforce earth fillplaced on top of soft sub-gradesoils.Geogrids aregenerallyusedwhere severalmetresof fillor a significantoverburden load willbe placed on topof them.Therefore, geogridconstruction generallyemploysearth-moving and compaction equipment.

For shallow burial and low-overburden reinforce-ment applications, geocells are generally employed.Geocells can be stacked to createverticalwallsup to severalmetres in height and can also be used for erosion control,as illustrated in Figure 7.Geocells filledwith concrete aresometimes used to create surfacewater channels. They canbe easily deployed and filled by hand using unskilledlabour and can be rapidly deployed and filledwith soil tocreate a stable roadway over a soft soil sub-grade. Ageotextile placed beneath the geocells for separation andreinforcement willfurther enhance the stabilityof ageocell-reinforced roadway.

Erosion control materials are deployed on slopes andas channel linings for storm water channels that aresubject to intermittent flow. When used to enhance"Surficialstability of a slope, the assumption is that veg-etation will eventually take root through the erosioncontrol net or mat. If the vegetation, once established, isanticipated to be sufficiently dense to stabilize the slopeon its own, temporary erosion control material is used.Permanent erosion control material used as a lining forintermittent channels provides supplemental resistanceto prevent scouring of vegetation during storm flows.

ARCHAEOLOGICAL APPLICATIONS

Archaeological applications of earth reinforcement using

386 EDWARD KAVAZAN]IAN,]R

geogrids or geocells have not been widely reported in thetechnical literature. However, at the Aztec Ruins NationalMonument in New Mexico, National Park Service per-sonnel have employed a geocell retaining system tominimize the lateral earth pressure against the wall of abackfilled room. Demas et al. [9] report on the use oferosion control material to aid in stabilization of theLaetoli trackway in Tanzania. The necessity of stabilizingthe exposed surface of an archaeological backfill againsterosion iswell-recognized [6],and erosion control mate-rial provides an effective and cost-effective means ofachieving this in many situations.

GEOSYNTHETICS ENGINEERING

Geo.rynthetics design

safety for the intended application. However, the cost ofgeosynthetics materials is generally proportional to theircapabilities, e.g. the cost of a non-woven geotextile isalmost directly proportional to its mass per unit area,which is in turn directly proportional to its cushioningability.Therefore, purchasing ageotextile with propertiessuperior to the minimum (or maximum) required valuewill result in unnecessary cost and may be prohibitivelyexpensive on larger-scaleprojects. However, for relativelysmall quantities of material (i.e. for most archaeologicalbackfill projects), the cost of the geosynthetic materials isprobably not a significant component of the overallexpenditure. Furthermore, often more than one type ofgeosynthetic material can serve the intended function, e.g.both geotextiles, geogrids and geocells can provide rein-forcement.

STANDARD TESTS FOR PROPERTIES OF GEOSYNTHETICS

A variety of standardized tests have been developed toquantify the properties of geosynthetics. The most com-monly cited standards are those developed by the Ameri-can Society for Testing Materials (ASTM). Table 2 listssome common properties of geosynthetic materials andthe ASTM standard test methods used to evaluate them.For most ASTM standards there are corresponding ISOstandards published by the European Community In-ternational Standards Organization. The GeosyntheticsResearch Institute (GRI) in Philadelphia also publishesstandards for geosynthetics materials and has a

DESIGN ANALYSIS

Engineering analysesof the necessary material propertiesfor most geosynthetics applications are available.K:.oerner[2] and Bouazza et al. [12] provide comprehensive sum-maries of such analyses.Sometimes the required propertymust fall within a range of values, e.g. the apparentopening size of a geotextile used for filtration applica-tions. At others, the required property has a minimum(or maximum) allowable value, e.g. a minimum mass perunit area is specified for most non-woven geotextileapplications. Procuring a product with greater than theminimum required value (or less than the maximumallowable value) may provide an additional margin of

Table 2. Standard tests for geotextiles and geomembranes (from [I ]).

Uniaxial tensile strength ASTM D 4595Multiaxial tensile or ASTM D 3786

burst testsPuncture resistance ASTM D 4833

Trapezoid tear strength ASTM D 4533Apparent opening size ASTM D 475\Permittivity ASTM D 4491Gradient ratio ASTM D 5101

Property

Thickness

Mass per unit areaGrab rupture

Transmissivity

Ultra-violet resistance

Seam strengthSeam strength

Test standard

ASTM D 5199

ASTM D 526\ASTM D 4632

ASTM D 47\6

ASTM D 4355

ASTM D \683ASTM D 4884

Test name

Standard test method for measuring nominal thickness of geotextiles andgeomembranes

Standard test method for measuring mass per unit area of geotextilesStandard test method for breaking load and elongation of geotextiles (grab

method)Standard test method for tensile properties by the wide-width strip methodStandard test method for hydraulic bursting strength of knitted goods and non-

woven fabrics - diaphragm bursting strength tester methodStandard test method for index puncture resistance ofgeotextiles, geomembranes

and related productsStandard test method for trapezoid tearing strength of geotextilesStandard test method for determining apparent opening size of a geotextileStandard tests methods for water permeability of geotextiles by permittivityStandard test for measuring the soil-geotextile system clogging potential by the

gradient ratioStandard test method for· constant head hydraulic transmissivity (in-plane flow) of

geotextiles-related productsStandard test method for deterioration of geotextiles from exposure to ultra-

violet light and water (xenon-arc type apparatus)Failure in sewn seams of woven fabricsStandard test method for seam strength of sewn geotextiles

USE OF GEOSYNTHETICS FOR ARCHAEOLOGICAL SITE REBURIAL 387

geosynthetics testing laboratory certification programme.Manufacturers of geosynthetics typicallycertify the prop-erties of their products with respect to the appropriatestandards listed in Table 2. Thus, in addition to provid-ing information for quantitative design analyses, thestandards in Table 2 provide a basis for comparingdifferent materials. On large projects and for criticalapplications, third-party testing is often carried out to giveindependent verification of manufacturer-certified prop-erties. However, on smaller projects and for non-criticalapplications, manufacturer-certified properties may berelied upon.

Protection) separation and filtration

GENERAL DESIGN CONSIDERATIONS

With the exception of soilreinforcement, most geotextilesused in engineering applications are non-wovengeotextiles. Mass per unit area is perhaps the mostfrequently specified property and can be generally relatedto the intended application. Mass per unit area is typicallyexpressed in grammes per square metre (g/ m2) or ouncesper square yard (oz/yd2). The mass per unit area ofcommerciallyavailablenon-woven geotextilesranges from110g/m2 (4oz/yd2) to 880g/m2 (32oz/yd2). Non-woven geotextiles with lower mass per unit area (110-220g/ m2) are typicallyemployed for separation, with thehigh end of this range used where the strength of thegeotextileis reliedupon. Non-wovengeotextiles with massper unit area values from 220g/ m2 to 330g/ m2 aretypically used in filtration. Protection (cushioning) anddrainage applications typically require non-wovengeotextileswith mass per unit area in the middle to upperend of the range of commercially available values, from275g/m2 to 880g/m2•

I PROTECTION (CUSHIONING)

Analyses to determine the minimum required mass perunit area for protection of a geomembrane against dam-age (e.g. scoring, puncture) from an overlying coarse-grained soilare described byNarejo and Corcoran [13]andI<.oerner [2].The puncture potential of the overlying soildepends upon the overburden pressure and the size andangularity of the soil particles. Overburden pressure is afunction of the depth of burial and additional surfaceloads, such as those resulting from construction equip-ment, vehicle traffic and/ or foundation loads. Punctureresistance depends primarily upon the mass per unit areaof the geotextile: the higher the overburden stress and the

coarser and more angular the backfill, the higher the massper unit area necessary for adequate protection. However,a 440g/ m2 non-woven geotextile will provide adequateprotection for ageomembrane overlain by sand or gravelsubject to tens of metres of overburden. It seems logicalthat ageotextile that can protect a geomembrane againstscoring or puncture from overlying soil should alsoprovide areasonable degree of protection for archaeologi-cal artefacts from the overlying backfill. For higher over-burden loads and critical applications, the analyses ofN arejo and Corcoran [13]or specializedlaboratory testing[2, 14] can be employed.

SEPARATION AND FILTRATION

For separation, the key engineering properties are thetensile strength, sometimes referred to as the 'burstresistance' as it is usually measured in the Mullen bursttest (ASTMD 3786), and the puncture resistance, meas-ured inASTM D 4833. I<.oerner[2]provides nomographsrelating the required burst and puncture resistance to theoverburden pressure at the geotextile interface and theaverageparticle sizeof the overlying soil.For fmer-grainedsoils (fme sands, silts and clays)the strength requirementsfor separation are minimal. Retention requirements for aseparation geotextile are based upon the apparent open-ing size (AOS) of the geotextile. A geotextile with anAOSof 75 will retain most soils, including silts and clays.

For filtration applications, the geotextile must meetseparation requirements and additional requirements forretention of soil particles. Retention criteria are basedupon the AOS of the geotextile and the coefficient ofuniformity (CD) and grain size of the soil. Giroud [15]presents recommendations for the relationship amongthese parameters for filtration applications.

Geotextiles used for filtration (or drainage) may besubject to mineral precipitation. This phenomenon issometimes referred as biological fouling, as the mineralprecipitation is induced by changes in the pH of the soiland pore water resulting from growth of biologicalorganisms. This type of fouling can create problemswhen it impedes fluid flow across the geotextile. It mayalso explain cases where geotextiles used:£or separationduring reburial have been reported to be 'stuck' to arte-facts upon exhumation. Defensive measures againstbiological fouling include the use ofbiocide-impregnatedfabrics, although the long-term efficacy of such measuresis questionable. Therefore, the typical approach used inengineering applications is to provide a substantial factorof safety (often of the order of 15 to 20) on the ability of

388 EDWARDKAVAZAN]IAN,]R

a filter geotextile to transmit fluid, asdiscussed in the nextsection, along with the ability to 'flush' the drainagesystem. However, these mitigation measures are de-signed to address flow capacity and will probably not beeffective in protecting archaeological artefacts from min-eral precipitation. Intimate contact (to eliminate micro-environments), pH control, infiltration control and theuse of biocides are among the bio-fouling preventionstrategies that are likelyto be applicable to archaeologicalreburial projects.

Drainage design

The required flow capacity of a sub-surface drainagesystem can be calculated based upon its tributary area, thesaturated hydraulic conductivity (permeability) of theadjacent soil, the anticipated or maximum possible hy-draulic gradient in the soil and the design safety factor.When the required fluid flow capacity is relatively small,a non-woven geotextile may be used as a drainage me-dium. However, for most practical situations, either adrainage core and filter geotextile(s) or a prefabricateddrainage composite are required to provide adequate flowcapacity.The capacityofanon-wovengeotextile or drain-age core to transmit fluid along the plane of the materialis called transmissivity. The transmissivity of ageosynthetic drainage layer is generally a function of thethickness of the drainage core or the mass per unit area ofthe geotextile (if the geotextileis the primary flow con-duit), the overburden pressure on the drainage layer andthe medium surrounding the drainage layer.Transmissivity of a geosynthetic drainage element willincrease with increasing mass per unit area for non-wovengeotextilesand with increasing thickness for drainagecores and will decrease with increasing overburden pres-sure, Transmissivity decreases when the geosyntheticdrainage element is surrounded by soil rather than a 'rigid'substrate, such asageomembrane or concrete, because ofpenetration of the soil particles into the geosyntheticelement. While manufacturers generally do specifytransmissivity ..as a function of overburden pressure,manufactur,e\!alues are generallymeasured byplacing thedrainage element between steel plates and thus they donot reflect potential decreases in transmissivity as a resultof soil particle penetration into the drainage element.

The abilityof ageotextile to transmit fluidperpendicu-lar to the plane of the fabric is referred to as 'permittivity'.Permittivity can be important when a geotextile is a partof a prefabricated drainage system (or for filtration): thepermittivity of the geotextile should be sufficiently great

to allowfree entry and discharge of fluid from the drainagecore (or through the layer being filtered). Permittivityvalues are influenced by the same factors as transmissivity,except that the permittivity of a non-woven geotextiledecreases with increasing mass per unit area.

Transmissivity and permittivity of a non-wovengeotextilemay also decrease because ofinternal cloggingwith fine-grained soilparticles and biological fouling. Tominimize the potential for soil clogging, non-wovengeotextiles used for drainage applications should alsomeet AOS requirements for filtration. Because of thepossibility of clogging with soil particles and biologicalfouling, factors of safety of the order of 15 to 20 are oftenused in engineeringpracticewhen specifyingtransmissivityand permitivity.

Most geotextiles are, to some extent, hydrophobic andthus will resist moisture intrusion until a certain waterentry pressure isexceeded.This property means geotextileshavealimitedabilityto functionasacapillarybreak.However,thewater entrypressure formost geotextilesisrelativelylow,and penetration of soil into the geotextile further reducesits· ability to resist water intrusion. Furthermore, oncewater intrudes upon the geotextile, 'breakthrough' oc-curs and water flows into and through the fabric quicklyand easily.

Infiltration barriers

GEOMEMBRANES

The thickness of geomembrane infiltration barriers isgenerally based upon puncture requirements although,in some. applications (e.g. geomembranes subject towind uplift), tensile strength of the geomembrane mayalso be an issue. The puncture resistance and punctureprotection of geomembranes with non-woven 'cushion'geotextiles are discussed by I<':'oerner[2]and Narejo andCorcoran [13]. Performance factors involved in the selec-tion of geomembrane polymer types are summarized inTable 3 for landfill applications, after Bouazza [1]. HDPEis the most commonly used geomembrane in landfillliner infiltration barrier layers owing to its exceptionalchemical resistance and durability. HDPE is somewhatmore rigid than other geomembranes but generally hasgood physical properties and can withstand the large I

stresses often imposed on it during construction.VFPEand PVC are the most commonly used geomembranematerials for landfill covers because of their enhancedductility compared with HDPE. The term VFPE encom-passes various low-density polyethylene grades such asvery low-density polyethylene (VLDPE) and certain types

USE OF GEOSYNTHETICS FOR ARCHAEOLOGICAL SITE REBURIAL 389

Table 3. Geomembrane selection criteria (from [I D.Criteria

Liquid barrier

Mechanical properties

Construction survivability

Installation

Chemical resistance

Long-term durability

Considerations for selection

All polymers generally have acceptable characteristics as liquid barriers, although HDPEgeomembranes have the best. All have extremely low hydraulic conductivity and areimpermeable for practical purposes.HDPE is relatively stiff and has a relatively small yield strain. PVC is relatively extensible anddoes not exhibit post-yield strength loss. The tensile properties of PP and CSPE often fallbetween those of HDPE and PVC but are difficult to generalize because CSPE is often madewith embedded reinforcing fabrics that affect tensile response. Mechanical properties varysomewhat with geomembrane thickness.All polymers have acceptable ability to maintain integrity when subjected to concentratedstresses. However, the best performance is obtained with more extensible geomembranes.Therefore, PVC offers the most favourable performance.PP, PVC and CSPE are easier to place than HDPE because their greater flexibility makes themconform more easily to the foundation and makes them less prone to thermal expansionwrinkles. Acceptable placement and wrinkle control, however, can be achieved with allpolymers if appropriate installation procedures are used. All polymers are easily seamed, withHDPE usually achieving the highest seam strength and quality.HDPE has the highest degree of compatibility with a wide variety of chemicals. PP has verygood chemical resistance. CSPE has good resistance to many chemicals but is attacked by somewhich are relatively common, including chlorinated solvents and hydrocarbons. PVC typically isthe least chemically resistant of the polymers.HDPE offers the best performance. HDPE is a highly inert and durable material that is notsusceptible to chemical degradation under conditions generally encountered in landfills. Thedurability of PVC geomembranes is significantly less favourable than that of HDPE becausePVC geomembranes are composed of approximately two-thirds PVC resin and one-thirdplasticisers. Over time physical degradation (extraction) may cause plasticiser loss, whichresults in reduced geomembrane flexibility. The durability of PP and CSPE geomembranes istypically between that of HDPE and PVC.

of linear low-density polyethylene (LLDPE).PP geomembranes are becoming more common for

both landfill liners and covers. They are more flexiblethanHD PE geomembranes, remain stable at higher tempera-tures and, while not as chemically resistant as HD PE, aremore. so thanVFPE or PVC and provide sufficientchemical resistance for municipal solid waste landfills.PVC geomembranes are very rugged and are often usedas liners for liquid impoundment and waste containmentfor relatively short-term applications, generally no morethan five years because oflong- term durability concerns.PVC geomembranes are generally less expensive thanpolyethylene geomembranes and can be manufactured inrelatively large panels. The large panel sizes allow easierinstallation since there are fewer field-fabricated seams.

The shear resistance of the interface between a smoothgeomembrane and a fine-grained (silt or clay) soil can bevery small.This low interface strength can lead to stabilityproblems when the geomembrane is deployed on a slopeand seismic stability can be an issue even when it isdeployed on a flat surface. HDPE geomembranes can betextured to increase the interface shear strength to be-tween 600/0and 90% of that of the overlyingsoilifnecessary.When agranular soilisplaced on top of the geomembrane,the interface strength is typically80-90% of that of the soil.However, frequently,puncture considerationsrequireplace-

ment of a geotextile between the granular soil and thegeomembrane. The interface resistance of a non-wovencushion geotextile and a smooth geomembrane is alsovery small, creating the potential for stability problems.In general, whenever geomembranes are deployed onsloping surfaces, a stability analysisis required to evaluatethe failure potential of the overlying soil.

The main pathways for transmission of fluid orvapour through a geomembrane are holes in thegeomembrane or defects in the seam. Carefulgeosynthetics installation and backfilling are essential tominimize flow through defects. In the early uses ofgeomembranes for waste containment applications, per-formance concerns included the chemicalcompatibilitybe-tween geomembranes and waste and the service life ofgeomembranes. Now, construction qualityissuesareviewedas the principal limitations to their performance. A prop-erlydesigned and constructed geomembrme barrier layerhas the potential for hundreds of years of service lifetime.

GEOSYNTHETIC CLAY LINERS

Application of GCLs is also often controlled by slopestability considerations. Saturated bentonite is one of thelowest strength natural soils. (In the 19th century, ben-tonite was used as a lubricant for wagon wheels in thewestern USA.)N eedle-punch-reinforced GCLs offer sig-

390 EDWARDKAVAZANJIAN,JR

nificantlyenhanced stabilityover unreinforced GCL prod-ucts, particularly at low confining (overburden) pres-sures. Another technique to enhance stability is to'encapsulate' the GCL, providing geomembranes aboveand below it to minimize the potential for hydration ofthe bentonite. This technique is generally used where thebentonite is fixed to a geomembrane carrier, as only oneadditional geomembrane is then required. However, formost reburial applications, the conftningpressure is likelyto be sufficiently low that a needle-punch-reinforcedgeomembrane will provide adequate stability.

COMPOSITE LINERS

For situations where infiltration control is critical, acomposite liner composed of a geomembrane on top ofa GCL willprovide the highes t level of infiltration resist-ance. Composite liners are subject to the same· slopestability constraints as geomembranes and GCLs sepa-rately.The performance of acomposite liner, or anybarrierlayerfor that matter, willbe greatlyenhancedifitis overlainby a drainage layer designed to minimize the hydraulichead on the liner. For situations where infiltration mustbe minimized absolutely (e.g.hazardous waste landfills),double composite liners (two composite liners separatedby a drainage layer) are used. However, for most practicalsituations, a single composite liner will limit infiltrationto a negligibly small value.

Reinforcement

Geogrids, high-strength woven geotextiles and geocellscan allbe used asreinforcement elements to mechanicallystabilize soiL By engaging the tensile strength of thereinforcement, horizontal layers can be used to··buildwallswith vertical and near-vertical faces, to reduce lateralearth pressure against backfilled structures and toim-prove the bearing capacity of soft soil sub-grades. Engi-neering analyses for various reinforcement applicationsare described by I<'oerner [2]and the US Federal HighwayAdministration· [16]. Manufacturers may also providedesign guidelines, typical details and designassistance fortheir product. Mowever, for critical reinforcement appli-cations, a site-specific design analysis by a registeredprofessional engineer is recommended.

Mechanically stabilized earth walls that are of substan-tial height or subject to substantial load generally employrelativelyfree-drainingbackfill,i.e.cohesionless soils(sandsand gravels).However, lowwallsand walls for non-criticalapplications can be constructed of almost any soil type,and earth reinforcement, if properly spaced, will reduce

lateral earth pressure from almostanytype of soilbackfill.If the local soil· is poorly drained and a high wall isnecessary, or the reinforced fill will have to support asubstantial load, geosynthetic drainage elements can beused within the reinforced earth mass.

Lowwalls (up to several metres in height) and limitedareas of backfill can be constructed easilyby hand usinggeocells.·However, geocell walls require backfilling withgranular, cohesionless soil, as cohesivesoil cannot bereadily packed into geocells in· a uniform and stablemanner (i.e. without voids and post-placement settle-ment). If cohesive soil is to be used as backfill, a wrap-around geotextile wallcan be constructed. Compaction of I

the soil lifts between layers of geotextile reinforcement isnecessary, primarily to reduce the lateral deformation atthe face of the wall as successive lifts of soil are placed. Ifdeformation of the wall face is not critical, compaction isnot required. For archaeological backfill situations, thefaceof thewallcan be setback from the structure and, oncecompleted, the space between can then be filled with afree-draining cohesionless soil. This will minimize thepressure against the backfilled wallwhile providing drain-age at its face.

Erosion control

When used for erosion control, geosynthetic materialsgenerally work in combination with vegetation, as illus-trated in Figure 11.Temporary erosion control materialsare designed only to hold the soil in place until vegetationis established. Permanent materials provide supplemen-tal reinforcement for interim periods when erosive forcescould potentially scour and remove the vegetation, e.g. instorm water control channels subject to intermittent highflows. Geocells filled with concrete ,or soil cement arepermanent erosion control features that do not relyuponthe establishment of vegetation.

The type of erosion control material most suitable fora particular application depends upon soil type, windvelocity, rainfall intensity and/or stream flow velocity.Design guidelines are provided by manufacturers to aidin selection of the appropriate type of product. There isalso some manufacturer-independent design guidanceavailable [2, 17].

OTHER GEOTECHNOLOGY APPLICATIONS

Other aspects of modern geotechnology that are poten-tiallyapplicable for archaeologicalreburialprojects includeevapotranspirative capping, and soil nailing and micro-

USE OF GEOSYNTHETICS FOR ARCHAEOLOGICAL SITE REBURIAL 391

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--/~'7J7

E

used to reinforce adobe or brick walls subject to lateralearth pressure from soil backfill. However, the intrusivenature of soil nailing may be a deterrent to its use onarchaeological projects. Design guidelines and analysesare available from the US Federal Highway Administra-tion [21].Special heavy equipment is available for rapidinstallation of soil nails on large jobs and in stiff soils.However, they can also be installed by hand using light-weight equipment for small jobs in loose sand and softsoils.

Figure 12. Soil nailing for excavation support [21].

Micro-piles are small diameter (15-20cm), pressure-grouted borings with a reinforcing bar placed down themiddle. Micro-piles were originally developed in Europefor preservation of historic structures and protection ofbuildings and utilities adjacent to excavations and tun-nels in urban environments. Micro-piles can reinforcefoundations, as illustrated in Figure 13. They have alsoproved to be an effective means to stabilize hillsidessubject to deep-seated landsliding. Design methodolo-gies are described by Lizzi [22]. Micro-pile installationrequires a small drill rig and pressure-grouting equip-ment. Low overhead drill rigs,with overhead clearance aslittle as 2m, have been used.

SUMMARY AND CONCLUSION

Geosynthetic materials can be employed in a variety ofways to facilitatearchaeological reburial projects. Applica-tions include the use of geotextiles for separation and

Micro-piles

piling for soilreinforcement. Over the past fewyears therehas been a tremendous amount of research into anddevelopment of methodologies to predict the perform-ance of evapotranspirative capping systems [18-20]. Thistechnology can be used to evaluate the optimal burialdepth required to minimize seepage and provide a stableunderground environment for a buried artefact.

A capillary break is sometimes employed in anevapotranspirative cover system to cut off soil suctionfrom below the evapotranspirative cap, reducing thepotential for infiltrating water to be drawn down belowthe zone of evaporative and transpirative action andenhancing the water storage capacity of theevapotranspirative layer. In mining, capillary breaks areused on tailings piles to stop mineral-laden water frombeing drawn up to the surface and into the oxidizingzone, thereby reducing acid run-off from tailings piles. Inthis manner, capillary breaks may be able to minimizemineral precipitation on the walls and surfaces of reburiedand backfilled archaeological sites. Generally, a layer ofcoarse-grained soil incapable of sustaining capillary ten-sion across the pore spaces (e.g.gravel or coarse sand) isemployed as the capillary break, though a drainagegeocomposite may also serve this function.

Soil nailing is an earth-reinforcement technique usedto stabilize excavations, slopes and walls. In soil nailing,reinforcing bars are placed in grout-filled holes drilledroughly perpendicular to the surface that is being rein-forced. In sands and other soils without cohesion, sometype of facing (e.g. wood lagging, shotcrete) may berequired to prevent soil from ravelling at the face of thewall. Soil nailing is particularly useful for supportingexcavations. As illustrated in Figure 12, soil nails areinstalled as the excavation proceeds, starting at the groundsurface (i.e. 'top-down' construction) and the excavationrequires no internal bracing. Soil nailing could also be

Figure 1I. Vegetation working in concert with erosioncontrol netting.

392 EDWARDKAVAZAN]IAN,]R

\f GROUND LEVEL

_ROOTPILES

Figure 13. Micro-piles for foundation stabilization [22].

protection, geomembranes for moisture migration con-trol, and the use of various erosion control materials.Other geosynthetic material applications that may beuseful include use of geotextiles and geocomposites forsub-surface filtration and drainage, use of GCLs as infil-tration barriers, and use of geomembranes asroot barriersand earth reinforcement. Furthermore, new and en-hanced geosynthetic materials with potential archaeologi-cal applications are continually being developed byengineers and manufacturers. In addition to the applica-tion of geosynthetic materials, other geotechnologiesthat offer potential benefits to archaeological reburialprojects include evapotranspirative capping design meth-odologies, and soil nailing and micro-piles. If properlyapplied, these geotechnologies can help contribute to thedevelopment of robust and cost-effective reburial sys-tems.

Edward Kavazanjian, Jr is a registered professionalgeotechnical engineer and Research Professor of CivilEngineering at the University of Southern California. Hisexpertise includes the use of geosynthetic materials forwaste containment systems.

Contact address: IraA. Fulton School of EngineeringArizona State University, Tempe,AZ 85287-5306, USA.Tel.: +1 480 727 8566, Fax: +1 480 965 0557,Email:edkavy@asu.edu

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L'utilisation de geosynthetiques pour Ie re-enterrement desites archeoIogiques

Edward Kavazanjian,Jr.

Resume

Les applications de materiaux geosynthetiques dans la pratiquede re-enterrement incluent 1'utilisation de geotextiles pour laseparation, la filtration et la protection (amortissement), degeomembranes et de doublures d'argile geosynthetiques pourIe controle des filtrations, des geofilets et des geocomposespour l'ecoulement des eaux sous la surface et des geocellules

pour Ie controle de l' erosion. Le renfort de terre mecaniquementstabilisee en utilisant des geocellules, des geofilets et desgeotextiles peut aussi apporter de benefices substantiels dans lesprojets de re-enterrement en relation avec la reduction depression laterale de la terre contre les structures remblayees.D'autres aspects de la geotechnologie moderne qui peuvent ctreutiles pour les projets de re-enterrement incluent des micro-piquets et Ie cloutage de sols pour des fondations et pour desappuis dans les fouilles et la technologie de recouvrementsevapotranspiratoires pour etablir la profondeur du recouvrementdu sol necessaire pour isoler une structure ou un artefact desfluctuations d'humidite et de temperature. L' application optimalede ces geotechnologies requiert une comprehension desprincipes de base de la construction mecanique associes avecleur implementation, en complement de la connaissance desfacteurs qui influencent la preservation des sites archeologiques.

EI uso de geosinteticos para el re-enterramiento de sitiosarqueoI6gicos

Edward Kavazanjian,]r.

Resumen

Las aplicaciones de materiales geosinteticos en la practica de re-enterramiento incluyen el uso de geotextiles para la separaci6n,filtraci6n y protecci6n (amortiguamiento), de geomembranas yforros de arcilla geosinteicos para el control de las filtraciones,de georedes y geocompuestos para el drenaje bajo la superficie,y de geocelulas para el control de la erosi6n. El refuerzo de tierrameca,nicamente estabilizada utilizando geocelulas, georedes ygeotextiles tambien puede proveer beneficios sustanciales enlos proyectos de re-enterramiento en relaci6n con la reducci6nde presi6n lateral de la tierra contra las estructuras rellenadas.Otros aspectos de la geotecnologia moderna que pueden serutiles para proyectos de re-enterramiento incluyen las micro-estacas y el clavado de suelos para cimientos y para apoyos enlas excavaciones y la tecnologia de recubrimientosevapotranspirativos para establecer la profundidad derecubrimiento de suelo requerida para aislar a una estructura 0

un artefacto de las fluctuaciones de humedad y temperatura. Laaplicaci6n 6ptima de estas geotecnologias requiere de unacomprensi6n de los principios basicos de ingenieria asociadoscon su implementaci6n, junto con el conocimiento de losfactores que influencian la preservaci6n de sitios arqueo16gicos.