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Non-mineral steepwall liner systems for landfills

By Eugene M Gallagher, Adrian D Needham and Derek M Smith, EDGE Consultants UK Ltd

Abstract Waste depositories are often constructed in former quarries. This practice makes economic use of voids, is a natural progression from quarrying activities at a site and can lead to reasonably sympathetic restoration of what might otherwise be scars on the landscape. The engineering challenge in constructing such landfills includes developing an appropriate lining system for the base and the often steep-sided quarry walls, while at the same time maximising void space.

The sidewall lining system can be a mineral liner formed from natural soils, usually a stiff clay, or a non-mineral lining system using artificial materials. Various non-mineral steepwall lining systems have been installed in the UK over the last decade and there has been an ongoing process of technical development. These lining systems include the use of rows ofHDPE tubes, reinforced soil with polystyrene formers and various manufactured revetment systems. This paper reviews and compares the main features of these non-mineral lining systems.

Introduction Three general types of non-mineral, patented steepwall lining systems have been used at 11 UK landfills to date and range from small-scale trials of prototype systems to complete operational sidewall lining systems.

Vertical barrier system: Prototype, at one site (Joint Stocks, Durham). A triple row of vertical HDPE pipes, the central row filled with low permeability slurry.

Reinforced soi l w ith polysty rene former : Sidewalls inclined at up to 70degrees from the horizontal.

Sites include Albury, Guildford; Offham, Kent; Howley Park, Leeds; Houghton le Spring, Sunderland; Lisbane, Northern Ireland; Trehir, South Wales; and Cookstown in Northern Ireland.

Revetments: A family of four systems typically inclined at 50degreesto 90degrees from the horizontal. This includes, in its earliest manifestation, double rows of gabions sandwiching a geomembrane, through to frameworks rock bolted to the quarry wall with first gabions and later waste-filled bulk sacks and, more recently, a 'geoplate' derivative. Sites include Kendal Fell, Cumbria; High Moor, Oldham; and Humberfield in Humberside.

Landfill sidewalls - particular engineering considerations General principles of landfill design relating to containment and exclusion are given in Waste Management Paper 26B 1. Sidewalls generally start well above the sump and occasional exposure to leachate can be managed by incorporating preferential drainage paths to the landfill side of the liner, linked to the basal leachate drainage system.

A suitable regime of active groundwater management may be necessary for landfills constructed below the groundwater level, to control uplift pressures on the basal liner and hydrostatic pressures on sidewalls.

The stability of any steep slope should be properly assessed as part of the review of suitable sidewall liner options. The verticality and integrity of any quarry wall depend on its material type and fabric (ie fractures, including bedding planes, joints and faulting). A comprehensive assessment of the suitability of all parts of any quarry should be made at an early stage by a geotechnical specialist 2. The most appropriate sidewall lining system is chosen in the context of and following the initial assessment of site conditions.

Geomembranes such as HDPE in sidewalls require adequate protection against the potential effects of damage from the waste body and the sidewall structure which may lead to perforations and/or stress cracking. Other factors to be considered include interface shear effects caused by settlement of the waste, incorporation of gas monitoring systems as required and interfacing with basal, capping and perhaps even other adjacent sidewall liners.

Vertical barrier system System description The vertical barrier system (VBS) was devised by Durham County Waste Management. A 50m long and 10m high prototype version was trialed in an 18 months study.

The system is based upon three rows of vertical HDPE tubes of 220mm od,8mm wall thickness and 1. 2m height (Figures 1 and 2). The central row is filled with a 25%/50%/25% sand/bentonite/PFA slurry of average permeability 7. 9 x 10 m/s. The outer tubes are filled with gravel. The tubes are prefabricated in groups of five, each group welded to its neighbouring group of five tubes on site. The gap between the VBS and quarry wall is infilled with geosynthetic-reinforced soil.

Difficulties were encountered in forming welds between groups of tubes and so, for the trial, the vertical and horizontal joints were sealed and taped. The next VBS may be based upon moulded interlocking hollow blocks of rectangular and triangular cross-section rather than tubes.

Comment The VBS uses a low permeability mineral barrier contained within other liner material (the HDPE of the tubes). The designers claim that their system thereby constitutes a triple barrier (HDPE/slurry/HDPE) which would make it unique among sidewall systems.

Arguably, the VBS has a built-in leak detection mechanism with the potential to show if a breach in the mineral liner has occurred by observation ofdepressions in the level of slurry at the top of the tubes - provided that the slurry remains plastic. The system is inherently self-healing in that leaks detected in this manner can be repaired by adding further slurry. A major advantage in the VBS is that geomembrane protection layers are not required.

The outer tubes in the pilot trial were filled with gravel. It is claimed that this enables the tubes nearest to the waste to act as a conduit for gas and leachate. This would only be true if gas or leachate were actually able to enter the tubes. The tubes are not perforated.

Similarly, it is claimed the gravel-filled tubes on the side nearest the quarry wall facilitate gas monitoring. This is questionable because the tubes are designed as sealed. Monitoring every individual tube for gas would not be a practical or economic proposition.

The VBS is designed for essentially vertical sides of quarries. It is able to follow changes in the plan profile of (ideally) near vertical quarry walls, but cannot yet accommodate benches, for example, while maintaining continuity of its central core.

Reinforced soil with polystyrene formers System description This system has been used for quarries with side slopes up to 70degrees from the horizontal and has been reported extensively in the literature 4. Essentially, a geogrid-reinforced soil is built in 3m to 3. 5m high lifts, typically 2. 5m to 3m wide. Polystyrene formers inclined at a comparable slope angle to that of the quarry wall, form a facing upon which the geomembrane is laid directly (Figure 3).

The formers do not act as structural elements of the reinforced soil wall. Reinforcement requirements and grade of polystyrene are site specific. The geomembrane is overlain by a geocomposite then by 500mm of selected inert material. The system relies on lateral support being provided by the waste body. The system (Figure 4) is marketed by Cordek of West Sussex, manufacturer of the polystyrene formers, under the trade name Tipform 5. Construction can be undertaken by competent contractors.

The geomembrane is textured on one side with this side placed on the polystyrene. The protection layer to the waste side of the liner, also acting as leachate drainage and gas venting layer, is a geocomposite. This comprises a geonet (in contact with the geomembrane) and a suitable geotextile. The geonet-geomembrane interface is designed as the slip surface. It is reported that a vertical revetment version of the system is to be trialed in 2000.

Comment The attractions of this system include the clean, planar surface for the liner. The polystyrene is lightweight and, except in windy conditions, easily handled on site. Generally straight, smooth lines are achieved, leading to straightforward installation of the geomembrane. Special polystyrene units, eg for corners, are pre-cut at the factory. Minor adjustments can be readily made on site to correct misalignments.

Although there are various methods to incorporate a suitable high permeability gas detection layer and groundwater drainage layer if required, these have generally not been included in the reinforced soil wall.

The designer must assess and demonstrate adequate chemical resistance of the polystyrene to the leachates particular to the site, in case of leakage through the geomembrane. In selecting a suitable grade of polystyrene, the designer must also assess the effects of future compression under service loads.

Initial setting out is important to successful installation of this sidewall. Choice of this system depends on an adequate supply of suitable fill.

Patented revetment systems Introduction In this section, a progression of systems is described, one which developed from the original idea of sandwiching an impermeable geomembrane between two vertical layers of stone-filled gabions.

These systems are marketed and installed by Geotex Ground Services of Market Harborough (Figure 5a-5d).

Common features of this family of systems are:a single geomembrane liner generally high geoprotection constraints significant engineering input during design and construction increasing flexibility in following the quarry wall leading to gains in usable void.

Gabion-gabion system System description This system comprises two near-vertical layers of stone-filled gabions each side of the liner (Figure 5a). The gabions are not tied to the quarry wall. A geotextile protection layer is located each side of the liner. The liner and geoprotectors are 'held' between the gabions. Construction of the inner gabions occurs after installation of the liner. Construction ofmost of the sidewall takes place from on top of the waste body.

Comment This system has attractions in that the components fulfil many of the basic engineering requirements for a sidewall. The inner gabions are free draining for gas and leachate flows. The welded geomembrane is the impermeable liner. The outer gabions are free draining, thus relieving build-up ofhydrostatic pressures outside the cell and allowing gas monitoring.

There are, however, flaws. Most critically, the system is dependent on the double layer of gabions being structurally stable after the first few metres. The ability of the gabions to form vertical, stable walls to height is questionable. The quarry wall can also vary significantly in both vertical and horizontal planes. These constraints mean that the option of inclining the gabions at a constant angle to the vertical and using the constant profile of the quarry wall for support is limited in practice. If the outer gabions are positioned vertically, the gap between them and the quarry wall must be filled with suitable fill material. The outer, vertical gabions are unsupported to the waste side of the landfill until the liner is installed and the inner gabions and waste have been placed. Even then, lateral support from the waste can only be relied upon to a limited extent. Vertical sidewall construction also requires increasing quantities of backfill material with height, which in turn reduces the volume of potentially usable void space, making the system progressively less economical.

When this patent 6was filed (1990), the need for adequate geomembrane protection against damage including stress cracking effects was not of primary concern. The first author's experience in using the cylinder test some years later, with typical profiles ofmaterials proposed in this original system, is that local strains would be at least one order ofmagnitude greater than the permissible local strain of 0. 25%.

Framework with gabion system System description The next system 8uses a galvanised steel framework rather than gabions as the outer layer (Figure 5b and Figure 6). This framework is a 1. 5m by 1. 5m grid using cruciform members forming nodal points and tied to the quarry wall using grouted rock bolts. A galvanised wire mesh is fixed to the front of this framework. The mesh-covered framework is then backfilled with self-compacting, free-draining fill. Stone-filled gabions are used to the landfill side and lie against the framework, sandwiching the liner and its geoprotectors in place (Figure 7).

Comment The framework system is better able to follow the quarry wall in two planes than the previous system. Although the framework allows the offset from the quarry wall to liner to be kept to a minimum, the whole system still uses gabions to the landfill side. These are dependent on their own stability with some contribution from the lateral pressure of waste. This restricts the amount by which the whole system can follow the quarry wall profile, limiting the advantages of this system.

Framework with bulk bag system System description The revised system is shown schematically in Figure 5c. This system still uses the framework on the quarry wall side of the liner. Minor modifications include the use of prefabricated double bays of framework and reduced stone size.

Various protection layers have been used to the quarry wall side of the liner. HDPE and flexible polyethylene have been used as liner geomembrane. The geomembrane and quarry side geoprotector are fixed to the framework at 3m vertical intervals (Figure 8).

A multifunctional combination ofmaterials is used between the geomembrane and the main body of waste, replacing the geotextile (protection layer) and vertical layer of gabions (leachate drainage and gas venting) of the previous system. The combination comprises a geocomposite in contact with the geomembrane and a layer of bulk sacks filled with fine pulverised waste (Figure 9). The geocomposite layer is not fixed, merely held in place by the pressure from the bulk sacks, and forms the leachate drainage and gas venting layer, also contributing to the geomembrane protection. The geocomposite to geomembrane forms a low shear interface. The waste-filled sacks act as a buffer zone and provide protection of the geomembrane.

Comment The two primary reasons to avoid using gabions between the liner and the waste were:

continued concerns for stability the high degree of geomembrane protection required.

There is the potential for interface shear forces to be transmitted to the liner by settlement of the waste (up to 30% of the height). The combined bulk sack/geocomposite layer takes account of these forces by:

incorporation of a low shear geomembrane to geocomposite interface provision of 1m horizontal overlaps (per 3m lift) in the geocomposite.

Extensive testing on a wide range of materials was commissioned to demonstrate adequate geomembrane protection efficiency.

Other variables included mesh sizing and stone grading.

A complication in using the cylinder test to model sidewall systems is that, unlike a typical basal lining situation where drainage aggregate is on one side only, sidewalls have drainage aggregate plus mesh plus protection layers to one side and, potentially, an equally complex profile of materials to the other.

The test was not devised for such situations, and engineering judgement is required to find a suitable solution.

Subject to a suitable waste stream, the use of waste-filled bulk sacks is attractive economically. Waste is used as a construction material, thereby saving on costs of materials as well as increasing usable void space. Using waste rather than primary aggregate in this context reduces the environmental impact of these works to some extent.

The inherent flexibility of this system allows fuller use to be made of the flexibility of the framework and has been capable of limiting the offset from quarry wall to geomembrane to about 0. 7m to 1. 5m.

'Geoplate'prototype System description An alternative 9, shown schematically in Figure 5d, has been developed comprising galvanised steel panels (Figure 10). The 1. 4m by 1. 4m 'geoplates'are joined with a simple channel and rock-bolted to the quarry wall at the nodes using a special anchor plate, then backfilled with stone (Figure 11). The geoplates form a relatively smooth surface for subsequent layers.

Comment Site construction time and the number of work operations required to reach the lining stage are considerably reduced compared to the framework system.

Setting out is straightforward and generally it is a simpler system to build which should lead to cost savings. The relatively smooth, planar surface for deployment of the liner means a geomembrane protection layer of reduced mass per unit area, and hence cost, is required. The developers hope there may be longer term potential to have no geoprotector on the quarry wall side of the liner.

The designer must assess the effects of future corrosion with this and all systems incorporating metal or degradable components.

The system is less flexible than the framework in that variations in two planes at one time can only be achieved by creating special units. The geoplates can readily change orientation in either the vertical or horizontal plane at one time. The geoplate system appears to be able to interface with the standard framework system and the potential exists for a combination approach at one site.

Discussion Table 1 compares the six systems. Only two of the systems have any significant penetration of the limited market for these specialised products, ie reinforced soil with polystyrene formers and the framework with waste-filled bulk sacks. Both are geo- membrane based systems.

The overall flexibility of these systems is an important consideration. The designer must assess, in terms of the particular site, how sensitive the system is to initial setting out, how well it can subsequently cope with minor variations and how well it can maintain a close distance to the quarry wall.

The framework with waste-filled bulk sacks probably scores higher under such an assessment.

Against these advantages must be weighed the need for increased engineering input at design and construction stages because of the added complexity inherent in the system.

Summary A number of innovative, patented steepwall lining systems are available.

A lining system must be designed and assessed as a complete package: small changes to one component can significantly influence the behaviour of the rest of the system.

Suitability must be assessed on a site by site basis.

Economic assessments should be made on a life cycle basis including the cost of void space.

The more developed systems described here can provide adequate, robust lining solutions in the appropriate circumstances.

References

1 Department of the Environment (1995). Waste Management Paper 26B, Landfill design, construction and operational practice, HMSO, London.

2 The Quarry Regulations (1999). Statutory Instrument 1999/2024, The Stationery Office, London.

3 UK Patent GB 2292577 B (1997). Barrier system, Durham County Waste Management Company.

4 Di Stefano AB and Needham AD (1994). Geosynthetic lining of steep wall quarry landfills - utilising polystyrene facings, Wastes Management, pp 26-29, February 1994.

5 UK Patent GB 2276899 (1994). Improvement in or relating to filling in a hollow in the ground, Cordek.

6 UK Patent GB 2239036 B (1993). Lining of landfill sites, Tarmac Econowaste.

7 Environment Agency (1998). A methodology for cylinder testing of protectors for geomembranes, Environment Agency.

8 UK Patent Application GB 2293849 A (1994). Lining for a landfill site, British Reinforced Concrete Engineering.

9 International Patent Application PCT/GB97/01692 (1997). Lining of landfill sites, Woodman IP Holdings.

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