RE-USE AND RECYCLING OF UTILITY TRENCH ARISINGS. Introduction

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1 This digest is one of a series produced by the Aggregates Advisory Service to provide information on aggregates efficiency issues. The aim of this service, funded by the Department of the Environment, Transport and the Regions, is to assist the Government to achieve its objective of reducing the construction industry s dependence on landwon primary aggregates and increasing the contribution from secondary and recycled materials. Further information on aggregates efficiency issues, whether relating to primary, secondary or recycled materials, can be obtained from the Aggregates Advisory Service on Freephone no , or visit the AAS website at RE-USE AND RECYCLING OF UTILITY TRENCH ARISINGS This digest briefly describes the current situation with regard to the re-use and recycling of utility trench arisings. It reviews current practice and describes some of the new and recent developments in waste minimisation. Contact details for some of the case studies are given at the end of the digest. Introduction The majority of our service infrastructure lies underground, often beneath our road network, in the form of buried water mains, sewers, gas mains, electricity and telecommunications cables etc. As a result, the utility companies frequently need to dig holes to repair and renew this infrastructure as well as provide new services to developments as required. Recent developments in trenchless technology mean that there are now circumstances where work can be carried out without having to dig up the road. However, in the majority of cases, it is still not feasible or cost effective to use such methods and excavating a hole or trench is the only way to carry out the work. In the backfilling and reinstatement of trenches, distinct priorities arise because of the differing interests of the parties involved. The main concern of the utility company is to protect its services whilst the highway authority s purpose is to maintain the standard of the road surface and its structural integrity. In addition, the highway authority seeks to minimise disruption to the public and the utility (and significantly, its contractor) seeks to complete work in the most cost effective manner. Legislation has been introduced to regulate these interests and to improve the quality of highway reinstatement (discussed later). The most common backfilling solution currently adopted is to dispose of the excavated material offsite and import high quality, primary granular material. This has cost implications in terms of: disposal of the excavated material, including landfill tax in some cases transport for disposal of the excavated material and for importation of new material purchase of new material In addition, there are environmental impacts associated with the transport of materials, the disposal of excavated materials and the use of primary aggregate resources. This Digest is concerned only with the backfilling of the trench i.e. that part of the reinstatement below the road formation. Road recycling is dealt with in Digests 056 and 060. This Digest reviews current practice and possible alternatives and investigates developing technologies which may contribute to a reduction in the reliance on primary aggregate and an increase in the use of Symonds 1

2 secondary and recycled materials for utility trench backfill. Legal Framework The current legislation governing the work of the utilities in the highway (defined as including roads, footways and verges) is the New Roads and Street Works Act 1991 (NRSWA), which is supported by secondary legislation (regulations) and approved codes of practice. The Approved Code of Practice regarding the reinstatement of trenches within the highway is the Specification for the Reinstatement of Openings in Highways, June 1992 produced by The Department of Transport, The Welsh Office and The Scottish Office and published by HMSO. This NRSWA Specification is commonly known as the HAUC Specification, after the Highway Authorities and Utilities Committee who administer and monitor its use. Technical Constraints The Specification sets out the types and depths of materials to be used for backfill/reinstatement. The requirements vary according to the type of road or footway, with more onerous requirements for heavily trafficked roads. The Specification requires the carriageway reinstatement to be bituminous or concrete to match the existing construction usually with a granular sub-base immediately below this. However, there are options for the use of class A graded granular materials as a sub-base in type 3 and 4 carriageways. The Specification is flexible in that it allows most types of materials to be used for the general backfill below the sub-base, subject to them meeting certain criteria. The Highway Agency s Type 1 sub-base is most frequently used for general backfilling because of its consistent quality and its handling and compaction properties. Whilst the Specification allows most materials to be used for general backfilling, it places strict limits on the placement moisture content because of its effect on the potential for future settlement; voids may form during compaction in material that is too dry and material that is too wet may shrink as it dries out. This precludes the use of many as-dug materials, because, in practice, cohesive materials (clays and silts) are less likely to meet the moisture requirements than granular ones (sands and gravels). In general, contractors tend to prefer to use granular material for backfilling because it is easier to place and compact, especially in the particularly difficult conditions encountered in a narrow trench. Often there is very little storage area for excavated material by the side of a trench especially in urban areas where it is desirable and necessary to minimise disruption to traffic and pedestrians. In this case it is necessary to remove excavated material from the site regardless of whether or not it is suitable for re-use. The type of work, the size and the timescale often dictate the methods to be employed. For example, trenchless technology is not a feasible option for the emergency repair of a leaking water main. There are two options for reinstatement permitted under NRSWA: i. first time reinstatement i.e. permanent reinstatement from the outset; ii. interim reinstatement followed by permanent reinstatement at a later date of those layers not completed to permanent standards first time. Methods to minimise waste Utility companies and contractors can minimise the use of primary aggregates by following the waste hierarchy : reduce the quantity of materials used re-use excavated materials at the work site Symonds 2

3 recycle previously excavated material stockpiled off-site, possibly treating unsuitable materials to render them useable finally dispose of unwanted arisings Reduce: First time reinstatement by its nature uses less material overall and is usually less disruptive, but is not always possible because of the specialist nature of the workforce, who may not be skilled both in trenching and in road surfacing. New excavation techniques, similar to keyhole surgery, can significantly reduce the quantity of reinstatement materials necessary. Re-use: Site operatives can be trained to decide, on a site specific basis, whether an excavated material is suitable for immediate re-use on site above the installation with granular sub-base used as the next layer. Re-use may not always be possible because the excavated material may be unsuitable, too variable, too wet or storage space may be limited. The next best alternative to re-use on site is to remove the material to a depot. In some cases, particularly if storage space at the work site is the only constraint, it may be re-useable immediately elsewhere. A recycling facility as described will require planning consent and is likely to require a Waste Management Licence for waste transfer and possibly for waste disposal, depending on the requirements of the Environment Agency. New and Recent Developments Proposed Revision to HAUC Specification The HAUC Specification is under review. Currently it allows for the re-use and recycling of materials including, where necessary, stabilisation or other measures to render them suitable for re-use. However, the specification requires all materials to be classified through laboratory or field testing against the same criteria applicable to new materials and their use is then limited to the permitted use of each class of materials. There is a limited opportunity for the development of new materials that do not fall into the established classifications, although they may provide an equivalent (or better) performance. Such development is restricted by the method type of specification and relatively short duration relating to performance requirements. Recycle: If the material is unsuitable for immediate reuse, it can be removed to a depot and processed to render it suitable. This can be difficult because materials returned to the depot are often a mixture of different types that require segregating in order to form a useful and reliable backfill. An alternative is to stockpile recycled material from external sources, such as crushed concrete or bituminous planings, to blend with the excavated materials to form a suitable backfill. Secondary aggregates, such as pulverised fuel ash (PFA), colliery spoil or slate waste, could be similarly used. Understandably, whilst highway authorities are generally supportive of the re-use and recycling of materials, they are reluctant to agree to departures from the specification since the long term risks are unknown. The revised specification therefore needs to address the requirement for performance of reinstatements to allow greater freedom of choice in materials whilst not placing greater risk on the highway authorities. Soil Stabilisation As stated previously, certain soils are unsuitable by virtue of being too soft or too wet. However, they can be treated by a process known as soil stabilisation, which involves mixing lime and/or cement to the Symonds 3

4 excavated materials. This can be carried out at the work site for immediate re-use or at the depot where the processed material can be stockpiled for future use. Lime/cement stabilisation is the subject of AAS Digest 058. A study for UK Water Industry Research Ltd (UKWIR), which included experimental trials at the Transport Research Laboratory (TRL), investigated the potential of this method. It was recommended that field trials were carried out and a Supervisor s Guide was produced. At present the technique is not widely used, mainly due to concerns over lack of supervision and control of the site mixing process, although this could be addressed by improved training. There is also uncertainty over the long-term performance of backfill treated in this way. However, the Highways Agency uses lime as a construction material and its use is known in the construction industry. Current research on the re-use of arisings In 1998/99, UKWIR continued their sponsorship of work into the re-use of arisings by re-commissioning TRL to review the potential for processing these materials for re-use in reinstatements. The research specifically dealt with the identification of the best treatment options available, their resultant engineering properties and their environmental impact. In 1999/2000, UKWIR extended the research to investigate the engineering properties by means of laboratory testing with the aim of trialling the most promising materials at reinstatement sites. A DETR Partners in Innovation project, involving a consortium of industrial partners, local authorities, private consultants, BRE and universities, is developing novel technology to enable trench arisings and quarry wastes, in particular cohesive materials (clays and silts), to be recycled into useable aggregates for a range of applications. Key to the research work is the production of synthetic aggregates by the addition of appropriate industrial byproducts and wastes to excavated materials to improve their engineering quality. Potential uses include backfill material, concrete aggregates and various other applications. The methodology adopted specifically addresses environmental considerations i.e. the need for sustainable technology and also the need for cost competitive production processes. The technology is currently undergoing pilot scale production trials. The findings will be disseminated as a Best Practice Guide for recycling and upgrading waste materials for use in the construction and maintenance of UK roads. Transco (a part of BG plc) is currently supporting a significant research programme in the area of reinstatement technology. The work is being carried out in conjunction with BG Technology (the research and development business unit of BG plc). The areas of interest are: recycling of trench arisings, their re-use as granular and bound fill material, investigation of means of determining the overall performance of completed reinstatements. The results of this work will be disseminated in due course. Recent Initiatives Recent initiatives have investigated the feasibility of single organisations taking responsibility for accepting and recycling streetworks waste from a large area, possibly from many different utilities and contractors. Such schemes can produce economies of scale with respect to the treatment and throughput of material and obtaining and complying with Waste Management licences. The environmental impact of transporting materials to and from the site must be taken into account, but overall such schemes are considered to have a Symonds 4

5 positive environmental impact and there may be cost savings in the long term once initial set up costs have been overcome. Several examples of such schemes are described below: BG Energy Services has been involved in the setting up of a plant in West Lothian for recycling surplus materials from streetworks. The service will be offered to the utilities and the plant expects to handle 100,000 tonnes per year. It is planned to expand the geographical coverage of the service. The initiative, which is part of a European study, is backed by Friends of the Earth. The Northern Local Distribution Zone of Transco (a part of BG plc) has recently been involved in the crushing and screening of utility arisings for their re-use in reinstatement operations. This smallscale local initiative included the trial crushing and re-use of materials for finefill (pipe surround) and as an aggregate for foamed concrete. Save Waste and Prosper (SWAP) has set up a Leeds based regional aggregates forum with a number of local authorities and three major utilities. Research was carried out on the factors enabling the recycling of excavation waste, and plans were developed for a pilot project recycling programme of road excavation waste. The clay content found in Yorkshire and the HAUC Specification put this programme on hold but developments in binding of the clay and the proposed HAUC revision mean that the project will soon be resurrected. 4. UK WATER INDUSTRY RESEARCH LTD A Supervisor s Guide for the Re-use of Excavated Material, UKWIR, 1996 Contact details TRL Research Projects Contact Marilyn Burtwell, Transport Research Laboratory, Old Wokingham Road, Crowthorne, Berkshire RG45 6AU Tel: Fax: mburtwell@trl.co.uk DETR Partners in Innovation Project Contact: Dr Nizar Ghazireh (Senior Research Technologist), Tarmac Quarry Products, Technical Centre, Millfields Road, Ettingshall, Wolverhampton WV4 6JP Tel: Fax: SWAP Project Contact: Karen Houghton, Save Waste & Prosper, 74 Kirkgate, Leeds LS2 7DJ Tel: Fax: References 1. The Street Works (Reinstatement) Regulations Statutory Instruments 1992 No.1689, HMSO 2. THE DEPARTMENT OF TRANSPORT, THE WELSH OFFICE AND THE SCOTTISH OFFICE, Specification for the Reinstatement of Openings in Highways, HMSO, June UK WATER INDUSTRY RESEARCH LTD Re-use of Excavated Material, UKWIR, 1996 Symonds 5