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Compacted fill is a major component of most construction works and primarily used in engineering constructions such as backfilling excavations and retaining structures, earth dams, road pavement, flood barriers, temporary and permanent construction platforms, landfill liners, and landscaping. The specific construction will determine the nature of the fill material to be used, however with an increasing awareness of sustainability, practising engineers are being urged to consider locally available fill materials, which include clayey based soils, as an alternative to freshly quarried granular materials.

In contrast, large depressions left within the landscape, both in the UK and globally, are usually the end result of coal and other industrial mining operations (Cairns, 2011). It is reported that for every tonne of coal extracted 20 t of overburden materials are removed (Grimshaw, 1992). The British Coal Authority has estimated that there are 10 000 abandoned mine workings across the UK (MMSD, 2002). The British Geological Society (BGS) published the results of a survey conducted across the UK, in conjunction with the coal and planning authorities, regarding opencast coal mining. In the period 2006–2007, 37 opencast sites were operational and a further 14 proposed sites (totalling 1548 ha) were granted planning permission. These statistics give some indication of the scale of existing and future derelict land created as a result of coal and other industrial mining operations in the UK alone. In general, the depressions caused by opencast coal mines have been previously backfilled with site-won materials (generally fine-grained soils). However, due to restrictions imposed to building development within green-field sites, the ever-increasing demand for urbanisation and the overdevelopment of inner cities, the limited availability of suitable sites has led to previously backfilled excavations becoming the target of major reclamation schemes, mainly in the form of housing developments.

‘Compaction' is a technique primarily used in placing fills, and the process essentially ‘densifies' the fills by reducing the air voids. For ease of construction and to achieve target performance the water content of the fill is often controlled. The practice of backfilling is the principal technique used in re-landscaping. Provided the fill is placed in a controlled manner – that is, engineered fill – the fill behaviour should be reasonably predictable on the basis of average properties. However, in the majority of cases encountered in practice the method of fill placement is largely unknown (in this case the fills are regarded as ‘unengineered') and therefore warranting an assessment of ground conditions through a detailed subsurface investigation, which is time-consuming, costly and unwelcome to a potential client (Cairns, 2011).

The problems associated with compacted fill can be better described in the following matter. The source materials can be stiff/very stiff with low compressibility. However, the excavation of these materials and subsequent reconstruction in the form of compaction will result in a complex micro/macro structure. Large aggregates (also called packets or lumps), constituted of fine particles and held together by suction (or bonding of other forms), are generally saturated and the space between them (macro voids) are filled with water and air. The first difficulty which is encountered as result of air in the voids is the invalidity of the traditional effective stress concept for predicting the performance of the end product. The presence of macro voids leads to rapid infiltration of water that can cause reduced strength and aggregate slippage, marked with potential collapse settlement, which is often the case in unengineered fills. It is also true that engineered fill can lead to swelling (Sivakumar et al., 2010b) (or heave) upon post-construction wetting which is equally undesirable and can contribute to differential settlement and excessive lateral pressures on retaining structures.

It is generally considered that structures can be constructed upon compacted fill provided that the fill has been compacted to the required specifications (Cairns 2011). However, it has been recognised that regardless of the level of control exercised in the characterisation of the proposed fill material, it is extremely rare that the method of fill placement can be regulated, primarily due to the continuing daily site traffic operations (Legget, 1967). Nonetheless, in relation to the performance of on-site compactions, specifications are outlined within the Design Manual for Roads and Bridges (Highways Agency, 1995) and the Specification for Highway Works (Highways Agency, 2004).

Research into the behaviour of compacted soils (unsaturated soil) has spanned the past 50 years, however a surge in activity and interest has occurred within the last two decades, largely instigated by environmental factors which include sustainability factors (Boyd and Sivakumar, 2011; Sivakumar et al., 2010b, 2010c). It is true to state that the outcome of the academic research is far from conclusive (there is a lack of user-friendly predictive tools for day-to-day applications) and practising engineers are faced with having to use statistical information and empirical approaches together with reliable site-monitoring tools. The articles presented in the themed issues shed useful light on this complex subject.

Puttock et al. (2011) report on the construction of a working platform for a holiday resort in Morocco. About 350 000 m³ of highly variable site-won materials were used to construct an 8 m thick compacted fill platform, covering 620 ha, to support low-rise terraced villas on pad or strip footing. The authors adopted a unique combination of UK techniques and French technical guidelines (GTR) to ensure the performance of the end product. This was also complemented with on-site measurements including pressuremeter testing.

McNicholl (2011) reports on the design and construction of filled building platforms and highlights the fact that some practitioners do not always follow the best practice; he proposes a 10-step approach (from desk study to final verification of end product using field tests) to ensure the quality of the end product. The paper also reports on some interesting case histories around the world which clearly demonstrate the consequence of not following good practice when dealing with compacted fills. These include catastrophic failure claiming many lives and the disruption of infrastructure.

Ground improvement, as a replacement to deep foundation is also seen as an environmentally friendly construction technique (Sivakumar et al., 2010a). In that respect, Hughes et al. (2011) report a carefully executed laboratory study on the benefit of using red gypsum–ground granulated blastfurnace slag (GGBS) binders (by–products of other industrial outputs) to improve the strength of weak soils. The authors conclude that a rapid increase in strength could be achieved by adding small amount of Portland cement. Rahmat and Kinuthia (2011) report on another extensive laboratory-based programme that investigated the stabilisation of sulfate-bearing soils with sustainable binders which include wastepaper sludge ash, lime, cement and blastfurnace slag. It was shown that certain combinations of binders resulted in increased unconfined compressive strength and reduced linear expansion.

Ground improvement can also be achieved through deep compaction, generally in granular fill or natural deposits. Watts and Cooper (2011) report on the benefit of using the rapid impact compactor (RIC) method for enhancing load-bearing capacity and settlement control based on several case studies around the world including on-site end-product verification. The paper also reports some interesting information on vibration data for the RIC process (critical distance between existing structures and point of application) and cautions on any attempt to mitigate the transmission of vibrations using trenches.

The soil–water characteristic curve is a key tool, widely used for assessing the behaviour of unsaturated compacted clay soils. Zielinski et al. (2011) report a large-scale model study carried out inside an environmental chamber at the University of Strathclyde, with the intention of reproducing the full-scale behaviour of the flood defence system that is under construction at Gaston, Scotland. The model study consisted of controlled wetting and drying cycles together with sufficient instrumentation for suction and water-content measurements. The authors conclude that the hydraulic cycles result in progressive reduction of water storage and increase in the air-entry value, both of which are associated with an accumulation of shrinkage.

The academic research in the area of unsaturated soils surged in recent years, and the paper by El Mountassir et al. (2011) makes an attempt to relate these studies to practical applications with respect to flood embankment in the Bengawan Solo River, Indonesia, which has a history of overflowing and associated failures. Laboratory tests on samples extracted from the site were carried out under saturated and unsaturated conditions with suction control in order to examine the volumetric and shear-strength characteristics. An interesting point to note from the conclusion is that the reduction in shear strength due to saturation was not necessarily the triggering mechanism for failure, rather poorly compacted fill (i.e. low dry density) associated with additional loading due to remedial work could have caused volumetric collapse.

The macro pores and cracks caused by shrinkage always favour the infiltration of surface water and that can be detrimental in terms of volumetric response of compacted clay fills, particularly if they are unengineered. Li et al. (2011) report on field permeability measurements using the double-infiltration method and associated laboratory studies on block samples. The authors conclude that the hydraulic conductivity at shallow depth is, understandably, significantly higher than at greater depths; this they attribute to macro pores. The authors also highlight the effects of sample size on the measured hydraulic conductivity if the sample contains large pores and cracks.

Czerewko et al. (2011) report an interesting case history with respect to the degradation of pyritic mudrocks when exposed to water and oxygen in the A46 road improvement scheme between Newark and Widmerpool in Nottinghamshire, UK. The paper provides interesting background information regarding the degradation of pyritic mudrocks and potential problems to structures and supporting strata, together with evidence of real-time weathering of mudstone and the effects of cycles of wetting and drying on the weathering process. The paper also reports on the procedure adopted for conditioning the material before placement.

One legacy of the industrialisation in the early twentieth century is the poor quality of land in many parts of the UK, often backfilled with uncontrolled clay-based excavated materials. Palmer and Wilson (2011) present a case history regarding the reclamation of former ironstone workings in Corby for 5500 new homes and associated amenities. The site investigation work carried out in 2005 as part of the project is compared with previous data published by Charles and Watts (2001) based on investigations in 1971. The paper highlights significant changes in the fill material over time with a general increase in void ratio. The reclamation project considered large-scale field trials conducted under various configurations, involving surcharge, in conjunction with cone penetration test evaluation before and after treatment, which confirmed some notable changes in the undrained strength. The paper concludes with an interesting point that, irrespective of the sources of the fill, it remains a young soil and, as such, is susceptible to rapid change, the effects of which need to be assessed and controlled in the geotechnical design.

Another case history regarding reclamation of derelict lands is reported by Jarvis (2011); it deals with a unique procedure adopted to reclaim two adjacent sites with a sustainable approach that utilised the site-won material for filling purposes. The development also includes one of northern Europe's largest ‘made slopes', with a maximum height of 38 m. The performance of the end product is also being monitored with series of displacement measurements together with pore-water-pressure-monitoring devices.

Most of the articles summarised above deal with reclamation of lands left derelict after their use during the industrial age. Davies and McIlquham (2011) report on reclamation of land in a different context for the Port Botany expansion project in Sydney, Australia. About 63 ha of land were reclaimed by filling dredged material from new navigation channel, making the project sustainable. Various compaction techniques, which included impact rolling, vibro compaction and dynamic compaction (depending on the location of the fill), were used to ensure the quality of the end product. Site monitoring, including an earth pressure cell located on the retaining structure, enabled comparison of the project's performance with predictions from finite-element analyses. Trigger levels were also set on the instrumentation to allow considerations for a possible design review.

High Speed 1, a landmark project that links the UK to the European high-speed rail network, has been in operational since 2007. Barker and Phear (2011) report on the construction techniques and strategy adopted in one of the project's largest earthworks contracts, located at Ebbsfleet in Kent. The work consisted of many different infrastructure elements and the minimisation of waste was the prime objective of the earthwork strategy. The paper reports on the success of the ‘single point compaction' strategy to deal with highly variable fill materials used at the site.

Brocklebank and Sharp (2011) report on the re-use of carbonate sedimentary rock excavated from the 75 km long Arabian Chanel in the United Arab Emirates. Due to natural variability of carbonate sediments and poor geotechnical properties, a site-specific compaction technique was developed for successful re-use of the material. An interesting approach was used to group materials in to two different types based on percentage of air voids, which allowed these materials to be deployed at different locations based on presumed post-construction events such as rapid water infiltration, which may cause collapse settlement of fill with a high proportion of air voids.

Unsaturated loessial soils are notorious, as the particles are aggregated together by weak bonding, and can go through rapid degradation upon wetting. When excavated from sites, these materials are usually stockpiled and often considered unacceptable for re-use. Roohnavaz et al. (2011) report a sustainable re-use of these materials for earthwork constructions; the study involved an extensive testing programme consisting of suction measurements, California bearing ratio, compaction and compressibility. The study highlights the importance of the type of compaction methods, which can be beneficial in terms of breaking down the aggregated structure of loessial fills at the time of placement.

The common usage in most, if not all of the papers is the desire to consider compacted fill using site-won material to meet sustainability requirements in the construction industry, in the knowledge that the compacted fills, if not placed with stringent conditions, could lead to adverse effects. Academic research in the area of compacted fill has progressed significantly over the last decade, though it must be admitted that there is no specific user-friendly model available to practising engineers for day-to-day design applications. However, it is my opinion that by taking account of the information in the papers presented here, practising engineers and academics could work together to enhance the understanding of this complex subject, which would be to the benefit of both parties.

These themed issues in Geotechnical Engineering on ‘compacted fills' have been a tremendous success. This could not have been achieved without the help of my peers: Dr P. Smith, Dr P. Ingram, Dr A. Pellew and Dr D. Shohet. I wish to extend my sincere thanks to Miss Sohini Banerjee for coordinating the review process in a meticulous manner. Sincere thanks also go to Mr P. Allanson for preparing the final articles in time for the scheduled publications.