Copyright © 2015 Dr T. Yalcin Irfan - Landline: 01912134546 Email: Y. Irfan@Cundall.com)

Dr T. Yalcin Irfan & Dr Eric Peng

KEY GEOTECHNICAL CHALLENGES:• Minimise significant risks to road/railway/river/residential areas/

aquifer.• Complex site and landfill geology/underground mineworkings.• Ground investigation techniques to characterise landfill materials.• Determination of geotechnical design parameters for extremely

variable waste materials.• Shear strength of waste & slope stability methods of analysis/FOS.• Prediction of ongoing/future settlement & methods of analysis• Foundations for buildings (piling through complex ground

conditions/aggressive ground/groundwater for concrete).• Car park areas; flexible design for future settlement/strict settlement

criteria for disabled car parking.• Design of piled wall to retain building platforms.• Gas emissions/economic off site disposal of surplus waste. • Design of (contamination) remediation strategy under strict UK

legislation without compromising slope stability / settlement.

• Geoenvironmen

Geotechnical Investigation and Design Aspects of Developing Sloping Landfill Sites

MATLOCK LANDFILL:Municipal landfill/side valley infill/1950-1974/550m long/37m high/9 platforms, variable thickness up to 11.6m deep.

SITE INVESTIGATION:Detailed desk study

Targeted intrusive geotechnical/geoenvironmental GI:• Percussion /rotary / window sampling/ trial pits• SPT/CPT • Groundwater/gas installations within landfill/perimeter• Regular monitoring (continued after construction)

Geotechnical/Chemical Laboratory Testing:• Classification. Bulk density/mc, Compaction• Undrained/triaxial strength ((c/Ø)), Consolidation• Rock strength (UCS, PLS)• Aggressivity/ combustability• Extensive contamination testing

Literature reviews for landfill slope stability/ settlement data

A regional leisure centre complex has recently been constructed at a 37m high sloping refuse landfill site, Matlock, UK. Design and construction insteeply sloping landfill sites present significant geotechnical challenges as well as environmental considerations. This is not only related to complexground conditions in these sites but also in characterising and testing variable landfill materials in the laboratory and insitu, as well as in theapplication of traditional slope stability, settlement and foundation design methods of analyses. This paper summarizes the results of a comprehensivesite investigation undertaken at Matlock and subsequently derivation of the design input parameters using a combination of traditional andnonstandard testing methods, and then describes the methods of analysis adopted for assessing the long term settlement and slope stability inundertaking the infrastructure and foundation designs at the landfill site.

CONCLUSIONS:

• Developing old and sloping landfill sites requires not only costly land and groundwater remediation measures, but also substantial slope and settlement remedial works.

• Geotechnical parameters for analysis of refuse can be derived from a combination of traditional and nonstandard testing/ investigation methods/back analyses and observational methods and used in approximate numerical relationships (both traditional and theoretical methods) for geotechnical analysis and design.

• Conservative approach is required for selection of design parameters and analysis methods.

Brownfield Briefing Remediation Innovation Awards 2010: Most Sustainable Remediation Project

Derbyshire Times Business Awards 2011:Most Innovative Design

DESIGN METHODOLOGY: For Matlock landfill site specific geotechnical design parameters were derived from a combination of traditional and other insitu and laboratory methods together with those derived from back analysis and data from other similar landfill sites.These were then used in approximate numerical relationships based on traditional and theoretically derived methods of analysis in carrying out the slope stabilisation, foundation and infrastructure design.

SETTLEMENT ANALYSIS:Short and Long Term Settlement Assessment using Sowers Method(10.5m thick landfill area)

Geotechnical investigation and design aspects of developing sloping landfill sites

Étude géotechnique et aspects de conception lies au développement de site d’enfouissement en pente

T. Y. Irfan*1, J. Peng1

1 Cundall, UK, * Corresponding Author

ABSTRACT A regional leisure centre complex has recently been constructed at a 37m high sloping refuse landfill site, Matlock, UK. De-sign and construction in steeply sloping landfill sites present significant geotechnical challenges as well as environmental considerations. This is not only related to complex ground conditions in these sites but also in characterising and testing variable landfill materials in the laboratory and insitu, as well as in the application of traditional slope stability, settlement and foundation design methods of analyses. The traditional theories may not be adequate for analysing refuse landfill materials that undergo large deformations without failure. This paper summarizes the results of a comprehensive site investigation undertaken to characterise the landfill materials at Matlock and subsequently derivation of the design input parameters from a combination of traditional and nonstandard testing methods together with the published values determined from back analyses and observational methods on similar landfill sites. It then describes the methods of analysis adopted for assessing the long term settlement and slope stability in undertaking the infrastructure and foundation designs at Matlock.

RÉSUMÉ A Matlock au Royaume-Uni, un centre de loisirs régional a récemment été construit sur un site d’enfouissement de déchets avec une hauteur de pente de 37m. La conception et la construction de bâtiments sur des décharges à sol incliné présentent d’importants défis géotechniques et enjeux écologiques. Ceux-ci ne sont pas seulement liés à la nature complexe du terrain, mais aussi à la characterisation des différents matériaux d’enfouissement en laboratoire et in situ, ainsi que, dans le cadre des applications de stabilité sur pente traditio-nelle, au développement des methods d’analyses pour la conception des fondations. Les theories traditionelles ne sont pas adaptées aux analyses des matériaux d’enfouissement qui subissent des deformation importantes sans échec. Cet article résume les résultats d’une en-quête menée sur site ayant pour but la characterisation des matériuax d’enfouissement à Matlock, et par la suite, la derivation des para-mètres de conception depuis une combinaison des méthodes de tests inédites et traditionnelles avec les valeurs publiées à partir d’analyses et observations réalisées sur des sites d’enfouissement similaires. L’article décrit ensuite les méthodes d’analyses employées pour évaluer l’établissement à long terme at la stabilité du sol incliné dans le cadre de la conception des fondations et des infrastuctures.

1 INTRODUCTION

More and more major infrastructure schemes are be-ing constructed in historical landfill sites, due to shortage of land in rapidly developing urban areas. These sites bring with them their own construction difficulties, not only related to somehow complex ground conditions but also significant land and groundwater contamination which may require costly engineering and remediation solutions. Furthermore, re-engineering steeply sloping landfill sites present

significant geotechnical challenges as well as envi-ronmental considerations.

One major difficulty is that the quantification of geotechnical properties of landfill can be very diffi-cult as such materials are very heterogeneous, which may also include a large proportion of regradable components that change their properties with time. Similarly, the soil mechanics methods of analysis de-veloped for traditional soils may not be wholly appli-cable to very heterogeneous and degradable materi-als, which undergo large deformations before failure.

It may be necessary to adopt novel techniques of in-vestigation and characterisation of these materials.

A regional leisure centre complex together with its associated infrastructure has been constructed at a steeply sloping historic municipal landfill site, Mat-lock, Derbyshire. A number of major challenges faced the geotechnical design team at the onset of the project as described in this paper.

2 CHARACTERISATION OF GEOTECHNICAL PROPERTIES OF LANDFILL SITES

The parameters required for geotechnical design vary with the waste type, composition, depth, method of compaction and the rate of decomposition amongst others. The rate of compression is further influenced by several factors, including the effects of time, tem-perature and other environmental conditions. In addi-tion, the geotechnical properties of the refuse chang-es with time. It is difficult to derive reasonable geotechnical parameters from variable refuse fill ma-terials. Testing in the laboratory of small samples is generally not possible due to the presence of many larger fragments even within the ‘clayey’ members.

Laboratory and field test methods that have been used to quantify the geotechnical properties of urban refuse can be grouped as: In situ CPT/SPT tests; large scale laboratory and field classification tests (Landva & Clark 1990); laboratory shear strength tests (Landva & Clark 1990; Stark et al 2009); full scale loading tests; geophysical methods (Sharma et al. 1990), back derivation from field tests (Singh & Murphy 1990; Huvaj-Sarihan & Stark 2008); and modelling studies (Sowers 1973; Edil et al. 1990; Golders 2008; Watts et al 2006).

3 SHEAR STRENGTH & SLOPE STABILITY

There are concerns that the Mohr-Coulomb theory may not adequately account for refuse materials which undergo large deformations without failure. Secondly, the incompatibility of strains that produce shear failure in soil and those that would produce shear failure in refuse suggests that stability analysis of a refuse fill may be related more to its settlement and foundation bearing capacity than to its slope fail-

ure. Singh & Murphy (1990) considered that for a typical refuse fill of moderate height (less than 60m), sitting on a strong foundation soil, a classical soil slope analysis can be performed using appropriate shear strength parameters.

Estimates of shear strength for refuse materials have been determined using; (i) laboratory testing, (ii) back calculations of field tests, and (iii) back analysis of failures. The laboratory investigations treated the refuse as ‘cohesionless’ material, whereas the back calculating approach indicated the refuse to possess both cohesion and frictional properties.

Figure 1. Recommended Shear Strength Parameters for Refuse

Landfills (after Singh & Murphy 1990)

An extensive review of shear strength of refuse-

landfills in North America is presented in Singh & Murphy (1990), who considered that the back calcu-lated data summarized in Figure 1 represented the minimum available strength of refuse and was there-fore conservative. Stark & Huvaj-Sarihan (2009) summarized worldwide published laboratory and field strength data and back analyses of case histories in order to develop a better understanding of shear strength of waste to be used in static and seismic slope stability analyses of landfills. Reported values of Φ’ ranged from 10 to 530, while c’ ranged from 0 to 67 kPa. The wide range is caused by the numerous factors that influence the test results including the in-herent heterogeneous composition of waste, age of waste, degree of decomposition, specimen size, unit weight, pre-test processing, test method and condi-tions. These factors together with non-standardized sampling methods and small sample size and limited

shear displacement or axial strain imposed during the laboratory shear testing may have created considera-ble scatter in reported results. They suggested that back-analysis of failed waste slopes should be used to guide the laboratory strength parameters.

Factors of safety applied to slope analysis of land-fills vary from 1.3 to 1.6 for new landfills, depending on the waste type and risk ranking and from 1.4 to 1.6 for old landfills (Koerner & Daniel 1970). Possi-ble presence of local weaker zones and other sources of low strength (e.g. seepage along the laminated composite structure of the refuse) could contribute to an overall low factor of safety.

4 SETTLEMENT IN REFUSE LANDFILLS

Large settlements occur within first few months of completion of the landfill, followed by substantial secondary settlement, with the magnitude of settle-ment decreasing over time and with increasing depth of fill. Historical records indicate that total settlement within a refuse landfill generally ranges from 5 to 30 percent of the original thickness, with most of the settlement occurring in the first two years.

The settlement mechanism in landfills is complex and controlled by many factors, commencing with immediate compression of successive layers of fill from crushing, distortion, bending and reorientation of particles, long term movements as biodegradation causes a reduction in volume and by creep compres-sion as the particles become more closely packed.

The geotechnical parameters required for settle-ment calculations vary with the waste type, composi-tion, depth, time, method of compaction and the de-composition rate amongst others. Laboratory/insitu tests as well as back derivation from field tests/records have been used by various researchers to quantify the geotechnical properties of refuse. Large plate load tests are useful in determining the settlement properties, but a large number of tests are required to characterize the variable refuse. Monitor-ing records and back derivations from full scale em-bankment loading can be used, which would proba-bly give more representative parameters.

Even though experimental records show similarity between the refuse settlement-time relationships and the primary and secondary stages of soil consolida-

tion, the application of consolidation theory to land-fill materials is at its best approximate because some of the assumptions of the classical theory are not sat-isfied. In particular, the presence of gas causes in-complete saturation, both the solids and the fluids are not incompressible and chemical and biological pro-cesses can dominate the consolidation itself.

Predicting large amounts of secondary settlement using analytical soil mechanics methods is difficult and therefore, a simple model combining all stages of compression is required. Three commonly used ana-lytical methods are those of Sowers (1973), Gibson & Lo (1961) and the power creep law. The power creep model provides a better representation of the settlement data than the rheological model of Gibson & Lo, but the latter has parameters that can be as-signed physical meaning and reflect the effects of certain refuse placement conditions (Edil et al. 1990). The model proposed by Sowers is the most widely used approach for settlement prediction, which con-siders primary and secondary consolidations sepa-rately. The simplicity of the model has encouraged its continued use. More recently, ISP model (Olivier et al 2003), Thomas & Cooke (2007) model, which considers the biological degradation processes, and the Hydro-Biological-Mechanical (HBM) model (Golders 2008), which is complex in its formulation and remains a complex tool to implement, have been proposed to analyse settlement in landfills. All these methods require detailed waste placement records, which are generally not available for old landfills.

5 MATLOCK LANDFILL SITE

The Matlock landfill site, 550m long, is located in a steep side valley of the River Derwent, surrounded by existing residential developments, and comprises eight platforms separated by steep (up to 400) slopes (Figure 2). The landfilling activity commenced circa 1955, constructed with no formally engineered clay capping or basal liner, and was closed in 1973/74. Little data existed from the landfilling operations. The site is underlain by mudstones overlying lime-stones, with the head deposits being present on the hillsides. Three disused shafts are present in Platform P7, related to a historically mined lead ore body within the limestone.

Figure 2. Site Location Plan of Matlock Landfill

Siting Considerations. Following consideration of a number of viable options and taking into account the architectural requirements, the client’s prefer-ences and the investigation results, the leisure centre itself was sited on P3. The car parks were located on P4 and P5 and the access road to the site was moved as far as possible to the perimeter of the landfill to minimize costs related to ground improvement. Cost-ly offsite disposal of contaminated landfill was min-imized by optimizing cutting and filling earthworks.

Figure 3. Representative E-W and N-S Geological Cross Sections

6 GROUND INVESTIGATION

A targeted ground investigation was carried out at the site to address the key geotechnical and geoenviron-mental issues and to provide data for scheme design and site remediation. The investigation comprised a number of cable percussion/rotary/window sample boreholes, machine dug trial pits, SPT/CPT tests and installation of gas/groundwater standpipes in selected boreholes.

The investigation indicated a variable thickness of landfill materials, up to 11.6m thick, underlain by up to 3m thick head deposits overlying completely weathered mudstones, progressing into very weak to weak rock (Figure 3).

The landfill materials typically comprised sandy gravelly clay/clayey sandy gravel, with varying pro-portions of rock and concrete cobbles/boulders. The upper 1m or so of the landfill was typically of a more soil like composition, with less than 5% by volume waste present and below 1m depth 5% to 20%, local-ly up to 70%. The waste typically comprised met-als/plastics/glass/newspapers/pottery/bulbs/clothes/plastic bags, slag and clinker. The CPTs indicated much more variable materials than those recorded by the boreholes. A shallow groundwater table within the landfill, and a deeper water table within the mud-stones exist at the site.

7 ENGINEERING PROPERTIES OF REFUSE FILL AND OTHER STRATA AT MATLOCK

Due to limited recovery of undisturbed samples, la-boratory testing was restricted to determination of compaction characteristics, PSD and unit weight. The strength and compressibility properties of the refuse were indirectly derived from CPT/SPT results, using approximate relationships developed for traditional soils and these were then compared with published data generally from field/laboratory studies.

The properties of the natural soils and mudstones were determined in the laboratory on the samples re-covered from the boreholes/trial pits. These results, together with those indirectly derived from in situ testing, were used in determining the design parame-ters and carrying out appropriate numerical analyses.

Unit Weight. Unit weight of refuse landfill is of-ten difficult to obtain because of erratic and often coarse nature of waste materials. Dry unit weights could be expected to fall within a range of 7 to 14kN/m3 (Landva & Clark 1990). It is expected that the unit weight would increase with the age of the re-fuse, but likely to be erratic within a landfill. Dry unit weight of the Matlock landfill varied between 8 and 15kN/m3, determined on U100 samples. These values probably overestimated the unit weight due to possi-ble densification during driving of the U100 tube.

Undrained Strength. Approximate undrained strength, Cu, values of the cohesive landfill units were indirectly determined from SPT/CPT results, using Stroud’s approximate equation, with the Cu be-ing varying from 22 to 60kPa for N=5 to 14. The ap-proximate relationships based on CPT (CIRIA, 1987) were used to derive the undrained strength/ deforma-bility/stiffness properties of the ‘cohesive’ and ‘gran-ular’ units of the landfill. No apparent strength in-crease with depth was observed in the landfill.

Shear Strength. Design shear strength adopted for the landfill was conservatively taken as Φ’=20o, c’=0kPa from Figure 1 and based on resistivity anal-ysis of existing Matlock landfill slopes (Φ’=15o to 25o, c’=0kPa). For sensitivity analysis a higher shear strength of Φ’=24o, c’=0kPa was also adopted. A lower strength of Φ’=18o, c’=0kPa was selected for the potential failure surface along the landfill/natural soil contact. The strength properties adopted for the soil/rock units underlying the landfill were based on the results of laboratory tests (not reported here).

Compressibility. Limited data is available in the UK for the compressibility characteristics of old re-fuse fills (Charles 1993, Watts et al 2006). Approxi-mate values of mv for the granular and cohesive sub-units of the landfill were derived from the CPT and SPT results, using approximate relationships estab-lished for traditional soils. The mv values derived from CPT results were generally between 0.20 and 0.45m2/MN. The values derived from N values gave similar results to those derived from CPT tests (gen-erally varying between 0.10 and 0.50m2/MN). The constrained modulus values for the granular subunits were generally between 7.5MN/m2 and 24MN/m2.

8 SLOPE STABILITY ASSESMENT

The traditional soil mechanics approach was adopted for the slope stability analysis in view of the overall height being 32m, the landfill materials being in gen-eral of solid constituents and with the landfill bearing on reasonably strong foundation soils.

A minimum FOS of 1.6 was adopted for assessing the long term stability of the existing waste slopes. The numerical analyses indicated adequate FOS for the overall slope and most batters, but grossly inade-quate for the slope between P3 and P4 behind the

proposed centre location. The existing steep slope (at 400) in this section was already showing signs of lo-cal failure. Cutting into the landfill deposits was re-quired elsewhere at the site. Based on the results of the stability analyses the cut gradient was recom-mended to be no steeper than 4(H) in 1(V).

Numerical analysis indicated that a retaining struc-ture would be required to improve the stability of the landfill slope between P3 and P4. A contiguous pile retaining wall (socketed into the mudstones) was considered as the most feasible stabilisation measure together with some regrading of the slope face above in order to achieve a minimum factor of safety of 1.6.

Sensitivity analysis was carried out in order to find the optimum wall height/alignment, taking into con-sideration the required FOS, the need to minimise settlement from backfill, the need to minimise waste disposal and reduce the land take from the upper plat-form, allocated as the main car parking area. These factors resulted in the design of a retained height of 2.5m, and the backslope trimmed to 4(H):1(V).

9 SETTLEMENT ASSESSMENT

Settlement analyses were carried out using the sim-plistic Sowers’ and Gibson and Lo’s methods in or-der to estimate and compare the likely past and future settlements at the site, including proposed ‘surcharge loads’ (e.g. clean cover layer in landscaped areas).

As the initial landfill properties were unkown, sensitivity analyses were carried out using a range of data from published sources. Initial void ratio, com-pression index and secondary compression factors used in the Sowers’ method were based on Sowers (1973).

The analyses indicated a settlement of up to 1.7m since completion of landfill in 1970 and between 0.1 and 0.25m for the next 50 years in areas where the initial landfill thickness was about 10.5m (Figure 4). This would increase up to 0.36m for the next 50 years if 1m of surcharge fill was to be placed on the landfill. Gibson and Lo’s method gave more variable results and generally higher settlements depending on the range of parameters chosen.

The future settlements are likely to be highly vari-able, the largest settlement occurring in the central areas where the landfill is thickest.

Figure 4. Settlement versus Time, 10.5m Thick Landfill

A great proportion of the settlement would take place within the initial few years of completion of the scheme. In central parts of the proposed car parks, the settlement would reach 0.20m in the next 10 years resulting from continuing consolidation of the landfill and that imposed by a 0.5m thick geogrid re-inforced stone layer to be placed as part of car park construction.

Due to anticipated significant differential settle-ment, special measures would be required at the in-terfaces between the piled structures and non piled access locations. Design of piled foundations and the retaining wall are outside the scope of this paper.

10 CONCLUSIONS

Developing old and sloping landfill sites for major building schemes requires not only costly land and groundwater remediation measures, but also substan-tial ground improvement including slope and settle-ment remedial measures.

The geotechnical parameters required for the anal-ysis of refuse landfills can be derived from a combi-nation of traditional and nonstandard testing methods together with those determined from back analyses and observational methods of similar sites. The pa-rameters such derived can then be used in approxi-

mate numerical relationships, both traditional soil mechanics and theoretically derived methods of analysis, for assessing settlement and slope stability and carrying out foundation designs in landfill sites as was done for this sloping site at Matlock. Howev-er, a conservative approach is necessary in the choice of the design parameters and factors of safety to be used in these analyses.

ACKNOWLEDGEMENTS.

This paper is published with the kind permission of the Derbyshire Dales District Council (the Client).

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