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Landfill Engineering

Structural integrity of steep side-wall landfill lining systems: the role of waste/barrier interaction

Research personnel: Neil Dixon, Project Leader; Samson Ng'ambi, Research Assistant; Andrew Connell, Research Student

Collaborators: Golder Associates (UK) Ltd; The Environment Agency; Cambridge Insitu; Shanks

Funding: EPSRC Research Grant; Golder Associates (UK) Ltd

Project duration: 36 months

Start date: 21 February 1999

Introduction:
Increasing demand in the UK for landfill facilities for the disposal of waste has led to the use of void spaces with steep (near vertical, >70°) side wall, e.g. quarries, to optimise the space. The performance and structural integrity of the lining systems for steep side-wall is controlled by the interaction between the waste body and lining system.

Aim:
To develop a numerical model for the assessment of the structural integrity and performance of steep side wall lining systems during waste placement and in the long term.

Objectives:
To investigate factors which influence the structural integrity and performance of steep side wall lining systems by:

1. obtaining the relevant waste material properties required as input parameters in the numerical model. The pressuremeter technique is being used to measure in situ stresses and stiffness parameters for municipal solid waste material (MSW) in landfills.
2. validating the numerical model by instrumentation of a steep side wall liner of a landfill in order to obtain deformations and stresses within the lining system and at the waste/liner interface. The data is used to validate the numerical model.

Outcome:
To disseminate the results of the study to researchers, designers, operators, regulators and relevant organisations. In addition, the ultimate and most important benefit is to the general public as a result of the design, construction and operation of cost effective full containment landfill facilities.

Current status:
A series of pressuremeter testing has been conducted in waste of various ages, <1 year old (fresh waste), 5 years old, and 15 years old (degraded). Instrumentation of a landfill steep side-wall clay liner is on going. Pressures cells, piezometers, settlement magnets, and inclinometer tubes have been installed in the liner that is now at 10m high from the datum base. Preliminary readings of the vibrating wire instruments and settlement magnets indicate that they are operating satisfactorily. Numerical modelling of the site being instrumented is also going on using FLAC.

Key outputs:
Reports of pressuremeter test results, a technical report on waste mechanics, a report on instrumentation proposal, and two research papers have been produced.

List of publications:
SC Ng'ambi, N Dixon, DRV Jones and AK Connell (2001), Structural integrity of a steep side slope mineral barrier: Field monitoring. Proc. 8th Waste Management and Landfill Symposium, Sardinia, October 2001, Vol. 3, 83-92.
N Dixon, SC Ng'ambi, DRV Jones, and AK Connell (2000), The role of waste deformations on landfill steep side wall lining stability. Proc.: WASTECON 2000, Cape Town, September 2000. Vol. 2 379-388.
N Dixon, DRV Jones, and RW Whittle (1999), Mechanical properties of household waste: in situ assessment using pressuremeters. Proc. 7th Waste Management and Landfill Symposium, Sardinia, October 1999, Vol. 4, 453-460.

Integrity of steep-sided landfill lining systems

Research student: Andrew Connell

Supervisors: Neil Dixon, Ashraf El-Hamalawi, Russell Jones, Golder Associates (UK) Ltd.

Funder: EPSRC Project Studentship

Start date: July 1999

Context/Background:
Modern landfill sites must prevent the leakage of leachate into the surrounding groundwater, and typically sites must be lined so as to provide an impermeable barrier. Landfill sites that are situated in rock voids can be lined with either mineral or geosynthetic systems. The structural integrity of the lining system and hence the impermeable performance as a barrier to gas and leachate, is primarily controlled by the interaction between the waste and the barrier. Investigations have demonstrated that optimisation of landfill barrier design can not be achieved without specific geotechnical properties and time dependent variations in these properties as the waste degrades.

However, the landfill industry is presently constructing barrier systems for steep sided slopes (e.g. reinforced earth, polystyrene face supports and buttressed clay barriers) without a full understanding of the factors controlling either the short-term construction related, or of the long-term waste degradation controlled deformations. The aim of this research project is to assess and model existing lining systems of various types, so that their structural integrity can be assessed. A suitable steep-sided landfill site was selected, and instrumentation of the site commenced in June 2000 (see above). The site was instrumented so that data could be provided for use in the validation of a computer model.

Aim:
To investigate the factors controlling the waste/barrier interaction, with specific consideration to the integrity of steep-sided landfill lining systems.

Objectives:

1. To review the factors which control the design, construction and regulation of steep sided landfill lining systems.
2. To identify and assess the time dependent changes in waste mechanics, including the relationships of waste type composition at various depths.
3. To develop a procedure for numerical modelling of waste/barrier interaction in steep sided landfill sites.
4. To validate the model using field data procured from instrumented landfill sites.
5. To apply the model to assess recently proposed steep sided lining systems using the modelling procedures developed.

Publications Produced:
Connell, A. K., 2000. Integrity of steep-sided landfill lining systems. Unpub. Paper presented to the 6th YGES, City University, London.
Connell, A. K. & Jefferson, I., 2000. Foundation of Frozen Ground. Proc. 6th Ground Improvement Techniques Symposium, Singapore.
Jones, R, Dixon, N. & Connell, A. Effect of Landfill Construction Activities on Mobilised Interface Shear Strength. Proc. 2nd European Geosynthetics Conference, Bologna. Vol. 2, pp581-586.

Investigating the stability of landfill capping systems

Research student: Patience Kamugisha

Supervisors: Neil Dixon; Russell Jones (Golder Associates (UK) Ltd.)

Funder: Golder Associates; School of Civil and Building Engineering, Loughborough University

Start date: 1 January 1999

Context/Background:
To date, landfills have been the most economical and environmentally acceptable method of disposing of residual solid waste. Landfill closure often involves the construction of engineered cover systems designed to prevent percolation of water into the underlying waste and to counteract the uncontrolled escape of methane gas. The design and construction of the final cover is as important as the liner at the base of the landfill.

There is an increasing use of thin plastic planar materials (geosynthetics) in combination with soils in cover systems. Steeper slopes are being constructed in order to maximise the available void space. Geosynthetics introduce planes of weakness, whose interface shear strength is crucial for slope stability. The large direct shear box that is used to measure interface shear strengths has to be specially adapted for testing geosynthetics and soils at low normal stresses suitable for capping systems. There is a dearth of information on shear strength at low normal stresses between typically used drainage geocomposites and other capping materials e.g. soil, geomembranes. The effects of the orientation of the geocomposite core on the interface shear strength are also unknown.

At present, there are no known methods available for measuring the magnitude, distribution and time dependent variation of pore water pressures generated at interfaces during shear. The controlling effective stresses need to be quantified for better design.

Aims and Objectives:
The specific aims and objectives of the project include:

• Developing methods for obtaining interface shear strength parameters at low normal stresses relevant to cover system design.
• Investigating the shear behaviour of recently developed geosynthetic geocomposite drainage materials.
• Assessing the role of time-dependent pore water pressures generated at interfaces during shearing, on the structural stability of cover systems.

Method and Current Status:
An on-going literature survey is being carried out to provide background information and to keep informed about related work around the world. The project is in its third year and is mainly laboratory based using internationally accepted testing standards in relation to current best practice on site. The large direct shear box (300mm x 300mm) is the main equipment involved. As at March 2001, objectives one and two have been achieved and objective three is in progress.

Benefits/Expected Outcomes:
Increased confidence in measured interface shear strength parameters and quantification of the controlling effective stresses during shearing will allow a better definition of the capping slope design parameters. This will lead to better slope designs and fewer failures. Measurement of pore water pressures during shear could lead to a reduction of shear testing time and cost of testing.

Publications:
Blümel, W., Stoewahse, C., Dixon, N., Kamugisha, P. & Jones, D.R.V. (2000). British-German cooperative research on geosynthetic friction testing methods. Proceedings of 2nd European Geosynthetics Conference, Bologna, Italy, October, Eds Cancelli, A., Cazzuffi, A. & Soccodato, C., 2, 923-927.
Dixon, N., Kamugisha, P. & Jones, D.R.V. (2000). Geosynthetic interface testing at low normal stresses: Design implications. Proceedings of 2nd European Geosynthetics Conference, Bologna, Italy, October, Eds Cancelli, A., Cazzuffi, A. & Soccodato, C., 2, 535-540.

Investigation of Libyan clays for its potential use as an engineered clay liner in waste disposal landfill sites

Research student: Abdussalam Ahmed Saleh

Supervisors: Ashraf El-Hamalawi, Paul Fleming

Funder: The Ministry of Higher Education - Libya

Start date: 1 October 2000

Context/Background:
The use of clay as a barrier or liner in order to retard the leachate produced from landfill sites has become very popular in recent years. In the case of Libya where the research will focus, the use of clay materials as a barrier or liner in waste disposal landfill sites has not been used yet, and no research has been conducted on the use of clays in the area of landfill sites. All industrial and domestic waste is dumped in large collection sites without protection. These waste sites have started causing severe problems to the environment and to groundwater, where water wells are drilled for drinking and irrigation purposes. The primary aim of this project will be to find out how to benefit from the natural clays in Libya, particularly, because there are large clay deposits that could be utilised as clay liners.

Aims and Objectives:
The aim of this research is to investigate and examine experimentally the suitability of Libyan clays to function as an engineered clay liner, and to investigate the use of additives to the clays to improve its performance.

Method and Current Status:
The research will involve a field study to collect clay samples for laboratory tests. A review of the literature related to this project is continuing in order to determine the state of the art on research in landfill liner engineering, and to identify areas where new research could be applied. A review of UK and international guidelines for landfill liners is also being carried out.

Benefits/Expected Outcome:
The research will focus on the suitability of clay in Libya in order to use it as an engineered clay liner for waste disposal landfill sites. It will be of great and direct benefit for the Libyan authorities to use the local natural resources, which have not been used before in this area.