Geosynthetics International: Vol. 6, No. 4, 1999

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Technical Paper by I. Alobaidi and D.J. Hoare

MECHANISMS OF PUMPING AT THE SUBGRADE-SUBBASE INTERFACE OF HIGHWAY PAVEMENTS

ABSTRACT: Research into the mechanisms of pumping of fines at the subgrade-subbase interface of highway pavements is presented and discussed with respect to the behaviour of the subgrade-subbase system without a geotextile/transition layer and with a geotextile/transition layer at the interface. When no geotextile layer is used, softening of the subgrade soil occurs both outside the subbase particle-subgrade areas of contact due to swelling, and under the subbase particle-subgrade areas of contact due to cyclic loading. This softening causes the penetration of the subbase particles into the subgrade soil with consequent reduction of subbase layer thickness. When a geotextile layer is placed between the subbase and subgrade layers, it acts as a separator and, thus, prevents the intermixing of the two layers. However, the geotextile allows for rapid dissipation of the cyclic pore pressures under areas of contact and this causes erosion (pumping) of the subgrade surface. The geotextile layer reduces the static stress under areas of contact, but has a less significant effect on reducing the cyclic stress. Thus, the net effect is to cause an increase in the amount of pumping of fines. Geocomposites, which have a high compression modulus, high in-plane extensibility, low flexural stiffness, and ability to spread point loads from a subbase onto the subgrade, are more effective in reducing pumping. An anti-pumping geocomposite should also have a low short-term permeability to reduce the rapid dissipation of pore pressures during a loading cycle and sufficient permeability to dissipate pore pressures in the long term.

KEYWORDS: Geotextile, Geocomposite, Pumping, Anti-pumping, Fines, Pavement, Reinforcement, Separation, Drainage, Cyclic loading.

AUTHORS: I. Alobaidi, Research Fellow, and D.J. Hoare, Senior Lecturer, School of Civil Engineering, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom, Telephone: 44/121-414-5089, Telefax: 44/121-414-5051, E-mail: alobaidi@bham.civ-fs3.ac.uk and d.j.hoare@bham.ac.uk, respectively.

DATES: Original manuscript received 24 September 1998, revised version received 2 July 1999 and accepted 5 July 1999. Discussion open until 1 March 2000.

REFERENCE: Alobaidi, I. and Hoare, D.J., 1999, "Mechanisms of Pumping at the Subgrade-Subbase Interface of Highway Pavements", Geosynthetics International, Vol. 6, No. 4, pp. 241-259.

Technical Paper by M.J. Lopes and M.L. Lopes

SOIL-GEOSYNTHETIC INTERACTION - INFLUENCE OF SOIL PARTICLE SIZE AND GEOSYNTHETIC STRUCTURE

ABSTRACT: This paper reports the results of pullout tests on five different geosynthetics embedded in two different granular soils. Soil and geosynthetic properties are described and soil-geosynthetic interaction behaviour is studied. Based on the results of pullout tests, the influence of soil particle size, geosynthetic structure, and the role of geogrid bearing members are discussed. The main conclusion is that the influence of soil particle size on soil-geosynthetic interaction is important, but its significance depends on several factors. With geogrids, the relative sizes of soil particles and geogrid apertures, and the thickness of the geogrid bearing members, determine soil-geogrid interface shear resistance. A marked increase in soil-geogrid interface shear resistance was observed when the soil contained a significant percentage of particles with sizes slightly greater than the thickness of the geogrid bearing members, but smaller than the geogrid apertures. Tests, on geogrids in which the bearing members had been cut, show a significant decrease in soil-geogrid interface shear resistance. The influence of soil particle size is less important for geotextiles and geocomposites. Although the structure of geotextiles and geocomposites has an effect on soil-reinforcement interface behaviour, mobilised pullout resistance is also affected by the axial tensile stiffness of geotextiles and geocomposites.

KEYWORDS: Pullout test, Geogrid, Geotextile, Geocomposite, Interaction, Soil particle size, Geosynthetic structure, Bearing member.

AUTHORS: M.J. Lopes, Ph.D. candidate, and M.L. Lopes, Assistant Professor, Department of Civil Engineering, Geotechnical Division, University of Porto, Rua dos Bragas 4099 Porto Codex, Portugal, Telephone: 351/2-2041945, Telefax: 351/2-2003640, E-mail: lcosta@fe.up.pt.

DATES: Original manuscript received 5 January 1999, revised version received 3 June 1999 and accepted 29 June 1999. Discussion open until 1 March 2000.

REFERENCE: Lopes, M.J. and Lopes, M.L., 1999, "Soil-Geosynthetic Interaction - Influence of Soil Particle Size and Geosynthetic Structure", Geosynthetics International, Vol. 6, No. 4, pp. 261-282.

Technical Paper by E.M. Gallagher, W. Darbyshire and R.G. Warwick

PERFORMANCE TESTING OF LANDFILL GEOPROTECTORS: BACKGROUND, CRITIQUE, DEVELOPMENT, AND CURRENT UK PRACTICE

ABSTRACT: Geomembrane basal landfill liners are typically overlain by a stone drainage layer with a geoprotector layer placed between the drainage layer and the geomembrane to protect the geomembrane from environmental stress cracking caused by pressures transmitted through the stone drainage layer. The effectiveness of the geoprotector layer (which may be mineral or synthetic, or a combination of both) is demonstrated by means of a cylinder test using site specific materials. Current United Kingdom (UK) practice is to follow the Environment Agency (EA) method for this performance test, which evolved from guidelines produced in the early 1990s by the German "Quo Vadis Schutzlagen" working party. This paper reviews the background information for the test, in particular, the perceived deficiencies in the original German guidelines, and, hence, the need for the new EA methodology. A commentary on the new EA test method from the perspectives of regulator, designer, manufacturer, and test institution is included. The EA test standardises the apparatus, testing procedure, method of measuring deformations, and calculation of local strains.

KEYWORDS: Landfill, Geosynthetic, Protection efficiency, Performance evaluation, Laboratory test, Test standard.

AUTHORS: E.M. Gallagher, Senior Engineer, EDGE Consultants UK Limited, Atlas House, Simonsway, Manchester, M22 5PP, United Kingdom, Telephone: 44/161-436-6767, Telefax: 44/161-499-7987, E-mail: edge@edgeuk.u-net.com; W. Darbyshire, Regional Waste Technical Manager, Environment Agency, P.O. Box 12, Richard Fairclough House, Knutsford Road, Warrington, WA4 1HG, United Kingdom, Telephone: 44/1925-653999, Telefax: 44/1925-242209, E-mail: bill.darbyshire@environment-agency.gov.uk; and R.G. Warwick, Managing Director, Geofabrics Limited, P.O. Box 7, Wellington Mills, Liversedge, WF15 7XA, United Kingdom, Telephone: 44/1924-412477, Telefax: 44/1924-412375, E-mail: geofabrics@legend.co.uk.

DATES: Original manuscript received 24 February 1999, revised version received 21 June 1999 and accepted 29 June 1999. Discussion open until 1 March 2000.

REFERENCE: Gallagher, E.M., Darbyshire, W. and Warwick, R.G., 1999, "Performance Testing of Landfill Geoprotectors: Background, Critique, Development and Current UK Practice", Geosynthetics International, Vol. 6, No. 4, pp. 283-301.

Technical Paper by H. Zanzinger

EFFICIENCY OF GEOSYNTHETIC PROTECTION LAYERS FOR GEOMEMBRANE LINERS: PERFORMANCE IN A LARGE-SCALE MODEL TEST

ABSTRACT: Geomembrane landfill basal liners must be protected from damage caused by overlying, large-particle leachate collection/drainage systems. Geosynthetics, such as geotextiles and geocomposites, are being used as protection layers for geomembrane liners. To date, there has been no systematic observation as to the efficiency of these protection layers in a landfill. Large-scale tests were carried out at LGA-Geotechnical Institute in Nuremberg, Germany to model field conditions at the base of a landfill. The geomembranes showed no signs of damage after long-term loads of approximately 800 kPa. For the large-scale tests, higher maximum local strain values were measured than in standard, small-scale protection efficiency laboratory tests. Although the maximum strain measured in the large-scale tests exceeded 0.25%, which is the limiting maximum strain value according to the German approval procedure, the protection layers performed satisfactorily.

KEYWORDS: Geocomposite, Geomembrane, Geotextile, Geomat, Laboratory test, Landfill, Performance evaluation, Protection efficiency.

AUTHOR: H. Zanzinger, Geosynthetic Laboratory Manager, LGA-Geosynthetics Institute, Tillystr. 2, D-90431 Nuremberg, Germany, Telephone: 49/911-655-5563, Telefax: 49/911-655-5592, E-mail: gbza@lga.de.

DATES: Original manuscript received 24 April 1999, revised version received 22 July 1999 and accepted 27 July 1999. Discussion open until 1 March 2000.

REFERENCE: Zanzinger, H., 1999, "Efficiency of Geosynthetic Protection Layers for Geomembrane Liners: Performance in a Large-Scale Model Test", Geosynthetics International, Vol. 6, No. 4, pp. 303-317.