Geosynthetics International: Vol. 9, No. 3, 2002

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Technical Paper by D.T. Bergado, G.A. Lorenzo, and P.V. Long

LIMIT EQUILIBRIUM METHOD BACK ANALYSES OF GEOTEXTILE-REINFORCED EMBANKMENTS ON SOFT BANGKOK CLAY – A CASE STUDY

ABSTRACT: The limit equilibrium method (LEM) was used to back analyze the performance
of three full-scale test embankments constructed over soft Bangkok clay. The
control embankment was unreinforced while the other two embankments were reinforced
with multiple layers of low-strength geotextile and with a single layer of highstrength
geotextile, respectively. The analyses results confirmed the embankment performance
indicating a two-stage failure mode of the reinforced embankments, namely:
(i) the critical stage followed by (ii) the collapse stage. The critical stage occurred at the
onset of plastic deformation of the soft clay foundation. The collapse stage occurred at
the onset of embankment collapse when the reinforcement was on the verge of experiencing
tensile failure or rupture. The critical reinforcement strain at the critical stage
was independent of geotextile stiffness and on subsoil conditions and was 2.5 to 3%.
The localized strain corresponds to the mobilized strain at the collapse stage in excess
of the critical strain and can be estimated using a new method requiring two parameters:
the in-soil stiffness and the reinforcement inclination factor. Back-analyses confirmed
the horizontal and bisectional directions of reinforcement tensile force corresponding
to the critical and collapse heights, respectively.

KEYWORDS: Geotextile, Critical strain, Localized strain, Inclination factor, Soft
ground.

AUTHORS: D.T. Bergado and G.A. Lorenzo, Professor and Doctoral Candidate,
respectively, School of Civil Engineering, Asian Institute of Technology, P.O. Box 4,
Klong Luang, Pathumthani 12120, Thailand, Telephone: 66/2 524-5521, Telefax: 66/
2-524-6050, E-mail: bergado@ait.ac.th; and P.V. Long, Senior Design Engineer,
Hydraulic Engineering Consultants No. 2, 169 Tran Quoc Thao, District 3, Ho Chi
Minh, Vietnam, Tel: 848/844-5225, Fax: 848/844-4958.

DATE: Original manuscript submitted 23 July 2002, revised version received 30 July
2002, and accepted 17 September 2002. Discussion open until 1 May 2003.

REFERENCE: Bergado, D.T., Lorenzo, G.A., and Long, P.V., “Limit Equilibrium
Method Back Analyses of Geotextile-Reinforced Embankments on Soft Bangkok Clay
- A Case Study”, Geosynthetics International, Vol. 9, No. 3, pp. 217-245.

Technical Paper by J.H. Greenwood

THE EFFECT OF INSTALLATION DAMAGE ON THE LONG-TERM DESIGN STRENGTH OF A REINFORCING GEOSYNTHETIC

ABSTRACT: Current practice in the design of reinforced soil is to calculate the longterm
strength of a reinforcement damaged during installation by multiplying the two
partial safety factors RFID and RFCR . This procedure assumes that there is no synergy
between them. To test this hypothesis, stepped isothermal method creep-rupture tests
were performed on a polyester geosynthetic in its undamaged and damaged states. The
geosynthetic chosen and the method of damage were specifically chosen to yield a 30%
reduction in strength with the minimum of scatter. From simple tensile testing, the
reduction factor for installation damage, RFID , was measured to be 1.42. Stepped isothermal
method tests on the undamaged geosynthetic gave a creep-rupture characteristic
with RFCR = 1.70 for 106 hours (114 years). Thus, the predicted reduction factor for
the long-term strength of the damaged geosynthetic is (RFCR)(RFID) = 2.41. The measured
reduction factor was 2.26, 7% less than predicted. It is recommended that the
practice of using (RFCR)(RFID) is upheld in the knowledge that the prediction will be slightly conservative.

KEYWORDS: Installation damage, Reinforcement, Creep, Design, Reduction factor.

AUTHOR: J.H. Greenwood, ERA Technology Ltd., Cleeve Road, Leatherhead, Surrey,
KT22 7SA, United Kingdom, Telephone: 44/1372 367005, Telefax: 44/1372
367070, E-mail: john.greenwood@era.co.uk.

DATE: Original manuscript submitted 11 April 2002, revised version received and
accepted 2 October 2002. Discussion open until 1 May 2003.

REFERENCE: Greenwood, J.H., 2002, “The Effect of Installation Damage on the
Long-Term Design Strength of a Reinforcing Geosynthetic”, Geosynthetics
International, Vol. 9, No. 3, pp. 247-258.

Technical Paper by H.B. Poorooshasb

SUBSIDENCE EVALUATION OF GEOTEXTILEREINFORCED GRAVEL MATS BRIDGING A SINKHOLE

ABSTRACT: Compacted gravel mats, reinforced with layers of geosynthetic, are
often used in areas prone to the occurrence of sinkholes. The purpose of the geosynthetic
reinforcement is to bridge the cavities formed by the sinkholes. Since compaction
of the overlying gravel inevitably involves the possibility of damaging the
reinforcement, as well as higher costs, it is desirable to have a method of analysis that
will provide a balance between cost and the best technical design. That is, enable the
engineer to choose between a less compact gravel mat with a higher degree of reinforcement
versus a more compact gravel mat containing less reinforcement. The
present paper shows that, for a very dense gravel mat, geotextile reinforcement acts
solely as a separator (it prevents the caving of the material into the sinkhole). For
medium dense gravel mats, and gravel mats with a minimum degree of compaction,
the geotextile reinforcement plays a more significant role in reducing the settlement at
the surface of the gravel mat.

KEYWORDS: Geosynthetic, Geotextile, Gravel mat, Sinkhole, Settlement,
CANAsand constitutive model, ID numerical technique.

AUTHOR: H.B. Poorooshasb, Professor, Department of Building, Civil and
Environmental Engineering, Concordia University, Montreal, Quebec, Canada, H3G
1M8, Telephone: 1/514-848-7806, Telefax: 1/514-848-2809, E-mail:
pooroos@civil.concordia.ca.

DATE: Original manuscript submitted 14 October 2001, revised version received 18
September 2002, and accepted 20 September 2002. Discussion open until 1 May 2003.

REFERENCE: Poorooshasb, H.B., 2002, “Subsidence Evaluation of Geotextile-
Reinforced Gravel Mats Bridging a Sinkhole”, Geosynthetics International, Vol. 9,
No. 3, pp. 259-282.

Technical Paper by D.J. Elton and I. Peggs

GEOMEMBRANE RESEARCH NEEDS

ABSTRACT: A US National Science Foundation/Auburn University Workshop was
held in Summer 2000 to identify geomembrane-related topics that require research.
Three publications were produced: a geomembrane state-of-practice paper, a paper on
research needs, and a final paper on organizing similar workshops. The present paper
focuses on geomembrane research needs, with the intent of moving the profession forward
by identifying what new knowledge/research is needed and how to get it. Stimulating
discussion on the subject is a secondary purpose of the paper. Ten professionals,
selected for their expertise and experience, attended the Workshop. Most facets of the
geomembrane/waste disposal industry were represented. The participants almost exclusively
represented United States geomembrane practice. Most of the discussions related
to high-density polyethylene, linear low-density polyethylene, polyvinyl chloride, and
polypropylene geomembranes. The Workshop considered seventeen topics related to
geomembranes in waste, water, and product containment systems. The topics were seam
tests, seam strength, wrinkles, lifetime predictions, seams types, geomembranes on
steep walls, thermal seams in PVC geomembranes, geomembranes in bioreactors, durability,
remote leak repair, defect significance, shear displacements, seams in different
geomembrane types, leak location and significance, puncture protection, and multiaxial
testing.

KEYWORDS: Geomembrane, Research, Liner, Landfill.

AUTHORS: D.J. Elton, Associate Professor, Civil Engineering Department, Auburn
University, Alabama, 36849, USA, Tel: 1/334-844-6285, Fax: 1/334-844-6290, Email:
elton@eng.auburn.edu; and I. Peggs, President, I-Corp International, 6072 N.
Ocean Blvd, Ocean Ridge, Florida 33435-5210, USA, Tel: 1/561-369-0795, Fax: 1/
561-369-0895, E-mail: Geoicorp@aol.com.

DATE: Original manuscript submitted 7 March 2002, revised version received and
accepted 12 November 2002. Discussion open until 1 May 2003.

REFERENCE: Elton, D.J. and Peggs, I., 2002, “Geomembrane Research Needs”,
Geosynthetics International, Vol. 9, No. 3, pp. 283-300.