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Geosynthetics International: Vol.9, No. 5-6, 2002
To gain access the full text of the papers below, you must become a member of the IGS - if you are already an IGS Member, please to the Geosynthetics International Journal Archives in the Members Only section of the site.
SPECIAL ISSUE ON GEOSYNTHETIC-REINFORCED SOIL WALL PERFORMANCE AND DESIGN IMPLICATIONS
Note of Appreciation to Paper Reviewers
The quality of the technical journal Geosynthetics International and, hence, its reputation as the premier peer-reviewed journal on geosynthetics and related topics depends on the dedication of the many reviewers who donate their time and expertise to assess submissions to the Journal. The Editor, Co-Editor, and Chair of the Editorial Board would like to thank the following individuals for completing manuscript reviews in 2002. If we have missed your name below please let us know and we will be sure to include you in the reviewers list to appear in the last issue of Volume 10 in 2003.
T.S. Ingold R.J. Bathurst J.P. Giroud
M. Abramento,K. Adalier, M. Alfaro, D. Bergado, A. Bouazza, J. Bray, S. Chandra, J. Cowland, M. Esfehami, R.J. Fannin, G. Fischer, A.B. Fourier,G. Ghataora, C. Ghosh, G. Gottardi, J. Han, K. Hatami, M. Heibaum, S. Helwaney, D.J. Hoare, J. Horvath, Y.G. Hsuan, G. Koerner, J. Lafleur, D. Leschinsky, H. Ling, J. Ling, N. Miura, E. Palmeira, I. Peggs, S. Perkins, R.K. Rowe, G. Skinner, P. Villard, D. Walters, J.-H. Yin, H. Zanzinger
Foreword by T.S. Ingold and J.P. Giroud
SPECIAL ISSUE ON GEOSYNTHETIC-REINFORCED SOIL WALL PERFORMANCE AND DESIGN IMPLICATIONS
It is with great pleasure that we close out Volume 9 of Geosynthetics International with this special double issue devoted to geosynthetic-reinforced soil walls. The authors have undertaken a landmark study focused on the analysis of the long-term performance of walls including structures that have been in place for up to 25 years.
The first paper provides details of a database of 20 well-documented geosynthetic-reinforced wall case studies representing 35 different analysis cases. To demonstrate the value of this database, the authors use the information to show that the current Simplified Method used in North America for internal stability design results in 1.5 to 4 times as much reinforcement as that needed to achieve acceptable wall performance. This paper provides an important impetus to improve the way geosynthetic-reinforced soil walls are currently designed, not only in North America, but also world-wide.
The second paper is focused on the interpretation of reinforcement strain measurements that are commonly used to infer wall behaviour. The authors have carefully examined the reliability of different measurement techniques, quantified their accuracy, and demonstrated how to calibrate strain measurements to deduce global strains as a first step to accurately predict reinforcement loads.
The third paper addresses the second step to accurately predict reinforcement loads - the conversion of reinforcement strains to loads through proper selection of reinforcement stiffness values. A valuable part of this paper is an assessment of the choice of creep, relaxation, or constant-rate-of-strain laboratory test results for this purpose.
The fourth paper uses the techniques presented in the second and third papers to estimate loads in 16 full-scale walls, assign error limits on load predictions, summarise trends in the data, and compare estimated load values to predictions based on current design practice. The comparisons presented support the argument advanced in the first paper that loads in geosynthetic-reinforced walls are generally much lower than values assumed using current limit-equilibrium based design practice. The authors demonstrate that the triangular distribution of reinforcement load assumed in current design practice is not observed in the distributions of load taken from the large database of case studies. Rather, a trapezoidal distribution is more applicable. The proposed trapezoidal distribution provides a fundamental shift towards a future empirical-based design method. Equally important, the authors introduce soil failure based on the peak shear strength of the soil as a new limit state for reinforced soil wall design and propose limiting the strain in the reinforcement to prevent this limit state from occurring.
The fifth and final paper in the series compares measured creep strain levels and strain rates recorded in full-scale walls with values measured in comparable laboratory creep testing. The authors show that, at working stress conditions, laboratory creep data can often be used to estimate creep in walls provided the load level in the wall reinforcement and the laboratory are approximately the same. At these relatively small strains, reinforcement creep can be expected to become nearly imperceptible after a short period of time following the end of construction. At relatively high strains sufficient to achieve a soil failure limit state, reinforcement load levels appear to increase, resulting in increased creep rates and significant long-term creep. Hence, wall behaviour from working stress conditions to failure can now be defined. Finally, this paper provides a valuable set of guidelines to identify the magnitude of wall deformations and reinforcement strains that, if exceeded, can lead to poor performance.
The authors are to be congratulated on the presentation of an important database of well-documented case studies. This collection of papers should serve to guide, and validate, the development of future design methods with a view to rationalising excessively conservative aspects of current design practice.
T.S. Ingold, Editor J.P. Giroud, Chairman of the Editorial Board
Introduction by T.M. Allen and R.J. Bathurst
Geosynthetic-reinforced soil walls have been in use for more than 25 years and in the vast majority of cases have performed very well. Nevertheless, the inherent margin of safety against poor performance with respect to internal stability has not been quantified in a systematic manner using a large database of walls. This lack of quantification can be argued to contribute to the current lack of acceptance for this technology by some practitioners and government agencies in North America.
We have been engaged in research for many years with the long-term objective of developing rational analyses and design methodologies for reinforced soil walls. Beginning with the Rainier Avenue wall built in Seattle by the Washington State Department of Transportation in 1989, a major research effort was launched through the University of Washington to investigate geosynthetic wall behavior. In 1996, we began extending what was learned from the Rainier Avenue wall by collecting and analyzing case study records of other well-documented instrumented walls. Our first objective was to quantify the level of conservatism in current North American design practice and to provide data against which new design methods could be validated. Parallel to these efforts, the results of work that had been underway at the Royal Military College of Canada (RMC) since 1986 involving a series of full-scale laboratory walls, was also used to increase our understanding of the behavior of geosynthetic-reinforced soil walls. These efforts were joined together in 1998 in a pooled fund study involving many US State Transportation Departments, the National Concrete Masonry Association, private sector geosynthetic and steel-reinforced soil wall companies, and grants from the Natural Sciences and Engineering Research Council of Canada and the Canadian Department of National Defence. This database now includes comprehensive high-quality measured wall performance data from 11 geosynthetic-reinforced field structures in North America, Europe and Scandinavia, and five full-scale laboratory walls constructed at RMC.
A fundamental requirement to assess conservatism in current design practice and to validate new design approaches is a reliable estimate of the load in reinforcement layers. However, direct measurement of reinforcement loads is rare in the case study literature. Instead we had to carefully examine reinforcement strain measurements and the conversion of strain to load through the use of a properly selected reinforcement stiffness value. This approach required a study of instrumentation techniques to record reinforcement strains and a quantification of the reliability and accuracy of these measurements. The next challenge was to develop a strategy to select appropriate stiffness values for geosynthetic reinforcement products based largely on interpretation of in-isolation laboratory testing, but recognizing the influence of soil confinement, strain level, duration of loading, and temperature amongst other factors.
Based on best estimates of reinforcement loads, we were able to investigate actual reinforcement load levels and their distribution within typical wall structures and to propose an empirical-based distribution that is trapezoidal in shape rather than triangular as assumed in current North America practice.
The database of case studies, which includes walls that have been taken to conditions in excess of working load levels, has allowed us to propose guidelines to differentiate between walls that have behaved well from those that have performed poorly based largely on the magnitude of post-construction reinforcement strains and wall deformations. This assessment has also led us to propose a new limit state for internal stability design: the development of a contiguous failure zone within the reinforced soil mass that can be prevented by limiting reinforcement strains to a prescribed maximum value.
We believe that this collection of papers will provide researchers with a comprehensive set of data that can be used to guide the development of new design methods, validate these new approaches, and test the results of advanced numerical models.
To establish the objectives in this series of papers, we have referred to the work of many other researchers whose names appear in the text of the papers. In many cases, these researchers have assisted us with the interpretation of their data and provided us with unpublished results and insight. To these individuals we are truly grateful. We are particularly indebted to R.R. Berg, B.R. Christopher, R.D. Holtz, and J.R. Bell who have assisted us over many years with valuable advice and, in the case of R.R. Berg, with co-authoring one of the papers. We are also grateful to D.L. Walters (Ph.D. candidate with the GeoEngineering Centre at Queen’s University-RMC) who co-authored two other papers in this series. Two anonymous reviewers materially improved the contents of this collection of papers by providing an exhaustive critique of the original manuscript submissions and helping the writers to focus the objectives and conclusions. To these two reviewers we express our thanks.
Finally, we wish to thank T.S. Ingold, Editor of Geosynthetics International, and J.P. Giroud, Chairman of the Editorial Board of Geosynthetics International for giving us the opportunity to publish our work in the Journal.
T.M. Allen R.J. Bathurst
Technical Paper by T.M. Allen, R.J. Bathurst, and R.R. Berg
GLOBAL LEVEL OF SAFETY AND PERFORMANCE OF GEOSYNTHETIC WALLS: AN HISTORICALPERSPECTIVE
ABSTRACT: A summary of 20 well-documented geosynthetic wall case histories representing a total of 35 analysis conditions is presented. These case histories cover a wide variety of wall heights, surcharge loading, foundation conditions, facing types and batter, reinforcement types and stiffness, and reinforcement spacing. All of the production walls, including some that have been in service for 25 years, have performed well with low reinforcement strains and minimal deflections. Some of the walls were research structures that, although purposely underdesigned, could not be taken to failure, demonstrating that the internal stability design of geosynthetic walls in North America is conservative. Each of the walls was characterized globally with respect to internal level of safety, or resistance to demand ratio. Even when using nonconservative estimates of soil property values and perfect matching of the reinforcement strength to demand, the Simplified Method resulted in approximately 1.5 to 4 times as much geosynthetic reinforcement as that needed to achieve acceptable performance based on actual long-term performance of many of the wall case histories. Based on the analyses presented here, there is a need to re-evaluate the current North American approach to design of geosynthetic walls against internal reinforcement instability.
KEYWORDS: Geosynthetic, Wall, Reinforcement, Stability, Deformation, Safety.
AUTHORS: T.M. Allen, PE, Washington State Department of Transportation, Olympia, Washington, 98504-7365, USA, Telephone: 1/360-709-5450, Telefax: 1/360-709-5585, E-mail: allent@wsdot.wa.gov; R.J. Bathurst, Professor, GeoEngineering Centre at Queen’s-RMC, Civil Engineering Department, Royal Military College of Canada, Kingston, Ontario, K7K 7B4, Canada, Telephone: 1/613-541-6000 Ext. 6479; Telefax: 1/613-541-6218; E-mail: bathurst-r@rmc.ca; and R.R. Berg, PE, Ryan Engineering and Design, Inc., 2190 Leyland Alcove, Woodbury, Minnesota 55125, USA, Telephone: 1/651-735-7622; Telefax: 1/651-735-7629; E-mail: ryanberg@worldnet.att.net.
DATE: Original manuscript submitted 9 December 2001, revised version received 25 November 2002, and accepted 6 December 2002. Discussion open until 1 November 2003.
REFERENCE: Allen, T.M., Bathurst, R.J., and Berg, R.R., 2002, “Global Level of Safety and Performance of Geosynthetic Walls: An Historical Perspective”, Geosynthetics International, Vol. 9, Nos. 5-6, pp. 395-450.
Technical Paper by R.J. Bathurst, T.M. Allen, and D.L. Walters
SHORT-TERM STRAIN AND DEFORMATION BEHAVIOR OF GEOSYNTHETIC WALLS AT WORKING STRESS CONDITIONS
ABSTRACT: The paper reviews geosynthetic reinforcement strain measurement techniques that have been reported in a database of well-documented case studies and more recent full-scale laboratory test walls. Interpretation of strain measurements, accuracy of readings, and advantages and disadvantages of different techniques are discussed. In general, properly calibrated strain gauges have proven useful to estimate reinforcement strains at low strain levels (0.02 to 2%). Extensometers are shown to be accurate at strains greater than 2% and to have marginal reliability at strains between 0.5 and 2%. A strategy to improve confidence with interpretation of strain readings is to use strain gauges and extensometers in the field and to adjust strain gauge calibration factors based on in situ measurements from both devices. Corrected reinforcement strains can be used together with appropriately selected reinforcement stiffness values to estimate reinforcement loads. Estimated loads can then be compared to predicted values using current and proposed design methods for the internal stability of geosynthetic-reinforced soil walls.
KEYWORDS: Geosynthetic, Reinforcement, Wall, Strain measurement, Deformation, Strain calibration, Coefficient of variation.
AUTHORS: R.J. Bathurst, Professor, GeoEngineering Centre at Queen’s-RMC, Civil Engineering Department, Royal Military College of Canada, Kingston, Ontario, K7K 7B4, Canada, Telephone: 1/613-541-6000 Ext. 6479; Telefax: 1/613-541-6218; E-mail: bathurst-r@rmc.ca; T.M. Allen, PE, Washington State Department of Transportation, Olympia, Washington, 98504-7365, USA, Telephone: 1/360-709-5450, Telefax: 1/360-709-5585, E-mail: allent@wsdot.wa.gov; and D.L. Walters, Ph.D. candidate, GeoEngineering Centre at Queen’s-RMC, Department of Civil Engineering, Queen’s University, Kingston, Ontario, K7L 3N6, Canada, Telephone: 1/613-541-6000 Ext. 6347; Telefax: 1/613-541-6218; E-mail: walters-d@rmc.ca.
DATE: Original manuscript submitted 9 December 2001, revised version received 25 November 2002, and accepted 6 December 2002. Discussion open until 1 November 2003.
REFERENCE: Bathurst, R.J.,
Allen, T.M., and Walters, D.L., 2002, “Short-Term Strain and Deformation
Behavior of Geosynthetic Walls at Working Stress Conditions”, Geosynthetics
International, Vol. 9, Nos. 5-6, pp. 451-482.
Technical Paper by D.L. Walters, T.M. Allen, and R.J. Bathurst
CONVERSION OF GEOSYNTHETIC STRAIN TO LOAD USING REINFORCEMENT STIFFNESS
ABSTRACT: Measurements indicative of the internal behavior of full-scale geosynthetic-reinforced soil walls typically consist of reinforcement strains and overall deformations. The focus of this paper is the development of a methodology that can be used to convert measured reinforcement strains to load using properly selected reinforcement stiffness values. The loading of the geosynthetic in the field can be simulated in the laboratory using creep, relaxation, and constant-rate-of-strain tests. It was found that in-isolation creep stiffness data is sufficiently accurate to estimate reinforcement loads from strain measurements, at least for geogrids and most woven geotextiles. The approach is validated using data from carefully instrumented wall case histories in which reinforcement loads were measured directly and compared to loads estimated from measured reinforcement strain data.
KEYWORDS: Geosynthetic, Strain, Load, Stiffness, Creep, Stress relaxation, Reinforcement, Installation damage, Wall, Coefficient of variation.
AUTHORS: D.L. Walters, Ph.D. candidate, GeoEngineering Centre at Queen’s-RMC, Department of Civil Engineering, Queen’s University, Kingston, Ontario, K7L 3N6, Canada, Telephone: 1/613-541-6000 Ext. 6347; Telefax: 1/613-541-6218; E-mail: walters-d@rmc.ca; T.M. Allen, PE, Washington State Department of Transportation, Olympia, Washington, 98504-7365, USA, Telephone: 1/360-709-5450, Telefax: 1/360-709-5585, E-mail: Allent@wsdot.wa.gov; and R.J. Bathurst, Professor, GeoEngineering Centre at Queen’s-RMC, Civil Engineering Department, Royal Military College of Canada, Kingston, Ontario, K7K 7B4, Canada, Telephone: 1/613-541-6000 Ext. 6479; Telefax: 1/613-541-6218; E-mail: bathurst-r@rmc.ca.
DATE: Original manuscript submitted 9 December 2001, revised version received 25 November 2002 and accepted 6 December 2002. Discussion open until 1 November 2002.
REFERENCE: Walters, D.L., Allen, T.M., and Bathurst, R.J. “Conversion of Geosynthetic Strain to Load using Reinforcement Stiffness”, Geosynthetics International, Vol. 9, Nos. 5-6, pp. 483-523.
Technical Paper by T.M. Allen and R.J. Bathurst
SOIL REINFORCEMENT LOADS IN GEOSYNTHETIC WALLS AT WORKING STRESS CONDITIONS
ABSTRACT: Knowing the load in geosynthetic reinforcement layers in full-scale walls is an important step to improving internal stability design methods. Interpretation of empirical reinforcement load data enables analytical models to be properly calibrated. High-quality empirical data also provides a baseline against which new design methods can be validated. In this paper, loads in soil reinforcement layers from 16 full-scale geosynthetic wall case histories were estimated from strain measurements and converted to load through the stiffness of the reinforcement material. The paper summarizes these estimated peak loads, describes general trends in the data, and compares these reinforcement loads to predictions using current design practice applied to the wall case histories. It was found that reinforcement loads derived from strain measurements are, in general, much lower than would be predicted based on current limit equilibrium design methods that use classical earth pressure theory. The low reinforcement strains and loads measured to date in geosynthetic walls point to the desirability of using peak soil shear strength rather than constant volume shear strength for design. This approach will help to reduce design conservatism and will be consistent with the philosophy of preventing failure of a major component of the reinforced soil system, the soil. Once the soil has failed, for all practical purposes, the wall has failed as well.
KEYWORDS: Geosynthetic, Wall, Reinforcement, Stiffness, Creep, Load, Strain, Coefficient of variation.
AUTHORS: T.M. Allen, PE, Washington State Department of Transportation, Olympia, Washington, 98504-7365 USA, Telephone: 1/360-709-5450, Telefax: 1/360-709-5585, E-mail: allent@wsdot.wa.gov; and R.J. Bathurst, Professor, GeoEngineering Centre at Queen’s-RMC, Civil Engineering Department, Royal Military College of Canada, Kingston, Ontario, K7K 7B4, Canada, Telephone: 1/613-541-6000 ext. 6479, Telefax: 1/613-541-6218, E-mail: bathurst-r@rmc.ca.
DATE: Original manuscript submitted 9 December 2001, revised version received 25 November 2002, and accepted 6 December 2002. Discussion open until 1 November 2003.
REFERENCE: Allen, T.M. and Bathurst, R.J., 2002, “Soil Reinforcement Loads in Geosynthetic Walls at Working Stress Conditions”, Geosynthetics International, Vol. 9, Nos. 5-6, pp. 525-566.
Technical Paper by T.M. Allen and R.J. Bathurst
OBSERVED LONG-TERM PERFORMANCE OF GEOSYNTHETIC WALLS AND IMPLICATIONS FOR DESIGN
ABSTRACT: Geosynthetic-reinforced walls have been viewed by the civil engineering profession as a new technology whose acceptable long-term performance is yet to be established. Nevertheless, geosynthetic walls have been in use for almost 25 years. Much of the uncertainty associated with acceptance of geosynthetic-reinforced wall technologies is related to time-dependent deformation. This paper summarizes the creep rates measured in full-scale walls and compares them to creep rates measured in-isolation in the laboratory where the applied load level matches values estimated for the same structures in the field. In the majority of cases, the laboratory in-isolation creep rates were the same as or greater than the measured reinforcement creep rates in full-scale walls, corroborating that reinforcement load levels can be estimated from measured strain data. At the end of wall construction, it appears that the reinforcement is primarily exhibiting creep, with only minor stress relaxation. However, in the long-term, there is a trend toward reinforcement stress relaxation. Furthermore, the long-term behavior observed in the full-scale walls indicates that the reinforcement loads are well below values required to cause creep rupture over the design life of the structures and, in some cases, creep appears to have stopped completely. Finally, the paper offers quantitative guidelines to delineate anticipated poor and good long-term wall performance based largely on level of reinforcement strains and magnitude of post-construction wall deformations.
KEYWORDS: Geosynthetic, Creep, Reinforcement, Wall, Stress relaxation, Deformation, Strain, Load.
AUTHORS: T.M. Allen, PE, Washington State Department of Transportation, Olympia, Washington, 98504-7365 USA, Telephone: 1/360-709-5450, Telefax: 1/360-709-5585, E-mail: allent@wsdot.wa.gov; and R.J. Bathurst, Professor, GeoEngineering Centre at Queen’s-RMC, Civil Engineering Department, Royal Military College of Canada, Kingston, Ontario, K7K 7B4, Canada, Telephone: 1/613-541-6000 ext. 6479; Telefax: 1/613-541-6218; E-mail: bathurst-r@rmc.ca.
DATE: Original manuscript submitted 9 December 2001, revised version received 25 November 2002, and accepted 6 December 2002. Discussion open until 1 November 2003.
REFERENCE: Allen, T.M. and Bathurst, R.J., 2002, “Observed Long-Term Performance of Geosynthetic Walls and Implications for Design”, Geosynthetics International, Vol. 9, Nos. 5-6, pp. 567-606.
LANDMARKS IN EARTH REINFORCEMENT VOLUMES 1 AND 2
Ochiai H., Yasufuku N., and Omine H., Editors,
2001, “Landmarks in
Earth
Reinforcement ”, Publisher A.A. Balkema, Swets & Zeitlinger
Publishers, 1157 pages (2
volumes). Hardbound in 173 mm × 247 mm format. Two volumes: ISBN
90 265 1863 3,
Volume 1 90 265 1864 1, Volume 2 90 265 1865 X
North America: A.A. Balkema,
c/o Ashgate Publishing Company, 2252 Ridge Road,
Brookfield, Vermont 05036-9704, USA, Telephone: 1/800-535-9544, E-mail:
info@ashgate.com. Price $352.80. International: A.A. Balkema Publishers,
P.O. Box
1675, 3000 BR Rotterdam, The Netherlands, Telephone: 31/10-414-5822,
Telefax:
31/10-413-5947, E-mail: orders@swets.nl. Price EUR306.80.
These two volumes
comprise the proceedings of the Fourth International Conference on
Earth Reinforcement (IS-Kyushu), which was held in November 2001. Published
early in
2002, the 767 page Volume 1 presents 137 papers set out under five
sections entitled
Testing and Materials, Embankments, Wall Structures, Foundations, and
Soil Nailing.
Just published is Volume 2 of the proceedings (390 pages), which presents
a special
lecture, five keynote lectures, five technical reports, summary discussion,
and reports for
Technical Committee TC-9 of the International Society for Soil Mechanics
and
Geotechnical Engineering (ISSMGE). The excellent special lecture, entitled “Full-scale
performance testing and numerical modelling of reinforced soil retaining
walls” is
followed by five equally informative keynote lectures, “Insights
from case histories:
Reinforced embankments and retaining walls”, “Performance
related issues affecting
reinforced soil structures in Asia”, “An outlook on recent
research and development
concerning long-term performance and extreme loading”, “The
durability of geosynthetics
for retaining walls and slopes for long term performance”, and “Actual
status of
the soil nailing to expressway cut slope construction in Japan”.
The five technical reports
consider aspects of the five main sessions presented in Volume 1, followed
by a highly
informative summary discussion on design procedures. Finally, Volume
2 sets out reports
from the five sub-committees of TC-9.
At over one thousand pages, these tomes represent good value for the
money and deserve
a place on the bookshelf of any practising engineer, or academic, involved
with
reinforced soil.