By Ryan R. Berg, P.E., and Ronald P. Anderson, P.E.
Some days it seems like we have been designing and constructing concrete-faced, geosynthetic-reinforced, mechanically stabilized earth (MSE) walls forever, and that this technique of constructing MSE walls has always been a conventional solution to grade separation projects.
Certainly today, geogrid-reinforced walls with drycast concrete modular block wall (MBW) facings are a conventional construction technology/methodology for private as well as public property development. Geogrid-reinforced precast panel faced walls and MBW-faced walls are also widely used for transportation works.
This technology has had an interesting, and sometimes challenging, journey to maturity. Not long ago, the idea of constructing a major, precast concrete-faced MSE highway wall with geogrid soil reinforcement was a novel and an unproven technology.
Use of geogrids
The first use of precast concrete panel faced MSE walls reinforced with geogrids in North America was designed and constructed 25 years ago (see Figure 1).
It was not a demonstration wall, or a single wall, or even a small or short wall. Forty-three separate MSE walls totaling more than a mile in length were designed and constructed from 1984–86 in Tucson, Ariz. This project raised and widened Tanque Verde Road and created underpasses for the cross streets of Pantano Road and Wrightstown Road. The walls were constructed for the Pima County Department of Transportation and Flood Control.
The walls on this project are up to 21.5ft (6.5m) high and are reinforced with high-density polyethylene (HDPE) geogrids (just a single grade/strength of uniaxial geogrid was available in those days). The geogrid has a peak tensile strength of approximately 5,400 lb/ft (wide-width test) and a long-term creep limited (10% strain limit) strength of 1,986 lb/ft. This geogrid has a mass of about 29 oz/yd2 and was supplied in rolls 3.3ft (1m) wide and 98.4ft (30m) long.
The facing is full-height precast concrete panels 10ft (3m) wide and 6in. (15.2cm) thick. These panels were cast on-site by the contractor, face down, with geogrid connection tabs on the opposite (top) side situated at predetermined, design intervals. (Connection detail comparisons from 1985 and 2009 are seen in Figures 3 and 4.)
Exposed faces were sandblasted, after wall construction, for aesthetic treatment. The bridge structures are supported on drilled shafts, and the geogrid soil reinforcement was placed around these shafts. A typical series of walls around a bridge abutment (shortly after construction) are shown in Figure 1 (1985).
Evolution of design
Much has evolved in the design methods for geosynthetic-reinforced highway MSE walls since the Tanque Verde project. Details of the 1984 design of the Tanque Verde walls are summarized in Table 1. Today’s treatment of these design items is also listed and illustrates evolution of design during the past 25 years.
Twenty-five years ago, no one had built a precast, concrete-faced retaining wall using geogrids in North America (one or two small projects had been built in the U.K.). It took a motivated geogrid manufacturer who hired experienced geotechnical engineers and design consultants to provide the technical support, plus site-specific research and development needed to allow the “owner” (Pima County) to be comfortable that this new, never-been-built-before technology would work on their critical highway project.
Keys for success
In hindsight, several ingredients were key to the success of this ground-breaking MSE wall project.
First, and perhaps foremost, a great contractor was critical for success with a pioneering project. These walls were skillfully and successfully constructed by M.M. Sundt Construction Co. of Tucson.
The advocacy of the geogrid manufacturer was another key. The manufacturer provided technical support directly and, with a local engineering consultant, addressed concerns of the owner through laboratory testing of connections and field measurements of temperature gradient through concrete panel and cosponsored an instrumentation program.
A progressive and detailed design engineer was another key. These walls were designed by Dames & Moore (now URS Corp.) of Phoenix.
And it helped to have a responsive owner—in this case, Pima County—that was serious about the V.E. (“value engineering”) provisions of their contract bid documents and performed a thorough review of the proposed new wall technology.
It was the collaborative efforts and frequent communication among these four entities—contractor, geogrid manufacturer, consultant, and owner—that made for a successful project.
Documentation & conclusion
Two wall sections were instrumented during and after construction to document and monitor performance.
Publications based on this instrumentation and interpretations of results are listed in the following Project Bibliography. In particular, FHWA-EP-90-001-005 (Bonaparte and Gross, 1989) is a comprehensive report on the design, construction, and performance of these retaining walls.
Tanque Verde project data has been used to support:
- Change of internal stability design methodology from Coherent Gravity or Tieback Wedge and, later, to Simplified Method.
- Consideration of temperature design parameters for precast concrete-faced walls reinforced with geosynthetics.
- Change of external bearing pressure computations via trapezoidal distribution to Meyerhof-type distribution.
The Tanque Verde project data also documented:
- Construction procedures of connection tensioning and fill placement/compaction adjacent to full-height panels.
- Actual geogrid reinforcement strains and maximum strains of about 1%.
- Full-height facing panel movements during construction to define initial required batter of about 1H:60V.
- Durability of HDPE geogrid soilreinforcements.
The Pima County Department of Transportation reports today that the Tanque Verde walls (see Figures 5 and 6) continue to perform well, and are now under the maintenance jurisdiction of the city of Tucson via annexation.
Ryan Berg is a geotechnical consultant in St. Paul, Minn. He is a coauthor of the FHWA Design and Construction Guidelines for MSE Walls and Reinforced Soil Slopes (2009, 2001) and of the NCMA Design Manual for Segmental Retaining Walls (1st Edition, 1993).
Ron Anderson is vice president of professional development with Tensar Corporation in Denver.
Both authors were previously project engineers with Dames & Moore, and prepared the initial design and value engineering submittal for the Tanque Verde project.
Sundt’s project manager for the Tanque Verde walls was Ken Brunker, now branch manager of Coffman Specialties Inc. in Scottsdale, Ariz.
Pima County’s project manager was Barry Berkovitz, assisted by Ali Fermawi. After the project was successfully underway, Berkovitz took a position with the FHWA, where he served as a geotechnical engineer until his death in 2004. Fermawi is now assistant division manager, field engineering, with the Pima County Department of Transportation.
Mike Cowell, then Tensar regional manager out of Denver and Dr. Rudy Bonaparte, technical director at Tensar, provided technical assistance. Cowell is now the president and CEO of GeoStructures in Purcellville, Va.. and Dr. Bonaparte is president and CEO of Geosyntec Consultants.
Instrumentation was installed and read by professors Chandrakant Desai and Dr. Ken Fishman (then a graduate student) of the University of Arizona, assisted by Bob Sogge of Desert Earth Engineering. Today, Dr. Fishman is president of Earth Reinforcement Testing Inc. of Buffalo, N.Y.
Allen, T.M., Bathurst, R.J., and Berg, R.R., “Global Level of Safety and Performance of Geosynthetic Walls: A Historical Perspective,” Geosynthetics International, Vol. 9, Nos. 5-6, 2002, pp. 395-450.
Berg, R.R. Allen, T.M. and Bell, J.R., “Design Procedures for Geosynthetic Reinforced Soil Walls—A Historical Perspective,” Proceedings of Sixth International Conference on Geotextiles, Geomembranes and Related Products, Atlanta, March, 1998, pp. 491-496.
Berg, R.R. and Chouery-Curtis, V.E., “Geogrid Reinforced Propped Cantilever Walls for Grade Separation, Tanque Verde Road,“ Tucson, Ariz., Geosynthetics Case Histories, International Society of Soil Mechanics and Foundation Engineers, 1993, pp. 262-263.
Berg, R.R., Bonaparte, R., Chouery, V.C., and Anderson, R.P., “Design, Construction, and Performance of Two Geogrid Reinforced Soil Retaining Walls,” Proceedings of the IIIrd International Conference on Geotextiles, Vienna, Austria, April, 1986, pp. 401-406.
Bonaparte, R. and Gross, B.A., Tensar-Geogrid Reinforced Soil Wall, Experimental Project Program, Experimental Project 1, Ground Modifications Techniques, FHWA-EP-90-001-005, Federal Highway Administration, Washington D.C., 1989, 119 p.
Bright, D.G., Collin, J.G. and Berg, R.R., “Durability of Geosynthetic Soil Reinforcement Elements in Tanque Verde Retaining Wall Structures,” Transportation Research Record No.1439 Durability of Geosynthetics, National Academy Press, Washington D.C., 1995 pp. 46-54.
Collin, J.G., Bright, D.G. and Berg, R.R., “Performance Summary of the Tanque Verde Project—Geogrid Reinforced Soil Retaining Walls,” Serviceability of Earth Retaining Structures, Geotechnical Special Publication No. 42, American Society of Civil Engineers, 1994, pp. 43-56.
Collin, J.G., Berg, R.R. and Anderson, R.B., “Temperature Effects on the Connection Strength of MSE Walls,” Proceedings of the Fifth International Conference on Geotextiles, Geomembranes and Related Products, Vol. 1, Singapore, September, 1994, pp. 229-232.
Collin. J.G. and Berg, R.R., “Permanent Geogrid Reinforced Soil Retaining Walls—U.S. Experience, Recent Case Histories of Permanent Geosynthetic-Reinforced Soil Retaining Walls,” Tatsuoka, F. and Leshchinsky, D., Eds., A.A. Balkema, Rotterdam, 1994, Proceedings of Seiken Symposium No. 11, Tokyo, Japan, November, 1992, pp. 61-68.
Desai, C.S. and El-Hoseiny, “Prediction of Field Behavior of Reinforced Soil Wall Using Advanced Constitutive Model,” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 131, No. 6, June 2005, pp. 729-739.
Fishman, K.L., Desai, C.S. and Sogge, R.L., “Field Behavior of Instrumented Geogrid Soil Reinforced Wall,” Journal of Geotechnical Engineering, ASCE, Vol. 119, No. 8, August 1993, pp. 1293-1307.
Fishman, K.L., Berg, R.R., and Desai, C.S., “Geosynthetic-Reinforced Soil Wall: 4-Year History, Transportation Research Record No. 1330, Behavior of Jointed Rock Masses and Reinforced Soil Structures, Transportation Research Board, Washington D.C., 1991, pp. 30-39.
Wayne, M.H., Bright, D.G., Berg, R.R. and Fishman, K., “Tanque Verde Retaining Wall Structure: Revisited after 11 Years,” 10th Annual GRI Conference, Drexel University, Philadelphia, Pa., December 10-11, 1996.