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Retaining-wall dialog: ‘A tale of two walls’

August 1st, 2006 / By: / From Our Readers

By Bob Barrett

Introductory note from the letter writer:

Geosynthetics editor, Ron Bygness, originally invited an article from me on the first GRS wall with negative batter in New Zealand. In order to explain why the Kiwis were different, I found it necessary to comment on why the U.S. is so far behind in GRS technologies. Ron and I agreed that my submission would be better served in a letter to the editor, where I would be allowed more latitude in opinion.

Al Ruckman and I own Soil Nail Launcher, Inc., a design/build geotechnical firm that specializes in soil and rock reinforcement. The Web site is www.soilnaillauncher.com. Al and I have been involved in reinforced soil research since 1971.

I was also chair of the TRB Committee on Geosynthetics, 1990-1997; and chair of NCHRP 12-59, Design and Construction Guidelines for Geosynthetically Reinforced Soil Abutments. The first publication from that effort is the landmark NCHRP Report 556 (2006).

To the editor:

The first GRS wall built in New Zealand leans outward and on purpose. GRS is the acronym for geosynthetically reinforced soil with generic components. The Kiwis are noted for punching above their weight, thus this was the right crowd for plunging into the deep end first.

Introduction of generic GRS technologies in New Zealand was part of the landslide control portfolio with launched soil nails. These innovative tensile inclusions create their own brand of reinforced soil in in-situ soils. However there are instances where lost soils must be replaced. A case in point would be on highways where the sliding has regressed into the traveled way.

Introduction of generic GRS technologies in New Zealand was part of the landslide control portfolio with launched soil nails. These innovative tensile inclusions create their own brand of reinforced soil in in-situ soils. However there are instances where lost soils must be replaced. A case in point would be on highways where the sliding has regressed into the traveled way.

Some background on how they jumped ahead of the world to get to this level:

GRS technologies have a tattered history. Modern geosynthetics are only a generation old. (As long as Al Ruckman, Bob Holtz, Jonathan Wu, Bob Koerner, Barry Christopher, Jim Collin, Richard Bathurst and I are still around—otherwise it will be two generations.)

These new polymers have, as Geosynthetics readers know, made a dramatic indentation into the practice of geotechnical engineering. This indentation has yet to reach the appropriate depth on the structural engineering community.

There was a group of dedicated researchers in the 1970s and ’80s whose mission was to decipher the code for how geosynthetic inclusions in compacted soil created a composite that behaved differently than the two elements that comprise that composite. Evolving at a more rapid rate were our wonderful capitalist interests. These folks with their proprietary offerings soon subverted the quest for absolute truth.

Instead of bringing the civil engineering community into the learning and growth process, the commercial sector offered an easy way out. No need to learn all that soil mechanics stuff. No one needs to know anything more than what the vendors and vendor associations provide. Our government agency engineers bought into this concept. And therefore, hardly anyone in the United States can properly design polymerically reinforced soil features. It is a level of sloth that defies explanation.

This was all explained to the brightest of the soil and structural engineers of New Zealand. They could see that it was important to learn the simple design criteria of generic reinforced soil technology. The alternative was to sit back and let vendors and structural engineering associations dictate what could and could not be done.

Certainly the quasi-tieback vendor offerings could not tolerate reverse batter.

After seeing their miserable failures in the 1999 Chichi earthquake and in the recent feature in Geosynthetics (“Eight ways to achieve improved retaining-wall performance” By Michael R. Simac, April/May), we come to realize how large our mistake was to rely on vendor designs for the last two decades. We can begin to see how much our sloth has cost.

Not so with the Kiwis. They quickly understood and assimilated what has taken 40 years and tens of millions of tax dollars to learn. To wit: Spacing is more important than the strength of the inclusion. Colorado DOT built a full-scale bridge pier with bed linen to demonstrate this.

Creep is not possible in compacted granular soil when the inclusion spacing is close (8-12 inches). This enlightened finding has been demonstrated with Colorado DOT and U.S. Forest Service funds at the University of Colorado/Denver and at the FHWA research facility in McLean, Va. It has been confirmed in hundreds of constructions under the seal of Albert Ruckman.

As shown in the bedsheet demo and in field constructions, in-air stiffness of the reinforcement is not necessary in these generic structures. Even paper towels behave stiffly under this level of confinement. These findings allow lighter, much less expensive, generic inclusions to be incorporated in permanent structures.

The width of the structure, that is the depth of the inclusion, is a function of external or global stability. The generic design criteria adopted by Hiway Stabilizers provides a structure that cannot fail internally. These design criteria were successfully demonstrated most recently by FHWA research under the direction of Dr. Scott Anderson of Central Federal Lands and Mike Adams of the Turner-Fairbank Highway Research Center.

Other departures in GRS from AASHTO and FHWA Demo 82 include the absence of mandatory embedment. This is a global issue, not internal. A concrete leveling pad under the facing is counter-productive. It creates cracking in the blocks. A compaction or drainage gravel at the face is unnecessary with close spacing.

In summary, I submit that prevailing guidelines for internally supported MSE walls are based on translations from externally supported cantilever walls and are generally inappropriate. The errors in these translations are not necessarily conservative. Look at the failure rates. But worse than just being wrong, quasi-tieback approaches are a dead end in technological progression.

The New Zealand construction exemplifies more than just their willingness to not be intimidated by myth and proprietary interests. It shows their commitment and respect for implementing technically correct and more economic technologies. It illustrates what can be accomplished when engineers’ priorities are focused to serve their clients.

Bob Barrett
Grand Junction, Colo.
bob@soilnaillauncher.com or bbarrett33@aol.com

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