To the editor:
“Magic” is an appropriate description where the two significant elements of very different properties are combined to produce a unique composite stronger than either element. Concrete and asphaltic concrete are two examples of “unique composites.” Unique composites exhibit behaviors that exceed the sum of the constituents and cannot be accurately modeled using an additive representation of their constituent elements.
We don’t fully understand these, nor do we have mechanistic designs to explain unique composites. Imagine the folly of trying to model pavement performance based only on the properties of a bucket of liquid asphalt and a sack of gravel.
Yet we can mathematically predict behavior of simple composites like tiebacks, soil nails, and MSE to some extent, whose properties are additive to their geoenvironment. This not the case for GRS.
Fortuitously, the June/July 2010 issue of Geosynthetics is a great resource for examples of mixing these two dimensions or paradigms of soil improvement (i.e., simple composites such as tiebacks, soil nails, and MSE vs. unique composites such as concrete and GRS).
In “Building bridges the geosynthetic-reinforced soil way” and “Geosynthetic materials play a major role in new underground stormwater detention system,” the [magazine’s] editor and author Terry Sheridan tout bridges on GRS abutments. Moreover, recent NCHRP-funded shake table testing shows that GRS abutments and walls can tolerate any credible earthquake.
In contrast, “MSE walls support laterally loaded drilled shafts” describes a classic MSE design to support a lateral load. Costs for that project would be significantly less had they used NCHRP Report 556 and proven GRS technologies. Both vertical and lateral load capacities in GRS are exponentially superior to MSE. (Go back to the articles mentioned in the preceding paragraph.)
The light, durable (cheap) facing blocks used for GRS would have seen much less distortion. And GRS would not have required a meter of embedment, which wastes of a lot of facing blocks (expensive in the case of MSE), and which misadventure made the wall taller and geogrids even wider, which favors the vendors, which is probably why they support AASHTO’s, FHWA’s, and NCMA’s often baseless guidelines.
(I can say this. I chaired the TRB Committee on Geosynthetics, 1990–1997, when these were developed and then immortalized. I was part of the problem!)
Deep patch: A good bad example of mixing paradigms
In the 1980s, the U.S. Forest Service developed an empirical technique for slide repair called the “deep patch.”
The deep patch was used on roads in mountainous terrain where cut/cast construction resulted in sliding in the cast material. Those innovators would excavate vertically 6–10ft and laterally to behind the failure scarp in the road, and replace that excavation with granular fill and sheets of nonwoven geotextiles on close spacing. It seemed to work most every time.
I partnered with Dr. J.T.H. Wu at CU/Denver to build a huge steel frame, inside of which we could build a full-scale embankment prototype. We discovered that this “deep patch” concept significantly unloaded the driving forces to the extent we were almost cantilevering dirt. Model this as a geomonolithic beam and the results are close to what we observed.
Paradoxically, USFS designers deduced that the only explanation for this behavior was the added tensile capacity of the inclusion and, therefore, concluded that the same results could be elicited with one sheet of stiff, high-strength inclusion. It became economical on paper to use just one layer of high-strength grid, which meant the excavation could be much shallower. I made an impassioned plea for them to change their design manual to reflect our research results.
Most engineers and professors cannot yet separate the concepts of simple tieback composite behavior (MSE) and geomonolithic, unique composite behavior of GRS. MSE has a failure rate! So will this misguided version of MSE as a deep patch. GRS and MSE are very different technologies.
It seems there are more paradoxes that not. This has been particularly the case in the checkered and confused history of MSE and GRS. We see this in practice and in organizations such as AASHTO and NCMA—and the FHWA, where one group does leading edge research and demonstrations showing the value of GRS and another continues telling the state DOTs that MSE is the way to go.
One of the most important articles in a while, also in this [June/July 2010] issue of Geosynthetics, is “Geosynthetic reinforced walls and steep slopes: Is it magic?” where Dr. Dov Leshchinsky presents a scathing rebuttal, in his classic understatement, to the morbidly flawed concept of reducing tensile strength and widening spacing of the stiff geogrids in MSE walls based on observed behavior in a few perfectly constructed test walls. (The K-something modification.)
A proposed design revision for MSE that includes either weaker grid or wider spacing is not appropriate. There are already enough contractor errors and outright failures with those constructions.
Finally, I am fascinated with the trend in MSE to keep lowering the quality of the backfill without field testing these combinations. The empirical formulae that sort of works for MSE is based on granular backfill. This is a major reason for failures in MSE.
Robert K. Barrett
President, Terratask LLC
Grand Junction, Colo.
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