Evolution of earth retention systems

June 2013 » Features » PROGRESSIVE ENGINEERING
Improved material properties, engineering knowledge, and advancements in construction equipment support new methods of stabilizing slopes.
Richard Short, P.E., G.E., D.G.E.
Early earth retention structures such as this stone gravity wall used the heavy weight of the stone to resist the lateral force of the soil being retained. Photo: Library of Congress

Engineers today face the same challenges designing earth retention systems that the master builders faced when they first started building large structures sometime around 2500 B.C. (Rocah, 2003). The necessity for earth retention systems has been a component of construction to be reckoned with from the start of civilization to modern times. The story of earth retention systems evolved over a period of approximately 4,500 years. During the first 4,200 years little change occurred in the basic methods used for earth retention. During the following three centuries up until the present time innovations brought huge advancements. New earth retention systems were based on the discovery of materials with superior properties, our increased knowledge of the engineering behavior of soils, and the significant advancements in construction equipment.

Early earth retention structures were constructed using available materials such as stone and timber. These structures utilized the heavy weight of the stone to resist the lateral force of the soil being retained. This most basic form of earth retention is referred to as a gravity wall. Early retaining walls were also constructed by driving vertical timbers called soldier piles, and then by stacking timbers horizontally behind the vertical soldier piles. The Romans constructed two parallel timber walls and filled the space between them with earth and stone to form a wide retaining wall called a cofferdam (Rowland and Howe, 2001). Gravity retaining walls made of quarried stone and the more temporary timber walls were the basic earth retention structures used during the pre-Roman era, the Roman-era, the Middle Ages, and the Renaissance up to the beginning of the eighteenth century.

During the Industrial Revolution, the need for earth retention structures grew, as the design of roads, bridges, ports, railroads, and canals became more complex. At this time, too, stronger, more efficient building materials became available such as structural steel and high quality cement called portland cement. (The equivalent of portland cement was used by the Romans but the formula was lost during the Middle Ages (Wayman, 2011).) Square steel rods were used for tensile reinforcement in concrete structures to create thinner, stronger, cantilevered structures having less reliance on gravity. In the nineteenth century, the cities of the developed world were expanding, and more and more structures required earth retention systems. With the invention of the steam engine, heavy equipment could move earth, pour concrete, drill deep holes in the ground, and drive piles, all of which encouraged the development of new methods for earth retention.

In the post World War II era, technologic advancements stimulated innovation in construction materials and techniques. In the 1950s, the list of innovations included prestressed concrete piles, precast crib wall members, prestressed concrete sheet piles, and closely spaced large-diameter drilled shafts. Generally speaking, these methods were used in a new category referred to as self-supporting structures.

Engineers took innovation to the next level and integrated the soil and the structure. An early soil/structure system used grouted tie-backs (or anchors) drilled into the retained earth behind the retaining wall to secure the wall structure, which may have been constructed of reinforced concrete, slurry walls, sheet pile walls, or side-by-side drilled shafts. The soil/structure system was advanced even further with the introduction of mechanically stabilized earth (MSE) structures. MSE systems are constructed by placing layers of steel tension reinforcing strips, polypropylene fabric, or geogrids between lifts of engineered fill. The fill is stabilized by the reinforcing while the face of the walls is protected by precast concrete shapes attached to the reinforcement.

Plate Pile elements are driven vertically through unstable soil into a competent layer in a staggered grid or honeycomb pattern.

Another reinforcing technique involved installing steel rebar, or soil nails, horizontally or at an angle into the undisturbed, retained earth to form an equally spaced grid of reinforcement. The face of the slope is covered with welded wire mesh with horizontal whalers of rebar connecting the soil nails. The face is then shotcreted to form a temporary, or completed, concrete surface. MSE structures use the strength of the soil itself to contribute to the stabilization, facilitating the move away from gravity walls and self-supporting structures.

What's fascinating about all these developments is how they made the transition from simply "innovative" to "state-of-practice." And engineers today are continuing to innovate. A more recent sensitivity to the environment is driving the innovation of design methods and construction procedures that have minimal impact on the environment. Engineers are developing systems that minimize the following:

  • impacts from construction equipment including carbon dioxide emissions, noise, and ground and groundwater contamination caused by fuel spills or oil leaks
  • disruption of habitat during the construction process
  • unsightly structures that obstruct natural scenery
  • the time required to complete construction
  • site ecology disturbance caused by temporary access roads and construction materials storage
  • removal of trees and vegetation to accommodate earthwork cut-and-fill operations

An earth retention system that reduces the impact to the environment by minimizing earthwork, concrete placement, use of large diesel-powered construction equipment, and construction time was developed for reinforcing slopes and repairing landslides. The Geopier SRT method is used to repair active shallow landslides or to reinforce existing slopes, eliminating the need for retaining structures. This concept uses slender steel Plate Pile elements, driven vertically through unstable soil into a competent layer in a staggered grid or honeycomb pattern. Similar to MSE retaining structures that rely on the strength of the soil, Plate Piles mobilize the strength of the soil through arching and transmit slide forces to the underlying stiffer soil.

The Geopier SRT method minimizes the site disturbance and leaves the slope and the natural vegetation in place. Today, tens of thousands of SRT Plate Piles have been installed successfully for use in reinforcing an existing slope or repairing a landslide. The Plate Pile method treads lightly on the ground and it saves thousands (and in some cases, millions) of dollars compared with traditional methods of constructing earth retention using heavy construction methods.

A considerable number of shallow landslides occur in North America annually. Data from state transportation departments, the Federal Highway Administration, and the U.S. Geological Survey indicate that approximately $750 million is spent annually to repair shallow slides up to 10 feet thick, not including the cost of structural damage (Schuster, 1996). Innovative methods of retaining earth can reduce this cost and stretch the funds available for slope repairs.

It has been said that most inventions grow out of other inventions, ideas, or concepts. For example, the ideas and benefits of three geotechnical concepts were incorporated into the Plate Pile design concept: Soil arching justifies using closely spaced reinforcing elements in rows instead of continuous solid walls; dividing the force to be resisted into small increments allows the use of relatively small reinforcing elements that can be carried into hard to reach areas; and the long-established method of inserting steel elements into the ground using pile-driving technology with impact or vibration energy allows Plate Pile installation.

Each new engineering innovation saves cost and improves the quality of our built environment. We can be certain that the need for creative solutions will continue, and it will be exciting to see what engineering innovations develop in the future.

Foundation Company, Inc., a subsidiary of Tensar Corporation.


References

  • Rowland, Ingrid and Howe, Thomas (editors), 2001, "Vitruvius – Ten Books on Architecture," Cambridge University Press.
  • Rocah, Louis I., 2003, "The Architecture Time Charts," University of Illinois.
  • Schuster, Robert L., 1996, "Landslides Investigation and Mitigation," Special Report 247, Transportation Research Board, Chapter 2 – Socioeconomic Significance of Landslides.
  • Wayman, Erin, 2011, "The Secrets of Ancient Rome's Buildings," www.smithsonianmag.com/history-archaeology/The-Secrets-of-Ancient-Romes-Buildings.html

Richard Short, P.E., G.E., D.G.E., is founder of SRT, now part of Geopier Foundation Company, Inc., a subsidiary of Tensar Corporation.


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