The Green Wall
Federal Reclamation Program, Appalachia Team, Office of Surface Mining, Pennsylvania.
Project Point of Contact
412-324-2828, ext 22
The Shawnee Wells Landslide Project near Pikeville, Kentucky featured the construction of a Green Wall which abated an OSM Abandoned Mine Lands emergency landslide. The wall was essentially compacted lifts of the site s native soil integrated with layers of geogrid reinforcement. Welded wire facing units were added to give it some geometry. Turf reinforcement matting was placed to keep the soil intact and serve as a medium to support vegetation. A stone blanket drain was incorporated to isolate the soil mass from re-saturation. All this combined to create a gravity wall that was able to support and abate the emergency and promote vegetation. A surprising aspect about this green initiative was that it did not come at a premium; it was both economical and practical. The green concepts utilized on this project can be adapted to abating other AML emergencies and reclamation projects by both OSM and state reclamation programs.
A landslide had developed behind two homes. This slide was being influenced by the seasonal drainage emanating from an abandoned coal mine. The unstable land mass had already damaged a wooden fence and was threatening to damage the Wells home as well as their neighbor, thus placing the occupants of the homes in extreme danger. When the site was initially reviewed the original abatement scheme involved the construction of a reinforced concrete or a gabion basket wall. For the last thirty-five years the Office of Surface Mining Federal Reclamation Program Division has abated landslide emergencies resulting from eligible abandoned mine lands with various techniques and have been very successful. The downside has been some of these measures have been expensive and time consuming. Typical remedial measures require large quantities of off-site materials. Steel beams, tons of rebar, grout, and concrete which are all utilized to construct various types of retaining structures. In a lot of landslide projects stone of various sizes are hauled to the site and used as backfill to buttress failing slopes and to replace the excavated void left by landslide removal. In addition, a tremendous amount of waste material (spoil) is generated that must be removed from the landslide construction site and permanently stored. Permanent waste areas are difficult to obtain and transportation of the landslide spoil to these facilities drives up the cost of abatement. Transportation of these materials during construction almost always creates a negative impact aesthetically and environmentally. This practice often resulted in physically damaging roads, bridges, and personal property.
Several designs have been developed and proven that have utilized native soil as the bulk of retaining medium; mostly in instances when the native soil possess good engineering qualities. The “green wall” featured in this design was constructed on a reasonably good foundation, which was for the most part was comprised of shale and compacted sandy clay. The site’s native soil consisted of mostly of sandy clay. The Standard Proctor test run on samples indicated that a density of 115.7 pounds per cubic foot at a moisture of 14. 7% could be achieved at 95% compaction. The design criteria followed a width to height ratio of 75%; a ten foot embedment depth to a height of fifteen feet. 18 by 18 by 10 welded wire “L” facing units were tied together and placed on the ground and served as the exoskeleton which gave the wall some geometric stability and served as a yard stick to help manage the soil placement. Soil was then placed behind the facing units in nine inch lifts and compacted. The soil was reinforced with layers of geogrid reinforcement placed every eighteen inches with a ten foot embedment. Turf reinforcement matting was placed on the fronts and the exposed tops of the facing units to keep the soil intact and serve as a medium to support vegetation. A stone aggregate wrapped in filter cloth was placed at the rear of the wall of the excavation to form a continuous drainage blanket that emptied into an 8 HDPE pipe sub-drain. This drainage blanket served to isolate the soil mass from re-saturation and to intercept migrating seepage from the rock\backfill interface to relieve any hydraulic pressure that may develop. It also provided a path for the remaining fifteen plus percent moisture remaining in the backfill to consolidate over time. All this combined to create a gravity wall that was able to support, abate the emergency, and promote vegetation for a more esthetic “green” appeal.
Results and Achievements
This project demonstrated to be an economical alternative when dealing with marginally strong soils where historically these sites would have been excavated, land slide material transported off-site, and replaced with more expensive and expedient concrete, gabion structures, or a rock toe buttress. This project has shown that the overall dependency on outside materials (stone, concrete, etc.) was significantly reduced. In addition, transportation of materials (unstable landslide soils) from the project site was eliminated which minimized adverse impacts from excessive over the road traffic thus reducing overall cost.
The total project construction cost was in the $60K range. If other more traditional landslide abatement techniques had been used the overall cost would have been over $100K. Another thing that this concept allowed us to do was sequence the landslide abatement. In traditional wall or buttress projects you usually have to cut all the material out at the same time near the toe of the slope to place the structures. With this concept we were able to buttress one section of the slide at a time thus reducing the exposed unsupported sections of the slide material. This is crucial when you are dealing with landslides very close to homes. Unlike more conventional concrete or stone retaining walls that can be an eyesore to some, it is expected that within three-five years from its completion any visual evidence of this wall will be eliminated by the growth of planted grasses and trees.
The Office of Surface Mining has been diligent in the search and selection of ideas from design systems that have been developed by other agencies and be adapted for our use. These selected techniques have already been incorporated into a handful of past OSM emergency abatement designs; but there has been an understandable resistance to change long standing successful abatement measures and to adopt methods that are perceived as radical. This project illustrates a turning point in the acceptance of some of these methods in certain circumstances. With the culmination of these little successes our team has demonstrated that these green techniques can now be adapted by OSM and other state reclamation programs to abate future AML emergencies and reclamation projects with similar circumstances. Countering the idea that most green projects come at a premium and are not usually an economical alternative.
A number of landslides that OSM encounters are comprised of water saturated coal refuse material and we have no other option but to remove the material or buttress it. On occasions it is comprised of saturated colluvial material. A great deal of this colluvial material has been sitting at rest for hundreds of years if not thousands of years at a safety factor a little greater than one. In many of our cases mine drainage disrupts this equilibrium and the material gets saturated and then fails. Designs that deal with the water and reinforcement of the soil raise that factor of safety back to a respectable level. The native soil in the next landslide may possess enough redeeming qualities to be reused as backfill material. One has to understand that it will require a little more attention to detail but this has proven that it can be a more economical solution and greener solution to the more traditional landslide abatement techniques.
Criteria to keep in mind:
- Have a staging area close-by to stockpile and keep dry during inclement weather.
- Isolate the backfill from re-saturation with a blanket or chimney drain system.
- Usually requires about 75% of the designed height for the depth of reinforcement.
- Can be built by readily available construction equipment and does not require special labor skills.
- Can be sequenced so issues with creating more instability while trying to abate the problem are reduced.