The Integration of Geosynthetics, Mechanical Structures & Vegetation for Slope and Embankment Stabilization

By Jerry B. Sanders, CPESC

The integration of the correct products and installation procedures can provide a successful recipe for the remediation of major slope failure.Just such a slope failure occurred during the spring of 2002 in Ft. Worth, Texas. The slope failure was 312 feet in length and extended 85 feet down an established earth bank. Located along the top of the bank was a series of large oak and elm trees. Many of the elm trees slid the total distance to the bottom of the slope, while two live oaks remained upright and traveled nearly 20 feet down the slide and were approximately 15 feet below their original elevation when they came to rest.

The owner placed several construction parameters and requirements on the design of the installation. The slide area was to be returned to a natural setting and the live oak trees were to be stabilized in their new location in the slide.

A series of broken sprinkler and drainage lines were located and determined to be the root cause of the slide. The broken lines in combination with a series of recent rain events, flooded the bank and resulted in the development of a slip plane between a limestone rock ledge located approximately 23 feet below the original lawn surface. Soil borings revealed a series of gravel and sand layers in the slope that served as percolation patterns for the movement of water through the slope. The repair system would need to specifically address the drainage of each of these granular layers as well as the drainage at the base of the slope.

It was imperative that all of the slide material be excavated to reveal the stable rock ledge where adequate drainage could be installed. It was determined that a perforated cellular confinement system would provide the perfect drainage mat along the top of the rock ledge while providing increased friction between the rock layer and the soil bank. To collect the flow of water passing through the existing granular seams in the earth bank, we selected a 12-inch composite drain. The composite drain provided for durability and ease of installation.

Due to limited material storage and equipment access the slope repair was constructed as a reinforced earth bank structure. The use of reinforced earth eliminated the need for storage of additional construction materials and made it possible to reconstruct the bank with small front-end loaders and excavators.

However, the reinforced earth installation did not provide a method to stabilize the existing Live Oak trees on the edge of the slope. To do this required the development of a structural support system to contain or retain the root ball of each tree. Each tree needed support independently of the slope reinforcement and the support was constructed as a container for the root ball of each. Two rows of galvanized steel pilings were installed along the downhill side of each of the root balls and a single row of pilings were installed along the uphill side. The rows of pilings were then fastened one to another with a single steel piling to control lateral movement along the top of the individual units and to provide a connection point for the installation of a series of braces between the two rows of pilings along the base. A series of tensioning rods were connected from the top of the first row of pilings along the base to the top of the pilings on the uphill side of the root ball locking the root ball between the piling installations. This was followed by the installation of a series of earth anchors driven into the stable earth bank and connected directly to the top row of pilings. (Figure 1 & 2)

Upon completion of the structural and mechanical connections to develop the skeleton of the container, a gabion wire mesh was installed across the downhill side of the root ball. The base of the wire mesh was anchored into the slope with earth anchors and the wire was then stretched between the first row of steel pilings. A filter fabric and gravel drain mat was then installed between the root ball and the gabion wire mesh. Following the installation of the drainage mat, the tensioning rods were tightened applying tension and completing the container construction.

With the trees secure on the slope, the area was cleared of all debris and the perforated containment system was installed along the top of the exposed rock ledge. Each of the granular seams located in the stable section of the earth bank were covered with a composite drain to provide for the collection and expulsion of any seepage from the granular seams directly to the base drain installation. The soil originally excavated in preparation for the repair was reinstalled and compacted over selected geogrid reinforcement. (Figure 3 “SancoFig3.gif”)

The final step of the repair was to stabilize the surface and control erosion of the newly developed slope. Using grass sod, seed and temporary erosion control mats.