Engineers in California have implemented a platepile slope-repair technique that, in certain slope creep and shallow landslide situations, offers greater efficiency and improved results as compared with conventional slope repair and reinforcement methods.
Platepiles, or small steel piles with thin steel plates welded to the upper section of the vertical steel elements, are inserted in a staggered grid pattern on a given slope. The plates stabilize sliding earth, forcing the energy downward into the stable earth below. The platepiles are installed using a hydraulic hammer or a vibratory hammer fitted with an adaptor and affixed to standard backhoe or excavator. Developed as a slope reinforcement technique, platepiles can prevent slippage of the slope surface soil 1 to 10 feet deep.
Compared with conventional repair and reinforcement methods-such as the installation of retaining walls, debris removal and slope reconstruction, or infilling with riprap-platepile slope repair has been less costly, required shorter construction periods, and had minimal environmental and public impact.
Platepile Possibilities
First developed in 2003 at California’s Blackhawk Geologic Hazard Abatement District, the platepile slope repair concept is patented and distributed by Slope Reinforcement Technology (SRT).
Richard Short, G.E., president and founder of SRT, explains, “The concept relies on closely spaced vertical reinforcing elements, inserted through the unstable layer into underlying stable soil or bedrock, which increases resistance to sliding. The spacing between piles can be adjusted in accordance with the SRT design manual charts and specific slope characteristics, including soil shear strength, depth of unstable material, and slope inclination.”
The platepile slope repair method can increase the factor of safety against slope movement and creep deformation by 20% to 50%. It can stabilize active shallow slides, manmade cut-and-fill slopes, and natural slopes. In addition, it has been shown to reduce the cost for slope stabilization by up to 50% compared to conventional methods. Platepile slope repair is backed by scientific testing, mathematical modeling, and eight years of successful projects.
Slope Access Versatility
The platepile slope repair technique is well suited for a wide variety of slope conditions, including road shoulder slope failures, road-widening projects that require steeper slopes, creeping slopes, natural streambank failures, unlined channel and levee slope stabilization, shallow landslide and debris flow prevention/stabilization, reinforcement for steep slopes, and stopping a slow-moving landslide.
For example, the San Diego-based Kleinfelder consulting firm used platepiles to repair a 33-acre hillside surrounding a commercial site in Santa Rosa, CA. The hillside had experienced progressive translation soil slips each winter after intense storms. The design/construction team used 13,500 platepiles on a 4-foot-by-4-foot grid, which ultimately saved the owner over $4 million as compared with slope reconstruction. This is the largest number of platepiles to be used on a single site, and there have been no slope failures since installation in 2003.
Another project, located at the Canyon Lakes Geologic Hazard Abatement District in San Ramon, CA, illustrated how an active slide can be stabilized. The Crow Canyon Road slide occurred in February 2008 when the upper 3 feet of the slope along this busy parkway began to slide toward the pavement. A small excavator with a hydraulic hammer was able to climb onto the moving slide mass and begin installing platepiles. After one row was installed, the slide mass movement stopped, enabling the completion of the stabilization. Three hundred platepiles were installed at a cost of about $60,000. The district manager estimated that this 10,000-square-foot slide would have cost $200,000 to repair using the conventional remove-and-replace earthwork method.
Major Interstate Repair
The California Department of Transportation (Caltrans) is also very interested in the platepile slope repair concept. Caltrans recently used platepiles on a $5 million pilot project to reinforce 1.2 miles of embankment at the intersection of Interstate 5 and Highway 20 near the city of Williams in northern California. The embankment demonstrated extensive creeping and shallow landslide failures in the expansive clay fill placed over a decade ago, which was causing tension cracks and sinking of the pavement in the roadway.
Kleinfelder was retained by Caltrans to evaluate several engineering mitigation options that were previously suggested to stabilize this site. Based on past successful experiences on other projects, the firm also provided slope stability assessment and design utilizing the platepile method. After assessing all the mitigation alternatives, on the recommendation of Kleinfelder, Caltrans selected platepiles as the most feasible solution with the time and space constraints, which included protecting sensitive wetlands. The platepile alternative was also the most affordable. The estimated savings amounted to more than $3 million versus typical grading or soil-treatment slope-stabilization methods.
The ramp stabilization project included the installation of 9,000 platepiles ranging from 6 feet to 10 feet in length. The 6-foot platepiles were used to stabilize areas with unstable materials down to a 3-foot depth, while 10-foot platepiles were used in areas with unstable materials 4 to 6 feet deep. The platepiles were laid out on a 4-foot horizontal by 8-foot vertical grid, stabilizing both soil creep and existing landslides. Platepile construction occurred during the summer of 2010 by North Bay Construction of Petaluma, CA. Kleinfelder assisted SRT with platepile grid layout and documentation during construction.
When asked if platepiles could have an application on other projects around the state, a Caltrans spokesperson replied, “Yes, depending on the evaluation of the experimental project.” Caltrans plans to monitor the performance of the platepile slope mitigation method for at least a year to assess its effectiveness and future use throughout the state.
SRT’s Short says, “Like any new engineering method, platepiles must go through the necessary testing and documentation. We appreciate Caltrans’s new product program-which allowed us to put the platepile method to work on the State Highway 5 repair project-and Kleinfelder, for recognizing the benefits of the platepile method. Kleinfelder verified the method using its own analysis and has used platepiles on numerous projects since [the method’s] inception in 2003.”
Future Stability and Erosion Control
Deciding whether the platepile method is an effective solution for a slope creep or landslide problem begins, like any other slope assessment, with a site evaluation. A geotechnical engineering firm usually conducts geologic mapping of the site (geomorphology, determining the extent and type of existing or potential slope instability), identifies the depth of movement (the preferred method is by subsurface exploration), and characterizes the soil and bedrock conditions underlying the slope.
The next step is to develop a cross-section of the slope through the axis of the slide, using available topographic data or field surveying measurements, and to plot the estimated depth and location of slide plane on the cross-section to confirm that the slide conforms to the criteria for the platepile method. The final step is to perform a slope stability analysis to determine the factor of safety against a deep-seated slide that would underlie the observed shallow slide. As with any other method, there are limitations to the platepile methodology. Currently, the platepile method is not intended to stabilize deep-seated slides where the slide plane is deeper than 10 feet below the surface of the slope.
The platepile method, by itself, is not an erosion protection method. Slope surfaces that have been stabilized by platepiles must be protected against erosion just as with any slope stabilization project. Likewise, many erosion protection applications are not capable of providing protection against an underlying slope failure. Kleinfelder, together with SRT, is currently considering projects that incorporate the slope stabilization benefits of platepiles with surface erosion protection capabilities of erosion control fabrics. This concept utilizes the platepiles as the “anchors” for the surface stabilization/erosion control fabrics.
SRT has joined with Propex Geosynthetics and is using its AmorMat turf reinforcement mat to combine slope stability with heavy-duty erosion protection in stream channel applications. The AmorMat tie-down cables are attached to the platepiles, which combine to provide a high-performance turf reinforcement mat with an underlying stable slope. Lighter-duty mats and netting are also routinely used with platepiles to complete the slope repair, depending on the potential for erosion. The combination of platepiles with mats or netting results in a slope that is stabilized, vegetated, and has a greatly reduced surface erosion potential.