Slope stabilization with platepiles

December 2010 » Features » PROGRESSIVE ENGINEERING
Caltrans tests a new slope repair alternative.
William McCormick, CEG

Chronic slope creep and shallow landslide failure affecting existing highway embankments and pavements, particularly those constructed in highly expansive clay fill soils, are problems facing transportation organizations across the country. Conventional repair and reinforcement methods — such as the installation of retaining walls, debris removal and slope reconstruction, or infilling with riprap — are costly, require lengthy construction periods, and cause traffic delays.

Creeping clay-rich soils have caused visible pavement distress.

The California Department of Transportation (Caltrans) and other private and public design professionals in California are testing an innovative alternative that is significantly less expensive than traditional solutions and has minimal environmental and public impact: the platepile slope repair concept — backed by scientific testing, modeling, and years of successful case studies.

Platepile method basics
The platepile slope repair concept was developed in 2003 at the Blackhawk Geologic Hazard Abatement District (GHAD) by Richard Short, G.E., president and founder of Slope Reinforcement Technology (SRT), which owns the patent and distributes the proprietary method and product.

“The concept relies on closely spaced vertical elements, called mini-piles, inserted into underlying stable soil or bedrock, which increases resistance to sliding,” Short said.

(above) Platepiles are installed on a slope using a vibratory hammer attachment on an excavator. (below) A close-up view of platepile installation shows the thin steel plates welded to the upper section of a steel pile.

Platepiles are small steel piles with thin steel plates welded to the upper section of vertical steel elements. The plates resist sliding earth, forcing the energy downward to be absorbed into the stable earth below. The platepiles are inserted in a staggered grid pattern on a slope using a vibratory hammer fitted with an adaptor and affixed to a standard backhoe or excavator. The offset grid pattern essentially divides the slope into individual increments or cells that are stabilized by each platepile.

“The spacing between piles can be adjusted in accordance with the SRT design manual charts and specific slope characteristic details, including soil shear strength, depth of unstable material, and slope inclination,” Short said.

The platepile slope repair method can significantly increase the factor of safety against slope movement and creep deformation by 20 percent to 50 percent for clay slopes and slope failures. It can stabilize active shallow slides on cut, fill, and natural slopes. In addition, it has been shown to reduce the cost for slope stabilization by as much as 50 percent of conventional methods.

Caltrans connections
Caltrans recently used platepiles on a $5 million pilot experimental 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 slope failures in the expansive clay fill placed decades ago, which was causing tension cracks and sinking of the pavement in the roadway.

Caltrans retained Kleinfelder to evaluate several engineering mitigation options that were previously suggested to stabilize the site. Based on past successful experiences on other projects, Kleinfelder 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 installation of 9,000 platepiles that ranged from 6 feet to 10 feet long. 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 and were utilized to stabilize both soil creep and existing landslides. Platepile construction was conducted during summer 2010 by North Bay Construction of Petaluma, Calif. Kleinfelder assisted SRT with platepile grid layout and documentation during construction.

A Caltrans spokesperson indicated that platepiles could be applied to other projects around the state, 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.

“Like any new engineering method, platepiles must go through the necessary testing and documentation,” said SRT’s Short. “We appreciate Caltrans’ new product program that 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 and verifying the method using its own analysis.” Kleinfelder has used platepiles on numerous projects.

When platepiles are right
The platepile slope repair method has been used on transportation, commercial, and public projects. Platepiles were used to repair a 33-acre hillside surrounding a commercial site in Santa Rosa, Calif., that had experienced progressive translation soil slips each winter after intense storms. Kleinfelder engineers used 13,500 platepiles on a 4-foot by 4-foot grid. There have been no slope failures since installation in 2005. The platepile concept saved the owner more than $4 million as compared with slope reconstruction.

Flags mark the offset grid pattern layout for platepiles. The pavement shows typical distress caused by creeping soils.

Another project located at the Canyon Lakes Geologic Hazard Abatement District in San Ramon, Calif., demonstrated how a slide can be stabilized while still moving. 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 driving platepiles. After one row was installed, the slide mass movement stopped, enabling 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.

Like any other slope assessment, deciding whether the platepile method is an effective solution for a slope creep or landslide problem begins with a site evaluation. A geotechnical engineering firm can conduct geologic mapping of the site (geomorphology and determining extent and type of existing or potential slope instability), identify depth of movement (preferred method is by subsurface exploration), and characterize 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. Plot the estimated depth and location of the slide plane on the cross section to confirm that the slide conforms to the criteria for a shallow translational-type slide 6 feet thick or less. 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.

There are limitations to the platepile methodology. Currently, the platepile concept 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 is not an erosion protection method, and slope surfaces must be protected against erosion as with other stabilization projects.

The technique is well suited for road shoulder slope failures, road widening projects that require steeper slopes, creeping slopes, stream bank and levee stabilization, shallow landslide and debris flow protection/stabilization, reinforcement for steep slopes, or to stop a slow-moving landslide.

William McCormick, CEG, is principal engineering geologist for Kleinfelder. He can be contacted at 707-571-1883 or bmccormick@kleinfelder.com.

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