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Slope stabilization with platepiles

December 2010 Features PROGRESSIVE ENGINEERINGCaltrans tests a new slope repair alternative.

William McCormick, CEG

Chronic slope creep and shallow landslide failure affecting existing highway embankments andpavements, particularly those constructed in highly expansive clay fill soils, are problems facingtransportation organizations across the country. Conventional repair and reinforcement methodssuch as the installation of retaining walls, debris removal and slope reconstruction, or infillingwith riprapare costly, require lengthy construction periods, and cause traffic delays.

The California Department of Transportation (Caltrans) andother private and public design professionals in California aretesting an innovative alternative that is significantly less

expensive than traditional solutions and has minimalenvironmental and public impact: the platepile slope repairconceptbacked by scientific testing, modeling, and years ofsuccessful case studies.

Platepile method basicsThe platepile slope repair concept was developed in 2003 atthe Blackhawk Geologic Hazard Abatement District (GHAD)by Richard Short, G.E., president and founder of SlopeReinforcement 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 intounderlying stable soil or bedrock, which increases resistance to sliding, Short said.

Creeping clay-rich soils havecaused visible pavementdistress.

(above) Platepiles are installedon a slope using a vibratoryhammer attachment on anexcavator. (below) A close-upview of platepile installationshows the thin steel plateswelded to the upper section of

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Platepiles are small steel piles with thin steel plates welded tothe upper section of vertical steel elements. The plates resistsliding earth, forcing the energy downward to be absorbed intothe stable earth below. The platepiles are inserted in astaggered grid pattern on a slope using a vibratory hammer

fitted with an adaptor and affixed to a standard backhoe orexcavator. The offset grid pattern essentially divides the slopeinto individual increments or cells that are stabilized by eachplatepile.

The spacing between piles can be adjusted in accordance

with the SRT design manual charts and specific slopecharacteristic details, including soil shear strength, depth ofunstable material, and slope inclination, Short said.

The platepile slope repair method can significantly increase

the factor of safety against slope movement and creepdeformation by 20 percent to 50 percent for clay slopes andslope failures. It can stabilize active shallow slides on cut, fill, and natural slopes. In addition, ithas been shown to reduce the cost for slope stabilization by as much as 50 percent ofconventional methods.

Caltrans connectionsCaltrans recently used platepiles on a $5 million pilot experimental project to reinforce 1.2 milesof embankment at the intersection of Interstate 5 and Highway 20 near the city of Williams innorthern California. The embankment demonstrated extensive creeping and shallow slopefailures 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 werepreviously 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, Caltransselected platepiles as the most feasible solution with the time and space constraints, whichincluded 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 treatmentslope stabilization methods.

The ramp stabilization project included installation of 9,000 platepiles that ranged from 6 feet to10 feet long. The 6-foot platepiles were used to stabilize areas with unstable materials down to a3-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 tostabilize both soil creep and existing landslides. Platepile construction was conducted duringsummer 2010 by North Bay Construction of Petaluma, Calif. Kleinfelder assisted SRT withplatepile grid layout and documentation during construction.

a steel pile.

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A Caltrans spokesperson indicated that platepiles could be applied to other projects around thestate, depending on the evaluation of the experimental project. Caltrans plans to monitor theperformance of the platepile slope mitigation method for at least a year to assess itseffectiveness.

Like any new engineering method, platepiles must go through the necessary testing anddocumentation, said SRTs Short. We appreciate Caltrans new product program that allowedus to put the platepile method to work on the State Highway 5 repair project and Kleinfelder forrecognizing the benefits of the platepile method and verifying the method using its ownanalysis. Kleinfelder has used platepiles on numerous projects.

When platepiles are rightThe platepile slope repair method has been used on transportation, commercial, and publicprojects. Platepiles were used to repair a 33-acre hillside surrounding a commercial site in SantaRosa, Calif., that had experienced progressive translation soil slips each winter after intensestorms. 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 $4million as compared with slope reconstruction.

Another project located at the Canyon Lakes Geologic HazardAbatement District in San Ramon, Calif., demonstrated how aslide can be stabilized while still moving. The Crow Canyonroad slide occurred in February 2008 when the upper 3 feet ofthe slope along this busy parkway began to slide toward thepavement. A small excavator with a hydraulic hammer wasable to climb onto the moving slide mass and begin drivingplatepiles. 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 theconventional remove-and-replace earthwork method.

Like any other slope assessment, deciding whether the platepile method is an effective solutionfor a slope creep or landslide problem begins with a site evaluation. A geotechnical engineeringfirm can conduct geologic mapping of the site (geomorphology and determining extent and typeof existing or potential slope instability), identify depth of movement (preferred method is bysubsurface 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 usingavailable topographic data or field surveying measurements. Plot the estimated depth andlocation of the slide plane on the cross section to confirm that the slide conforms to the criteriafor a shallow translational-type slide 6 feet thick or less. The final step is to perform a slopestability analysis to determine the factor of safety against a deep-seated slide that would underliethe observed shallow slide.

Flags mark the offset gridpattern layout for platepiles.The pavement shows typicaldistress caused by creepingsoils.

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There are limitations to the platepile methodology. Currently, the platepile concept is notintended to stabilize deep-seated slides where the slide plane is deeper than 10 feet below thesurface of the slope. The platepile method is not an erosion protection method, and slopesurfaces must be protected against erosion as with other stabilization projects.

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

Will iam McCormick, CEG, is principal engineering geologist for Kleinfelder. He can be

contacted at 707-571-1883 orbmccormick@kleinfelder.com.

mailto:bmccormick@kleinfelder.commailto:bmccormick@kleinfelder.commailto:bmccormick@kleinfelder.commailto:bmccormick@kleinfelder.com