Stability challenges

October 2012 » Features » ENGINEERED SOLUTIONS
Slope repair method proves versatile for shallow slides
Richard Short, P.E.
Workers install Plate Piles on a failed embankment on Caltrans Route 20/Highway 5 Separation.

Civil Engineers responsible for maintaining new and existing embankments face the challenge of keeping slopes stable. Ground improvement solutions have been used for decades to address global stability for new embankments and mechanically stabilized earth walls. However, many existing slopes are also unstable with shallow slides occurring to depths of 3 to 10 feet below the slope face. Historically, these failures were addressed by excavating the sliding mass and replacing the soil after flattening of the slope. However, this is expensive and time consuming.

The Geopier SRT method, developed in 2003, has proved its versatility on a wide variety of slope conditions that have been analyzed and successfully repaired. The system is composed of vertical steel reinforcing elements – Plate Piles – that are rapidly driven into the ground through the unstable soil into an underlying competent layer. Working with Geopier SRT engineers, University of California, Berkeley researchers showed that Plate Piles could increase the factor of safety of a slope by 30 to 50 percent. The system works because portions of the failing slope are each supported by a single structural element. Significant cost and time savings are achieved by eliminating the need for substantial earthwork operations to remove the original slide mass.

Plate Piles can be installed in tight sites and steep slope constraints that typically prohibit large equipment access. This soldier pile wall in Oakland, Calif., is repaired by installing Plate Piles with a hammer suspended from an excavator boom.

The Geopier SRT system is designed to stabilize slopes where soil conditions consist of an upper relatively shallow zone of weathered, loose, soft, or disturbed soil over a stronger zone of soil or soft rock located several feet below the slope surface. Since slides occur on an infinite variety of slope conditions, with variations including slope steepness, soil strength, site accessibility, and groundwater conditions, each new slope stabilization application requires a site-specific slope stability analysis to determine spacing and number of Plate Piles required. The Plate Piles are then engineered into a staggered spacing based on slope grade and soil properties. Lateral spacing is constant at 4 feet center to center, while transverse spacing up and down the slope is determined based on slope steepness and the shear strength of the underlying supporting layer. The closely spaced Plate Piles form a barrier where the soil arches between the plates and limits downslope movement. Slide forces are transmitted downward to the underlying stiffer soil, resisting lateral movements and increasing the factor of safety against failure. The overall solution increases slope stability without the cost and time of major earthwork operations.

Demonstrated performance
The fill for the overpass ramp embankments of Caltrans Route 20/Highway 5 Separation consists of clayey, highly expansive soils. After about 10 years, the stability of the embankment slopes began to deteriorate as evidenced by slope creep and shallow slides. The slope movement encroached on roadways and deteriorated the pavement causing numerous cracks parallel to the pavement edge combined with lateral spreading. Additionally, guard rail posts were undermined by shallow slides.

Caltrans considered several methods for stabilizing the slopes: lime treatment of the slope, slope inclination reduction using imported fill, non-expansive fill slope overlay, and the Geopier SRT slope stabilization method. The final project design included a combination of slope reduction, imported fill, and Geopier SRT stabilization. Plate Piles were selected to stabilize approximately 1.5 miles of ramp slopes as a cost-saving measure, with minimal earthwork operations and shorter construction time.

For construction of the Caltrans Route 20/Highway 5 Separation, 100 working days were scheduled to complete the installation of approximately 8,800 Plate Piles. Actual installation was completed in only 70 days at an average rate of 125 Plate Piles per day. Approximately 2,700, six-foot-long Plate Piles, and 6,100, ten-foot-long Plate Piles were installed using both hydraulic hammers and vibrator plates adapted for installation. The Geopier SRT system saved Caltrans more than $3 million and months of additional construction time versus traditional slope stabilization methods. Caltrans has been monitoring the stabilized slopes for many years with no signs of failures or creep.


The Plate Pile system works because portions of the failing slope are each supported by a single structural element. Significant cost and time savings are achieved by eliminating the need for substantial earthwork operations to remove the original slide mass.


Applications
Through site-specific engineering, the Geopier SRT system can solve problems on many new slopes and active shallow slides. The patented method allows for immediate stabilization in tight sites and steep slope constraints where large equipment access is typically prohibited, making it an ideal alternative to traditional methods in a variety of slope applications.

Natural slopes – Slides occurring on natural slopes often are caused by weathering combined with excessive groundwater conditions. Slides are also initiated by excavations (cut) into a slope that remove the toe or steepen the slope, excessive load placed at the top of a slope, water diverted over the face of a slope, or any combination of these causes. The unstable zone can vary from a few feet to tens of feet thick. The Geopier SRT system is designed for slides as deep as 10 feet. Plate Piles are ideally suited for natural slopes because they can be installed in most site conditions using track-mounted or handheld equipment.

Slides occurring on natural slopes often are caused by weathering combined with excessive groundwater conditions.

Fill slopes – Fill slopes composed of fine grain soils with slope angles steeper than 27 degrees experience shallow slides that occur in the zone affected by seasonal moisture change. Cycles of swelling and shrinking cause the soil to have a net volume increase with a corresponding decrease in shear strength. Runoff penetrates into the shrinkage (desiccation) cracks creating a rise in pore pressure and subsequent saturation of the near-surface layer. Slope failures occur when the near-surface layer becomes saturated and the shear strength has degraded so that the near-surface soil layer can no longer resist the lateral force of soil weight along the slope.

Installation of Plate Piles stops slide movement in progress at Crow Canyon Road in San Ramon, Calif.

The addition of reinforcing steel Plate Piles stabilizes the slope and/or prevents the slide from occurring because the steel pile resists the shear force along the slide plane. Soil does not flow between the plates because the spacing has been designed to take advantage of soil arching between Plate Piles. The reinforced layer will not trap water and will resist down slope movement. Tracked equipment can navigate common fill slope angles ranging from 27 to 32 degrees.

Road shoulder failures – Shallow slope failures are a common hazard along roads located in mountainous terrain where the slide scarp encroaches into the shoulder or pavement surface. These types of slope failures create a dangerous condition for roadway traffic and require immediate attention. In most cases, the depth of the slide is clearly shown by the exposed scarp left by the slide. Knowing the depth of the slide, the original slope configuration, and the soil properties enables the rapid design of the Geopier SRT repair. In most cases, the slide can be stabilized with minimal regrading and the installation of Plate Piles with equipment located on the road or with workmen on the slope secured with safety lines using air hammers.

Levee slope failures – One of the most critical slope conditions can be a levee failure. Levee embankments are often constructed of poor soils that have been excavated from the adjacent waterway. Levees also have differential slope conditions, which often mean that one side is under water and saturated while the opposite side remains in the dry. Because of the typically weak embankment soils, levees are often at risk of slope failure due to earthquakes. Repair or reinforcement is costly because the existing embankment must be maintained functional during the construction operations. Geopier SRT reinforcement is ideally suited for levees because it is minimally disruptive to the existing levee. A slope failure can be repaired with minimal grading and the installation of Plate Piles. Seismic stability has been researched at the University of California using shake table tests to simulate actual field conditions during an earthquake with results showing that sliding and loss of freeboard can be prevented with Geopier SRT reinforcement.

Shallow slope failures are a common hazard along roads located in mountainous terrain. This shallow slide undermined a soldier pile wall in Oakland, Calif.

Creek bank failures – High stream flows can cause creek bank failures that create environmental concerns for agencies such as fish and wildlife departments, U.S. Army Corps of Engineers, and others. The potential for stream siltation caused by the repair operations is a major concern with most agencies and requires a minimally invasive repair. Repairs using Plate Piles in combination with erosion protection can be accomplished with minimal earthwork operations. Typically, the wetted surface of the slope is protected with a heavy duty erosion protection material, which can include tree stumps or other natural materials. The underlying slope is reinforced with the Geopier SRT system to prevent a slope failure by increasing the factor of safety. Site preparation often involves cutting the slope to remove irregular unstable areas followed by installation of the Plate Piles. Upon completion, the underlying slope is reinforced against future failure and the surface is protected against erosion from the stream and surface runoff.

Unlined canal slopes – Along the same lines as creek bank failures, unlined canal slopes are threatened by toe erosion and rapid drawdown resulting from intermittent high flow in the channel. The erosion of the toe creates a near vertical slope. At the same time, the soil underlying the slope has become saturated up to the high water level. The sudden cessation of rain will drop the channel water level faster than the water can exit the slope, creating a condition called "rapid drawdown." The combination of toe erosion creating a near vertical slope plus the saturation of the soil causes a collapse of the slope. The Geopier SRT system of soil reinforcement underlying the channel slope prevents slides from occurring

Thousands of Plate Piles have been successfully installed on various sites plagued by historic landsliding and have been well-tested and proven by many seasons of heavy rainfall. When considering solutions for emergency repairs for shallow slides along failing embankments or designing cost-effective solutions for prevention of slides, Geopier SRT can provide a simple, rapid, and economical approach to long-term stability.

Richard Short, P.E., is founder-SRT at Geopier Foundation Company.


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