Stabilizing major landslides

December 2011 » Progressive Engineering
Tiebacks and micropiles can cost effectively increase resisting forces to provide required factors of safety.
John R. Wolosick, P.E., D.GE, M.ASCE

Many major landslides that have occurred throughout the United States in the last 15 years have been repaired using tiebacks and micropiles. This has enabled owners to repair their projects for reasonable costs and within tight schedules. The projects include public and private facilities for many types of owners including state transportation departments, railroads, casinos, shopping malls, and apartment complexes. This article presents case histories from several projects located in a wide range of geological conditions and terrains, including Southern California, the Rocky Mountains, the Appalachian Mountains, middle Tennessee, and the southern Mississippi River region.

The Signal Hill Slide threatened US 61 in Mississippi and required closure of the southbound lanes.

The design for the repair of landslides is most often assessed in simple terms of forces and factors of safety. The factor of safety, in its simplest form, is defined as the ratio of the resisting forces divided by the driving forces. Drilled and grouted elements add to the resisting forces in this equation. The required factor of safety provided for any project depends on decisions made by the design team. Typical factors of safety provided usually vary between 1.3 and 1.5. However, slightly lower numbers such as 1.2 have been used when huge amounts of force have been required and technical or budget reasons have limited the installed systems.

Table 1 summarizes the projects presented in this article listed in order, based on the required horizontal force applied by the tiebacks and/or micropiles to achieve the desired factor of safety.

Table 1: Major landslide stabilization projects
Project Name Location
Required Horizontal Force (kips/linear foot)
Static Factor of Safety
Seismic Factor of Safety
Blue Trail Slide Alpine, Wyo.
172
>1.5
1.1
Oso Creek Slide Orange Co., Calif.
160
1.5
1.1
Ameristar Casino I Vicksburg, Miss.
130
1.3
-
Ameristar Casino II Vicksburg, Miss.
130
1.3
-
Mission Viejo Mall Mission Viejo, Calif.
102
1.5
1.1
Signal Hill Slide Vicksburg, Miss.
100
1.2
-
Lexington Apartments Nashville, Tenn.
72
1.3
-

 

Oso Creek Landslide
In 2000, heavy rains saturated the hillside and caused Oso Creek in south Orange County, Calif., to swell and undercut the toe of the slope downhill from a roadway called Camino Capistrano and the Orange County Transportation Authority railroad line. The site consisted of loose fill materials overlying residual weathered Capistrano formation and deeper unweathered Capistrano.

Repair of the 640-foot-long Oso Creek Landslide in Orange County, Calif., included 324, 70- to 75-foot-long micropiles and 60, 150- to 170-foot-long tiebacks.

The landslide was almost 640 feet long. The repair included 324 micropiles, 5-1/2 inches O.D., which were 70 to 75 feet long. Sixty tiebacks were installed with capacities of 400 kips and 550 kips, and were 150 to 170 feet long. The system was designed for a static factor of safety of 1.5 and a seismic factor of safety of 1.1 for a design earthquake with a 0.15g horizontal acceleration.

Blue Trail Slide
The Blue Trail Slide occurred on US 26/89 within the Snake River canyon in west-central Wyoming, just north of the city of Alpine. The slide consisted of a block failure of various types of detrital rocks with sand, silt, and clay sliding on shales and siltstones.

To stabilize the Blue Trail Slide near Alpine, Wyo., 475 micropiles were installed through 4-foot by 4-foot reinforced concrete cap beams on multiple retaining walls.

A combination micropile retaining wall and tieback anchor system stabilized the slide. Because of the huge dimensions of the slide, which included about 150 feet in elevation change from the roadway level to the river, multiple retaining walls were required for stabilization. A last-minute uptick in the design horizontal ground acceleration to 0.11g required large tieback anchor forces for the lower wall to provide a seismic factor of safety of 1.1. The static factor of safety exceeded 1.5.

The micropiles and tiebacks were installed through 4-foot by 4-foot reinforced concrete cap beams. The elements included 475 micropiles, 4-1/2 inches in outside diameter (O.D.), varying in length from 40 feet to 80 feet. The tieback anchors included 16 anchors for the upper walls (350 kips), which were about 105 feet long, and 23 anchors for the lower wall (630 kips), which were about 85 feet long.


Steeply inclined (45 degrees) tieback anchors with large wales hold a retaining wall at the Ameristar Casino in Vicksburg, Miss., consisting of driven steel soldier piles and treated wood lagging.

Ameristar Casino I
This project, completed in 1993, was located near the intersection of Interstate 20 and the Mississippi River in Vicksburg, Miss. Two retaining walls were required to support the existing bluff while providing grade separation for an access road to the casino and a parking lot. The walls consisted of driven steel soldier piles and treated wood lagging with steeply inclined tieback anchors with large wales. The surficial soils included a thick loess layer underlain by high-plasticity clay terrace deposits overlying a relatively thin limestone shelf at depth.

The anchors were drilled into the underlying limestone at a 45-degree angle and were 480 kip capacity. Lengths varied from 100 feet for the top row to 75 feet for the bottom row. The system was designed for a global factor of safety (static) of 1.3.

At the Ameristar Casino in Vicksburg, Miss., a mechanically stabilized earth retaining wall was used as the back form for a 550-foot-long, cast-in-place concrete wall that included two rows of 220-foot-long tieback anchors drilled at shallow angles.

Ameristar Casino II
A very large landslide was documented at the north end of the casino and was repaired in 2005-2006 using tieback anchors. The landslide encompassed a large mechanically stabilized earth (MSE) wall. The MSE wall was incorporated into the fix as a back form for a 550-foot-long, cast-in-place concrete wall that included two rows of 220-foot-long tieback anchors drilled at very shallow angles of about 10 degrees. The 385-kip anchors were installed within a hard marl layer. The anchors were spaced at 6 feet horizontally and 15 feet vertically. The design of the slope stabilization system provided an overall global factor of safety of 1.3.

A retaining wall at the Mission Viejo Mall in Orange County, Calif., is made from 42-inch-diameter reinforced concrete drilled shafts with 95- to 170-foot-long tieback anchors. A concrete facing was installed after construction.

Mission Viejo Mall
In 1990, a huge landslide occurred at the Mission Viejo Mall in Orange County, Calif. The slide was located between the mall and Interstate 5. The slide mass was about 350 feet long and 90 feet high. A permanent fix of a tieback-supported drilled shaft retaining wall was selected to stabilize the slope.

The slope consisted of clay fill overlying weathered Capistrano formation and unweathered Capistrano formation materials. The retaining wall consisted of 42-inch-diameter reinforced concrete drilled shafts on 6-foot center-to-center spacing. The anchors were 250 kips capacity each and varied from 95 feet to 170 feet in length. The design factor of safety (static) was 1.5, including assumed elevated groundwater levels.

Signal Hill Slide
In 2006, the Mississippi Department of Transportation decided to use deep tieback anchors to repair this huge landslide, which encompassed 20 acres (pictured on page 36). The slide threatened the four-lane highway. Traffic was rerouted onto the northbound lanes, with one lane utilized in each direction. The soils onsite included loess fill, loess, and terrace deposits overlying stratified deposits of sands and high-plasticity clays.

The anchors were installed at an angle of 45 degrees from horizontal in five rows and ranged from 170 feet to 265 feet long. Anchor loadings were typically 466 kips each. Large concrete reaction blocks, typically 13.7 feet square, were precast onsite. A total of 253 anchors were installed. The design of the slope stabilization system provided a factor of safety of 1.2.

Lexington Apartments
In late February 2003, after a very heavy rain, the 2:1 to 4:1 cut-slope behind three units of the Lexington Apartments in Nashville, Tenn., began to move toward the buildings. Each unit contained 24 apartments.

The slope stabilization plan included a tieback-anchored soldier beam retaining wall, socketed into limestone bedrock. The wall was augmented with two additional rows of tiebacks uphill, installed through 10-foot by 10-foot precast concrete reaction blocks.

The design of the slope stabilization system provided a factor of safety of 1.3. A total of 80 tieback anchors were installed at a 30-degree angle, including 28 in the upper row and 29 in the middle row. Twenty-three tiebacks were installed through the soldier pile wall. Anchor loadings were 300 kips. Anchor lengths varied from 85 to 110 feet.

Conclusions
Although tieback anchors and micropiles are sometimes considered expensive by some engineers and owners, they often offer the best solution to repair major landslides. These systems have been used very effectively in many varied geologic settings around the United States to stabilize very large landslides. The design teams that worked on these projects selected design factor of safety values that matched the needs of the project, considering technical concerns and budget constraints.

John R. Wolosick, P.E., D.GE, M.ASCE, is director, Hayward Baker Inc., in Alpharetta, Ga. He can be contacted at jrwolosick@haywardbaker.com.

This article is revised and condensed from a paper presented at the American Society of Civil Engineers Geo-Institute's Earth Retention 2010 conference. It is published here with permission from ASCE.


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