Side-Hill Retaining Walls: An Overview and Alternatives (Part 2 of 2)
By Randy Post, PE, GIT
Owner and Editor, GeoPrac.net
Project Engineer, NCS Consultants, LLC
Originally Published on April 2, 2009
Download the PDF version of Side-Hill Retaining Walls: An Overview and Alternatives by Randy Post (PDF document).
It is recommended that you read Part 1 of Side-Hill Retaining Walls: An Overview and Alternatives prior to reading this part.
Wall Alternatives
We have looked at some of the general issues and problems with side-hill walls, covered what types of investigations may be needed, and looked at the tools to be used for analysis. Now we will discuss various wall types and alternatives to walls that can be used for side-hill applications. We will briefly describe each option and hit some of the significant pluses and minuses for their use. The various alternatives I will be discussing are given in the following list.
- Fill Slope or Embankment
- Reinforced Soil Slope (RSS)
- Gabion Wall / Gravity Wall
- Cast-in-Place Concrete Wall (CIP Wall)
- Mechanically Stabilized Earth Wall (MSE Wall)
- Soldier Pile – Lagging Wall (SPL Wall)
- Tangent or Secant Pile Wall
- Sheet Pile Wall
- Micropile Wall
- CIP Wall on Piles or Shafts
- Chemically Stabilized Wall (Ground Improvement)
- CIP Wall with Lightweight Fill
- Geofoam
- Anchored Walls or Tie-Back Walls
- Shored MSE Walls (SMSE Wall)
- Hybrid Soil-nail / Geofoam Wall
- Simply-Supported Slab (Conceptual)
Fill Slope or Embankment
Fill slopes or embankments are the simplest and often most cost effective option for these types of side-hill applications. But there is probably a reason why you are considering retaining walls, this option doesn’t always work for the constraints at hand. (Figure 15: Example of an existing highway embankment slope. Photo courtesy of Dave Perrotti, URS Corporation.)
Advantages: Cost effective, no special construction techniques
Disadvantages: Space requirements, significant borrow requirements
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Reinforced Soil Slope (RSS)
RSS are Slopes with some form of planar reinforcement, usually geosynthetic. They can be constructed with a slope of up to 70°. Facing can be wrap-around geotextile, or more of an erosion control type approach. (Figure 16: Example of a RSS slope. Image from Strata Systems.)
Advantages: Can be cost effective when compared with shallower embankment slopes requiring more borrow and/or right-of-way, can help provide improved compaction at the edges of the slope, geotextiles with in-plane drainage can help with drainage problems in the slope, can typically use on-site backfill
Disadvantages: Wide footprint for construction sometimes requiring temporary shoring, arguably fewer options for aesthetic treatment, can’t go vertical, weight of fill can increase driving forces for global stability, must analyze compound stability as well as conventional global stability.
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Gabion Wall / Gravity Wall
Gabion walls have wire mesh baskets that are filled with rock. They are a subset of gravity retaining walls that resist lateral pressures by simply being massive and heavy. Walls that would be considered gravity walls include crib walls, concrete modular walls, and bin walls in addition to the gabion walls. (Figure 17: Gabion retaining in a cut situation. Photo by lviescas.)
Advantages: Cost effective, easy to construct
Disadvantages: Weight significantly increases driving forces for global stability and provides no additional resistance, potentially prone to vandalism
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Cast-in-Place Concrete Wall (CIP Wall)
One of the more common types of retaining walls, CIP walls are constructed by excavating sufficient room to layout the reinforcement, form the wall footing and stem and pour it. Many times they have a shear key to help resist sliding. (Figure 18: CIP retaining wall along SR 90 in Sierra Vista, AZ. Photo courtesy of NCS Consultants, LLC.)
Advantages: Straightforward construction, many agencies have standard dimensions for typical cases
Disadvantages: Relatively high weight of fill, high bearing pressures, large temporary excavation, frequently requires select (structural) backfill material, no additional resistance for global stability failure surface, expensive compared to other walls if over 12-ft.
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Mechanically Stabilized Earth Wall (MSE Wall)
MSE walls use internal reinforcing elements, either steel strips, steel bar mats (steel grids), welded wire mats, geogrids, or geotextiles to stabilize the compacted soil mass. Facing elements complete the system and can be as simple as wrap-around geotextiles, to pre-cast concrete panels, gabion-type rockfill with wire facing, and modular blocks. Most of these wall systems are relatively flexible, allowing them to tolerate some deformations without causing problems. Modular blocks are arguably not as well suited to side-hill walls because they are not as flexible as other wall systems. For normal MSE walls on level ground, you start with reinforcement lengths of approximately 0.7*H. For side-hill walls, you might end up with 0.9-1.2*H in order to satisfy global stability. Beyond the reinforced backfill zone (with the reinforcement) it is advisable to have a zone of retained backfill with similar select properties and higher compaction than typical embankment. This increases the space required to construct the wall. (Figure 19: Wire-faced MSE wall along Catalina Highway, Mt. Lemmon, near Tucson, Arizona. Photo courtesy of NCS Consultants.)
Advantages: Flexible, tolerable of settlement, wide width of reinforcement lowers bearing pressure, can be cost effective, plenty of aesthetic options
Disadvantages: Select fill required to prevent corrosion of metallic reinforcement or prevent installation damage of geosynthetic reinforcement, wide footprint means large temporary excavation (don’t forget the retained backfill zone), wide widths of reinforcement needed to satisfy global stability, proprietary wall systems make specifying the type you want tricky, if on-site materials don’t meet material specs, import can drive cost sky high, must analyze compound stability as well as global stability.
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Soldier Pile – Lagging Wall (SPL Wall)
SPL walls consist of discrete vertical elements (soldiers) at some spacing with lagging used in between to retain the soil face and transmit the lateral earth pressures to the soldiers. The soldiers can consist of H-piles, wide flange sections, double channel sections, pipe sections, precast concrete or cast-in-place concrete. CIP SPL walls are similar to Tangent Piles described below. The lagging may consist of wood, light steel sheeting or precast concrete.
Figure 20: Soldier Pile Lagging Wall on US 70 near Ruidoso, NM. Photo courtesy of Mike Pegnam, Golder Associates, Inc.
Advantages: Materials are readily available, can be installed through hard strata (with proper equipment!), can resist high bending moments, can support large vertical loads, piles can be spliced, generally free draining, can be anchored if needed
Disadvantages: Steel elements are susceptible to corrosion, higher displacements than some systems, relies on passive resistance in front of wall
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Tangent Pile or Secant Pile Wall
These walls are constructed as a line of bored piles. If the piles are continuous, they are tangent to each other, if they overlap, they are secant. This wall type is often used in urban environments as a temporary or permanent shoring system for basement excavations for large high-rises and other structures.
Advantages: Constructed with conventional drilled shaft equipment, shafts can be designed to penetrate below global stability failure zone
Disadvantages: Costly, access in side-hill situations might be difficult, without consideration for drainage hydrostatic pressures can build up behind the wall, relies on passive resistance in front of wall.
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Sheet Pile Wall
Sheet pile walls are commonly found in coastal and marine applications, as cofferdams and as cutoff walls in levees and dams. They consist of a cold-rolled steel section usually in a Z-shape (at least for retaining wall applications) that has a mechanical ball and socket connection to link two adjacent sections. The sections are driven by pile-driving type equipment to the desired depth. They can be combined with H-Piles for a stiffer section. Although these walls are typically used in cut-type situations, they can be used in fill situations, allowing the sheet pile to project up in the air and then backfilling behind it. If additional lateral resistance is required, anchors or tiebacks can be utilized.
Figure 21: Sheet pile wall from City of Lincoln, Nebraska stream bank repair project. Photo by the City of Lincoln.
Advantages: Straightforward construction, easily driven/pressed/vibrated into most clays and sands, section can be optimized for required strength and stiffness (to a point), can be combined with anchors and tiebacks.
Disadvantages: Not likely to penetrate gravelly or rocky soil, water tight joints, wall heights have practical limitation for cantilevered wall.
Micropile Wall
A micropile is a small diameter (less than 300 mm or 12-in) drilled pile with some form of steel reinforcement that is bonded to the earth with grout and may or may not be placed under pressure. Micropile walls are made up of a cluster of micropiles. Three common approaches to these wall types are reticulated micropile (RMP) walls, “A-frame” walls, and anchored micropile walls. RMP walls can be thought of as analogous to reinforced concrete where the soil is the concrete, and the micropile is the reinforcing steel. The RMP is designed to assume that the load is transferred to the soil and the micropiles rather than just to the micropiles themselves as in conventional pile design. This soil/micropile interaction is referred to as the “knot effect”. “A-frame” walls are typically used to stabilize slopes from a global stability failure and obtain their name from the “A” shape of the wall in cross-section composed of vertical and battered micropiles. Anchored micropile walls, as the name suggests, are walls where micropiles act as vertical elements and anchors are used to reduce wall deflection or to overcome global stability problems.
Figure 22: SR 89a Banjo Bill Rockfall Containment project constructed by DBM Contractors. On the right, one of two anchored micropile walls on the project can be seen under construction. Notice the exposed micropiles as well as the first two rows of anchors and structural shotcrete. Photo from NCS Consultants, LLC.
Advantages: Very adaptable to site conditions, can be built on steep slopes and with relatively small and light-weight equipment.
Disadvantages: Cost, requires underground easements, specialized design procedures, relies on passive resistance in front of wall.
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CIP Wall on Piles or Shafts
Micropiles, driven piles or drilled shafts can be used to create a solid foundation for a CIP retaining wall to build one where you otherwise could not.
Advantages: Vertical elements can help increase FOS against global stability failure, minimize settlement
Disadvantages: Cost, refer to disadvantages from other wall types.
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Chemically Stabilized Wall (Ground Improvement)
A chemically stabilized wall is one that is created by improving the ground using one of the numerous methods of improving the ground. This wall system is more commonly used for cut walls than fill walls. I’ve never encountered a wall like this in side-hill applications over my relatively short career with one notable exception. My former firm was involved in the design of chemical stabilization of a failing side-hill MSE wall on a project. The contractor selected Jet Grouting as the method. I don’t’ think this is a common enough solution to worry about advantages and disadvantages, particularly because it would be among the most expensive wall systems you could construct, but I wanted to mention it for completeness.
Figure 23: Photo of exposed Jet-Grouted columns as shoring or possibly a basement wall. Photo by tirissoner.
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CIP Wall with Lightweight Fill
One of the drawbacks of CIP walls is that the additional load from the wall and backfill can decrease the stability of a slope in a side-hill application. By using lightweight backfill such as lightweight concrete fill (LCF), you can reduce the driving forces and make it so that no net increase in stress is “felt” by the slope. Unit weights of between 30 and 40 pcf are commonly attained with LCF.
Figure 24: CIP wall with LCF backfill from SR-90 in Sierra Vista, Arizona. Photo courtesy of NCS Consultants, LLC.
Advantages: Reduction in driving forces, strength is controlled by mix design and QC/QA (no backfill compaction to worry about)
Disadvantages: Cost of LCF, specialized contractor for LCF placement
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Geofoam
Geofoam or Expanded PolyStyrene (EPS) is another way of reducing the driving forces in a side-hill retaining wall application. It won’t work in every situation, but it is a nice option to keep in the back of your tool box. It is sometimes used in embankments where settlements are a concern. (Figure 25: Example of geofoam blocks. Photo by r.i.c.h.)
Geofoam comes in large blocks and can be hand placed and trimmed when necessary. They treat the adjacent soil for termites when they place it and put down a geomembrane for protection from gasoline spills when used in roadway applications.
Advantages: Reduction in driving forces, easy material to work with
Disadvantages: Must be protected from termites and fuel spills, costly, does not resist lateral pressures on its own
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Anchored Walls or Tie-Back Walls
Some of the above-referenced systems rely on passive resistance to resist the lateral earth pressure acting on the wall. In side-hill applications, this resistance is often not enough. Anchors or tie-backs as they are often called can augment a number of different wall systems described above, including SPL walls, micropile walls, CIP walls, and Tangent/Secant Pile walls. The basic system components are a bar or steel strand anchor with corrosion protection, a bonded zone which develops the resistance to the axial load, grout which transfers the load from the surrounding soil or rock to the anchor, the unbonded zone, and the structural connection to the face of the wall. (Figure 26: Strand type anchor connection at wall face with permanent load cell. Photo by NCS Consultants, LLC.)
Advantages: Additional lateral resistance, can shorten embedded portion of cantilevered wall systems
Disadvantages: Specialty geotechnical contractor needed, underground easements required, cost
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Shored MSE Walls (SMSE Wall)
As mentioned in the MSE Wall section, one limitation of MSE walls in side-hill applications is that you need a large temporary excavation to construct the wall. This frequently involves some type of temporary shoring, but in cases where the shoring is left in place (like a soil nail wall), the benefit of the long term stabilizing effect of the shoring is not considered when designing the MSE wall. A shored MSE wall is one where the shoring system is considered a part of the permanent wall system, and the stabilizing effect of the shoring reduces the length of reinforcement required in the MSE wall to resist lateral earth pressures. This type of wall system is not universally accepted, but there is guidance provided by The FHWA Central Federal Lands Highway Division’s document titled “Shored Mechanically Stabilized Earth (SMSE) Wall Systems Design Guidelines”. I’ve been told that some of the guidelines in that document might be making it into the revised FHWA MSE Wall Manual due out later this year.
Figure 27: Schematic diagram of a shored MSE wall (SMSE wall) in a highway application. From Morrison et. al. (2006), Shored Mechanically Stabilized Earth (SMSE) Wall Systems Design Guidelines
Advantages: Less temporary excavation compared with regular MSE walls, reinforcement lengths as little as 1.54-m or 30% of the wall height (whichever is larger) are possible.
Disadvantages: Joint between two systems must be handled properly to avoid a crack at the top, specialized design procedures and more complex analysis, not completely accepted in the industry consequently not all owners or agencies will accept this type of system, compound stability must be analyzed.
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Hybrid Soil-nail / Geofoam Wall
This approach was used on the SR 264: Second Mesa project in Northeastern Arizona. An existing highway with steep existing embankment slopes was being widened but had significant constraints, such as no new right-of-way acquisition, no cutting into the upslope cuts, and maintaining traffic during construction. It was originally designed to use MSE Walls, but the temporary shoring was thought to add significant cost to the project. NCS Consultants became involved in a value engineering effort on behalf of ADOT. (Figure 28: Soil nail rig installing nails adjacent to geofoam blocks. Photo by Bharat Kandel, ADOT via NCS Consultants, LLC.)
As mentioned in the geofoam section above, it does not support lateral loads because of its low unit weight. To overcome this, a permanent soil nail wall was used to take up the lateral loads from the existing embankment while allowing the creation of enough space to construct the geofoam portion and all while maintaining traffic on the highway. The geofoam was used to ensure that no net increase in unit weight was “felt” by the slope. The wall was finished with a precast concrete facing wall. For more photos and details on the hybrid soil-nail geofoam wall on SR 264, Second Mesa are available at the NCS consultants.com website. (Figure 29: Partially completed hybrid geofoam-soil-nail wall. Note footer for precast facing elements in foreground. Photo courtesy of Bharat Khandel, ADOT via NCS Consultants, LLC.)
Figure 30: Completed SR 264: Hydrid Geofoam / Soil-Nail Wall. Photo by Bharat Kandel, ADOT – courtesy of NCS Consultants, LLC.
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Simply Supported Slab (Conceptual)
Although not actually a retaining wall, the simply supported slab concept was developed for one of our projects that was an extreme example of side-hill retaining walls. The site was on the edge of an existing open pit copper mine in Bisbee, Arizona. The local officials wanted to have a sidewalk along the highway that followed the edge of the pit from a scenic overlook to another tourist area. The existing slope is approximately 15 to 20-ft high and sloped at approximately 1/2H:1V (about 63 degrees) in fractured bedrock. ADOT did not wish to shift the curb of the highway, so widening on the outside was investigated to support the proposed sidewalk.
Because of the extreme angle, slope stability and bearing capacity could not be satisfied with any of the side-hill retaining wall options we tried (and believe me, we tried most of the ones on the list above). So an alternative concept was investigated that we called the “simply supported sidewalk” because the sidewalk slab was supported by vertical support elements (micropiles etc) along the outside edge, and by a free end on a simple footer or on the existing sidewalk much like a simply-supported beam in statics. A schematic diagram of the concept is shown below in Figure 31.
Figure 31: The “Simply-supported sidewalk” concept which was never constructed. Courtesy of NCS Consultants, LLC.
Although we feel the concept was workable, there were other significant hurdles related to the acidic nature of the native materials that made the lifespan of regular concrete and steel virtually nothing. All of those things led to a significant cost for the project so the concept was never taken beyond the conceptual stage and the project was ultimately killed. If nothing else, it is an excellent example of the opportunities for outside the box thinking and hybrid systems when it comes to side-hill walls.
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Conclusions
Side hill retaining wall projects have been some of the most challenging and fun of any I’ve been lucky enough to work on. For starters, the investigations are anything but conventional…helicopter or crane moves, track mounted drill rigs, beautiful scenery (usually). And the analysis is challenging, the wall selection process is unique and finally, there’s nothing quite like seeing a completed retaining wall on that nasty slope.
Hopefully this article gave a good overview of the considerations for dealing with side-hill walls, maybe even a few ideas for how to handle them a little differently than you might have otherwise. By having an idea of what types of systems have been utilized on projects previously, you can be in a better position to analyze what might be the most cost effective wall system for your project. Perhaps you’ll even invent a new hybrid wall system that meets the project objectives.
In closing, I want to repeat the three most important points to remember when it comes to designing with side-hill retaining walls:
- Global stability
- Global stability
- GLOBAL STABILITY!
Acknowledgements
I would like to thank my colleagues at NCS Consultants, Drs. Naresh Samtani and Wolfgang Fritz for their comments on this article. Their experience and insight is most appreciated.
