Terraced Retaining Walls

Tiered retaining walls at the Plum Creek project provide stability, maximize buildable space, and enhance aesthetics through engineered design and global stability analysis.

Tiered Retaining Walls Provide Site Solutions

Tiered, or terraced, retaining walls are used for many reasons including aesthetics, functionality, constructability, and economics. The Plum Creek subdivision, developed by Richmond American Homes in Castle Rock, Colorado, incorporated tiered walls to address several of these needs.

To increase the buildable footprint, improve lot layout, and help restore the financial viability of the project, a feasibility study was conducted. CTL Thompson, a local engineering consultant, determined that the retaining walls needed to fit between Castle Rock’s skyline and ridgeline protection boundaries while still providing the building envelope requested by the developer. In addition, Castle Rock officials had strong concerns regarding public safety and long-term wall integrity due to recent retaining wall issues in the area.

To meet these expectations, the CTL Thompson team designed a system of tiered retaining walls. Each wall tier reaches heights of up to 6 ft (1.8 m), with the overall system rising approximately 40 ft (12 m). The structure gradually climbs the hillside, reinforcing the soil behind it while maintaining stability across the site. A global stability analysis was performed to evaluate the overall performance of the entire wall system.

Approximately 15,000 ft² (1,400 m²) of wall has been constructed so far, with additional phases planned as the development expands. The results have been successful for both the developer and the town. The developer gained the buildable space needed for the project, while the town received a high-quality retaining wall system with engineered stability and attractive aesthetics.

Global Stability and Tiered Walls

For the Plum Creek project, the solution involved constructing a series of tiered retaining walls reaching a total retained height of approximately 40 ft (12 m). When designing tiered wall systems, engineers must evaluate several factors including native soil conditions, slopes above and below the wall, excavation methods, and the performance of the retaining wall system as a whole. This evaluation is commonly performed through a global stability analysis.

Native Soil Conditions

Understanding site-specific soil conditions is essential for proper wall design. Soil testing reported in the geotechnical investigation allows engineers to evaluate key soil properties such as internal friction angle and cohesion.

These properties indicate the soil’s natural ability to resist movement and maintain stability when a retaining structure is introduced. One major indicator of potential global stability concerns is a low internal friction angle. Lower friction angles indicate weaker soil strength, which can increase the lateral pressures applied to a retaining structure.

Cohesion values may also be incorporated into global stability models. However, because cohesion can be unpredictable in the field, designers commonly use less than 10% of the reported cohesion value. The final amount used should be determined by the design engineer based on the specific project conditions.

Water conditions must also be considered. The presence of groundwater within the soil mass can significantly reduce soil strength. Oversaturated soils may behave in a buoyant manner, decreasing their resistance to shear and increasing the potential for global stability failure.

Influence of Slopes

Slopes located above or below retaining walls can influence overall wall performance. Slopes at the top of a wall may help with drainage and grading, but steeper slopes also increase the surcharge loads acting on the retaining wall.

Slopes of 3:1 or steeper often raise design concerns, particularly in seismic regions, and may indicate the need for a global stability analysis in any location.

Slopes at the base of a retaining wall create additional concerns regarding embedment and base stability. For walls constructed on level ground, a common embedment guideline is 1 inch per foot (25 mm per meter) of wall height. When walls are built on sloping ground, additional embedment is required.

A common design practice is to provide a minimum 5 ft (1.5 m) flat bench in front of the wall, often referred to as a “bench to daylight.” While this practice improves stability, it does not eliminate the potential need for a global stability evaluation.

Excavation Methods

Excavation practices can also influence wall performance. When constructing retaining walls, especially tiered systems, it is recommended that excavation occur in a series of stepped cuts rather than a continuous slope.

Stepped excavation improves the interaction between newly placed compacted soil and the undisturbed native soils, which helps increase overall stability during and after construction.

Tiered Retaining Walls

The behavior of a tiered retaining wall system is similar to a retaining wall with a sloped surcharge above it. The lower wall must resist additional loads created by the upper wall tiers.

A common rule of thumb is that the zone of influence extends a horizontal distance equal to two times the wall height. In other words, if an upper wall is located within a distance equal to twice the height of the lower wall, it is considered to be within the zone of influence and will apply surcharge loading to the lower wall.

Even when this guideline is followed, global stability concerns may still exist, and additional engineering analysis may be required.

Global Stability

Global stability analysis evaluates the behavior of the entire soil mass to determine whether it can maintain its intended shape and stability. This type of analysis may consider a single retaining wall or, as in the Plum Creek project, a full tiered wall system interacting with the surrounding soils.

Several analytical methods are commonly used for global stability analysis. Two widely used approaches are the Bishop Method and the Modified Bishop Method. In these analyses, potential failure surfaces are divided into multiple shear wedges, allowing engineers to evaluate the resistance along each potential slip surface.

Computer modeling evaluates various potential slip surfaces represented by arcs, curves, or planes. The minimum shear resistance required for stability along each surface is compared with the available shear strength of the soil.

Two primary failure modes are typically evaluated:

Deep-seated failure occurs when the critical slip surface begins in front of the retaining wall and extends below and beyond the reinforced soil zone.

Compound failure occurs when the critical slip surface begins along the face of the retaining wall or in front of it, passes through the reinforced soil zone, and continues into the retained soil mass.

A special type of compound failure is known as Internal Compound Stability (ICS). In this case, the slip surface begins at or above the foundation soil level and passes through the reinforced zone of the retaining wall. Additional information on ICS can be found in AB Tech Sheet #807: Internal Compound Stability.