Special Design Considerations for Excavation Support

Stepped Structures

In some cases, the use of a stepped facing on a soil-nailed retaining wall with horizontal setbacks between individual wall sections (Figure 1) may be appropriate.
When the horizontal setback relative to the horizontal bench height is small, the structure behaves equivalently to a wall with an inclined facing.
Using design guidelines specific to Mechanically Stabilized Earth (MSE) walls, this situation occurs when the setbacks are less than 1/20 of the height of the stepped retaining wall, resulting in an equivalent slope of approximately 3 degrees.
When the horizontal setbacks are larger than the height of the lower retaining wall section (H2), each retaining wall acts independently, and each must be analyzed and designed as two separate soil-nailed retaining wall structures.
If the horizontal setbacks are smaller than the height of the lower wall, the lower retaining wall must be analyzed under the additional load equivalent to the weight of the upper retaining wall.

Composite Structures

Composite structures refer to a combination of soil-nailed retaining walls and ground anchor systems.
For example, a composite system may include nails (or soil nail reinforcements) that are in contact with anchors (Figure 2).
Nails (or reinforcements) and anchors are installed progressively from the top downward during excavation.
The main objective of using an anchoring system is to contribute to overall stability and significantly reduce deformations of the soil-nailed retaining wall, especially when anchoring is applied in the upper part of the wall (Figure 2a).
Anchoring can also be installed along the entire height of the wall with the help of precast concrete struts (Figure 2b).
Composite systems have been used for walls approximately 25 meters high.

The design method depends on the configuration and structure of the support system, particularly the relative contribution and expected performance of the nails (or reinforcements) and the anchors.
For instance, in the design of the system shown in Figure 2a (where nailing provides greater stability against shallow failure surfaces, while anchors contribute more to deep failure stability), it is recommended to analyze the wall only with nails (or reinforcements) and limit the slip surface in the shallow failure analysis (e.g., within a distance less than 1.5 times the proposed height behind the toe of the retaining wall).
Then, the nailed zone is considered as a rigid block.
Anchors must provide stability against deep failure surfaces, so their length is controlled by the stability requirements of the nailed block.

Note:

  • C and T are derived from the horizontal and vertical equilibrium of nails (or reinforcements) inclined at 15 degrees and highly inclined nails at 45 degrees.

  • W: The weight of the facing supported by the nails (or reinforcements).

Figure 1: Example of a stepped soil-nailed retaining wall (Source: Byrne et al., 1988)

Figure 2: Composite soil-nailed retaining wall structures (Source: Byrne et al., 1988)