?What special considerations should be taken into account in the design of a retaining wall stabilized by the nailing method

Special Considerations in Excavation Stabilization Design

Stepped Structures

In some cases, the use of stepped facing in a nailed (or soil nailing) retaining wall with horizontal setbacks between individual wall segments is appropriate.

When the horizontal setback is small relative to the horizontal bench height, the structure behaves like a wall with an inclined face.

According to MSE (Mechanically Stabilized Earth) wall guidelines, this occurs when setbacks are less than 1/20 of the retaining wall height, resulting in an equivalent slope of approximately 3 degrees.

If the horizontal setbacks exceed the height of the lower wall section (H2), each retaining wall acts independently and must be analyzed and designed as separate nailed retaining walls.

When setbacks are smaller than the lower wall height, the lower wall must be analyzed under an additional load equivalent to the weight of the upper wall.

Composite Structures

Composite structures refer to a combination of nailed retaining walls and ground anchoring systems.

For example, a composite system may include soil nails (or reinforced nails) in contact with ground anchors.

Nails and anchors are installed progressively from the top down during excavation. The main purpose of ground anchoring is to provide overall stability and significantly reduce deformation of the retaining wall, especially if anchoring is applied in the upper section of the wall. Ground anchors can also be installed along the entire height of the retaining wall using concrete struts.

Composite systems often involve retaining walls approximately 25 meters high.

The design approach depends on the configuration and behavior of the support system, especially the relative contribution and expected performance of the nails and ground anchors. For instance, in system design, wall analysis with only nails may be limited to controlling slip surface failure zones (e.g., within 1.5 times the proposed height behind the wall crest).

Then, the nailed zone is considered a rigid block. Ground anchoring should ensure stability against deep slip surfaces, so anchor length is controlled by stability requirements of the nailed block.