It depends on the construction sequence and support conditions; however in my experience most walls like these are built by installing the piles and then excavating the open side.
As you've noted, consolidation settlement is a concern for piles because the 'downdrag' force effectively reduces their capacity as a percentage of the vertical resistance is now being used to counter the downdrag.
Walls, however, provide capacity against lateral movement, and therefore loss of vertical capacity is insignificant. The consolidation, however, could effect the lateral pressures (depending on the construction method / support conditions; see note at end).
The lateral force applied to the retaining structure is calculated as:
$$\sigma_h = K \sigma_v$$
Where:
- $\sigma_h$ is is the resulting lateral earth pressure
- $K$ is the lateral earth pressure coefficient
- $\sigma_v$ is the applied vertical pressure at the calculation location
If the pile is installed in situ prior to excavation then the lateral earth pressure can be based on the 'at rest' condition (see note at end), as the soil is never allowed to displace enough to generate the full 'active' stress.
The at rest lateral earth pressure coefficient for solis is:
$$K_0 = (1 - \sin \phi ' ) \times OCR ^ {( \sin \phi ' )} $$
Where:
- $K_0$ is the 'at rest' lateral earth pressure coefficient
- $\phi '$ is the effective stress friction angle of the soil being retained
- $OCR$ is the overconsolidation ratio ( > 1 for overly consolidated soils)
As the soil consolidates, therefore, the at rest lateral earth pressure increases; and therefore the lateral loading that the wall must structurally resist and retain increases.
Note
The 'at rest' condition can only be maintained so long as the retaining wall does not move. As the wall displaces, the lateral earth coefficient of the retained soil tends towards the smaller, 'active' coefficient (that is not proprotional to consolidation).
It is therefore a conservative assumption to say that the wall must sustain 'at rest' pressures.
Tomlinson suggests that $K_0$ should only be used when the movement of the structure is less than $5 \times 10^{-4} $ the retained height in normally consolidated soils; I would expect that limit to be lower for overly consolidated soils, as they require a larger strain to mobilse the 'active' condition.
It should be noted that, to the old British Standards at least, these kinds of walls are typically designed considering only the bending moment determined from the active / passive pressures, with an applied factor of safety (circa 1.4 - 1.5); potentially accounting for this.
