# Does downdrag affect cantilever pile retaining wall design?

Downdrag
Downdrag in foundations occurs when soil layers consolidate and settle under additional load. This typically occurs when additional fill is placed on top of the existing soil. The actual downdrag force comes from the friction of the soil against piles as the soil moves down. It is assumed that soil settlement goes along with downdrag.

Retaining Walls
The controlling load for cantilever retaining walls is usually the lateral earth pressure. Any vertical load on the sheet pile or soldier pile is usually so small that it is ignored.

This retaining wall is designed to retain additional fill over consolidating soil. The piles are embedded below the consolidating soil and will not settle. Would the downdrag or settling soil affect the calculation of the horizontal soil forces?

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.

• Plenty of retaining walls are built and then backfilled on one side afterwards. The method has been used to widen the M25 where it is on embankment. Sheet piles were installed part way down the slopes, and then backfilled. This widened the crest of the embankment without widening the base. Mar 2 '15 at 12:46