6.21 Consolidation settlement of pile groups
When a pile group consisting of friction piles is embedded in, or rests above a soft clay layer, it may transfer compressible stresses to the subsoil below the pile toe sufficient to cause substantial consolidation settlement, that must be added to the group settlement calculated using the methods described in Chapter 6.20.
A simplified procedure to estimate this settlement is based on the assumption that the total serviceability load transferred by the pile group Qw,group is acting at a depth equal to (2/3)L below the pile cap (Figure 6.79), and is distributed in the underlying soil following a 2:1 stress distribution (Das 2007, Budhu 2011).
![Schematic of a floating pile group consisting of four friction piles embedded in saturated clay. The distance between the centre of the piles in the longitudinal direction is B and the distance between the centre of the piles in the transverse direction is Z. The piles are connected with a pile cap, on which an axial compressive force Qw,group is applied. The length of the piles is L. At distance H from the pile toe an incompressible layer is found. The vertical stress applied at depth 2L/3 from the head of the piles is Δσz = Qw,group/(ZB). At depth z + 2L/3 from the ground surface the vertical stress is reduced to Δσz = Qw,group/[(Z+z)(B+z)], following a 2:1 distribution.](https://oercollective.caul.edu.au/app/uploads/sites/143/2025/03/6.79-corrected-typo-e1747025987161.png)
Under this assumption, the increase in the vertical stress in the clay layer is calculated as (see also Chapter 3.6):
(6.102) 
where Z and B are the dimensions of the pile group, depicted in Figure 6.79, and z is measured from the level where the total load is assumed to act (at 2/3L from the surface).
Following the estimation of the distribution of Δσz with depth along the compressible layer of thickness H+(1/3)L (Figure 6.79) we can estimate its consolidation settlement using the 1D (or 2D if the thickness of the compressible layer is comparable to the pile cap dimensions) linear or non-linear consolidation methods described in Part 4. Note that settlement of the soil layers above a depth equal to (2/3)L are not considered.
