Settlement of pile group

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• Settlement of Pile Group

Dr. Karl Von Terzaghi and Peck suggested that total settlement of a group of driven or bored piles under the safe design load not exceeding 1/3rd to 1/2 of the group capacity can generally be estimated roughly as for an equivalent pier foundation.

The deformation and compressibility properties of soil under the pile group can be estimated from empirical correlations of penetration tests(SPT, CPT, DCPT) and Plate or Pile load Test or from the laboratory tests on undisturbed samples of the cohesive soils.

These estimates however should be cross checked by conducting Pile Load Test under actual conditions.

• Stress on the underlying Strata:

• 1)Soil strata underneath pile group is homogeneous from ground level itself.

As we can see in the plotted diagram, the stress is assumed to get dispersed from the ground level itself in a 2:1 ratio or at 30 degrees angle.

• 2) In case if the strata is not homogeneous, the distribution is assumed to occur at a distance of 1/3rd of the length of the pile from the bottom.
  

• 3) In case Pile group is resting on hard strata, then assume the dispersion to take place at the hard strata itself, again in a ratio of 2:1, or at about 30 degrees of angle.

• Empirical Correlations:

1) Skempton's formula:

Sg/Si = [ (4.Bg + 2.7)/(Bg+3.6)]^2

Here, Sg= Settlement of pile group

           Si= Settlement of individual pile.

           Bg=  Width of the pile group in m.

2) Meyerhof's formula:

Sg/Si = [ S(5-S/3B)]/[B(1+1/m)]

Here, S = Pile spacing in mm.

• A) Settlement of piles in the granular, cohesionless soil.

As a preliminary guide for normal loads(factored load with FOS 3), the settlement Si of displacement piles can be estimated by the following expression given by Vesic

  Si = B/1000 + Ss

Here, B is the pile diameter in mm, Ss is the static compression of pile in mm. Si, is the settlement of single pile in mm.

                             Static compression of pile can be found as follow:

            Ss = 1000. QL/AE

   Here, Q = Applied load in kN

             A = average cross section area in m2

             L = Length of the pile in m

             E = Modulus of Elasticity in kPa.

• B) Settlement of piles in the Cohesive soil:

Sci = {H.Cc.Log10[(P+dP)/P]}/(1+e)

For normally consolidated soils, Cc = 0.009(Wl-10)

For over-consolidated soils,       Cc = 0.001(Wl-10)

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