Foundation on Clay


A load take down has revealed that the characteristic load at the column positions is 3500 kN. A small site investigation has been undertaken and the following reported:

  • clay from ground level to 30m depth
  • an SPT trend equal to 22 + 2z (z = depth below ground level).
  • average Plasticity Index value of 35%

Consider a 1.2m deep square pad footing:

R / A = 5.14 x Sc x Cu + q

  • Sc = 1.2 for a square or circle (EC7: Appendix D)
  • Cu = 99 + 9z = 109.8 kPa (as PI > 30 % use cu = 4.5 N cu base)
  • q = 20 x 1.2 = 24 kPa (unit weight of soil = 20 kg/m3)
  • (V / A) ult = (5.14 x 1.2 x 109.8) + 24 = 700 kPa
  • (V / A)max = 700 / 2.5 = 280 kPa (Apply overall FoS of 2.5)
  • A = 3500 / 280 = 12.5

Use a 3.6m square pad

Check settlement:

? = q B (1 – ?2) Is / E

  • q = 280 kPa
  • B = 3.6 m
  • ? = 0.5
  • Is= 1.2 (centre of a square foundation)
  • Ev = 200 cu = 200 (99 + (9 x 1.2)) = 21960 kPa
  • ? = (280 x 3.6 x (1 – 0.52) x 1.2) / 21960 = 41.2 mm

Consider a 0.75m diameter pile 25m long (contract pile tests will be done):

  • Cu at base = 99 + (9 x 25) = 324 kPa
  • Cu ave = (99 + 324) / 2 = 211.5 kPa
  • qs = ? cu ave = 0.60 x 211.5 = 126.9 kPa
  • Qs = qs x As = 126.9 x 0.75 x ? x 25 = 7475 kN
  • qb = Nc cu at base = 9 x 324 = 2916 kPa
  • Qb = 2916 x 0.752 x ? x 0.25 =1288 kN
  • (Qs+ Qb ) / 2.5 = 7475 + 1288) / 2.5 = 3505 kN
  • Qs / 1.2 = 7475 / 1.2 = 6229 kN

Now: max pile load = 0.25 fcu A or fcu = (3500 / 0.44) x 4 = 31818 kPa ( e.g use C35 concrete)

Check settlement using equivalent raft:

  • Load applied at 2/3 pile depth = 25 x 2/3 = 16.6m
  • Load spread a 1:4, so equivalent diameter = (2 x (16.6 / 4)) + 0.75) = 9.05m
  • equivalent stress = 3500 / (9.052 x ? x 0.25) = 54.4 kPa
  • ? = 0.5
  • Is= 1.0 (centre of a circular foundation)
  • Ev = 200 cu = 200 x (99 +( 9 x 16.6)) = 49680 kPa
  • ? = (54.4 x 9.05 x (1 – 0.52) x 1.0) / 49680 = 7.4 mm

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