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 = 12.5Use a 3.6m square padCheck 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 mmConsider 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