Pile Design and Construction Practice, Fifth edition

Resistance of piles to compressive loads 235

Calculating the base resistance, qcat base 9.7 MN/m^2 Dinner/DCPT 0.438/0.036 12.2
which is less than 0.83  9.7  103 /100 80.5. Therefore a rigid basal plug will develop.
From equation 4.35, qb 9.7(0.5 0.25 log 914/36) 1.45 MN/m^2 which is not less
than 0.15qc.Therefore Qb 1.45  0.656 0.95 MN, which is less than that of the
unplugged pile (Qb 9.7  0.1046 1.01 MN.

Total pile resistance 0.95  1.01 1.96 MN

Example 4.7

A bored and cast-in-place pile is required to carry a working load of 9000 kN at a site where
4 m of loose sand overlies a weak jointed cemented mudstone. Core drilling into the
mudstone showed partly open joints and RQD values increased from an average of 15% at
rockhead to 35% at a depth of 10 m. Tests on rock cores gave an average unconfined
compression strength of 4.5 MN/m^2. Determine the required depth of the pile below rock-
head and calculate the settlement of the pile at the working load.

The stress on the shaft of a 1.5 m pile is

This is satisfactory for concrete with a cube crushing strength of 25 MN/m^2 (allowable
stress 6 MN/m^2 ).
Load carried in shaft friction in the loose sand will be negligible.
From Figure 4.33 for The mass factor, j, for RQD from 15%
to 35% is 0.2. Therefore, , from Figure 4.34 is 0.65.
From equation 4.44, rock socket shaft friction 0.20.65 4500
585 kN/m^2
Taking a 7 m socket length, ultimate shaft friction 585 1.5 7 19 297 kN

Therefore, factor of safety on shaft friction

Because of the open joints in the rock it will be advisable to assume that the base
resistance does not exceed the unconfined compression strength of the rock.

19297

9000

2.1

quc 4.5 MN/m^2 , 0.2.

9

( (^) 4)1.5^2
5.1 MN/m^2
Depth of h(m)
Segment
(m bql)
17 – 16 0.5 0.599 1.055 9.5 0.174 0.087 0.250
16 – 15 1.5 0.395 1.047 8.9 0.107 0.053 0.152
15 – 14 2.5 0.325 1.037 8.4 0.082 0.041 0.118
14 – 13 3.5 0.286 1.026 7.9 0.067 0.033 0.095
13 – 12 4.5 0.260 1.015 7.4 0.057 0.028 0.080
12 – 11 5.5 0.241 1.003 6.8 0.048 0.024 0.069
11 – 10 6.5 0.226 0.989 6.3 0.041 0.020 0.057
10 – 9 7.5 0.214 0.974 5.8 0.035 0.017 0.049
9 – 8 8.5 0.204 0.958 5.2 0.029 0.015 0.043
8 – 6.5 7.25 0.217 0.935 4.4 0.026 0.013 0.037
Total Qs 0.95 MN
(MN/m^2 )
(MN/m^2 ) (MN/m^2 ) rc tan (^) cv
qc rc f Qs (MN)

vo
po
0.13

h
R*
0.38