Pile Design and Construction Practice, Fifth edition

Depth of h(m)
Segment
(m bql)
� h
�0.38
R *�
� �� vo
p o � 0.13
Resistance of piles to compressive loads 235
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
Calculating the base resistance, q c at base � 9.7 MN/m 2 D inner/D CPT � 0.438/0.036 � 12.2
which is less than 0.83 � 9.7 � 10 3 /100 � 80.5. Therefore a rigid basal plug will develop.
From equation 4.35, q b � 9.7(0.5 � 0.25 log 914/36) � 1.45 MN/m 2 which is not less
than 0.15q c. Therefore Q b � 1.45 � 0.656 � 0.95 MN, which is less than that of the
unplugged pile (Q b � 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/m2 . Determine the required depth of the pile below rockhead
and calculate the settlement of the pile at the working load.
The stress on the shaft of a 1.5 m pile is
(MN/m 2 )
9
� 5.1 MN/m2
2 (��4) � 1.5
This is satisfactory for concrete with a cube crushing strength of 25 MN/m2 (allowable
stress � 6 MN/m2 ).
Load carried in shaft friction in the loose sand will be negligible.
From Figure 4.33 for quc � 4.5 MN/m The mass factor, j, for RQD from 15%
2 , � � 0.2.
to 35% is 0.2. Therefore, �, from Figure 4.34 is 0.65.
From equation 4.44, rock socket shaft friction � 0.2 � 0.65 � 4500
� 585 kN/m2 Taking a 7 m socket length, ultimate shaft friction � 585 � �1.5 � 7 � 19297 kN
Therefore, factor of safety on shaft friction � 19297
�
� 2.1
9000
(MN/m2 ��rc )
(MN/m 2 �f �
�� rc tan �cv )
Q s (MN)
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.
q c