Pile Design and Construction Practice, Fifth edition

Calculating the allowable horizontal load which limits the lateral deflection at ground
level to 25 mm, from equation 6.50:
Depth to point of fixity � 1.4R � 1.4 � 3.3 � 4.62 m
From equation 6.20 with e � 0, H � 3 � 0.025 � 837.38 � 10 3 / 4.62 3 � 637 kN
Therefore the allowable load is governed by the resistance of the pile to overturning. A factor
of safety of 1.5 on the ultimate load of 477 kN will be appropriate giving an allowable load
of 318 kN.
Example 6.5
A group of 36 steel box piles are spaced at 1.25 m centres in both directions to form six rows
of six piles surmounted at ground level by a rigid cap. The piles are driven to a depth of
9 m into a medium-dense water-bearing sand and carry a working load of 240 kN on each
pile. Calculate the bending moments, deflections and soil-resistance values at various
points below the ground surface at the working load. Calculate the horizontal deflection of
the pile cap if the horizontal load is applied in the direction resisted by the maximum resistance
moment of the piles. Moment of inertia of the pile in direction of maximum resistance
moment � 58064 cm4 and elastic modulus of steel � 20 MN/cm2 .
From Figure 6.20, Terzaghi’s value of nh for a medium dense sand is 5 MN/m3 . Then from
equation 6.12:
5 20 � 58064
T �� � 188 cm.
�6 5 � 10
Because the embedded length of 9 m is more than 4T the pile behaves as a long elastic
fixed-headed element. The ultimate moment of resistance of pile (modulus of section �
2950 cm 3 ) � 2950 � 0.0247 � 72.86 MNcm for a working stress on BS 4360 Grade 40A
steel of 247 MN/m 2 .
From equation 6.51, depth to point of fixity � 1.8 � 188 � 338.4 cm.
From equation 6.19, ultimate horizontal load � H u � 2 � 72.86 � 10 3 /338.4 � 431 kN.
Factor of safety on applied load � 431/240 � 1.8, which is satisfactory if the pile head
deflections and the pile group behaviour are within acceptable limits.
The deflections, bending moments and soil-resistance values for the single pile at the
working load can be calculated from the curves in Figure 6.28. From equation 6.27:
deflection y F �
From equation 6.28:
bending moment M F � 240 � 188 F m � 45120F m kNcm � 451.2F m kNm.
From equation 6.29:
Piles to resist uplift and lateral loading 365
240 � 188 3
20 � 10 3 � 58 064 F y � 1.373F y cm � 13.73 F y mm.
soil reaction P F � 240 F p/188 � 1.28 F p kN per cm depth � 128 F p kN per m depth.
Z max � L/T � 9.0/1.88 � 4.8. Tabulated values of y F, M F, and P F are as follows: