Pile Design and Construction Practice, Fifth edition

360 Piles to resist uplift and lateral loading
70 � 20 � 1100
Total uplift on underside of floor � � 1540 MN.
1000
Therefore
factor of safety against uplift � � 1.25 (which is satisfactory and a closer
estimate of the rock volume is not needed).
1931.2
1540
Therefore the anchorage should consist of a 3 m square grid of 450 mm shell piles driven
on to rockhead, each pile being anchored 9 m below rockhead with a seven 15.2 mm/7-wire
Dyform strand cable having the bottom 7 m bonded to the rock.
Example 6.3
A piled dolphin carrying a horizontal pull of 1800 kN consists of a pair of compression piles
and a pair of tension piles, raked at angles of 1 horizontal to 3 vertical. Design ‘dead’
anchors for the tension piles, which are driven through 3 m of weak weathered chalk to
near-refusal on strong rock chalk (having an average submerged density of 0.5 mg/m3 ).
From the triangle of forces (Figure 6.43) the uplift load on a pair of tension piles is 2800 kN.
The load to be carried by a single pile is thus 0.5 � 2800 � 1400kN
From Table 4.15, the ultimate shaft friction in weathered chalk is 30 kN/m2 . For a safety
factor of 2.5 in uplift, in weathered chalk on a 600 mm steel tubular pile:
allowable load
� � � 0.6 � �(32 � 12 ) � 30
� 71 kN
2.5
The load to be carried by the dead anchor is thus 1400 � 71 � 1329 kN . Use a steel tube
having an overall diameter of in (168.3 mm). A wall thickness of in (16 mm) provides
a cross-sectional area of 7600 mm2 5
6 8
. Thus
5
8
Working tensile stress in anchor �
0.60 steel
tubular piles
Weathered chalk
Figure 6.43
1800 kN B
A
2800 kN
C
3.00
Hard chalk
Toe of piles
1
3
2800 kN
1329 � 1000
� 175 N/mm
7600
2
b
a
c
Sea bed
‘Dead’ anchor