Pile Design and Construction Practice, Fifth edition

Total uplift on underside of floor

Therefore

factor of safety against uplift 1.25 (which is satisfactory and a closer

estimate of the rock volume is not needed).

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/m^3 ).
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/m^2. For a safety
factor of 2.5 in uplift, in weathered chalk on a 600 mm steel tubular pile:

allowable load

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 mm^2. Thus

Working tensile stress in anchor 1 329 ^1000

7600

175 N/mm^2

5

(^68)
5
8

  0.6  (3^2  12 )  30

2.5

71 kN

1931.2

1540

70 ^20 ^1100

1000

1 540 MN.

360 Piles to resist uplift and lateral loading

1800 kN B

A

C

0.60 steel

tubular piles

Toe of piles

‘Dead’ anchor

Hard chalk

Weathered chalk

Sea bed

3.00

2800 kN

2800 kN c

ab

1

3

Figure 6.43