Pile Design and Construction Practice, Fifth edition

48 Types of pile
where the safety factor on driving resistance is not greater than 2. For jacked piles or where
end-bearing piles are driven through relatively soft soils on to very dense granular soils
or sound rock the limit is 50% of yield strength. Eurocodes are based on limit state
design, hence BS EN 1993 Part 5: Piling (EC3–5) makes no reference to working stresses.
BS EN 12699 allows for the peak calculated stress in steel piles during driving to be
0.9 times the characteristic yield strength of the steel; it is stated that this may be
increased by 20% if the stresses are monitored during driving. BS 5950-1: 2000, the current
code of practice for steelwork design in buildings simply states that foundation design
should be in accordance with BS 8004 and the design strength of steel may be taken as
the yield stress for steel thickness less than 16 mm. EC3 calls up a suite of EN codes for
the production and composition of steel and manufacture of steel sections by hot rolling
and cold forming which apply to bearing and sheet pile design. For example, unalloyed
hot rolled structural steels, grades, designations and strengths are defined in BS EN
10025: 2004. The Steel Construction Institute’s H-Pile Design Guide, 2005 (2.11) is based
on limit state design as provided in the Eurocodes, and therefore does not consider
prescribed limits on steel working stresses. The Guide makes reference to the offshore
industry’s recommended practice for steel tubular piles based on North Sea experience
as described in the ICP Design Methods for Driven Piles in Sands and Clays (see
Chapter 4).
The American Petroleum Institute (3.5) states that the dynamic stresses during driving
should not exceed 80–90% of yield strength depending on specific circumstances such as
previous experience and confidence in the method of analysis.
The selection of a grade of steel for a particular task depends on the environmental
conditions as well as on the design working stresses. For piles wholly embedded in the
ground, or for piles in river and marine structures which are not subjected to severe
impact forces, particularly in tropical or temperate waters, a mild steel conforming to
Grade 43A (minimum yield strength 247 N/mm 2 ) or a high-tensile steel to Grade 50
(minimum yield strength 355 N/mm 2 ) should be satisfactory. The BS EN 10025-2: 2004
designated equivalent grades are S270GP (270 N/mm 2 ) and S355GP (355 N/mm 2 ) now
normally used for bearing piles. Corus tubular sections suitable for general piling correspond
to BS EN 10210 (hollow sections) grades S275J2H and S355J2H, both with
Charpy impact values of 27J at �20�C, (i.e. T 27J), well above the BS 4360 requirement.
Cold-formed, welded tubes in accordance with BS EN 10219 (hollow sections) for
non-alloy steels have similar yield strength and impact designations. Steel sheet piles
should conform to the yield strengths in BS EN 10248 for hot rolled sections or to BS EN
10249 for cold-formed.
Piles for deep-water platforms or berthing structures for large vessels are subjected to
high dynamic stresses from berthing impact and wave forces. In water at zero or sub-zero
temperatures, there is a risk of brittle fracture under dynamic loading, and the effects of
fatigue damage under large numbers of load repetitions and also of salt water corrosion
need to be considered. Steels must be selected to have a high impact value when tested at
low temperatures. Corus produces a special steel tube for offshore applications, 335 NH
‘Modified’, with a yield strength of 355 N/mm 2 and mechanical and chemical properties
superior to BS EN 10210 and BS 4360 grades 50E and 55E. The Charpy impact value is 60 J
at �50�C. Piles or bracing members for deep-water structures may be required to be
fabricated from plates 30 mm or more in thickness. The steel for such plates should have a