Pile Design and Construction Practice, Fifth edition

Types of pile 21

BS8004 requires that piles should be designed to withstand the loads or stresses and to
meet other serviceability requirements during handling, pitching, driving and in service in
accordance with BS8110 for the structural use of concrete (Table 2.4). EC7 requires the
structural design of piles to conform to the serviceability requirements in the relevant
material Eurocodes – EC2, EC3, and EC5 and the relevant National Annexes. EC2-1-1
provides common rules for concrete for building and civil engineering which are not very
different from BS8110 in terms of general design approach. Concrete performance, quality
and production are subject to BSEN 206-1: 2000, which must be read in conjunction with
the UK’s complementary rules for strength classes, cover, etc. in BS8500. If nominal
BS8110 mixes are adopted, a 40-grade concrete with a minimum 28-day cube strength of
40 N/mm^2 is suitable for hard to very hard driving and for all marine construction. For
normal or easy driving, a 25-grade concrete is suitable (i.e. 28-day cube strength of
25 N/mm^2 ). Depending on exposure conditions defined in EC2-1-1 and appropriate cover to
reinforcement, BS8500 recommends strength classes of concrete from C20/25 (i.e. grading
based on minimum characteristic strength of a cylinderat 28 days/minimum characteristic
cubestrength at 28 days in N/mm^2 ) in dry or permanently wet conditions to C45/55 in tidal
splash zones. BSEN 12794: 2005 Precast concrete products – Foundation piles does not give
specific requirements for the design strength of concrete for piles, but refers to BSEN
13369: 2004 Common rules for precast products, BSEN 206-1 and EC2. BSEN 12794
defines two classes of piles – ‘Class 1’with distributed reinforcement or prestressed piles
and ‘Class 2’with single central reinforcing bar. The only difference this division makes is
to the detailing of pile reinforcement. BSEN 13369 Clause 4.2.2 requires reinforced
concrete products to have a minimum strength class of C20/25 and prestressed concrete a
minimum of C30/37. Foundations in brownfield sites are not covered in BS8500 and the
recommendations in BRE Special Digest 1(2.6)should be followed for both in-situ foundation
concrete and precast units. It should be noted that the design strengths in EC2 are based on
the characteristic cylinderstrengths. High stresses, which may exceed the handling stresses,
can occur during driving and it is necessary to consider the serviceability limit of cracking.
BS8110 states that National Standards and Codes of Practice require cracks to be controlled
to maximum widths close to the main reinforcement ranging from 0.3 mm down to 0.1 mm
in an aggressive environment, or they require that crack widths shall at no point on the
surface of the structure exceed a specified width, usually 0.3 mm. EC2-1-1 Clause 7.3
provides for maximum crack widths of 0.3–0.4 mm in reinforced concrete elements taking
account of the proposed function of the structure and exposure of precast and prestressed
elements. Application to precast piles is not considered, and neither BSEN 12699 nor
BSEN 12794 comments on cracks due to driving.

Table 2.3Working loads and maximum lengths for ordinary pre-

cast concrete piles of square section

Pile size Range of working loads Maximum length

(mm^2 ) (kN) (m)

250 200 – 300 12

300 300 – 450 15

350 350 – 600 18

400 450 – 750 21

450 500 – 900 25