Pile Design and Construction Practice, Fifth edition

Types of pile 53

all cast-in-place displacement piles unless otherwise specified. BS8004 limits the working
stress to 25% of the 28-day cube strengths, but BSEN 12699 specifies concrete strength
classes of C20/25 to C30/37 which are 25% stronger than the cubestrengths usually adopted
in the UK under BS8004, that is, a range of 20–30 N/mm^2. EC2-1-1 Clause 3 refers to char-
acteristic cylinder strengthsfor the determination of design compressive strengths, and if
the 25% limit is applied the allowable stresses range from 5 to 7.5 N/mm^2 (i.e. similar to the
BS8004 limits, but for the stronger mixes). For these values, allowable loads for piles of var-
ious shaft diameters are as shown in the following table:

Nominal shaft Allowable working

diameter (mm) load (kN)

300 350 – 500

350 450 – 700

400 600 – 900

450 800 – 1000

500 1000 – 1400

600 1400 – 2000

The higher ranges in the above table should be adopted with caution, particularly in difficult
ground conditions.
Maximum working loads are as shown in the following table:

Nominal shaft Nominal maximum

diameter (mm) working load (kN)

350 440

400 590

450 740

500 930

550 980

600 1500

715 2000

The spacing of bars in the reinforcing cage should give ample space for the flow of
concrete through them. Bars of 5 mm diameter in the form of a spiral or flat steel hoops used
for lateral reinforcement should not be spaced at centres closer than 100 mm (80 mm when
using 20 mm aggregate).
The Vibrex pileinstalled by Fundex Verstraeten BV employs a diesel or hydraulic hammer
to drive the tube which is closed at the end by a loose sacrificial plate. An external ring
vibrator is then employed to extract the tube after the reinforcement cage and concrete has
been placed. A variation of the technique allows an enlarged base to be formed by using the
hammer to drive out a charge of concrete at the lower end of the pile. The Vibrex pile is
formed in shaft diameters from 350 to 600 mm.