Pile Design and Construction Practice, Fifth edition

Types of pile 27

in mm^2 for piles not exceeding 10 m in length, 0.01% cross-sectional area pile length
for piles between 10 and 20 m long and 0.2% for piles greater than 20 m long. It may be
desirable to specify a maximum load which can be applied to a precast concrete pile of any
dimensions. As in the case of timber piles this limitation is to prevent unseen damage to
piles which may be over-driven to achieve an arbitrary set given by a dynamic pile-driving
formula. BSEN 12699 limits the calculated stress (including any prestress) during driving
of precast piles to 0.8 times characteristic concrete strength in compression at the time of
driving; a 10% increase is permitted if the stresses are monitored during driving.
Concrete made with ordinary Portland cement (CEM I) is suitable for all normal exposure
conditions but sulphate-resisting cement may be needed for aggressive ground conditions as
discussed in Chapter 10.
Metal shoes are not required at the toes of precast concrete piles where they are driven
through soft or loose soils into dense sands and gravels or firm to stiff clays. A blunt pointed
end (Figure 2.7a) appears to be just as effective in achieving the desired penetration in these
soils as a more sharply pointed end (Figure 2.7b) and the blunt point is better for maintaining
alignment during driving. A cast-iron or cast-steel shoe fitted to a pointed toe may be used
for penetrating rocks or for splitting cemented soil layers. The shoe (Figure 2.7c) serves to
protect the pointed end of the pile.
Where piles are to be driven to refusal on a sloping hard rock surface, the ‘Oslo point’
(Figure 2.7d) is desirable. This is a hollow-ground hardened steel point. When the pile is
judged to be nearing the rock surface, the hammer drop is reduced and the pile point is seated
on to the rock by a number of blows with a small drop. As soon as there is an indication that
a seating has been obtained the drop can be increased and the pile driven to refusal or some
other predetermined set. The Oslo point was used by George Wimpey and Co. on the piles
as illustrated in Figure 2.5b, which were driven on to hard rock at the site of the Whitegate
Refinery, Cork. A hardened steel to BS970 with a Brinell hardness of 400 to 600 was

(a) (b) (c) (d)

Cast iron

or cast steel

shoe

Hardened

steel point

M.S. straps

Figure 2.7Shoes for precast (including prestressed) concrete piles (a) For driving through soft or
loose soils to shallow penetration into dense granular or firm to stiff clays (b) Pointed end
suitable for moderately deep penetration into medium-dense to dense sands and firm to
stiff clays (c) Cast-iron or cast-steel shoe for seating pile into weak rock or breaking
through cemented soil layer (d) ‘Oslo point’ for seating pile into hard rock.