Pile Design and Construction Practice, Fifth edition

Safety factors generally used in the Netherlands in conjunction with the ‘4D – 8D’
method to obtain the allowable pile load are given by te Kamp (4.26) as:
Timber 1.7
Precast concrete, straight shaft 2.0
Precast concrete, enlarged shaft 2.5
Resistance of piles to compressive loads 179
An upper limit is placed on the value of the ultimate base resistance obtained by either of
the methods shown in Figure 4.18. Upper limiting values depend on the particle-size
distribution and over-consolidation ratio and are shown in Figure 4.19.
The relationship q b = q c in equation 4.18 is valid for piles up to about 500 mm in diameter
or breadth provided, when designing by permissible stress methods, that a pile head
displacement of one-tenth of the diameter is taken as the criterion of failure and that a safety
factor of 2.5 is adopted on the calculated total resistance. The reduction of the q b/q c ratio
with increase in diameter is discussed in Section 4.3.7.
Cone-resistance values cannot be used to obtain the end-bearing resistance of bored and
cast-in-place piles because of the loosening of the soil caused by drilling as described in the
preceding chapter.
A further factor must be considered when calculating pile shaft friction and end-bearing
resistance from CPT data. This is the effect of changes in overburden pressure on the q c (and
also local friction) values at any given level. Changes in overburden pressure can result from
excavation, scour of a river or sea bed, or the loading of the ground surface by placing fill.
The direct relationship between q c and overburden pressure is evident from Figure 4.11.
Taking the case of a normally consolidated sand when the vertical effective stress is reduced
by excavation, the ratio of the horizontal to the vertical stress is also reduced, but not in the
same proportion depending on the degree of unloading. The effects are most marked at shallow
depths.
Limiting value of ultimate
end-bearing resistance (MN/m2 )
20
10
Fine-coarse sands
OCR=1
Limit 15MN/m 2 for all coarse-grained soils
Very gravelly coarse sand
and sand with OCR in
range of 2 to 4
Fine gravel and sand with OCR
in range of 6 to 10
0
0 5 10 15 20 25 30 35 40
Calculated value of ultimate
end-bearing resistance
(qub ) (MN/m2 )
Figure 4.19 Limiting values of pile end-bearing resistance for solid end piles (after te Kamp (4.26) ).