weathering grade and the unconfined compression strength of the rock given in Table 4.18
and Section 5.5.
These relationships are not applicable to high porosity chalk or weathered silty mudstone
(Mercia Mudstone). The relationships given in Section 5.5 assume fairly low stress levels.
Therefore calculated values based on the unconfined compression strength of the rock
should take into account the high-bearing pressures beneath the base of piles.
In CIRIA Report 574(4.43)the deformation modulus of chalk is related to the weathering
grade and standard penetration test N-values. For Grade A chalk where the N-value is greater
than 25, the deformation modulus is 100 to 300 MN/m^2. For Grades B, C and D with N-values
less than 25 the modulus is 25 to 100 MN/m^2.
Pells and Turner(4.60)have derived influence factors for calculating the settlement of a
bored pile where the load is carried by rock socket shaft friction only using the equation:
Settlement = (4.49)where
Q total load carried by the pile head
Ip influence factor
B diameter of the socket
Ed deformation modulus of the rock mass surrounding the shaft.The influence factors of Pells and Turner are shown in Figure 4.36. Where the rock
sockets are recessed below the ground surface or where a layer of soil or very weak rock
overlies competent rock, a reduction factor is applied to equation 4.49. Values of the reduction
factor are shown in Figure 4.37.
4.7.5 Eurocode recommendations for piles in rock
EC7 makes no specific recommendations for the design of piles carrying axial compression
loads in rock. The design methods described in the preceding Sections 4.7.1 to 4.7.3 are
based either on relationships with unconfined compression strengths or by correlation with
SPT N-values.
Where the calculations are based on relationships with SPT N-values, the correlation
factors based on the number of test profiles are applied as in equation 4.19 together with a
calibration factor of 1.05. Where design total pile resistances are obtained from static or
dynamic pile tests the correlation factors are obtained from Tables 4.7 and 4.8 respectively.
EC7(Clause 7.6.4.2) states that ‘when the pile toe is placed in a medium-dense or firm
layer overlying rock or very hard soil, the partial safety factors for the ultimate limit state
conditions are normally sufficient to satisfy serviceability limit-state conditions’.
4.8 Piles in fill – negative skin friction
4.8.1 Estimating negative skin friction
Piles are frequently required for supporting structures that are sited in areas of deep fill. The
piles are taken through the fill to a suitable bearing stratum in the underlying natural soil
QIp
BEd212 Resistance of piles to compressive loads