should be allowed between driving and load testing. The authors recommend a minimum
period of 28 days.
Some other observed values of the shaft resistance of piles in weak rocks are shown
in Table 4.16.
4.7.2 Driven and cast-in-place piles
Driven and cast-in-place piles terminated on strong rock can be regarded as end bearing. Their
working load is governed by the permissible working stress on the pile shaft at the point of
minimum cross-section, or by code of practice requirements. Where these piles are driven into
weak or weathered rocks they should be regarded as partly friction and partly end-bearing piles.
CIRIA Report 574(4.43)recommends that the base resistance of driven and cast-in-place
piles in chalk should be taken as 250 NkN/m^2 where Nis the SPT N-value. A lower bound
should be 200 NkN/m^2 with the recommendation to make a preliminary test pile whenever
possible. For calculating the unit shaft resistance the effective overburden pressure
should be multiplied by a factor of 0.8 where is less than 100 kN/m^2. If is greater
than 100 kN/m^2 the design should be confirmed by a loading test.
4.7.3 Bored and cast-in-place piles
Where these piles are installed by drilling through soft overburden onto a strong rock the
piles can be regarded as end-bearing elements and their working load is determined by the
safe working stress on the pile shaft at the point of minimum cross-section, or by code of
practice requirements. Bored piles drilled down for some depth into weak or weathered
rocks and terminated within these rocks act partly as friction and partly as end-bearing piles.
Wyllie(4.39)gives a detailed account of the factors governing the development of shaft
friction over the depth of the rock socket. The factors which govern the bearing capacity and
vo vovo204 Resistance of piles to compressive loads
Table 4.16Observed ultimate shaft friction values for piles driven into weak and
weathered rocksPile type Rock description Ultimate Reference
shaft friction
(kN/m^2 )
H-section Moderately strong slightly weathered 28 a 4.47
slaty mudstone
H-section Moderately strong slightly weathered 158 b 4.47
slaty mudstone
Steel tube Very weak coral detrital limestone 45 Unpubl.
(carbonate sandstone/siltstone)
Steel tube Faintly to moderately weathered 127 Unpubl.
moderately strong to strong mudstone
Steel tube Weak calcareous sandstone 45 Unpubl.
Precast Very weak closely fissured argillaceous 130 4.48
concrete siltstone (Mercia Mudstone)Notes
a Penetration 1.25 m.
b Penetration 2.2 m.