resistance. If the safety factor to obtain the allowable load is too low, the resulting shaft
settlement could break down the bond between the rock and concrete thus weakening the
total pile resistance in cases where the design requires the load to be shared between the
shaft and the base. A reduction in shaft resistance of 30% to 40% of the peak value has been
observed where shear displacements of the rock socket of little more than 15 mm have
occurred. It may also be difficult to remove soft or loose debris from the whole base area at
the time of final clean-out before placing the concrete.
Because of the porous cellular nature of chalk and the consequent break down and soft-
ening of the material under the action of drilling tools (similar to that described in Section
4.7.1), conventional methods of calculating the base resistance and rock socket shaft friction
cannot be used for bored piles in chalk. CIRIA Report 574 states that these two components
of bearing capacity are best determined from relationships with the standard penetration test
N-values uncorrected for overburden pressure. These give a rough indication of the weathering
grade to supplement the classification based on examination of rock cores and exposures in
the field. The recommendations of Report 574 are
Ultimate base resistance of bored piles: 200 N(kN/m^2 )
Ultimate base resistance of continuous flight auger (CFA) piles: 200 N(kN/m^2 )
Ultimate shaft resistance of bored piles in low to medium-density chalk: 0.8 vo
The above relationships for base resistance are subject to the pile bore being certified as
clean. Also where the average effective overburden pressure, , is less than 400 kN/m^2
(based on final ground levels and omitting the contribution from made ground and fill) the
calculated shaft friction must be confirmed by load testing. For high-density Grade A chalk
the pile should be treated as a rock socket and the shaft friction taken as 0.1 times the uni-
axial compression strength. The report makes a distinction between made ground and fill.
The former is regarded as an accumulation of debris resulting from the ‘activities of man’,
whereas fill is purposefully placed.
A somewhat modified recommendation is made for continuous flight auger piles. A later
CIRIA Report PR86(4.58)states:
Average ultimate shaft resistance
vo (4.48)vo210 Resistance of piles to compressive loads
Table 4.18Relationships between weathering grades, undrained shear
strength, and elastic properties of weak rocks (see also
Seedhouse and Sancters(4.56)).
Weathering grade Undrained shear Shear modulus Youngs modulus
strength (cu, kN/m^2 ) (G, MN/m^2 ) (E, MN/m^2 )
V–VI 250 80 115
IV 850 100 230
III 1330 350 820
III 1270 265 615
III 1230 210 490
III 1150 175 405
III 1090 150 350