Pile Design and Construction Practice, Fifth edition

and RQD of the rock as shown in Table 4.14 can be used. The q uc values are determined
from tests on core specimens of the intact rock to obtain its point load strength
(Section 11.1.4).
It is important to note that to mobilize the maximum base resistance from equation 4.40
the settlement of the pile toe is likely to be of the order of 20% of its diameter, requiring an
ample safety factor, at least 2.5, to ensure that settlements at the working load are within
allowable limits (Section 4.7.4).
Driving a closed-end pile into low to medium density chalk causes blocks of the rock to
be pushed aside. Crushed and remoulded chalk flows from beneath the toe, and the cellular
structure of the rock is broken down releasing water trapped in the cells to form a slurry.
This flows into fissures and causes an increase in pore pressure which considerably
weakens the shaft resistance, although it is possible that drainage from the fissures will
eventually relieve the excess pore pressure thereby increasing the shaft resistance.
Very little penetration is likely to be achieved when attempting to drive large closed-end
piles into a high-density chalk formation with closed joints, but penetration is possible with
open-end or H-section plies.
Because of the effects of driving piles into chalk, as described above, equations 4.39 and
4.40 cannot be used to calculate base resistance. From the results of a number of plate and
pile loading tests, CIRIA Report 574 (4.43) recommends that the base resistance should be
related to the standard penetration test N-values (Section 11.1.4). The report gives the
relationship for driven piles as
Base resistance � q ub � 300 N kN/m 2 (4.41)
where N is the SPT resistance in blows/300 mm. A lower bound is of the order of
200 N kN/m 2 .
No correction should be made to the N-values for overburden pressure when using
equation 4.41. Use of this equation is subject to the stress at the base of the pile not
exceeding 600 to 800 kN/m 2 for low to medium density chalk, and 1000 to 1800 kN/m 2
for medium to high-density chalk. Also the allowable load on the pile should be the lesser
of either:
or,
P a � Q s � Q b
2
Resistance of piles to compressive loads 201
Pa � Qs 1.0 � Qb 3.5 (Pa � Qs 1.5 � Qb if the settlement is to be less than 10 mm)
3.5
(4.42)
(4.43)
Dynamic testing (Section 7.3) of trial or working piles is frequently used to determine
permissible working loads in end bearing on chalk. CIRIA Report 574 states that instrumented
dynamic tests using the CAPWAPC program can give a good estimate of end-bearing resistance
provided that the hammer blow displaces the toe at least 6 mm during the test. Definitions of
the density grades of chalk and their characteristics for use with equations 4.42 and 4.43 are
given in Appendix 3.