Pile Design and Construction Practice, Fifth edition

Resistance of piles to compressive loads 211
Table 4.19 Presumed safe vertical bearing stress for foundations on horizontal ground in
Hong Kong
Category Weathering Total core Uniaxial compression Equivalent point Presumed
grade recovery strength (MN/m 2 ) load index strength bearing stress
(%) (MN/m 2 ) (MN/m 2 )
1(a) �
II �95 of grade �50 �2 7.5
1(b) � II-III �85 of grade �25 �1 5
1(c) �III-IV
�50 of grade — — 3
Notes
Category 1(a): Fresh to slightly decomposed, strong.
Category 1(b): Slightly to moderately decomposed, moderately strong.
Category 1(c): Moderately decomposed, moderately strong to moderately weak.
The factor � should be based on SPT N-values. For low values of N (less than or equal
to 10) or a cone resistance q c between 2 and 4 MN/m 2 , and in the absence of pile test
results, � should be taken as 0.45 throughout. Where N is greater than 10, or q c greater
than 4 MN/m 2 , and in the absence of flints, � is taken as 0.8 throughout. The allowable
pile load P a should be determined by using the partial factors in equation 4.42.
Throughout reports 574 and PR86 it is emphasized that load testing is desirable at some
stage as a means of confirming load capacity, and achieving economy in design. It is pointed
out that a single test made to 3 times the working load is a much better aid to judgement than
two tests to 1.5 times the working load.
For granites and volcanic rocks, the practice in Hong Kong is to relate the allowable base
bearing pressure for bored piles to the weathering grade of the decomposed material. The
recommendations of the Government Geotechnical Office (4.45) as quoted by Ng et al. (4.59)
are shown in Table 4.19.
The Hong Kong Government recommends that completely weathered granite should be
treated as a soil. Also the rock socket shaft friction in weak to moderately weak and strong
to moderately strong granites should be determined from correlation with the uniaxial compression
strength of sedimentary rocks using the method of Horvath et al. (4.50) Ng et al. (4.59)
point out that observations made in loading tests in granites suggest that the value for b in
equation 4.45 of 0.2 is appropriate.
4.7.4 The settlement of the single pile at the working
load for piles in rocks
The effects of load transfer from shaft to base of piles on the pile head settlements have
been discussed by Wyllie (4.39) . Because of the relatively short penetration into rocks which
is needed to mobilize the required total pile resistance, the simpler methods of determining
pile head settlement described in Section 4.6 are suitable in most cases. For piles having
base diameters up to 600 mm the settlement at the working load should not exceed 10 mm
if a safety factor of 2.5 has been applied to the ultimate bearing capacity.
The settlement of large diameter piles can be calculated from equation 4.38. The modulus
of deformation of the rock below the pile toe can be obtained from plate bearing or
pressuremeter tests or from empirical relationships developed between the modulus, the