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Semi-empirical correlations have been developed relating both shaft and end-bearing resistance of piles founded in granular soils to SPT N values. Such a procedure would provide an approximate means of allowing for variability of the strata across a site in normalising and extrapolating the results of loading tests. In most of the correlations that have been established, the N values generally refer to uncorrected values before pile installation.

Because of the varying degree of weathering of the parent rocks in Hong Kong, the local practice is that SPT is often continued to much higher N values than in most other countries (Brand & Phillipson, 1984). However, the carrying out of SPT to very high values may damage the shoe which can subsequently lead to erroneous results. The guidance given in Geoguide 2 : Guide to Site Investigation (GCO, 1987) concerning termination of the test in very dense soils should be followed.

6.4.5.2 End-bearing resistance

Malone et al (1992) analysed the results of pile loading tests carried out on instrumented large-diameter bored piles and barrettes embedded in saprolites in Hong Kong. They found that the end resistance (in kPa) mobilised at the base of the pile at a settlement corresponding to 1% pile diameter is in the range of 6 to 13 times the uncorrected average SPT N values at the base of the pile.

A rule-of-thumb method for use in the design of caissons and bored piles has been in use in Hong Kong for some years (Chan, 1981). This method is based on the correlation that the allowable end-bearing pressure is equal to 5 times the SPT N for soils below the groundwater table. The allowable end-bearing pressure can be doubled for soils in dry condition.

6.4.5.3 Shaft resistance

For caissons and bored piles, the allowable shaft resistance has been either ignored or limited to 10 kPa, so as to avoid the need to be justified by loading tests. However, as discussed by Malone (1987), this rule-of-thumb generally results in unrealistic distribution of mobilised resistance and gross over-design of large-diameter bored piles founded in saprolites. Similarly, Lumb (1983) showed, on the basis of his interpretation of pile tests in

Hong Kong, that significant shaft resistance can be developed in granitic saprolites. This is also evident from the instrumented pile loading tests carried out in bored piles and barrettes founded on saprolites (Figure A2).

For saprolites in Hong Kong, loading tests on instrumented large-diameter bored piles and barrettes (Appendix A) suggest that the ratio of the average mobilised shaft resistance (kPa) to N ─ value generally ranges between 0.8 and 1.4. It is found that the shaft resistance is, in some cases, practically fully mobilised at an average relative pile/soil settlement of about 1% pile diameter. The mobilised shaft resistance was found to be dependent largely on the construction method and workmanship, as well as the geology and undisturbed ground conditions. Compared to bored piles in other tropically weathered soils, it appears that the above observed ratio of τs / N─ is low. For instance, Chang & Broms (1991) reported a ratio of τs / N─ ranging from about 0.7 to 4 (kPa) for bored piles in residual soils and weathered rocks in Singapore for N─ values up to 60, and suggested the relationship of τs / N─ of 2 (kPa) for design purposes. This is also supported by Ho (1993) for piles in weathered granite in Singapore for N─ values up to 75. The discrepancy may be due to differences in geology, methods for supporting empty bores during excavation, and methods of interpretation.

For preliminary design of large-diameter bored piles, barrettes and hand-dug caissons in sandy granitic saprolites below sea level in Hong Kong, the relationship of τs / N─ of 0.8 to 1.4 (kPa) may be used, with N value limited to 200. Limited data suggest the ratio of τs / N─ may be lower in volcanic saprolite (Appendix A).

Based on limited data in Hong Kong, the shaft resistance for small-displacement piles such as steel H-piles can be taken as 1.5 N─ to 2 N─ (kPa) for design, for a N─ value up to about 80 (Appendix A). N─ is the uncorrected mean SPT value in the soil strata where shaft resistance is being mobilised.

Based on observations of loading tests on precast prestressed concrete piles in Hong Kong, Ng (1989) proposed that τs in the range of 4 N─ to 7 N─ (kPa) may be taken for design in saprolites with a limiting average shaft resistance of 250 kPa. This is generally consistent with the 'rule-of-thumb' adopted in Hong Kong that τs = 4.8 N─ (kPa) (Siu & Kwan, 1982) for N

─ values up to about 60 for driven piles. It is recommended that the relationship of τs = 4.5 N─ (kPa) may be used for design of large-displacement driven piles in saprolites.

In traditional design of small-diameter bored piles involving pressure grouting or pressurising the concrete in Hong Kong, the empirical relationship of τs = 4.8 N─ to 5 N─ (kPa), ignoring the contribution from the base, is generally used for N─ values up to about 40, usually with a factor of safety of 3 (Chan, 1981). Lui et al (1993) reported a design of post-grouted mini-piles based on the relationship of τs = 5 N─ (kPa), where N─ is limited to 100 and the factor of safety is taken to be 3, which has been satisfactorily verified by instrumented pile loading tests.

The design method involving correlations with SPT results is empirical in nature, and the level of confidence is not high particularly where the scatter in SPT N values is large. If loading tests on preliminary piles are not carried out, this design approach should be checked

using the effective stress method based on soil mechanics principles (Section 6.4.4.3), and the smaller calculated capacity adopted for design.