Discussion: Session 1—chalk

I understand fully and sympathize with Mr Hobbs' aim in attempting to rationalize the data

now available concerning end-bearing and shaft friction components. However, while accept

ing that at present there is little alternative, I must count myself among the non-believers

regarding empirical correlation with SPT results. Like Dr Lord, I see little physical similarity

in the responses being correlated, particularly in the case of shaft friction on preformed driven

piles.

I think Mr Hobbs' graphs are useful in indicating trends and relative performance of types of pile but I am concerned about the quantification of the relationships for several reasons. Firstly, as Dennehy (1975) and Lord and Smith (1976) have indicated, the SPT is not a reliably consistent indicator of the type of chalk at various locations. Secondly, there is the obvious point regarding depth of the test. For example, I would not be happy to assume similar properties between a site giving say ^=1 5 at 2 m depth and one giving a similar result at 20 m depth, without a good deal of additional data. Thirdly, Mr Hobbs' graphs demonstrate the same characteristic as the Wakeling (1970) elastic modulus correlation. That is, as Ward (1970) pointed out when discussing the earlier correlation, the SPT is not a sensitive index and small variations in the relatively crude SPT result produce large differences in the parameter under consideration.