Civil Engineering Reference
In-Depth Information
Figure 5.21
Correlation of Young's Modulus for chalk against SPT value:
●
Borded piles category 4,
Driven piles category 3(a),
◦
Driven piles category 3(b),
+
Driven piles category 2(b)
(West Shell Piles, Portsmouth), C-C Slope derived from calculations to obtain a reasonable
relationship between modulus,
N
value and depth.
Numerical values are stress on base (MPa) at the end of linear (
L
) portion of these curves.
(Hobbs and Healy, 1979).
In soft rocks, the appropriate stiffness will be greatly affected by the degree of
weathering, and by the method of pile installation. Hobbs and Healy (1979) have
correlated values of soil modulus, back-analyzed from pile load tests in chalk, with
the SPT number,
N
. Their correlation is shown in Figure 5.21. There is consider-
able scatter in the results, much of it reflecting varying amounts of disturbance due
to pile installation. Even with good quality chalk, pile driving can loosen the block
structure and reduce the stiffness by an order of magnitude. Randolph and Wroth
(1978) quote values of shear modulus, deduced from load tests on driven cast-in-situ
piles end bearing on good quality chalk, which are as low as 36 MPa for grade I-II
chalk.
