Pile Design and Construction Practice, Fifth edition

Pile groups under compressive loading 273

might occur if the bedding is unfavourably inclined to the direction of loading (Figure 5.32).
The possibility of such occurrences must be studied in the light of the information available
on the geology of the site.
The settlement of a pile group may be of significance if the piles are heavily loaded.
Immediate settlements can be calculated as described in Section 5.2.2, and equations 5.14
and 5.22 are applicable, where Edis reasonably constant with depth.
It is possible to obtain a rough estimate of the deformation modulus of a jointed rock mass
from empirical relationships with the unconfined compression strength of the intact rock.
BS8004 gives a relationship as where jis the mass factor (see Section 4.7.3
for values) and Mris the ratio of the elastic modulus of the intact rock to its unconfined
compression strength. BS8004 gives the following values for Mr:

Values for Mr

Group 1 Pure limestones and dolomites 600

Carbonate sandstones of low porosity

Group 2 Igneous 300

Oolitic and marly limestones

Well-cemented sandstones

Indurated carbonate mudstones

Metamorphic rocks including slates and schists (flat

cleavage/foliation)

Group 3 Very marly limestones 150

Poorly cemented sandstones

Cemented mudstones and shales

Slates and schists (steep cleavage/foliation)

Group 4 Uncemented mudstones and shales 75

Chalk and Mercia Mudstone (Keuper Marl) are excluded from the above groups. Some
observed values of Edfor chalk are given in Table 5.3 and for Mercia Mudstone in Table 5.4.

Ed jMrqc

Clay-filled

bedding joints

Figure 5.32Instability of pile group bearing on sloping rock surface.