Pile Design and Construction Practice, Fifth edition

274 Pile groups under compressive loading

It is likely that weathered rocks will show an increase in Edwith depth as the state of
weathering decreases from complete at rockhead to the unweathered condition. If it is
possible to draw a straight line through the increasing values the influence factors in
Figure 5.19 can be used in conjunction with equation 5.21 to obtain the settlement at the
centre of the loaded area. These curves were established by Butler(5.12)for a Poisson’s ratio
of 0.5, but most rock formations have lower ratios. Meigh(5.23)stated that the Poisson’s
ratio of Triassic rocks is about 0.1 to 0.3.
Meigh(5.23)derived curves for the influence factors shown in Figure 5.33 for various
values of the constant kin equation 5.39 where

(5.39)

and for a Poisson’s ratio of 0.2. He applied further corrections to the calculation of the
settlement at the cornerof the foundation where

Settlement at corner (5.40)

The corrected settlement is given by

c(corrected) (5.41)

where

FB correction factor for roughness of base (Figure 5.34)

FD correction factor for depth of embedment (Figure 5.35)

qnBIp

Ef

FBFD

i

qnBIp

Ef

k

(EdEf)B/H

Ef

Table 5.3Values of deformation modulus of Chalk (after Lord et al.(4.43))

Density Grade ‘Yield’stress Ultimate bearing Secant modulus at ‘Yield’modulus
(MN/m^2 ) capacity (MN/m^2 ) applied stress of (MN/m^2 )
200 kN/m^2 (MN/m^2 )

Medium/high A — 16 1500 – 3000 —
B 0.3–0.5 4.0–7.7 1500 – 2000 35 – 80
C 0.3–0.5 4.0–7.7 300 – 1500 35 – 80
Low B and C 0.25–0.5 1.5–2.0 200 – 700 15 – 35
(Low) Dc 0.25–0.5 — 200 20 – 30
Dm —— 6 —

Table 5.4Values of deformation modulus

of Mercia Mudstone (Keuper

Marl) at low stress levels (after

Chandler and Davis(5.27))

Zone Deformation modulus (MN/m^2 )

I26– 250

II 9 – 70

III 2 – 48

IV 2 – 13