Civil Engineering Reference
In-Depth Information
Recorded values of shaft friction for piles driven into low-grade chalk vary from as
low as 8 kPa (Hodges and Pink, 1971) up to 26 kPa (Hobbs and Robins, 1976) for
similar pile penetrations.
4.1.3.1 End-bearing pressure
It has been customary for design codes to stipulate allowable bearing pressures on rocks
of different types. Tomlinson (1986) presents an extensive list of allowable pressures,
in terms of type, quality and joint spacing of the rock. In view of the great variability
in strength (and allowable end-bearing pressure) of individual rock types, it appears
best to express allowable bearing pressures for pile design in terms of the unconfined
compressive strength of the rock. Theoretical (and experimental, where failure has
actually been achieved) values of ultimate end-bearing pressure are generally in excess
of 10
q
u
(where
q
u
is the unconfined compressive strength). Large settlements would
be required to mobilize such values of end-bearing, and, in practice, a rather lower
'ultimate' end-bearing pressure of about 3
q
u
is often adopted (Rowe and Armitage,
1987).
A rational design approach is to limit allowable bearing stresses to below that which
would initiate crushing of the rock beneath the pile. Even where the joints are relatively
open, this value will not fall below the unconfined compressive strength of the rock.
Thus, allowable end-bearing stress,
q
ba
, may be expressed as (Rowe and Armitage,
1987)
q
ba
=
q
u
(4.23)
The above value is rather higher than that of 0.3
q
u
suggested by, for example, Poulos
and Davis (1980). However, the higher value is supported by experimental evidence
and is consistent with a factor of safety of 3 applied to the (conservative) ultimate
bearing pressure of 3
q
u
discussed above.
A more extensive database has been presented by Zhang and Einstein (1998), where
end-bearing pressures mobilized at a displacement of 10% of the pile diameter were
found to be proportional to the square root of the unconfined compression strength,
according to
15
q
u
p
a
q
b
p
a
≈
(4.24)
where
p
a
is atmospheric pressure. This expression has been compared with results from
cone penetration tests and model pile tests in calcarenite by Randolph
et al.
(1998),
as shown in Figure 4.15.
4.1.3.2 Shaft friction
Shear transfer along the shaft of a rock socket is a complex phenomenon that depends
on the frictional characteristics of the interface, the degree of roughness of the socket
and the strength properties of the host rock. Values of peak friction will depend
strongly on the normal stress along the shaft, and the amount of dilation that occurs
