294
BEHAVIOR OF PILES SUPPORTING OFFSHORE STRUCTURES
for the three strata, starting with the top: 0.83, 0.61, and 0.48. With these
numerical values, équation (11-1) becomes
Q, t 3tt [(50)(l/2)(0.83) + (50)(1.8/2)(0.61) + (40)(2.5/2)(0.48)] = 680 tons
The reasonable assumption is made that the pile plugged at some point during
driving and thus the end bearing can be computed as if the pile were solid. The
following resuit is found by substituting values into équation (11-2).
Qb = (2.5/2)(9.0)tt(1.5)2 = 80 tons
Thus, the total load the pile can sustain was computed to be 760 tons or 1520
kips. The safe load would be found by using an appropriate factor of safety.
The load-settlement curve for the pile can be computed by implementing
load-transfer curves. With regard to the load transfer in skin friction for piles
in clay, Coyle and Reese (1966) examined experimental data and proposed the
results shown in Table 11.1.
Table 11.1 Pile Load-Settlement Data
// fuit
Pile Movement, in.
0
0
0.18
0.01
0.38
0.02
0.79
0.04
0.97
0.06
1.00
0.08
0.97
0.12
0.93
0.16
0.93
0.20
0.93
>0.2
With regard to the load transfer in end bearing for piles in clay, the work
of Skempton (1951) is used, where the end bearing of a plate loaded in clay is
shown to correlate with the laboratory stress-strain curve. He noted that the
settlement wb at one-half the ultimate unit end bearing of the base is equal to
“'un
SO
(11-3)
2
In the absence of a laboratory stress-strain curve for the soil, the following
values of C50 can be taken as a function of the unconfined compressive strength
(consistency) of the clay: soft (<0.5 tons/ft2) 0.0.2; medium (0.5 to 1.0 tons/ft2)
0.01; and stiff (>1.0 tons/ft2) 0.005. For the example problem, the value of qu
at the base of the pile was 2.5 tsf, so the value of £50 was selected as 0.005, with
wuit/2 equal to 0.36 in. Numerous stress-strain curves for soil hâve been plotted
on log-log paper and found to be a straight line, many with a slope of about