Pile Design and Construction Practice, Fifth edition

where

qo ultimate base resistance in the lower weak layer
ql ultimate base resistance in the upper stiff or dense stratum
H distance from the pile toe to the base of the upper layer and
B width of the pile at the toe
The following procedures were adopted for the piled foundations of British Coal’s bulk-
handling plant at Immingham, where a layer of fairly dense sandy gravel was shown to exist
at a depth of about 14.6 m below ground level. The thickness of the gravel varied between
0.75 and 1.5 m and it lay between thick deposits of firm to stiff boulder clay. The end-
bearing resistance in the gravel of the 508 mm diameter driven and cast in-situ piles
was more than 3000 kN as derived from loading tests to obtain separate evaluations of shaft
friction and base resistance. It was calculated that if the toe of the pile reached a level at
which it was nearly breaking through to the underlying clay, the end-bearing resistance
would then fall to 1000 kN and the safety factor of the pile would be reduced to 1.2 at the
working load of 800 kN. This safety factor was inadequate, and it was then necessary to
drive the pile some 3.6 m deeper to mobilize additional shaft friction so as to raise the safety
factor to a satisfactory value. A record was made to compare the driving resistance of piles
driven completely through the gravel to a deeper penetration and those terminating on
the gravel layer (Figure 4.26). An evaluation of this record led to the establishment of the
following rules:

(1) When the driving resistance in the gravel increased rapidly from 20 mm per blow to
5 mm per blow for a complete 300 mm of driving it was judged that the pile was properly
seated in the gravel stratum

Resistance of piles to compressive loads 191

Driving resistance in blows/300 mm

0

5

Pile terminating in gravel layer

Depth below ground level (m) Pile driven through gravel

134

Firm clay

Soft clay

Peat

Firm to stiff

boulder clay

Dense gravel

with sand

Firm boulder

clay

Very stiff

boulder clay

10

15

20

25

20 40 60 80 100

Figure 4.26Resistance to driven and cast-in-place piles provided by a thin layer of dense sand and
gravel at Immingham.