Pile Design and Construction Practice, Fifth edition

where
Resistance of piles to compressive loads 191
q o � ultimate base resistance in the lower weak layer
q l � 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 bulkhandling
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 endbearing
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
Firm clay
Soft clay
Peat
Firm to stiff
boulder clay
Dense gravel
with sand
Firm boulder
clay
Very stiff
boulder clay
Depth below ground level (m)
5
10
15
20
25
0
Driving resistance in blows/300 mm
20 40 60 80 100
Pile terminating in gravel layer
134
Pile driven through gravel
Figure 4.26 Resistance to driven and cast-in-place piles provided by a thin layer of dense sand and
gravel at Immingham.