Small reductions in overburden pressure cause only elastic movements in the assembly of
soil particles. Larger reductions cause plastic yielding of the assembly and a proportionate
reduction of horizontal pressures. Broug(4.28)has shown that the threshold value for the
change from elastic to elastoplastic behaviour of the soil assembly occurs when the degree
of unloading becomes less than 0.4.
The effect of unloading on cone resistance values was shown by de Gijt and
Brassinga(4.27). Figure 4.20 shows qc/depth plots before and after dredging to a depth of 30 m
in the normally consolidated alluvial sands of the River Maas in connection with an extension
to the Euroterminal in the Netherlands. Large reductions in overburden pressure within
the zone 10 m below the new harbour bed caused the reduction in cone resistance. The
difference between the observed new cone resistance and the mean line predicted by Broug
did not exceed 5%.
The effects are most marked where the soil deposits contain weak particles such as mica-
ceous or carbonate sands. Broug(4.28)described field tests and laboratory experiments on
sands containing 2% to 5% of micaceous particles. These studies were made in connection
with the design of piled foundations for the Jamuna River bridge in Bangladesh where scour
depths of 30 to 35 m occur at times of major floods(3.17).
The static cone penetration test, which measures the resistance of the undisturbedsoil, is
used as a measure of the resistance to penetration of a pile into a soil which has been
compacted by the pile driving. Heijnen(4.29)measured the cone resistance of a loose to
medium-dense silty fine sand before and after installing driven and cast in-situ piles. The
increase in resistance at various distances from the 1 m diameter enlarged base caused by
the pile driving was as follows:180 Resistance of piles to compressive loads
Distance from pile axis (m) Increase in static cone resistance (%)
150 – 100
2 About 33
3.5 NegligibleIn spite of the considerable increase in resistance close to the pile base, the ultimate
resistance of the latter was in fact accurately predicted by the cone resistance value of
theundisturbedsoil by using equation 4.18. This indicates that the effect of compaction
both in driven and driven and cast in-situ piles is already allowed for when using this
equation.
Field trials to correlate the static cone resistance with pile loading tests are necessary in
any locality where there is no previous experience to establish the relationship between the
two. In the absence of such tests the base resistance should be taken as one-half of the static
cone resistance with the application of a factor of safety of 2.5 to obtain the allowable unit
pressure on the base of the pile. Experience has shown that if a safety factor of 2.5 is applied
to the ultimate base resistance as calculated from the cone resistance the settlement at the
working load is unlikely to exceed 10 mm for piles of base widths up to about 500 mm. For
larger base widths it is desirable to check that pile head settlements resulting from the design
end-bearing pressure are within tolerable limits. Pile head settlements can be calculated
using the methods described in Section 4.6.