Civil Engineering Reference
In-Depth Information
The sampler, connected to a sequence of drive rods, is lowered down the borehole until it rests on the
layer of cohesionless soil to be tested. It is then driven into the soil for a length of 450 mm by means of
a 63.5 kg hammer free-falling 760 mm for each blow. The number of blows required to drive the last
300 mm is recorded and this figure is designated as the N-value or the penetration resistance of the soil
layer. The first 150 mm of driving is ignored because of possible loose soil in the bottom of the borehole
from the boring operations. After the tube has been removed from the borehole it can be opened and
its contents examined.
In gravelly sand damage can occur to the cutting head of the sampler and a 60° solid cone can be fitted
in its place. In such a case the test is recorded as SPT(C). The N-value derived from such soils appears to
be of the same order as that obtained when the cutting head is used in finer soils.
Correction factors to the measured N-value
The N-value observed from the test is affected by different features of the testing procedure and ground
conditions. To take these into consideration, a number of correction factors can be applied to achieve a
more appropriate N-value. The most significant factors address energy losses delivered by the hammer
assembly and the effect of overburden pressure acting on the soil under test. EN ISO 22476-3:2005, Annex
A offers the following corrections:
i)
Energy delivered to the drive rods
An energy ratio, E
r
, measuring the ratio of the energy applied to the driving rods to the theoretical
energy available from the hammer, can be used to assess the energy loss in the hammer system. The
N-value is adjusted to a reference energy ratio of 60% through the following expression:
E
N
r
N
=
60
60
where N
60
denotes the N-value corrected for energy losses for a system operating with an energy
ratio E
r
.
ii)
Effect of overburden pressure in sands
An important feature of the standard penetration test is the influence of the effective overburden
pressure on the N-value. Sand can exhibit different N-values at different depths even though its density
index is constant. The effect of the overburden pressure can be taken into account by combining the
The N
60
value can now be corrected against a normalised effective vertical stress
σ
v
′ =
100
kPa
:
E N C
r
× ×
N
(
N
) =
1
60
60
Table 6.5
Correction factors for overburden effective vertical stress,
σ
v
kPa
′
(
)
.
Type of consolidation
Density index, I
D
(%)
Correction factor, C
N
200
100
+
′
Normally consolidated
40-60
σ
v
300
200
+
′
60-80
σ
v
170
70
+
′
Overconsolidated
-
σ
v
