Geoscience Reference
In-Depth Information
•
While the SPTN - value is the summation of the 300mm test drive, the incremental
change should also be noted, as this may signify loose fall in of material (ie incorrect
values) or change in strength (or layer) profile over that 450mm.
4.7 Standard penetration test in rock
•
The SPT procedure in rock is similar to that in soils but extending the refusal
blows to refusal. This requires at least 30 blows in less than 100mm, for both a
seating and a test drive before discontinuing the test.
•
Tabulate both the seating and the test drive. The driller may complain about
damage to the equipment.
•
A solid cone (apex angle of 60
◦
) is used for tests in gravelly soils, boulders and
soft weathered rock.
•
Values of N
60 that cannot be extrapolated to a value of 120 or above is of very
little quantitative value to the designer or assessing rock strength.
>
Table 4.7
Standard penetration test in rock.
Symbol
Test
N
=
23 (eg)
Penetration resistance (blows for 300 mm penetration following 150 mm seating
drive, example of 11
12
23).
+
=
30/50 mm,
Partial penetration, example of blows for the measured penetration, but allowing
−
30/20 mm (eg)
for measuring both seating and test drive.
N*
Inferred SPT Value.
•
There is a debate on whether inferred values should be placed on a factual log.
However, the debate then extends to how much on the log is factual. For example,
is the colour description (person dependent) more factual than N*.
4.8 Overburden correction factors to SPT result
•
An overburden correction factor applies for granular materials.
•
N
o
=
C
N
N.
Table 4.8
SPT correction factors to account for overburden pressure (adapted from Skempton, 1986).
Effective
Correction
Approximate depth of soil (metres) to achieve nominated
overburden
factor, C
N
effective overburden pressure for various ground water level (z
w
)
(kPa)
Fine
Coarse
At surface
z
w
=
2m
z
w
=
5m
z
w
=
10 m
sands
sands
z
w
0m
=
0
2.0
1.5
0.0 m
0.0 m
0.0 m
0.0 m
25
1.6
1.3
3.1m
1.4 m
1.4 m
1.4 m
50
1.3
1.2
6.2 m
3.7 m
2.8 m
2.8 m
100
1.0
1.0
12.5 m
10.0 m
6.2 m
5.6 m
200
0.7
0.8
25.0 m
22.5 m
18.8 m
12.5 m
300
0.5
0.6
37.5 m
35.0 m
31.2 m
25.0 m
400
0.5
0.5
50.0 m
47.5 m
43.7 m
37.5 m
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