Geoscience Reference
In-Depth Information
•
Average saturated unit weight of 18 kN/m
3
used in Table. Unit weight can
vary.
•
Borehole water balance is required for tests below the water table to avoid blowout
at the base of the hole with loosening of the soil, and a resulting non representative
low N - value.
•
In very fine or silty sands below the water table, a pore pressure may develop and
an additional correction factor applies for N
>
2(N
−
15. N
=
15
+
1
/
15).
4.9 Equipment and borehole correction factors for SPT result
•
An equipment correction and borehole size correction factors apply.
•
The effect of borehole diameter is negligible for cohesive soils, and no correction
factor is required.
•
The energy ratio is normalized to 60% of total energy.
•
(N
o
)
60
=
C
N
C
ER
N.
•
C
ER
=
C
H
C
R
C
s
C
B
.
Table 4.9
Energy ratio correction factors to be applied to SPT value to account for equipment and
borehole size (adapted from Skempton, 1986 and Takimatsu and Seed, 1987).
To account for
Parameter
Correction factor
Hammer - release - country
Hammer (C
H
)
Donut - free fall (Tombi) - Japan
1.3
•
Donut - rope and pulley - Japan
1.1
•
•
Safety - rope and pulley - USA
1.0
Donut - free fall (Trip) - Europe,
1.0
•
China,Australia
Donut - rope and pulley - China
0.8
•
Donut - rope and pulley - USA
0.75
•
Rod length (C
R
)
10 m
1.0
•
10 m to 6 m
0.95
•
•
6 m to 4 m
0.85
4 m to 3 m
0.75
•
Sampler (C
s
)
•
Standard
1.0
US sampler without liners
1.2
•
Borehole
•
65 mm - 115 mm
1.0
Diameter (C
B
)
150 mm
1.05
•
200 mm
1.15
•
4.10 Cone penetration test
•
There are several variations of the cone penetration test (CPT). Electric and
mechanical cones should be interpreted differently.
•
The CPTu data is tabled below. The CPT would not have any of the pore pressure
measurements.
•
The CPT has a high production rate (typically 100m/day but varies depending
on number, soil type, distance between tests, accessibility, etc) compared to other
profile testing.
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