Civil Engineering Reference
In-Depth Information
TABLE 5.1
Factors That Can Affect the Standard Penetration Test Results (
Continued
)
Factors that can affect
the standard penetration
test results
Comments
Use of drill rods heavier
With heavier rods, more energy is absorbed by the rods, causing an
than standard
increase in the blow count.
Not recording blow
Incorrect
N
values are obtained.
counts and penetration
accurately
Incorrect drilling
The standard penetration test was originally developed from wash bor-
ing techniques. Drilling procedures which seriously disturb the soil will
affect the
N
value, for example, drilling with cable tool equipment.
Using large drill holes
A borehole correction is required for large-diameter boreholes. This is
because larger diameters often result in a decrease in the blow count.
Inadequate supervision
Frequently a sampler will be impeded by gravel or cobbles, causing a
sudden increase in blow count. This is often not recognized by an inex-
perienced observer. Accurate recording of drilling sampling and depth
is always required.
Improper logging of soils
The sample is not described correctly.
Using too large a pump
Too high a pump capacity will loosen the soil at the base of the hole,
causing a decrease in blow count.
Source:
NAVFAC DM-7.1 (1982).
TABLE 5.2
Correlation between SPT
N
60
Value and Density of Clean Sand
N
60
(blows per foot)
Sand density
Relative density
D
r
,
percent
0- 4 Very loose condition 0 -15
4-10 Loose condition 15-35
10-30 Medium condition 35-65
30-50 Dense condition 65-85
Over 50 Very dense condition 85-100
Note:
Relative density
D
r
100(
e
max
e
)
(
e
max
e
min
), where
e
max
void ratio corresponding to the
loosest possible state of the soil, usually obtained by pouring the soil into a mold of known volume (ASTM D
4254),
e
min
void ratio corresponding to the densest possible state of the soil, usually obtained by vibrating the
soil particles into a dense state (ASTM D 4253), and
e
the natural void ratio of the soil.
Sources:
Terzaghi and Peck (1967) and Lambe and Whitman (1969).
TABLE 5.3
Correlation between (
N
1
)
60
and Density of Sand
(
N
1
)
60
(blows per foot)
Sand density
Relative density
D
r
,
percent
0 -2
Very loose condition
0 -15
2-5
Loose condition
15-35
5-20
Medium condition
35-65
20 -35
Dense condition
65-85
Over 35
Very dense condition
85-100
Source:
Tokimatsu and Seed (1987).
