Civil Engineering Reference
In-Depth Information
Although the soil specimen may be considered to be an undisturbed specimen, there
could still be disruption of the soil structure during all phases of the sampling operation.
The greatest disturbance will probably occur during the physical pushing of the sampler
into the soil.
2.
Block sampling:
Another approach for near-surface soil is to temporarily lower the
groundwater table. Then a test pit or trench is excavated into the soil. Because the ground-
water table has been lowered, the partially saturated soil will be held together by capillar-
ity. A block sample is then cut from the sides of the test pit or trench, and the block sample
is transported to the laboratory for testing.
If the soil does not have enough capillarity to hold itself together, then this method will
not work. In addition, the soil could be disturbed due to stress relief when making the exca-
vation or when extracting the soil specimen.
3.
Freezing technique:
The essential steps in the freezing technique are to first freeze
the soil and then cut or core the frozen soil from the ground. The freezing is accomplished
by installing pipes in the ground and then circulating ethanol and crushed dry ice or liquid
nitrogen through the pipes. Because water increases in volume upon freezing, it is impor-
tant to establish a slow freezing front so that the freezing water can slowly expand and
migrate out of the soil pores. This process can minimize the sample disturbance associated
with the increase in volume of freezing water.
From a practical standpoint, the three methods described above are usually not eco-
nomical for most projects. Thus laboratory testing is not practical, and the analyses of
earthquake hazards (such as liquefaction) are normally based on field testing that is per-
formed during the subsurface exploration. The two most commonly used field tests are the
standard penetration test (SPT) and the cone penetration test (CPT), as discussed in the
next two sections.
5.4.3
Standard Penetration Test
Test Procedure.
The standard penetration test can be used for all types of soil, but in gen-
eral the SPT should only be used for granular soils (Coduto 1994). The SPT can be espe-
cially valuable for clean sand deposits where the sand falls or flows out from the sampler
when retrieved from the ground. Without a soil sample, other types of tests, such as the
standard penetration test, must be used to assess the engineering properties of the sand.
Often when a borehole is drilled, if subsurface conditions indicate a sand stratum and sam-
pling tubes come up empty, the sampling gear can be quickly changed to perform standard
penetration tests.
The standard penetration test consists of driving a thick-walled sampler into the granu-
lar soil deposit. The test parameters are as follows:
●
Sampler:
Per ASTM D 1586, the SPT sampler must have an inside barrel diam-
eter
D
3.81 cm (1.5 in.) and an outside diameter
F
5.08 cm (2 in.), as shown in
Fig. 5.10.
●
Driving hammer:
The SPT sampler is driven into the sand by using a 63.5-kg (140-lb)
hammer falling a distance of 0.76 m (30 in.).
●
Driving distance:
The SPT sampler is driven a total of 45 cm (18 in.), with the number
of blows recorded for each 15-cm (6-in.) interval.
●
N value:
The
measured SPT N value
(blows per foot) is defined as the penetration resis-
tance of the soil, which equals the sum of the number of blows required to drive the SPT
