Soil Exploration - Soil Properties: Testing, Measurement, and Evaluation - page 14

The water table can be located by measuring down to the water
level in existing wells in an area. It can also be determined from bor-
ing holes. The level to which groundwater rises in a boring hole is the
groundwater elevation in that area. If adjacent soil is pervious, the
water level in a boring hole will stabilize in a short period of time; if
the soil is relatively impervious, it may take much longer for this to
happen. General practice in soil surveying is to cover the boring hole
(e.g., with a small piece of plywood) for safety reasons, leave it for at
least 24 hours to allow the water level to rise in the hole and stabilize,
and return the next day to locate and record the groundwater table.
The hole should then be filled to avoid subsequent injury to people or
animals.

The standard penetration test (SPT) is widely used in the United States.
Relatively simple and inexpensive to perform, it is useful in determin-
ing certain properties of soils, particularly of cohesionless soils, for
which undisturbed samples are not easily obtained.
The SPT utilizes a split-spoon sampler (see Figure 2-7). It is a 2-in.
(51-mm)-O.D. 1 3 ⁄ 8 -in. (35-mm)-I.D. tube, 18 to 24 in. (457 to 610 mm) long,
that is split longitudinally down the middle. The split-spoon sampler is at-
tached to the bottom of a drilling rod and driven into the soil with a drop
hammer. Specifically, a 140-lb (623-N) hammer falling 30 in. (762 mm) is
used to drive the split-spoon sampler 18 in. (457 mm) into the soil.
As a sampler is driven the 18 in. (457 mm) into the soil, the num-
ber of blows required to penetrate each of the three 6-in. (152-mm)
increments is recorded separately. The standard penetration resistance
value (or N -value) is the number of blows required to penetrate the last
12 in. (305 mm). Thus, the N -value represents the number of blows per
foot (305 mm). After blow counts have been obtained, the split-spoon
sampler can be removed and opened (along the longitudinal split) to
obtain a disturbed sample for subsequent examination and testing.
SPT results (i.e., N -values) are influenced by overburden pressure
(effective weight of overlying soil) at locations where blow counts are
made, by drill rod lengths, by whether or not liners are present, and by
borehole diameters.
Through empirical testing, correlations between (corrected) SPT
N -values and several soil parameters have been established. These
are particularly useful for cohesionless soils but are less reliable for

STANDARD
PENETRATION
TEST (ASTM
D 1586)

Figure 2-7
Split-Spoon Sampler for
the Standard Penetration
Test

32 in.

7 in. 22 in. 3 in.

Flat, for Wrench Flat, for Wrench

2 in.
Diameter

1 3 / 8 in.
Diameter

Center Section, Split Lengthwise
Tool Steel Drive Shoe

Thread for
Wash Pipe

Water Ports, 5 / 8 in. Diameter

3 / 4 in.

Total Weight 15 lb