Building Design and Construction Handbook - Merritt - Ventech!

6.12 SECTION SIX
from the hole, and the soil lodged in the groves of the flight auger or contained
in the bucket of the bucket auger is removed. A casing is generally not used for
auger borings, and the hole may cave-in during the excavation of loose or soft
soils or when the excavation is below the groundwater table. Augers are probably
the most common type of equipment used to excavate borings.
Hollow-Stem Flight Auger. A hollow-stem flight auger has a circular hollow
core which allows for sampling down the center of the auger. The hollow-stem
auger acts like a casing and allows for sampling in loose or soft soils or when
the excavation is below the groundwater table.
Wash-Type Borings. Wash-type borings use circulating drilling fluid, which
removes cuttings from the borehole. The cuttings are created by the chopping,
twisting, and jetting action of the drill bit, which breaks the soil or rock into
small fragments. Casings are often used to prevent cave-in of the hole. Because
drilling fluid is used during the excavation, it can be difficult to classify the soil
and obtain uncontaminated soil samples.
Rotary Coring. This type of boring equipment uses power rotation of the drilling
bit as circulating fluid removes cuttings from the hole. Table 6.3 lists various
types of rotary coring for soil and rock.
Percussion Drilling. This type of drilling equipment is often used to penetrate
hard rock, for subsurface exploration or for the purpose of drilling wells. The
drill bit works much like a jackhammer, rising and falling to break up and crush
the rock material.
In addition to borings, other methods for performing subsurface exploration include
test pits and trenches. Test pits are often square in plan view, with a typical
dimension of 1.2 m by 1.2 m (4 ft by 4 ft). Trenches are long and narrow excavations
usually made by a backhoe or bulldozer. Table 6.4 presents the uses, capabilities,
and limitations of test pits and trenches.
Test pits and trenches provide for a visual observation of subsurface conditions.
They can also be used to obtain undisturbed block samples of soil. The process
consists of carving a block of soil from the side or bottom of the test pit or trench.
Soil samples can also be obtained from the test pits or trenches by manually driving
Shelby tubes, drive cylinders, or other types of sampling tubes into the ground.
(See Art. 6.2.3.)
Backhoe trenches are an economical means of performing subsurface exploration.
The backhoe can quickly excavate the trench, which can then be used to
observe and test the in-situ soil. In many subsurface explorations, backhoe trenches
are used to evaluate near-surface and geologic conditions (i.e., up to 15 ft deep),
with borings being used to investigate deeper subsurface conditions.
6.2.3 Soil Sampling
Many different types of samplers are used to retrieve soil and rock specimens from
the borings. Common examples are indicated in Table 6.3. Figure 6.1 shows three
types of samplers, the ‘‘California Sampler,’’ Shelby tube sampler, and Standard
Penetration Test (SPT) sampler.
The most common type of soil sampler used in the United States is the Shelby
tube, which is a thin-walled sampling tube. It can be manufactured to different
diameters and lengths, with a typical diameter varying from 5 to 7.6 cm (2 to 3 in)
and a length of 0.6 to 0.9 m (2 to 3 ft). The Shelby tube should be manufactured