Conducting Subsurface Environmental Investigations in Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 139

FIGURE 4.8
Example test pit. (Photo by Daniel T. Rogers.)
4.3.1.2 Excavating Test Pits
When possible, excavating test pits using a backhoe is a preferred method of collecting
and characterizing shallow subsurface geology. Test pits expose much more of the subsur-
face than other methods and give the geologist greater opportunity to observe and gather
geologic information. An added advantage with test pits is the ability to select optimal
locations for collecting soil samples for analysis (Figure 4.8).
While test pits may be the preferred method of characterizing subsurface geology and
providing optimal sample collection points, excavating test pits in an urban setting is
uncommon. In many cases, test pits cannot be excavated because they disturb much larger
areas than drilling a soil boring using a Geoprobe or other similar method. In addition,
many sampling locations are located near utilities, buildings, or beneath paved areas mak-
ing test pits very difficult, dangerous, or impossible to excavate.
4.3.1.3 Drilling Using a Hand Auger
Using a hand auger to characterize subsurface geology has severe limitations. The twist-
ing required to advance the auger deeper into the subsurface causes the loss of most, if not
all depositional structures and features in the recovered samples. For this reason, a hand
auger is used to characterize soil or sediment type and is not used to interpret most com-
plex geological features or structures. A hand auger may also be used to drill the first few
feet to explore and avoid buried utilities before a drill rig completes the rest of the boring.
Figure 4.9 shows a typical hand auger.
4.3.1.4 Mechanical Drilling Methods
For drilling in unconsolidated materials, preferred mechanical drilling methods include
a Geoprobe, hollow-stem auger, or Roto-sonic methods. Characterizing subsurface geol-
ogy from soil borings obtained by mechanical methods is much more difficult than test
pits, because these methods create a long and very narrow sample (4-10 ft long, and only
5-10 cm [2-4 in.] in diameter). These dimensions of sample size increase the difficulty of
the analysis. In addition (1) the outer edges of soil samples collected from a mechanical