Fundamental Soil Mechanics - Mathematics and Statistics for the Environment - page 315

• Contact utility companies to ensure that underground installations are identiied and
located.
• Ensure that underground installations are protected, supported, or removed as necessary
to safeguard workers.
• Remove or secure any surface obstacles (trees, rocks, and sidewalks, for example) that may
create a hazard for workers.
• Classify the type of soil and rock deposits at the site as stable rock, type A, type B, or type
C soil. One visual and at least one manual analysis must be performed to make the soil
classification.
Let's take a closer look at the requirement to classify the type of soil to be excavated. Before an
excavation can be accomplished, the soil type must be determined. The soil must be classified as
stable rock, type A, type B, or type C soil. Remember, commonly you will find a combination of
soil types at an excavation site. In this case, soil classification is used to determine the need for a
protective system. Following is a description of the various soil classifications:
• Stable rock is a natural solid mineral material that can be excavated with vertical sides.
Stable rock will remain intact while exposed, but keep in mind that, even though solid rock
is generally stable, it may become very unstable when excavated (in practice you never
work in this kind of rock).
• Type A soil , the most stable soil, includes clay, silty clay, sandy clay, clay loam, and some-
times silty clay loam and sandy clay loam.
• Type B soil , moderately stable, includes silt, silt loam, sandy loam, and sometimes silty
clay loam and sand clay loam.
• Type C soil , the least stable, includes granular soils such as gravel, sand, loamy sand, sub-
merged soil, soil from which water is freely seeping, and submerged rock that is not stable.
To test and classify soil for excavation, both visual and manual tests should be conducted. Visual
soil testing looks at soil particle size and type. Of course, a mixture of soils will be visible. If the
soil clumps when dug it could be clay or silt. Type B or C soil can sometimes be identified by the
presence of cracks in walls and spalling (breaks up into chips or fragments). If you notice layered
systems with adjacent hazardous areas—buildings, roads, and vibrating machinery—a professional
engineer may be required for classification. Standing water or water seeping through trench walls
automatically classifies the soil as type C.
Manual soil testing is required before a protective system (e.g., shoring or shoring box) is selected.
A sample taken from soil dug out into a spoil pile should be tested as soon as possible to preserve its
natural moisture. Soil can be tested either onsite or offsite. Manual soil tests include a sedimentation
test, wet shaking test, thread test, and ribbon test.
A sedimentation test determines how much silt and clay are in sandy soil. Saturated sandy soil is
placed in a straight-sided jar with about 5 inches of water. After the sample is thoroughly mixed (by
shaking it) and allowed to settle, the percentage of sand is visible. A sample containing 80% sand,
for example, will be classified as type C.
The wet shaking test is another way to determine the amount of sand vs. clay and silt in a soil
sample. This test is accomplished by shaking a saturated sample by hand to gauge soil permeability
based on the following facts: (1) shaken clay resists water movement through it, and (2) water flows
freely through sand and less freely through silt.
The thread test is used to determine cohesion (remember, cohesion relates to stability—how well
the grains hold together). After a representative soil sample is taken, it is rolled between the palms
of the hands to about 1/8-inch diameter and several inches in length (any child who has played in