OBJECTIVES OF A SOIL SURVEY
The overall objective of a soil survey is to gather
(explore) as much information of engineering
significance as possible pertaining to the subsurface
conditions in a specified area. Soil samples are collected
for laboratory tests to determine if the existing soil
conditions could support the type of structure planned
for construction without adding other material for
stabilization. The exploration is conducted in a specific
manner to determine the following information:
1. Location, nature, and classification of soil layers
2. Condition of soils in place (density and moisture
content)
3. Drainage characteristics
4. Groundwater and bedrock
5. Development of a soil profile.
Location, Nature and Classification
of Soil Layers
Adequate and economic earthwork and foundation
design of a structure can be done only when the types
and depths of soil are known. By the classification of
the soils (discussed later in this chapter), you can predict
the extent of problems concerning drainage, frost action,
settlement, stability, and similar factors. While you can
estimate the soil characteristics by field observations,
for laboratory testing, you should obtain samples of the
major soil types as well as less extensive deposits that
may conversely influence design.
Condition of Natural Soils
The moisture content and density of a soil in its
natural state plays an important part in design and
construction. The moisture content of a soil in place may
be so high as to require the selection of a different site.
If the natural soil is sufficiently dense and meets the
required specifications, no compaction of subgrade is
required. On the other hand, extremely dense soil lying
in cut sections maybe difficult to excavate with ordinary
tractor-scraper units. Such dense soil often needs to be
scarified or rooted before excavation.
Drainage Characteristics
Drainage characteristics, both surface and sub-
surface, of a soil greatly affect the strength of the soil.
This characteristic is controlled by a combination of
factors. Some of these factors are void ratio, soil
structure and stratification, temperature of soil, depth to
water table, and the extent of local disturbance by roots
and worms. Coarse-grained soils have better internal
drainage than fine-grained soils.
Groundwater and Bedrock
All structures must be constructed at an elevation
that ensures they will not be adversely affected by the
groundwater table. If a proposed grade line lies below
the elevation of the water table, either the grade line
must be raised or the water table must be lowered by
artificial drainage.
The unexpected discovery of bedrock within the
limits of an excavation greatly increases the time and
equipment required to excavate. If the amount of rock
is extensive, a change in grade or even a change of site
may be the only way out.
Field Notes and Soil Profile
The engineer or EA in charge of the soil survey must
keep accurate field notes and logs. This person is
responsible for surveying, numbering, and recording
each boring, test pit, or other exploration investigation.
A log is kept of each test hole. It should show the
depth below the surface (or the top and bottom
elevations) of each soil layer, the field identification of
each soil present at the site, and the number and type of
each sample taken. Other items of information you need
to include in the log are the density of each soil, changes
in moisture content, depth to groundwater, and depth to
rock. Keep a detailed field log of each auger boring or
test pit made during the soil survey. A typical boring log
is shown in figure 16-1.
When you complete the survey, consolidate the
information contained in the separate logs. Classify and
show the depth of soil layers in each log. It is also helpful
for the log keeper to show the natural water contents of
fine-grained soils, when possible. Record this along the
side of each log. Note the elevation of the groundwater
table. This elevation is simply that of any free water
standing in the test hole. To permit the water to reach
maximum elevation, the engineer or EA should allow
24 hours to elapse before measuring it. This gives a more
accurate measurement.
16-5
