Lesson 3/Learning Event 1
The deliberate survey does not dismiss the fact that the time factor may be important; therefore, the
scope of a deliberate survey may necessarily be limited in some cases. A deliberate survey is often
performed while topographical data is being obtained, so that the results of the soil survey may be
integrated with other pertinent information.
The principal methods of exploration used in soil surveys for roads, airfields, and borrow areas are soil
samples obtained either by using hand augers or by digging a test pit. Other methods that may be used
are power-driven earth augers, sounding rods, or earth moving equipment under expedient conditions to
permit a hasty approach to the underlying soil.
A soil survey includes gathering data on soil layers, condition of soil in place, drainage characteristics,
depth of water table, and soil profile. The explorations are conducted to determine the following
Location, Nature, and Classification of Soil Layers. Adequate and economic earthwork and
foundation design of a structure can only be accomplished when the types and depths of soil to be
encountered are known. By classifying the soils encountered, you can predict the extent of problems
with drainage, frost action, settlement, stability, and similar factors. While an estimate of the soil
characteristics may be obtained by field observations, you should also obtain samples of the major soil
types as well as less extensive deposits, which may influence design.
Condition of Soils in Place. The moisture content and density of a soil in its natural state sometimes
play an important part in design and construction. Moisture content of some soils, in place, may be so
high that you should consider selecting another site for the airfield. If the natural soil is sufficiently
dense or compact to meet the required specification, no further compaction of the subgrade will be
required. Very compact soils in cut section may be difficult to excavate with ordinary tractor scraper
units. Scarification or rooting before excavation may be necessary.
Drainage Characteristics. The drainage characteristics, in both surface and subsurface soils, are
controlled by a combination of factors such as the void ratio, soil structure and stratification,
temperature of soil, depth to water table, height of capillary rise, and the extent of local disturbances by
roots and worms. The coarse-grained soils have better internal drainage than fine-grained soils.
Remolding a soil also may change its drainage properties. Observations of the soil should be made in
both the disturbed and undisturbed condition.