ERRORS AND MISTAKES
Some of the common errors and mistakes made in
leveling are as follows:
l Inaccurate adjustment of the instrument: The
most common instrument error is caused by a level out
of adjustment. The instrument must be adjusted, so the
line of sight is horizontal when the bubble is in the
center of the tube.
l Errors in sighting: If the eyepiece of the tele-
scope is not properly focused, the rod reading appears
to change, because the position of the eye is changed
with respect to the eyepiece.
. Errors due to changes in the position of the
instrument: When the instrument is not properly leveled
or if it is set up in an unstable position, errors due to
settlement will result. An unstable instrument setup
makes the level bubble tremble slightly, even though it
appears to be properly centered. Check the position of
the bubble before and after each rod reading to make
sure that the bubble has remained in the center of the
. Faulty handling of the rod: The rod may not be
properly plumbed. If the rod is not held plumb, such as
if it leans toward or away from the instrument, the result
will be an excessive reading.
l Erroneous rod length: Check the length of the
extended leveling rod with a steel tape.
. Failure to clamp the rod at the proper place when
an extended leveling rod is used: This error could result
in reading the wrong mark on the rod or reading the
wrong cross hairs. Inspect the clamped positions before
and after each sight to make sure that the extended rod
has not slipped down.
The soil is an important part of a solid foundation.
A poor foundation will eventually cause roads,
runways, buildings, and other temporary or permanent
structures to collapse.
Soils are formed through the breakdown of a solid
rock mass or parent material into smaller particles,
You may have seen rocks that have been crumbled up
or that were softer than others. This is one step in the
breakdown of rock into soil. Rocks wear away when
they are in contact with moving water, as seen in
stream beds or rivers. Rocks also break up when they
freeze and thaw. When rocks heat up by the sun and
then cool quickly, they crack.
During formation, soils are in a natural profile
made up of three distinct layers (fig. 15-58).
The upper layer, A-horizon, is made up mostly of
organic materials. Because these materials are spongy,
drain poorly, and do not compact, they are normally
removed before building anything on this layer.
The B-horizon lies directly beneath the view
A-horizon. This layer is lighter in color and is made
up of sand, gravel, silt, and clay. Seldom is soil in its
natural state made up of only sand, gravel, silt, or clay.
Most soil is made up of a mixture of the four. How
strong and free-draining the soil is depends on the type
and amount of each in the mixture. The B-horizon is
usually the base for all types of pavement construc-
The C-horizon is rock in its natural state. It is
sometimes called parent material, because this is
where B-horizon material comes from. Very seldom
are projects built on the C-horizon.
With experience, you will learn that you can use
different properties of soil to your advantage. Soil
properties are as follows:
Expansion and contraction are undesirable
characteristics for a solid foundation that must be
monitored closely. Clays and some forms of silt
expand and contract with changes in moisture content.
Plasticity is the ability of a soil to be molded into
shapes. Some clays and silts are also plastic and can
be a problem if not controlled properly. Cohesion is
the ability of soil to stick together when dry, and a
good example are clays which are very cohesive. The
more plastic a soil is when wet, the more cohesive it
is when dry.