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 telescope 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 tube.

. 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.

SOILS

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.

SOIL FORMATION

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.

Soil Profile

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 construction.

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.

Soil Properties

With experience, you will learn that you can use different properties of soil to your advantage. Soil properties are as follows: 

. Expansion 

l Contraction 

l Plasticity

c Cohesion

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.

Soil Sizes

Soils are grouped by the size of their particle grains. One method used to distinguish sizes is through the use of sieves (fig. 15-59). A sieve is a screen attached across the end of a cylindrical metal frame. The screen allows particles smaller than its openings to fall through and retains larger particles. Sieves with screen openings of different sizes allows you to sort soil into particle groups, based on size.

Sieve sizes are designated by the screen opening size; for example, a 3-inch sieve has a screen with openings 3 inches square. A No. 4 sieve has four openings per linear inch, thus having 16 openings per square inch.

Figure 15-58.-Soil profile.

Soil Classification

The soils you normally work with in earthwork operations are classified as follows:

. Coarse grained

. Fine grained

l Organic