ERRORS AND MISTAKESSome of the common errors and mistakes made inleveling are as follows:l Inaccurate adjustment of the instrument: Themost common instrument error is caused by a level outof adjustment. The instrument must be adjusted, so theline of sight is horizontal when the bubble is in thecenter of the tube.l Errors in sighting: If the eyepiece of the tele-scope is not properly focused, the rod reading appearsto change, because the position of the eye is changedwith respect to the eyepiece.. Errors due to changes in the position of theinstrument: When the instrument is not properly leveledor if it is set up in an unstable position, errors due tosettlement will result. An unstable instrument setupmakes the level bubble tremble slightly, even though itappears to be properly centered. Check the position ofthe bubble before and after each rod reading to makesure that the bubble has remained in the center of thetube.. Faulty handling of the rod: The rod may not beproperly plumbed. If the rod is not held plumb, such asif it leans toward or away from the instrument, the resultwill be an excessive reading.l Erroneous rod length: Check the length of theextended leveling rod with a steel tape.. Failure to clamp the rod at the proper place whenan extended leveling rod is used: This error could resultin reading the wrong mark on the rod or reading thewrong cross hairs. Inspect the clamped positions beforeand after each sight to make sure that the extended rodhas not slipped down.SOILSThe soil is an important part of a solid foundation.A poor foundation will eventually cause roads,runways, buildings, and other temporary or permanentstructures to collapse.SOIL FORMATIONSoils are formed through the breakdown of a solidrock mass or parent material into smaller particles,You may have seen rocks that have been crumbled upor that were softer than others. This is one step in thebreakdown of rock into soil. Rocks wear away whenthey are in contact with moving water, as seen instream beds or rivers. Rocks also break up when theyfreeze and thaw. When rocks heat up by the sun andthen cool quickly, they crack.Soil ProfileDuring formation, soils are in a natural profilemade up of three distinct layers (fig. 15-58).The upper layer, A-horizon, is made up mostly oforganic materials. Because these materials are spongy,drain poorly, and do not compact, they are normallyremoved before building anything on this layer.The B-horizon lies directly beneath the viewA-horizon. This layer is lighter in color and is madeup of sand, gravel, silt, and clay. Seldom is soil in itsnatural state made up of only sand, gravel, silt, or clay.Most soil is made up of a mixture of the four. Howstrong and free-draining the soil is depends on the typeand amount of each in the mixture. The B-horizon isusually the base for all types of pavement construc-tion.The C-horizon is rock in its natural state. It issometimes called parent material, because this iswhere B-horizon material comes from. Very seldomare projects built on the C-horizon.Soil PropertiesWith experience, you will learn that you can usedifferent properties of soil to your advantage. Soilproperties are as follows:. Expansionl Contractionl Plasticityc CohesionExpansion and contraction are undesirablecharacteristics for a solid foundation that must bemonitored closely. Clays and some forms of siltexpand and contract with changes in moisture content.Plasticity is the ability of a soil to be molded intoshapes. Some clays and silts are also plastic and canbe a problem if not controlled properly. Cohesion isthe ability of soil to stick together when dry, and agood example are clays which are very cohesive. Themore plastic a soil is when wet, the more cohesive itis when dry.15-34
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