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 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 construc- tion. 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. 15-34