Compaction  helps  the  soil  to  be  more  resistant  to soaking  up  moisture  from  below. Fills   are   built   up   in   compacted   layers.   In earthwork operations, these layers are called  lifts. Lifts are from 4 inches to 1 foot in depth, depending upon  the  compaction  necessary,  compaction equipment available, and material used for the fill. The fill material must have the right amount of moisture, referred to as optimum moisture content. To obtain maximum compaction, wet the fill, when necessary, before it is compacted. Compaction may be obtained  by  using  a  pneumatic,  tandem,  or  vibratory roller. SOIL STABILIZATION There  are  three  purposes  for  soil  stabilization.  The first  one  is  strength  improvement.  This  increases  the strength  of  the  existing  soil  to  enhance  its  load-bearing capacity.  The  second  purpose  is  for  dust  control.  This is done to eliminate or alleviate dust, generated by the operation   of   equipment   and   aircraft   during   dry weather  or  in  arid  climates.  The  third  purpose  is  soil waterproofing,  which  is  done  to  preserve  the  natural or constructed strength of a soil by preventing the entry of surface water. There  are  two  methods  used  to  apply  soil stabilization materials. The first is the admix way. This  is  used  where  it  is  necessary  to  combine  two different soils together for stabilization. This can be done as follows: .  In-place  mixing:  accomplished  by  blending  of soil  and  stabilization  materials  on  the  jobsite. . Off-site mixing: accomplished by using station- ary mixing plants. . Windrow mixing: accomplished by mixing the materials using a grader. The  second  way  is  the  surface  penetration application, which is accomplished by placing a soil treatment  material  directly  to  the  existing  ground surface by spraying or other means of distribution. Some of the additives used in soil stabilization are cement,   lime,   bituminous   products,   and   calcium chloride.   Cement-treated   bases   are   the   most commonly used for the purpose of upgrading a poor quality soil. Soil-cement is a mixture of pulverized soil and measured amounts of portland cement and water, compacted to a high density. There are three types of soil-cement. The first type is  compacted  soil-cement  that  contains  sufficient amounts  of  cement  to  harden  the  soil  and  enough moisture for both compaction and hydration of the cement.  The  second  type  is  cement  modified  soil which is an unhardened or semihardened mixture of soil  and  cement.  Only  enough  cement  is  used  to change the physical properties of the soil. The third is plastic soil-cement. It is a hardened mixture of soil and cement that contains at the time of placing, enough water  to  produce  a  consistency  similar  to  that  of plastering mortar. The three basic materials needed when  working  with  soil-cement  are  soil,  portland cement,  and  water.  The  soil  can  almost  be  any combination of gravel, sand, silt, or clay. Three major control factors when working with soil-cement are as follows: 1. The proper cement content is needed. A rule of thumb: use one 50-pound bag per square yard. 2. Proper moisture content. On a soil sample, a firm cast should be made when squeezed in your hand without  squeezing  out  any  water. 3.   Adequate   compaction.   The   principles   of compacting soil-cement are the same for compacting the  same  soils  without  cement  treatment.  The soil-cement  mixture  at  optimum  moisture  content should  be  compacted  to  maximum  density  and  finished immediately.  Moisture  loss  by  evaporation  during compaction, as indicated by the graying of the surface, should  be  replaced  with  light  applications  of  water. Occasionally  during  compaction,  the  treated  area may yield under the compaction equipment. This may be due to one or more of the following causes: (1) the soil-cement  mix  is  much  wetter  than  optimum moisture  content,  (2)  the  soil  may  be  too  wet  and unstable, and (3) the roller may be too heavy for the soil. If the soil-cement mix is too damp, it should be aerated by using the scarifier on the grader. After it has dried to near optimum moisture content, then it is compacted. TECHNIQUES OF EARTHWORK OPERATIONS Techniques  of  earthwork  operations  consist  of knowing  the  equipment  needed  and  the  operations  of pioneering,  clearing,  grubbing,  stripping,  draining, and  grading  and  excavating.  These  operations  are done primarily with heavy construction equipment, 15-37