most  suitable  treatments  for  various  soil  types  to stabilize these soils for different objectives. MECHANICAL  METHOD Mechanical  stabilization  is  accomplished  by  mixing or blending soils of two or more gradations to obtain a material meeting the required specification. The soil blending may take place at the construction site, at a central plant, or at a borrow area. The blended material is  then  spread  and  compacted  to  required  densities  by conventional  means. ADDITIVE METHOD Additive  refers  to  a  manufactured  commercial product  that,  when  added  to  the  soil  in  the  proper quantities, will improve the quality of the soil layer. This chapter is directed towards the use of portland cement, lime, lime-cement-fly ash, and bitumen, alone or in combination,   as   additives   to   stabilize   soils.   The selection  and  determination  of  the  percentage  of additives depend upon the soil classification and the degree  of  improvement  in  soil  quality  desired. Generally,  smaller  amounts  of  additives  are  required  to alter soil properties, such as gradation, workability, and plasticity, than to improve the strength and durability sufficiently to permit a thickness reduction design. After the additive has been mixed with the soil, spreading and compacting  are  accomplished  by  conventional  means. Stabilization by Cementing Action This  method  requires  the  addition  of  chemical agents to the soil to produce the hardened product. There are three main stabilizing agents that can be added, and the method of treatment bears the name of these agents: soil-cement, soil-lime, and lime-fly ash. The methods of chemical  stabilization  have  much  in  common  and involve  somewhat  similar  construction  practices.  They depend  upon  hydration,  pozzolanic  action  of  lime  with silica and alumina, alteration of the clay material, or a combination of these actions. The result is a semirigid, fairly brittle material with considerable compressive strength  and  moderate  flexural  strength  when  tested either  statically  or  dynamicaly.  The  ultimate  strength depends to a great degree on the density that is achieved during compaction and before the mix cures. Bituminous  Stabilization In  bituminous  treatment,  the  end  product  performs differently—at least initially, and the product is much less  brittle.  Additionally,  its  behavior  depends  on  the nature  of  the  loading  (static  or  dynamic)  and  the temperature when the load is applied. MODIFICATION METHOD Soil stabilization by modification usually results in something less than a thoroughly cemented, hardened or semihardened material. This type of stabilization may be  accomplished  by  compacting,  by  mechanical blending,  by  adding  cementing  materials  in  small amounts, or by adding chemical modifiers. Cement and lime   modifiers   (cement-modified   soil   and lime-modified  soil)  are  used  in  quantities  too  small  to provide  high-strength  cementing  action.  They  reduce the plasticity of clay soils. Calcium chloride or sodium chloride is added to the soil to retain moisture (and also control   dust),   to   hold   fine   material   for   better compaction, and to reduce frost heave by lowering the freezing point of water in the soil. Bituminous materials, such  as  cutback  asphalts  or  asphaltic  penetrative  soil binder (APSB), and certain chemicals, such as polyvinyl acetate emulsion (DCA-70), are used to waterproof the soil surface and to control dust. GENERAL REQUIREMENTS FOR USE OF STABILIZERS This  section  discusses  different  types  of  stabilizers. It also provides a method of selecting the type or types of  stabilizers  that  you  can  use  for  various  conditions. Before a proper stabilizer can be selected, however, you must first perform, or have performed, a sieve analysis and Atterberg limits tests for the particular type of soil you  are  concerned  with.  Both  sieve  analysis  and Atterberg  limits  testing  are  discussed  in  the  EA3 TRAMAN  and  in  Materials   Testing,   NAVFAC MO-330. LIME Experience  shows  that  lime  will  react  with  many medium,  moderately  fine,  and  fine-grained  soils  to produce decreased plasticity, increased workability, reduced  swell,  and  increased  strength.  Soils  classified according to the Unified Soil Classification System (USCS) as CH, CL, MH, ML, OH, OL, SC, SM, GC, GM,   SW-SC,   SP-SC,   SM-SC,   GW-GC,   GP-GC, ML-CL,   and   GM-GC   should   be   considered   as potentially capable of being stabilized with lime. CEMENT Cement can be used as an effective stabilizer for a wide  range  of  materials.  In  general,  however,  the  soil should have a PI less than 30. For coarse-grained soils, the amount passing the No. 4 sieve should be greater than 45 percent. 18-2