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sieve indicates that lime or a combination LCF will be the  best  additive  for  Stabilization SOIL-CEMENT STABILIZATION In  general,  there  are  three  types  of  soil-and-cement mixtures as follows: l Plastic soil-cement is a hardened mixture of soil and cement that contains, at the time of placing, enough water to produce a consistency similar to plastering mortar. It is used to line or pave ditches, slopes, and other areas that are subject to erosion. It also maybe used for emergency  road  repair  by  mixing  high-early-strength cement into the natural material in mudholes. l  Cement-modified  soil  is  an  unhardened  or semihardened  mixture  of  soil  and  cement.  When relatively small quantities of portland cement are added to  granular  soil  or  silt-clay  soil,  the  chemical  and physical properties of that soil are changed. Cement reduces  the  plasticity  and  water-holding  capacity  of  the soil  and  increases  its  bearing  value.  The  degree  of improvement depends upon the quantity of the cement used and the type of soil. In cement-modified soil, only enough cement is used to change the physical properties of the soil to the degree desired. Cement-modified soils may be used for base courses, subbases, treated sub- grades, highway fills, and as trench backfill material. .  Compacted  soil-cement,  often  referred  to  as simply soil-cement, is a mixture of pulverized soil and calculated amounts of portland cement and water that is compacted to a high density. The result is a rigid slab having  moderate  compressive  strength  and  resistance  to the disintegrating effects of wetting and drying and freezing and thawing. The remainder of our discussion of  soil-cement  is  directed  towards  this  type  of soil-and-cement  mixture. MATERIALS FOR SOIL-CEMENT Soil, portland cement, and water are the three basic materials needed to produce soil-cement. Low cost is achieved  mainly  by  using  inexpensive  local  materials. The soil that makes up the bulk of soil-cement is either in place or obtained nearby, and the water is usually hauled  only  short  distances. The word  soil, as used  in  soil-cement,  means  almost any combination of gravel, sand, silt, and clay, and includes such materials as cinder, caliche, shale, laterite, and  many  waste  materials  including  dirty  and  poorly graded  sands  from  gravel  pits. The quantities of Portland cement and water to be added and the density to which the mixture must be compacted  are  determined  from  tests.  The  water  serves two  purposes:  it  helps  to  obtain  maximum  compaction (density) by lubricating the soil grains and it is necessary for hydration of the cement that hardens and binds the soil into a solid mass. Properly produced soil-cement contains enough water for both purposes. The cement could be almost any type of portland cement that complies with the requirements of the latest ASTM (American Safety for testing and Materials), AASHTO(American  Association  of  State  Highway  and Transportation  Officials),  or  federal  specifications. Types I (normal) and IA (air entrained) portland cements are the most commonly used. The water used in soil-cement should be relatively clean and free from harmful amounts of alkalies, acid, or organic matter. Water fit to drink is satisfactory. Sometimes seawater has been used satisfactorily when fresh  water  has  been  unobtainable. Practically all soils and soil combinations can be hardened with portland cement. They do not need to be well-graded  aggregates  since  stability  is  attained primarily  through  hydration  of  cement  and  not  by cohesion and internal friction of the materials. The general  suitability  of  soils  for  soil-cement  can  be  judged before they are tested on the basis of their gradation and their  position  in  the  soil  profile.  On  the  basis  of gradation,  soils  for  soil-cement  construction  can  be divided into three broad groups as follows: 1.  Sandy  and  gravelly  soils  with  about  10-  to 35-percent  silt  and  clay  combined  have  the  most favorable characteristics and generally require the least amount of cement for adequate hardening. Glacial-and water-deposited  sands  and  gravels,  crusher-run limestone, caliche, lime rock and almost all granular materials work well if they contain 55 percent or more material passing the No. 4 sieve and 37 percent passing the No. 10 sieve. Stones over an inch or two in diameter are  undesirable.  Exceptionally  well-graded  materials may contain up to 65-percent gravel retained on the No. 4  sieve  and  have  sufficient  fine  material  for  adequate binding.  These  soils  are  readily  pulverized,  easily mixed and can be used under a wide range of weather conditions. 2  Sandy  soils  deficient  in  fines,  such  as  some beach  sands,  glacial  sands,  and  windblown  sands,  make good  soil-cement  although  the  amount  of  cement needed for adequate hardening is usually slightly greater than with the soil in Group 1 above. Because of poor 18-5

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