Sources of Water in Soils Surface  water  results  from  precipitation  or runoff and enters the soil through the openings between the particles. This moisture may adhere to the different particles, or it may penetrate the soil  to  some  lower  layer. Subsurface water is collected or held in pools or layers beneath the surface by a restricting layer of soil or rock. This water is constantly acted upon by  one  or  more  external  forces. Water  controlled  by  gravity  (free  or  gravita- tional  water)  seeks  a  lower  layer  and  moves through the voids or spaces until it reaches some restriction. This restriction may be a bedrock or an  impervious  layer  of  soil  whose  openings  or voids are so small as to prevent water passage. The  voids  or  spaces  in  a  soil  may  form  con- tinuous tunnels or tubes and cause the water to rise  in  the  tubes  by  capillary  action  (capillary moisture). The smaller the tube, the stronger the capillary action; therefore, the water rises higher in  the  finer  soils,  which  have  smaller  intercon- nected voids. This area of moisture above the free water layer or pool is called the capillary fringe. Another force acting on soil water is absorp- tion   by   the   atmosphere.   As   the   moisture evaporates from the soil surface, more moisture is drawn from the soil below and is, in turn, also evaporated. This process continues until the soil is  in  an  airdry  condition  in  which  the  moisture in  the  soil  is  in  equilibrium  with  the  moisture vapor in the air. In this airdry state, the moisture remaining in the soil is in the form of thin films of water surrounding the individual soil particles and  is  called  the  hydroscopic  moisture.  These moisture  films  are  due  to  naturally  occurring  elec- trical  forces,  which  bind  the  water  molecules  to the  soil  particles.  Hydroscopic  moisture  films  may be  driven  off  from  airdried  soil  by  heating  the material in an oven at a controlled temperature for  24  hr  or  until  constant  weight  is  attained. To  define  the  amount  of  water  present  in  a soil  sample,  the  term  moisture  content  (symbol w)  is  used.  It  is  the  proportion  of  the  weight  of water  to  the  weight  of  the  solid  mineral  grains (weight of dry soil) expressed as a percentage or w  = weight  of  water x 100 weight  of  dry  soil When wet soil is dried in air in the laboratory, the  amount  of  hydroscopic  moisture  remaining in the airdried soil, expressed as a percentage of the weight of the dry soil, is called the hydroscopic moisture  content. Plasticity Plasticity  is  a  property  of  the  fine-grained  por- tion of a soil that allows it to be deformed beyond the point of recovery without cracking or chang- ing volume appreciably. Some minerals, such as quartz  powder,  cannot  be  made  plastic  no  mat- ter how fine the particles or how much water is added. All clay minerals, on the other hand, are plastic and can be rolled into thin threads at a cer- tain  moisture  content  without  crumbling.  Since practically  all  fine-grained  soils  contain  some  clay, most of them are plastic. The degree of plasticity is  a  general  index  to  the  clay  content  of  a  soil. The term fat and lean are sometimes used to distinguish  between  highly  plastic  and  slightly plastic soils. For example, lean clay is only slightly plastic,  whereas  fat  clay  is  highly  plastic.  In engineering practice, soil plasticity is determined by observing the different physical states that a plastic soil passes through as the moisture condi- tions  change.  The  boundaries  between  the  dif- ferent  states,  as  described  by  the  moisture  content at the time of change, are called consistency limits or  Atterberg  limits. The liquid limit (LL) is the moisture content corresponding to the arbitrary limit between the liquid and plastic states of consistency of a soil. Above  this  value,  the  soil  is  presumed  to  be  a liquid and behaves as such by flowing freely under its  own  weight.  Below  this  value,  it  deforms  under pressure without crumbling, provided the soil ex- hibits a plastic state. The plastic limit (PL) is the moisture content at  an  arbitrary  limit  between  the  plastic  and semisolid state. It is reached when the soil is no longer pliable and crumbles under pressure. Bet- ween  the  liquid  and  plastic  limits  is  the  plastic range. The numerical difference in moisture con- tent  between  the  two  limits  is  called  the  plasticity index  (PI).  The  equation  is  PI  =  LL  –  PL.  It defines the range of moisture content within which the soil is in a plastic state. The   shrinkage   limit   is   the   boundary   in moisture  content  between  the  solid  and  the semisolid states. The solid state is reached when the soil sample, upon being dried, finally reaches a  limiting  or  minimum  volume.  Beyond  this  point, further  drying  does  not  reduce  the  volume  but may cause cracking. The limit tests are described later in this chapter. Effects of Soil Moisture Moisture  affects  coarse-grained  soils  much  less than  fine-grained  soils.  One  reason  for  this  is  that 15-5