soils  are  not  classified  on  the  basis  of  grain size  distribution  but  according  to  plasticity  and compressibility. HIGHLY   ORGANIC   SOILS   are  those organic  soils,  such  as  peat,  that  have  too  many undesirable  characteristics  from  the  standpoint  of their behavior as foundations and their use as con- struction  materials.  A  special  classification  is reserved  for  these  soils,  and  no  laboratory  criteria are established for them. Highly organic soils can generally  be  readily  identified  in  the  field  by  their distinctive  color  and  odor,  spongy  feel,  and  fre- quently  fibrous  textures.  Particles  of  leaves,  grass, branches,  or  other  fibrous  vegetable  matter  are common  components  of  these  soils. CLASSIFICATION  TESTS The above is by no means a thorough descrip- tion of the USCS and the methods used to classify soils; nor is it intended to be. However, the results of  certain  tests  (sieve  analysis  and  Atterberg  limits) that you will be performing as an EA3 will be used for  the  purpose  of  soil  classification.  The  pre- ceding  discussion  is  presented  so  that  you  have an understanding of why you perform the tests, what the results are used for, and the importance of ensuring that your test results are correct and reliable. A full discussion of the test procedures will  be  presented  later  in  this  chapter,  Should  you desire  to  learn  more  about  the  USCS  and  soils classification,  you  may  refer  to  the  EA1 TRAMAN,  to  NAVFAC  MO-330,   Materials Testing,   or  to  one  of  numerous  commercial publications  on  soil  mechanics. SOIL  SAMPLING In the planning and execution of construction operations, it is vital to know as much informa- tion of engineering significance as possible about the  subsurface  conditions  in  the  construction  area. That information includes not only the location, extent,  and  condition  of  the  soil  layers  but  also the  elevation  of  the  groundwater  table  and bedrock; the drainage characteristics of the sur- face  and  subsurface  soils;  and  the  location  of possible borrow areas from which soil and other mineral-product   materials   may   be   “borrowed” for  a  construction  operation.  Soil  surveys  are  con- ducted  to  gather  (explore)  this  information.  These are multifaceted surveys that consist of the follow- ing: gathering soil samples; soil testing by either laboratory or field procedures, or both; soil clas- sification;  and  the  development  of  soil  profiles. In the full scope of soil surveying, your primary concern, as an EA3, is gathering soil samples and conducting  certain  of  the  laboratory  soils  tests. Should you desire to learn more about soil survey- ing,  an  excellent  source  is  NAVFAC  MO-330, Materials  Testing. SAMPLING   METHODS The  gathering,  or  collecting,  of  soil  samples in  the  field  for  testing  is  called  SOIL  SAMPLING. The three principal methods of sampling are the taking of samples from the surface, from already existing excavations, and from test pits and test holes. The extent and methods used will be depen- dent  upon  the  time  available. The method that provides the most satisfac- tory  results  for  both  studying  the  natural  soil  con- ditions  and  for  collecting  undisturbed  soil  samples is the taking of samples from test pits. A test pit is an  open  excavation  that  is  large  enough  for  a  per- son to enter. Usually, these pits are dug by hand; however,  when  power  equipment  is  available, power  excavation  by  clamshell,  dragline,  bull- dozer,  backhoe,  or  a  power-driven  earth  auger  can expedite  the  digging.  Excavations  below  the groundwater table require the use of pneumatic caissons or the lowering of the water table. Addi- tionally, excavations that extend to 5 ft or more in depth may require adequate shoring and bracing to prevent cave-ins, as discussed in the previous chapter.  Load-bearing  tests  can  also  be  performed on  the  soil  in  the  bottom  of  the  pit. Test hole exploration, with the use of the hand auger, is the most common method of digging test holes.  It  is  best  suited  to  cohesive  soils  but  can be  used  on  cohesionless  soils  above  the  water table, provided the diameter of the individual ag- gregate  particles  is  smaller  than  the  bit  clearance of the auger. Auger borings are usually used for work  at  shallow  depths,  but  if  pipe  extensions  are added,  the  earth  auger  may  be  used  to  a  depth of about 30 ft in relatively soft soils. Samples ob- tained  by  this  method  are  completely  disturbed but are satisfactory for determining the soil pro- file,  classification,  moisture  content,  compaction capabilities,  and  similar  soil  properties. In a hasty soil survey, which is made under expedient conditions or when time is limited, the number  of  test  pits  and  test  holes  is  kept  to  a minimum  by  the  use  of  existing  excavations  for soil sampling. In a deliberate survey, where time and  conditions  allow  a  more  thorough  sampling operation, test holes are used extensively and are augmented  by  test  pits,  governed  by  the  judgment of  the  engineering  officer. 15-7