dragline,  clamshell,  bulldozer,  backhoe,  or  a  large 24-inch  (diameter)  power-driven  earth  auger  can expedite  the  digging-if  the  equipment  is  available. Excavations  below  the  groundwater  table  require  the use of pneumatic caissons or the lowering of the water table. Load-bearing tests can also be performed on the soil in the bottom of the pit. The use of the hand auger is the most common method of digging test holes. It is best suited to cohesive soils;  however,  it  can  be  used  on  cohesionless  soils above the water table, provided the diameter of the individual aggregate particles is smaller than the bit clearance of the auger. By adding a pipe extension, you may use the earth auger to a depth of about 30 feet in relatively  soft  soils.  The  sample  is  completely  disturbed but  is  satisfactory  for  determining  the  soil  profile, classification,   moisture   content,   compaction capabilities,  and  similar  properties.  Auger  borings  are principally used for work at shallow depths. Wash boring is probably the most common method used commercially to make deep test holes in all soil deposits except rock or other large obstructions. The test hole is made by a chopping bit fastened to a wash pipe inside a 2-, 4-, or 6-inch (diameter) steel casing. The wash pipe is churned up and down, while the bit, from which water flows under pressure, loosens the soil. The water then carries the soil particles to the surface where they collect inside the casing. An experienced operator can detect from the appearance of the wash water when a  change  in  the  type  of  soil  being  penetrated  has occurred. Wash samples are samples taken directly from the  wastewater.  They  are  so  disturbed,  however,  that their value is limited. This method of sampling should not be used if any other means is available. Dry-sample boring makes use of the wash boring method to sink the hole. When a change of soil type occurs or sometimes at specified depth intervals, the washing is stopped and the bit is replaced by a sampler. The  sampler  (an  open-end  pipe)  is  driven  into  the relatively disturbed soil in the bottom of the hole to extract  a  sample.  The  sample  is  removed  and  preserved in a sample bottle until tested in the laboratory. The undisturbed  sampling  process is used to obtain  samples  with  negligible  disturbance  and deformation   for   testing   for   shear   strength, compressibility,  and  permeability.  These  samples  can best  be  obtained  from  relatively  cohesive  soils.  Methods that you can use to obtain undisturbed samples are discussed  in  the  EA3  TRAMAN. The core boring  process  is  used  to  obtain  samples from boulders, sound rock frozen ground, and highly resistant  soils.  The  cutting  element  may  consist  of diamonds, chilled shot, or steel-tooth cutters. The drill cuts an angular ring in the rock leaving a central core which enters the core barrel of the drill and is retained by a holding device when the drill is removed from the hole.  This  is  the  best  method  for  determining  the characteristic  and  condition  of  subsurface  rock PLANNING FIELD EXPLORATIONS The location of test holes or test pits depends upon the  particular  situation.  Soil  tests  should  be  made  on samples that are representative of the major soil types in the area. In view of this, the first step in exploration is to develop a general picture of the subgrade conditions to assist in determining the representative soils. Field reconnaissance  should  be  made  to  study  landforms  and soil conditions in ditches and cuts. Techniques have been developed whereby aerial photographs can be used for delineating areas of similar soil conditions. Full use should be made of all existing data. Subgrade  Areas To determine subgrade conditions in an area to be used for road or for airport runway, taxiway, and apron construction, the next step after field reconnaissance is usually to make preliminary borings at strategic points. An  arbitrary  spacing  of  these  borings  at  uniform intervals  does  not  give  a  true  picture  and  is  not recommended. Intelligent use of various procedures permit  strategic  spacing  of  the  preliminary  borings  to obtain maximum information with a minimum number of  borings. Obtain  soil  samples  for  classification  purposes  in these preliminary borings. After these samples are classified, develop soil profiles. Representative soils should then be selected for detailed testing. Test pits or larger diameter borings should then be made to obtain the samples needed for testing or to permit in-place tests to be made. The types and number of samples required depend on the characteristics of the subgrade soils. Subsoil  investigations  in  areas  of  proposed  pavement must include measurements of in-place water content, density, and strength to determine the depth to which compaction must extend and to ascertain whether soft layers exist in the subsoil. 16-9