Quantcast UNDISTURBED  SAMPLES

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Figure 15-7.-Taking a composite sample from a small windrow. in drawing the soil profile, and in estimating the physical  properties  of  soils  encountered  in  the field. Generally,  100  g  of  soil  is  enough  to  determine the moisture content of fine-grained soils. Larger samples  are  required  for  soils  that  contain  gravel. Normally,  moisture  content  samples  are  placed  in metal  dishes  (canisters)  that  have  tight-fitting covers;   however,   any   other   clean   container that   can   be   adequately   sealed   may   be   used. When  the  moisture  content  test  is  to  be  per- formed   within   1   day   after   the   sample   is Figure 15-8.-Sealing a container to retain moisture content of a sample. obtained,   sealing   of   the   container   is   not required.  If  a  longer  time  interval  will  elapse between  sampling  and  testing,  the  containers may  be  sealed,  as  shown  in  figure  15-8. UNDISTURBED  SAMPLES UNDISTURBED soil samples are those that are  cut,  removed,  and  packed  with  the  least possible disturbance. They are samples in which the  natural  structures,  void  ratio,  and  moisture content  are  preserved  as  carefully  as  possible. Samples of this type are used for determining the density (unit weight) of soil in the laboratory and investigating the strength of undisturbed soils in the  laboratory  by  the  CBR  or  unconfined  com- pression tests. These samples may be shipped to more completely equipped laboratories for shear, consolidation,  or  other  strength  tests. Types  of  undisturbed  samples  are  chunk samples, cut by hand with a shovel and knife, and cylinder samples, obtained by use of a cylindrical sampler or the CBR mold equipped with a sampl- ing   cutter.   Expedient   methods   of   obtaining cylinder  samples  are  also  used. The method of sampling chosen depends upon the equipment available, the tests required, and the type of soil. All undisturbed samples must be handled  with  care.  Cohesionless  soil  samples  must be kept in the container until ready for testing, and  the  container  should  be  handled  without jarring  or  vibration.  Some  soils  are  too  hard  or contain too many stones to permit sampling with the cylindrical samplers and can be sampled only by  cutting  out  chunks  by  hand.  Taking  of  un- disturbed  samples  frequently  requires  a  great  deal of ingenuity in adapting the sampling devices to job conditions and in devising schemes for their use. Whatever method is used, the sample must be taken and packed in the container for shipment without allowing its structure to change. Protec- tion  against  change  in  moisture  content  during sampling  and  shipment  is  also  required. Chunk Samples The  simplest  type  of  undisturbed  sample  is the chunk sample. It should be noted, however, that  these  can  be  obtained will  not  deform,  break,  or removed. only  from  soils  that crumble while being 15-11



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