• Home
  • Download PDF
  • Order CD-ROM
  • Order in Print
VISUAL EXAMINATION - 14070_392
ROLL OR THREAD TEST - 14070_394

Engineering Aid 1 - Advanced Structural engineering guide book
Page Navigation
  365    366    367    368    369  370  371    372    373    374    375  
subangular or subaounded, depending on which shape predominates. Laboratory  analysis  must  be  performed  when accurate grain size distribution is to be determined. However,  you  can  approximate  the  distribution  by visual examination using the following steps: 1. Separate the larger grain particles from the rest of the soil sample by picking them out one at a time. 2. Examine the remainder of the soil and estimate the proportion of visible individual particles (larger than the No. 200 sieve) and the fines. 3.  Convert  these  estimates  into  percentages  by weight of the total sample. If the fines exceed 50 percent, the soil is considered fine-grained (M, C, or O); if the coarse  material  exceeds  50  percent,  the  soil  is coarse-grained (G or S). 4. Examine the coarse-grained soil for gradation of particle  sizes  from  the  largest  to  the  smallest.  A  good distribution of all sizes without too much or too little of any  one  size  means  the  soil  is  well-graded  (W). Overabundance or lack of any size means the material is poorly graded (P). 5.  Estimate  the  percentage  of  the  fine-grained portion of the coarse-grained soil. If nonplastic fines are less than 5 percent of the total, the soil maybe classified either as a GW, GP, SW, or SP type, depending on the other information noted above. 6.  If  the  fine-grained  portion  (Step  5  above) exceeds 12 percent, the soil is either silty (M) or clayey (C) and requires further testing to identify. 7. Fine-grained portions (Step 5 above) between 5- and  12-percent  (nonplastic  fines  or  fines  not  interfering with drainage, or 0 to 12 percent plastic fines) total are border line and require a double symbol (GW-GM or SW-SM). 8.  Fine-grained  soils  (M,  C,  or  O)  from  Step  3 above  require  other  tests  to  distinguish  them  further. Grain size distribution of fine portions is normally not performed in field identification. However, should it become necessary, you can approximate the grain size of  the  fines  by  shaking  them  in  a  jar  of  water  and allowing the material to settle. The materials settle in layers of different sizes from which the proportion can be estimated. It should be kept in mind that gravel and sand settle into a much denser mass than either clay or silt. If you use the characteristics determined up to this point, it is possible to evaluate the soil as it appeared in place (undisturbed). Gravels or sands can be described qualitatively as loose, medium, or dense. Clays maybe hard, stiff, or soft. The ease or difficulty with which the sample  was  removed  from  the  ground  is  a  good indicator. Soils that have been cultivated or farmed can be further evaluated as loose and compressible. Highly organic soils can be spongy and elastic. In addition, moisture content of the soil influences the in-place characteristics.  This  condition  should  be  recognized  and reported  with  the  undisturbed  soil  properties. BREAKING  OR  DRY-STRENGTH TEST The breaking test is done only on the material passing the No. 40 sieve. This test as well as the roll test and the ribbon test, is used to measure the cohesive and plastic  characteristics  of  the  soil.  The  test  normally  is made on a small pat of soil about 1/2 inch thick and about 1 1/2 inches in diameter. The pat is prepared by molding a portion of the soil in the wet plastic state into the size and shape desired and then allowing the pat to dry completely. Samples may be tested for dry strength in their natural condition as they are found in the field However, you should not depend too much on such tests because  of  the  variations  that  exist  in  the  drying environment   under   field   conditions.   You   may approximate the dry strength by such a test however, and verify it later by a carefully prepared sample. After the prepared sample is thoroughly dry, attempt to break it using the thumbs and forefingers of both hands (fig. 16-5). If you are able to break it, then try to powder it by rubbing it with the thumb and fingers of one  hand. Figure 16-5.-Breaking or dry-strength test. 16-17







Western Governors University

Privacy Statement
Press Release
Contact

© Copyright Integrated Publishing, Inc.. All Rights Reserved. Design by Strategico.