The Atterberg plastic limit^ (PL) is determined as the lowest 

 water content at which a sediment sample of approximately l£ grams 

 can be rolled to form a l/8-lnch thread of sediment without cracking, 

 crumbling, or breaking into pieces (fig. 6). A sediment which cannot 

 be rolled into a thread is recorded as nonplastic. 



i I ill I I V/q" 



ABOVE PLASTIC LIMIT I PLASTIC LIMIT BEIOW PLASTIC LIMIT 



Figure 6, Schematic diagram illustrating a 15-gram 

 sediment sample above and below the plastic limit 



The Rigidense instrument is designed especially for use on 

 sediment cores and consists essentially of a metal base and vertical 

 bar, a hinged brass sample container, an adjustable brass base to 

 support the container, and a steel-tipped aluminum cone with a vertex 

 angle of 37 degrees fitted to the base of a lead penetrometer weight 

 with attached millimeter scale (fig. 7). To determine the rigidense 

 value of a sample, the cup is filled with a section of a core and 

 placed on the adjustable base which is then raised until the pene- 

 trometer point just touches the surface of the core section. The 

 penetrometer is then released, by pressing the penetrometer spring 

 release, and allowed to settle for 2 minutes at which time a reading 

 is taken* The critical features of the instrument are the total 

 weight of the lead penetrometer, cone, and scale which must total 

 1*35 grams, and the vertex angle of the cone which must be 37 degrees. 



III. Results 



Seven sediment types were selected for this investigation,, 

 Each sample was analyzed for rigidense, Atterberg limits, water 

 content, and the percent of sand, silt, and clay,, The water con- 

 tent of each sample was varied three times and ten rigidense meas- 

 urements made for each variation. A total of 230 rigidense meas- 

 urements were made. For a given sample, the reproducibility of this 

 test was within 10 percent. Only five of the seven sediment types 

 contained a sufficient amount of material in the less than U20-micron 

 range to perform Atterberg limit tests. Approximately 100 determi- 

 nations were made on these five sediments. Reproducibility for the 

 Atterberg tests generally was not within 20 percent. 



^TbTdTTTP" 202-203 



