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17 CHAPTER 4- CONCLUSIONS AND RECOMMENDATIONS 4.1 Conclusions Reliable determination of organic content of a soil is an integral part of evaluating suitability of a soil for pavement applications. AASHTO T267 test method âDetermination of Organic Content in Soils by Loss on Ignitionâ lacks precision estimates, which are required for reliable measurement of organic content. An interlaboratory study was designed and conducted for the purpose of preparing precision estimates for AASHTO T267. Samples from three types of soils (clay, silt and sand) were each blended with three different percentages (2%, 5%, and 8%) of fine walnut shell grits as organic material and sent to 30 laboratories for organic content measurement. The laboratories were instructed to test three replicates of each organic content level of each soil type. The results were obtained from 27 different laboratories. Based on the results of AASHTO T267 interlaboratory study, the following conclusions are reached: ï· Clay has the most amount of inherent organic material and sand has the least amount of inherent organic material. ï· The within-laboratory and between-laboratory standard deviations were very consistent for different organic content levels of clay or silt blend. Therefore, for these two blends, the standard deviations corresponding to 2%, 5%, and 8% organic material were pooled together. ï· For the sand blend, the within-laboratory and between-laboratory standard deviations of 5% and 8% organic content were significantly larger than those of 2% organic content. Therefore, the standard deviations corresponding to different organic content levels were not pooled together. ï· The large variability in measurement of organic content of sand blends with 5% and 8% organic material is speculated to be caused by segregation of organic material during shipment due to non-adhesive nature of sand. ï· Since sand has typically less than 2% organic material in its natural state, the precision estimates for sand were prepared based on the standard deviations of the blend with 2% organic content and the standard deviations corresponding to 5% and 8% organic content were not included in precision estimate development. ï· The within-laboratory and between-laboratory standard deviations of the silt and sand blends were statistically similar and they were combined. ï· The within-laboratory and between-laboratory standard deviations of the clay blend were significantly different from those of sand and silt blends and were reported separately. Based on the results of the ILS described in this report, a precision statement was prepared for AASHTO T267 that includes separate precision estimates for clay blends and combined precision estimates for silt and sand blends. The proposed precision statement is provided in Appendix E.
18 4.2 Recommendations Currently, AASHTO Test Method T267 lacks precision estimates that would define the accuracy requirements for organic content measurements in soils. It is recommended that the precision statement in Appendix E, which is prepared based on analysis of the data collected through an interlaboratory study be published in AASHTO T267.
