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Investigation Regarding Fines in Aggregate Used for Soil Absorption Systems

Published: 2006


Commonwealth of Massachusetts Regulations pertaining to the installation of onsite septic systems require that base aggregate used for leaching structures be “free of iron, fines and dust in place.” Absent of any field testing procedure for aggregate, it is difficult for contractors or inspectors to reasonably ensure adherence to the intent of the regulation. The purpose of the present study was to determine whether any rapid field tests for aggregate quality were in use in other jurisdictions and to research, in a preliminary way, factors for consideration in the assessment of aggregate in the field. This study made use of the extensive network of practitioners in the field of onsite wastewater and the regulator community available through the Environmental Protection Agency internet listserve. Three separate inquiries were made. The survey verified that, although the problem of “dirty stone” (referring to what is perceived as aggregate with excessive fine material) is universally recognized, there is no consensus regarding reasonable methods for determining quality of aggregate with the exception of ASTM tests, which might impose unreasonable delays during septic system installation. In addition there is no universal agreement as to what amount of fine material, using standard laboratory tests such as American Standards of Testing Materials Method C177, would constitute an unacceptable level. To research some of the issues relative to aggregate quality and its impact on soil absorption system performance, field test cells were constructed and test were performed on aggregate of differing quality relative to fine material content. Test cell conditions were optimized to favor the entrainment of fine material from the aggregate surface to the soil interface, and thus represent the worst possible conditions. The results of field tests were compared with results from subsamples tested using the ASTM Method C177. The results of laboratory tests indicated that the range of fine material in all cells including controls was 0.1-0.5% fine material by weight. No significant correlation between percent fines and percolation rate was observed. Of note is the fact that the unwashed aggregate, which we believe would have been universally rejected for use for septic systems, performed better than a number of double washed test aggregates including the “clean” control in some cases. These results suggest that subjective means of evaluation of aggregate may be inadequate as a tool to ensure compliance with requirements of the Massachusetts regulation in similar situations. As a substitute for subjective tests performed at the time of aggregate placement, we recommend that a requirement for periodic random sampling of aggregate supply be considered (perhaps twice per year). These results suggest that up to 0.5% fine material, when the receiving soil is a predominantly sand, can be tolerated with little impact to system performance. The reader is cautioned to consider that the present study was performed using loamy sand as a soil interface and a limited (0.1-0.5% fines by weight) quality of aggregate. There are no data to suggest that these results extrapolate to finer-grained base soils or higher percentages of fine material. Literature reviewed revealed little understanding of the quantitative effects of fines in coarse aggregate used for soil absorption systems. While some authors comment and speculate on the impact of fines during initial placement of the soil absorption system, none comment on the possible attenuating effect the biological component of wastewater disposal on the entrainment of fines to the soil interface. Conversely, some allied literature reviewed suggests that wastewater biota could serve to stabilize and hold fines in place on the coarse aggregate. Additional research is necessary in order to extrapolate these results to finer-grained receiving soil and different hardness coarse aggregate.