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MINNESOTA EXTENSION SERVICE UNIVERSITY OF NIINNESOTA Department of Agricultural Engineering � — -,� 201 Agricultural Engineering Building > 1390 Eckles Avenue � St.Paul,Minnesota 55108 i (612)625-9733 � FAX(612)624-3005 August 20, 1992 �� u� � 5 F�:�� Mike Gaffron City of Orono P.O. Box 66 Crystal Bay, N, 55323 1 ��`�,_� Dear Mr. Ga�{fr � This letter is in regard to the inspection and review of the mound system at 2280 French Creek Circle on the morning of August 18, 1992. At the site were a!I person� inv�!�!ec! including the designer, the contractor and the builder. At the inspection we excavated a section of the mound rock bed. In reviewing the section of rock bed that was open, it became apparent that the rock bed was filled with water and the rock itself was covered with black material. As excavation continued, into the rock bed there was a barrier layer of soil particles in the rock. This layer of particles appears to be the cause of failure with the mound system. As was discussed at the site, the problem is not a new problem but had been on-going for approximately 4 years. The critical situation is to identify what caused this layer. There appears to be two potential sources. One is the use of dirty rock and the second is the washing in of soil particles. The use of dirty rock typically would not cause a layer this thick, but dirty rock could facilitate clogging and that clogging would create a washing effect carrying more soil. The other potential problem is the cover material was hay, straw and building paper. At the inspection there was no building paper found, but some hay and stray. Because of lawn watering and other water sources running through the soil above the mound system, the potential to wash clay particles into the rock is quite high. Currently, the use of hay, straw and building paper does meet the requirements of Chapter 7080, but it looked like the hay, straw and building paper did not keep the soil particles out of the mound system. The other qu2stion ruised at t"e site was did tl�e afternati;r2 d2sign of ,he system lea� to the failure. Because of the brown color of the sand that was used to replace the removed damagec� material, it did not appear to be saturated, highlighting it is not a cause to the type of failure that was noted on the site. The design itself, even though it did not meet the standard system requirements of Chapter 7080, would fall under an alternative system design and would be considered engineered to meet the intent of the rule. If there are ot er q�stions, please feel free to call me at (612) 625-6711. r Si� er�ly, 1 1�y ��,; � . Dave Gilsiafson ,_._----�---� Assistant Ex��nsion Specialist DG:rm UNIVERSITY OF MINNESOTA,U.S.DEPARTMENT OF AGRICULTURE,AND MINNESOTA COUNTIES COOPERATING