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, <br /> ' l s r ����'/SC��=�i <br /> ADVANCE SURVEYING & ENGINEERING C0 . <br /> 5811 Cedar Lake Road, Minneapolis, MN 55416 Phone 541 0500 <br /> December 31, 1987 f�`� � <br /> JAN - 41988 <br /> Mr. Michael P. Gaffron <br /> Assistant Planning & Zoning Administrator <br /> City of Orono . <br /> Post Office Box 66 <br /> Crystal Bay, Minnesota 55323 <br /> Dear Mike: <br /> We received your letter dated December 23 , 1987 today. We <br /> would like to clear up some misunderstandings that the <br /> letter seems to indicate. We enumerate our responses to <br /> correspond to the numbering in your letter: <br /> l. We are aware that the longitudinal axis of a <br /> typical mound system should be aligned with the contour <br /> lines on site and that is our normal practice. The <br /> underlying reason for this provision of the code is <br /> that this practice gives a maximum amount of ground <br /> surface under the mound which is in the path of <br /> downward percolating effluent (lets call that surface <br /> the "wetable interface") . The effluent from the rock <br /> bed percolates straight down from the rock bed until it <br /> reaches the sand topsoil interface and then if the <br /> underlying soil is less permeable than the sand, the <br /> effluent will have to spread out to some extent to be <br /> absorbed. Each drop of effluent which cannot be <br /> absorbed directly under the rock beds will proceed in a <br /> downhill direction (at right angles to the underlying <br /> contour lines) until the area wetted is sufficient to <br /> absorb all the effluent. One goal of a mound design is <br /> to provide the maximum "wetable interface" since if the <br /> wetted area required to absorb the effluent exceeds �he <br /> "wetable interface" provided then hydraulic failure <br /> will occur at the toe of the mound. <br /> In the rather unusual case of this double mound, the <br /> goal of providing the maximum "wetable interface" is <br /> best met by aligning the diagonal rather than the <br /> longitudinal axis of the mound with the contour lines. <br /> We enclose a diagram illustrating this relationship. <br /> This was our motivation for the proposed alignment and <br /> the fact that this better fits the boundary lines with <br /> this alignment is a fortunate coincidence. <br />