03-23-1992 Council Packet - Laserfiche WebLink

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m- V' L_l <M> racaj also depend uponChan 5 minutes pei: inch. The allowable percolation , . uj the slope of the original ground surface. A table 't the^e re.atxonships is presented on pagr E-8. It should be noted that mound construction be gins with the 12-inc . layer of clean sand upon whicn the roc.< is place-. The design material presented in section E of this Manual suggejrs a possible "cookbook” approach and is intenued to deal pr'.mariiy witn mounds or "berms" for single family residences, or daily sewage flow rates of no more than 1,200 gallons. A flow of 1,200 gallons per day can be treated with a rock bed 10 feet wide by 100 feet xong in a propeny construe.e- mound or "berm." However, the proper hydraulic operation of a mound depends upon lateral a.s v;el? as vertical seepage. Wiile there is little douot that rock beds wider chan 10 feet will operate satisfactorily on some soils as far as flow hydraulics is concerned, a ca "ul analysis must be made of the ground slope and soil permeability underlying Clio clean sand layer Ot t..e mound. A vertical separation of at least 3 feet is required cetween the hot.cm of Che rock bed and any restricting layer in order to maintain aerobic condi tions in Che clean sand under the rock layer. (Mien consolidated impermeable_ bedrock is present the vertical separation distance is 7 feet.) Mien aerobic conditions exist in the clean sand, the long-term acc<_ycance rate will be 1.- gallons per day per square foot. If Che depth to the restriccing layer is inadequate or the rock bed is too wide, anaerobic conditions may exist and cause a much slower acceptance race. To evaluate Che possitLlity or anaerobi conditions and the subsequent hydraulic failure is Che ma]cr design problem when sizing mounds larger chan those required for single :ami_y resicen-es. Thus, the design criteria of section E cannot be simply multiplied by a scale factor and expected to properly treat larger flows. The hydrau.iv,s of ..aceral and vertical movement in the clean sand layer and the soil uruer the elevated rock bed must be carefully analyzed to ascertain that anaerobic condici'jns will not exist. Thus, both lateral and horizontal permeabilities of the.under lying soil layers must be utilized to analyze the flow regime to estimate the height of the saturated zone. Where heavy clay soils with slow permeabilities and high seasonal saturated conditions generally exist over an area, ic is far better to utilize mounds for one or two single family residences Chan to collect the effluent from many residences than attempt to dispose and treat ic at a single location. The flow hydraulics in clay soils will require ecther large depths of fill, or underdrainage, or beth, in order to design a proper sewage treatment sy -.em to prevent anaerobic conditions under the rock layer. As an example, a mout.-J designed to treat 450 gallons per day may function very well under certain clay soil conditions, while a single mound serving 5 or 10 residences will fail hydraulically if constructed according to the same vertical separation specifications. Proper construction practices for mounds are extremely important but^when carefully followed will produce a sewage treatment system that will .unction effectively on a long-term basis. There are an estimated 5.000 single family mounds successfully treating sewage in Minnesota. Many Minnesota have found that properly designed and constructed mounds or^ berms ^ effective method of sewage treatment and accept taem as a s.ar.uard s, s