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HomeMy WebLinkAbout1981-11-27 Subdivision Soil Analysis ReportEXHIBIT November 27, 1981 P.O. Box 209 Wayzata, Minnesota 55391 Mr. Judd Ringer 405 South Willow Drive i Long Lake, Minnesota 55356 i Dear Mr. Ringer: We have completed a site evaluation for on -site sewage treatment facilities of 15 proposed lots located Fast of Willow Road and North of Trunk Highway No. 12 in Orono, Minnesota. A location map has been enclosed showing the location of the soil borings and Percolation tests. Soil boring, logs and i percolation test data is also enclosed. The soil boring show a uniformly high water table throughout the area tested. This condition prohibits the installation of conventional soil absorption systems. The gentle slopes and percolation rates encountered indicate that Mound Systems should be satisfactory on these sites. We recommend that the future homes be proviued with Mound Systems of soil absorption. Please call if you have any questions. T%.an' Yours very truly, CARL V. BORG, P.E. r� _ CWB:vl cc: Mr. Michael Gaffron City of Orono Enc. 3 copies - Location Map Sheets 1 - 3 - Soil Borings 1 Sheet Percolation Test Data C ilt ) 5 ilia MA �ulAnd Ow %mm &# AM) 4 .6 ON 5 DICKEYS LAKLE -two dFs Lo %L ILA Sam Alm wo." &mi* 146 oftk 5W**fwa V MOO 3 f k Xl oil A. XLJ& r Unu rm ITI rNT A TA COMPILATION BY LOT Lot 1, Block 1 Lot 2,. Block i Lot 3, Block 1 Lot 4, Block 1 Lot 5, Block 1 Let 6, Block 1 Lot 7, Block 1 Primary Secondary Primary Secondary Primary Secondary Primary Secondary Pr imary Secondary Primary Secondary Primary Secondary Tverage pert. # of rate _ holes 3 mpi. 4 52 mpi 2 9 mpi* 3 59 mpi 2 18 mpi* 3 11 mpi 2 12 mpi 4 28 mpi 2 14 mpi 4 148 mpi 2 39 mri* 3 20 mpi* 1 87 mpi* 3 135 mpi 2 Average Depth to slope mottling u-3 ;a 2.5 ft. 0-3 % 1.7 ft. 0-3 36 2.2 ft. 0-2 % 2.0 ft. 4-6 % 1.5 ft. 8-10 % 1.5 ft. 6-6 0 2.8 ft. 5-7 k 2.0 ft. 6 % 2.7 ft. 2-3 % 2.0 ft. 2-4 % 2.4 ft. 2 % 2.5 ft. 2-3 % 2.5 ft. 2 % 2.5 ft. Recommended Svstem Mound Mound * * Mound Mound Mound * * Mound Mound Mound Mound Mound / 1 Mound Mound Mound Mound /1 Lot 1, Block 2 Primary 73 mpi* 3 0-2 % 2.7 ft. Mound Secondary (a) 54 mpi* 1 0-2 % 2.2 ft. Moun3 Secondary (b) 84 mpi 2 0-2 % --- Mound Lot 2, Block 2 Primary 63 mpi 4 0-2 % 2.3 ft. Mound Secondary 84 mpi* 1 0-2 % 2.0 ft. Mound * Average perc rate is based on one additional test hole which did not percolate fast enough to be measurable, hence average may be somewhat higher. ** Mound system should have at least 24" sand base. Due to high water tables, all other mounds noted on this page should have at least 18" sand base. /1 Average percolation rate is higher than normally allowed for a mound system by code. Data compilation by lot (cont'd.) Lot 3, Block 2 Lot 4, Block 2 Lot 5, Block 2 Lot 6, Block 2 Lot 1, Block 3 Lot 2, Block 3 Pr ima z y Secondary Primary Secondary Primary Secondary Primary Secondary Future Pature Average perc. # of rate holes_ 60 mpi* 3 61 mpi 2 2c mpi 4 136 mpi* 1 107 mpi* 3 124 mpi* 1 90 mpi*/2 2 34 mpi 2 21 mpi 4 56 mpi 4 Average Depth to slope mottling 2-5 % 3.5 it*** 2 % 2.0 ft. 0-2 % 2.0 ft. 0-2 % 2.3 ft. 0-2 % 1.2 ft. 0-2 % 2.0 ft. 0-2 % 1.5 f.t. 0-2 % 2.5 ft. 6-8 % 5.0 ft./3 6-8 % 3.5 ft System ro s Recommended M u Mound F Mound rt F Mound Mound /1 r t t Mound ** F < Mound /1 a F c Mound ** Mound Shallow trenches or mound Shallow trenches or mound * Average perc rate is based on one additional test hole which did not percolate fast enough to be measurable, hence average may be somewhat higher. ** !mound system should have at least 24" sand base. Due to high water tables, all other mounds noted on this page should have at least 18" sand baste. ** Actual water level at 2.5 ft. /I Average percolation rate is higher than normally allowed for a mound system by code. /2 Tl*o test holes of four were too slow to measure #615 Ringer Subdivision Wage 4 Lot 2, Block 1 Peres PB21 thrL. PB26. moorings BB21 thru BB23. Average perc rates for bDth primary and alternate sites are acceptable. Slopes are nearly level a'_ the tested sites, ranging from 0 to 3%. Mottling indicated below 2', with actual water gable at 3.2 to 4.2'. A standard mound system should be used due to the presence of shallow saturation zones. Lot Block 1 Peres PB31 thru PB26. Borings BB31 thru BB33. Slopes in the primary site are approximately t-8%, on a slight flattening Of the 10-12% slopes surrounding it. Borings in the primary site indicate 1 to 2 ' of loamy topsoil over a 6" layer of organic clay topsoil over a plastic sandy c' r. Actual water table at the primary site was below 5'. ho, er, the high plasticity indicates a heavy clay that is probably saturated. The secondary site was mottled below 1.5 feet with an actual saturated zone at 1' depth. Coupled with a slope of 10% the secondary site is marginal for a mound system and would require at least 2' of sand base as part of the design. I suggest that a standardpressure mound system be required for both sites with added sand depth for the secondary system. Please note that this lot is the most tenuous in the subdivision in terms of slope and soils consideration for using a mound system. Should a proposed house location encroach on either of the tested sites, careful review and possible additional testing will be needed Lo ensure that a mound can still be used. The site does not appear capable of supporting any kind of a trench system. Lot 4, Block 1 Peres PB41 thru PB46. Borings SB41 thru BB43. Slopes average 5% - 7% overall. Borings indicate actual water table at 4' with mottling at a depth of 3-4' for the primary and 2' at the secondary site. Perc rates were fairly uniform throughout these tested sites. A depression/drainageway to the immediate south of the tested sites must be avoided during construction of the systems. I would recommend a standard pressure mound for both sites. Lot 5, Block 1 Peres PB51 thru PB56. Borings BB51 thru BB53. Slopes are 3% to 68 overall. Mottling at the primary site started at 2.7 to 3.5' with actual. water table at 3.7 to 5.0'. This site is acceptable for a standard mound system. The alternate site has a perc rate considered too slow for any system by code. However, close scrutiny shows that the peres were taken at depth in the sandy clay subsoil, not in the topsoil. In general, throughout the entire subdivision the test'J.ng done at a depth of 12", in the topsoil, indicates acceptable percolation rates, with testing at 24" or more indicating slower, marginal perc rates. In my opinion, this site is no different than most of the others, and would have an acceptable perc rate if tested at the 12" depth in the sandy loam topsoil. No furt.her'testing should be required; a mound system is recommended. #615 Ringer Subdivision Page 5 Lot 6, Block 1 Peres PB61 thru PB66. Borings BB61 thru BB63. Average perc rates at both sites are acceptable. Slopes range from 2 to 4%. Mottling started at a depth of approximately 2.4 feet, with actual water level at 2.5 to 4.4'. A standard mound system should be used at both sites. Lot 7, Block 1 Peres PB71 thru PB76. Borings BB71 thru BB7?. Slopes average 2-3%. Perc rates were fairly slow, with the secondary site being somewhat slower than the limit (however, the justification for Lot 5, Block 1 also applies here). Actual water level ranged from 1.7 to 3.5' with mottling generally below 2.5'. A standard mound is recommended for both sites. Lot 1, Block 2 Peres PA11 thru PA18. Borings BAll thru BA13. Slopes are very flat. Mottling was indicated below 2.2 ' with actual water encountered below 2.0'. Two secondary sites were tested. All sites are acceptable for a standard mound system. Lot 2, Block 2 Peres PA21 thru PA26. Borings BA21 thru BA23. Very flat slopes. Mottling occurs at 2.0 to 2.4' with actual water encountered below 2.41. Perc rates on both sites are acceptable. A mound system is recommended for both sites. Lot 3, Block 2 Peres PA31 thru PA36. Borings BA31 thru BA33. Slopes averaged 2 to 5%. Mottling starts at a depth of 2.0-3.5'. Actual water level was below 2.5'. Perc rates are acceptable. Mound systems are recommended for both sites. Lot 4, Block 2 Peres PA41 thru PA46. Borings BA41 thru BA43. Slopes are very flat. Mottling generally was found below 2.01, with actual water level at about 4.01. Perc rate for primary site is acceptable; perc rate for secondary site is slightly slow but acceptable for a mound based on the justification referred to for Lot 5, Flock 1. I recommend a mound system for both sites. Lot 5, Block 2 Peres PA51 thru PA56. Borings BA51 thru BA53. Slopes are very flat. Mottling occurred below 1.2 feet with actual water below 4.0 feet. Perc rates are acceptable for the primary site and slightly slow at the secondary site. (Refer to Lot 5, Block 1) This lot has the overall worst set of percolation rates of the division. However, only two of the peres were taken in the topsoil, the others, two of which were very slow, were at depth. I recommend that a mound system be used on this site. If peres were re -taken in the topsoil, it is likely they would pass as did nearly all others done in this manner. #615 Ringer Subdivision Page 6 Lot 6, Block 2 Peres PA61 thru PA66. Borings BA61 thru BA63. Slopes are nearly level. Mottling occurred below 1.5', with actual water encountered below 5.7'. (Note that soil appears saturated below 2.5') Perc rates were acceptable, although L of 6 taken at depth very very slow. Both primary and secondary sites should use —ind systems. Lot 1, Block 3 Peres PCll thru PC14. Boring BC11 and BC12. This lot contains an existing house, garage and barn. The existing septic system is unknown and was not located during my routine inspection in July 1981, although no evidence of failure was noted. A future drainfield site has been tested to the south of the house. Average percolation rate is 21 minutes per inch; soil borings indicate soils suited for a shallow trench or mound system, with mottling beginning at a depth of 5'. This is the highest elevated point in the entire subdivision and appears to be the only lot possibly suited for a shallow trench system. An attempt should be made to locate the existing system to ensure it is within the lot boundaries, although there is only a remote possibility that it ..._V Lot 2, Block 3 Peres PC21 thru PC24. Borings BC21 and BC22. This lot contains an existing house and small shed. The existing septic system is unknown and was not located during my inspection last July. No evidence of system failure is noted. While the existing system undoubtedly consists of trenches, soils data provided for a future drainfield site indicates mottling at a 3.5' depth. This indicates that the replacement system, should it -3ver be necessary, would probably have to be a mound. Percolation rates averaged 56 mpi. It would alto be advisable to locate the existing system to ensure it is within the new lot boundaries. B. Discussion of Mound Systems Since the proposed Ringer subdivision appears to need mound systems throughout, it might be worthwhile to review the conditions which favor use of mound systems over conventional trench systems. As you are aware, a biomat forms at the rock -soil interface in a standard trench system. This biomat acts as a "valve" to slow the downward percolation of septic tank effluent to a rate where the flow is unsaturated, i.e. air in the soil. pores is in contact with the percolating effluent. Under these essential conditions, adequate sewage treatment will occur. However, in many soils, either a seasonal saturated condition or extremely fine, impermeable soil texture (or both) tends to eliminate the air/effluent contact and also severely limits the soils capacity to absorb large amounts of effluent. To ensure that a standard trench system is not #615 Ringer Subdivision Page 7 installed in these limiting soils, a et of governing criteria have evolved over -the last 50 years. Specifically, trench systems are not allowed: 1) where the percolation rate is slower than 60 minutes per inch of water level. drop in a percolation test hole; and 2) where the highest known water table or indicated saturated soil layer is less than 3' below the bottom of the trench. In most cases, these same soils have a varying amount of topsoil which is usually "loamier" and will accept and treat a limited amount of effluent. The pressure -mound system is designed specifically to use this loamy topsoil layer for effluent treatment and disposal. In brief, a 12-24" layer of sand (placed over the natural roughened topsoil) is fed effluent from a pressure distribution system in an overlying rock bed. The biomat "valve" forms at the rock/ sand interface which is 12-24" above the natural soil. Because of the pressure distribution, an unsaturated air/effluent contact is maintained as treatment occurs in the sand bed. This treated effluent is then dispersed into the topsoil and allowed to move laterally through the topsoil,possibly even past the boundaries of the mound system (yet still below the surface) until it eventually seeps downward or evaporates over a wide area. It would be poor planning for Orono to allow installation of standard trench drainfields in soils which have high-water table characterisL;.(-s, since many systems likely would eventually fail, as was the case in Medina-Morningside. Since Orono has shown a strict desire to limit the extension of sanitary sewers, it appeals reasonable to allow (and, in fact, promote) the use of alternate on -site sewage treatment methods where development pressures persist. Mound systems have been installed over the last 10 to 15 years around the country as an alternative to trenches. In Orono, pressure distribution mounds were first installed in 1978. To date, the City'has 13 pressure mounds in service, with no recorded failures. In my opinion, under specified site conditions, mound systems are a safe, sanitary, reliable method of sewage treatment.