11-17-2025 - Agenda Packet Planning Commission - Full Packet

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Report of Geotechnical Exploration North Arm Bay Boat Launch; Mound, MN April 10, 2023 AET Report No. P-0016964 AAMEMICAN The swamp deposited soils found at 12 feet in depth in every boring consisted of sapric peat, organic clay, and bog lime. Swamp deposited soils had water contents that ranged from 58% to 232%, and single -digit N-values. 5.3 Groundwater Groundwater was observed in borings B-2 and B-4 at depths of 9 and 10 feet, respectively. This correlates to an elevation of approximately 926'/2 feet based on the surface elevations recorded. Groundwater levels fluctuate due to varying seasonal and annual rainfall and snow melt amounts, as well as other factors. Given the proximity to the lake, the groundwater level is generally expected to be very close to the adjacent lake levels. Minnesota Department of Natural Resources (MnDNR) provides an ordinary high water level of 929.4 feet for the North Arm of Lake Minnetonka. 6.0 Storm Water Management The planned bottom elevation of the infiltration system is planned to be 2 to 4 feet below the 10- foot depth water table. The MPCA highly recommends that infiltration practices not be used in existing fill soils (see Design criteria for infiltration - Minnesota Stormwater Manual (state.mn.us), refer to the Soils subsection) due to the potential variability in these materials. Fill soils were found extending to a depth of about 12 feet at the boring locations. The Minnesota Pollution Control Agency's (MPCA) Minnesota Stormwater Manual provides recommendations for design infiltration rates based on soil type (Design infiltration rates - Minnesota Stormwater Manual (state.mn.us)). Further, organic soils have relatively high variability in infiltration capabilities and are generally not recommended for infiltration practices. The native soil types encountered below the fill per the Unified Soil Classification System are sapric peat and organic clay. Based on the soil types encountered in the borings and the depth of fill, we recommend excavating the fill and organic soils to the desired depth below the water table. Based on the conversation with IMO, we understand the infiltration trench system will be constructed of a base layer of filter fabric with coarse aggerate placed between another layer of filter fabric. A layer of pea gravel will then be placed between the final layer of filter fabric and the backfilling granular material. We also understand drain tile may be implemented as added assurance stormwater runoff will get to the infiltration trenches. Based on our sieve analysis results and visual -manual classification, the corresponding MPCA Design infiltration rate for the limiting soil types, organic clay (USCS classification OL/OH), is Page 3 of 5 iV*A