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HomeMy WebLinkAbout8-1 Geology, soils and slope 2019-09-24 1 Geology, Soils and Slope This section is a response to the requirements of the City of Bozeman Unified Development Ordinance Section 38.220.060 relative to Geology - Soils - Slopes. Geology Hazards / Unusual Features Alluvial deposits derived from the mountains south of Bozeman underlie the Bozeman Gateway Subdivision. The geology does not offer geologic hazards or other unusual features that limit the suitability of the land for the proposed uses. The project area generally slopes from south to north at approximately 1.25 percent. The site is essentially flat from east to west with mild variations adjacent to streams that run from the south to the north. Ground water issues are addressed in the section titled Groundwater. The site located in Seismic Zone 3, is a seismically active region. The risk of earthquake damage, however, is no greater than in the remainder of the Gallatin Valley. This risk will be minimized by adherence to appropriate building standards. Soils According to NRCS information, two soil types exist on the site. The map boundaries and distribution of soil types within the boundary of the development are displayed on the Bozeman Gateway Subdivision Soils Map included in this section. This exhibit is derived from the NRCS Gallatin County Soil Survey Map. Detailed NRCS data is provided at the end of this section. Soils underlying the Bozeman Gateway Subdivision are similar to those found throughout Bozeman. They are typically dark colored loams underlain by gravel. Limitations to development are primarily depth to groundwater, bearing capacity, frost heave potential, shrink- swell potential and flooding. The limitations posed by these soils are well known in the Bozeman area. Engineering and common construction practices have proven that the limitations can be overcome successfully without extreme or undo expense. Area street and utility construction demonstrate that ground water issues are very manageable. Cuts and Fills Owing to the flat terrain, cuts and fills in excess of three feet are limited to areas where over- excavation is called for in road, drainage and sanitary sewer construction, or to improve the natural drainage of the site. To minimize erosion, the construction contract shall provide for seeding of slopes in the cut and fill areas. Material for site grading will generally be obtained on site and all disturbed areas will be seeded. Mulching or placement of erosion control mats will be used as necessary. Methods of erosion control and revegetation shall be in conformance to the Montana Department of Environmental Quality (MDEQ) Montana Sediment and Erosion Control Manual. An MPDES Stormwater Erosion Control Plan using Best Management Practices (BMP’s) will be prepared for all construction activities for review and approval by the MDEQ. Typical Best 2 Management Practices for disturbed areas typically include: ground cover and topsoil replacement, mulching, hydromulch seeding and hand seeding native grass species, sediment trapping fabric fences, protection of culvert outlets, and temporary straw bale swale protection. The construction contract will include erosion control measures to address erosion during construction. Included as pay items in the contract will be financial incentives for the contractor to comply with the Stormwater Master Plan. Payment to the contractor will be contingent on the completion of the measures prior to the contractor starting other work on the project that will disturb soil. Roads The designed section of the internal road system within the Bozeman Gateway Subdivision was based upon the attached NTL Geotechnical Investigative Report for the area. See section 3.2 Alternate 3, which recommends a section of 3” asphalt, 6” base, and 9” subbase. The section for the Bozeman Gateway internal roads will have a section of 3” grade “B” asphaltic concrete, 6” of 1-1/2” minus type “A” crushed base course, and 18” of 6” minus pit-run. (See attached road section details)