HomeMy WebLinkAbout8-1 - Geology, soils and slope 2020-03-03 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
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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)