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HomeMy WebLinkAbout19 GroundwaterThe Lakes at Valley West Subdivision – Phases 1 and 2 Preliminary Plat Application Groundwater A geotechnical investigation was complete for the project area and the findings of the report, dated October 17th, are included in Section 20. Groundwater was addressed as part of this geotechnical investigation and findings are available in this report. Groundwater depths across The Lakes at Valley West site are well documented by the original groundwater monitoring program, ongoing construction activities associated with the gravel pit and pond construction, and test pits excavated during the late summer of 2014. In one form or another groundwater has been continually monitored over the past decade. The overall conditions appear to be very similar to the portion of Valley West that is previously developed adding the construction of several hundred homes to list of construction experience is dealing with these conditions. Groundwater present in the area does not limit the residential development planned for the site. In general, groundwater depth varies across the site but appears to equalize with the elevation of the ponds on the downstream or north side of the property. The ponds are naturally fed by groundwater. The level of the pond can be controlled by a discharge structure in the northwest corner. The discharge structure is used to equalize the water levels in the pond when necessary. Over the years the pond has equalized to the current water level and has fluctuated very little since 2010. That was the last time the water levels in the pond were adjusted. Another consistent throughout the site is a layer of native gravels that tends vary from between 3’ and 5’ below the natural finished grade elevation. The finished grade of the site has been modified over the years with the construction of the ponds and associated movement of materials. In areas not influenced by the pond there seems to be a direct correlation between groundwater depth and the depth to gravel. This is a reoccurring condition throughout Valley West Subdivision and very similar to the groundwater conditions in the portions of Valley West that are already developed. During the excavation of test pits in the late summer of 2014 groundwater depths varied from 2’ to 10’ below existing natural grade. This correlates with the groundwater conditions encountered in the existing developed area of Valley West. A difference is that the soil conditions in this area of Valley West appear to be much more conducive to the construction of foundations. During high groundwater months the depth to groundwater upstream of the ponds appears to follow the boundary between the underlying gravels and overburden of soils. It appears that the ponds The Lakes at Valley West Subdivision – Phases 1 and 2 Preliminary Plat Application constructed on the site tend to improve groundwater conditions downstream (to the north) by equalizing the depth to groundwater with the elevation of ponds. They have little or no effect on the groundwater conditions upstream (to the south). The groundwater information obtained from the test pits, monitoring program, and construction activities corresponds to the gravel and groundwater depths identified by soils information available through the NRCS. Limitations posed by groundwater are common throughout Bozeman and the Gallatin Valley. The groundwater conditions at The Lakes at Valley West are very similar to the conditions seen and successfully mitigated throughout the portions of Valley West that are previously developed. Simple engineering and construction practices can successfully mitigate these limitations without extreme or undo expense. These conditions will limit the use of basements in the area. As one mitigation measure, where needed, a note can be placed on the final plat to advise future owners of the groundwater depths and limitations to the use of basements. A geotechnical report will be required for each foundation constructed in the project, as is the requirement in the existing Valley West subdivision to establish the footing elevations and finished floor elevations of each home to be constructed. Also, as recommended by the geotechnical report provided with this application, it will be recommended that at least 2-feet of separation be provided between the bottom of footing and groundwater elevation for 16”- or 18”-inch wide footings. The above write-up, as well as the attached information following this write-up, was submitted to the Gallatin County Department of Environmental Health. Please review this correspondence and information as well.