Agenda - Council - 01/13/2015 - Laserfiche WebLink

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More detailed subsurface information is illustrated on the attached boring logs. The attached diagram illustrates the approximate boring locations at the respective project area. Attached are fence diagrams (cross sections) that illustrate the soil types encountered in the soil borings within the alignment of the proposed Center Street. The intent of this fence diagram is to provide a general summary of the site soil conditions encountered at each specific boring location. Use of the fence diagram to determine earthwork quantities or to extrapolate soil conditions between the soil boring locations for any purpose is prohibited. Groundwater and Groundwater Control Our drill crew observed free groundwater during or at the completion of the borings. Groundwater was observed in borings SB-1 and SB-2 at depths ranging from 17 to 19 feet below existing grade, respectively, corresponding to elevations of 867.4 and 870.0 feet. Due to the depth to water levels measured in the soil borings, groundwater is not expected to be a significant issue for the proposed utility and street reconstruction. However, excavations extending into sand layers over clay or silt-based soil may encounter inflows of perched groundwater. These inflows are expected to be effectively controlled by excavating sump pits and removing water from the trenches with pumps. Recommendations and further discussion for groundwater control are described in greater detail in Appendix B should they be required during construction. Utilities The stable non-organic fill and native alluvial soils observed in soil borings were generally suitable for utility support. If encountered, removal of organic soils and unstable clay is recommended if they are encountered below proposed utility inverts during construction. If left in place, organic soils will likely biodegrade and settle with time, thus requiring the utility to support the load of the overlying soil. Excess stress may subsequently cause damage to the pipes if the pipes are supported by compressible or organic soils. The underlying soft alluvial clay in boring SB-2 likely has completed primary consolidation and is not considered to be highly compressible if grades and subsurface conditions do not change. However, additional effort may be required to stabilize the soft soils at the base of the trench during construction in areas with similar conditions. The Geotechnical Engineer of Record or their designated representative should observe the project excavations to determine that unsuitable materials have been properly removed and adequate bearing support is provided by the exposed soils. The exposed soil at the base should be compacted to 95 percent standard Proctor dry density (ASTM D698).Such observations and testing should be performed prior to and during backfilling. The onsite, non-organic soils are anticipated to be suitable for reuse if properly moisture conditioned and compacted. Replacement backfill required in utility trenches should consist of non-organic material similar to the surrounding soil. All import fill should be approved by NTI or the City’s representative. 3