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Grand Island Resources,LLC Page 17 <br /> Idaho Tunnel Portal—Slope Stability Analysis <br /> 5.0 CONCLUSIONS <br /> For geotechnical stability of the Country Road a required minimum FoS is not defined by the current <br /> Boulder County Multimodal Transportation Standards'. The Colorado Department of Transportation <br /> (CDOT) Geotechnical Design Manuals requires a minimum FoS of 1.1 during construction and 1.3 under <br /> long term static loading conditions for embankment and cut slopes except where failure or significant <br /> deformation will affect bridges or critical facilities. Design for seismic loading conditions is not required <br /> by CDOT for non-critical slopes. <br /> The results of the analyses and discussion presented herein are sufficient to demonstrate that the slope <br /> meets the CDOT stability criteria for the construction case in its current condition. With a calculated <br /> minimum FoS of 1.36 for the slope under drained conditions, the 2D analyses also demonstrate that the <br /> slope will meet the CDOT criteria for long-term static loading conditions provided that adequate drainage <br /> can be maintained. <br /> The presence of seepage during spring thaw indicates that some saturation of the slope may occur during <br /> seasonally high water levels. This can have a significant deleterious effect on stability of the slope. <br /> Parametric analyses suggest that should the surficial colluvium and regolith soil materials become <br /> saturated over approximately half their thickness then instability could occur. However, seepage from <br /> the slope does not appear uniformly which suggests isolated areas of seepage flow through discrete <br /> pathways typical in mountainous terrain, rather than complete saturation. The shotcrete facing should <br /> be provided with weep holes to prevent the buildup of water pressure in the slope behind the shotcrete. <br /> The Idaho Tunnel also serves as a drain to some extent to limit water pressures in the slope. The collapse <br /> void will be mitigated by backfilling it with pervious cellular concrete which will increase this effect. <br /> Stability of the slope could be further enhanced with horizontal drains if necessary. <br /> The cellular concrete void fill is much stronger than the colluvial material it replaces and increases the FoS <br /> significantly when included in the analyses by imparting a buttressing effect. There are areas of the <br /> excavated slope on either side of the portal which would have none of this material in section. However, <br /> the actual FoS on these sections should be higher than 2D analyses results due to 3D effects related to <br /> the concaved slope orientation and adjacent areas which are buttressed by the cellular concrete void fill. <br /> Spillings installed through the cellular void fill and similar ground control elements installed when <br /> rehabilitation of the tunnel resumes will serve to underpin the portion of the slope directly above the <br /> Idaho Tunnel and below the County Road. Permanent tunnel lining ground support installed as the tunnel <br /> is rehabilitated will ensure that stability of the tunnel itself does not impact the road. <br /> The slope stability evaluation presented was based largely on observation and professional judgement as <br /> limited engineering data was available. Data deficiencies at this time include: <br /> • As-built configuration of the portal excavation <br /> • Accurate topographic data for the slope and road <br /> • Subsurface geology beneath the slope <br /> 4 https://assets.bouldercountv.org/wp-content/uploads/2017/02/multi-modal-standards.pdf <br /> 5 https://www.codot.gov/business/designsupport/materials-and-geotechnical/programs/geotech/docs/cdot-gdm <br /> Appiied GeoLogic LLC <br />