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Grand Island Resources, LLC Page 17 <br />Idaho Tunnel Portal – Slope Stability Analysis <br />Applied GeoLogic LLC 5/7/2020 <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 Standards4. The Colorado Department of Transportation <br />(CDOT) Geotechnical Design Manual5 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.bouldercounty.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