Laserfiche WebLink
3.3 Subsurface Conditions <br />As noted above, the material exposed on the slope above the portal primarily consists of <br />colluvium. These materials consist of dense to very dense angular to subrounded cobbles from 2 <br />to 6 inches in diameter in a sandy silt/clay matrix; occasional boulders up to 18 inches are also <br />present. <br />Test pits excavated on the portal bench indicate fill materials consists of loose to medium <br />dense lean gravelly sandy silt/clay overlying colluvium. Bedrock was not encountered, but <br />based on mapping performed by Dr. Gonzales and surrounding oucrops, it is estimated to be less <br />than 10 feet deep. <br />As discussed above, some colluvial material has been removed directly above the portal <br />during its 2009 reconstruction which, in turn, undermined the slope and created the unstable <br />condition in Figure 3. <br />4.0 Conclusions <br />4.1 Portal Slope Stability <br />On the basis of the geotechnical investigation and analysis, we conclude that the portal <br />repair is feasible. The primary stability concerns include the following: <br />• Loose, poorly or uncompacted fill on the portal bench and on the outboard side of <br />the above Incas Access Road <br />• Steep marginally stable slopes above the portal <br />The potential impacts associated with these concerns can be minimized by incorporating <br />the following recommendations in the portal and Little Deadwood Gulch drainage channel <br />design. <br />5.0 Recommendations <br />5.1 Portal Slope <br />Following removal of fill and debris from the drainage channel, a buttress should be <br />constructed to establish a pad on which portal improvements can be supported and the slope <br />above the portal stabilized. Figure 4 presents recommendations for fill over slope (buttress) <br />construction. The buttress should extend at least 10 feet beyond the lateral extent of the portal, <br />and be keyed into stiff colluvium or rock beneath the fill. The buttress width and keyway depth <br />should be observed by the geotechnical engineer to check that suitable bearing materials are <br />exposed. The slope of the buttress fill should be no steeper than 1.5:1 horizontal:vertical. <br />Materials used as fill should be free of organic material, have a liquid limit and plasticity <br />index less than 30 and 15, respectively, with the largest particle sizes less than six inches. The <br />investigation indicates onsite materials will generally be suitable for use as fill. The fill should <br />Rule 6.5 Geotechnical Stability Technical Revision <br />Chief Portal <br />March 16, 2013 <br />Page 3 <br />