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Dan Hernandez <br />September 10, 2012 <br />Construction Plan and Grout Cutoff <br />Page 2 <br />The construction plan addresses sealing off the alluvial water by constructing a steel- reinforced <br />concrete collar from ground surface down to and embedded into the siltstone /sandstone bedrock. <br />The collar will be embedded to a sufficient depth to provide a cutoff from groundwater entering <br />the shaft opening should alluvial groundwater be encountered. For the remainder of the hole if <br />water - bearing zones are encountered arrangements will be made to seal these zones by means of <br />grouting from the surface. Should groundwater flow be present in discrete zones after the raise <br />boring and slashing are completed, another round of grouting will take place. AAI proposes to <br />line shaft sidewalls with 6 inches of shotcrete. The drilling and grouting plan and grout cutoff <br />proposal as opposed to constructing a concrete liner from top to bottom is what is in question <br />with regard to its effectiveness for sealing off groundwater inflow to the shaft. After talking with <br />industry experts, I conclude that this is an acceptable engineering practice and in fact is more <br />targeted to stopping the groundwater flow as opposed to just controlling the flow. <br />Considering the following, drilling and grouting may be more appropriate than requiring a <br />concrete liner from the top to bottom of the shaft. The operator is committed to preventing <br />groundwater migration into the shaft and they have developed a design that should be as good as <br />or better than a concrete liner. With a concrete liner you have cold joints that are not water tight <br />and if groundwater is present these may leak. With a concrete liner the grout would be typically <br />placed in 20 foot vertical intervals. A cold joint is formed when fresh concrete is poured against <br />partially set or hardened concrete. Another potential problem could be porous areas in the <br />concrete liner. The concrete needs to be vibrated after placement to achieve full consolidation <br />otherwise you can have porous areas. Further as pointed out by AAI you will likely have <br />accumulation of groundwater behind the liner at the contact between the concrete and rock <br />interface, which over time could be problematic. A concrete liner will control groundwater but <br />by targeting the water - bearing zones with vertical and if necessary horizontal drilling and grout <br />injection you can more aggressively stop the groundwater inflow. The long term effectiveness <br />of a concrete liner would be no better than the pre -shaft construction grouting and post -shaft <br />construction grouting plan proposed by AAI. <br />Adequacy Comments <br />The following should be incorporated into your final adequacy letter to the operator regarding <br />the adequacy of the Bates Shaft construction plan. <br />1. It appears that AAI's grouting plan targets secondary porosity (groundwater flowing <br />through fractures) as opposed to primary porosity (groundwater in the pores between <br />grains of the siltstone /sandstone bedrock). It is stated on Page 3 of the AAI letter that the <br />grout mix would infill fractures and other possible void spaces within the water - bearing <br />zone. Specific type of grout is necessary such as acrylamide grout to be effective at <br />sealing off primary porosity, if needed. Please provide more specific information <br />regarding the grouting technique and specifications including the type(s) of grout to be <br />used. <br />