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Memo to Bill Carter <br />Sanborn Creek - New Pile Preliminary Review <br />page 2 <br />regarding the possible depth of colluvial/alluvial material in the erosional <br />trough beneath the proposed location of the sediment pond. The sediment <br />pond and its concrete wall dam are proposed to occupy a relatively narrow <br />erosional trough cut through the massive Rollins Sandstone outcrop. The <br />Rollins at this location is massive and competent. The applicant proposes <br />to key the concrete wall into the sandstone at each abutment. I have no <br />doubt that this will effectively assist in preventing overturning of the <br />wall under hydrostatic head. My concern relates to the applicant's lack of <br />a determination of the actual depth and character of the colluvial fill <br />occupying the erosional pocket immediately beneath the proposed pond <br />location. <br />During our walking tour of the site last fall, we specifically discussed <br />this concern with the applicant and WESTEC's representatives. The <br />prospective contractor indicated a possibility of being able to access the <br />site with a backhoe from the adjoining railroad right-of-way. OXBOW <br />intended to request permission from the railroad. However, the only <br />subsurface investigation reported within the design analysis is an auger <br />boring in the west yard, several hundred feet from the pond site. My <br />concern relates to the possibility of hydraulic effects which might result <br />from uncontrolled flow beneath the concrete wall. <br />I do not consider this to be a critical flaw in the application. In order to <br />satisfy my concern, it will be necessary for OXBOW to verify the depth to <br />the Rollins Sandstone bedrock beneath the concrete pond wall prior to its <br />installation. If possible, I believe the concrete wall should be founded <br />directly upon the competent sandstone. This would require excavation to <br />the bedrock and the preparation of a keyway trench. f would not be <br />surprised, however, if competent bedrock was several tens of feet beneath <br />the current depositional surface. The high energy environment of these <br />ledge sandstone outcrop valleys can result in the development of <br />surprisingly deep, narrow notches. The location of the railroad <br />embankment immediately downslope may have impounded considerable <br />colluvium during the past several decades. <br />