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• 2.05.3(4) Ponds, Impoundments and Diversions 24. Last fall the Division prepared preliminary comments for Oxbow, based upon an informal examination of the applicant's DRAFT design for the proposed coal refuse pile. Within Dr. Pendleton's preliminary adequary comments, a concern was mentioned regarding the possible depth of colluvial/alluvial material in the erosional trough beneath the proposed location of the sediment pond. The sediment pond and its concrete wall dam are proposed to occupy a relatively narrow erosional trough cur through the massive Rollins Sandstone outnop. The Rollins at this location is massive and competent. The applicant proposed to key the concrete wall into the sandstone at each abutment. There is no doubt that this will effectively assist in preventing overturning of the wall under hydrostatic head. However, our concern relates to the applicant's continued lack of a determination of the actual depth and character of the colluvial fill occupying the erosional pocket immediately beneath the proposed pond location. During our walking tour of the site last fall, we specifically discussed this concern with the applicant and WESTEC's representatives. The prospective contractor indicated a possibility of being able to access the site with a backhoe from the adjoining railroad right-of--way. OXBOW intended to request permission from the railroad. However, the only subsurface investigation reported within the design analysis is an auger boring in the west yard, several hundred feet from the pond site. Our concern relates to the possibility of hydraulic effects which might result from uncontrolled flow beneath the concrete wall. WESTEC Drawing N95255-04 an Balaz fr Associates Drawing NOXWVF04, which are essentially identical, both depict the connete wall founded on "random fill" at near original grade with no attempt to limit permeability of the • sediment pond subgrade. In order to satisfy our concern, it will be necessary for OXBOW to verify the depth to the Rollins Sandstone bedrock beneath the concrete pond wall prior to its installation. If possible, the concrete wall should be founded directly upon the competent sandstone. This would require excavation to the bedrock and the preparation of keyway trench. It would not be surprising, however, if competent bedrock was several tens of feet beneath the current depositional surface. The high energy environment of these ledge sandstone outnop valleys can result in the development of surprisingly deep, marrow notches. The location of the railroad embankment immediately downslope may have impounded consulerab[e colluvium during the past several decades. If OXBOW can complete the excavation and determination prior to' final application we recommend that they do so. If they cannot, we recommend that the Division address this concern by placing a stipulation upon the approval. That stipulation should require OXBOW complete the excavation and determination of depth to competent bedrock upon mobilizing to the site. Once the determination is completed the plans should be amended to ensure foundation ojthe concrete sediment wall on bedrock. In the event that depth to bedrock is great enough to favor an alternative construction method, OXBOW shall provide an amended plan design to the Division for review. In no case shall OXBOW proceed with construction ojthe connete pond wall until approval is obtained from the Division. Response: A test pit in the proposed embankment foundation area was completed to a maximum depth of approximately 15 feet on 9/16/96. Based on this test excavation, • foundation chazacteristics are described in a 6/3/97 letter from Lincoln Devote provided with these responses for Insertion in Appendix L. The results of this field investigation indicate that bedrock in the foundation area as represented by the Rollins Sandstone is excessively deep rendering it infeasible to found the embankment wall in bedrock. Lincoln DeVore has, therefore, recommended an alternative construction method involving