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-5- <br />• of internal friction occurs at pressures above 100 kilograms per square <br />centimeter (approximately 1500 psij. In order Fo'r this to be a considera- <br />tion, t;;e fill above yranuiar colluviu,;, would have to be more than 1700 <br />feet in height. <br />In our opinion, the strength parameters used far the bedrock are quite <br />conservative. The d4eraQe blow: count sno'.vn rn the Golder' Asscc~ates' <br />'report (2) is i60 blows for 12 incnes of penetration. The *_est data cited <br />eras obtained From The lighter report (22) and svgs based on direct shear test- <br />ino conducted on a Shelby tube sainole. It should be noted that Shei6y <br />tube SdmDlinQ lS n0 rm311y used on SOTt t0 5itff, cohesive 50115 svi th bl O`d <br />counts in the range or 2/12 to 15/12. Our experience indicates that strength <br />upwards of 8,000 psf and angles of internal friction of 20 degrees are <br />common far medium hard ciaystone. Strengths exceeding 50,000 psf are not <br />uncommon for hard ciaystone oedrock. <br />Paragraph o of the letter deals with the assumption that the angle of internal <br />friction of the spoil is equivalent to the observed angle of repose. It <br />should be noted that this is a rather common assumption and has, in fact, <br />been used by other consultants, such as Golder Associates (20, 21 ). There <br />is little doubt that for a granular material, the angle of •repose represents <br />the lo'.ver bound of the true angle or" internal friction (14). The primary question <br />is vrhether or not the spoil can be assumed to be granular. Based. upon our <br />Ob52.'V3ti0n5 and u~052 OT OthcrS (~~> 2I, 22, 1,~5) Sp011 75 m05t'ly composed <br />of coarse materials. In addition, the research reported by Huang (7) states <br />• that mine spoils in KenTUCky behave essentia?ly as a granular material, ble <br />agree that r.;aterials which are removed as strippable overburden should not <br />be considered granular in nature. Hovrever, vrhe.^• overburden becomes so dif- <br />ficult to exca"ate *_hat blasting is required, we believe that the character <br />of the spoil is granular. Observations reported by Golder Associates (2) <br />and a':so observed during the field trip of June ?3, 1979, indicate that tiie <br />fill material is predominantly granular in nature and contains re c:: frann+ents <br />of up to 2 ro 3 feet in size. Orill loos presented in the 41. A. Wainer and <br />Pssociates report (22) indicate The overburden consists essentaiily of sand- <br />stone, shale and siltstone. Laboratory strength tests conducted on over- <br />burden samples indicate unconfined compressive strengths in the ran.ae of 3300 <br />psi to 10,400 psi (23 ). For intact rock strengths in this range, the charac- <br />ter of The blasted spoil vrill be hard, sound fragments or" a granular nature. <br />Although not specifically mentioned in the letter, some thought should be <br />given to weathering and its effer_t upon the strength. 41e believe there will <br />be a zone of ':leathering extending a`ew feet into the spoil.. The depth o. <br />this :reathered material will depend upon °nvironmenta? factors, cover, <br />moisture and time. In areas where we have investigated other spoil piles, <br />we have Found weathered materials uo to 2 feet into the spoil. The weathered <br />u <br />