Show PETERSON fIlES DETAIL REPORT ON DAM PROJECT State Engineer Receives Data Showing Mountain Dell Formation to Be Safe and and Satisfactory Professor William Peterson Petersen who j j Inspected d the Mountain Dell jeser- jeser resel eser- eser olt Oit site at the request of ot State Stata I Engineer Lloyd Lloy-d Garrison submitted tl 1 his report on the tile geological formation formation forma forma- tion of ot the earth in the locality where the dam l is situated and the I proposed addition is given approval in that he ile found conditions such that he believes es bedrock foundation has been reached and that It is safe I and d satisfactory On October 20 20 1924 I visited the I Mountain Dell reservoir located Parleys Parley's can canyon on with the following men Mr Sli Lloyd Garrison state engineer R. R A. A Hart drainage en engineer engineer en- en bureau of roads U. U S. S de department department de de- of agriculture Mr Er Reid Jerman of ot the state engineers engineer's of office of- of fice flee and Mr Burton and Mr Cannon Can non of ot the city engineering depart depart- ment A careful examination of the tile geology a ag as it has Ilas a bearing on on foundation of ot the Mountain Dell dam was made The dam is located in sections 9 9 and 10 10 township 1 south range i-ange 2 2 I east The canyon canon in which the reservoir is located has been largely produced d b by erosion and no major faulting faulting- slipping or folding is ap P apparent parent within the vi of ot the dam FORMATION EXPOSED The formation of ot the dam Is especIally es- es es especially well vell exposed allowing detailed examination both of the north and south sides of the stream and for mo mole e than a thousand fee feet upstream and for several thousand feet downstream m from the dam The rock exposed at the tile damsite damsite dam damsite site is Triassic lassie in the tile geological ta tabia ta- ta ble bia and is interpreted as being lo located 10 near noor the upper portion of ot limestone The limestone i.- i. is composed In western areas essentially of ot lime shale both thick and thin with some layers of ot sandstone and The rock formation I in the tIle canyon has hag been disturbed previous to th the erosion so that tile the formation in th the vicinity of the dam damIs Is tilted and truncated The Wasatch Wasatch Wa- Wa satch conglomerate lies les ably Q-n Q top of the truncated strati strath but the tile contact Is 13 ls at a higher elevation than even the tho high line of the newly proposed dan dam The rock in the Immediate vicinity of the tile dam on Oil the north northside northside northside side was noted to dip to 46 degrees to the east and strike north 70 de degrees degrees de- de grees west The Tile lodge hedge dge exposed is made up of thin bands of ot greenish gray shale with some brown above which is more than thirty It Ieee of ot dark gray shale con con- taming of ot sandstone and ar argillaceous an- an lime Very Verv careful examination was wag made ot of the scales to determine whether or 01 not the they contained any material water-soluble water or whether the showed any fracturing due to movement NO FAULT DISCLOSED Two hundred feet north Of df the north end of ot the dam the shale material s 's somo some small local folding but no faulting nd no open crevices or soluble material Examination was made of the I south side exposure in the spillWay spillway spill spill- way of the tile rock exposed in the tho I third fourth fifth sixth eighth I ninth and tenth arch rings countIng count count- lug Ing from the tIle south and careful readings were made on Cad each to toI I determine the nature of the r i cic cicI and its position The Tile rock uniformly uni on- I dips 46 degrees to the east and strikes north from 66 to 70 de degrees degrees degrees de- de grees west depending on the posItion position tion Lion in the slight local folding The exposure in the tile spillway shows ya two la layers ers of of argillaceous sandstone sandstone sand sand- stone each about one foot Jn irs thickness which makes It rather rathel to trace the forma formation tion easy en entirely across the canyon The rhe position of ot these was checked from tho spillway to the dug road north of ot the tile dam The These e layers definitely n tely extend throughout the tho area area In question in sequential position showing showIng- that the cross section of or orthe the dam has suffered no disturbance disturbance disturbance dis dis- dis- dis see blueprint for cross crosa section Logs Loss of oC tour four our holes were tur fur fur- These holes had been beeD Continued I on page 4 l. 4 PETERSON PETERSON FILES DETAIL REPORT ON DAM PROJECT Continued from from- page 1 bored for a report of the earlier structure Each of these shows about twenty five feet of Incoherent Once Inco herent debris made up of of- gravel graveland anti and clay lying on top of the bed sock rock The logs show the buttresses buttresses but but- tresses of the present dam clam to bo be extending from three and a half to five reel feet into the solid bed rock MINIMIZES DANGER darri- darri The material of which the site is composed Is the very best from th the standpoint and holding water There Is no danger of cracks and crevices de developing jie- jie which woul Increase with leakage a condition which sometimes sometimes some some- times occurs in limestone The positioning of the dam Is unique unique that irI the the- structure structure ture is so positioned as to follow almost exactly the strike line of the formation and tile the arch ring slope agrees almost identically with the dip of the strata The projection pro pro- of the rock underground Is isto to the eastward and the strata slightly flattens as It goes farther east This Inclination of the strata develops a natural structure res reservoir not only with the projecting rock at the damsite but the same strata extends underneath the theres reservoir res assuring the basin against leaks leals Shale material weathers very slowly unless exposed directly to or abrasion Its crushing strength is many times that pressure pressure pres pres- sure that will be necessary to support support sup sup- port the proposed structure Excavations should be made madewell madewell well into the shale for the ex extension cx- cx tension of the buttresses The solid rock should be carefully before the concrete i is poured to see tolt to-it that no weathered weathered ered or partially disintegrated material material ma ma- is left as a foundation for forthe forthe the buttresses As soon as the Concrete is set the excavation should be filled back to the level of the creek bed RECOMMENDS CONCRETE I In the construction construction- of the proposed proposed pro pro- posed new spillway at the north northend end eM the excavation should be concreted concreted con con- for a distance sufficiently far so that the darn dam structure will willbe willbe be rendered entirely safe from the erosion which might develop the water through the spillway It would be safer to carry the concrete west vest of the small cottage on the hill and then safely land it back into the river without allowing allow ing it to tear a. a great hole or 1 gorge Into the side of the hill It ItIs Itis 1 is much better to make this thi safe In tile the beginning than to make repairs later 1 In conclusion it is my I that the rock material for the foundation is entirely safe and satisfactory Th The position of the structure Is such that the inclination tion of the rock beds add strength to the proposed dam If proper precaution Is maintained in the tho building the dam should maintain with absolute safety |