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Book Chapter

Summary of Six Rocky Mountain Case Histories

By
Charles S. Content
Charles S. Content
U. S. Bureau of Reclamation, Denver, Colorado
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Published:
January 01, 1958

Abstract

CASE I Problem: Canal location along side of alluviated valley cut in sediments of shallow, fresh-water deposition. Before review by competent engineering geologist, bedrock had been falsely determined by hand-auger explorations to be at 1 to 3 feet depth. Specifications were to be prepared indicating this. Geologist, suspicious that auger may have been stopped by float from buried ehannel gravels or lava cap, had test pits excavated at three pertinent points along questionable streteh of proposed canal line. These were carried to grade entirely in alluvial sands, thus changing assumed rock excavation to common.Development of Problem: Stage 1. Valley cut in Miocene sediments Stage 2. Lava flow fills valley burying Tertiary gravels Stage 3. Erosion of soft sediments leaving old valley as lava-capped ridge.

CASE II Problem: Series of attractive dam sites at narrow gorges "bottling" open valleys apparently suitable for water-storage sites. Geologic studies of aerial photos and in field indicated narrow gorges formed only where superimposed stream was cut in rock. Wider portions of valley–i.e., reservoir areas–were located where old and new streams were coincident. Therefore, leakage potential was greatest at topographically best dam sites since permeable gravels of old stream were present beyond one or the other abutment.Development of Problem: Stage 1. Valleys developed Stage 2. Land subsides, valleys fill Stage 3. Rejuvenation and recutting. New valleys formed, at places coincident with old and at others newly established in rock.

CASE III Problem: Tunnel location. From most apparent surface outcrops and topographic expression, tunnel

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Contents

GSA Engineering Geology Case Histories

Engineering Geology Case Histories Number 2

Parker D. Trask
Parker D. Trask
Prepared for the Division on Engineering Geology of The Geological Society of America
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Geological Society of America
Volume
2
ISBN electronic:
9780813759272
Publication date:
January 01, 1958

GeoRef

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