Laserfiche WebLink
Disclaimer <br />Statements and views presented in this paper are strictly those of the author, and do not necessarily <br />reflect positions held by their affiliations, the Highway Geology Symposium (HGS), or others <br />acknowledged above. The mention of trade names for commercial products does not imply the approval <br />or endorsement by HGS. <br />Copyright Notice <br />Copyright © 2012 Highway Geology Symposium (HGS) <br />All Rights Reserved. Printed in the United States of America. No part of this publication may be <br />reproduced or copied in any form or by any means — graphic, electronic, or mechanical, including <br />photocopying, taping, or information storage and retrieval systems — without prior written permission of <br />the HGS. This excludes the original authors. <br />ABSTRACT <br />In the arid western United States it is not uncommon to observe slopes in weakly cemented <br />alluvial deposits that stand at ' /2:1 (63 degrees) or steeper. These deposits are often conglomeratic in <br />nature with clasts ranging in size from cobles to boulders enclosed in a matrix of finer material. <br />Estimating accurate Mohr- Coulomb shear strengths for design presents a challenge because one cannot <br />obtain a large enough sample to test in a standard shear box. <br />One of the best empirical methods to establish the shear strengths is back analysis of failed slopes <br />coupled with general Hoek -Brown criterion. For back analysis, only one strength parameter can be used, <br />I prefer friction. I have found that a practical method to estimate base friction is to measure the slopes of <br />the small conical colluvial talus and scree fans of loose debris that have eroded from the slope face that <br />are just at equilibrium and at the static angle of repose. The angle of repose represents the minimum angle <br />of internal friction for these materials with cohesion at zero. I then employ a computer program such as <br />SLIDE Version 6 by RocScience , to estimate the cohesion value assuming the slopes are in equilibrium <br />and at a factor -of- safety of 1.0. The resultant shear strengths are compared to the Mohr- Coulomb <br />equivalent strengths developed using generalized Hoek -Brown criterion for the rock masses and charts <br />developed by Hoek and Bray (1981) on back analyses of failed slopes of similar material. The results <br />show that 34 degrees friction and 500 psf cohesive strength are conservative and therefore may be used <br />for design of weakly cemented cut slopes in arid regions. <br />