Geotechnical Manuals for Slope.pdf

GEOTECHNICAL MANUAL FOR SLOPES

GEOTECHNICAL ENGINEERING OFFICE Civil Engineering Department The Government of the Hong Kong Special Administrative Region

GEOTECHNICAL MANUAL FOR SLOPES

GEOTECHNICAL ENGINEERING OFFICE Civil Engineering Department The Government of the Hong Kong Special Administrative Region

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© The Government of the Hong Kong Special Administrative Region First published, November 1979 Reprinted with minor corrections, November 1981 Second Edition, May 1984 First reprint, June 1991 Second reprint, March 1994 Third reprint, June 1997 Fourth reprint, February 2000 Prepared by: Geotechnical Engineering Office, Civil Engineering Department, Civil Engineering Building, 101 Princess Margaret Road, Homantin, Kowloon, Hong Kong.

This publication is available from: Government Publications Centre, Ground Floor, Low Block, Queensway Government Offices, 66 Queensway, Hong Kong. Overseas orders should be placed with: Publications Sales Section, Information Services Department, Room 402, 4th Floor, Murray Building, Garden Road, Central, Hong Kong. Price in Hong Kong: HK$70 Price overseas: US$14.5 (including surface postage) An additional bank charge of HK$50 or US$6.50 is required per cheque made in currencies other than Hong Kong dollars. Cheques, bank drafts or money orders must be made payable to The Government of the Hong Kong Special Administrative Region.

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Page No. TABLES

171

LIST OF TABLES

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TABLES

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FIGURES

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LIST OF FIGURES

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FIGURES

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PLATES

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LIST OF PLATES

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PLATES

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ADDENDUM

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47 excessive. Filter paper side drains should not be used in triaxial tests, because they can lead to errors in strength measurement and are generally unnecessary for the soils of Hong Kong. Membrane corrections must be made in the usual way. Saturation by back pressure can only be obtained by applying a small effective stress to the specimen. Specimens that start with a very low degree of saturation can be difficult to saturate. In these cases, saturation can be carried out by first percolating deaired water under a small hydraulic gradient through the specimen until air stops bubbling from it. A back pressure can then be applied to complete the saturation. The strain rate for drained tests with pore pressure monitoring should be such that the pore water pressure fluctuation is negligible, and in any case the fluctuation should be no greater than 5% of the effective confining pressure. For undrained tests, the rate should be selected so as to allow complete equalisation of pore water pressure throughout the specimen. It is desirable that the strain rate does not exceed 2% per hour. For undrained tests, failure can be defined either as the maximum deviator stress or as the maximum obliquity (σ1'/σ3'). For fully-drained tests, these two criteria coincide.

3.8.2

Interpretation of Results

For ease of interpretation, it is recommended that the results of CU triaxial tests are plotted as p'-q stress paths (Figure 3.1), where p' = (σ1' + σ3')/2 and q = (σ1 - σ3)/2 (Lambe & Whitman, 1969). The shape of a stress path indicates the tendency for a specimen to compress or dilate during shear. The p'-q plots also enable the most sensible strength envelope to be drawn as the boundary to a family of stress paths. For CD tests, the p'-q stress paths are of no significance. Actual volume changes during drained tests should be measured throughout the shear process. Strength envelopes determined from triaxial tests will often not be linear, and they will sometimes exhibit an apparent break-point in the region of a definite ‘critical’ pressure. This is because the stress-strain behaviour of the material is dependent upon the confining pressure under which it is sheared. Specimens that are tested at low confining pressures in the triaxial test tend to dilate during shearing. At high confining pressures, specimens tend to compress. These different stress-strain behaviours are indicated clearly by the different shapes of the respective stress paths (Figure 3.1). In Hong Kong soils, the critical pressure can be considered to be analogous to the maximum past pressure for a sediment. It is important to remember that, where a strength envelope is not linear, the portion of the envelope used for design purposes must be that for the correct design stress range. In the interpretation of triaxial test data, especially in the low stress range, the following sources of error should be borne in mind :