Assessment of stability of nailed soil cut slopes under seismic excitations in Hong Kong

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Assessment of stability of nailed soil cut slopes under seismic excitations in Hong Kong

 

Author: Ieong, Kwok-lun
Title: Assessment of stability of nailed soil cut slopes under seismic excitations in Hong Kong
Degree: M.Sc.
Year: 2000
Subject: Slopes (Soil mechanics) -- China -- Hong Kong
Soil stabilization -- China -- Hong Kong
Earthquake hazard analysis -- China -- Hong Kong
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Dept. of Civil and Structural Engineering
Pages: xi, 124, [76] leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1504166
URI: http://theses.lib.polyu.edu.hk/handle/200/3481
Abstract: This dissertation was initiated by the author to assess the stability of soil-nailed cut slopes under seismic excitations in Hong Kong. Wong and Ho (1998) concluded that the risk of earthquake-induced landslides at slopes designed or upgraded to current geotechnical standards is much smaller than the risk of rain-induced landslides for pre-1978 man-made slopes that have not been upgraded to current standards. However, Wong and Ho (1998)'s conclusion was only based on the preliminary analysis of quantitative risk assessment, without carrying out any stability analyses to soil-nailed cut slopes. Therefore, the performance of soil-nailed cut slopes in Hong Kong under seismic excitations remains uncertain. The purpose of this dissertation was to calculate the factors of safety of typically designed soil-nailed cut slopes subjected to four levels of horizontal peak ground acceleration (0.10g, 0.15g, 0.20g and 0.25g) and the associated vertical peak ground acceleration (assumed to be 40% of horizontal peak ground acceleration), by adopting a two-part wedge failure mode. In the assessment, the contribution of cyclic pull-out resistance of soil nails and seismic bearing capacity of nail heads to maintaining slope stability were examined. As an upper-bound estimate, the author estimated the seismic bearing capacity of soil nail heads by combining the seismic bearing capacity factors derived by Richards et al. (1993) and Vesic (1973)'s static bearing capacity equation. It is demonstrated that bearing failure mode of soil nail heads is not more critical than pull-out mode of failure of soil nails for 2 to 7 rows of prescriptive soil nails. It is found that the stability of low cut slopes (3 m high) reinforced by one row of prescriptive soil nails against pull-out failure and bearing failure of nail heads cannot meet the current geotechnical standards and their stabilities are recommended to be improved. In addition, for soil cut slopes upgraded by prescriptive soil nails with a static factor of safety of 1.4, it was demonstrated that the factor of safety would only be reduced to unity with a minimum horizontal peak ground acceleration of 0.20g. The findings of the assessment also confirm that cut slopes in Hong Kong upgraded by 2 to 7 rows of prescriptive soil nails with a length ratio (nail length/slope height) exceeding 0.65 would have good stability by withstanding earthquakes with horizontal peak ground acceleration of up to 0.25g. Concern should be given to the stability of soil-nailed slopes higher than 21.5 m and steeper than 40 degree, as their calculated factors of sagety under horizontal peak ground acceleration of 0.20g were found to be below unity.

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