A 37 m high cut slope excavated in a highly weathered, poor quality rock mass on the flanks of a large hillside in Hong Kong has been showing signs of progressive failure since 1983. This paper describes the results of the extensive investigations and discusses the possible mode and mechanism of this progressive failure as related to the complex geology and groundwater conditions.
Une pente taillee haute de 37 mètres et creusee dans une masse rocheuse de mauvaise qualite et extrêmement desagregçe sur les côtes d'un grand coteau à Hong Kong faillit progressivement depuis 1983 à la suite d'une precipitation importante. Cet expose decrit les resultats des etudes approfondies enterprises et discute le mode et le mecanisme possibles de cette faillite progressive en ce qui se rapporte à la geologie complexe et les conditions d'eaux de fond.
Ein 37 M. hoher, abgegrabener Hang, ausgegraben in einer stark verwitterten, schlechter Qualitaet, Gesteinmasse, auf den Seiten eines grossen Huegels in Hong Kong, hat seit dem Jahre 1983 Symptome eines progressiven Verfalls gezeight. Dieser Aufsatz beschreibt die Ergebenisse ausfuehrlicher untersuchungen und bespricht die moegliche Art und Weise und den Mechanismus dieses progressiven Verfalls mit Beziehung auf die komplizierte Geologie und Grundwasser-Verhaeltnisse.
A large number of steep cut and fill slopes formed in the urban areas of Hong Kong directly overlook major roads or multistorey residential, commercial or community developments. Tropical cyclones and low pressure troughs bring short intense rainstorms and longer periods of heavy rainfall to Hong Kong. The effect of such rainfall is to cause a large number of landslides in the steep hilly terrain, which is extensively covered with a thick mantle of colluvium and deeply weathered rock. The failures are generally sudden in both the weathered mantle and the underlying rock and these are usually related to rapid changes in pore pressure and the consequent reduction in shear strength. Some slow moving and progressive failures have been reported in weathered rocks from Hong Kong and elsewhere, (Wolle 1985, Sassa 1984) but these are not common in Hong Kong. Only a few of the failures in weathered rocks have been monitored closely thus precluding detailed studies of failure mechanisms and critical examination of the methods of analysis used which traditionally treated them either as conventional soil mechanics or rock mechanics problems (Powell & Irfan 1986). Brand (1984) stressed that because of the nature of weathered rock profiles and colluvium, stability assessments in these materials must necessarily combine the classical analysis of soil mechanics and rock mechanics with a knowledge of the engineering geology of each situation, and with sound engineering judgement and experience. However, there are situations where the complexity of the site geology confounds conventional analysis (Hencher et al 1984). A 37 m high cut slope in a highly weathered, poor quality rock mass underlying a colluvial mantle at Tin Wan Hill, Hong Kong, has been showing signs of progressive failure since 1983. Sudden failures in the colluvial mantle have also occurred. The slope was excavated in 1963 on the southwestern flank of a large hill (Mt Kellett) in connection with the development of a housing estate (Figure 1). The cut slope originally had three berms with interberm angles of 45° to 50°, giving an overall slope angle of 34°. The detailed investigation and monitoring undertaken by the Geotechnical Control Office (GCO) to design remedial works for the failed slope, which is threatening an urban road, has provided insights into the mode and mechanism of such failures. This paper describes the history of the failure and the extensive investigation undertaken, and discusses the possible mode and mechanism of this progressive failure as related to the complex geology and groundwater conditions in a poor quality rock mass.
An examination of aerial photographs and the few available records indicate that the slope has been subject to continuous instability since it was cut in 1963, and major remedial works in the form of regrading, surface protection (sealing) and subsurface drains were carried out between 1968 and 1982. Detailed study by the GCO commenced in early October 1983 when localised instability in the form of some minor rock falls and displacement of a surface drainage channel by 50 to 100 mm at the toe of the slope were observed. These movements indicated that the toe of the slope was being overstressed, which subsequently led to a larger scale shearing in mid-October 1983 after a heavy rainfall.
