Landslide Mechanisms in Hong Kong

S. R. Hencher Halcrow China Ltd., School of Earth & Environment, the University of Leeds, UK Dept of Earth Sciences, Hong Kong University

Abstract: This paper reviews the nature and mechanics of landslides in weathered terrain in Hong Kong. The vast majority of landslides occur during intense rainstorms and a relationship is presented that relates severity of landsliding to 24- hour rainfall. Deeper seated landslides are often delayed in that they occur days after intense rainstorms. The reasons for this appear to be chiefly hydrogeological as explained in this paper. A hydrogeological classification of landslide mechanisms is introduced. The conclusions are supported by case examples of numerous slope failures, the study of some of which have been taken to a forensic level.

1 INTRODUCTION

Landsliding is a common hazard in Hong Kong and this hazard has been addressed over many years by the concerted efforts of the Hong Kong Government, their consultants and contractors through a specialized office now known as the Geotechnical Engineering Office. This office vets the designs of all new slopes and is going through a process of upgrading old slopes through the implementation of extensive landslide preventive works, wherever necessary. Over the last few years, it has also begun to tackle, in a managed way, the risks from landslides issuing from the steep and rugged natural terrain that is typical of most of the territory (Figure 1). The early history of the HK Government on dealing with the risk of landslides in Hong Kong is reviewed in Brand (1984). The current situation is described by GEO (2004), Chan et al (2007) and Tang et al (2007).

Fig. 1. a) Natural rock exposure above the Mid Levels District of Hong Kong Island b) typical coastal exposure with failure on sheeting joints

2 GROUND CONDITIONS

Geology Current understanding of the geology of Hong Kong is summarised in two Government publications (Sewell et al, 2000; Fyfe et al, 2000). The major urban areas are underlain by Jurassic and Early Cretaceous volcanic rocks - largely crystal tuffs, intruded by granitic rocks of similar age. The volcanic rocks which in their fresh state can have unconfined compressive strength well in excess of 200 MPa largely make up the higher mountainous areas surrounding Hong Kong harbour and peaks on Lantau Island. The central harbour basin, other low lying areas and more flat lying islands such as Lamma and Cheung Chau comprise granitic rocks which include granite, granodiorite and monzonite. Differences in these petrologies are reflected to a degree in engineering behaviour. In particular the monzonites are linked to ground problems such as those encountered in construction of the Aberdeen tunnel (Cochrane, 1984) and in the Yip Kan Street landslide (Hencher, 1981).

Weathering Hong Kong rocks are often but not universally deeply weathered to depths of up to 100 metres and rarely more. The distribution of weathering is not straightforward. Sometimes hills are made up of thick saprolite (soil-like grades VI to grade IV of GEO, 1988 and BS 5930:1999), elsewhere the hills comprise 100% rock from the ground surface with no weathering profile evident. Valleys are sometimes the location of deep weathering but elsewhere rivers run directly on exposed rock. The distribution of weathering clearly relates only poorly to current geomorphological and groundwater distribution which is why prediction is rather difficult. The most informative description of weathered rocks in Hong Kong remains that of Ruxton & Berry (1957). Current weathering processes and the implications for geotechnical engineering are discussed by Hencher (2006). The terminology for weathering used in this paper is that of BS 5930:1999 and of GeoGuide 3 (GEO, 1988) rather than that adopted recently for Eurocode 7 (see critique in Hencher, 2008). The weathered rocks in Hong Kong range from completely restructured and quite dense residual soils through soils retaining original 3 rock features but dry densities as low as 1.2 Mg/m . Such a value is less than half the density of rock in its unweathered state and indicative of a very porous and open texture due to flushing out of the weathering products in a region of occasional very intense rain storms. At a mass scale the weathering front is sometimes very sharp with a zone of uniform soil-like weathered rock overlying almost fresh rock (Figure 2). Elsewhere the zonal change is more gradual with mixtures of rock and soil occurring between zones that are fully soil or fully rock (Figure 3). Elsewhere again the weathering profile can be completely haphazard (see Figure 4). The difficulties for investigation, characterisation, testing and analysis are obvious. Ruxton & Berry's (1957) interpretation of maturity of weathered terrain is presented in Figure 5 and illustrates some of the types of profile that can be encountered.

Figure 2 Sharp weathering front with grade IV Figure 3 Mixture of core stones (grades II and III)

overlying grade II material (granite, Tai Tam with grade IV along discontinuities (tuff, Victoria

Reservoir) Peak)

Engineering Properties Properties are sometimes difficult to ascertain because sampling of materials without disturbance is almost impossible or because of the heterogeneous nature of mass zones. These problems are addressed partly by Hencher & McNicoll (1995) and by Hencher (2006). Geological structure, fabric and texture survive at least in part right through until the final stage of weathering - residual soil where the soil collapses and becomes reworked, so relict joints and joints that develop during weathering (Hencher, 2006; Hencher & Knipe, 2007) will dominate stability in many slopes. Larger structures such as dykes, sills and faults often result in differential weathering both

Figure 4 Highly heterogeneous profile with single very large core stone. Right hand photo shows detail with very rapid transition from grade V, completely decomposed rock to almost fresh rock within the corestone. in terms of degree and in terms of the weathering product. Mismatches in soil particle grading and texture result in permeability contrasts that control groundwater flow and partitioning as discussed below with respect to landsliding. Flow through the rock mass can cause erosion pipes to form, typically at discontinuity intersections but also commonly as part of the overall degradation and progressive failure of slopes (Nash & Dale, 1981; Hencher, 2006). These erosion pipes can lead to rapid (relatively) infiltration and are a common feature associated with large, deep seated landslides. Properties of typical weathered rock materials in Hong Kong are discussed by Irfan (1996, 1999), Hencher (2006) and Ebuk (1991). In saprolite, friction angles are typically 30 + degrees (Martin, 1986) and tests on dilatant grade IV granite, corrected for dilation angle at peak strength, which varied proportionally with dry density and inversely with normal stress level, gave a basic friction angle of 38 to 40 degrees (Hencher, 1983c, 2006) which tallies with corrected shear strengths of granite joints (Hencher & Richards, 1982) and actually for most silicate rocks (Papaliangas et al, 1993). Volcanic rocks sometimes give lower dilation-corrected shear strengths on joints, down to 32 degrees for fine grained varieties. Coatings on joints can lead to considerably reduced shear strength of 17 degrees at low stresses for chlorite (Hencher, 1981; Brand et al, 1983) and joints infilled with clay can of course again be much lower, of the order of 10 to 20 degrees (Koor et al., 2000). True cohesion is present in weathered rock soil-like material (grades VI to IV) although reported values are typically less than 200 kPa which is surprisingly low given the upper bound unconfined compressive strength of grade IV is about 10 to 15 MPa (break by hand). It is suspected that the lack of higher reported values is due to a lack of systematic study. Suction clearly provides some temporary effective cohesion in slope stability and loss of cohesion by wetting is a factor in some landslides.

Figure 5 Variety of weathering profiles to be anticipated in different parts of slope profile (schematic and not to be relied upon). Redrawn from Ruxton & Berry (1957) 3 LANDSLIDES AND RAINFALL

Almost all landslides in Hong Kong are associated with periods of high rainfall which is highly seasonal. Intense storms are to be anticipated from about April through to September although storms sometimes occur outside that period and similarly cause extensive failures (e.g. Wong & Ho, 1995). The strong link between landsliding and intense rainfall may be inferred from Figure 6 which gives data from 1997, a particularly severe year for rainfall. Lumb (1975) related the occurrence of landslides to rainfall over the preceding 15 day period and rainfall on the day of the majority of landslides. Careful analysis of calls to emergency services later identified the importance of short-period rainfall intensity of about 70 mm/hour or more which often coincides with the onset of numerous failures (Premchitt et al, 1994) although this criterion alone does not correlate particularly well with landslide occurrence - over 1/3rd of such rainstorms between 1979 to 1995 did not trigger landslides according to Malone (1996).

Fig. 6. Landslide incidents reported to GEO vs. monthly rainfall, 1997.

Two of the intense storms listed by Malone (1996) occurred in the same year, 1982, and caused large numbers of landslides, many of the major ones of which were investigated by a team led by Hencher (1983a). One of the things that was most striking about the landslides caused by those storms was the way in which they were geographically discrete. Areas devastated by the 1st storm in May 1982 were relatively untouched by the 2nd storm in August and vice versa. The same was true of landslides triggered by Typhoon Rusa in Korea in 2002 (Lee & Hencher, 2007) - the area devastated by landslides was very discrete. Following the 1982 storms it was even noted that in some natural terrain, sides of valleys facing in one direction were often scarred by many landslides whilst the opposing valley sides were unscarred. It is recalled that along one road, in Tsing Yi Island, almost every cut slope showed some signs of distress. The intense storm had “tested” each slope and found at least one section with an insufficient reserve of strength to resist the triggering action. Following those storms in 1982, landslide incidents reported to the Geotechnical Control Office (now GEO) were plotted against maps showing the maximum 24 hour rainfalls as in Figure 7.

Figure 7 Rainfall isohyets for 24 hour period, 28/29th May 1982 vs reported incidents.

Figure 8 Twenty four hour rainfall vs. reported incidents (after Hencher et al 2006).

Analysis of these and other data allowed a plot to be developed showing intensity of landslides per square kilometre vs. maximum 24 hour rainfall (Figure 8). It can be seen that for the 1982 rainstorms, in areas where the 24 hour rainfall exceeded about 500 mm, a density of reported incidents of about 5 to 10 might be anticipated. Extrapolating to the worst rainfall anticipated for Hong Kong (GEO, 2004) indicates a density of incidents in the areas of most extreme rainfall of perhaps 20 to 50 per km2.

Figure 9 Hydrogeological pathways and processes in a hill slope (after Hencher, 2000) It is acknowledged that the above analysis is crude not least because there is no definition of reported "incident" which might range from a very minor rock fall to a deep seated landslide. Also incidents will go unreported in non-urban areas and no account might be taken of incomplete detachments. Incidentally, recent study of Malaysian landslides in weathered granite shows that serious incidents can be triggered at much lower rainfall than would be associated with similar incidents in Hong Kong. The reason for this is not clear but it does indicate that the relationship in Figure 8 is not generally applicable, probably because, in some, way, terrain susceptibility is in balance with local climate. Perhaps the Malaysian saprolite has different density profiles for equivalent weathering grades as is true of the hydrothermal weathering profiles in the UK (Hencher et al, 1990) with knock on effects on permeability, wetting, suction and cohesion and the impact of particular type of rain storms - intensity and duration.

4 LANDSLIDE MECHANISMS AND OCCURRENCE

The landslides that occur in the weathered terrain of Hong Kong are quite varied because of the relatively complex geology and structure for a small area and the diverse geomorphological environments that range from steep rock cliffs to deeply weathered foothills. Furthermore there is potential for natural terrain landslides of all types and for failures in man-made cuts and embankments. The influence of the built up areas, especially on hydrogeology, is significant (Nash & Dale, 1984).

To illustrate the styles of landslide that occur in Hong Kong one might attempt to use a geometrical form classification such as that of Varnes (1978) or a process classification (Brunsden, 1993) but actually neither of these quite fit the bill for Hong Kong landslides. One of the chief aspects of HK landslides is the link to severe storms and therefore a "hydrogeological classification" is an attractive concept.

Figure 9 is a schematic representation of some of the processes that the author imagines is taking place in typical hillsides in Hong Kong. It is evident that there are many different ways that water can influence stability ranging from surface erosion through to wetting infiltration and loss of suction and rise in water pressure, locally above an aquiclude or more generally through a rise in water table. It is also immediately obvious that the time frame for these various influences will be very different. Erosion will probably happen during a storm but rises in groundwater table will probably be delayed by hours if not days and weeks.

To illustrate the styles of landslide a six-fold classification has been developed based on hydrogeological factors, as follows:

Type 1. Shallow landslides caused by wetting and erosion. Surface flow concentration may be natural or anthropogenic for example due to a blocked drain or broken surface channels. Type 2. Loss of suction Type 3. Infiltration leading to rise in water table above aquiclude (perching) or partitioned by low permeability zones (e.g. dykes) Type 4. Infiltration leading to a rise in general water table Type 5. High permeability zone Type 6. Complex hydrogeological control

Type 1 Shallow landslides caused by wetting and erosion.

Shallow landslides caused by erosion or shallow processes such as cleft water pressures behind loose rock blocks are the commonest types of failure in Hong Kong and probably the most significant in terms of risk. They typically occur during or very shortly after severe rain storms. Rock and boulder falls cause damage and disruption. Seventy four such incidents were reported between 1978 and 1995. There have been 9 reported fatalities since 1926. The processes are illustrated in Figure 10. Figure 11 shows the serious Shum Wat landslide that may have been triggered by concentrated surface water flow along roads.

Figure 10 Type 1: Shallow failures, generally caused by concentrated surface flow, either natural (left) or due to man's influence (right).

Figure 11 The Shum Wan failure of 1995 that killed 2 people and was considered to have been triggered by flow down roads at the head of the landslide exacerbated by blocked drains (GEO, 1996).

Type 2 Loss of suction

Suction essentially provides additional apparent cohesion of the order of a few tens kPa to a slope in its partially saturated condition which is of the same order as true cohesion due to relict bonding in weaker weathered rocks. This may be significant to stability though not generally to be relied upon for design because it may be lost during a storm (e.g. Rodin et al, 1982). Shen (1998) reviews the main work done on this subject. Suction probably plays a role in many landslides although it is generally difficult to prove as a primary candidate because of the many other factors and lack of definitive monitoring. The landslide in Figure 12 was however judged to be such a case largely because its lack of catchment ruled out the likelihood of positive pore pressure development (Hencher 1983b; Hencher et al, 1984).

Type 3 Infiltration leading to rise in water table above an aquiclude (perching) or otherwise partitioned

This type of landslide is quite common and is caused because infiltration is restricted by the presence of a low permeability layer so that water pressure can build up above that layer. The concepts are illustrated in Figure 13. Weathered rock profiles have many such features - persistent discontinuities can become filled with clay, derived from weathering of the rock above and around. Dykes and sills of fine grained rock such as basalt and dolerite will weather differently to the host parent rock, say granite, with a resultant permeability contrast at the interfaces.

Figure 12 Type 2: Loss of suction due to wetting (saturation) of partially saturated material. Right hand photos are of Chung Hom Kok failure in highly decomposed granite (Schmidt hammer rebound values up to 17) before and after (Hencher, 1983b).

Figure 13 Type 3: Partitioning of ground water pressures within slopes due to permeability contrasts.

Examples of this type of failure are reported by Hencher (1983c &1983d), Martin & Hencher (1984), Hudson & Hencher (1984) and Au (1986). The slope illustrated in Figure 14 is on the Tuen Mun Highway in Hong Kong. Post failure investigation and analysis established that positive pore water pressure was necessary to explain the landslide yet a piezometer through the landslide scar and field observation established that the main groundwater level a few days post failure was much deeper than the failure surface. A series of decomposed dolerite dykes with lower permeability than the granitic country rock were mapped through the slope with a disposition such that the development of perched water was likely during high rainfall and it was deduced that this was the triggering factor for the failure (Hencher, 1983c). An example of groundwater being partitioned as in the right side of Figure 13 was identified at Ching Cheung Road (Figure 15) (Hencher, 1983d; Hudson & Hencher, 1984). The presence of a thick dyke was established through mapping of the failure scar and its presence was used to explain various unusual aspects of the delayed failure.

Type 4 Infiltration leading to a rise in general water table

Type 4 (Figure 16) is perhaps the most simple concept of hydrogeological control of landslides with infiltration leading to a general wetting and eventually rise in the main groundwater table, increased water pressure and then failure. Lumb (1975) discusses infiltration rates in detail and calculates that the upper several metres could become saturated following continuous heavy rainfall for more than half a day in granite and rather longer in volcanic rocks. Blake et al (2003) provide a more sophisticated numerical analysis for build up of water pressure behind a retaining wall illustrating that significant pressure can develop within a few hours. Rises in deeper seated groundwater table may take much longer (Figure 9) and a delayed response may therefore be expected for deep-seated landslides. Documented cases are remarkably rare for Hong Kong despite this (hydrogeology) and shear strength having been identified long ago as key issues for slope stability, knowledge of which might be improved considerably through systematic study (Sweeney & Robertson, 1979; Hencher et al, 1984; Brand, 1985).

Figure 14 Tuen Mun Highway landslide, 1982.

Figure 15 Landslide in section of Ching Cheung Road, 1982 that failed 4 days after heavy rain. A steeply dipping, thick decomposed dolerite dyke was mapped across the failure scarp with seepage from above, after failure.

One example of a deep seated failure illustrating most of the key characteristics - evidence of deep recharge through a transient and continuously changing natural pipe system and delayed response of piezometric head to rainfall and delayed failure is illustrated in the right side of Figure 16. This failure is described in detail by Halcrow Asia Partnership (1998) and Hencher (2000, 2006). Other examples of delayed rises in deep groundwater level are given by Insley & McNicoll (1982).

Figure 16 Type 4: Rise in groundwater table. Right hand figure is for Ching Cheung Road landslide of 1997. Note the number of choked natural pipes sampled in each borehole and that they extend to considerable depth (even below the 1997 slip surface) which indicates the length of recharge path to the groundwater table.

Figure 17 Type 5: Highly permeability zone allowing rapid through flow and build up of artesian pore pressure. Right hand picture is of slope on Chai Wan Road, 1982 with large pipes at boundary between colluvium and underlying volcanic rock.

Type 5 High permeability zones

Type 5 comprises landslides associated with rapid through flow of water along highly conductive layers as illustrated in Figure 17. These will affect the pore pressure distribution within a slope and the timing of water pressure development (e.g. Cowland & Richards, 1986 and Jiao et al 1993). Such zones can comprise preferentially fractured layers such as faults or shear zones, or pipes which can coalesce to become proper underground stream systems (Hencher et al, 2008). Examples of failures associated with such highly transmissive zones include the Chai Wan 1982 landslide (Hencher, 1983e; Hencher et al 1984), South Bay Close (Hencher, 1983f) and Lai Ping Road (Koor & Campbell, 2005).

Type 6 Complex hydrogeological control

Type 6 comprises those landslides that do not fall readily into other classes but perhaps where the hydrogeological influences leading to failure include several of the earlier types. One such example, illustrated in Figure 18 is a partial failure that occurred on Tsing Yi Island in 1982 (Hencher 1983g). The failure comprised the dilation and partial detachment of a part of a cut slope (see central image in Figure 18). Water was seen issuing below the distressed area several days after rainfall and an area to the left of the failure was noted to be very wet and with vegetation. The interpretation was that heavy rainfall above the slope transferred down an old river valley towards the left side of the slope which had been dammed by a hard surface cover. Water pressure then built up behind the concrete and moved laterally through relict joints into the area where the failure then took place. The initial movement involved rock mass dilation which allowed the driving water pressure to drop and the slope to temporarily stabilise as illustrated in the right hand image in Figure 18.

Figure 18 Tsing Yi (2) landslide after Hencher (1983g) and Hencher (2000).

9 CONCLUSIONS

Landslides occur every year in Hong Kong and these are almost always associated with intense rainstorms. Shallow failures, including minor rock falls are the most common, generally occur during the storm when the intensity of rainfall is highest and pose the greatest risk because of their frequency. Deeper seated landslides are often delayed, even many days after the rainstorm that is subsequently identified as the triggering factor. Infiltration paths and pore pressure distributions in slopes can be assumed to be tortuous on the available evidence with channel flow often dominating. The eventual mechanism leading to failure can be very complex with evident implications for the potential adequacy of investigation and design, if the controlling model is not recognised.

10 ACKNOWLEDGEMENT

The author acknowledges the many contributions from colleagues including Dick Martin and Andrew Malone in drawing these conclusions. Diarmad Campbell was very helpful in discussing the development of a "hydrogeological classification" of landslides. Much of the work reported here was carried out in association with the Hong Kong Government, either as an employee of the Geotechnical Control Office more than 20 years ago or more recently through various consultancies and general discussions with colleagues from the Geotechnical Engineering Office as it now is. The author’s research is partially supported by a grant (NEMA-06-NH-05) from the Natural Hazard Mitigation Research Group, National Emergency Management Agency, Korean Government.

12 REFERENCES

Au, S.W.C. (1986). Decomposed dolerite dykes as a cause of slope failure. Hong Kong Engineer, 14, 2, pp 33-36. Brand, E.W. (1984). Landslides in Southeast Asia: A State-of-the-Art Report. Proceedings, 4th International Symposium on Landslides, Toronto, Volume 1, pp 17 - 59 plus addendum (2p). Brand, E.W. (1985) Predicting the performance of residual soil slopes. Proceedings of the Eleventh International Conference on Soil Mechanics and Foundation Engineering, San Francisco, 5, pp 2541-2548. Brand, E.W., Hencher, S.R. & Youdan, D.G. (1983). Rock slope engineering in Hong Kong. Proceedings of the Fifth International Rock Mechanics Congress, Melbourne, vol. 1, pp C17-C24. Blake, J.R., Renaud, J.-P., Anderson, M.G. & Hencher, S.R. (2003) Prediction of rainfall-induced transient water pressure head behind a retaining wall using a high-resolution finite element model. Computers and Geotechnics, pp xx-xx. Brunsden, D. (1993) Mass movement; the research frontier and beyond: a geomorphological approach. Geomorphology, 7, pp 85-128. Chan, R.K.S, Mak, S.H. & Au-Yeung, Y.S. (2007). Partnering with the community to reduce landslide risk in Hong Kong over the past thirty years. Proceedings of the Geotechnical Advancements in Hong Kong since 1970s, Hong Kong, pp 183-195. Cochrane, R.A. (1984). Ground problems at Hong Kong's Aberdeen Tunnel. (Report by O. Bevan of British Tunnelling Society Meeting, 24 November 1983). Tunnels & Tunnelling, vol. 16, no. 5, pp 33-34. (Discussion, pp 34-36). Cowland, J.W. & Richards, L.R. (1985) Transient groundwater rises in sheeting joints in a Hong Kong granite slope. Hong Kong Engineer, 13, pp 27-32. Ebuk, E. J. (1991) The Influence of Fabric on the Shear Strength Characteristics of Weathered Granites. Unpublished PhD thesis, The University of Leeds, 486p. Ebuk, E. J. Hencher, S. R. & Lumsden, A. C. (1993) The influence of structure on the shearing mechanism of weakly bonded soils. Proceedings 26th Annual Conference of the Engineering Group of the Geological Society, the Engineering Geology of Weak Rock, Leeds, pp 207-215. Fyfe, J.A., Shaw, R., Campbell, S.D.G., Lai, K.W. & Kirk, P.A. (2000). The Quaternary Geology of Hong Kong. Geotechnical Engineering Office, Civil Engineering Department, Government of Hong Kong Special Administrative Region, 209 p. Geotechnical Engineering Office (1996). Report on the Shum Wan Road Landslide of 13 August 1995, vol. 1. Geotechnical Engineering Office, Hong Kong, 123 p. Geotechnical Engineering Office (2004) The Landslide Preventive Measures (LPM) Programme. Information Note 18/2004, July 2004, 6p http://www.cedd.gov.hk/eng/publications/information_notes/doc/in_2004_18e.pdf Halcrow Asia Partnership (1998). Ching Cheung Road, Geotechnical Engineering Office, Hong Kong, 140 p Full Reference Halcrow China Ltd (2001). Detailed Study of Selected Landslides above Leung King Estate of 14 April 2000. Landslide Study Report LSR 9/2001, Geotechnical Engineering Office, Hong Kong, 140 p. Halcrow China Limited (2003). Interim Report on Detailed Hydrogeological Study of the Hillside near Yee King Road, Tai Hang. Landslide Study Report LSR 4/2003, Geotechnical Engineering Office, Civil Engineering Department, Hong Kong. Hencher, S.R. (1981) Report on Slope Failure at Yip Kan Street (11SW-D/C86) Aberdeen on 12th July 1981. Geotechnical Control Office Report No. GCO 16/81, Geotechnical Control Office, Hong Kong, 26p. Hencher, S.R. (1983a). Summary Report on Ten Major Landslides in 1982. Special Project Report No. SPR 1/83, Geotechnical Engineering Office, Hong Kong, 28 p. Hencher, S.R. (1983b). Landslide Studies 1982 Case Study No. 5 Chung Hom Kok. Special Project Report No. SPR 4/83, Geotechnical Control Office, Hong Kong, 34p Hencher, S.R. (1983c). Landslide Studies 1982 Case Study No. 5 Tuen Mun Highway. Special Project Report No. SPR 6/83, Geotechnical Control Office, Hong Kong, 42p Hencher, S.R. (1983d). Landslide Studies 1982 Case Study No. 5 Ching Cheung Road. Special Project Report No. SPR 11/83, Geotechnical Control Office, Hong Kong, 38p Hencher, S.R. (1983e). Landslide Studies 1982 Case Study No. 5 Chai Wan Road. Special Project Report No. SPR 1/83, Geotechnical Control Office, Hong Kong, 34p Hencher, S.R. (1983f) Landslide Studies 1982 Case Study No. 4 South Bay Close. Special Project Report No. SPR 5/83, Geotechnical Control Office, Hong Kong, 38p. Hencher, S.R. (1983g) Landslide Studies 1982 Case Study No. 9 Tsing Yi (2). Special Project Report No. SPR 10/83, Geotechnical Control Office, Hong Kong, 44p. Hencher, S.R. (2000). Engineering geological aspects of landslides. Keynote paper. Proceedings Conference on Engineering Geology HK 2000, Institution of Mining and Metallurgy, Hong Kong, pp 93-116 Hencher, S.R. (2006). Weathering and erosion processes in rock – implications for geotechnical engineering. Invited Paper. Proceedings Symposium on Hong Kong Soils and Rocks, March 2004, Institution of Mining, Metallurgy and Materials and Geological Society of London, pp 29-79. Hencher, S.R. (2008) Rock descriptions Ground Engineering In press. Hencher, S.R., Ebuk, E.J., Abrams, J.H. & Lumsden, A.C. (1990) Field description and classification of hydrothermally altered granite from SW England. Proceedings 10th SouthEast Asian Geotechnical Conference, Taipei, ROC, pp 303-308. Hencher, S.R., Anderson, M.G. & Martin R.P. (2006). Hydrogeology of landslides. Proceedings of International Conference on Slopes, Malaysia, pp 463-474. Hencher, S.R. & Knipe, R.J, (2007) Development of rock joints with time and consequences for engineering. Proceedings of the 11th Congress of the International Society for Rock Mechanics, Lisbon, 1, pp 223 – 226. Hencher, S.R., Massey, J.B. & Brand, E.W. (1984) Application of back analysis to some Hong Kong landslides. Proceedings of the 4th International Symposium on Landslides, Toronto, 1, pp 631-638. Hencher, S. R. & McNicholl, D.P. (1995) Engineering in weathered rock. Quarterly Journal of Engineering Geology. 28(3), pp 253-266. Hencher, S.R., Sun, H.W., & Ho. K.K.S. (2008). The investigation of underground streams in a weathered granite terrain in Hong Kong. Proceedings 3rd International Conference on Site Characterisation, Taipei, (in press). Hudson, R.R. & Hencher, S.R. (1984). The delayed failure of a large cutting in Hong Kong. Proceedings of the International Conference on Case Histories in Geotechnical Engineering, St. Louis, Missouri, 2, pp 679-682. Insley, H.T.M. & McNicholl, D. (1982) Groundwater monitoring of a soil slope in Hong Kong. Proceedings of the 7th Southeast Asian Geotechnical Conference, Hong Kong, 1, pp 63-75. Irfan, T.Y. (1996). Mineralogy, fabric properties and classification of weathered granites in Hong Kong. Quarterly Journal of Engineering Geology, Vol. 29, pp 5-35. Irfan, T.Y. (1999). Characterization of weathered volcanic rocks in Hong Kong. Quarterly Journal of Engineering Geology, vol. 32, part 4, pp 317-348. Jiao, J.J., Leung, C.M. & Ding, G.P. (2003). Confined groundwater at No. 52, Hollywood Road, Hong Kong. Proceedings of the International Conference on Slope Engineering, The University of Hong Kong, 1, pp 316-321. Kirk, P.A., Campbell, S.D.G., Fletcher, C.J.N. & Merriman, R.J. (1997) The significance of primary volcanic fabrics and clay distribution in landslides in Hong Kong. Journal of the Geological Society of London, Vol.154, pp 1009-1019. Koor, N.P., Parry, S. & Yin, J.H. (2000) The shear strength of infilled and slickensided discontinuities of some Hong Kong saprolites. Proceedings of the Conference on Engineering Geology HK 2000, Institution of Mining and Metallurgy, Hong Kong Branch, November 2000, pp 187-192. Koor, N.P. & Campbell, S.D.G. (2005). Geological characterisation of the Lai Ping Road Landslide, GEO Report No. 166, Geotechnical Engineering Office, Hong Kong, 194p. Lumb, P. (1975) Slope failures in Hong Kong. Quarterly Journal of Engineering Geology, 8, pp. 31-65. Malone, A.W. (1996) Areal extent of intense rainfall in Hong Kong 1979 to 1995. GEO Report No. 62, 86p. Malone, A.W. (1998) Slope movement and failure : evidence from field observations of landslides associated with hillside cuttings in saprolites in Hong Kong. Proceedings 13th Southeast Asian Geotechnical Conference, Taipei, v2, pp 81-90. Martin, R.P. (1986). Use of index tests for engineering assessment of weathered rocks. Proceedings of the Fifth International Congress of the International Association of Engineering Geology, Buenos Aires, vol. 1, pp 433-450. Martin, R.P. (2000) Geological input to slope engineering in Hong Kong. Proceedings Conference on Engineering Geology HK 2000, Institution of Mining and Metallurgy, Hong Kong, pp 117-138. Martin, R.P. & Hencher, S.R. (1984) The failure of a cut slope on the Tuen Mun Road in Hong Kong. Proceedings of the International Conference on Case Histories in Geotechnical Engineering, St. Louis, Missouri, 2, pp 683-688. Nash, J. M. & Dale, M.J. (1984) Geology and hydrogeology of natural tunnel erosion in superficial deposits in Hong Kong. Geology of Superficial Deposits in Hong Kong. Geological Society of Hong Kong Bulletin, 1, pp 61-72. Papaliangas, T., Hencher, S. R., & Lumsden, A.C. (1996) A comprehensive peak shear strength criterion for rock joints. Proceedings 8th International Congress ISRM, Tokyo, 1, pp 359-366. Parry, S., Campbell, S.D.G. & Fletcher, C.J.N. (2000) Typical kaolin occurrences in Hong Kong – a model for their origin and implications for landslide development. Landslides in research, theory and practice. Proc. 8th International Symposium on Landslides, Cardiff, 3, pp1177-1181. Premchitt, J., Brand, E.W. & Chen, P.Y.M. (1994). Rain-induced Landslides in Hong Kong, 1972-1992. Asia Engineer, June, pp43 -51. Rodin, S., Henkel, D.J. & Brown, R.L. (1982). Geotechnical study of a large hillside area in Hong Kong. Hong Kong Engineer, vol. 10, no. 5, pp 37-45. Ruxton, B.P & Berry, L. (1957) The weathering of granite and associated erosional features in Hong Kong. Bulletin of the Geological Society of America, 68, pp 1263-92. Sewell, R.J., Campbell, S.D.G., Fletcher, C.J.N., Lai, K.W. & Kirk, P.A. (2000). The Pre-Quaternary Geology of Hong Kong. Geotechnical Engineering Office, Civil Engineering Department, Government of Hong Kong Special Administrative Region, 181 p Shen, J.M. (1998). Soil suction in relation to slope stability: a summary of research carried out in Hong Kong in 1978-1997. Proceedings of the Geotechnical Division Seminar on Slope Engineering in Hong Kong, Hong Kong Institution of Engineers, pp 93-99. Sweeney, D.J. & Robertson, P.K. (1979). A fundamental approach to slope stability problems in Hong Kong. Hong Kong Engineer, 7, 10, pp 35-44. Tang, M.C., Ho, K.K.S., Chan, T.C.F. & Chan, N.F. (2007). The landslip preventive measures programme of the Hong Kong SAR Government - reflections on achievements, advancement and lessons learnt in past 30 years. A Commemorative Volume Published in Conjunction with the 40th Anniversary of the Southeast Asian Geotechnical Society, Kuala Lumpur, pp 337-359. Varnes, D.J. (1978). Slope movements: types and processes. In Landslides Analysis and Control (Ed. E.B. Eckel). Transp. Res. Board, Spec. Rep. 176, pp. 11-33. Wong, H.N. & Ho, K.K.S. (1995). General report on landslips on 5 November 1993 at man-made features in Lantau. Geotechnical Engineering Office, Hong Kong Government, GEO Report No. 144, 79p.