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The vast majority of the landslides in Hong Kong are triggered by rainfall. Surface drainage commonly provided on man-made slopes aims to prevent surface erosion and reduce infiltration. It plays an important role to avert slope instability. However, inadequate management of surface drainage could hamper its efficiency and often results in uncontrolled surface water flow contributory to landsliding. This paper provides insights on the importance of slope surface drainage management under the dense urban settings in Hong Kong from the landslide perspective. Selected landslides involving inadequately managed surface drainage are presented to highlight the key lessons learnt and areas deserving attention on various drainage aspects, including the design and detailing. It is noteworthy that inadequate surface drainage maintenance could exacerbate and lead to sizeable landslides even on engineered slopes. A review of landslide records over the years suggests that some landslides could have been averted had there been proper surface drainage maintenance. [ABSTRACT FROM AUTHOR]

Debris-flows are forms of landslides in mountainous regions that can potentially cause significant damage. Structural countermeasures to mitigate an entire debris-flow may become unrealistically massive and expensive. If the flow cannot be stopped completely, one may alternatively consider reducing the impact and velocity of the flow using energy dissipating structures. A debris-flow screen is such a countermeasure designed to dissipate energy. A screen is made by parallel grids, with some gaps, placed in the direction of the debris-flow on an elevated foundation. This structure acts as a filter for separating water from the saturated debris-flow to reduce its flow energy. This paper presents a laboratory model test investigating the effect of the screen with length (0.5m and 1.0 m) and opening width (2 mm, 4mm and 6 mm) in dissipating the debris-flow energy. The effectiveness of the screens was determined in terms of reductions in the run-out distance and the flow velocity. The importance of the screen length and the opening width is demonstrated. A hypothesis that the optimum opening size should be close to d50 of the solid material seems to be validated. The application of the laboratory observations to the field is indicated based on the energy line and scaling principles. [ABSTRACT FROM AUTHOR]

A new type of flexible debris-resisting barrier has been designed using the state-of-the-art analytical force approach for mitigation of channelised debris flow hazards in a hillside study area in Hong Kong. Unlike the conventional three-panel rectangular form, these flexible barriers were configured in a valley shape to geometrically fit in the incised drainage lines. The "design event" resulted in a calculated debris impact pressure of up to 400 kPa and over 10,000 kJ in terms of impact energy necessitating a site-specific design. The design involved advanced structural modelling of the highly non-linear behaviour of flexible barriers under debris impact giving rise to a large deformation. This paper presents the design and detailing of these valley-shaped flexible barriers, involving formulation of the design methodology, derivation of the multi-phase impact load model and optimisation of the barrier configuration. To support the design, numerical parametric studies were conducted to examine the barrier's impact behaviour. Two design initiatives were highlighted for enhancing the design robustness, including a rigorous approach to analyse the effects of the energy-dissipating devices on barrier performance and a robustness check to cater for adverse scenarios that could conceivably happen. Some suggestions on configuring a valley-shaped flexible barrier were given. [ABSTRACT FROM AUTHOR]

The Geotechnical Engineering Office (GEO) has been steering technical development work to facilitate enhancement in natural terrain landslide risk management in Hong Kong. Collaborations between GEO, the academia from both local and overseas institutes and geotechnical professions have been initiated in recent years to carry out studies on various aspects, including the development of design methodologies for debris-resisting barriers, the investigation of debris impact mechanics and debris-barrier interaction as well as the improvement of detailing of debris-resisting barriers. The concerted efforts have put Hong Kong in a leading position amongst other countries in the area of combating landslide risk. Many of the studies have brought about significant impacts to the engineering practice in many different regions, and have been made reference with by practitioners worldwide in establishing directions of technical development work on the subject. This paper presents the technical advances in natural terrain landslide risk mitigation measures in Hong Kong. It covers a wide spectrum of innovative studies jointly carried out by the GEO and various experts in respect of advanced numerical modelling, laboratory flume testing, centrifuge modelling and large-scale physical modelling, etc. These studies shed light on the potential of rationalising the present barrier design approaches. [ABSTRACT FROM AUTHOR]

An intense rainstorm on 21 May 2016 resulted a total of 34 natural terrain landslides in the vicinity of Sai Kung Sai Wan Road in the Sai Kung East Country Park, Hong Kong. One of these landslides was a large-scale failure with a source volume of about 2100m3. It was almost an order of magnitude larger than all the others. The debris severely eroded a road embankment, undermining the road and resulting in road closure. The nature of the landslide was unusual in that it was a structurally controlled rockslide, with a basal failure surface formed from a persistent, planar sheeting joint within columnar jointed tuff rock with a maximum depth of about 12 m, while the other landslides that occurred in the country park during the same rainstorm involved only shallow failures within the regolith. An investigation of this unusual landslide was undertaken to diagnose the probable mechanism and causes of the failure. As there was concern about further large-scale instability occurring in the adjacent hillside where the sheeting joint was inferred to extend into, the potential for further large-scale instability was assessed as part of the investigation. The key components of the investigation included detailed engineering geological mapping, ground investigation and engineering analyses. This paper presents the findings of the investigation. [ABSTRACT FROM AUTHOR]

Mitigation of natural terrain hazards through the use of flexible and rigid barriers has become a common type of geotechnical work in Hong Kong. The current design approach for these structural countermeasures typically involves separate debris mobility modelling and structural analyses of barriers subject to pseudo-static impact loadings. This practical approach, however, could not realistically account for the dynamic debris-barrier interaction. With a view to better capturing the response of barriers under the debris impact, effort has recently been devoted to developing an advanced numerical technique to model debris flows, barrier structural responses and the impact dynamics in a coupled manner. This paper presents some recent advances made in establishing the numerical technique of coupled analysis using a three-dimensional finiteelement computer code LS-DYNA. The methodologies of modelling the landslide debris mobility, the large-deformation non-linear behaviour of a flexible barrier under impact and the coupled analysis of debris-barrier interaction are introduced. Back analyses of selected landslide events and debris impact tests of flexible barriers for verification of the computer models are discussed. The coupled analysis enables detailed investigation of the energy balance involved in the impact process. Preliminary results of this energy analysis are also presented. [ABSTRACT FROM AUTHOR]