Challenges in Improving Slope Safety in Hong Kong through the Landslip Prevention and Mitigation Programme
By Ken K.S. Ho and Raymond W.M. Cheung
ABSTRACT: In 1977, the Hong Kong Government embarked on a systematic retrofitting programme, known as the Landslip Preventive Measures (LPM) Programme, to systematically upgrade existing substandard man-made slopes to modern safety standards. By 2010, some 4,500 high-risk government man-made slopes have been upgraded through engineering works, and the overall landslide risk arising from man-made slopes has been reduced to less than 25% of the 1977 level. Over the years, the programme has evolved progressively in response to Government’s continuous improvement initiatives and rising public expectations in respect of slope safety and slope appearance. In 2010, the Government launched the Landslip Prevention and Mitigation (LPMit) Programme to dovetail with the LPM Programme, with the focus being on retrofitting the remaining moderate-risk substandard man-made slopes and systematically mitigating natural terrain landslide risk. This paper presents the challenges, technical advances and achievements of the LPM and LPMit Programmes.
Rock Caverns – Hong Kong’s Hidden Land
By K.C. Ng, K.J. Roberts and Y.K. Ho
ABSTRACT: The hilly terrain and underlying geology of Hong Kong offer an excellent opportunity for placing urban facilities underground. About two-thirds of Hong Kong’s land is found to be suitable for rock cavern development. Given the potential for multi-layer cavern development, a substantial usable area could be created. In September 2012, the Civil Engineering and Development Department of the Government of the Hong Kong Special Administrative Region commenced a study on “Long-term Strategy for Cavern Development”, to develop a holistic approach in planning and implementing cavern development and render it a sustainable means for expanding land resources. The study also places emphasis on private sector participation as facilities, such as storage, warehousing and data centres, can benefit from rock caverns’ stable and secure setting. Implementation of a long-term strategy for cavern development could provide a sustainable approach in easing the pressure of land shortage. Developing a systematic relocation programme for suitable Government facilities could release surface sites for other uses including housing, and placing nuisance or potentially hazardous facilities in caverns could remove incompatible land uses. Reserving rock cavern space to accommodate future public and private sector facilities underground could further reduce the land take. The Hong Kong Government has also commenced an initiative to explore the potential of underground space development in the urban areas. Facilitating rock cavern development at the urban fringes and underground space development in the urban areas could enhance Hong Kong’s utilisation of land resources in pursuit of sustainable development.
The First Subsea TBM Road Tunnel in Hong Kong
By Albert Liu, Stephen Chan, Conrad Ng, Joseph Lo, C. K. Tsang and Dunson Shut
ABSTRACT: Subsea tunnels for transportation are traditionally constructed in the form of Immersed Tunnel (IMT). With the technical advancement of mechanized Tunnel Boring Machine (TBM) construction, subsea TBM bored tunnels were successfully constructed in different parts of the World over the last decade. Using a TBM has benefits over the IMT when excavating beneath the sea, since it does not require dredging and marine access. This makes it particularly favourable when coping with environmental concerns and constraints within existing shipping passages. Since the first subsea tunnel across the Victoria Harbour in Hong Kong was constructed in 1972 by immersed tunnel method, four other additional subsea immersed tunnels were constructed across the same Victoria Harbour between 1979 and 1997. The subsea tunnel of Tuen Mun – Chek Lap Kok Link (TM-CLKL) was also originally proposed using immersed tunnel method in the feasibility study stage. However, the tunnel scheme was changed to TBM bored tunnel in the Investigation and Preliminary Design Stage. The TBM bored tunnel scheme was further developed in the Detailed Design Stage and the project is now under construction. This would be the first subsea TBM road tunnel in Hong Kong and this paper discusses the key considerations and rationales in changing the original IMT scheme to the TBM bored tunnel scheme for the subsea tunnel section of TM-CLKL.
KEYWORDS: Subsea tunnel, Tunnel boring machine (TBM), Immersed tunnel (IMT), Mixed ground conditions
Achievements of and Challenges to the Hong Kong Landslide Risk Management
By Rick CK Tam and Michael MK Chang
ABSTRACT: Landslide is one of the common natural hazards in Hong Kong. With the Government and public’s concerted efforts, landslide risk in Hong Kong has been drastically reduced since the establishment of a comprehensive slope safety system in 1977. However, given Hong Kong’s climatic and geographical conditions and the current state of technology, occurrence of serious landslides that could potentially cause multiple fatalities remains a distinct possibility, particularly during extreme rainfall events.
Subsea Horizontal Directional Coring (HDC)
By C. K. Tsang, S. F. Chau and Jimmy Chan
ABSTRACT: The Tuen Mun – Chek Lap Kok Link comprises a 9 km long dual 2-lane carriageway between Tuen Mun and North Lantau, with approximately 5 km long sub-sea tunnel between Hong Kong Boundary Crossing Facilities and Tuen Mun. This is a major highway infrastructure constructed to alleviate the increase in cross boundary traffic due to projected developments in the Northwest New Territories and North Lantau in Hong Kong, including the Airport developments and the Hong Kong-Zhuhai-Macao Bridge. The proposed subsea tunnel is to be constructed by large diameter Tunnel Boring Machines (TBM) which will bore underneath two sets of existing submarine power cables providing power supply to the Hong Kong International Airport. Ground investigation using conventional vertical marine drillholes is not allowed within the cable protection zone with the considerations of the potential risk of damaging the power cables. To provide sufficient ground information for the design of the proposed TBM tunnel, Horizontal Directional Coring (HDC) with a total length of 660m was proposed at the invert level along the tunnel alignment. It was anticipated that the HDC would go through rock, soil or soil/rock interface and terminate at interface of soft / mixed ground. The HDC works has been completed in mid-2013. This paper describes the design considerations and the trajectory planning of the HDC work, with construction of a marine platform (of size 15m x 20m to facilitate the installation of the HDC). The difficulties and problems encountered during the subsea horizontal drilling is also discussed.
7th Lumb lecture 10th October 2012 “Peter Lumb’s legacy, Soil Mechanics = Simple concepts + mathematical processes + lateral thinking”
By John Endicott
SYNOPSIS: Professor Peter Lumb’s legacy to the Hong Kong geotechnical engineering profession was 32 years of service at the University of Hong Kong. For this he is fondly remembered by his many students as a quiet teacher, a contemplative man. The majority of his time Peter had grappled with tropical weathering and its consequence in engineering properties as well as the performance of soils and rock in an industry that was mostly not very enlightened for some 24 years before the Geotechnical Control Office (GCO), was established.
In his early days reliable laboratory testing was not common. Peter built the first testing laboratory in Hong Kong . Computers were under development and not in use. Peter taught assessment, insight and auditable hand calculations. Faced with a heavily regulatory system designed to compensate for inadequacies of the not well informed amongst the practitioners, he shied away from getting involved with day to day projects. As a profound thinker, when Ken Roscoe at Cambridge University was working on Critical State Soil Mechanics and Alan Bishop at Imperial College London was trying to perfect uni-axial compression tests, Peter realised that statistics was a means of handling variation, uncertainty and risk. Like some other geotechnical people, trained to investigate, he branched out into a new field and became a worldwide specialist in statistical theory not related to applications to soil mechanics.
He retired 26 years ago. What have been the fruits of his legacy? The most obvious results are dozens of his former students who have carried on his tradition, not necessarily in soil mechanics, and have achieved high positions and led worthwhile lives. The industry has changed. Testing laboratories are accredited. Deep excavations with lateral support and foundations are designed rationally. Much reclamation have been completed without the mud waves of the kind that were generated in the 1970’s. Thanks to the efforts of the Geotechnical engineering Office (GEO), landslide risk has been significantly reduced. The subject of stability of slopes is complex and there is fascinating on-going research into the performance of slopes. Computers are taken for granted. Computations can be carried out quickly and more intricately than he imagined. Mathematics was a predictive tool, now it is hidden behind icons which can be invoked without thought.
Mathematics has been a principal tool behind the soil mechanics that Peter taught. Coulomb and Terzaghi were mathematicians. However solutions have given place to processes. Numerical modelling is very useful and is now made freely available to engineers. The collapse of the Nicholl Highway in Singapore was initially blamed on the mis-use of numerical modelling. Within limits debris flow can be analysed but prediction of flow remains difficult. Numerical models can predict slopes moving uphill in the dry season.
Statistics are being adopted to a limited degree. Quantitative Risk Assessment and Fractal Analysis require large supplies of relevant data. Today gigabites of data are transmitted in minutes. One wonders whether Peter would have approached statistics in a less theoretical way had he been working 26 years later?
Geotechnical Engineers file data spatially as Geographic Information Systems (GIS). Very much as Peter thought laterally and was attracted to statistics likewise GIS people, thinking laterally, have moved into asset management and a variety of other fields.
The legacy of Peter Lumb lives on; it is the better side of human nature.
Singapore Case Histories on Omission of Strut by Observation Approach For Circle Line and Down Town Line Projects
By David Ng C. C. and Simon Low Y. H.
ABSTRACT: This paper will describe in details the issues and challenges involved in the procedures for strut omission by observational approach for two case histories from two different projects – Circle Line Contract C824 and Down Town Line Stage 3 Contract C922. The case history of Overrun Tunnel (ORT) of C922 is basically an underground facility building functions as both Railway Facility (Operation Control Centre) and Electrical Substation (ESS) which is to be built next the Expo Station. ORT is located in old alluvium (OA). The proposed underground overrun tunnel is a box structure with dimensions of approximately 23m wide, 25m deep and approximately 440m long. The proposed diaphragm wall function as the earth retaining system (ERSS), it designed for both temporary loading conditions during excavation and permanent load conditions in accordance with LTA Civil Design Criteria. Bottom-up construction sequence is adopted where lateral supports using four (S3 to S6) or six (S1 to S6) layers of steel strutting were installed as excavation progresses downward. The most challenging part is the omission of the last layer of strut S6 for the whole ORT by using observational approach. The case history of C824 Nicoll Highway Station demonstrates that Jet Mechanical Mixing (JMM), if properly installed, has major benefits in controlling the stability and movements induced by deep excavations in soft ground. The reasons can be attributed to the fact that the inner soil column is comprehensively mixed, combined with the attributes of the outer jet grouted column with sufficient overlapping. The whole process undergoes tight quality control and rigorous testing to ensure a continuous and comprehensive slab. In addition to the JMM slab, there is the major benefit of the discrete soil mixing columns formed above the JMM slab during the withdrawal of the auger. This case history also shows that with observational approach, if used appropriately, the design of temporary works can be effectively streamlined to achieve a more economical and yet safe design, This is illustrated by the approach to omit the intermediate layer of strut in the original design after observing the better than expected performance of the JMM. Based on the limited usage to date it is difficult to suggest what parameters should be used for future design. The approach to the back analyses and forward analyses in the observational approach is presented in this paper. This paper will also discuss the design and construction considerations by focusing on the challenge of strut omission by observation approach. The instrumentation monitoring results will also be presented as evaluation of the performance of the ERSS. The site observation and instrumentation result is in line with the forward analysis prediction for the omission of strut. This proposal has helped to expedite the project with a more economical design. With the implementation of observational approach, we will be able to achieve a more sustainable development of underground infrastructure projects.
KEYWORDS: Deep excavation, Strut omission, Back analyses, Forward analyses, Observational approach
Vacuum preloading methods: an update
By Jian Chu, Shuwang Yan and Wei Guo
ABSTRACT: It has been more than 60 years since the concept of vacuum preloading was proposed. The vacuum preloading method has been evolving. There have been considerable improvements in the techniques as well as new applications. In this paper, several vacuum preloading methods including some new variations are introduced. The advantages and disadvantages of each method are compared. Technical issues such as improvement depth, vacuum pressure distribution in soil, and evaluation of degree of consolidation for soil under vacuum consolidation are discussed. A case history using a combined vacuum and fill surcharge preloading method for soft soil improvement is also used to illustrate the changes in the pore pressure versus depth profiles and the application of the method to calculate degree of consolidation using pore water pressure distributions.
A New Lithostratigraphical Framework Proposed for Singapore
By K.K Lat, K.H Goay, S.G Lau, S.L Chiam and K.C Chew
ABSTRACT: A study was initiated in mid-2013 by Building & Construction Authority of Singapore (BCA) to review the existing stratigraphy framework of Singapore. The new lithostratigraphical framework follows the recommendations of International Commission of Stratigraphy (ICS) and it was developed based on geological fieldworks observations and rock cores examination obtained from new deep boreholes. This paper will only cover on the Jurong Formation, Fort Canning Boulder Bed and Old Alluvium. The Jurong Formation has been upgraded to Jurong group according to ICS stratigraphy guidelines and the Jurong group is sub-divided into three (3) formations, known as Tuas formation, Bukit Resam formation and Pasir Panjang formation. The Fort Canning Boulder Bed and Old Alluvium have been re-classified as Fort Canning formation and Bedok formation respectively.
KEYWORDS: Lithostratigraphical Framework, International Commission on Stratigraphy (ICS), Lexicon
Economical Design for NSF Piles in Soft Clays using Soil-Structure Interaction
By Siew Ann Tan
ABSTRACT: Code based design of piles with NSF consider the NSF force as a dragload to be imposed on the pile as an unfavourable design action. These codes like Singapore CP4, UK BS 8004 and the recent EC7 would indirectly factor up the value of the dragload while at the same time factor down the positive shaft friction below the neutral plane. Thus the pile design in very deep soft clays typical of Singapore and Asean coastal plains will lead to very conservative pile lengths to meet the code requirements. The Unified pile design method of Fellenius recognized this deficiency and it allows for better pile design with NSF taking into account the need for both force and settlement equilibrium between pile and soil. Fortunately, EC7 also allows for interactive pile/soil analysis using modern FEM tools that can optimise pile design for NSF, particularly when the remaining consolidation settlements around the piles are relatively small. This paper will compare these methods and provide insights into the proper understanding of NSF effects on pile behaviour, and recommend the way forward for rational and economical pile design in settling soils.
KEYWORDS: Geotechnical Design Codes, CP4, BS8004, EC7, NSF piles, and Consolidation of soft clays
Towards a Design Framework for Spatial Variability in Cement Treatment for Underground Construction
By Y. Liu, Y. Jiang and F. H. Lee
ABSTRACT: The most common form of ground treatment used to facilitate underground construction in Singapore is cement treatment. However, there is currently no indication on how safe and how conservative this adopted strength is since the prescribed strength bears no relationship to the probability of failure or factor of safety. This paper examined several sources leading to non-uniformity and spatial variation in cement-treated soils, including curing time effect, influence of operating parameters on slurry concentration, in-situ water content and column positioning errors. A framework for design and monitoring of ground treatment by cement was proposed.
KEYWORDS: Cement-treated ground; Underground construction; Unconfined compressive strength; Spatial variability
Advances and Challenges in Underground Space Use in Singapore
By Y. Zhou and J. Zhao
ABSTRACT: Despite its promise and many benefits for sustainable urban development, the use of underground space has tended to be the last resort, due to high development cost and the complexities in the planning and development of underground space. In 2010, the Economic Strategies Committee of the Singapore government made developing underground space part of the government’s long-term economic strategy with specific recommendations on master planning, geological investigations, investment in research and development, and various policy issues. With this, the use of underground space has been elevated to a strategic level and has become an economic imperative in landscarce Singapore. The ESC report also recommended that the government should take the lead in catalyzing the use of underground space. Based on these recommendations, the Singapore government has taken various initiatives and studies, and initiated various research projects in support of these initiatives. This top-down strategy has also made it possible to plan and coordinate the development of underground space in a holistic manner, and helps overcome the key challenges of the systems. This paper gives a review of advances in underground space development, highlights some key challenges, and discusses the various recent studies and planning issues, and examines possible strategies for future use of underground space in Singapore.
KEYWORDS: Underground space, Rock caverns, Singapore
Effect of Ageing Environment on Fiber-Reinforced Polymer/Granular Interface Shear Behaviour
By H.A. Shaia and H.M. Abuel-Naga
ABSTRACT: The aim of this paper is to investigate the ageing induced changes in Fiber-Reinforced Polymer (FRP)/Granular interface shear behaviour under different aging environments. The testing materials in this study include two different FRP materials; Glass Fiber Reinforced Polymer (GFRP) and Carbon Fiber Reinforced Polymer (CFRP), and two different granular materials in terms of particle shape and D50. Acidic and alkaline aging environments were adopted in this study. The experimental program involves assessing the ageing effect on the testing FRP materials in terms of the changes in their hardness and surface roughness properties. Furthermore interface shear tests were conducted, using the unaged and aged FRP materials, to evaluate the effect of aging environments on FRP-granular interface shear coefficient. The test results indicated that FRP-granular interface shear behaviour was improved after subjected to the adopted aging environments. This behaviour could be attributed to the observed increase in the surface roughness of the aged FRP testing specimens.
KEYWORDS: Interface shear, FRP, Laboratory study, Long-term
Groundwater Flow Modeling and Slope Stability Analysis for Deepening of Mae Moh Open Pit Lignite Mine
By A.B.N. Dassanayake, N. Phien-wej and P. H. Giao
ABSTRACT: This study deals with modeling the groundwater pressure effect and slope stability analysis of C1 pit on deep pit mining of Mae Moh open pit lignite mine, Thailand. Groundwater flow model was constructed and run using Visual Modflow 3D. The groundwater model consists of two aquifers and one aquitard, which were then divided into nine material types for property designation. The predictive simulations were carried out for seven years (2010 to 2017) according to the mine development plan. Results showed that low permeable Argillite underlying the west wall of C1 pit is problematic as low drawdowns were obtained for short term pumping from Argillite formation. Long term pumping schedules must be initiated to lower the potentiometric head within Argillite formation by pumping from limestone under Northeast (NE) pit. Stability of the west wall of the C1 pit for 2017 pit slope was evaluated in terms of the safety factor by the limit equilibrium method. Results obtained in this study indicated that the west wall is susceptible to failure due to water pressure associated with it. To maintain a safe slope, potentiometric head within west wall of C1 pit should be maintained below 170m, MSL.
KEYWORDS: Groundwater flow modeling, slope stability, open pit, Mae Moh mine.
A Study on Internal Erosion of Low-Plasticity Silty Sand
By Jing-Wen Chen, Bo-Rung Lin, Wei F. Lee and Yie-Ruey Chen
ABSTRACT: Internal erosion phenomenon of low plasticity silty sand causing by migrant of fines content within soil matrix is of great interest to engineers. A novel direct shear device was developed to enable controlled seepage flows to be introduced during testing. In this study, this new developed seepage flow direct shear test device was used to investigate the effects of internal erosion to non-plastic silty sand prior to shearing. Specimens prepared with different fines contents and relative densities were tested to examine the influences of fines and density state to planar shear strength of non-plastic silty sand after experiencing internal erosion.
The results revealed that coarse particles existing in the test specimens still dominate the behavior of soil. Hence, when all soil specimens had similar void ratios, the strength of the eroded soil is greater than that of the uneroded soil ones. Moreover, fines contents had noticeable influence on soil behaviors, regardless of whether an internal erosion process was applied to the samples. Results of presented study is hoped to improve engineers’ understandings in engineering behavior of low plasticity silty sand.
KEYWORDS: Seepage flow direct shear (SFDS) device, Fines content, Friction angle, Low plasticity silty sand
Analysis of Influencing Factors on Brazilian Test Results Based on A Complex-shaped Grain Model for Brittle Rock
By Guangcheng Yang and Xinghua Wang
ABSTRACT: Brazilian test is a commonly used laboratory method to measure the tensile strength of brittle rock, but there are also many controversies. In this paper, influencing factors including the Poisson’s ratio and the rock specimen thickness on Brazilian test results are investigated using PFC3D program based on a complex-shaped grain model which can capture all the characteristics of brittle rock in threedimensional environment. The relationship between the Poisson’s ratio and the tensile strength is observed through the comparison among three different models which presenting similar macro-properties except the Poisson’s ratio and the tensile strength. The study of specimen thickness effect indicates that the Brazilian test results significantly overestimate the real tensile strength of the specimens with relatively larger thickness. Through investigating the stress-strain curves and crack developing processes of the Brazilian test specimens, conclusions are made that the Brazilian tensile strength will increase with the specimen thickness due to the great loading increment.
KEYWORDS: Brazilian test, Tensile strength, Poisson’s ratio, Specimen thickness, Complex-shaped grain model.
Effect of Infill Moisture Content and Thickness on Shear Behavior of Planar and Rough Rock Joints
By Tsu-Chiang Cheng, Shuh-Gi Chern, Shin-Ru Wu and Yu-The Lin
ABSTRACT: The shear behavior of rock joints have normally been investigated in the past by conducting conventional direct shear test in the laboratory, where during the shearing process, the effect of infill material is often neglected.
It is well known that the shear strength of rock joints decreases significantly due to the presence of gauge on infill materials. In addition, the joint strength is highly dependent on the shear behavior of infill material and joint asperity during shear movement. Though many researches have been done about the influence of infill material and asperity on joint shear strength, however, only a limited number of studies are found in relation to the influences of infill thickness and water content on the shear strength of infilled rock joints. The current study is an attempt to investigate the shear behavior of soft rock joints under Constant Normal Load v (CNL) conditions, with special reference to the influences of infill thickness and moisture content on shear behavior of planar and rough joints. The results of this study show that infill water content could influence shear strength of planar and rough rock joints, more significant than infill thickness.
KEYWORDS: Rough rock joints, Shear behavior, Infill, Moisture.
Role of Bentonite in Improving the Efficiency of Cement Grouting in Coarse Sand
By T.G. Santhoshkumar, Benny Mathews Abraham, A, Sridharan, and Babu T Jose
ABSTRACT: Grouting generally is used to fill the voids in the ground (fissures and porous structures) with the aim of increasing resistance against deformation, to increase cohesion, shear strength and uniaxial compressive strength or finally (even more frequently) to reduce conductivity and interconnected porosity in an aquifer. In the case of loose sandy soils, the very low bearing capacity of the foundation bed causes shear failure and excessive settlements. Cement grouting technique is one of the possible solutions to the foundation problems for improving the properties of soil at shallow depths. Various authors have recommended a number of additives that can be used in cement grouting. Admixtures like antibleeder increases viscosity of the cement grouts, at the same time reducing sedimentation to a considerable extent. Bentonite can be considered as a cheap and effective admixture for cement grouts with regard to stability. This paper presents the results of experimental studies conducted in the laboratory, in this direction. It was found that addition of small percentages of bentonite and detergent increases the lateral flow of cement grout in coarse sand. The results clearly indicate that addition of even a small amount of bentonite to the cement grout increases the grouting efficiency in coarse sand.
KEYWORDS: Cement grout, Loose sand, Bentonite, Grouting efficiency.
