SPECIAL PROJECT REPORT SPR1/2003 STUDY OF COASTAL SUBSOIL GEOTECHNICAL CONDITIONS W.P. Lee, T.C. Chan, W.C. Siu, C.K. Chu April 2003 Technical Services Division
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- 5 - CONTENTS Title Page 1 FOREWORD 3 CONTENTS 5 1. INTRODUCTION 7 2. METHODOLOGY 9 2.1 General 9 2.2 Difficult Ground Conditions 9 2.3 Study Stages 10 2.4 Subsoil Profiles 12 3. KOWLOON AND NEW TERRITORIES 13 3.1 Nim Wan to Castle Peak Bay 13 3.2 Siu Lam to Sham Tseng 15 3.3 Ting Kau to Stonecutters Island 17 3.4 Stonecutters Island to Tsim Sha Tsui (West) 20 3.5 Tsim Sha Tsui to To Kwa Wan 22 3.6 Kai Tak to Lei Yue Mun 24 3.7 Lei Yue Mun to Tseung Kwan O 26 3.8 Tai Po, Shatin and Ma On Shan 29 4. HONG KONG ISLAND AND ISLANDS 31 4.1 Kennedy Town to Central 31 4.2 Central (East) to Causeway Bay 33 4.3 North Point to Chai Wan 35 4.4 Ap Lei Chau and Stanley 38 4.5 Tai O and Tung Chung to Yam O 39 4.6 Discovery Bay and Penny s Bay to Kap Shui Mun 43 4.7 Hei Ling Chau and Lamma Island 45 5. SUMMARY OF DIFFICULT GROUND CONDITIONS 47 5.1 General 47 5.2 Locations and Characteristics 47 Page No.
- 6-6. GUIDELINES FOR GROUND INVESTIGATION 49 Page No. 6.1 Assessment of Type of Ground Conditions 49 6.2 Ground Investigation Involving Difficult Ground Conditions 50 6.2.1 Points of Exploration 50 6.2.2 Depth of Penetration 51 6.2.3 In-situ Field Tests and Soil Sampling for Laboratory Testing 51 6.3 Review of Ground Conditions During Construction 53 7. REFERENCES 55 FIGURES 57 List of Figures 59 Figures 61
- 7-1. INTRODUCTION At present, large amount of geotechnical data collected from ground investigations are kept in Geotechnical Information Unit (GIU) of Civil Engineering Library. However, they are still in their elementary form of providing description of individual borehole information. As such, a study on the existing ground investigation records along the coastline of the territory was undertaken to consolidate and compile these data in order to facilitate project engineers to undertake foundation design of marine works in the future. The objectives of the study are : Determine nearshore subsoil profiles along the coastline. Identify locations of difficult ground conditions. Recommend guidelines for site investigations in areas of difficult ground conditions. It should be noted that the locations of difficult ground conditions in the coastal areas of Hong Kong indicated in this study are intended to serve as source reference based on the existing borehole information and are by no means exhaustive. Designers should assess the actual ground conditions of their sites with reference to the additional information collected and the nature of the projects. The suggested site investigation guidelines provide a general framework for designers to plan the ground investigation works and details should be adjusted to suit individual site conditions. For more specific details on ground investigation, such as the selection of ground investigation methods, in-situ tests, sampling techniques and laboratory tests, reference can be made to Geoguide 2 (CESD, 1987), Geospec 3 (CED, 2001a), GEO Technical Guidance Note No. 5 (CED, 2001b) and Port Works Design Manual : Part 1 (CED, 2002a).
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- 9-2. METHODOLOGY 2.1 General This study concentrated on those regions where there are or may be coastal developments. The study area is shown in Figure 1. The distribution of existing boreholes in the study area is shown in Figure 2. No additional ground investigation was carried out for the purpose of this study. The findings of the study are categorized under the following regions, namely, Kowloon and New Territories, and Hong Kong Island and Islands. In these regions, there are sub-regions classified according to geographic locations, in order to facilitate readers to extract relevant information. 2.2 Difficult Ground Conditions Difficult ground conditions generally refer to those circumstances in which unfavourable or potentially unfavourable subsoil strata exist at the site and the presence of which, if not properly handled, may lead to problems in construction of marine structures such as seawalls and breakwaters and affect the future land use. In the coastal waters of Hong Kong, the subsoil profiles normally consist of a sequence of marine and alluvial deposits overlying in-situ rock of various states of weathering. The marine deposit is generally soft and its properties have been studied extensively in recent years (Ho & Chan, 1994). On the other hand, the underlying alluvial layers are highly variable, and may be gravelly, sandy, silty or clayey in nature. Amongst the different types of alluvium, the soft clayey alluvial deposit, which may behave in a way similar to marine deposit, require particular attention when designing marine works. When soft clay-rich deposits are loaded, the increase in external shear and vertical stresses will cause deformation leading to stability problems on marine structures if the deformation is excessive. In a reclamation site, the presence of soft deposits may induce long-term settlement to the land if they are not properly removed or treated. These soft deposits can normally be handled without much difficulty if they are not present at great depth. However, if they exist below 35 mpd, they may not be easily removed because of the limitation on the dredging depth of typical marine plant. The situation will be even more complicated if highly variable or interbedded clayey soils are present, as they must be clearly
- 10 - located to avoid potential slip surfaces in the foundation and long-term uneven settlement of the seawalls and breakwaters. In addition, the interbedded soft layers in alluvium will pose difficulties in soil treatment works, such as the installation of band drains in reclamation, as the installation mandrel may not be able to penetrate easily through the firmer alluvium to the lower soft layers. If the settlement due to the soft layers is excessive, costly preboring works may be required to allow treatment of these layers. Hence, this study concentrates on identifying these difficult ground conditions and recommends appropriate ground investigation guidelines for such conditions. For piled structures, difficult ground conditions are normally associated with the presence of obstructions in the soil strata and deep bedrock level. Ground investigations under these conditions can normally be catered for by the guidelines given in Geoguide 2 (CESD, 1987) and Practice Note for Authorized Persons and Registered Structural Engineers No. 66 issued by Buildings Department (BD, 2002). If cavernous marble is encountered, reference can be made to Practice Note for Authorized Persons and Registered Structural Engineers No. 161 (BD, 1993). Although relevant ground conditions are also indicated in this study, the associated ground investigation guidelines are not repeated here. 2.3 Study Stages The study was carried out in two stages : Stage One Stage One mainly involved searching for existing borehole records and plotting of subsoil profiles. The Comprehensive Ground Information System (CGIS) developed and maintained by the Jockey Club Research and Information Centre for Landslip Prevention and Land Development of the University of Hong Kong was used to search for the required information. This system provides a one-stop service platform for users to access the geotechnical information database through the internet. It is equipped with geographical information system features to help users to obtain the data, thus saving the time and effort in retrieving data manually from GIU. However, the required borehole records in the study area were not all available in CGIS. About 60% of the records were retrieved from the ground investigation reports in GIU.
- 11 - The following tasks were involved in Stage One : Review and summarize borehole data within the study area. Plot subsoil profiles, including the soil types, strata and results of standard penetration tests/vane shear tests. Correlate soil data and identify relationships. Based on the borehole descriptions together with the in-situ test results, it is considered adequate to assess the nature and general properties of the subsoil. As such, reference to laboratory testing results was not made in this study. Stage Two Stage Two involved refining the findings of Stage One. The work of Stage Two included additional borehole search to fill in data gaps. Besides, areas with thick clayey/silty deposits and interbedded soft clayey/silty material at great depth (e.g. below 35 mpd) as well as large variation of borehole information within short distance were studied with care to see if difficult ground conditions were present. After completing these tasks, guidelines on planning marine ground investigation at difficult ground locations were prepared. In this study, the ground conditions are classified into the following three categories : (1) Ground conditions that are difficult These are difficult ground conditions with very thick marine deposit and/or extensive/thick interbedded soft alluvium below 35 mpd as indicated by available ground investigation information. (2) Ground conditions that are likely to be difficult These are ground conditions where marine deposit and/or soft alluvium are present down to/below about 35 mpd or where the soil strata are variable but the available ground investigation information is not sufficient to conclude that they are difficult. (3) Ground conditions that are unlikely to be difficult These are ground conditions where signs of difficult ground conditions such as marine deposit and/or soft alluvium at great depth are not present.
- 12-2.4 Subsoil Profiles The following were adopted when preparing the subsoil profiles : Interpretation of subsoil conditions was not carried out for area where boreholes are very far apart. Section lines were selected to pass through as many boreholes as possible and parallel to the coastline where possible. Boreholes on land close to the shore or located in reclaimed areas were also included if marine borehole data were not adequate. To avoid misleading interpretation of the results, data of these boreholes above the seabed level at the section line of the subsoil profile were not plotted unless indicated otherwise. In such cases, only the length of the boreholes are shown in the figures. No adjustment of borehole information was made when plotting the subsoil profiles. Boreholes are indicated with offset distance when such distance is greater than 200 m from the section line of the profile. The description of the type of soil layer (marine/alluvial deposit) and rock, and whether the soil is soft or soft/firm, is based on logger s interpretation in the borehole logs. The standard penetration test and vane shear test results are marked on the subsoil profiles for readers reference of the in-situ strength of the soil. Where the positions of the soil interfaces cannot be ascertained from the borehole information, the interfaces are marked with?. The borehole number as shown in the subsoil profiles is the same as that shown in relevant ground investigation report.
- 13-3. KOWLOON AND NEW TERRITORIES 3.1 Nim Wan to Castle Peak Bay The study area zoned under Nim Wan to Castle Peak Bay includes coastal areas located from Nim Wan, Lan Kok Tsui, Lung Kwu Tan, Tap Shek Kok, Pillar Point and Castle Peak Bay. The coverage and the subsoil profile of this region are shown in Figures 3a and 3b. ` The seabed in this region is generally covered by a layer of very soft, brownish grey/greenish grey marine deposit ranging from 3 m to 20 m thick. Below the marine deposit, greenish brown and grey alluvium is recorded with thickness ranging from 0 m to 18 m. Interbedded soft silty/clayey alluvial deposit with variable thickness is present in some coastal locations. Granite with various degrees of decomposition is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 5 mpd to 35 mpd. Nim Wan to Lan Kok Tsui (Between Points A-D of Figure 3b) A layer of marine deposit about 5 m to 16 m thick is present below the seabed at the sea frontage of Nim Wan. The marine deposit generally lies above 25 mpd. Underlying the marine deposit is sandy alluvium about 5 m to 18 m thick. Interbedded soft silty material of 2 m to 6 m thick is present within the sandy alluvial layer. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 15 mpd to 26 mpd. In this area, the marine deposit and the interbedded soft alluvium are not very thick. No difficult ground conditions are identified. Lung Kwu Tan to Tap Shek Kok (Between Points D-G of Figure 3b) Marine deposit ranging from 3 m to 20 m thick is present around the coastal areas of Lung Kwu Tan to Tap Shek Kok. The marine deposit generally lies above 30 mpd. The underlying alluvium is about 0 m to 10 m thick. However, soft alluvium about 1 m thick is recorded at about 1300 m away from the sea frontage of Lung Kwu Tan. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 5 mpd to 35 mpd.
- 14 - As the marine deposit generally lies above 30 mpd and only some localized soft alluvial pockets of limited size are recorded in this area, no difficult ground conditions are identified. Pillar Point to Castle Peak Bay (Between Points G-L of Figure 3b) The marine deposit at the coastline from Pillar Point to Castle Peak Bay is about 0 m to 12 m thick. The marine deposit generally lies above 20 mpd. A layer of alluvial deposit of thickness less than 11 m is found underneath the marine deposit. Interbedded silty or clayey soft alluvial deposit with thickness of 2 m to 5 m is recorded between 10 mpd and 26 mpd at Pillar Point and Castle Peak Bay. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 12 mpd to 28 mpd. Isolated soft alluvial pockets are present along the coastline at Pillar Point and Castle Peak Bay. As they lie almost immediately underneath the marine deposit and are not located at great depth, no difficult ground conditions are anticipated. Summary of Subsoil Profiles Marine deposit Nim Wan to Lan Kok Tsui 5 16 m thick, generally above 25 mpd Lung Kwu Tan to Tap Shek Kok 3 20 m thick, generally above 30 mpd Pillar Point to Castle Peak Bay 0 12 m thick, generally above 20 mpd Alluvial deposit 5 18 m thick 0 10 m thick 0 11 m thick Decomposed rock from 15 mpd to 26 mpd from 5 mpd to 35 mpd from 12 mpd to 28 mpd Presence of soft alluvium Difficult ground conditions 2 to 6 m thick interbedded soft alluvium is found at depth less than 22 mpd Some interbedded soft alluvium is found at 24 mpd at about 1300 m away from the sea frontage of Lung Kwu Tan 2 to 5 m thick interbedded soft alluvium is present at 10 mpd to 26 mpd Unlikely Unlikely Unlikely
- 15-3.2 Siu Lam to Sham Tseng The study area zoned under Siu Lam to Sham Tseng includes coastal areas located from Siu Lam, Tai Lam Kok, Tai Lam Chung, Tsing Lung Tau and Sham Tseng. The coverage and the subsoil profile of this region are shown in Figures 4a and 4b. The seabed in this region is generally covered by a layer of very soft, brownish grey/greenish grey marine deposit of thickness ranging from 0 m to 16 m. Below the marine deposit, greenish brown and grey alluvium is recorded with thickness ranging from 0 m to 17 m. Interbedded soft silty/clayey alluvial deposit with variable thickness occasionally exists in some coastal locations. Granite with various degrees of decomposition is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 10 mpd to 30 mpd. In addition, small amount of igneous rock named basalt (dyke rock) is recorded near Tsing Lung Tau to Sham Tseng. Siu Lam to Tai Lam Kok (Between Points A-C of Figure 4b) A layer of marine deposit of thickness ranging from 0 m to 14 m is present below the seabed at Siu Lam to Tai Lam Kok. The marine deposit generally lies above 18 mpd. Underlying the marine deposit is a layer of sandy alluvium of variable thickness from 0 m to 17 m. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 15 mpd to 30 mpd. Despite the presence of a small, soft alluvial pocket of 2 m thick at Tai Lam Kok sea frontage at 14 mpd, no difficult ground conditions are anticipated in this area. Tai Lam Chung to Tsing Lung Tau (Between Points C-E of Figure 4b) Marine deposit ranging from 1 m to 8 m thick is present near the sea frontage of Tai Lam Chung to Tsing Lung Tau. The marine deposit generally lies above 20 mpd. The underlying alluvium is about 2 m thick and sandy in nature. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 10 mpd to 25 mpd. No soft alluvium is found from the limited borehole information within this area.
- 16 - Sham Tseng (Between Points E-G of Figure 4b) The marine deposit at Sham Tseng is about 1 m to 16 m thick. A layer of alluvial deposit about 2 m thick is found underneath. The marine deposit generally lies above 26 mpd. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 15 mpd to 30 mpd. Despite the presence of a 3 m thick soft alluvial pocket at 18 mpd near Point F, no difficult ground conditions are identified in this area. Summary of Subsoil Profiles Marine deposit Siu Lam to Tai Lam Kok Tai Lam Chung to Tsing Lung Tau 0 14 m thick, generally above 18 mpd 1 8 m thick, generally above 20 mpd Sham Tseng 1 16 m thick, generally above 26 mpd Alluvial deposit 0 17 m thick 0 2 m thick 0 2 m thick Decomposed rock from 15 mpd to 30 mpd from 10 mpd to 25 mpd from 15 mpd to 30 mpd Presence of soft alluvium Difficult ground conditions A soft alluvial pocket/lens is found at 14 mpd Not found A soft alluvial pocket/lens is found at 18 mpd Unlikely Unlikely Unlikely
- 17-3.3 Ting Kau to Stonecutters Island The study area zoned under Ting Kau to Stonecutters Island includes coastal areas located from Ting Kau to the southwest of Stonecutters Island via Rambler Channel. The coverage and the subsoil profile of this region are shown in Figures 5a and 5b. The seabed in this region is generally covered by a layer of very soft, brownish grey/greenish grey marine deposit of thickness ranging from 2 m to 22 m. Below the marine deposit, greenish brown and grey alluvium is recorded with thickness ranging from 0 m to 34 m. Interbedded soft silty/clayey alluvial deposit with variable thickness is present in some coastal locations. Granite with various degrees of decomposition is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 13 mpd to 44 mpd. Ting Kau to Tsuen Wan (Between Points A-C of Figure 5b) At Ting Kau, a layer of marine deposit of thickness ranging from 10 m to 22 m is present below the seabed. The marine deposit generally lies above 30 mpd but can reach 32 mpd locally. Underlying the marine deposit is a layer of thin sandy alluvium ranging from 0 m to 2 m thick. No silty/clayey alluvium is recorded. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 20 mpd to 35 mpd. At Tuen Wan, marine deposit ranging from 2 m to 10 m thick is present below the seabed. The marine deposit generally lies above 20 mpd. The underlying alluvium is less than 4 m thick. Small pockets of soft alluvial deposit of thickness 1 m to 3 m are found between 10 mpd and 20 mpd. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 13 mpd to 25 mpd. Although the marine deposit is relatively thick in this area, it does not extend down to very deep level. Hence, the marine deposit can be removed or treated by typical marine plant without much difficulty. Only small soft alluvial pockets are recorded locally. Hence, no difficult ground conditions are anticipated in this area. Kwai Chung (Between Points B-D of Figure 5b) The thickness of marine deposit at the sea frontage of Kwai Chung ranges from 4 m to
- 18-20 m. It generally lies above 30 mpd but can reach 37 mpd locally near the eastern end of Kwai Chung. The underlying alluvium ranges from 1 m to 34 m thick. Soft interbedded clayey material of 2 m to 8 m thick is present at the upper part of the alluvial layer. In general, the soft alluvium is located between 19 mpd to 38 mpd. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 16 mpd to 44 mpd. Thick marine deposit down to 37 mpd and soft alluvial pockets at about 38 mpd are found locally near the eastern end of Kwai Chung. In view of the depth of such soft deposits, difficulty may be encountered in dredging or soil treatment in this area. For the rest of Kwai Chung area, no significant difficulty is anticipated. Stonecutters Island (Between Points D-E of Figure 5b) The marine deposit near the southwestern part of Stonecutters Island is about 9 m to 19 m thick. It generally lies above 30 mpd. A layer of 1 m to 12 m thick alluvial deposit with 2 m to 3 m thick soft silty/clayey pockets is found underneath. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 20 mpd to 44 mpd. As the soft alluvial pockets generally lie above 30 mpd and just below the marine deposit, they can be removed or treated by typical marine plant without much difficulty. No difficult ground conditions are anticipated in this area.
- 19 - Summary of Subsoil Profiles Marine deposit Ting Kau Tsuen Wan Kwai Chung Stonecutters Island 10 22 m thick, generally above 30 mpd but can locally reach 32 mpd 2 10 m thick, generally above 20 mpd 4 20 m thick, generally above 30 mpd but can locally reach 37 mpd 9 19 m thick, generally above 30 mpd Alluvial deposit 0 2 m thick 0 4 m thick 1 34 m thick 1 12 m thick Decomposed rock Presence of soft alluvium/thick marine deposit Difficult ground conditions from 20 mpd to 35 mpd Thick marine deposit is present but not at great depth from 13 mpd to 25 mpd 1 to 3 m thick interbeded soft alluvium is found between 10 mpd and 20 mpd from 16 mpd to 44 mpd 2 to 8 m thick interbedded soft alluvium is found between 19 mpd and 38 mpd, thick marine deposit down to 37 mpd is present locally Unlikely Unlikely Likely near the eastern end of Kwai Chung area from 20 mpd to 44 mpd Thick marine deposit and some soft alluvial pockets are present but not at great depth; they generally lie above 30 mpd Unlikely
- 20-3.4 Stonecutters Island to Tsim Sha Tsui (West) The study area zoned under Stonecutters Island to Tsim Sha Tsui (West) includes coastal areas located from the south of Stonecutters Island, Yau Ma Tei to Tsim Sha Tsui Ferry Pier. The coverage and the subsoil profile of this region are shown in Figures 6a and 6b. The seabed in this region is generally covered by a layer of very soft, brownish grey/greenish grey marine deposit of thickness ranging from 3 m to 13 m. Below the marine deposit, greenish brown and grey alluvium is recorded with thickness ranging from 1 m to 46 m. Interbedded soft silty/clayey alluvial deposit with variable thickness is present in some coastal locations. Granite with various degrees of decomposition is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 12 mpd to 36 mpd. Stonecutters Island (Between Points A-C of Figure 6b) A layer of marine deposit ranging from 4 m to 13 m thick is present below the seabed at the south of Stonecutters Island. The marine deposit generally lies above 20 mpd. Underlying the marine deposit is sandy alluvium of variable thickness from 1 m to 12 m that blended with silty material locally. Interbedded soft alluvial deposit, about 1 m to 3 m thick, is present between 10 mpd to 30 mpd. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 15 mpd to 35 mpd. As the soft alluvial pockets are not present at great depth, they can be readily removed or treated by typical marine plant. It is unlikely that there will be any difficulty in design and construction of marine works in this area. Yau Ma Tei (Between Points D-E of Figure 6b) Marine deposit ranging from 3 m to 10 m thick is present underneath the seabed at Yau Ma Tei. It generally lies above 15 mpd. The underlying alluvium is 10 m to 46 m thick. No record of soft alluvial deposit is found. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite generally lies below 25 mpd.
- 21 - Since the available ground investigation records are limited, a very conclusive remark of the geotechnical conditions of this area cannot be given. However, the limited borehole information does not reveal any sign of difficult ground conditions. Tsim Sha Tsui (West) (Between Points E-G of Figure 6b) The marine deposit at Tsim Sha Tsui (West) is about 3 m to 12 m thick, underlain by a layer of 2 m to 21 m thick alluvial deposit. It generally lies above 20 mpd. Interbedded silty or clayey soft alluvial deposit is found from 15 mpd to 26 mpd. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 12 mpd to 36 mpd. The soft alluvial deposit in this region is rather thick and embedded with sandy material. Localized soft pockets are also present. Although such deposit is located above 26 mpd, difficult ground conditions may be anticipated in view of the variable distribution of soft material within the alluvial layer. The intermediate firmer sandy material may pose some difficulty to the installation of band drains inside the soft alluvium. Summary of Subsoil Profiles Marine deposit Stonecutters Island Yau Ma Tei Tsim Sha Tsui (West) 4 13 m thick, generally above 20 mpd 3 10 m thick, generally above 15 mpd 3 12 m thick, generally above 20 mpd Alluvial deposit 1 12 m thick 10 46 m thick 2 21 m thick Decomposed rock from 15 mpd to 35 mpd Top level lies below 25 mpd generally from 12 mpd to 36 mpd Presence of soft alluvium Difficult ground conditions Soft alluvium is found between 10 mpd and 30 mpd Unlikely No soft alluvium is identified from the limited information The limited borehole information does not reveal any sign of difficult ground conditions Variable soft alluvium is found between 15 mpd and 26 mpd Likely
- 22-3.5 Tsim Sha Tsui to To Kwa Wan The study area zoned under Tsim Sha Tsui to To Kwa Wan includes coastal areas located at the sea frontage of the Hong Kong Cultural Centre, Tsim Sha Tsui East, Hung Hom and To Kwa Wan. The coverage and the subsoil profile of this region are shown in Figures 7a and 7b. The seabed in this region is generally covered by a layer of very soft, brownish grey/greenish grey marine deposit of thickness ranging from 3 m to 9 m. Below the marine deposit, greenish brown and grey alluvium is recorded and its thickness ranges from 0 m to 22 m. Interbedded soft silty/clayey alluvial deposit with variable thickness is present at the coastline of Hung Hom and To Kwa Wan. Granite with various degrees of decomposition is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 4 mpd to 42 mpd. Tsim Sha Tsui to Tsim Sha Tsui East (Between Points A-D of Figure 7b) Only limited ground investigation information is available in this area. Generally, the marine deposit is about 3 m to 8 m thick and lies above 24 mpd. A thin layer of alluvium between 0 m and 2 m thick is present underneath the marine deposit. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 4 mpd to 30 mpd. Hung Hom (Between Points D-E of Figure 7b) A layer of marine deposit, about 4 m to 9 m thick, is present below the seabed at Hung Hom. The marine deposit generally lies above 20 mpd. Underlying the marine deposit is sandy alluvium of variable thickness from 0 m to 22 m that blended with silty material locally. Interbedded soft alluvial deposit from 1 m to 6 m thick is found between 20 mpd to 34 mpd. A layer of 5m thick soft clayey alluvium is recorded at 38 mpd to 42 mpd at about 650 m from the sea frontage of the Tai Wan Shan Park. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 25 mpd to 42 mpd. The 5 m thick soft clayey alluvium present at 38 mpd to 42 mpd in front of the Tai Wan Shan Park may pose construction difficulty due to the limitation on the working depth of typical marine plant. Additional ground investigation is required to determine the extent of this
- 23 - layer of soft alluvium. For reclamation, the soft interbedded alluvial layers between 20 mpd and 34 mpd may not be easily reached by band drains due to the presence of stiff alluvium above. Preboring may be required for the installation of band drains through the stiffer layers. To Kwa Wan (Between Points D-E of Figure 7b) The thickness of marine deposit present near To Kwa Wan and Ma Tau Kok ranges from 3 m to 6 m. The marine deposit generally lies above 14 mpd. The underlying alluvium is between 15 m to 20 m thick. Soft pockets are found in the alluvium. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 26 mpd to 32 mpd. The soft pockets from 16 mpd to 28 mpd may pose concern on instability and longterm settlement if they are not properly removed and treated. Although they are not present at great depth, difficult ground conditions may be anticipated in view of the variability of the alluvial layer at the adjacent Hung Hom area. Additional ground investigation is required to determine the variability of the soil layers in this area. Summary of Subsoil Profiles Marine deposit Tsim Sha Tsui to Tsim Sha Tsui East 3 8 m thick, generally above 24 mpd Hung Hom 4 9 m thick, generally above 20 mpd To Kwa Wan 3 6 m thick, generally above 14 mpd Alluvial deposit 0 2 m thick 0 22 m thick 15 20 m thick Decomposed rock from 4 mpd to 30 mpd Presence of soft alluvium Difficult ground conditions No soft alluvium is identified from the limited information from 25 mpd to 42 mpd Thick interbedded soft alluvium is present between 20 mpd and 42 mpd from 26 mpd to 32 mpd Pockets/lenses of soft alluvium are found between 16 mpd and 28 mpd; soil strata may be variable if more ground investigation data are available Unlikely Likely Likely
- 24-3.6 Kai Tak to Lei Yue Mun The study area zoned under Kai Tak to Lei Yue Mun includes coastal areas located from the runway of ex-kai Tak Airport to Lei Yue Mun. The coverage and the subsoil profile of this region are shown in Figures 8a and 8b. The seabed in this region is generally covered by a layer of very soft, brownish grey/greenish grey marine deposit of thickness ranging from 2 m to 9 m. Below the marine deposit, greenish brown and grey alluvium is recorded and its thickness ranges from 2 m to 31 m. Interbedded soft silty/clayey alluvial deposit with variable thickness is present in some coastal locations. Granite with various degrees of decomposition is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 20 mpd to 44 mpd. South of Ex-Kai Tak Airport Runway (Between Points A-B of Figure 8b) Marine deposit ranging from 2 m to 9 m thick is present below the seabed at the southern side of the Kai Tak Airport Runway. It generally lies above 20 mpd. Underlying the marine deposit is sandy alluvium of 9 m to 31 m thick blended with silty material locally. Interbedded soft alluvial deposit about 2 m to 6 m thick is found between 10 mpd to 29 mpd. In addition, a low SPT N value of 7 is recorded at about 36 mpd in one borehole (MBH-05) in the western portion of the area. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 20 mpd to 44 mpd. In the eastern portion of the area, no major soft layer is identified. In the western portion, the soft alluvium above 30 mpd can readily be removed or treated using typical marine plant. However, as soft alluvium is recorded at about 36 mpd in one borehole, additional ground investigation is required to ascertain the extent of soft alluvium around this location. Kowloon Bay to Kwun Tong (Between Points C-D of Figure 8b) Marine deposit of thickness ranging from 3 m to 9 m is present below the seabed of Kowloon Bay, Ngau Tau Kok and Kwun Tong. It generally lies above 15 mpd. The underlying alluvium is about 5 m to 18 m thick. However, some soft alluvial pockets about 1 m to 3 m thick are found between 10 mpd and 22 mpd near the coastline of Kowloon Bay/Ngau
- 25 - Tau Kok. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 20 mpd to 32 mpd. No significant interbedded soft alluvium is found in this area. However, attention should be paid to the stability of existing marine structures around the coastline of Kowloon Bay and Ngau Tau Kok if the soft pockets are dredged away. Cha Kwo Ling to Lei Yue Mun (Between Points C-D of Figure 8b) The marine deposit between Cha Kwo Ling and Lei Yue Mun is about 2 m to 7 m thick. It generally lies above 24 mpd. A layer of 2 m to 19 m thick alluvium is found underneath. Interbedded silty or clayey soft alluvium with thickness of 1 m to 3 m is recorded at about 15 mpd to 25 mpd in a few locations, and a SPT N value of 7 is recorded in one borehole (Borehole MBH-15) at 29 mpd. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 25 mpd to 34 mpd. No extensive interbedded soft alluvium is found. The ground conditions are unlikely to be difficult. Summary of Subsoil Profiles Marine deposit South of Ex-Kai Tak Airport Runway 2 9 m thick, generally above 20 mpd Kowloon Bay to Kwun Tong 3 9 m thick, generally above 15 mpd Cha Kwo Ling to Lei Yue Mun 2 7 m thick, generally above 24 mpd Alluvial deposit 9 31 m thick 5 18 m thick 2 19 m thick Decomposed rock from 20 mpd to 44 mpd from 20 mpd to 32 mpd from 25 mpd to 34 mpd Presence of soft alluvium Difficult ground conditions Interbedded soft alluvium is found between 10 mpd and 29 mpd with low SPT N value recorded at 36 mpd Soft alluvial pockets are found between 10 mpd to 22 mpd. Dredging may affect stability of existing structures Interbedded soft alluvium is found between 15 mpd and 29 mpd Likely Likely Unlikely
- 26-3.7 Lei Yue Mun to Tseung Kwan O The study area zoned under Lei Yue Mun to Tseung Kwan O includes coastal areas located around Tiu Keng Leng, Tseung Kwan O Town Centre, Siu Chik Sha and Fat Tong Chau. The coverage and the subsoil profile of this region are shown in Figures 9a and 9b. The seabed in this region is generally covered by a layer of very soft, greenish grey marine deposit of thickness ranging from 2 m to 26 m. Below the marine deposit, greenish brown and grey alluvium of thickness ranging from 2 m to 30 m is found. Interbedded soft silty/clayey alluvial deposit with variable thickness is present in many locations. Volcanic tuff with various degrees of decomposition is found underneath the alluvium. The interface between the alluvium and the decomposed tuff varies from 14 mpd to 55 mpd. Lei Yue Mun to Tiu Keng Leng (Between Points A-B of Figure 9b) The seabed at Lei Yue Mun to Tiu Keng Leng is covered by a layer of 2 m to 15 m thick marine deposit underlain by alluvium of thickness from 4 m to 20 m. The marine deposit generally lies above 22 mpd. Completely decomposed tuff is found underneath the alluvium. The interface between the alluvium and the decomposed tuff varies from 14 mpd to 42 mpd. Soft silty/clayey alluvial pockets are found between 12 mpd and 38 mpd. Attention should be paid to the presence of the soft alluvial deposit between 34 mpd and 38 mpd, as the removal and treatment of such deposit may be difficult due to the limitation on the dredging depth of typical marine plant and the presence of the overlying stiff alluvium. Tseung Kwan O Town Centre (Around Point B of Figure 9b) The marine deposit located near Tseung Kwan O Town Centre is around 11 m to 19 m thick. It generally lies above 24 mpd. A layer of 11 m to 23 m thick alluvial deposit is found underneath. Interbedded silty or clayey soft alluvial deposit with thickness of 2 m to 10 m is recorded between 20 mpd and 44 mpd. Completely decomposed tuff is found underneath the alluvium. The interface between the alluvium and the decomposed tuff varies from 32 mpd to 46 mpd. Attention should be paid to the presence of the soft alluvial deposit between 40 mpd and 44 mpd, as the removal or treatment of such deposit will likely be difficult due to the
- 27 - limitation on the dredging depth of typical marine plant and the presence of the overlying stiff alluvium. Siu Chik Sha (Between Points B-D of Figure 9b) Marine deposit of 10 m to 15 m thick is present at Siu Chik Sha. It lies above 25 mpd generally. The underlying alluvium is 2 m to 26 m thick. Interbedded soft alluvium of thickness from about 1 m to 5 m is found between 18 mpd to 42 mpd. Completely decomposed tuff is found underneath the alluvium. The interface between the alluvium and the decomposed tuff varies from 23 mpd to 44 mpd. There are numerous soft alluvial pockets/lenses in this area. Marine works should be planned with care as the soft alluvium, if not properly removed or treated, will cause instability and settlement problems to the development. Removal or treatment of the soft pockets/lenses will be difficult, because of their wide coverage and presence at great depth. In addition, thin layers of soft alluvial deposit may be displaced due to additional loading imposed by new infrastructures. This may affect the ground conditions of adjacent areas. In this regard, additional ground investigation works should be undertaken, as far as practicable, to determine the exact locations and properties of the soft alluvium. Fat Tong Chau (Between Points D-E of Figure 9b) A layer of marine deposit of thickness ranging from 5 m to 26 m is present below the seabed near Fat Tong Chau. The marine deposit generally lies above 30 mpd but can extend down to 35 mpd locally. Underlying the marine deposit is sandy alluvium of variable thickness from 7 m to 30 m that blended with silty material locally. However, numerous soft alluvial pockets are found between 24 mpd and 51 mpd. In addition, a localized layer of marine deposit of thickness ranging from 3 m to 13 m is present between 26 mpd and 40 mpd within the alluvium near borehole PBH 1 and V2. Completely decomposed tuff is found underneath the alluvium. The interface between the alluvium and the decomposed tuff varies from 34 mpd to 55 mpd. The soil strata of the area are characterized by numerous interbedded sandy and clayey alluvium. The widespread distribution of soft layers at various depths down to 51 mpd will pose great difficulty in the design and construction of marine structures and reclamation due to concerns on stability as well as long-term settlement. In view of the large variation of soil strata
- 28 - in the region, dredged samples should be taken during construction and verified against the soil profile adopted in design. If the discrepancies are great, the design should be carefully reviewed. Detailed ground investigation is required to obtain sufficient information to address the soil variability. Summary of Subsoil Conditions Marine deposit Lei Yue Mun to Tiu Keng Leng 2 15 m thick, generally above 22 mpd Tseung Kwan O Town Centre 11 19 m thick, generally above 24 mpd Siu Chik Sha 10 15 m thick, generally above 25 mpd Fat Tong Chau 5 26 m thick, generally above 30 mpd but can be present down to 35 mpd locally Alluvial deposit 4 20 m thick 11 23 m thick 2 26 m thick 7 30 m thick Decomposed rock from 14 mpd to 42 mpd from 32 mpd to 46 mpd from 23 mpd to 44 mpd from 34 mpd to 55 mpd Presence of soft alluvium/thick marine deposit Difficult ground conditions Soft alluvial pockets are found from 34 mpd to 38 mpd 2 to 10 m thick interbedded soft alluvium is found from 20 mpd to 44 mpd Numerous interbedded soft alluvial pockets of thickness 1 to 5 m are found from 18 mpd to 42 mpd Very thick marine deposit is present. The alluvial layer is interbedded heavily with soft clayey material from 24 mpd to 51 mpd, and contains a localized layer of 3 to 13 m thick marine deposit from 26 mpd to 40 mpd Likely Likely Yes Yes
- 29-3.8 Tai Po, Shatin and Ma On Shan The study area includes the coastal areas adjacent to Tai Po Hoi and Sha Tin Hoi including Tai Po Kau, Pak Shek Kok, Ma Liu Shui, Sha Tin Sewage Treatment Works on the one side of Sha Tin Hoi, and Ma On Shan and Wu Kai Sha on the other. The coverage and the subsoil profiles of this region are shown in Figures 10a and 10b. It should be noted that a large number of the boreholes shown in the figure are land-based boreholes, which are somewhat offset from the section line, and the seabed level shown are the inferred seabed level along the section. The seabed is generally covered by marine deposit about 5 m to 15 m thick. A thick layer of fill to replace in-situ soft material is commonly found on reclaimed lands. Depending on the location, a layer of alluvium with thickness ranging from 1 m to 30 m is found below the fill and marine deposit. Soft alluvial pockets/lenses are present at some locations. Igneous, pyroclastic and sedimentary rock such as granite, tuff and sandstone/siltstone with various degrees of decomposition are present. The top level of the decomposed rock varies from 13 mpd to 45 mpd. Debris flow deposit and cavernous marble are also found in Ma On Shan. Tai Po (Between Points A-F and Points P-R of Figure 10b) A layer of marine deposit ranging from 5 m to 10 m thick is found below the seabed near the Tolo Highway from Tai Po Kau to Ma Liu Shui. The marine deposit lies above 15 mpd generally. The underlying alluvium is about 1 m to 5 m thick, except that at a few locations the thickness of the alluvium can vary from 10 m to 20 m. Several soft pockets of alluvium about 1 m to 2 m thick are recorded at about 10 mpd underneath the marine deposit. The top level of completely decomposed tuff, or completely decomposed sandstone/siltstone near Sha Tin Hoi, generally lies below 15 mpd. In view of relatively shallow marine deposit and the small isolated soft pockets at relatively shallow depth, no difficult ground conditions are identified. Shatin (Between Points F-H of Figure 10b) The marine deposit from Ma Liu Shui to Sha Tin Sewage Treatment Plant is about 10 m to 15 m thick and lies generally above 20 mpd. Near the Shing Mun River, a layer of alluvium of about 5 m to 10 m thick is found underneath the marine deposit. Completely decomposed granite and tuff is found underneath the alluvium. The interface between the alluvium and the decomposed rock varies from 13 mpd to 30 mpd. No difficult ground conditions are identified in this area.
- 30 - Ma On Shan (Between Points H-N of Figure 10b) The marine deposit in this area is about 10 m to 15 m thick, and is underlain by alluvium about 10 m to 30 m thick. It generally lies above 16 mpd. Small, soft alluvial pockets/lenses about 1 m to 3 m thick are found locally at about 15 mpd. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 13 mpd to 45 mpd. Debris flow deposit and cavernous marble are present at some locations. The problematic areas are likely confined to those regions near the scheduled area of Ma On Shan (Area Number 4 of the Scheduled Areas in the Buildings Ordinance) in which debris flow deposit and cavernous marble can be found. Ground investigation should be focused into identifying the locations of these materials and their engineering properties when involving sensitive structures or deep foundations. Summary of Subsoil Profiles Tai Po Shatin Ma On Shan Fill Varies Varies Varies Marine deposit 5 10 m thick, generally above 15 mpd 10 15 m thick, generally above 20 mpd 10 15 m thick, generally above 16 mpd Alluvial deposit 1 5 m thick, but can be up to 10 20 m thick in some locations 5 10 m thick 10 30 m thick Decomposed rock Presence of soft alluvium Difficult ground conditions Top level below 15 mpd Soft alluvial pockets are occasionally found at about 10 mpd from 13 mpd to 30 mpd Not found from 13 mpd to 45 mpd Soft alluvial pockets are occasionally found at about 15 mpd Unlikely Unlikely Unlikely
- 31-4. HONG KONG ISLAND AND ISLANDS 4.1 Kennedy Town to Central The study area zoned under Kennedy Town to Central includes coastal areas located from Kennedy Town, Sai Ying Pun to Central. The coverage and the subsoil profile of this region are shown in Figures 11a and 11b. The seabed in this region is generally covered by a layer of very soft, greenish, brownish or pinkish grey marine deposit of thickness ranging from 0 m to 28 m. Below the marine deposit, greenish brown and grey alluvium is recorded and its maximum thickness is about 10 m. The top level of decomposed rock ranges from 15 mpd to 51 mpd. Granite is the main bedrock although rhyolite/tuff with various degrees of decomposition is present in some locations. Kennedy Town (Between Points A-C and Points L-M of Figure 11b) The thickness of marine deposit near Kennedy Town is from 7 m to 28 m. The bottom level of the marine deposit generally lies above 44 mpd and locally above 20 mpd. The underlying alluvium is relatively thin with maximum thickness of about 7 m. The top level of the decomposed rock varies from 18 mpd to 51 mpd. The rock stratum includes granite, basalt, tuff and rhyolite. Particular attention should be paid to the presence of thick marine deposit down to 44 mpd in this region as such depth exceeds the dredging limit of typical marine plant. Soil treatment works such as installation of band drains at great depth should be carefully evaluated and monitored in the design and construction stage to ensure their effectiveness in speeding up the consolidation. Sai Ying Pun (Between Points C-G and Points N-P of Figure 11b) The thickness of marine deposit in front of Sai Ying Pun coastline ranges from 2 m to 23 m. The marine deposit generally lies above 30 mpd but can reach 34 mpd locally. An underlying layer of alluvium about 0 m to 9 m thick is present. The top level of completely decomposed granite, rhyolite or tuff occurs at 25mPD to 35mPD. At the western end of Sai Ying Pun, the decomposed rock is generally volcanic in nature.
- 32 - The marine deposit down to 34 mpd is rather thick in this region. Although the records show that they are still within the working depth of typical marine plant, additional ground investigation is required to ascertain their extent before detailed design. Central (Between Points G-K of Figure 11b) Marine deposit of thickness ranging from 0 m to 13 m is found below the seabed near Central. It generally lies above 25 mpd. The underlying alluvium is mainly silty sand from 0 m to 10 m thick. Locally, soft alluvial pockets/lenses are found between 15 mpd and 24 mpd. The alluvium is generally classified as firm. Completely decomposed granite is found underneath the alluvium. The interface between the alluvium and the decomposed granite varies from 15 mpd to 33 mpd. The marine deposit and alluvial layer are not very thick, and is unlikely to pose difficult engineering problems for typical marine works. Summary of Subsoil Profiles Marine deposit Kennedy Town Sai Ying Pun Central 7 28 m thick, generally lies above 44 mpd 2 23 m thick, generally above 30 mpd but can reach 34 mpd locally 0 13 m thick, generally above 25 mpd Alluvial deposit 0 7 m thick 0 9 m thick 0 10 m thick Decomposed rock from 18 mpd to 51 mpd from 25 mpd to 35 mpd from 15 mpd to 33 mpd Presence of soft alluvium/thick marine deposit Difficult ground conditions Very thick marine deposit is present at great depth Thick marine deposit is present but not at very great depth Soft alluvial pockets are found occasionally from 15 mpd to 24 mpd Yes Likely Unlikely
- 33-4.2 Central (East) to Causeway Bay The study area zoned under Central (East) to Causeway Bay includes coastal areas located from East of Central Reclamation Area to Causeway Bay. The coverage and the subsoil profile of this region are shown in Figures 12a and 12b. The seabed in this region is generally covered by a layer of very soft, greenish, brownish or pinkish grey marine deposit of thickness ranging from 2 m to 9 m. Below the marine deposit, greenish brown and grey alluvium from 0 m to about 26 m thick is recorded. The top level of decomposed rock layer ranges from 9 mpd to 33 mpd. The bedrock in the region is granite. Central (East) to Wan Chai (Between Points A-C and Points G-H of Figure 12b) The thickness of marine deposit from Central (East) to Wan Chai ranges from 2 m to 9 m. The underlying alluvium is about 6 m thick. The marine deposit generally lies above 18 mpd. Soft alluvial pockets are found between 14 mpd and 20 mpd. The top level of completely decomposed granite varies from 18 mpd to 24 mpd. Although pockets of soft material are found within the alluvial layer, they are generally limited in size and depth. The top level of the decomposed rock is rather shallow. Difficult ground conditions are not anticipated in this area. Causeway Bay (Between Points C-F of Figure 12b) A layer of 2 m to 9 m thick marine deposit generally above 14 mpd is recorded below the seabed near Causeway Bay. The alluvium underneath ranges from 0 m to 26 m thick, of which sandy and silty materials are interbedded. At some locations, soft or soft/firm alluvium with thickness up to 7 m is present down to 22 mpd. In general, the alluvium is classified as firm. The top level of the underlying decomposed granite ranges from 9 mpd to 33 mpd. Although pockets of soft material are found within the alluvial layer, they are generally limited in size and depth. Difficult ground conditions are not anticipated in this area.