Blasting in Hong Kong, A Review of Current Blasting Assessment Requirements

Transcription

1 Blasting in Hong Kong, A Review of Current Blasting Assessment Requirements 19 June 2008 Guy Bridges Maunsell Geotechnical Services 1

2 Brief Introduction to the control of explosives in Hong Kong Basic principles of blasting Current guidance documents relating to Blasting Assessments Contents of a Blasting Assessment Typical methods adopted for the Assessment 2

3 The control of explosives in Hong Kong Regulated under the Dangerous Goods Ordinance Cap. 295 Controlling authority is the Commissioner of Mines Applies to the manufacture, storage, transport and use of explosives Covers Category 1 Dangerous Goods (Explosives) 3

4 Category 1 Dangerous Goods Divided into 8 classes based on chemical compositions of the explosives: Class 1 Gunpowder Class 2 Nitrate mixture Class 3 Nitro-compound Class 4 Chlorate mixture Class 5 Fulminate (detonators) Class 6 Ammunition Class 7 Firework (aerial shells, fire crackers) Class 8 Other Explosives typically in Class 2, Detonators typically in Class 6 4

5 UN Classification Class 1: Explosives Division 1.1: Substances and articles which have a mass explosion hazard Division 1.2: Substances and articles which have a projection hazard but not a mass explosion hazard Division 1.3: Substances and articles which have a fire hazard and either a minor blast hazard or a minor projection hazard or both, but not a mass explosion hazard Division 1.4: Substances and articles which present no significant hazard Division 1.5: Very insensitive substances which have a mass explosion hazard Division 1.6: Extremely insensitive articles which do not have a mass explosion hazard Explosives typically in Division 1.1, Detonators typically in Division 1.4 5

6 Basic Principles of Blasting Typically use Emulsion or ANFO for bulk blasting Consist of a mixture of fuel oil and oxidiser Detonation causes explosive oxidation of the fuel Explosion creates both shockwave and large volumes of gas Shock wave cracks rock Gas heaves the rock 6

7 Basic Principles of Blasting Shockwave creates high compressive force in the rock Rock next to blasthole is crushed Rock further out is fractured Ground vibration beyond the fractured zone 7

8 Basic Principles of Blasting Confine energy within blasthole to get efficient transfer into rockmass Compressive wave transmitted through rockmass Compressive wave reflected as tensile wave from free air surfaces Free surface could be a blast face or ground surface Gas will penetrate the cracks to enlarge them and heave the rock 8

9 Basic Principles of Blasting Different explosive products have different characteristics High rate of detonation gives greater shockwave Good for massive brittle rock Large gas volume produces greater heaving effect Good for fractured rockmass 9

10 Basic Principles of Blasting Detonators required to initiate the explosion Electric, Non-electric and Electronic detonators Electric Detonators Activated by electric current 30 different time delays Connect all detonators in sequence All detonators initiated at the same time Non-electric Detonators Activated by shockwave within plastic tube Use surface connectors to complete the circuit Detonators initiated in sequence due to surface delays Electronic Detonators Activated by electric current Programmeable delay Connect all detonators in sequence Unlimited number of delays 10

11 Basic Principles of Blasting Larger quantity of explosive creates greater shock wave Shockwave from many blastholes may overlap Minimum 8milliseconds delay between each detonation Electric detonator timing is fixed at 25ms intervals, maximum 30 intervals Non-electric detonators can adjust surface delays to create a large number of intervals Electronic detonators can programe any number of delays 11

12 Current Guidance Documents Mines Division Guidance Note on How to Apply for a Blasting Permit Project Administration Handbook for Civil Engineering Works, 2006 Edition Buildings Department Practice Note for Authorized Persons and Registered Structural Engineers 178 GEO Circular No. 27 Geotechnical Control of Blasting Give advice on what to include in the Assessment 12

13 Current Guidance Documents Mines Division Practice Note No. 1 GEO Report No. 15 Assessment of Stability of Slopes Subjected to Blasting Vibration GEO REPORT No. 102 A Study of the effects of Blasting Vibration on Green Concrete General Specification for Civil Engineering Works Vol. 1 WSD Departmental Instruction No MTRC M&W Specification for Structures and Utilities Give advice on typical methods and limits to be applied 13

14 Mines Division Guidance Note on How to Apply for a Blasting Permit: Regulation 46 of the Dangerous Goods (General) Regulations, no person shall carry out any blasting without the permission of the Commissioner of Mines Pre-Contract Blasting Assessment prepared by Designer and reviewed by CEDD (GEO and Mines Division) and BD (for private projects) Contractor prepares updated Blasting Assessment and submits to CEDD (GEO and Mines Division) and BD (for private projects) for approval Contractor prepares his Method Statement for Blasting Contractors Blasting Assessment and Method Statement are required as supporting documents for the Contractors application for a Blasting Licence and Blasting Permit 14

15 At project design stage 15

16 At project construction stage 16

17 Item 2.2 (d) - site plan of scale 1:500 or 1:1000 showing: iii) all sensitive receivers, including streets, structures, foundations, railways, public utilities, water mains, drains, sewers, gas mains and other services, geotechnical features such as slopes, retaining walls, boulders, tunnels, caverns etc. within a plan radius of 150m for tunnel/shaft blasting or 300m for open cast 17

18 Contents of a Blasting Assessment 18

19 Contents of a Blasting Assessment a) Site plans clearly indicating the proposed areas of blasting and locations of all sensitive receivers. Topographic survey information Location of existing buildings and foundations Utility layout plans Location of geotechnical features The proposed blasting area 19

20 Contents of a Blasting Assessment b) A report containing the results of a study, including the site topography, geology, ground, groundwater and surface water conditions, and the physical site constraints, sensitive receivers and site history. Geological plan Geological sections Review of borehole information API 20

21 Contents of a Blasting Assessment c) A report containing examination of the conditions of the sensitive receivers on and adjacent to the site. Inspection records for buildings and structures Identification of any unusually sensitive structures Inspection records for geotechnical features Identification of any unstable features or records of past instability. 21

22 Contents of a Blasting Assessment d) A report containing an assessment of the effects of blasting works to demonstrate that the proposed blasting would not cause any injury to persons or damage to property and sensitive receivers. Risk of damage to structures due to ground vibrations Risk of damage or injury to structures or people due to flyrock Risk of damage or injury to structures or people due to noise 22

23 Contents of a Blasting Assessment e) Proposals of preventive measures to be carried out for sensitive receivers, if considered necessary. Strengthening works Erection of protective barriers Provision of facilities to test the effects of the blasting 23

24 Contents of a Blasting Assessment f) A list of the action limits to be specified... Based on the identified allowable ground vibration or air overpressure limits Typically adopt AAA criteria Identifies requirements for investigation and reporting at each implementation level Typically sets stop blasting criteria 24

25 Contents of a Blasting Assessment f) documentary evidence of consultation and agreement, where appropriate, with the key stakeholders... Correspondence with utility companies Correspondence with Government Departments Confirmation on the feasibility of proposed road closures Correspondence with owners of particularly sensitive or affected facilities (eg: schools, hospitals) 25

26 Contents of a Blasting Assessment g) An outline of the blast design to demonstrate that the blasting works could be safely carried out Practicality of blasting within identified limits Requires knowledge of the mechanism of blasting Poor blast design will increase risk 26

27 Contents of a Blasting Assessment h) A document setting out the safety management system to be employed... Site supervision arrangements Qualification / Experience of staff Checking procedure for carrying out the blast Contingency plans for misfire or cancelled blast 27

28 Contents of a Blasting Assessment i) Particulars of the site inspections, surveys and monitoring to be carried out... including plans showing the locations of the monitoring stations, the performance criteria and the action limits. Identify the location of monitoring points Identify the vibration and/or air overpressure limits at each monitoring point Identify the criteria for carrying out monitoring at each location Include detail of the monitoring installation 28

29 Contents of a Blasting Assessment j) Proposals of protective and precautionary measures to be taken... Surface cover on blast area Cages to cover blast area Screens to surround blast area Evacuation areas Road closures Special measures to include in the blast design 29

30 Contents of a Blasting Assessment k) Proposals of the arrangement for delivery of explosives to the site... Planned duration of blasting Frequency and timing of blasting Quantity of explosive required at each stage Suitable location for delivery and handover of the explosives 30

31 Contents of a Blasting Assessment Explosive is stored in a Government Magazine on Lantau Mines Division can provide one delivery per day to each construction site An assessment of required explosive quantities is required to show the construction programme could be met Prediction of advance rate Must match the available explosives supply 31

32 Contents of a Blasting Assessment l) If an on-site explosive store is considered necessary, a report containing an assessment of its feasibility and proposed arrangement. Required storage capacity Layout of the magazine site Clearance distances to nearby public areas 32

33 Effects of Blasting Ground Vibration Flyrock Noise (air overpressure) 33

34 Ground Vibration Ground vibration is proportional to the scaled distance (m/kg -0.5 ) PPV = K(R/W B ) A PPV peak particle velocity (mm/s) K rock transmission constant R distance between blast and measuring point (m) W maximum charge weight per delay interval (kg) B charge exponent, normally taken as 0.5 A attenuation exponent. 34

35 Ground Vibration PPV = K(R/W B ) A Prediction is based on historical data analysed by Li and Ng K is typically 644 A is typically 1.22 Above limits are based on 84% confidence limit Location K A Blasting Type Lam Tin Surface Shek Lei Surface Lamma Power Station Surface Castle Peak Power Station B Surface Kornhill surface blasting Surface Tsing Ma Bridge Surface Jordan Valley Surface Average Kornhill underground blasting Tunnel Cheung Ching Tunnel Eagles Nest North Tunnel Eagles Nest South Tunnel Eagles Nest Vent tunnel Tunnel Average Mines all 35

36 Ground Vibration Constants may be adjusted subject to trial blasting allowing evaluation of more accurate site specific constants Can vary depending on the type of blasting carried out Large scatter due to variations in ground response give a normal distribution Comparison of project attenuation constants, 10 kg explosive Predicted PPV Distance Mines Lam Tin Shek Lei Lamma Castle Peak Kornhill (1) Kornhill (2) Cheung Ching Tsing Ma Bridge Jordan Valley Eagles Nest North Eagles Nest South Eagles Nest Vent 36

37 Ground Vibration Ground vibration can be measured as acceleration, velocity or displacement. Vibration considered to be a sinusoidal oscillation Relationship between each parameter depends on the frequency Vector sum is the sum of three orthogonal measurements at any instant in time Peak Particle Velocity(PPV) adopted to determine control limits 37

38 Ground Vibration Blast Vibration frequency can be over 100Hz High frequency vibration rapidly attenuated over distance. Lower frequencies dominate at larger distance from the blast Structures have natural response frequencies at which damage may occur Typical frequency adopted for assessment is 30Hz 38

39 Ground Vibration High frequencies less damaging to structures Typical frequency adopted for assessment is 30Hz Blast vibration levels for various damage levels from BS Hz 15 Hz 40 Hz Cosmetic Minor damage Major Damage

40 Flyrock Throwing of rock through the air from the blast area All blasting must generate movement of rock in order to crack and loosen it Dangerous when throw is violent and uncontrolled Can be either from the front face of a surface blast, or top surface blast area Guidelines for flyrock hazard are related to the diameter of the blasthole 40

41 Flyrock Hong Kong Guidelines 210m for 89 mm holes 150m for 76mm holes May need to increase the distance to allow for differences in elevation Apply for normal blasting with stemming 41

42 Air Overpressure Is related to the quantity of explosive and distance Can directional effect for surface blasting May be affected by atmospheric conditions Blast design details especially stemming and detonator cord arrangement can be significant Audible noise measured as dba, causes discomfort to humans Air pressure measured as dbl, causes damage to structures Current target limit is 120dBL 42

43 Safe Limits for Ground Vibration Structures based on form of construction and condition of building Extensive studies carried out by USBM in the US Limits specified in terms of particle velocity Basic guidelines given the General Specification for Civil Engineering Works 43

44 Safe Limits for Ground Vibration Geotechnical features based on stability assessment Requires detailed knowledge of ground conditions and structural form Will vary depending on groundwater conditions Assessment based on GEO Report 15 Retaining walls assessed using Earthquake design codes 44

45 Safe Limits for Ground Vibration Geotechnical features, need to consider: Slip failure within soil slopes Sliding and wedge failure in rock slopes Sliding and overturning failure of retaining structures Instability of boulders Very often a lack of data for older features 45

46 Safe Limits for Ground Vibration Utilities based on nature of utility Basic guidelines given the General Specification for Civil Engineering Works Cable joints and major substations more sensitive Some utilities may require particular assessment Most utilities and equipment is constructed to withstand earthquakes 46

47 Preventive Measures Ground cover - Gunny Sacks - Heavy wire mesh - Sand Bags Blast Cages - Steel I-beam structure covered with heavy wire mesh - Weigh 5 to 6 tonnes - Cover all blastholes 47

48 Preventive Measures Vertical Screens - Steel I-beam structure with heavy wire mesh - 12 to 14m high - Up to 10 tonnes each Blast Doors - Steel I-beam frame with steel plate - Sound proofing and rubber mats - Hanging wire mesh 48

49 Blast Design Aim to reduce Maximum Instantaneous Charge Use separate detonator delays for each blasthole Actual sequencing of each blasthole timing can affect safety Timing of each detonation not a factor in vibration calculation 49

50 Environmental Permit Blasting noise not covered under EPD regulations Designated projects that plan to use blasting may be required to carry out Hazard to Life Assessment Risk due to site storage of Explosives Risk due to off site transport of explosives 50

51 Explosives supply Blasting noise not covered under EPD regulations Designated projects that plan to use blasting may be required to carry out Hazard to Life Assessment Risk due to site storage of Explosives Risk due to off site transport of explosives 51

52 Conclusion The requirements for carrying out Blasting Assessments are specified Content of the Assessment has been expanded recently Established procedures are conservative Procedure for approval may be lengthy Blasting may still be the best option 52

53 Thank you 53