NB/T 11454-2023Specification for risk management of freezing sinking method (English PDF)

Foreword

This document was issued on 28 December 2023 by the National Energy Administration of the PRC and takes effect on 28 June 2024.

It is a NB/T standard: recommended rather than compulsory, but it is the text a Chinese reviewer applies when assessing a submission.

It is classified under ICS 73.020, Chinese classification D 15.

This document is drafted in accordance with the rules given in GB/T 1.1-2020 Directives for standardization - Part 1: Rules for the structure and drafting of standardizing documents.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. The issuing body of this document shall not be held responsible for identifying any or all such patent rights.

This document was proposed by the China National Coal Association.

This document is under the jurisdiction of the Coal Industry Standardization Technical Committee for Special Equipment of Coal Mines.

Drafting organizations of this document: Beijing China Coal Mine Engineering Co., Ltd.; China Electronics Engineering Design Institute Co., Ltd.; Third Engineering Division of China Coal No. 5 Construction Co., Ltd.; Henan Guolong Mine Construction Co., Ltd.; China Coal Special Shaft Sinking Co., Ltd.; Dadi Engineering Development (Group) Co., Ltd.; Henan Yingchuang Mine Engineering Co., Ltd.; Anhui Huaiwu Engineering Project Management Co., Ltd.; Zhongyun International Engineering Co., Ltd.; Kailuan Construction (Group) Co., Ltd.; Beijing Meikelian Applied Technology Research Institute; Henan Haodi Engineering Consulting Co., Ltd.

Chief drafters of this document: Li Gongzhou, Jiang Jun, Gao Wei, Wang Zhiyang, Liu Jiezhong, Wang Shengli, Pan Jianzhong, Liu Qingli, Wei Guoqiang, Wang Guoqi, Zhang Bujun, Zhang Daohai, Gao Gangrong, Guo Zhongqiang, Chen Yuankun, Cui Jianjun, Zhao Yuming, Chen Zhangqing, Zhao Yong, Cao Dadian, Li Xiaowei, Wang Yi, Zeng Fanyi, Ma Yuefeng, Song Wei, Li Dongsheng, Jiang Xinchun, Hou Jinping, Xin Peihong, Liu Wenmin.

This standard is an energy industry standard of the People's Republic of China, issued by the National Energy Administration on 2023-12-28, implemented from 2024-06-28, and published by Standards Press of China.

Introduction

This document provides technical reference for risk management of shaft sinking by the freezing method. The safety requirements for shaft sinking by the freezing method are subject to laws and regulations, mandatory national standards and other relevant provisions.

1 Scope

NB/T 11454-2023 governs risk management for mine shafts sunk by the artificial ground freezing method. Where a shaft must pass through deep water-bearing sand, gravel and clay, the ground around it is frozen by circulating chilled brine through a ring of boreholes, creating a wall of frozen soil inside which the shaft is excavated and lined. It works, but its failure modes are severe: a gap in the frozen wall lets water and sand rush into the shaft, a freeze pipe breaks, the lining cracks under frost heave or thaw settlement. The specification sets the scope, the normative references and a full set of defined terms, then the basic requirements of the risk management system and the process it follows - identification, analysis, evaluation, response and monitoring - with the responsibilities of owner, designer, contractor and supervisor. It then applies that process stage by stage: the design of the shaft lining, the design of the freezing scheme with its borehole layout and frozen wall thickness, the refrigeration plant and its reliability, the drilling of the freeze holes and their deviation control, the active freezing period and the verification that the wall has closed, the excavation and lining inside the frozen ground, and the grouting that follows thawing. Risk grades, emergency response and the records of the risk management process complete the standard.

This document specifies the risk management requirements for shaft sinking projects by the freezing method using a brine refrigeration system, covering shaft lining design, freezing scheme design, freezing system and power supply system design, freezing hole drilling construction, refrigeration plant installation construction, the freezing process, shaft excavation and lining construction, and grouting construction between shaft linings, as well as the requirements for special risk control measures for shaft sinking by the freezing method.

This document is applicable to risk management in the design, construction and management of shaft sinking by the freezing method in mining engineering. Freezing method construction in other engineering fields may refer to it.

2 Normative references

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

GB/T 29639 Guidelines for enterprises to develop emergency response plans for production safety accidents

GB/T 39963 Technical specification for C80 to C100 concrete applied to the shaft lining of vertical shafts sunk by the freezing method

GB 50016 Code for fire protection design of buildings

GB 50072 Code for design of cold store

GB 50148 Code for construction and acceptance of power transformers, oil-immersed reactors and mutual inductors in electric equipment installation engineering

GB 50169 Code for construction and acceptance of grounding connections in electric equipment installation engineering

GB 50171 Code for construction and acceptance of switchboards, cabinets and secondary circuit wiring in electric equipment installation engineering

GB 50204 Standard for acceptance of construction quality of concrete structures

GB 50213 Code for quality acceptance of shaft and roadway engineering in coal mines

GB 50231 General code for construction and acceptance of mechanical equipment installation engineering

GB 50235 Code for construction of industrial metallic piping

GB 50236 Code for construction of field equipment, industrial pipe welding engineering

GB 50274 Code for construction and acceptance of refrigerating and air separation equipment installation engineering

GB 50384 Code for design of vertical shafts and chambers of coal mines

GB 50415 Code for design of inclined shafts and chambers of coal mines

GB 50511 Code for construction of shaft and roadway engineering in coal mines

GB/T 51277 Standard for construction and quality acceptance of the freezing method for mine vertical shafts

GB 55008 General code for concrete structures

AQ 1029 Specification for the management of safety monitoring systems and detecting instruments in coal mines

AQ 1083 Safety specification for coal mine construction

AQ 7015 Safety code for ammonia refrigeration enterprises

JGJ 46 Technical code for safety of temporary electricity use on construction sites

JGJ 107 Technical specification for mechanical splicing of steel reinforcing bars

NB/T 10053 Technical specification for freezing construction of inclined shafts in coal mines

SBJ 12 Code for construction and acceptance of ammonia refrigeration system installation engineering

TSG 21 Supervision regulation on safety technology for stationary pressure vessels

3 Terms and definitions

For the purposes of this document, the following terms and definitions apply.

3.1 ground freezing method: A construction method in which, before construction of a shaft or underground structure, the water-bearing strata around the shaft or underground structure are frozen by artificial refrigeration to form a frozen wall that bears temporary loads, isolates water and meets the safety needs of construction, after which excavation and lining of the shaft or structure are carried out under the protection of the frozen wall.

3.2 refrigeration plant (also: freezing station; refrigerating station): The site near the proposed shaft or underground structure where refrigeration equipment and facilities are concentrated. Note: A refrigeration plant is mainly composed of the refrigeration system, brine system, cooling water system and power supply system.

3.3 freezing systems: The collective name for the refrigeration system, brine system and cooling water system of a refrigeration plant. Note: Refrigerants commonly used in the refrigeration system are mainly ammonia and freon; the secondary refrigerant commonly used in the brine system is calcium chloride solution.

3.4 freezing process: The process in which the strata are frozen through the operation of the freezing system, power supply system and monitoring and detection system.

3.5 risk: The combination of the likelihood of an adverse event or accident and its loss.

3.6 risk factors: The various subjective and objective potential causes leading to risk events.

3.7 risk management: A series of management and coordination activities for engineering construction risks, including risk definition, risk identification, risk analysis, risk evaluation and risk control.

3.8 risk definition: The process of analyzing and setting the objectives and objects of engineering construction risk management, establishing the risk management grading standards and dividing the risk assessment units.

3.9 risk identification (also: risk recognition): The process of investigating the types, locations, timing and causes of potential risks in engineering construction and screening and classifying them.

3.10 risk analysis: Defining, identifying and estimating risks by qualitative or quantitative analysis methods.

3.11 risk acceptance criteria: The grade standards for analyzing and deciding on risks and judging whether a risk is acceptable.

3.12 risk evaluation: The process of grading engineering risks, ranking risks and making risk decisions according to the established risk grading standards and acceptance criteria.

3.13 risk assessment: The process of identifying, analyzing and evaluating risks and deciding on the hazard of the risks and their treatment measures.

3.14 risk control: The process of formulating risk treatment measures and emergency plans and implementing risk monitoring, tracking and recording.

3.15 loss of life and personal injury: Health hazards, bodily injuries, deaths and the like suffered by all kinds of persons after an engineering construction risk occurs.

3.16 economic loss: The collective term for all direct or indirect costs incurred by the project as a result of engineering construction risks.

3.17 project delay: The failure to complete construction on the planned date due to engineering construction risks, causing extension of the construction period or an unreasonable advance of the construction period.

3.18 third party: Other organizations or persons in the surrounding environment or social groups who are not directly involved in the engineering construction but are affected by the construction activities.

3.19 harm to surroundings: Pollution of the natural environment and damage to the surrounding area and adjacent buildings (structures) caused by engineering construction risks.

3.20 harm to society: Abnormal emergency relocation, negative social impact or instability, loss of government credibility and the like caused by engineering construction risks.

3.21 hazard: Unsafe conditions of workplaces, equipment and facilities, unsafe acts of persons, and management deficiencies. Note: Hazards are the direct cause of safety accidents. Where risk control measures fail or are not implemented, hazards are formed.

4 Basic requirements

4.1 Risks of shaft sinking projects by the freezing method should be classified according to risk loss. Risk types may be divided into: a) risk of loss of life and personal injury; b) risk of economic loss; c) risk of project delay; d) risk of harm to surroundings; e) risk of harm to society.

4.2 According to the content and implementation process of shaft sinking projects by the freezing method, risk management may be divided into: a) risk management of shaft lining design; b) risk management of freezing scheme design; c) risk management of freezing system and power supply system design; d) risk management of freezing hole drilling construction; e) risk management of refrigeration plant installation construction; f) risk management of the freezing process; g) risk management of shaft excavation and lining construction; h) risk management of grouting construction between shaft linings.

4.3 Risk management of shaft sinking projects by the freezing method shall follow the procedure shown in Figure 1 (Risk management procedure flow): risk definition, risk identification, risk analysis, risk evaluation and risk control, supported throughout by risk communication and exchange and closed by hazard elimination and risk monitoring, which feed back into the cycle.

4.4 Risk definition for shaft sinking projects by the freezing method shall follow the basic principles below.

4.4 a) Risk assessment units shall be divided by stage and by objective.

4.4 b) Shaft sinking projects by the freezing method involve the owner, design unit, survey unit, supervision unit, construction contractor, suppliers and other construction-related parties; risk communication and exchange during the implementation of construction risk management shall be strengthened. For high-risk projects, research institutions and technical guidance units shall also be invited to participate.

4.4 c) The allocation of risk management responsibilities shall adhere to the consistency of responsibility, authority and interests.

4.5 Risk identification for shaft sinking projects by the freezing method may include steps such as risk classification, determination of participants, collection of relevant data, risk screening and preparation of the risk identification report. Risk identification may use methods such as the checklist method and the expert survey method, and may be completed with reference to Annex A. For the checklist method and expert survey method, see the explanations in GB 50652-2011. After risk identification is completed, a risk identification report shall be prepared, stating the methods used, the scope of identification, the participants and the risk list; the risk list may be completed with reference to Annex B.

4.6 Risk analysis methods for shaft sinking projects by the freezing method may include the qualitative analysis method, quantitative analysis method and comprehensive analysis method. For these methods, see the explanations in GB 50652-2011.

4.7 For shaft sinking projects by the freezing method, a risk assessment of the hazard of risks shall be made through risk definition, risk identification, risk analysis and risk evaluation, and risk control and treatment measures shall be proposed for decision.

4.8 Risk grades for shaft sinking projects by the freezing method shall be divided according to the grade standards for the likelihood of risk occurrence and the severity of risk loss. The specific grade standards shall comply with Table 1 and Table 2.

Table 1 Grade standard for likelihood of risk occurrence: grade 1, frequent, probability or frequency P not less than 0.1; grade 2, likely, P not less than 0.01 and less than 0.1; grade 3, occasional, P not less than 0.001 and less than 0.01; grade 4, rare, P not less than 0.0001 and less than 0.001; grade 5, improbable, P less than 0.0001.

Table 2 Grade standard for severity of risk loss, grade A (very serious): minor injuries to 11 or more persons, or serious injuries to 6 to 9 persons, or death of 2 or more persons; economic loss M greater than 10 million yuan; project delay T greater than 6 months; environmental impact over a very large scope with relatively serious pollution or damage to the surrounding ecological environment; very serious social impact, or emergency relocation of 301 or more persons.

Table 2, grade B (serious): minor injuries to 6 to 10 persons, or serious injuries to 2 to 5 persons, or death of 1 person; economic loss greater than 5 million yuan and not more than 10 million yuan; project delay greater than 3 months and not more than 6 months; environmental impact over a large scope with pollution or damage to the ecological environment within the area; serious social impact, or emergency relocation of 101 to 300 persons.

Table 2, grade C (considerable): minor injuries to 3 to 5 persons, or serious injury to 1 person; economic loss greater than 1 million yuan and not more than 5 million yuan; project delay greater than 1 month and not more than 3 months; environmental impact over a small scope with slight pollution or damage to the ecological environment of the adjacent area; relatively serious social impact, or emergency relocation of 31 to 100 persons.

Table 2, grade D (slight): minor injuries to 1 to 2 persons; economic loss not more than 1 million yuan; project delay not more than 1 month; environmental impact over a small scope with minor pollution or damage to the ecological environment of the construction area; slight social impact, or emergency relocation of 30 persons. Note: The number of casualties includes construction personnel and third parties. Economic loss includes losses of the project itself and of third parties.

4.9 Risk evaluation of shaft sinking projects by the freezing method should be divided into four grades: Grade I (extremely major), Grade II (major), Grade III (relatively major) and Grade IV (general). The matrix of risk grade standards shall comply with Table 3.

Table 3 Matrix of risk evaluation grade standards: likelihood grade 1 (frequent) gives Grade I for severity A, B and C and Grade II for D; grade 2 (likely) gives Grade I for A and B, Grade II for C and Grade III for D; grade 3 (occasional) gives Grade I for A, Grade II for B and Grade III for C and D; grade 4 (rare) gives Grade II for A, Grade III for B and C and Grade IV for D; grade 5 (improbable) gives Grade III for A and B and Grade IV for C and D.

4.10 Different risk control measures shall be taken for risks of different evaluation grades. The acceptance criteria for risks of each evaluation grade shall comply with Table 4.

Table 4 Risk acceptance criteria: Grade I (extremely major) is unacceptable; risk control measures shall be taken to reduce the risk, at least to an acceptable or conditionally acceptable level, and a risk early-warning and emergency response plan shall be prepared, or the relevant scheme shall be corrected or adjusted, or the risk avoided. Grade II (major) is conditionally acceptable; risk management shall be implemented to reduce the risk, the cost of reducing the risk should be less than the loss after the risk occurs, and risk prevention and monitoring shall be implemented and risk treatment measures formulated. Grade III (relatively major) is acceptable; risk status monitoring and routine review and inspection should be strengthened. Grade IV (general) is negligible; risk status monitoring and routine review and inspection should be carried out.

4.11 The persons responsible for risk evaluation shall be clearly defined. Basic data shall be collected, a work plan and assessment strategy shall be formulated according to the risk evaluation grade standards and acceptance criteria, risk evaluation methods shall be proposed, and a risk evaluation report shall be prepared.

4.12 Risk control methods shall be based on the planning principles of risk control and selected in light of actual conditions, including setting objectives, indicators and plans, implementing relevant procedure documents, preparing work instructions and carrying out the necessary monitoring of risk factors.

4.13 For major and extremely major risks, special risk demonstration shall be conducted, and a risk monitoring plan, special emergency plan and risk treatment measures shall be prepared.

4.14 For vertical shaft sinking projects by the freezing method in complex strata such as alluvium deeper than 400 m or soft water-rich rock deeper than 600 m, inclined shaft sinking projects by the freezing method with a vertical freezing depth greater than 100 m, or freezing projects with considerable technical difficulty under complex conditions such as high groundwater velocity or special engineering structures, expert review and demonstration shall be organized to confirm the freezing scheme, special risk demonstration shall be carried out, and a risk monitoring plan, special emergency plan and risk treatment measures shall be prepared.

4.15 a) Risk control measures shall adhere to the principle of 'people first, safety first, prevention first and comprehensive management', and economical, feasible and proactive treatment measures shall be adopted to reduce or lower risks.

4.15 b) The risk control plan shall be organized by the owner, with the joint participation of all construction parties, and prepared according to the risk treatment countermeasures.

5 Risk management of shaft lining design

5.1.1 Shaft lining design shall comply with the requirements of GB 55008, GB 50384 and NB/T 10053; where C80 to C100 concrete is used for vertical shaft linings, GB/T 39963 shall also be met.

5.1.2 Shaft lining design shall include: a) determination of the shaft lining structural form; b) determination of the excavation and lining depth of the frozen section and the level of the wall base; c) determination of shaft lining loads; d) determination of the maximum strength grade of shaft lining support concrete; e) determination of the levels of inner lining segments, the structure and levels of yieldable shaft lining devices, and the level of the junction between frozen and unfrozen sections; f) design calculation of the shaft lining structure and the height and strength of the wall seat; g) design calculation of shaft lining reinforcement; h) checking design of internal forces and bearing capacity of the shaft lining.

5.1.3 Shaft lining design shall collect the following data: a) preliminary mine design, mine field development layout, shaft design features and other data; b) the geological report of the shaft check hole; c) geological and hydrogeological data and construction records of similar shafts already built around the mine.

5.1.4 Shaft lining design shall fully reflect safety, scientific soundness, economy and advancement.

5.1.5 Risk management in the shaft lining design stage shall include strict implementation of design qualification and design process management regulations and of the requirements of current technical specifications, identification, analysis and assessment of risk factors in the design, determination of the engineering risk grade, and formulation of risk control measures.

5.1.6 The main objective of risk management in the shaft lining design stage is to identify and analyze risk factors in the shaft lining structural design and construction, study risk countermeasures, make the shaft lining structure meet the safety requirements of construction and long-term service, and create the basic conditions for reducing construction and operation risks and keeping risks controllable.

5.2.1 The engineering geological, hydrogeological and other geological data obtained for the shaft are inaccurate; the geological data of the shaft check hole have not gone through the review and approval procedures.

5.2.2 The selected shaft lining structural form is unreasonable or infeasible and does not comply with GB 50384, GB 50415 and NB/T 10053. The structural form shall be reasonably determined according to the engineering geological and hydrogeological conditions of the shaft, the shaft diameter and function, the vertical additional force borne by the shaft, the freezing depth of the shaft and other data.

5.2.3 The determined excavation and lining depth of the frozen section is unreasonable; the freezing depth and the excavation and lining depth do not comply with GB 50384, GB 50511 and NB/T 10053. The shaft lining support depth shall meet the safety requirements of shaft sinking in the frozen section.

5.2.4 The determined load calculation values do not meet the requirements of GB 50384, GB/T 39963 and NB/T 10053. The standard radial and vertical load values of the inner and outer shaft linings shall be reasonably determined according to the geological conditions of the shaft.

5.2.5 The calculation methods for shaft lining strength and thickness are unscientific or unreliable and fail to meet the requirements of GB 50384, GB 55008, GB/T 39963 and NB/T 10053.

5.2.6 The materials and material strength grades selected for the shaft lining structure do not take into account the technical feasibility of local implementation.

5.2.7 The setting of shaft lining segment levels, the structure and levels of yieldable shaft lining devices, and the level of the junction between frozen and unfrozen sections do not meet reasonable, safe, economical and feasible requirements.

5.2.8 The grouting design between the inner and outer shaft linings and the grouting timing are unreasonable and do not comply with GB 50384 and GB 50511.

5.2.9 The position and thickness of polystyrene foam boards and polyethylene boards are unreasonable or infeasible. The physical and mechanical performance indexes of the polystyrene foam boards and polyethylene boards selected for the shaft lining structure shall comply with GB 50384 and the technical requirements.

5.2.10 During shaft construction, where the exposed geological conditions differ considerably from those used in the shaft lining design or the construction scheme changes, the rationality and structural strength safety of the existing shaft lining structure have not been analyzed.

5.2.11 The risks to the shaft lining from factors such as excavation segment height, shaft sidewall temperature, sidewall exposure time, frozen wall displacement, expansion rate of expansive clay layers and early strength development of concrete during construction have not been analyzed.

5.3.1 Evaluation of risk factors in shaft lining design for shaft sinking by the freezing method shall carry out hazard analysis, grading and ranking of shaft lining design risks according to Clause 4.

5.3.2 According to the complexity of the engineering geological and hydrogeological conditions exposed by the shaft and the shaft characteristics, and in combination with the application of similar domestic projects, qualitative analysis and evaluation shall be made of the technical difficulties, construction difficulty and operation risks of the shaft lining design.

5.3.3 a) Major deviation of geological data: confirm whether the freezing excavation depth affects the setting of the wall base and wall seat.

5.3.3 b) Reliability analysis of the shaft lining strength design calculation: confirm whether the load calculation values are reliable and their influence on the design strength of the shaft lining, including whether the determination of standard load values for the frozen shaft lining and the strength and thickness calculation parameters are reliable, and whether the internal force and bearing capacity calculations are clear and verifiable.

5.3.3 c) Based on the excavation segment height, sidewall temperature, sidewall exposure time, frozen wall displacement, expansion rate of expansive clay layers, early strength of concrete, laying thickness of polystyrene foam boards, laying method of polyethylene boards and grouting between inner and outer linings during construction, qualitatively analyze and evaluate the risks of shaft lining implementation.

5.3.4 Qualitative or quantitative analysis and evaluation shall be made of the important technical difficulties in the shaft lining design and the engineering risks that may exist.

5.4.1 The mine geological and hydrological data required for the engineering design, such as the geological, hydrological and frozen soil test data provided by the shaft check hole, shall meet the design requirements; where they do not meet the required design depth, they shall be supplemented. The design unit shall analyze the reliability of the data provided and, where necessary, collect and compare such data from adjacent projects to further improve the completeness and reliability of the data.

5.4.2 The shaft lining structural form and design scheme shall be reviewed to improve their rationality and reliability.

5.4.3 Engineering materials selected in the design shall comply with GB 50384 and the technical requirements; where C80 to C100 concrete is used for the shaft lining, GB/T 39963 and its technical requirements shall be met.

5.4.4 The calculation loads, structural safety factor values and calculation methods selected for vertical shaft lining structural design shall comply with GB 50384, GB 55008, GB/T 39963 and their technical requirements; the determination of design calculation loads for the whole and each part of an inclined shaft lining shall meet NB/T 10053.

5.4.5 New materials, technologies and processes selected in the design shall undergo the necessary testing and demonstration research; important technical issues such as new shaft lining structural forms and design theories shall be subject to special research, testing and review, and a risk control plan shall be formulated.

5.4.6 After the design drawings are completed, the design unit shall give technical disclosure of the construction drawings to the owner, the construction contractor and other parties; the owner shall organize technical experts to review the construction drawings, which shall be revised and completed according to the technical disclosure and expert review opinions.

5.4.7 For projects with relatively complex engineering geological conditions, during construction the construction scheme and process shall be adjusted in time according to the monitoring of frozen wall displacement and changes in shaft lining loading, and the timing and level of inner lining casting and the shaft lining structural strength shall be revised and the construction drawing design improved in time.

5.4.8 During construction, where the hydrological and geological conditions exposed by the shaft differ from those used in the shaft design or the construction scheme is adjusted, the construction drawing design shall be revised and improved in time according to actual site conditions.

6 Risk management of freezing scheme design

6.1.1 Freezing scheme design for vertical shafts shall comply with GB 50511; freezing scheme design for deep alluvium thicker than 200 m shall also comply with GB/T 39963. Freezing scheme design for inclined shafts shall comply with NB/T 10053.

6.1.2 Freezing scheme design shall include: a) determination of the freezing depth; b) determination of the freezing control levels; c) determination of the calculated strength of frozen soil; d) design of the freezing brine temperature; e) design of the sidewall temperature at the freezing control levels and the effective average temperature of the frozen wall; f) design of the frozen wall thickness; g) design of the number of rings, ring diameter layout and depth of each ring of freezing holes for vertical shafts, and design of the layout and depth of freezing holes for inclined shafts; h) determination of freezing pipe specifications; i) design of temperature measuring holes and water level observation holes; j) design of the required refrigeration capacity.

6.1.3 Freezing scheme design shall collect the following data: a) geographical location of the mine field, overview of the mine design, shaft characteristics and shaft lining structural design; b) geological, hydrological and frozen soil test data from the shaft check hole; c) the construction scheme and method for shaft excavation and lining, including the excavation and lining schedule, the construction machinery and the overall capability of the construction contractor; d) the professional level of managers, system building and management level of the owner, supervision unit and construction contractor.

6.1.4 Freezing scheme design shall fully reflect safety, scientific soundness, economy and advancement.

6.1.5 Risk management in the freezing scheme design stage shall include strict implementation of design and design process management regulations and of current technical specifications, identification, analysis and assessment of risk factors in the design, determination of the engineering risk grade, and formulation of risk control measures.

6.1.6 The main objective of risk management in the freezing scheme design stage is to identify and analyze risk factors in the freezing scheme design and construction, study risk countermeasures, and formulate a safe and efficient freezing scheme that ensures construction and operation safety, creating the basic conditions for reducing construction risks and keeping risks controllable.

6.2.1 The hydrogeological data obtained are unreliable, including non-standard operation by technical personnel during geological exploration, exploration data not having gone through review and approval, exploration data not meeting freezing scheme design requirements, failure to analyze and verify whether surface water source wells and rivers are hydraulically connected with groundwater, and failure to analyze the influence of groundwater flow direction and velocity, permeability coefficient, water temperature, salt content and freezing temperature of water on frozen wall formation.

6.2.2 The design theory and calculation methods for frozen wall thickness and strength are unreasonable, including freezing depth not meeting specification requirements or the need for normal connection with construction of the unfrozen section; unscientific or unreasonable selection or determination of frozen wall design control levels, freezing brine temperature, sidewall temperature at control levels, freezing pipe specifications and freezing hole spacing; unscientific or unreliable selection of the frozen wall average temperature formula, frozen wall thickness formula and calculated strength values of frozen soil (frozen rock); unreasonable determination of excavation and lining process control parameters such as excavation segment height and exposure time in the frozen section; and deformation checking of the frozen wall not meeting the needs of safe and continuous shaft excavation and lining.

6.2.3 The freezing hole layout and the design of required refrigeration capacity are unreasonable, including failure to analyze and confirm whether the freezing hole layout of a vertical shaft can achieve the designed frozen wall thickness, frozen wall average temperature and sidewall temperature at the control levels at the expected time and depth, or whether effective regulation can be achieved during freezing to meet safe excavation and lining. For inclined shafts, the roof and floor freezing holes do not follow the principle that the roof and floor and both sidewall frozen walls close at the same time, and the design and layout of sealing-head and sealing-tail holes and the freezing hole layout of the upper and lower connecting sections do not comply with NB/T 10053; cores are not taken at the start and end freezing boreholes of an inclined shaft to verify the rationality of the frozen zone; the layout of temperature measuring holes and water level observation holes does not meet the needs of quality evaluation and control of frozen wall formation during construction; and the required refrigeration capacity is calculated incorrectly.

6.3.1 Evaluation of risk factors in freezing scheme design shall carry out hazard analysis, grading and ranking of freezing scheme design risks according to Clause 4.

6.3.2 a) In the freezing scheme design stage, analyze whether the basic design data are true and sufficient, whether the properties and levels of strata in the frozen section have changed significantly, and whether the termination depths of alluvium, soft unstable water-bearing layers and fractured rock zones deviate significantly.

6.3.2 b) Analyze whether the freezing depth design and frozen wall calculation methods comply with current standards, whether the parameter values in the frozen wall thickness formula are reliable, whether the frozen wall thickness calculation is clear and verifiable, and whether the layout of freezing holes, temperature measuring holes and water level observation holes is reasonable; whether there is experience from similar projects under special stratum conditions, whether the feasibility of any new technology applied has been sufficiently demonstrated, and whether the design and design process management are compliant.

6.3.2 c) Analyze the influence of water quality, groundwater velocity and similar factors on frozen wall formation and the influence of stratum freezing temperature on frozen wall formation and strength; whether the calculation of refrigeration capacity required for the frozen soil layers is reasonable, whether the heat dissipation coefficient, thermal conductivity or heat transfer coefficient of the freezing pipes has been reasonably determined, and whether the verification method for frozen wall formation is reliable.

6.3.2 d) Focus on analyzing the technical difficulty and feasibility of shaft sinking by the freezing method under complex engineering conditions such as thick alluvium and high groundwater velocity, and assess the risk of major quality and safety accidents in shaft sinking under such conditions.

6.3.2 e) Analyze whether the management organization and systems are complete. The mechanisms and organizations of the construction contractor, supervision unit and owner for reviewing the freezing scheme shall be sound.

6.3.3 Risk factors in freezing scheme design shall be evaluated by a method combining qualitative and quantitative analysis.

6.4.1 Reliable hydrological, engineering geological and environmental data shall be obtained. Shaft check hole data shall be reviewed according to current regulations, and before freezing scheme design the geological report of the shaft check hole shall be analyzed and evaluated. The hydrological and engineering geological data for vertical shaft freezing shall comply with GB 50511, and those for inclined shaft freezing shall comply with NB/T 10053.

6.4.2 A reliable frozen wall design calculation system shall be adopted. For vertical shaft freezing, the frozen wall thickness, frozen wall average temperature formula and calculated strengths of cohesive and sandy soil layers shall comply with GB/T 39963 and GB 50511. For inclined shaft freezing, the frozen wall thickness shall comply with NB/T 10053.

6.4.3 The freezing hole layout shall be rationally designed. Freezing holes shall be arranged according to soil properties, water content, groundwater velocity and direction, frozen wall thickness, frozen wall average temperature, sidewall temperature and schedule requirements, meeting the requirements of safe and continuous shaft construction. The determination of basic design parameters such as brine temperature, brine flow, freezing pipe specifications and heat exchange load, and freezing hole spacing, as well as the selection of the layout pattern, shall comply with GB/T 39963, GB 50511 and NB/T 10053. The feasibility of the freezing hole layout design shall be verified by frozen wall formation calculation, and freezing efficiency shall be improved by optimizing the layout.

6.4.4 The reliability of the frozen wall closure time and the shaft excavation and lining schedule shall be analyzed, and prediction analysis of the frozen wall formation process shall be carried out to confirm that the frozen wall thickness and average temperature at each control level reach the design requirements on time, meeting safe and continuous construction; the sidewall temperature distribution design shall be reasonable to meet excavation and lining needs. For multi-ring freezing holes in deep thick alluvium, a layout with the main freezing holes on the outer side is preferred.

6.4.5 Temperature measuring holes and hydrological holes shall comply with GB/T 39963, GB 50511 and NB/T 10053.

6.4.6 The required refrigeration capacity shall be calculated for the different stages of frozen wall formation, fully considering the influence of uncertainties such as changes in engineering geological and hydrogeological conditions and changes in the construction plan.

6.4.7 Where new technologies related to engineering construction risk factors are adopted, special technical feasibility demonstration shall be carried out.

6.4.8 The freezing scheme design shall meet the needs of safe and continuous shaft excavation and lining according to plan.

6.4.9 The owner and the construction contractor shall coordinate and improve the freezing scheme design and construction risk management measures. The freezing scheme design shall balance reducing construction technical difficulty, controlling construction cost and improving construction safety reliability.

6.4.10 Design contingency plans shall be prepared for quality and safety accidents that may occur during construction, such as freezing pipe rupture, frozen wall collapse, water leakage and damage to the frozen shaft lining.

6.4.11 A design calculation book shall be prepared for the freezing scheme design. In complex strata such as deep thick alluvium and deep soft water-rich rock, expert review and demonstration shall be organized to confirm the feasibility of the freezing scheme design.

6.4.12 Design process management shall be scientific, rigorous and standardized.

7 Risk management of freezing system and power supply system design

7.1.1 Design of the freezing system and power supply system shall comply with GB 50511, GB/T 39963, GB 50072, NB/T 10053 and other relevant standards.

7.1.2 Design of the freezing system and power supply system shall include: a) calculation of the design refrigeration capacity of the refrigeration plant; b) selection of refrigeration equipment and design of the piping system; c) design of the brine circulation system; d) design of the cooling water circulation system; e) calculation of the power load of the refrigeration plant and design of the power transformation and distribution system; f) layout and installation drawings of the freezing system.

7.1.3 Risk management of freezing system and power supply system design shall include strict implementation of design and design process management regulations and of current technical specifications, identification, analysis and study of risk factors in the design and construction, and formulation of risk control measures.

7.1.4 The main objective of risk management in the freezing system and power supply system design stage is to identify and analyze risk factors in the design and construction of the freezing system and power supply system, study risk countermeasures, and ensure that the freezing system delivers the refrigeration capacity required to form the design frozen wall within the specified time, creating the basic conditions for reducing construction and operation risks of the freezing system.

7.2.1 Risk factors in the design of refrigeration capacity of the refrigeration plant for vertical shafts shall include: a) the heat absorption load per unit area of the freezer is taken too low, without comprehensively considering actual ground temperature, geological and hydrogeological characteristics, thermal conductivity of rock (soil) layers, groundwater velocity, brine cooling design and other factors; b) the surface cold loss coefficient is unreasonably selected, without fully considering the distance from the refrigeration plant to the shaft, the insulation quality of low-temperature pipelines, ambient temperature and other factors; c) for deep shaft freezing, contraction of the brine main pipe is not considered and anti-freeze expansion devices are not installed on the brine main supply and return pipelines; d) where one refrigeration plant serves several shafts, the cold supply coefficient during the frozen wall maintenance period of the shaft frozen first is improperly taken.

7.2.2 Risk factors in the design of refrigeration capacity of the refrigeration plant for inclined shafts shall include: a) the heat absorption load per unit area of the freezer is taken too low, without comprehensively considering actual ground temperature, geological and hydrogeological characteristics, thermal conductivity of rock (soil) layers, groundwater velocity, brine cooling design and other factors; b) the surface cold loss coefficient is unreasonably selected, without fully considering the length of the brine main pipe, the insulation quality of low-temperature pipelines, ambient temperature and other factors; c) the calculation of maximum refrigeration demand of the shaft fails to cover at the same time the sum of the demands of all freezing segments in the formation period and the maintenance period; d) the refrigeration capacity of the plant does not meet the maximum refrigeration demand of the inclined shaft; e) the freezing time of each segment does not match the excavation and lining progress.

7.2.3 Risk factors in the design of working refrigeration capacity of refrigerating machines shall include: a) the design operating conditions of the refrigeration system do not meet the technical requirements of the selected equipment; b) the conversion coefficient from standard refrigeration capacity to working refrigeration capacity is not reasonably taken according to the design evaporating temperature.

7.2.4 Risk factors in the number of refrigerating machines (units) provided shall include: a) service life and working efficiency of the equipment are not considered when calculating actual working refrigeration capacity; b) a certain number of standby units for equipment maintenance is not reserved; c) the influence of local seasonal air temperature is not considered.

7.2.5 Risk factors in the selection of auxiliary equipment of the refrigeration system shall include: a) the selection calculation of auxiliary equipment is unreasonable; b) the influence of the age of equipment on its working efficiency and of ambient temperature and humidity is not considered in selection; c) the selected evaporator area does not meet the peak heat load requirement.

7.2.6 The purity of the refrigerant selected for the refrigeration plant does not comply with the relevant standards.

7.2.7 The initial charge of refrigerant for the refrigeration plant and the make-up for losses during operation are calculated too high or too low, affecting the normal operation of the refrigeration plant.

7.2.8 Risk factors in brine system design shall include: a) the calculated total brine circulation is too low, the design brine flow per hole is too low, or the brine specific gravity design is unreasonable; b) the inner diameters of the brine main pipe, the collecting and distributing ring pipes and the supply pipes are too small, resulting in high brine velocity and increased system pressure loss; c) the dynamic viscosity coefficient of the brine is taken too low, making the calculated system pressure loss too small and the brine pumps undersized; d) the calculated motor power of the brine pumps is unreasonable and cannot meet normal system operation; e) the basis for brine pump selection is insufficient; f) no standby pump is provided when the number of brine pumps is configured.