NB/T 11028-2022
Technical specifications for sealing of underground coal mine waste gas drainage boreholes (English Version)
- Standard No.
- NB/T 11028-2022
- Language
- Chinese, Available in English version
- Release Date
- 2022
- Published By
- Professional Standard - Energy CN / NB
- Latest
- NB/T 11028-2022
- Introduction
In-depth Analysis and Implementation Guide for NB/T 11028-2022 Standard
The publication of NB/T 11028-2022 'Technical Specifications for Sealing Abandoned Gas Drainage Boreholes in Coal Mines' marks a key step forward in the refinement and standardization of coal mine gas control in China. If abandoned gas drainage boreholes are not sealed properly, they can become major hazards leading to abnormal gas outbursts or even accumulation and explosions underground. This document was drafted by the National Engineering Research Center for Coal Mine Gas Control in collaboration with multiple research institutes and enterprises. It aims to provide a scientific, reliable, and operable technical basis for the permanent sealing of abandoned boreholes in coal mines, filling the gap in specialized technical standards in this field.
I. Core Framework of the Standard and Technical Evolution
The core of this standard lies in clarifying the applicable scope, process flow, and quality control requirements for two mainstream sealing processes: polyurethane sealing and cement mortar sealing. Its technical evolution reflects the transition from 'experience-based sealing' to 'scientific sealing'. For the first time, it quantifies key parameters such as 'over-diameter coefficient' and 'sealable depth' at the industry standard level and introduces differentiated plugging structure designs based on borehole inclination.
Comparison Dimension Polyurethane Sealing Process Cement Mortar Sealing Process Significance of Technical Evolution Applicable Conditions Abandoned boreholes without water inside Abandoned boreholes with or without water inside (especially recommended for boreholes with water) Precise selection based on hydrogeological conditions to improve sealing reliability Core Materials Polyurethane liquid components A and B, toweling cloth, support rods Expansive cement with compressive strength >32.5MPa, sieved sand, clean water Standardize material performance indicators to ensure the strength and durability of the sealing body from the source Sealing Depth Strata-boreholes ≥8m, cross-strata boreholes ≥5m (based on sealable depth) Strata-boreholes ≥8m, cross-strata boreholes ≥5m (based on sealable depth) Unify minimum safety depth to prevent failure due to insufficient sealing length Process Characteristics 'Liquid-wrapping method', rapid reaction foaming, good sealing 'Grouting plugging method' (one plug one grout / multiple grouts, two plugs one grout / multiple grouts), high strength, strong resistance to disturbance Process descriptions are specific and visualized to reduce on-site operational deviations Key Equipment/Tools Probe rods, support rods, mixing buckets Mud pumps, grouting pipes, return pipes, gate valves, bags (optional) Clarify equipment requirements, e.g., bags must comply with NB/T10358 to ensure the quality of auxiliary materials
II. In-depth Interpretation of Key Technical Points
1. Scientific Definition of Terminology
The standard defines 'abandoned gas drainage boreholes' and 'over-diameter coefficient' for the first time. The former refers to 'boreholes that have not been sealed or gas drainage operations were not carried out due to reasons other than meeting design requirements'. This definition includes all boreholes stopped due to changes in geological conditions, meeting drainage standards, or the end of engineering projects, thereby avoiding management loopholes. The introduction of the 'over-diameter coefficient' (K = actual maximum diameter / nominal drill bit diameter) is a major technical advancement. Coal mine boreholes often experience 'necking' or 'over-diameter' during construction due to formation stress and drill tool wear. Calculating grouting volume based on nominal diameter traditionally leads to severe shortages. The standard suggests taking K as 1.2~1.5, providing a scientific basis for the precise calculation of material quantities.
2. Refined Control of Polyurethane Sealing Process
This process is suitable for dry boreholes, with its core being the 'liquid-wrapping method'. The operational key points are speed and uniformity. After mixing the liquid, stirring should stop immediately when the color changes from yellow-brown to milky white, followed by immediate wrapping and insertion. This aims to utilize the rapid foaming and expansion characteristics of polyurethane to form a dense, seamless sealing body inside the borehole. The standard requires the installation of rubber gaskets at both ends of the sealed section and at least 200mm of yellow mud (or equivalent material) for external sealing within 0.5m of the borehole mouth, forming a dual guarantee system of 'internal expansion sealing + external mechanical compaction'.
Application Case: A strata borehole in a certain mine, with a design depth of 100m, was abandoned due to a sudden decrease in gas outflow upon encountering rock fractures. The borehole was dry, and the sealable depth was measured at 10m. Polyurethane sealing was adopted. A 10m long support rod was selected. Following the standard process, toweling cloth soaked in liquid was wrapped and quickly inserted. 24 hours after sealing, the gas concentration at the borehole mouth dropped below 0.1%, demonstrating significant results.
3. Systematic Design of Cement Mortar Sealing Process
This is the focus of the standard, especially suitable for environments with water. Its systematic nature is reflected in:
- Standardized Material Preparation: Cement must have a compressive strength >32.5MPa and be expansive. Sand particle size should be ≤1.0mm. The water-cement-sand ratio is 1:1:0.65~0.70. This ratio balances fluidity, strength, and micro-expansion, facilitating the filling of the borehole and tight bonding with the borehole wall.
- Differentiated Plugging Structure: Depending on the borehole inclination (≥25° upward boreholes or ≤-25° downward boreholes vs. other angles), the use of 'one plug one grout' or 'two plugs one grout' structures is recommended. For near-horizontal boreholes, the 'two plugs one grout' structure (two plugs, inner and outer) effectively defines the grouting section, ensuring the grout pressure fills the space. For high-inclination boreholes, the 'one plug one grout' structure is simpler, relying on the self-weight or pressure of the grout to achieve effective plugging.
- Quantified Usage Calculation: The standard provides complete calculation formulas (Formulas 1-4) for calculating the usage of cement (Qc), sand (Qs), and water (Qw) from the usage per meter of borehole (Q). Where Q = 0.785D²KLγ, parameters such as borehole diameter (D), over-diameter coefficient (K), sealing length (L), and mortar density (γ) are all quantified, achieving precise management of material budgets and avoiding waste or shortages.
Calculation Example: To seal a borehole with a diameter of 113mm (D) and a length of 8m (L), taking K=1.3 and γ=1940 kg/m³. The cement mortar usage per meter of borehole Q = 0.785 * (0.113)² * 1.3 * 1 * 1940 ≈ 24.9 kg. The total usage is approximately 199.2 kg. Based on the ratio 1:1:0.65, the required cement is approximately 199.2 / (1+1+0.65) ≈ 75.2 kg, sand 75.2 kg, and water 48.9 kg.
III. Safety Technical Measures and Standard Implementation Suggestions
1. Analysis of Safety Measures
The safety technical measures in Chapter 8 are the bottom-line requirements for ensuring the safety of personnel and the quality of sealing. Highlights include:
- Organizational Guarantee: Requires the 'person in charge of the shift to command on-site uniformly', clarifying the subject of safety responsibility.
- Equipment and Process Safety: Inspecting mud pumps, pipelines, and joints before operation is key to preventing mechanical failures and grout leakage. Installing filters on grouting pipes can prevent impurities from clogging pipelines or gate valves.
- Personnel Protection and Operational Safety: 'Do not operate facing the borehole mouth directly' is an important regulation to prevent sudden ejection of pressure or foreign objects from the borehole. Wearing protective glasses is a basic requirement to prevent grout from splashing into the eyes.
2. Standard Implementation Suggestions
To effectively implement this standard, coal mine enterprises should take the following measures:
- Establish a Management Ledger for Abandoned Boreholes: Conduct a census and registration of all abandoned boreholes in the mine, recording information such as location, borehole diameter, depth, and hydrogeological conditions as the basis for sealing decisions.
- Conduct Specialized Technical Training: Organize construction personnel to deeply study the standard provisions, especially practical key points such as the mixing and stirring time for polyurethane liquid, control of cement mortar ratios, and judgment of grout 'return flow', and conduct simulation drills.
- Equip with Standardized Tooling and Testing Tools: Such as standard probe rods, graduated mixing containers, mortar consistency meters, etc., to ensure that operating conditions meet standard requirements.
- Improve Sealing Quality Acceptance and Post-Operation Monitoring Systems: Conduct airtightness checks at the borehole mouth after sealing and regularly monitor gas concentrations in the surrounding area. Include sealing projects in the mine's safety quality standardization assessment system.
In summary, NB/T 11028-2022 is not only a technical specification but also an engineering guide containing risk management thinking. Its comprehensive implementation will greatly improve the engineering quality and safety level of sealing abandoned gas drainage boreholes in China's coal mines, laying a solid technical foundation for safe coal mine production and comprehensive governance of gas resources.
NB/T 11028-2022 Referenced Document
- NB/T 10358 General technical conditions for filter bag sealers
NB/T 11028-2022 history
- 2022 NB/T 11028-2022 Technical specifications for sealing of underground coal mine waste gas drainage boreholes
