DGMS Technical Circular No. 9 (2008) & Circular No. 1 (2009) – Corrections, Modifications & Updated Requirements

Roof Bolts in Mines – DGMS Standards, Composition, Properties, Testing & MCQs

🧾 DETAILED NOTES

1️⃣ Introduction

Roof bolts are the primary support system used in underground coal and metal mines for strata control, reinforcement of roof beds, and preventing roof falls. They work by binding together several layers of rock into a single beam, improving the structural integrity of mine workings.

2️⃣ Types of Roof Bolts

Mechanical Anchored Bolts: Utilise an expansion shell to grip the rock. Best in hard, competent roof.
Resin-anchored Bolts: The most widely used in coal mines. Anchored by fast-setting or slow-setting resin capsules.
Friction Bolts (Split Set): Provide radial pressure. Used in metal mines.
Cable Bolts: Long reinforcement (2–6 m), ideal for deep or highly fractured roofs and major intersections.

3️⃣ Standard Specifications (DGMS + IS Standards)

Parameter	Specification
Bolt Length (Coal Mines)	1.2 m to 1.8 m
Bolt Length (Metal Mines)	1.5 m to 3.0 m (or more for cable bolts)
Bolt Diameter	Typically 16–25 mm
Material	High tensile steel (hot rolled or cold drawn)
Steel Grade	Minimum IS 1367 / IS 2062 (or equivalent)

4️⃣ Composition of Roof Bolts

Roof bolts are usually made from: Low-carbon alloy steel, Manganese steel, or Boron-treated steel (for high strength). Properties required: High tensile strength, High shear resistance, Controlled elongation, and Corrosion resistance.

5️⃣ Properties Required for Roof Bolts

Yield Strength: ≥ 240 MPa (for general applications), often 300 MPa or higher for specialized bolts.
Ultimate Tensile Strength: ≥ 410 MPa.
Shear Strength: Must be high to resist forces attempting to cut the bolt.
Elongation: 12–20% (to allow for strata movement without brittle failure).
Anchorage: Must provide strong, reliable anchorage to the rock strata.

6️⃣ Testing Requirements (DGMS Mandate)

Roof bolts must undergo stringent testing at DGMS-approved laboratories (e.g., ISM Dhanbad, CMPDI Ranchi, NCCBM):

Tensile Test, Yield Test, Shear Test, Elongation Test, Bend Test.
Pull-out Test: Essential for resin bolts to verify anchorage strength.
Torque Test: For mechanical bolts to ensure proper tightening.

⚡ QUICK ONE-LINERS (REVISION)

Roof bolts reinforce rock strata.
Most common type: Resin-anchored bolt.
Bolt length in coal mines: 1.2–1.8 m.
Yield strength ≥ 240 MPa.
Resin capsules: fast-set + slow-set.
Testing required at DGMS-approved labs.
Split set = friction bolt.
Cable bolts used in deep mines.
Steel grade IS 2062 is common.
Elongation of 12–20% required.

🧠 DESCRIPTIVE MODEL ANSWER

Q. Describe the standard composition and properties required for roof bolts used in mines.

Answer:
Roof bolts are essential strata support elements used to reinforce the mine roof. As per DGMS and IS standards, they are primarily made from high-tensile alloy steel, often low-carbon steel enhanced with manganese or boron for increased strength and durability. Typical bolt diameters range from 16–25 mm, with lengths of 1.2–1.8 m in coal mines and up to 3 m in metal mines.

The key properties required are a minimum yield strength of 240–300 MPa and an ultimate tensile strength exceeding 410 MPa. Bolts must also exhibit good shear resistance, an elongation of 12–20% (to absorb strata movement), reliable anchorage, and corrosion resistance. All roof bolts must pass tensile, yield, shear, and pull-out tests in DGMS-approved laboratories to ensure their reliability in underground support systems.

🧮 25 MCQs (Dynamic Answers A–E)

Q1. Common length of roof bolts in coal mines:

Solution: Standard lengths for roof bolts in coal mines are typically within the 1.2 to 1.8 meter range to bind superficial rock layers.

Q2. Main material of roof bolts:

Solution: High tensile steel is essential for roof bolts due to its strength and ability to withstand significant loads.

Q3. Most common bolt type in coal mines:

Solution: Resin-anchored bolts provide superior, durable anchorage in varying rock conditions, making them preferred in coal mines.

Q4. Minimum yield strength:

Solution: A yield strength of at least 240 MPa is a statutory requirement to ensure the bolt can withstand forces without permanent deformation.

Q5. A friction bolt is also called:

Solution: "Split set" is a common trade name for a type of friction bolt that works by radial pressure.

Q6. Resin capsule types:

Solution: Resin capsules come in different setting times to allow for varied installation speeds and full encapsulation.

Q7. Roof bolts prevent:

Solution: The primary function of roof bolts is to secure overhead strata and prevent dangerous roof falls.

Q8. Bolt diameter usually:

Solution: Common bolt diameters in mines are between 16 and 25 mm, providing a balance of strength and ease of installation.

Q9. Bolt testing includes:

Solution: Tensile testing is fundamental to assess the load-bearing capacity of the bolt before it yields or fractures.

Q10. Cable bolts are used for:

Solution: Cable bolts provide long-term reinforcement in challenging ground conditions due to their length and flexibility.

Q11. Resin anchors provide:

Solution: Full encapsulation ensures maximum contact between the bolt and the rock, leading to strong and durable anchorage.

Q12. Mechanical bolts use:

Solution: Mechanical bolts anchor by expanding an outer shell against the borehole walls.

Q13. Steel grade used:

Solution: IS 2062 (or IS 1367) defines the standard structural steel grades commonly used for manufacturing roof bolts.

Q14. Elongation requirement:

Solution: The material must possess adequate ductility (elongation) to withstand strata movement without brittle failure.

Q15. Anchor mixing in resin bolt achieved by:

Solution: The rotation of the bolt shreds the capsule and thoroughly mixes the resin and catalyst.

Q16. Bolts must resist:

Solution: High shear strength is required to prevent the bolt from being cut or sheared by strata movement.

Q17. Roof bolts are part of:

Solution: Roof bolting is the most widely used method for active strata control in underground mining.

Q18. Pull-out test checks:

Solution: The pull-out test verifies the holding power of the bolt in the rock strata.

Q19. Friction bolts provide:

Solution: Friction bolts (Split Set) generate support capacity through radial pressure against the borehole walls.

Q20. Bolt plates used for:

Solution: Bearing plates spread the pretension force over a wider area of the roof surface.

Q21. Failure may occur due to:

Solution: Incorrect drilling, mixing, or bolt tightening (poor installation) are primary causes of failure.

Q22. Testing at DGMS labs ensures:

Solution: DGMS testing ensures the manufacturer's product meets the specified mechanical and material standards (quality compliance).

Q23. Resin bolt length range:

Solution: In coal mines, the standard length for resin bolts is typically 1.2m to 1.8m.

Q24. Cable bolts are made from:

Solution: Cable bolts are constructed from high-tensile steel wire ropes, providing flexible, long-term reinforcement.

Q25. Purpose of roof bolting:

Solution: The fundamental purpose is to reinforce the roof strata (strata control) to ensure safety.

🔗 INTERNAL LINKS

Related Topic	Read More On...
Cement Capsules	Revised DGMS Standards for Cement Capsules
Strata Monitoring	Strata Monitoring in Longwall Workings
Alternative Support	Friction Stabilizers (Split Sets)
Testing	Pull-Out Test Procedures for Rock Bolts

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