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. Primary function of steel roof bolts:

Solution: Roof bolts work by actively reinforcing the rock mass, consolidating the strata to create a stable roof arch.

Q2. Standard bearing plate size:

Solution: 150 mm x 150 mm is the widely adopted standard size for square bearing plates in Indian mines.

Q3. Typical tensile strength of roof bolts:

Solution: High-strength steel bolts are required to have a minimum tensile strength exceeding 500 MPa.

Q4. Pull test checks:

Solution: The pull test verifies the integrity of the connection between the bolt, grout (resin/cement), and the rock.

Q5. Resin capsules provide:

Solution: Resin capsules are known for their fast polymerization, providing high anchorage strength almost immediately.

Q6. Bolt threads are used for:

Solution: The threads allow the nut/washer assembly to be securely fastened and pretensioned against the plate.

Q7. Roof bolt torque ensures:

Solution: Applying torque creates pretension in the bolt, which actively compresses the rock strata layers together.

Q8. Friction bolts (Split Sets) are typically used in:

Solution: Friction bolts are often favored in hard rock metal mines where they provide immediate frictional support.

Q9. Yield strength of bolts should be at least:

Solution: The material must have a high minimum yield strength (typically 300–450 MPa) to ensure it resists permanent deformation under load.

Q10. Corrosion resistance is improved by:

Solution: Applying a protective zinc coating (galvanizing) is a standard method to enhance corrosion resistance in humid mine air.

Q11. Bolt anchorage is improved by:

Solution: Resin or cement grout fills the annulus between the rock and bolt, providing the necessary bonding strength.

Q12. Roof bolt accessories include:

Solution: A torque wrench is essential for applying the correct pretension to the bolt.

Q13. Bolt failure may result from:

Solution: Applying a load greater than the bolt's or rock mass's capacity is a direct cause of failure.

Q14. Washer distributes:

Solution: Washers ensure that the compressive load is evenly spread across the bearing plate and surrounding rock.

Q15. Roof bolt length in coal mines (typical range):

Solution: Standard roof bolt lengths typically range between 1.2m and 2.4m, determined by the thickness of the unstable roof strata.

Q16. Fatigue strength helps resist:

Solution: Fatigue refers to the weakening of material under repeated stress (cyclic loading), which bolts must withstand over their life span.

Q17. DGMS requires testing of installed roof bolts via:

Solution: Mandatory pull testing ensures the bolt achieves the required anchorage capacity in the actual mine strata.

Q18. Mechanical bolts use:

Solution: Mechanical bolts anchor by expanding an outer shell against the borehole wall when the nut is tightened.

Q19. Elongation % ensures:

Solution: Adequate elongation ensures the bolt is ductile and can stretch slightly under load instead of fracturing suddenly (brittleness).

Q20. Cement grout is suitable for:

Solution: Cement provides a very strong, permanent anchorage used for long-term stability in main roadways and permanent workings.

Q21. Split set bolt is a type of:

Solution: Split set (or Swellex) bolts are members of the friction bolt family, anchoring through continuous radial contact pressure.

Q22. Bolt rod must be made of:

Solution: The rod material must be high-strength alloy or carbon steel to resist the extreme tensile forces.

Q23. Main danger controlled by roof bolts:

Solution: Roof bolts are used directly for strata control, making the prevention of roof falls their main safety function.

Q24. Bearing plates must withstand:

Solution: The plate transmits the high compressive force created by bolt pretensioning onto the rock surface.

Q25. Roof bolting must be performed by:

Solution: DGMS mandates that installation of support systems be done only by workers who are properly trained and authorized in strata control practices.

🔗 INTERNAL LINKING SUGGESTIONS

Related Topic	Read More On...
Roof Bolting Standards	Revised DGMS Standards for Resin Capsules
Strata Monitoring	Strata Monitoring in Longwall Workings
Alternative Support	Friction Stabilizers (Split Sets)
Testing	Pull Test & Torque Test Methods

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