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Standard Components & Mechanical Properties of Steel Roof Bolts in Mines – DGMS Notes & MCQs

Standard Components & Mechanical Properties of Steel Roof Bolts in Mines – DGMS Notes & MCQs

🧾 DETAILED NOTES

1️⃣ Introduction

Steel roof bolts are the primary support system in underground coal and metal mines. They prevent roof falls by reinforcing rock strata, distributing load, and creating a stable rock arch. DGMS mandates strict standards for type, material specification, mechanical strength, and installation to ensure safety and structural reliability.

2️⃣ Standard Components of a Steel Roof Bolt System

1. Bolt Rod: Made of high-strength alloy or carbon steel. Standard lengths: 1.2 m to 2.4 m (coal mines), up to 3 m or more (metal mines). Threaded or forged head type.

2. Bearing Plate: Transfers load from bolt head to surrounding rock. Standard sizes: 150 mm × 150 mm, 4–6 mm thickness. Should be corrosion-resistant and capable of withstanding high compressive load.

3. Nut / Washer: Nuts secure the bolt head. Hardened steel; Washers help distribute load uniformly.

4. Grouting / Anchorage Material: Resin capsules / Cement capsules / Friction anchors. Responsible for bonding bolt to rock and creating anchorage.

5. Accessories: Torque wrench, Roof bolt tester, Pull tester, Drill rod and drill bit.

3️⃣ Mechanical Properties of Steel Roof Bolts

Property DGMS / BIS Requirements
Tensile Strength ≥ 500–650 MPa depending on grade
Yield Strength ≥ 300–450 MPa
Elongation Minimum 12–16%
Shear Strength High resistance to lateral forces
Corrosion Resistance Galvanizing or epoxy coating recommended
Fatigue Strength Withstand long-term cyclic loading

4️⃣ Types of Roof Bolts Used in Mines

  • Mechanical / Expansion Shell Bolts: Used in hard rock. Provide immediate anchorage.
  • Resin-Grouted Bolts: Excellent for moist or fractured roofs. Quick setting & high strength.
  • Cement-Grouted Bolts: Used in permanent support, metal mines. Long-term anchorage.
  • Friction Bolts (Split Set / Swellex): Used in metal mines for dynamic loading. Immediate support.

5️⃣ DGMS Requirements for Roof Bolts

  • Must comply with IS 1363 / IS 1367 / IS 4000 standards.
  • Bolt rod must have clear marking of grade, batch, and manufacturer.
  • Mandatory pull testing in each working district.
  • Periodic torque test to ensure pre-tensioning.
  • Installation must be carried out by trained support workers.

6️⃣ Testing of Roof Bolts

1. Pull Test: Checks anchorage strength (Minimum pull requirement: 7–10 tonnes).

2. Torque Test: Ensures bolt is tightened as per specification (Typical torque: 150–300 Nm).

3. Visual Inspection: Check for corrosion, bending, improper seating of bearing plate.

4. Grout Testing: For resin/cement capsules: check curing, set time, and bonding.

⚡ QUICK ONE-LINERS

  • Roof bolts reinforce rock strata through anchorage.
  • Tensile strength must exceed 500 MPa.
  • Bearing plate size: 150 × 150 mm (standard).
  • Pull testing is mandatory in all districts.
  • Resin capsules provide quick-setting anchorage.
  • Torque test ensures bolt pretensioning.
  • Corrosion-resistant coating improves life span.
  • Washers distribute load evenly on bearing plates.
  • Mechanical bolts give immediate anchorage.
  • FIFO method ensures quality grout usage.

🧠 DESCRIPTIVE MODEL Q&A

Q. Describe the standard components and required mechanical properties of steel roof bolts used in underground mines.

Answer:
Steel roof bolts consist of a bolt rod, bearing plate, nut/washer assembly, and a grouting mechanism (resin or cement capsules). The bolt rod must be manufactured from high-strength alloy or carbon steel with tensile strength ranging from 500–650 MPa and yield strength above 300 MPa. Bearing plates distribute load to the surrounding strata, while nuts and washers ensure secure fastening. Roof bolts must undergo pull testing and torque testing to meet DGMS support standards. These components together provide long-term stability and prevent roof falls in underground workings.

🧮 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.

🔗 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 Hydraulic Cappel Testing

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