Safety Features in HEMM & Auxiliary Equipment in Opencast Mines – DGMS Compliance Guide

🧾 DETAILED NOTES

1️⃣ Introduction

Rigid steel props, chocks, and cogs are vital support systems used in underground mines to prevent roof falls, stabilize roadways, and maintain safe working conditions. DGMS specifies detailed mechanical and structural requirements to ensure their strength, durability, and performance under load.

These supports resist vertical and lateral loads and are widely used in depillaring, development headings, galleries, and junctions.

2️⃣ Rigid Steel Props – Components

1. Prop Body: Seamless steel tube or thick-walled pipe, high crushing strength. Length varies depending on seam height (1.0 m to 3.0 m).

2. Base Plate: Thick steel plate to distribute vertical load evenly and prevent sinking into floor strata.

3. Cap Plate / Head Plate: Receives load from roof, prevents eccentric loading, typically 150–200 mm wide.

4. Welded Joints: High-quality welds ensuring compliance with IS welding standards.

3️⃣ Chocks & Cogs – Components

Chocks: Stable structures made of fabricated steel sections (I-beams, channels) or timber blocks arranged in layers. They utilize heavy gauge steel Cap & Base Plates and Side Bracing to prevent lateral movement.

Cogs: Rigid pillar-like structures formed by crosswise stacking of short timber blocks or steel sections. They use a cap piece and packing material, tightened by wedges.

4️⃣ Mechanical Properties (DGMS Recommended)

Property	Requirement
Compressive Strength	Very high (≥ 300–400 kN for steel props)
Yield Strength	250–350 MPa
Buckling Resistance	Must resist lateral deformation
Corrosion Resistance	Coated or painted steel
Impact Resistance	Withstand dynamic roof weighting

5️⃣ Installation & Maintenance

Props should be installed vertically, with the base secured on a firm floor.
Proper packing is needed between the prop head and the roof.
Cogs must be layered crosswise to prevent shifting, and tightened using wedges.
Regular inspection is mandatory for deformation, rust, and signs of buckling.
Props must not be used if bent or cracked.

⚡ QUICK ONE-LINERS

Rigid steel props provide vertical support to the roof.
Chocks are stable support structures made of steel or timber blocks.
Cogs resist both vertical & horizontal (lateral) loads.
Compressive strength ≥ 300 kN is expected for steel props.
Must comply with DGMS & BIS support standards (e.g., IS 13224).
Widely used in depillaring operations.
Cap plate distributes the roof load uniformly.
Corrosion-resistant coating (paint/galvanizing) is essential.
Buckling is the major failure risk for props.
Crosswise packing and wedges are used to tighten cogs.

🧠 DESCRIPTIVE MODEL Q&A

Q. Describe the standard components and required mechanical properties of rigid steel props, chocks, and cogs used in mines.

Answer:
Rigid steel props consist of a solid steel tube, base plate, cap plate, and welded joints designed to support the roof vertically. They must have high compressive strength (≥ 300–400 kN), a high yield strength, and essential corrosion resistance. Chocks are stable, wider area supports built from steel/timber blocks with heavy cap and base plates. Cogs are column-like structures formed by crosswise stacking of blocks and tightened by wedges, providing long-term support. All systems must comply with DGMS-approved support standards and undergo periodic inspection for deformation or buckling to ensure safety.

🧮 25 MCQs (Dynamic Answers A–E)

Q1. Rigid steel props are primarily used for:

Solution: Steel props are structural components used to provide vertical resistance against roof load.

Q2. Main load-bearing property of steel props:

Solution: Since the prop resists the roof load vertically, its ability to resist crushing (compressive strength) is paramount.

Q3. Chocks are built using:

Solution: Chocks are stable crib-like structures commonly built using either steel sections or timber blocks.

Q4. Cogs resist:

Solution: Due to their broad, stacked structure, cogs resist both the downward vertical load and lateral shifting forces.

Q5. Steel props must comply with:

Solution: Indian Standards (e.g., IS 13224) and DGMS approval are mandatory for all support material used in mines.

Q6. Failure of props is mainly due to:

Solution: Buckling (lateral bending under vertical load) is the primary mode of failure for vertical support structures like props.

Q7. Cap plate is used for:

Solution: The cap plate sits between the prop and the roof to evenly distribute the roof load across a wider area.

Q8. Steel props are usually made from:

Solution: To achieve the high crushing strength required, props are constructed from high-quality, often seamless, steel tubes.

Q9. Chocks provide:

Solution: Chocks and cogs, due to their large size and layered structure, are suitable for stable, long-term support, especially in depillaring.

Q10. Cogs are installed in:

Solution: Cogs are most commonly used in the depillaring area and junctions to provide mass support and control the caving of the goaf.

Q11. Base plate prevents:

Solution: The base plate increases the contact area with the floor, preventing the prop from punching into soft floor strata.

Q12. Steel props must be stored:

Solution: Support materials, especially steel, must be stored in dry, clean conditions to minimize rust and corrosion.

Q13. Compressive strength of steel props generally:

Solution: Steel props are designed to handle very high loads, typically requiring a minimum compressive yield load of 300-400 kN.

Q14. Timber cogs depend on:

Solution: Since cogs are built from timber, their strength and capacity depend directly on the quality and species of wood used.

Q15. The topmost part of a cog assembly that receives load from the roof is the:

Solution: The cap piece is the topmost block or steel plate that spans the top of the cog to interface with the roof.

Q16. Chocks resist:

Solution: Chocks are massive support systems primarily designed to resist the downward weight (pressure) exerted by the roof strata.

Q17. Steel props must undergo inspection for:

Solution: To ensure safety, steel props must be inspected or tested to verify their resistance against vertical crushing and lateral buckling.

Q18. Cogs are usually placed:

Solution: Cogs are intended to act as pillars, providing vertical support, hence they are installed vertically.

Q19. Corrosion of steel props is prevented by:

Solution: Applying protective coatings like galvanizing or industrial paint is the standard engineering method to achieve corrosion resistance.

Q20. Chocks and cogs help control:

Solution: They resist strata movement, limiting the vertical closure (convergence) between the roof and floor.

Q21. Props must be replaced if:

Solution: Visible structural damage, such as bending or cracking, indicates failure and compromises the safety capacity of the prop.

Q22. Cogs are tightened using:

Solution: Wedges are driven into the stack of timber/steel blocks to pretension the cog against the roof.

Q23. Steel prop height depends on:

Solution: The length of the prop must match the distance between the roof and floor (seam height) of the working place.

Q24. Steel props provide:

Solution: As rigid supports, their primary mechanical function is to resist the downward (vertical) force of the roof.

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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