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HOLLOW BARS /ANCHOR BARS & ROCK BOLTS

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The hollow Anchor bar system is easily the most popular method for holding in position collapsing or loose soil at tunnel portals. A threaded steel bar is drilled into the soil and grouted into place. The hollow centre in the bar acts as a conduit for simultaneous grouting and drilling. It features a rope thread that connects to drilling equipment. Drill bits, rock bolts and soil nails normally accompany the hollow bars in the portal stabilization process.

Being results of cold rolling manufacturing process, the hollow bars feature refined grain structure. With incredibly high yield strength, the material is suitable for a wide range of applications. Previous users will confirm that these bars offer excellent support in different ground conditions and types of soils around the tunnel portal.

 
THE APPLICATION OF SELF-DRILLING ROCK BOLTS IN TUNNEL CONSTRUCTION:

In tunnel construction, the main function of Self-Drilling Hollow Bar is to reinforce rock mass, improve the stability of surrounding rock give full play to rock self-supporting ability and control the deformation and rock collapse of surrounding rock. Self-drilling Hollow Bar support has played an important role of supporting and reinforcing on surrounding rock control technology, which sets drilling, grouting, and anchoring in one. Compared with the traditional support, it has been widely applied in the world tunnel construction for its not only notable features but also technical and economic benefit.

Self-drilling Hollow Bars are commonly used throughout the civil engineering and tunnelling industries. They are used to improve the stability and load bearing characteristics of rock mass.

 
ROCK BOLTS/ANCHOR BOLTSOLTS/HOLLOW BARS USED IN INDERGROUND EXCAVATION WORKS LIKE:
  • Mining tunnels
  • Road & rail tunnels
  • Hydro & irrigation tunnels
  • Underground caverns
 
IT INCLUDES REQUIREMENTS FOR:
  • Types of rock bolts (mechanically anchored, resin-grouted, cement-grouted, friction bolts, self-drilling bolts, etc.)
  • Material specifications (steel grades, mechanical properties)
  • Dimensions & tolerances
  • Testing requirements (pull-out strength, tensile load capacity, shear strength,
    elongation, corrosion resistance
APPLICATION IN SELF DRILLING ANCHOR BOLTS:
  • Self-Drilling Anchor Bolts (SDAB) are widely used in tunnelling, slope stabilization, and underground mining.
  • As per IS 12427, SDAB can be considered a variant of grouted rock bolts, since they allow simultaneous drilling and grouting.
  • Manufacturers following IS 12427 must ensure steel chemistry, yield strength, elongation, and testing procedures are met.
 
RELATING TESTING STANDARDS:
  • IS 11309 – Method for conducting pull-out test on anchors in concrete
  • IS 1343 – Prestressed concrete (anchorages in civil applications)
  • IS 1367 – Mechanical properties of fasteners
  • A steel tube or slotted pipe driven into slightly undersized hole.
  • Holds by frictional resistance between bolt and rock surface.
  • Easy and quick installation, often used in mining.
  • Moderate load capacity compared to grouted types
SELF-DRILLING ROCK BOLTS (SDAB)/  SELF DRILLING ANCHOR / HOLLOW BARS
  • Hollow bolt with a sacrificial drill bit; drilling and grouting done simultaneously.
  • Grout injected through hollow core.
  • Eliminates need for pre-drilling; suitable for loose ground, soil, and fractured rock.
  • High efficiency in tunnelling, slope stabilization, and ground improvement
 
ADVANTAGES:
  • There is an obvious anti-pulling advantage in serious damaged rock mass.
  • The effect of grouting is excellent and suitable for all ground conditions, about 1.5-2 times than ordinary hollow bar.
  • Have high construction efficiency. No casing required in collapsing ground conditions, self-drilling Hollow Bars can get into shape once without these problems, such as, pipe-sticking, inserted bar difficulties and other issues. The effect is about 2-4 times than ordinary hollow bar and the economic advantages are obvious.
 
APPLICATIONS:
  • Stabilization of tunnel portals
  • Fore-poling
  • Face stabilization
  • Spilling
  • Root piling
  • Radial systematic anchoring
 
 
 
 
 SELF-DRILLING ROCK BOLTS (SDAB)/  SELF DRILLING ANCHOR / HOLLOW BARS
  • Self-drilling rock bolts are usually designated by outer diameter (OD) of the hollow bar and the length. The bar is threaded over its full length, with a sacrificial drill bit at the end. Self-Drilling Anchor Bars are a fast, efficient, and versatile ground support system designed for conditions where conventional bolt installation (pre-drilling + bolt insertion + grouting) is difficult.
  • A hollow, fully threaded steel bar with a sacrificial drill bit at the front.
  • Serves both as the drill rod and the final installed anchor.
  • Grouting is done through the hollow core immediately after or during drilling.
 
KEY FEATURES:
  • Combines drilling, grouting, and anchoring in a single operation.
  • Effective in loose soil, fractured rock, water-bearing strata, and overburden ground.
  • Eliminates the need for casing pipes.
  • Can be extended to large depths using couplers
SELF DRILING ANCHOR BOLT​
Outer Dia (mm) Inner Dia (mm) Tensile Capacity (kN) Applications
R25 25 14-16 200-240 Soil nails, shallow anchors
R32 32 16-20 280-350 Tunnel support, slope stabilization
R38 38 19-22 400-500 Mining & cavern supports
R51 51 25-28 600-800 Heavy tunnelling foundations
T76 76 38-42 1200+ Deep foundations, dams, heavy loads
SELF DRILING ANCHOR BOLT​
  • (R = round thread, T = trapezoidal thread profile)
  • Lengths
  • Standard: 2 m – 6 m
  • Extendable up to 12+ m with couplers
COMPONENTS/ACCESSORIES:
  • Hollow bar (anchor rod) – threaded full length
  • Drill bit – sacrificial (cross, button, clay, or tungsten-carbide)
  • Couplers – to extend bar length
  • Plate & nut – to apply load to surface
  • Grout adapter – for pumping cement grout through the bar
APPLICATIONS:
  • Tunneling (NATM / SEM methods) – temporary & permanent support
  • Mining – weak rock strata support
  • Slope stabilization – soil nailing, retaining walls
  • Foundations – micropiles, underpinning, retaining systems
  • Hydro & dams – anchors in water-bearing rock
ADVANTAGES:
  • Drilling and grouting in one step (save time).
  • Works in loose / fractured / collapsing ground.
  • Can be installed in overhead or inclined positions.
  • Adaptable with different drill bits for varying geology
COMMON DIAMETERS (HOLLOW BAR SIZES)
  • R25 -Ø 25 mm
  • R32 – Ø 32 mm
  • R38 – Ø 38 mm
  • R51 – Ø 51 mm
  • T76 -Ø 76 mm (heavy-duty)
  • (R = Round thread, T = Trapezoidal thread)
TYPICAL LENGTHS
  • Range: 1.5 m to 6 m (standard)
  • Can be extended up to 12 m or more using couplers
  • Chosen based on tunnel span, rock quality, and design load
SELF-DRILLING ROCK BOLTS (SDRB / SDAB)
  • Hollow threaded bar with sacrificial drill bit drilling & grouting done simultaneously.
  • Best for weak ground, soil, fractured rock, and water-bearing strata.
  • Eliminates separate drilling and grouting.
  • Common in NATM tunnelling, slope stabilization, and metro projects
OTHER KEY PARAMETERS
  • Hollow core diameter: ~10–38 mm (for grout injection)
  • Thread type: Continuous rolled thread (R or T Thread)
  • Ultimate tensile strength: 200–800 kN (depending on size & steel grade)
  • Accessories: Drill bits (cross, button, clay bits), couplers, plates, nuts, grout adapters
Size Table
Bolt Size Outer Dia (mm) Inner Dia (mm) Tensile Load (kN) Common Lengths (m)
R25 25 14-16 ~200-240 1.5-4.0
R32 32 16-21 ~280-350 2.0-6.0
R38 38 19-22 ~400-500 2.0-6.0
R51 51 25-28 ~600-800 3.0-9.0
T76 76 38-42 >1200+ 6.0-12.0

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