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DSI Underground  ›  Speciality Bolts  ›  DYWI® Drill Self-Drilling Bolt

Speciality Bolts

Bar Size
R25N R32N R32S R38NR51N
Diameter over threads[in] / [mm]  0.98425 1.260 321.26032 1.496 38 2.008 51
Cross sectional area of bar (a) [in2] / [mm2]  0.378 244 0.6143960.756 488 1.111 717 1.455 939
Ultimate Strength [1000 lbs]  45.0  62.980.9  112.4  179.8 
Yield Strength [1000 lbs]  33.7  51.762.9  89.9  141.6 
Weight [lbs/ft] / [kg/m]  1.5 2.3 4.1 4.0 6.0 5.6 8.4
Maximum Drilling Depths (c)[ft] / [m] 39.4 12 52.51665.6 20 78.7 24 98.4 30
Self Drilling Bolt

  • The DYWI® Drill Self-Drilling Bolt is a fully threaded hollow bar that initially serves as a drill rod, utilizing a lost bit. The thread is made to 15010208.
  • For use in unconsolidated ground or other difficult ground conditions.
  • Excellent for spiling (fore-poling).
  • Bolt is drilled in and grout is pumped through the center hole with pumpable resin or cement grouts. The couplers are designed to exceed the ultimate load of the bar by 20%. The couplers are designed to minimize the loss of drill energy.
  • Several drill bits are available. Most widely used is the EXX Tungsten carbide chisel bit for handling UCT* up to 12.5N/mm² (RQD 100%) and up to 50 N/mm² (RQD 75%).
  • Standard length is 3m (9ft-10").
  • Longer lengths are available: 4m, 6m

*UCT is unconfined compressive strength


The DYWI® Drill Self-Drilling Bolt is typically installed using rotary percussive drilling. This technique enables high rates of installation, good directional stability and also helps to consolidate the grout within the borehole.

Rotation speeds should be sufficient to cut a true borehole (120-150 RPM for soil nails; 100-130 RPM for mini piles), as opposed to displacement of the soil with the drill bit through percussion and heavy feed pressures (driven installation). Drilled boreholes ensure enlarged grout bodies together with better permeation of the grout into the surrounding ground. Feed pressures on the drill rods should be regulated in accordance with the cutting performance of the drill bit.

Simultaneous Drilling and Grouting

Suitable for granular soils and fills. This installation method utilizes a Grout Swivel, grout pump and drifter. The technique combines drilling and grouting as a single operation, ensuring that grout is placed over the full length of the borehole.

For ground conditions where borehole collapse is anticipated or where subsequent grout injection down the center of the bar is problematical, simultaneous drilling and grouting is the preferred solution.

Grouting pressures should be regulated to maintain circulation at all times (typically up to 100 psi), with a small amount of grout return visible at the mouth of the borehole. Pressures in excess of 100 psi are generally only required for specialist applications (i.e. anchors in cohesive soils or mining applications). The choice of grout pump varies between applications, but basic requirements are as follows:

  • thorough mixing of the grout – to avoid blockages at the drill bit,
  • delivery of a continuous volume – to ensure consistent grouting,
  • maintenance of sufficient pressure.


Grout Swivels

Grout Swivels are used for simultaneous drill and grout installation, to inject grout into the bore of a rotating DYWI® Drill Hollow Bar. The unit comprises of a heat treated shaft (to withstand the impact energy from the hammer drive) and a housing into which the grout is pumped. Inlet ports within the shaft enable grout to be pumped into the bore of the bar.

A locator frame is required (bolted to the drill sledge) to prevent the housing from rotating with the shaft, as well as positioning it at the correct location to enable unrestricted operation of the hammer.

Grout Swivel (rig installation)Grout Swivel (hand held installation)
Grout Swivel
(rig installation)
Grout Swivel
(hand held installation)

Drill Bits

Hardened cross cut drill bit, suitable for the majority of applications including narrow bands of soft rock.
(EXR denotes retroflush version)

Soil types: Fills, Shales, and Gravels.

EX / EXR Drill Bit

Tungsten carbide chisel drill bit. EXX is the hardest drill bit available, for strong rock, hard seams and concrete footings.

Rock types: Strong rock, Metamorphic and Igneous Rocks i.e. Slate and Granite.

EXX Drill Bit

Diameters Available for Both Drill Bit Types

  • R25 1.65", 2"
  • R32 2", 3"
  • R38 3", 3.5"
  • R51 3.5", 4.53"

Drill Tooling

A range of drill accessories are available, to enable connection between the output drive of the drilling head and the different DSI Bar diameters.

Air Flush Shank Balance Rod
Air Flush Shank
(hand held installation)
Balance Rod
Reducing CouplerReducing Coupler (with center bridge)
Reducing CouplerReducing Coupler
(with center bridge)
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