Self-Drilling Anchor Bolts

Self-Drilling Anchors are a bolting solution for unstable ground conditions such as sand, gravel, silt, and clays, and in soft to medium fractured rock formations.

For projects facing such ground conditions,  self-drilling anchors should be considered as the main productivity solution. Self-Drilling Anchors from MDTI are designed for optimized installation, tailored to the project’s needs.

By drilling a hole in collapsing soil or loose rock, with a sacrificial drill bit and a hollow rod, and after the drilling operation injecting cementitious grout or resin into the hollow rod and surrounding cavity, self-drilling anchors are a top productivity solution.

Self-drilling anchors from MDTI consists of:

  • A hexagonal nut
  • A bearing plate
  • Extension couplings, if the anchor consists of several anchor rod sections
  • Hollow anchor rod(s)
  • A sacrificial drill bit

Applications of Self-Drilling Anchors

Slope stabilization

Self-Drilling Anchors are used to stabilize unstable rock/soil formations. The unconsolidated or weathered ground conditions favor the SDA technique for a fast and simple method of installation compared to the traditional methods.

Foundations with micropiles

SDA micropiles can be defined as drilled piles, composed of injected grout with the anchor rod as steel reinforcement. They can be installed in sites with limited space using light drilling equipment, can be installed into most ground conditions including rock and are suitable for tension and compression loads. The big advantage is the combined drilling and grouting making high piling outputs possible.

Tunneling

Self drilling anchors are the most economical solution for the reinforcement of the tunnel circumference and advanced roof support for tunnel excavation in overburdened and soft fractured rock conditions.

Self-Drilling Anchor Equipment & Drill Bits

Sacrificial drill bits

The sacrificial drill bit is the most crucial part of the anchoring system and is responsible for the productivity of the installation. MDTI offers a large range of drill bits to suit the changing geology encountered during projects. In order to improve on performance and cost efficiency, data is collected from projects around the world and incorporated into the design with the goal to improve penetration rate and bit quality and to reduce manufacturing and application costs.

The selection of drill bits

A successful installation of the SDA system depends on the selection of the most suitable drill bit. Compared to conventional drill bit types offered globally for maximum standing performance for rock or soil, the criteria for SDA drill bits are defined by consideration of the following factors:

  • High quality for a limited service life
  • Installed MAI SDA length
  • Geology
  • Geometry

SDA Anchor Components

Hollow anchor rods

The anchor rod features a hollow bore for flushing, or simultaneous drilling and grouting, and has a left-hand thread for connection to standard drill tooling. It is manufactured from API standard thick wall steel tubing, cold rolled to a standard ISO rope thread profile. The rolling process refines the crystaline structure of the steel, increasing the yield strength, and producing a durable drill rod suitable for a wide range of applications. The standard rope thread of the anchor rod produces an excellent bond between the rod and grout, as well as enabling connection to the drilling rigs and a wide range of drill steel accessories.

Extension couplings

The coupling features a patented design that enables direct endto-end energy transmission between each bar, reducing losses and ensuring maximum percussive energy at the drill bit. To enable the correct seating of each bar within the coupler, all bars are chamfered with precision to enable the bar ends to have face-to-face contact.

Bearing plates

The bearing plates are forged steel plates with a center hole, allowing articulation of seven degrees in all directions.

Hexagonal nuts

The hexagonal nuts are manufactured from high precision steel with chamfered edges on both ends from high precision steel, and tempered to meet the stringent demands on anchor specifications and the daily operations of underground work. All nuts exceed the ultimate strength of the bar.

Galvanized anchors

Corrosion is a very complex process, and corrosion rates are very hard to predict accurately. However, analytical evaluations can be helpful to recommend the need for corrosion protected rock bolts. For this reason, MDTI has developed a hot dip galvanizing corrosion protection. The full range of SDA bars and accessories is available with this corrosion protection.

Gap Spacer

A gap spacer is available for specialist applications. For general use the centralizer is not required as it snags during rotation and interferes with grout circulation in the borehole. However, the use of a centralizer does not guarantee comprehensive grout cover along the length of the bar as it is impossible to control the grout within the borehole.

Coupling boxes

Manufactured from thick-wall steel which is machined and case hardened, coupling boxes are used at high wear locations, typically just below the hammer drive. A locking torque is normally applied to the coupling box at the permanent joint to avoid release when additional anchor bars are added. All the coupling boxes are supplied with a bridge to act as a center stop. When a step down (reducer) drive arrangement is used i.e. R38 shank to drive to install R32 anchor bar, a locking torque should be applied at the R38 end to prevent the R32 bar from climbing into the R38 chamber. Step up drive arrangements are not recommended as the additional torque overloads the smaller drive.

Shanks

Rotary percussion is the preferred method of installation. Either a hydraulic or air hammer is suitable. To transfer the rotation and percussion from the hammer to the bar‘s system, there is a need for a shank adapter. MDTI provides a full range of different shank adapters for different hammers.

Grout coupling

After using standard flushing media (water or air), grouting must be done. For connecting the grouting hose from the pump with the anchor bar, the grout coupling is used. The grout coupling will be removed and reused after the grouting process is done

Components of the Self-Drilling Anchor

  1. Hex Nut
  2. Termination Plate for new construction or bracket for underpinning
  3. HDPE Pipe for unbonded length or additional corrosion protection
  4. Soil Cement Grout Body
  5. Final Injection Grout
  6. Magnacore Self-Drilling Anchor Bar allows injection while serving as drill strong
  7. Coupler
  8. Centralizer to keep the micropile centered in the hole to allow for optimal grout cover
  9. Sacrificial Bit
  10. Grout Flushing Holes

Solid Bar Compared to Hollow Bar

Solid Bar

  • Predrilling required
  • Limited to shallow rock
  • Very stiff throughout soil strata
  • Casing needed in soft soils
  • Pre/Post grouting required
  • Several pieces of equipment

Hollow Bar Self-Drilling Anchor Bolts

  • No predrilling
  • Rock at any depth
  • Can be installed in all soil types
  • Casing for lateral loads
  • Grouting completed during install
  • Only need rock drill & grout plant

SDA Applications

  • New construction
  • Underpinning
  • Settlement/stabilization
  • Earth retention
  • Slope stabilization
  • Tunneling

Benefits of Self-Drilling Anchors

  • Difficult soil conditions
  • High installation rate
  • Limited access
  • No casing required
  • Extra corrosion protection
  • Decreased labor & material cost

Why MDTI?

In addition to materials and service, MDTI will provide continual support through the entire project, including engineering/design, on-site support, load testing, installation expertise, competitive pricing, and rental equipment. Call us at 888-262-7797 for more information.