Hollow bar is our most popular micropile as it offers quick installation rates and can penetrate boulders and rock using the rotary percussive action of our Marini ID MA100 rock drill. Normal design loads using DYWI ® R32 / R38 or Ischebeck 30/40 range from about 50kN to 250kN both in tension and compression.
The tendon of this micropile acts as a sacrificial drilling rod, load bearing element and injection point all in one. These micropiles are designed so that the high capacity steel hollow bar carries the majority of the load. The grout or drilling fluid suspension acts as a casing, preventing collapse of the borehole whilst also providing buckling protection for the bar through ground improvement and the geotechnical bond at the grout/soil interface.
A system of micropile classification from Sabatini et al. (2005) is based on the construction of the micropile. It provides four categories, denoted by the letters A to D, based on the grouting method, but includes the installation method and reinforcement type as sub-classifications. Bruce et al. (2010) proposes that the hollow bar system be placed in a fifth classification, Type E. Gomez et al. (2007) and Mitchel et al. (2007) agree with this proposition and suggest that the bond strengths achieved through the simultaneous drill and grout system is much greater than those provided by Sabatini et al. (2005).
We used hollow bar from two German manufacturers, Friedr. Ischebeck GmbH and Dywidag-Systems-International (DSI). Although carrying a much greater material cost, hollow bar micropiles offer quicker installation rates compared to traditional methods such as augurs.
Micropiles are generally designed in friction due to the large surface area of the shaft compared to the surface area at the toe; the loading is dependent on the frictional forces at the pile/soil interface and the surface area of the shaft. In areas of poor ground where the overburden is relatively shallow, the primary factor in our design is the diameter and length of the rock socket.
Hollow bar cannot be integrity tested due to the small diameter of the grout body. Normally, we complete dynamic load testing on 10% of all micropiles. We can also offer static load tests if required, but many designers consider the dynamic tests sufficient due to the costs.
Our rig can work in restricted access and low headroom areas and is powered by a separate diesel power pack, so fumes are not a concern. This system is also dust-free due to the simultaneous drill and grout system.
- Works in tension and compression,
- Can penetrate boulders and rock,
- Minimal vibration,
- Low head room,
- Limited access,
- Good installation rates,
- Can be installed at angles (raking),
- No dust.
- Cost of bar and accessories are relatively expensive,
- Relatively slow penetration rates in bedrock.