Raising the bar
21 February 2013
Joe Patterson of TEI Rock Drills explains how installation standards have improved the use of hollow bar soil nails in the US ground stabilisation market.
The use and installation of hollow bar soil nails (HBSN) in soil stabilising applications has been increasing since their introduction to the US market over 15 years ago.
Contractors have long known the advantages of using HBSN in collapsing soils and those containing cobbles and the significant cost saving they offer over the use of temporary casing. However, until relatively recently, there were no unbiased sources of test information for HBSN and for this reason, the federal and state transportation departments were reluctant to use them.
In order to promote and standardise the installation of HBSN, the US Department of Transportation Federal Highway Administration (FHWA) and the Federal Lands Highway Department asked the International Association of Foundation Drilling (ADSC-IAFD) to develop, and fund, a test programme to be used when employing HBSN in permanent, soil nail walls on federal or state highways and privately owned projects.
The process for standardising the installation and testing of HBSN originally began in conference rooms and on job sites around the country. Engineers, manufacturers and contractors all worked together to share their knowledge and establish a set of universal standards by which all installations could be measured. In 2006 the FHWA published a HBSN State of Practice document based on the results of the programme. After this was circulated around the foundation industry, it was obvious that a more detailed study needed to be performed, especially for testing. So in 2007, the ADSC-IAFD financed a further test programme with funding from the Industry Advancement Fund, and equipment and time donations from contractors and manufacturers. After three years of testing, analysing, drilling, grouting and in some cases freezing test equipment, the FHWA published the HBSN Test Program in 2010 that describes the standard for installation and testing of HBSN.
TEI Rock Drills has established an annual Hollow Bar Installation School, designed to help train drillers and engineers in the use of these standards and apply best practice principals during HBSN installations. Some of the main areas that contractors need to consider when embarking on such projects are as follows:
As with any type of construction, having the correct people and equipment to undertake the work is the most important aspect of a good job. The people part is different for every company. Whether its engineers, contractors or owners, not everyone will be familiar with this technology. Some people are uncomfortable with HBSN because it’s a new technology new to them and they haven’t received appropriate training. But most people will become believers in the use of HBSN after viewing the results and seeing the cost savings they offer.
There are three main components for the installation of HBSN: the hollow bar and drill bit, the grout plant and the drill rig.
Today there are many manufacturers of hollow bars. They are available in diameters from 32-130mm, with available ultimate loads up to 1,700kps. Ischebek, DSI and Williams are some of the better-known suppliers, but there are more and more suppliers coming out of Asia.
Each hollow bar manufacturer has their own thread design along with their own argument as to which design is the best and why. As with most consumable items, it is important to have a good relationship with your local distributor. You may have a preference for a certain brand of hollow bar, but if the product is not available when needed on the job site, this is irrelevant. So choose a good distributor and a good manufacturer that can provide mill certificates proving the ingredients of the steel used to manufacture the hollow bars.
Drilling equipment varies as much as the geographical areas where HBSN are used. A drifter head (rotary/percussion) is usually required on any drill rig for soil nail installation. Sometimes rotary only will work in sands and clays, but the use of percussive drilling ensures that the HBSN can be installed through any material. With the added power of the percussion it is important not to install the hollow bars too quickly. To ensure good grout coverage the rate of penetration should be no more than 1m every 30-60 seconds.
When the bottom of the hole is reached, plunge the rods up and down 5-10 times to create a “ball” at the end of soil nail. This ball makes pulling on the HBSN like “pulling a nail backwards through a board”. As with all drilling, listen to the rig and adjust the rotation speed and crowd pressure to best accommodate the diameter of hollow bar being installed. Monitor the grout flow as well, or the lack of grout coming out of the hole.
The simplest method for installing HBSN allows the use of almost any drill rig, from tiny limited access models, to excavator-mounted drill masts and the use of full-size rigs. Because no temporary casing is used, it is not necessary to use an expensive anchor rig to install HBSN. It’s best to use the anchor rig to install permanent tiebacks when required.
A drill-mast mounted on an excavator with a drifter is the best machine for installing HBSN. The excavator arm can reach up, over and down, whereas a traditional anchor rig cannot line-up on these types of holes. A bench is also required to accommodate full sized anchor drill rigs which is time consuming and expensive to construct.
The grout plant
The most overlooked piece of equipment used for installation of HBSN is the grout plant. The constant pressure generated by the drill bit can often cut more soil than the drill bit itself, creating the “grout balls” around the HBSN that we all like to see.
The grout mixer also ensures that all of the cement particles are hydrated and that the correct water-to-cement ratio is maintained throughout the installation process. The only types of grout mixers that can be used for these applications are high-shear (colloidal) mixers. Paddle mixers are not acceptable for the installation of HBSN, because they are not capable of hydrating all of the cement particles. In addition, a twin-piston pump is required to ensure a constant grout pressure is maintained at the bit face.
Complete grout coverage is not just important for the capacity of the HBSN, but also for corrosion protection. When drilling, a high water-to-cement ratio is used in the grout, this allows for better permeation of the grout into the soil.
When the final depth is reached, a full strength grout is introduced and pumped the entire length of the hole. This is the time to run the drill head up and down the mast 5-10 times, pumping the grout through the hole and creating the grout ball at the end of the HBSN. Not only does the grout form a layer of physical protection over the steel, but it also forms a passive film that reduces the rate of corrosion. All of this would be lost if the grout cracked due to poor installation methods. Again, look at the grout plant as the most important piece of equipment when installing HBSN.
Testing of HBSN is performed using typical verification testing methods on a sacrificial nail prior to construction. The same installation methods are used as with production nails.
The set-up for testing is similar to testing tiebacks- a reaction frame with cribbing and a steel plate is laid out perpendicular to the HBSN. A hydraulic jack and pump are used to develop the load and two-dial gauges on a support are used to measure any movement during testing. A typical verification test is to develop a load on the HBSN that is two times greater than the design load.
Below is a check list that can be used to ensure best practice when installing HBSN:
- Choose a stocking supplier for your hollow bars.
- Use hollow bars from a manufacturer with steel specifications.
- Whatever rig you use, have a drifter installed on it.
- While drilling, run the hole often to help the grout permeate the soil.
- When drilling is complete, run the drill head 5-10 times to create a grout ball.
- Use centralisers that are plastic- steels will corrode faster.
- Don’t use epoxy or zinc coated bars, use sacrificial steel instead for corrosion protection. The coatings rub off on the borehole during drilling.
- Use only a high-shear grout mixer with a dual piston pump.
- Finish grouting with a .45 water to cement ratio
- Ensure a minimum of 35mm of grout coverage around the bar for corrosion protection.
Although this article is meant to lightly describe the state of practice for installing HBSN, I believe the most important point is that this state of practice was devised by many different people working together to make a good product a great product. Rather than allow individuals to go off and start competing camps of thought, the ADSC and the FHWA pulled competitors together to improve industry standards across the board.
Now that there are standards that must be met, contractors not capable of performing to this standard don’t win contracts; manufacturers that are unable to provide quality materials are not used, and owners now have access to better costing and time saving technology.
This process is proof that great a camaraderie and work ethic that exists in the foundation drilling industry today, and is a shining example of good work.
This process is proof that a great camaraderie and work ethic exists in the foundation drilling industry today, and this cooperation is a shining example of good work.