Hiway delivers another winner

An innovative company’s launched soil nails save time and money in certain slip remedial work.

Hiway.jpgHiway Stabilizers, long regarded as a leader in the development of stabilisation techniques in New Zealand, is successfully employing an innovative technology for slope remediation – launched soil nails.

Hiway Stabilizers Environmental, formed in 2004 by the parent company as a standalone business, is promoting launched soil nails as a fast and effective tool in the construction and remediation of over-steep slopes and/or over-steep excavations in soils.

Hiway recently completed slip-remediation work at Biscuit Bend on State Highway 2 in the Whareratas south of Gisborne, where it was subcontracted to Quality Roading & Services. Hiway had to install the soil nails at the base of the slope, excavate slip materials, install subsoil drainage, and build two geosynthetically confined soil walls.

The work, an alternative to the specified construction, produced significant cost savings, shorter construction time, and reduced safety risks.

In another recent project, for Rodney District Council, Hiway constructed a three-metre-high masonry-face wall on top of a foundation system consisting of Shotrods installed vertically as micropiles, with a reinforced concrete beam connecting them. This method avoided having to excavate into the soft, silty clay.

Launched soil nails have been used on some 400 projects worldwide and on more than 40 in New Zealand. Since early 2006 Hiway has had a licence agreement with United States company Soil Nail Launcher Inc to use the technology, known as Shotrods, in this country.

Launched soil nail technology was developed in Britain in the 1950s for military purposes but about 20 years ago was adapted and patented for use in the UK and Europe for civilian use, mainly on road and rail landslide projects. In 2002 Soil Nail Lancher Inc began operations in the US.

The Shotrods high-pressure, compressed-air launcher is mounted on a 20-tonne excavator and is able to fire a 40mm-diameter tube up to six metres long into suitable soils at more than 300km/h. Most applications involve installing the rods in horizontal rows in a slope, with the rods spaced one-metre apart and staggered in a diamond pattern.  

Launched soil nails can be used for slips, settlements, small landslides, temporary shoring and excavation support, and failed retaining walls.

Hiway Stabilisers Environmental’s engineering manager Andy O’Sullivan says launched soil nails have a number of advantages. They can be installed rapidly (“a hundred in a day”) and once installed are at full capacity. This means the remedial work can be completed in days against the weeks of a more conventional solution, leading to minimal disruption and environmental benefits.

He says the technology is also versatile – installation equipment is highly mobile and flexible, the soil nails can be used as micropiles (fired vertically) for foundation support, and the nails can act as horizontal drains. 

Hiway Stabilizers Environmental is also promoting the advantages of geosynthetically confined soil (GCS) walls. O’Sullivan says more traditional solutions such as deep soil mixing, post and panel retaining walls and gabions have limitations, depending on the nature of the slope failure. GCS walls, on the other hand, cannot fail as long as the spacing is kept narrow and the soil is sufficiently confined.

O’Sullivan says GSC walls can greatly reduce the width required, when compared with conventional reinforced soil structures. They are also quick to build; they are relatively low tech, so no specialist training is required; they lead to better quality control on site by forcing the contractor to adopt good work practices as the compacted geotextile layers must be no more than 200mm thick; and they allow several facing options – shotcrete, soil socks, masonry blocks, and architectural blocks.

O’Sullivan says that while launched soil nails and GSC walls are not suitable for all situations, they are technologies with proven international track records that reach back 15 years.

“When the technologies are combined, projects can often be completed in 20 percent of the time of conventional solutions ­– days as opposed to weeks – and with very significant cost savings,” he says.

“We get a huge amount of repeat work. Clients are very happy when they see the results.”

• This article was adapted from a paper presented by Andy O’Sullivan at this year’s annual conference of Ingenium, the organisation for professionals engaged in the management and engineering of public infrastructure.  

Contractor Vol.32  No.7  August 2008
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