Launched soil nails for geohazard mitigation

There are many options available for slope stabilisation work and one of them is launched soil nails. The Soil Nail Launcher technology used by GeoStabilization International was originally developed by the British military, explains CEO Colby Barrett, to launch chemical weapons but is now used to accelerate soil nails into the ground at up to 250mph in a single shot.
Launched soil nails for geohazard mitigation Launched soil nails for geohazard mitigation Launched soil nails for geohazard mitigation Launched soil nails for geohazard mitigation Launched soil nails for geohazard mitigation

A tracked excavator is described by GeoStabilization International as the perfect launched nail installation tool for environmentally challenging applications

Colby Barrett

Launched soil nails are a unique technology for geotechnical hazard mitigation. These 6m long, 38mm diameter nails are installed in a single shot using a compressed air cannon at velocities of up to 400kph and rates approaching 250 nails/day. The nails reinforce an unstable or potentially unstable soil mass by transferring the nail's tensile and shear capacity into the sliding soil.

The cannon unit weighs over 2,700kg and is typically mounted on a tracked excavator that has been converted to carry a specialised compressor unit rather than a rear counterweight. The units may also be suspended from a crane or mounted on a long-reach excavator for greater range.

Initial nail installation trials used 38mm diameter, 6m long, galvanized solid steel bars. These solid bars were both costly and heavy, so in early in 2002, galvanized steel tubes were introduced.

Research conducted at project sites confirmed that the lighter tubes could be inserted up to 30cm farther into similar soils than the heavier solid bars. The tubes could also be perforated to act as drains, or pressure grouted to improve bond strength, corrosion protection, and capacity. The typical launched soil nail installation is now a three-step process:

  • A perforated, galvanized outer tube is launched to full depth at pressures between 5515 - 31,000kPa
  • The hollow tube is pressure grouted with neat cement grout
  • A #6 epoxy-coated inner bar is immediately inserted into the wet grout

Fibreglass outer tubes have also been installed in corrosive soil environments and many projects have been completed where pressure-grouted, launched soil nails were installed in combination with un-grouted, perforated launched drains (note that drains should be launched after the grouted nails are installed to prevent grout intrusion into the drains).

Project applications

As of 2020, tens of thousands of launched soil nails and drains have been installed in the US, Canada, the UK, New Zealand, and Australia. Primary applications have been to stabilise shallow landslides, although the technology has been used to stabilise failing sheet/H-pile walls, for temporary shoring, for pipeline stabilisation, and as micro-pile foundation supports for retaining walls. Launched soil nails have been used in a variety of soil and slope conditions, especially in mountainous areas, where rugged terrain limits construction options.

Launched soil nails are primarily fired into sand, silt, clay, and even soils with some cobbles or boulders. However, they are not suitable for sites with large/numerous boulders or very hard, shallow bedrock, in very stiff clays, or in areas where failure surfaces exceed 5m deep.

Launched soil nails have also been specified on sensitive riparian projects where drill cuttings/grout spoils and excavations often associated with traditional drilled, and grouted soil nails would not meet environmental mandates.

 tabilising a roadway with launched nails during active flooding conditions Stabilising a roadway with launched nails during active flooding conditions

Launched soil nails have been used on many notable projects and are credited with saving Interstate 75 in Northern Tennessee from total collapse in 2005 and again in 2012 due to their speed of installation and ability to provide an immediate structural contribution.

In 2005, the use of launched soil nails for temporary embankment shoring prevented slope failure during excavation for the emergency installation of a rock buttress. Because the nails bond with the soil seconds after installation, with no delay required for grout set, shoring of the slope could be conducted at the same rate as the slope excavation advanced without compromising worker safety.

A few miles down the road in 2012, large cracks had developed in the southbound lanes as part of a large, active landslide. Within hours, both southbound lanes had failed, and cracks were progressing into the northbound lanes. Emergency stabilisation work commenced, and within 48 hours, over 250 launched soil nails were installed into the northbound lane and pressure grouted, which prevented scarp regression of the large landslide. With traffic on the northbound lanes safely restored, the southbound lanes could be stabilised with post-tensioned strand anchors and then reconstructed using a large rock buttress.

Installation theory and corrosion protection

The compressed air cannon induces tensile stresses in each tube as it penetrates the ground. This tension counteracts the compressive stresses caused by the displaced soil and thereby prevents nail buckling. The single impulse, high-installation velocity creates a shock wave at the nail tip, which displaces the adjacent soil as the nail penetrates. It is important to note that during the majority of the nail's flight into the soil, the main frictional resistance occurs at the nail tip (not along its length) due to the elastic over-deformation of the soil induced by the rapid impulse.

To demonstrate this phenomenon, paper stickers were placed on the outside of nails that were launched into a gravel pile, and the nails were later carefully exhumed. The stickers remained unabraded even after travelling up to 5m into the gravel.

This phenomenon also explains the higher-than-expected bond strengths seen in launched soil nails versus driven soil nails. Like driven nails, the soil displaced by the nail is densified (thus creating higher normal stresses along the nail shaft), but unlike driven nails, launched soil nails create minimal disturbance to the surrounding soil because of the rapidity of the single impulse. Consequently, launched soil nail unit bond strengths typically exceed those of driven nails, and can exceed those of conventionally drilled soil nails using open-hole drilling techniques.

Launched soil nail bond stresses also tend to increase over time, with studies showing up to 30 per cent increases over a three-year period. Experts theorise that the mechanism for this time-dependent bond strength increase is due to excess pore water dissipation and soil/nail cohesion increase over time.

Launched soil nail design

Launched soil nail design methodology is outlined in the joint USFS/FHWA Application Guide for Launched Soil Nails and relies on the theory that launched soil nails resist soil movement by acting in both tension and shear. In a drilled and grouted nail, by contrast, nail shear contributions are typically ignored.

To understand this difference in design assumption, it is important to understand that unlike traditional drilled and grouted soil nails, launched soil nails have a much higher shear capacity to axial capacity ratio. Shear capacities of up to 20 per cent or more of axial pull-out capacity have been observed in launched soil nails (compared with typical values well below five per cent for traditional drilled and grouted nails). Because of this difference, the shear component of a launched soil nail is not ignored as it would be in traditional drilled and grouted soil nail design.

The ultimate shear resistance of the nail is not controlled by the shear strength of the nail material, but by the ultimate bearing capacity of the soil in a localised area near the active failure surface. This localised bearing failure develops over a short section of the nail on either side of the failure plane, typically 1m or less. Typical shear resistance values range from 14 - 57kPa.

 he small footprint of the machinery used during launched nail installs allow for active traffic during mitigation activities The small footprint of the machinery used during launched nail installs allow for active traffic during mitigation activities

Although the USFS/FHWA manual provides a detailed discussion of the equations and mechanisms behind launched soil nail capacity, the manual models shallow landslides and embankment failures as a planar sliding wedge, ultimately presenting simplified charts to determine nail spacing for various slopes. These charts, however, do not allow for non-uniform slopes, water tables, or slopes with non-uniform materials.

Between 1994 and 2013, if designers wished to model a slope that did not fit neatly into the charts, they were forced to employ a more tedious design method using nail input parameters from the USFS/FHWA manual. In 2013, the programmers who developed FHWA's SNAP (Soil Nail Analysis Program) and SNAP-2 created the free programme LSNAP (Launched Soil Nail Analysis Program). This software allows designers to quickly perform calculations that previously required many hours. In addition, designs can be produced using either ASD or LRFD formats, with both static and dynamic loading, and with highly complex soil geometries. Today, most launched soil nail design is completed using Rocscience's SLIDE software, which includes launched soil nails as a reinforcement type.

Launched soil nails have also been mentioned in other federal design documents. In the 2003 version of Geotechnical Engineering Circular (GEC) No. 7 - Soil Nail Walls, FHWA noted that launched soil nails were "bare bars" that were "only used for temporary nails" and that the method was "not currently used in FHWA projects." Twelve years and many federally funded, permanent launched soil nail projects later, the 2015 rewrite of GEC #7 eliminated the "temporary" restriction and noted that the "technique is applicable to landslide repairs, and to roadway and embankment widening."

Perhaps the most accurate federal guidance on the technology since the publication of the USFS/FHWA manual in 1994 can be found at geotechtools.org (see "GeoTech Tools - Your Ground Modification Website" in the November / December 2015 issue of GEOSTRATA, pp. 38-42, 44). The GeoTech Tools website contains case studies, cost data, and other useful information on launched soil nails and myriad other innovative, geotechnical construction technologies, and notes that the advantages of launched soil nails "include rapid construction, easy monitoring and testing, construction with limited headroom and right-of-way, and ability to withstand large deformations."

New applications and the future

Because of their speed of installation, technical characteristics, and relative cost compared to drilled soil nails, novel applications for launched soil nails continue to be developed.

From foundation supports for solar farms to gas vents for landfills, new non-slope stabilisation ideas for the technology may be viable in the future. At GeoStabilization International, engineers continue to explore a range of innovative ideas emerging through this technology.

 

topics