May, 2009 MEETING
Wednesday, May 13, 2009

TECHNICAL PROGRAM

Recent Advancements in Reinforced Soil Technologies and Landslide Mitigation using Geosynthetically Confined Soil and Launched Soil Nails

Speaker: Mr. Colby Barrett President of Soil Nail Launcher, Inc.(SNL), Grand Junction, Colorado, Tel: 303-909-6083

PRESENTATION SUMMARY

To an audience of about 55 at the HESS Club, Colby Barrett, who holds degrees from The University of California at Berkeley and Yale University in Chemical Engineering and Law, and who is now working toward his PhD in Geotechnical Engineering and has been with Soil Nail Launcher, Inc. since 2004, gave a slide presentation titled, "Recent Advancements in Reinforced Soil Technologies and Landslide Mitigation using Geosynthetically Confined Soil and Launched Soil Nails."

Mr. Barrett's presentation for the first half was the result of 40 years of research by the Colorado DOT (CDOT), the FHWA, the U.S. Forest Service and several universities in the U.S. and beyond. He first discussed the history of reinforced soil which dated back to 1000 B.C. in Irag, which became active again in modern times in the mid '70's. He showed details of CDOT's research of a different type Mechanically Stabilized Earth (MSE) wall in the '80's that was much stronger than calculations showed. In fact it was so strong that when tested, the wall could not be failed with increasing surcharge.

Now called, Geosynthetically Confined Soil (GCS), this system has since become common for bridge abutments and other applications that have significant surcharges. It is constructed out of granular soil (with minimum phi angle = 34 degrees, maximum PI = 6) compacted with vibratory rollers in 8" lifts separated by geo fabric. The current standard used to design GCS walls is the Transportation Research Board's National Cooperative Highway Research Program (NCHRP) Report 556: "Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing".

For the second half of his presentation, Mr. Barrett discussed Soil Nailing as a method of stabilizing existing embankments. His company launches the soil nails using a single shot, compressed air canon adapted from a concept that was developed by the British military for launching chemical weapons. The launching mechanism pulls the nail out of the gun such that the nail will not buckle as it is almost instantaneously driven into the soil. The penetration is ideally about 18 feet, though penetrations as much as 34 feet have (unintentionally) been achieved in a single shot. Because the pipe is under tension during the travel, its diameter is slightly reduced, which reduces the skin friction resistance from the soil during driving.

The nails are typically 20 ft. joints of 36 ksi yield steel pipes although fiberglas is sometimes used in a marine environment to avoid corrosion. Because pipes are used, the nails are able to drain the slope later, thus reducing the weight of slope and increasing the soil's shear strength, both helpful in improving the slope stability. The muzzle velocity is 220 MPH, creating a noise level of about 85 decibels, less than that of conventional impact pile driving which according to Mr. Barrett creates 93 dB. To stabilize a slope, the soil nail launcher and personnel are able to work off the upper surface of the embankment as shown in the photo.

Typically the soil nails are driven on a 3 ft. grid and are trimmed with torches after driving. Reload times are 1 to 2 minutes making it a much quicker job than installing conventional retaining walls and also less expensive. The installed cost is around $400 - $600 per nail.

For a video of the soil nail launcher in action, click here

To download Mr. Barrett's slide presentation, click here