TEI customer GeoFirma LLC, Nashville, Tennessee recently completed a difficult earth retention project near Nashville, Tennessee for a natural gas pipeline requiring new techniques and unique equipment. GeoFirma teamed up with TEI Rock Drills to solve the technical and mechanical problems associated with this job.
The project consisted of the construction of a new, 13- mile long, by 20-inch diameter, natural gas transmission line. As an upgrade of the system, the new line allowed the pipeline owner to comply with the new Federal DOT
pipeline inspection and safety standards. The alignment includes several areas of steep to very steep terrain in the rolling hills south of Nashville. A portion of the steep terrain is within the Fort Payne Geologic formation. This formation is known to include colluvial soils. Colluvial soils are gravity-placed soils that build up on, and below the steep slope in this formation. The soil being gravity-placed is at or near its stability equilibrium, thus providing a small factor of safety against slope failure or landslides. Any disturbance of these soils, removal of stabilizing vegetation, or increased water saturation can cause a loss in stability and produce slope failures. A slope stabilization system was needed to assure the pipeline would operate safely during its expected lifetime.
GeoFirma LLC Scope
GeoFirma LLC was contracted by Sheehan Pipeline, the project’s general contractor, to design and install a system to increase the slope stability of the high-risk areas of the South Nashville Pipeline alignment. This included 1,400 linear feet of a 50-foot wide alignment with grades as shallow as 4Horizontal to 1Vertical, and as steep as 1Horizontal to 1Vertical. The system had a dual purpose of reducing the risk of movement of the actual buried pipeline and increasing the stability of the entire 50-foot wide pipeline right-of-way. It included soil nails and rock anchors drilled and grouted perpendicular to the slope surface and Geobrugg Tecco 3mm wire mesh. Over 2,000 holes were drilled for this project. The holes were 10-20 feet deep with a 4-inch diameter, and were installed in a 6-foot by 6-foot grid across the right-of- way. All of the holes were drilled “open-hole.” A solid bar steel element with a tremmie tube was inserted and then grouted to complete the hole. There were several holes located in the ditches that would not stand open during drilling. Hollow bar soil nails were used in order
to solve this problem and to avoid having to install casing in these holes.
GeoFirma used the Geobrugg Tecco® System3 3mm wire mesh with a tensile strength of over 1770 N/mm2 as the facing for the pipeline slope stability project. The mesh came in 11.5 ft by 98 ft rolls. Once positioned at the top of the slope, the mesh was very easy to roll out down the slope. The T3 clips connect the seams of the mesh without any tools. Additionally, the T3 clips made it possible to connect the mesh at the seams without any overlap since the connection is full strength or they could overlap the mesh in any pattern needed to contour to odd shapes of the slope. The mesh was preloaded to reduce slope movement after installation. This involved digging divots around each nail prior to rolling out the mesh. The divots were approximately 2 foot in diameter and 8 to 12 inches deep. The mesh was loaded by drawing the mesh into the divot with the spike plates and hexnuts. The hex nuts were tightened with a large pneumatic impact wrench. All in all the Geobrugg Tecco slope stability system is one of the quickest systems to install. In addition, Ruvolum® dimensioning software made it possible to quickly and accurately plan safety measures allowing for optimal spacing and depth of the soil nails and rock bolts. The TEI MT100 Mountain Drill proved to be a perfect match with the Tecco slope stabilizing steel mesh.
Eric Snyder, owner of GeoFirma LLC, describes some of the drilling challenges encountered while planning the job. “The drilling for the shallow slopes (less than 2H/1V) was primarily accomplished with the TEI HEM550* excavator mounted drill. The hollow bars, when needed, were installed with a TEI Skid Steer drill and a TE350 drifter. It was our original intent to utilize the excavator- mounted drill on the 2H/1V slopes as well, however, the rainy weather of the fall and winter prevented the excavator from traversing a 2H/1V slope. We already knew that a wagon-type drill would be necessary for the 1H/1V slopes and now we knew we would need the wagon for more of the project. Our dilemma came from trying to decide on whether to have a custom-built wagon manufactured for our existing Skid Steer drill and dealing with the issues of the long hydraulic hoses and a remote hydraulic power pack, or mounting a diesel engine on the wagon and routinely changing the mounting angle of the engine to match the slope angle.” Neither of these options seamed easy or practical. Both of the problems were solved and eliminated by utilizing the new TEI MT100 Mountain Drill. The drill uses an onboard electric motor to turn the hydraulic pump which “has proven to be exceptionally easy to use” says Snyder. Pulling a single electrical cord is much easier compared to dragging multiple heavy hydraulic hoses. “Having only the air and electric line to deal with is huge,” added Snyder. Another bonus in using an electric motor with a diesel generator is that it is much more fuel-efficient
than a traditional diesel/hydraulic power pack, burning 50% less fuel per day. Snyder reports that his crews would fill the air compressor with fuel once or twice a day while filling the generator only once every three to four days. “The drill’s hydraulic winch and rear-steer wheels made it easy to move and position the drill, and the MT100 provided a very stable drilling platform” said Snyder. The most unique component on the drill is the variable volume hydraulic reservoir. This feature and an electric motor are what give the Mountain Drill the ability to drill at any angle. Since this formation was known to be unstable the soil nails and netting were part of the original construction plans. The pipeline was successfully installed during the summer months. This project highlights how qualified and creative geo-construction specialty subcontractors using innovative equipment can tackle the most challenging projects.