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These tunnels were constructed to provide a conduit for storm water to be more efficiently transported to the ocean in areas known to be prone to landslides initiated by water inundation. This work was performed just south of the Portuguese bend area of Rancho Palos Verdes, CA. The tunnels were constructed using modified SEM techniques supported by TBM equipment.

Drill Tech constructed the smaller slant drain tunnel using a digger shield supported by continuous driven steel casing, and the longer rib and lag tunnel using an open face TBM supported by steels rib and lagging through sandstone and siltstone. Both tunneling machines were 7’ diameter and consisted of an open shield with a carriage mounted roadheader attachment. Additional work included constructing the inlet and outlet structures. After the tunnels were excavated, 60″ diameter welded steel conveyance pipes were installed in the tunnels and the annuluses were filled with cellular grout.  

This job was part of The Colorado Department of Transportation’s project to reconnect Little Thompson River to Muggins Gulch. Since 1937, the river has been channeled into US 36’s roadside ditch, where floods have washed out the river channel and caused substantial damage to the roadway embankment. This drainage structure contributes to the permanent solution of directing the river path beneath the highway to minimize erosion damage and road closures in the event of future flooding.

Drill Tech constructed an 11′ diameter horseshoe tunnel for the East Culvert under the historic US 36, approximately 60′ below the road. Although the tunnel method was originally specified as shield, part of Drill Tech’s Value Engineering changed the method to conventional excavation utilizing an AM75 roadheader and mini-excavator due to the high risk of encountering large granitic boulders within the tunnel envelope. The project was successfully completed on schedule despite multiple challenges including encountering boulders in excess of 7′ diameter and multiple shutdowns due to nearby forest fires.

Drill Tech is contracted to perform the excavation and support for shaft and tunnel construction as part of Loudon Water’s project to fully develop an abandoned quarry into a raw water storage and pumping facility for water sourced directly from the Potomac River. The new reservoir will store over 1 billion gallons of water for its 5 million patrons. The new shaft and tunnels are constructed near and underneath multiple waterways and adjacent to historic construction, which requires careful coordination and a small job-site footprint.

The 2,400 LF of tunneling is split between three horseshoe tunnels that intersect the shaft at different elevations. Additional Drill Tech scopes on this project include the excavation of a new 6ft diameter vertical shaft utilizing raise bore methods, a new 36in and 42in diameter steel water pipe in the shaft and tunnels (installation, welding, and annular backfill), installation and contact grouting of a new 20ft thick concrete tunnel plug, and draining approximately 250M gallons of water from the existing abandoned quarry.

Consistent deterioration of the Lenihan Dam’s steel pipe outlet structure required construction to replace it with a new outlet pipe at Lexington Reservoir, Santa Clara Valley Water District’s second largest reservoir. Since tunneling occurred directly adjacent to and under the active reservoir, extensive probe hole drilling, spiling, and cut-off grouting was required. Drill Tech excavated through a Franciscan Mélange material ranging from extremely hard rock to crushed clay. A heavy invert slab was required to control the squeezing ground conditions in the clay, and extensive convergence monitoring was performed.

Drill Tech excavated the tunnel and performed initial lining that consisted of W6x25 steel sets spaced on variable centers and variable thickness steel fiber reinforced shotcrete lagging over timber lagging and crown bars. After driving the tunnel, a final cast in place concrete lining was installed and a 54″ diameter outlet pipe was installed in the tunnel. The tunnel was connected to the surface of the lake by a 16′ inside diameter, 50′ deep shaft, that was lined with secant piles and constructed less than 10 feet from the lake’s edge.

There were several logistically challenging aspects of this project that included small work areas, difficult environmental protection regulations, and the potential for high penalty liquidated damages in the event of late pipe service connection. Additionally, crew members had to access the site through public walking and biking trails that remained open during construction. Drill Tech finished this project ahead of schedule and performed the necessary planning and coordination among contractors, stakeholders, and the public for maximum efficiency of space. 

For a new raw water intake structure, Drill Tech installed a shaft and three tunnels at the edge of the South Fork Holston River. From the shaft, a 9′ diameter horseshoe tunnel was driven 1,100′ from the river to the existing water treatment plant on the opposite side of TN State Route 93. Four 48″ diameter shafts were drilled down to intersect the tunnel from the surface of the treatment facility. Extensive cut off grouting and chemical grouting were required throughout tunnel excavation to minimize groundwater inflows of up to 300 gallons per minute. Mechanical piping, electrical pumps, and CIP concrete structures were installed to pump water vertically into the treatment facility for processing.

Roughly 320′ of the tunnel was constructed in Limestone using jacklegs and drill/blast methods. The remaining 780′ was constructed in claystone and excavated using an AM-50 roadheader. Hollow bar spiling was implemented as extra ground support in areas of minimal cover. Drill Tech was responsible for all ground vibration monitoring during blasting activities, with blasting that occurred within 7′ of existing structures.

The rich history of Colorado’s Carlton Tunnel, originally known as the Busk Ivanhoe Tunnel, spans over 130 years. Although initially constructed for rail access in the late 19th century, it has served various purposes for vehicular transit and water conveyance (the latter being its current operations under the Busk Ivanhoe Water System Authority).  The existing tunnel is 16’ wide by 20’ tall, with both bald headed and timber supported sections. Many of the timber sets and associated blocking have degraded since installation, resulting in substantial raveling and caving in many areas of the tunnel. While there have been several repairs throughout the decades, substantial obstructions have affected water flow through the tunnel’s 36″ concrete pipe. 

Drill Tech is performing ongoing tunnel rehabilitation (1000 LF to date in the 9,500 LF tunnel) to establish safe access to the multiple cave-ins affecting the conveyance pipe and has encountered portions of the collapsed tunnel reaching over 30′ in height.  Once the cave-ins are safely reached, debris is removed to restore flow in the pipe, which is to be replaced with a new HDPE pipe. Drill Tech provided Early Contractor Involvement recommendations to the design and construction phase services, and this project has remained a collaborative effort between the owner, engineers, and Drill Tech services.

At nearly 11,000′ elevation, with the closest paved road being 14 miles away, the remote location of this project has presented logistical challenges of access that require careful coordination of the team as materials are ferried up to the jobsite. The goal of this project is to ultimately increase water capacity and re-establish the tunnel’s ability to safely support traversing service vehicles.