Microtunneling Vertical Curves: New Solutions for Utility Installations

Developments in the trenchless construction industry are resulting in new methods to install critical utilities through challenging conditions. The rise of the direct pipe method and advances in horizontal directional drilling (HDD) are providing owners and engineers with different options to allow for completing their projects.

The microtunneling method continues to evolve, as well. While continuing to offer the well-established quality control characteristics that have led to its popularity in North America, microtunneling innovations are allowing engineers and contractors to design and construct more unique, cost-effective installations. At the forefront of these developments is the design and construction of curved tunnel alignments, including vertical curves, significantly minimizing shaft depth requirements needed for tunnel clearances.

Bradshaw Construction Corp. recently completed such a project on the SWTP Water Transmission Line in New Braunfels, Texas, the first vertical curve in company history and one of the few ever designed and constructed in the United States. The project involved the installation of 2.5 miles of water main, with a 412-ft by 48-in. ID tunnel underneath the Guadalupe River. Pesado Construction of San Antonio, Texas, was awarded the project by the owner, New Braunfels Utilities. Project engineer Plummer partnered with Aldea Services Inc., the tunnel sub consultant on the project.

To maintain the required 12 ft of clearance between the river-bottom and the top of tunnel, 60-ft and 40-ft deep shaft excavations installed into challenging claystone would have been required for a flat tunnel grade. Instead, a 30-ft deep launch shaft was proposed, with the tunnel to be installed on a steep downslope (19.73 percent) through a tight 620-ft radius vertical curve, and recovering on a steep upslope (17.10 percent).

The construction of a tunnel on a steep downward launch slope and into a tight vertical curve presented unique challenges necessitating preparation and collaboration beyond a typical microtunneling project. With the belly of the vertical curve 30 ft deeper than the launch shaft, maintaining safe access and egress into and out of the tunnel was critical. Slurry and lubrication management had to be precise to prevent any issues with safe footing. All connections, including lighting, communication and ventilation systems, were regularly inspected as they passed through the curve. Water cut-off capabilities needed to be available in the event that the opening of the pipe joints on the outside of the curve allowed for any water inflows. With such a tight radius curve, Bradshaw sourced 48-in. WEKO seals to seal joints as safety precaution.

Coordination with critical suppliers and the collaborative engineering of their components was a significant key to the project. Bradshaw utilized 48-in. reinforced concrete jacking pipe provided by Vianini Pipe, Inc. and hydraulic joints provided by JackControl AG out of Switzerland. This was the fifth curved alignment performed by Bradshaw, and all five were built in partnership with Vianini and JackControl. The hydraulic joints allow for appropriate load distribution through the RCP joints as they pass through the curve. The RCP was designed to allow for passage through the curve without joints opening beyond the needed bearing surface of the pipe gaskets. While a 620-ft radius curve was achievable, it should be noted that any significant steering deviations would have resulted in joints opening and seal challenges.

With careful attention to detail and constant effort, the tunnel was installed within the contract tolerances and without significant steering corrections. Bradshaw utilized the TUnIS Navigation MT Gyro system by VMT, a combined gyro/hydrostatic water level guidance system. For all other curved drives previously installed, Bradshaw has utilized the VMT SLS Microtunneling LT system, a pipe mounted motorized theodolite system. However, given the tight radius of the curve and the vertical alignment, the positioning of the theodolite system within the 48-in. RCP would be challenging, located in the way of workers and requiring repeated updates. Ultimately, the selected guidance system aided the operator in successfully installing the tunnel to the project tolerances.

One of the more unique challenges was launching the tunnel on the -19.73 percent downslope from the launch shaft. Bradshaw originally considered pouring the slab to grade with over 5 ft of fall across the shaft. However, concerns over constructing a safe environment for the workforce with this much fall within the shaft lead to exploring alternative solutions. Bradshaw instead constructed a steel wedge to be mounted below grade on a flat working slab, against which the thrust block was poured and the jacking frame was set on grade for the launch and installation of the tunnel.

The tunnel was an immense success. Just over four weeks after launching, the MTBM broke through on the far side of the Guadalupe River. Rather than requiring a recovery shaft, installing the tunnel on the vertical curve allowed for retrieving the MTBM out of a 10 ft deep unsupported trench.

One of the few unexpected challenges of the tunnel was a pipe-jacking paradox. Typically, any pipe-jacking operation involves management and minimization of the required jacking forces to advance the pipe string. For this project, jacking forces were nearly non-existent for the first 360 ft all in claystone, with loads typically in the same 40 to 50 ton range as the contact force needed to excavate the face. The final 60 ft transitioned into alluvial gravel, seeing an increase to 100 tons by the end of the drive, indicating that the annulus within the claystone was being fully maintained and well lubricated. With the steep slope of the alignment and lack of any notable friction along the pipe string, the RCP unexpectedly moved while removing the intermediate jacking system cylinders, closing the IJS and opening joints higher in the curve. While the joints were closed and leaking annulus inflows were sealed, it was a unique issue worthy of future consideration. Immediately thereafter, the tunnel annulus was grouted, the HDPE was pulled through on casing spacers, and the tunnel was backfill grouted and completed.

Vertical curves are an important addition to the toolbox owners and engineers have available to them when designing projects. Microtunneling minimizes the construction footprint, equipment layout and surface disruption relative to other trenchless methods such as Direct Pipe. Unlike the HDD method, the borehole can be continuously supported with RCP during operations, and the required slurry volume is significantly minimized. Bradshaw Construction Corporation is proud of the successful installation of the vertical curve tunnel on this project and will continue to pursue innovative microtunneling methods to meet the utility needs of the future.

Jordan Bradshaw is project manager at Bradshaw Construction Corp.

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