THE FORT WORTH EXPERIENCE - An Auger Boring Machine used to Jack Concrete Box Culverts under the Railroad

THE FORT WORTH EXPERIENCE - An Auger Boring Machine used to Jack Concrete Box Culverts under the Railroad
by
Rajesh Tanwani, P.E.
Montgomery Watson, Houston, Texas

As part of the West Vickery Blvd. Drainage Relief System, two sets of twin concrete box culverts were installed under the railroad tracks in the southwest part of Fort Worth, Texas. These concrete box culverts were jacked in place under the railroad without any interruption to the railroad service.

Design Alternatives

Several design options were considered to provide the necessary hydraulic capacities under the railroad. Diversion structures were required at two different locations under the railroad. Precast railroad bridge structures were considered but did not prove to be economical. The bridge spans would have been extremely long due to restrictive head requirements and bridge deck depths. A tunneled multi-plate pipe arch and ellipse section were also considered. The existing soil conditions were not able to withstand the concentrated lateral loads produced at the corners of the multi-plate structures. Also, the clearance depths prohibited this design from being feasible as well as structurally sound. The final design solution consisted of precast box culvert section designed for the railroad loadings. To make this option feasible, the construction methods necessary to build such facilities were investigated.

Construction Alternatives

The general contractor initially proposed a temporary bridge design under the railroad so that an open cut and cast-in-place construction method could be used, but this proposal was not approved by the railroad. Therefore, precast concrete box culverts were installed by jacking them under the railroad.

Ground Conditions

The soil encountered was a split face condition with clayey silt at the bottom overlain by railroad fill. This did not present any problems as the face was open and there was full control over the excavation at the face. The soil cover was very shallow and care was taken to avoid over-excavation in the front of the shield to prevent a collapse. Hence the excavation was limited to 6 to 9 inches in front of the shield.

Method Description

For the installation of the boxes, a boring machine was utilized to install two 16-inch diameter steel casings on line and grade at the bottom of the proposed box culverts. These casings were then filled with concrete and served as guide rails for the box to slide on. Since it was very important that these casings be on a precise line and grade, a non-magnetic tracking and guidance system was used on this project.

   After the installation of the casings, the jacking pit was prepared for the installation of the boxes. The jacking pit was 29 feet long and 20 feet wide and had a concrete floor with a concrete backstop sufficient to withstand one million pounds of thrust. Even though a shorter jacking pit would have been sufficient, it was economical to modify the jacking pit used for the installation of the steel casings.

   The boxes were 12-feet wide and 11-feet high with a 12 inch wall thickness, and each box weighed about 32,000 pounds. On the leading box, a steel shield was bolted to assist in the excavation, to prevent over-excavation, and for the safety of the personnel working inside the box culvert during installation. The shield was 1-inch thick and extended 2-feet in front of the leading edge of the box. The shield was reinforced to prevent buckling. The shield ensured that the over-cut was limited to 1-inch as desired. This over-cut reduced the friction between the boxes and the soil. Bentonite ports were installed directly behind the shield to ensure that the space between the boxes and soil was filled with bentonite. The bentonite lubricated the surface of the boxes and reduced the friction between the soil and the boxes.

   A solid steel frame conforming to the dimensions of the box was constructed to serve as a push ring. Another steel frame was constructed to uniformly transfer the jacking force to the push ring. The boxes were jacked in place using an American Augers 60-inch machine rated at 1 million pounds of thrust. The push frame exerted a uniform pressure on the boxes to ensure that the boxes were jacked in the desired direction. The steel casings under the boxes ensured that the boxes were vertically aligned at all times while a laser beam was used to check the horizontal alignment.

   Soil was excavated at the face using a small loader and a small backhoe. A portable platform was constructed to enable the movement of the loader and the backhoe in and out of the boxes. Progress was slow in the beginning, but in the later stages up to 12 linear feet was jacked in a 12-hour shift. Bentonite ports were installed in each section of box culvert and bentonite was used liberally to reduce the skin friction. The maximum thrust encountered was 580,000 pounds. Initially, the jacking was performed only during one shift. After the boxes were allowed to stand over the first weekend, the jacking force greatly increased, and it was decided that the work should proceed around the clock to prevent a freeze-up. Final construction consisted of jacking 105 linear feet of twin 12-feet wide and 9-feet high concrete box culverts and 54 linear feet of twin 12-feet wide and 8-feet high concrete box culverts under the railroad over two month period.

Conclusions

In spite of difficult project conditions and with utilization of innovative technique, the project was completed successfully without service interruption to the railroad. The pipe jacking method is proved to be a viable alternative for tunnel construction.