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Record Jacking Project In San Antonio
“The shaft was located on the downstream end of the pre-existing 9 foot box culverts due north of the new box culvert location,” he continued. “The northeast rear corner of the shaft fell within inches of an eroded river bank, further complicating shaft construction, forcing us to re-position the front of the shaft within inches of the access ramp to Interstate 37 north. In order to accommodate the diameter of the boxes, the width was increased to 18-feet wide by 24-foot in length. The overall shaft construction was completed in approximately five days; this included the jacking setup used to move the massive boxes underground. Spoil from the shaft was distributed around the job site for removal at a later date.”
The size and weight of the boxes (13-foot outside diameter, each weighing approximately 50,000 pounds) mandated the use of a 100-plus ton crane. Logistically, because of the geographical location, delivery of the box culverts was restricted to between 9 a.m. and 4 p.m. Each box had to be transported individually due to size and weight by the manufacturer from a plant located near Dallas.
Next came jacking the boxes in place. The process is quite simple, said Ritchards. Four 500-ton Rogers jacks, operating in unison, were powered by a proprietary hydraulic system. They moved the huge boxes through the shaft. A laser provided accurate monitoring of the line and grade of the string of boxes.
“Each individual box was jacked inline the total distance of 294 linear feet of the shaft, one at a time,” said Ritchards. “We did it the old-fashioned way -- hand mining via pneumatic clay shovels. Once each box exited the shaft, another was placed directly behind and pushed through. We anticipated moving approximately one box per shift. Double shifting would come later when jacking pressure became a concern. However, the reality became approximately one to three foot per shift due to the rocky subsurface conditions.”
With each box weighing approximately 50,000 pounds, the entire string weighed in at about 2,450,000 pounds or 1,224 tons of reinforced concrete and steel.
A primary concern during the jacking was the location and angle in which the rear of the shaft came in contact with a river bank and created a difficult situation with regard to space and design. There was concern about blowing out the back of the shaft from jacking pressures.