Austin’s bike bridge at Barton Creek delayed by geology hiccup

Updated Nov 11, 2014

The city of Austin’s quest to span the Barton Creek greenbelt and nearby Loop 360 with bridges for cyclists and pedestrians, close to a decade in the planning, is six months behind schedule as the city and its state partner grapple with a geology problem.

Construction on the almost $11 million, two-phase project began early this year with tree clearing south of the creek and construction of bike lanes on the MoPac Boulevard frontage roads abutting the greenbelt. But the project has been mostly put on hold as consulting engineers redesign underground structures that would support the 1,045-foot-long, 14-foot-wide bridge over the creek and greenbelt. The bridge’s concrete path will be 12 to 14 feet below the northbound highway bridge just to the west, officials said.

The problem, city of Austin and Texas Department of Transportation engineers said, is limestone bedrock that, when crews began drilling shafts for the bridge supports, turned out to be farther underground than preparatory soil borings had indicated.

“It’s not solid rock like we thought,” said TxDOT area engineer Mike McKissick. “We hit a lot of sand and gravel.”

Chad Crager, a civil engineer and the city’s bicycle program division manager, said the borings, some conducted recently and others dating to MoPac’s construction in the 1980s, had indicated that the limestone bedrock was generally just a couple of feet below the surface of the steep hillsides on either side of Barton Creek. Instead, when work began south of the creek on what will be seven columns, the solid rock was four to 15 feet below the surface.

The depth of that hard rock is important, Crager said, because the project lies over the Barton Springs segment of the Edwards Aquifer. The city, in consultation with environmental groups, had hoped to keep its drill shafts above the highest historical point of the aquifer’s water table. The concern was that the construction process would put pollutants in the underground waters, which feed Barton Springs several miles to the north.

Instead of the single concrete columns found on highway bridges, including on MoPac just feet away from where this bicycle bridge will sit, the city and TxDOT had planned instead to sink a series of 9-inch-diameter “micro-piles,” with as many as 18 of them at each bridge support location.

“We wanted something that was shallower but would have the same effect” as TxDOT’s typical larger columns, Crager said.

The unexpectedly low bedrock, however, has forced engineers to make some subtle changes in that piling design, including adding more of them and going deeper. That will take them into the water table, or at least below that historical maximum level, Crager said.

“That’s obviously going to change the schedule” and add cost, Crager said. How much cost? Both Crager and McKissick said that amount is under negotiation. But Crager said the city, which is paying for about half of the project with a federal grant and $4 million approved by voters in a 2012 bond election, and TxDOT have enough set aside to cover the overage.

Work on both the south and north hillsides has been almost completely shut down for about three months during the trip back to the drafting table, McKissick said, and resumption of work on the bridge is still a few months away. The current estimate, he said, is that the bridge over Barton Creek and the associated concrete paths should be done by about a year from now at the latest.

Meanwhile, TxDOT has accepted a bid from another contractor to build the second phase, which includes a 100-foot-long bridge over the southbound MoPac-to-eastbound Loop 360 ramp, a 260-foot-long bridge over Loop 360 and a connecting ground-level concrete path alongside MoPac’s northbound frontage road. That path will connect to an existing sidewalk at Tuscan Terrace.

That second and final phase should begin in January and be done by spring 2016, McKissick and Crager said.

City officials hope the bridges and paths, abetted by a future 5-mile-long path from the greenbelt to beyond the Oak Hill “Y,” will encourage people in Southwest Austin to travel to downtown by bicycle, at least some of the time, rather than always using a car.