Paula Wallis, TunnelTalk

(Supplied by Espey Consultants Inc.)

Construction is underway on a mile long tunnel to alleviate deadly flooding in downtown Austin, Texas, and reclaim land in the 100-year floodplain for future development. City leaders and community members broke ground on the $144 million Waller Creek Tunnel Project earlier this month.

Over the decades flooding along Waller Creek has devastated the downtown. In 1915, 35 people died when floodwaters submerged entire sections of the downtown and on Memorial Day 1981, 13 people died in a flood that caused $36 million dollars of damage. The flood control project will greatly reduce the risk of flooding along the creek and reclaim about 28 acres of the 100-year floodplain for urban development.

Contract award
After decades of planning and design, the City awarded the tunnel contract to S.J. Louis Construction of Texas in February and issued a notice to proceed this month. The contractor submitted the lowest of five bids that ranged from $48.68 million to nearly $60 million (Table 1).

Table 1.    Bidders Bid Amount S.J. Louis Construction of Texas $48,683,500 Quest/Sak JV $51,769,769 Southland Contracting $52,088,867 JMD Constructors $55,239,094 Kiewit/Atkins JV $59,781,167

"We were very happy with the industry's response," said Gary Jackson, City of Austin Project Manager. "We had a large turnout at our pre-bid meeting in September last year and we received five very competitive bids." The 5,249ft (1,600m) tunnel will divert floodwaters from Waller Creek, just upstream of 12th Street at Waterloo Park, and discharge them into Lady Bird Lake at Waller Beach (Fig 1).

"The tunnel depth varies from 50ft to 70ft and the excavation will be at least a single tunnel diameter into solid rock or Austin Limestone," said Jackson. "The primary alignment is down the right of way of Sabine Street. We do come adjacent to some buildings but not under any."

Fig 1. Tunnel alignment

Tunnel design
The contract consists of the main tunnel and two connector tunnels, one at 4^th and 5^th street, the other at 8^th and 9^th street. The tunnel consists of three segments. Segment 1 is 26.5ft finished diameter and extends from the outlet structure about 2,275ft northward to Segment 2. Segment 2 is 22.5ft finished diameter extending about 1,570ft to Segment 3 that is 20.5ft finished diameter and extends about 1,490ft to connect with the inlet structure (Fig 2). The 4^th and 5^th Street connector tunnel is 22.5ft i.d. and extends eastward about 223ft from the tie-in point at the main tunnel to the 17ft i.d. 4^th Street shaft. The 8^th and 9^th Street connector is a 22.5ft i.d. tunnel and extends eastward for about 40ft.

Construction
S.J. Louis is mobilizing to the site with construction set to begin early summer on the shaft between 4^th and 5^th Street. "We haven't received the contractor's formal proposal yet," said Jackson, "but the 4^th-5^th Street shaft is to be the access shaft for tunneling activity. Following excavation of the short connector tunnel, two roadheaders will excavate the main tunnel in both directions. Once excavated, the tunnel will be finished with a cast-in-place final lining."

The project designer, KBR/Espey JV, a joint venture of Kellogg Brown & Root Services and Espey Consultants, is also servicing as Contract Manager for the City.

Fig 2. Tunnel schematic

Inlet structure
The inlet and outlet structures will be built under separate contracts. Flood flows of 8,247ft³/sec will be divert into the main tunnel at the inlet structure. The structure will also remove flood debris and to recirculate water during dry weather. The design includes a 42ft diameter morning glory spillway, a 20ft diameter tunnel inlet shaft, a short segment of the main tunnel, guide piers, a vortex wall, six 16ft wide x 30ft high mechanical bar screens, two 8ft monorail traveling gripper rakes for debris removal, an approach channel with a turning vane, an in-channel concrete dam, and a 20,000ft² operations and maintenance building to house debris handling and tunnel recirculation equipment.

Project groundbreaking
Photo by City of Austin

The contract for the inlet structure is to be advertised next month (May 2011) for a construction start in September or October 2011. The outlet structure contract will be advertised in early 2012 for a start of construction in late spring 2012, said Jackson.

Redevelopment
All three components of the project are schedule to be complete in Spring 2014. Once operational the City will reclaim about 26 acres from the 100-year-floodplain that has limited development in the past in the eastern sector of the downtown area. The City estimates the value of the reclaimed land to be almost $4 billion and anticipates a construction boom as developers transform the land into prime downtown real estate.

"This is an amazing example of how you can have park land, open space, health and safety and economic development all in one project," said Travis County Commissioner Sarah Eckhardt.

The project is financed in part through a tax-increment finance district, where a portion of the money collected from the sale of the reclaimed land is used to pay back the City and the County.

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