by Dan Foscalina |
BC WATER NEWS
AUSTIN—When
the Environmental Protection Agency’s Region 6
came knocking on the Austin Water Utility’s door,
it didn’t come
empty-handed. The regulatory agency brought an Administrative
Order (AO) requiring the central Texas utility to eliminate
sanitary sewer overflows by December 2007.
Austin
was under the gun: It needed to quickly implement land
acquisition, permitting, design and construction of
a large number of projects across its five-plant, 2,316-mile
collection system. The city put together a rotation list
of engineering firms to work on several fast-track collection
system projects, one of the most challenging and critical
of which was the Little Walnut Creek Tunnel Interceptor
Project.
Infiltration and inflow had been a problem on the Little
Walnut Creek interceptor, with residents and neighbors
complaining about wastewater discharges for more than 15
years. The existing 42-inch pipeline runs beneath a streambed,
with manholes rising out of the water at 100-yard intervals.
During wet weather events, wastewater overflows into the
creek.
But in the late 1980s when the city originally floated
the idea to replace the interceptor, residents blocked
the project, concerned that the proposed open cut construction
would disrupt nearby neighborhoods and harm the environmental
integrity of the creek. The city went back to the drawing
board and redesigned the improvement project, only to have
it blocked again by dissatisfied neighbors.
Fast-forward
to July 2005: Under the City of Austin’s
Clean Water Program, this third—and current—design
is being led by national engineering and consulting firm
Brown and Caldwell. To gain public buy-in, BC produced
a design that uses Tunnel Boring Machine (TBM) technology
to construct a new 10,000-linear-foot, 96-inch-diameter
tunnel in one continuous run with no intermediate shafts.
The $12.7-million project design also calls for a 60-inch
fiberglass carrier pipe, which will Significantly increase
the pipeline’s useful life over more traditional
reinforced concrete pipe.
Mining
crews are 140 feet below the Austin hill country and
stifling Texas heat, pushing a 300-foot-long
TBM along pipeline route. The best part? The tunnel is
bounded within right-of-way limits of existing surface
streets, minimizing land acquisitions and ensuring
zero impact to the neighborhood, traffic or the creek.
Also, construction shafts at each end of the
tunnel are on undeveloped
property, further keeping the project
out of the public eye.
"Things
aren't always as they seem from the surface."
|
“Things aren’t always as they seem from the
surface,” says Brown and Caldwell Project Manager
Susan Kelly. “Following the surface streets is not
only economical but also less risky.”
The project also includes open cut construction of 3,700
linear feet of 60-inch wastewater interceptor, allowing
AWU to abandon a similar length of deteriorated pipeline
and lay the groundwork for a city park near one of the
construction shafts.
“We
listened to the community and decided to invest in the
more expensive tunnel design,” says Reynaldo
Cantu, AWU assistant director. “The design successfully
addresses residents’ concerns regarding the impact
of construction activities on the neighborhood, traffic
and the creek, thereby preserving their quality of life."
The
design phase, including field investigations, was performed
within six months and completed $100,000 under
budget. The tunnel construction is well under way to meet
the EPA’s AO deadline.
Aug. 22, 2005
