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2011-04-21

Medina Dam sets engineering records, again!

By Carol L. Smith Special to the Courier

When it was completed in 1912, the Medina Dam project set several engineering records.

According to engineers with the Hayward Baker Company of California, tension cable projects specialists, when the current repairs to the dam are completed, this will be the largest cable tension anchor system completed in North America.

Once again, the Medina Dam will set engineering records.

The work being completed on the downstream side of the dam started with the construction of a concrete apron called an erosion protection slab.

This will keep the "wings" or abutments to the dam from being eroded away should water ever overtop the dam itself.

During a record-breaking flood in July 2002, water came within 18 inches of the top of the dam, making the evacuation of Rio Medina, Castroville and La Coste necessary.
For more history, refer to a front page article in the Dec. 30, 2010 edition of the Bandera County Courier.

While Austin Road and Bridge Company has been responsible for most of the excavation and construction up to this point, Hayward Baker engineers were scheduled to begin their work on the dam on April 11.

They will drill through the erosion protection slab and into the bedrock anywhere from nine to 11 feet down.

"L" shaped pieces of steel rebar will be installed and anchored in place.

"They will be drilling approximately 70 holes per day," said Albert Vega, a San Antonio River Authority engineer.

"Hundreds of these metal anchors will be installed within the slab."

Metal cages will be placed onto the erosion protection slab with the steel rebar anchors protruding through it. Straight rebar anchors will be also placed horizontally into the abutments in the same manner. Both sets of metal anchors will then be covered completely by concrete. This process will effectively tie the abutments into the bedrock behind the dam and the concrete slab that covers them will protect the dam from erosion. This process is expected to take about three weeks.

Hayward Baker will then concentrate on anchoring the abutments into the bedrock beneath them from above.

Large rectangular sections of concrete are being removed from the top of the dam and holes approximately 12- to 16-inches in size will be drilled through the entire height of the abutments and into the bedrock below.

Huge post tension anchor cables will be tied into the bedrock and strung up through the holes and into a metal plate with a tension guide at the top. The enormous cables, 12- to 14-inches in diameter, will be pulled taut. Once the correct amount tension is achieved, the cables will be tied onto a metal plate, which will then be covered with concrete.

Ed Berger, business manager of the Bexar Medina Atascosa Water Control and Improvement District (BMA), has been keeping close tabs on the repair project. This is just one of the many improvement projects that will have completed by the BMA during his tenure. "This is a big check mark off of my 'to do' list," stated Berger. "This will really take a huge load off my mind."

The lack of appreciable rainfall has necessitated the release of water from Medina Lake for the spring planting season for the farmers in the BMA district. A good soaking rain may keep the BMA from irrigating although a severe flood event might prove problematic during the construction repairs - another reason for the expedited time schedule.

The entire Medina Dam renovation project should be completed in October, a little less than a year from start to finish. The work appears to be right on schedule.


Pictured: Photo by Carol L. Smith

From left, David Wilson, project engineer with the San Antonio River Authority, and Ed Berger, BMS business manager of the BMA, survey construction at Medina Dam.