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2012-07-26

Medina Dam renovation heads to finish line

By Carol L. Smith Special to the Courier

Heading toward the finish line, The Medina Dam renovations when completed will set yet another engineering record. Now in its centennial year, this historic dame will be the largest post tension cable anchoring system completed in the United States.

At its start, the renovations project was estimated to take the same amount of time as the original building of the dam, one year. The most recent date for final completion was July 18 - over two years to finish.

As reported in the Bandera County Courier on July 21, 2011, an accident that month involving three Hayward Baker employees caused some delay. Additionally, in spring 2012, the breeding season of the endangered Golden Cheeked Warbler set the project back almost four months.

The Medina Dam consists of three sections - the dam itself, the tall, straight section in the middle, and two abutments or wings, on either end. The wings anchor the dam to the original canyon walls.

During the 2002 flood, the waters of Medina Lake came within eight feet of going over the top of the dam. At that time, State of Texas dam safety engineers became concerned about the stability of the wings and the integrity of the dam itself.

A study determined that the wings needed additional anchoring to prevent erosion by water churning at the bases should water go over the top of the dam. The Medina Dam itself has not had any construction on or to it, only the wings and at the base of the wings.

The renovations entailed securing the wings into the bedrock underneath and building an erosion protection slab (or apron) at the base of each wing to prevent erosion should water ever go over the dam. Workers drilled 32 anchors through the abutments - 21 on the west side and 11 on the east end.

Thirteen anchors have been totally completed with the others nearing their 30-day hardening, or set, period.

Each anchor will have undergone a load torque test of 2.5 million pounds per square inch.

The anchors consist of a group of individual cables with a diameter of about 12 to 14 inches that have been run through a circular cavity tunneled through the wing and secured into the bedrock. The top of this enormous cable rope has a cap on it, to which the hydraulic engine was placed to torque test each one.

Once the cable and the cavity have met their tolerance limits, the cavity was filled with grout. A concrete cap will be placed over the top off each anchor to match the original surface. Other than the telltale sign of new versus old concrete, the construction will be virtually indiscernible.

Kris Roberson, San Antonio River Authority (SARA) project managing engineer, noted, "The thicker the abutment the further into the bedrock we had to go." The shallowest anchor is 75' and the deepest is 170', in order to reach its needed depth. "We just put some suspenders on the old man," he quipped.

SARA workers will also pressure wash the front and back of the wings to removed excess grouting that may have spilled over the sides. They will also place metal collars on some of the handrail columns that have rusted through at the bases where they attach to the top of the dam.

Interestingly, the Medina Dam itself contained no steel except for the handrails across the top.

In 1912, the Medina Dam set engineering records for the amount of concrete poured in a 24-hour period and for the speed in which the entire project was completed. One of the oldest concrete dams in the US - the oldest is 120 - it has within 10 percent of the same cubic capacity of concrete as the Hoover Dam. Finished in 1935, the Hoover Dam is known as the tallest solid concrete dam in the Western Hemisphere.

Additional backstory information can be found in Courier articles from April 21, 2011 and Dec. 30, 2011. The Medina Lake Preservation Society will have a pictorial slide show presentation during the Medina Dam Centennial Celebration on Saturday, August 25, detailing the entire project. The presentation will be shown at the Bedrock Resort in Mico and the Lakehills Civic Center on Park Road 37.

Pictured: The concrete erosion protection slab or "apron" on the back side of the dam.