“Staff determined that the retaining wall was not built in accordance with the design provided by (the) design engineer,” Assistant City Manager T.C. Broadnax wrote in an e-mail to his boss, Sheryl Sculley, Mayor Julián Castro and the City Council.

“For example, the building plans for the wall show limestone mortared throughout the wall. Based on field observations of the failed portion of the wall, mortar was not installed according to the building plans.”

Centex officials said the city's findings were based on visual observations — not on surveys, soil sampling or other data it has been collecting at the site. The developer and builder of the neighborhood believes soil shifting put pressure on the wall.

“The city did say that the wall was not built according to plan,” Centex spokeswoman Valerie Dolenga said. “At this point, we're still trying to substantiate that. We're trying to find some answers. We're all pushing pretty much around the clock.”

The city's findings highlight a problem with the type of retaining wall constructed at the Hills of Rivermist.

Known as a “gravity wall,” the stone-and-mortar structure relies on its own weight to remain stable. Using less mortar could make the wall lighter — and unstable.

“One of the key components of a gravity wall is a solid mortar section, creating a solid mass,” said Richard Jenkins, an engineer in Round Rock whose firm exclusively designs retaining walls.

“You don't want to jump to any conclusions before any facts are in,” Jenkins said of the collapse in Rivermist. “But in the same breath, I will say a gravity wall system has a weak point. Its Achilles' heel is that many of its components are not verifiable through inspections and testing.”

Roderick Sanchez, director of the city's Planning and Development Services Department, said he and his inspectors checked the portion of the wall that had cracked apart and looked inside. There appeared to be less mortar inside the core of the wall than its facade, Sanchez said.

“The biggest concern was, we did see mortar, but it seemed like it only went in 2 feet,” Sanchez said.

“We're very, very concerned,” he added. “That obviously plays a key role in the structural stability of that wall.”

On Wednesday, Centex provided the city with designs for the wall that collapsed at Rivermist.

Engineer Russell Leavens of Enterprise Engineers Inc. in Fort Worth drew up a set of plans for the wall. A company aptly named Gravity Walls Ltd. was the contractor. Leavens declined to comment for this report and messages left at Gravity Walls were not returned.

Leavens' design was titled “Retaining Wall Rebuild,” suggesting it replaced an earlier wall at the site that Centex replaced in 2007 because it “was not performing to expectations.”

“The decision was made to completely rebuild the wall and substantially reinforce its foundation,” the company said in a statement.

Leavens designed several segments of the wall that varied in height — one segment was 25 feet tall. A cross-section of the wall looks like a slice of pie that measures 4 feet thick at its highest point and widens to a base that is 7 feet thick. The base was supposed to be buried about 7 feet underground and rest on a base of reinforced concrete. The plans call for weep holes to allow water behind the wall to drain.

Sanchez said the rainy weather Friday prevented inspectors from checking the concrete base underground and whether it was built correctly.

Engineer Barry Archer, assistant director of the city's Planning and Development Services department, said the design by Leavens appeared to be sound.

“This is a substantial wall — if you built it that way,” Archer said on Thursday, before the city inspected the retaining wall.

The paperwork submitted by Centex also included a hand-drawn design by Gravity Walls Ltd. for a shorter segment of the wall. That diagram doesn't bear the stamp of a licensed engineer.

The wall described in that plan does not have a concrete footing or a wide base. Sanchez said it's unclear if any section of the existing wall was built under that plan.

Properly compacting the soil behind a retaining wall is just as important as building a good wall. Centex provided the city a stack of reports documenting stages of soil compaction. Arias & Associates Inc. is listed as the geo-technical firm that monitored the compacting process, and studied the soil composition.

The firm's owner, Robert Arias, did not return repeated phone calls for comment last week.
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