slope failure endangers homes in San Antonio Texas subdivision 1

[roncity]

From
<

http://www.mysanantonio.com/news/local_news/Sinkhole_prompts_evacuation_on_Northwest_Side.html>

( click on " slide show " for wall failure photos)

San Antonio Express-News jan 24 2010
Authorities evacuated about 80 homes in a Northwest Side neighborhood Sunday when ground caved in behind several houses, pushing earth down a 30-foot hill and into two retaining walls that cracked and threatened residences below.

No one was injured, and agencies acted quickly to address the endangered homes near West Hausman Road and Loop 1604.

Describing the collapse as a “slope failure,” authorities at a Sunday night meeting told residents from The Hills at Rivermist subdivision that in some areas the crevices grew to 12 to 15 feet deep and 6 to 8 feet wide. But they didn't know the cause.

The meeting, held at O'Connor High School, attracted about 200 residents wanting answers from the builder of the homes and emergency personnel.

Laurin Darnell, a vice president for Pulte Homes Inc., thanked residents for their patience but had no information on what caused the problem and said engineers were assessing the area.

 

[msucog]

This looks like a rubble wall I've seen from designers in TX...except this one looks especially thin for the height. Obviously loose backfill, storm/sewer structures in backfill that have rotated, possibly improper setbacks (front and back), tiered wall desgin, slope above wall, yards likely shedding water toward wall, most likely very liberal design parameters chosen by designer with no backup test data on soils, likely no construction oversight by designer, possibly no materials testing required by designer prior to or during construction, backfill material type??, don't see drainage material behind wall, foundation conditions (who knows-appears wall slid forward), etc etc. for foundation, with a fill height that thick, I'd expect a substantial amount of fill induced settlement...so what does that do to the storm/sewer structure hanging in the fill?? (can you say broken pipes? may help explain some of the pics i've seen posted).

And the homeowners will likely be the biggest loser in this fiasco. The wall designer and contractor should own most of the responsibility. The developer should own a significant portion too. The building official should even own some of the responsibility too for allowing the project to move forward without the necessary paperwork, test data, etc and sign off stamp from the design engineer.

That is my take on it.

http://www.eng-tips.com/viewthread.cfm?qid=264084&page=1

http://www.eng-tips.com/viewthread.cfm?qid=263740&page=1

other discussions going on multiple threads

Ryan Coggins, P.E., S.I.

http://www.linkedin.com/in/ryancoggins

 

[Spartan5]

More info here:

http://www.mysanantonio.com/news/local_news/Collapsed_wall_wasnt_built_according_to_specs.html

The retaining wall that collapsed last week and jeopardized a neighborhood built by Centex Homes was built with less mortar than what engineering plans called for, according to city officials who inspected the wall Friday.

“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.

 

[bridgebuster]

Two thoughts:

"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."

A gravity wall doesn't necessarily need to be mortared. A dry rubble wall has no mortar joints. I've inspected several of these in NYC that are over a century old and still stable. My objection is that someone will read the quote, see a dry rubble wall, and panic.


"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."

25 feet tall and 7' at the base? No way. Too narrow.

 

[dgillette]

Curiously, it appears to be a sliding failure, rather than overturning like you might expect with aspect ratio over 3.

"The base was supposed to be buried about 7 feet underground and rest on a base of reinforced concrete."

One thing I couldn't tell from the pictures is whether there was a 7' deep passive wedge pushed up ahead of the wall, to go with the 7' depth. Is there one, or did the wall shear off at ground level, or was there a slope with a free face at the elevation of the footing (so the soil pushed ahead of the wall would just translate, rather than being lifted), or what?

 

[brandoncdg]

Does anyone have more pictures of what happened?

Since Pulte bought Centex, who is going to get killed from this?

Civil Development Group, LLC
Los Angeles Civil Engineering specializing in Hillside Grading

http://civildevelopmentgroup.com

http://civildevelopmentgroup.com/blog