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slope failure endangers homes in San Antonio Texas subdivision

slope failure endangers homes in San Antonio Texas subdivision

slope failure endangers homes in San Antonio Texas subdivision

(OP)
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.
 

RE: slope failure endangers homes in San Antonio Texas subdivision

Wow...  Hope you aren't the design engineer on this one!

My first day of my first engineering job, my boss says to me, "you know what the three most important things in civil engineering are?"

"Water, Water and Water...  You can design and build a bunch of pretty stuff - walls, bridges, roads, etc., but if you don't consider water, it can destroy it all."

Looks to me like someone neglected proper surface and subsurface drainage behind that wall, which likely caused this failure.  And in areas where you are creating a fill slope wall like this, getting solid soil compaction behind the wall is also imperative, and may have contributed to the slide.

If I didn't know better, I would have thought this video was taken in California - happens there all the time.
 

RE: slope failure endangers homes in San Antonio Texas subdivision

The forensic investigation might not take very long:

D9DF2SBO0@news.ap.org&client=landingpage&qid=n1gmole5demdse45laxd4v45" target="_blank">http://kai03.qwest.com/WindowsLive/Media/News/NewsDetail/National/Official_No_permit_for_crumbling_retaining_wall.aspx?id=D9DF2SBO0@news.ap.org&client=landingpage&amp;qid=n1gmole5demdse45laxd4v45

Official: No permit for crumbling retaining wall
Jan 25, 2010 (5:50p CST)
By MICHELLE ROBERTS  (Associated Press Writer)

SAN ANTONIO -  A San Antonio planning official says a retaining wall designed to hold up the land beneath a group of homes that now sit precariously on a crumbling hilltop did not have a permit.

Planning Director Roderick Sanchez says Centex Homes was required to get a permit for the 1,000-foot-long stone wall used to shore up a hill where several homes were built - but no permit was acquired.

He says the builder's parent company, Pulte Homes Inc., assured the city that it had a certified engineer design the wall and it was built to specifications.

But so far, Sanchez said Monday that the city has not seen any verification.

About 25 homes remain evacuated after a landslide that began Sunday threatened to collapse the wall and send several houses tumbling downhill.
 

RE: slope failure endangers homes in San Antonio Texas subdivision

It appears the wall had no design, straps, underdrain/weepholes.

The slideshow did not slide too well for me, but it looks as though the wall was just constructed of aggr./stone and not tied into the fill with anything.

RE: slope failure endangers homes in San Antonio Texas subdivision

Yup.  That's their reputation here too.  

A little birdie just flew by and said "Cheap, Cheap".

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: slope failure endangers homes in San Antonio Texas subdivision

Interesting the City said they did not issue a permit for the retaining wall.

I wonder though, if the city issued certificates of occupancy for the houses.

I bet the design engineer comes forward with acceptable drawings, only to find out the contractor didn't build it that way.

The contractor will probably say the developer made them to build it another way, but it won't be in writing.

Let the finger pointing officially begin!!

RE: slope failure endangers homes in San Antonio Texas subdivision

For more Pulte follies:
http://justanotherlemon.com/CCFL/pulte.htm
Obviously, this lady has an agenda.  But it seems like it would be cheaper to buy her house than listen to her squawk.
At what point is the price of proper engineering and construction become more than the costs of mitigating these disasters?   

RE: slope failure endangers homes in San Antonio Texas subdivision

Horrible situation for everyone who bought these houses.

We have had some possible work for keystone retaining walls that were built wrong on a slope holding up a house, and I don't think the owner wanted to pay us to fix the badly constructed walls.  Do you guys think the subdrain water is the issue with this wall?  Or the footings/piles not done correctly?

Civil Development Group, LLC
Los Angeles Civil Engineering specializing in Hillside Grading
http://www.civildevelopmentgroup.com

RE: slope failure endangers homes in San Antonio Texas subdivision

Too early to say, Brandon, but there is suggestive evidence:  I read somewhere that there had been heavy rains lately.  Also, there was a sewer line in the fill, but I have seen no report on whether it had leaked.  Either of those could have been the straw that broke the camel's back.

Was there a drain?  Would it have made a difference in a sustained period of wet weather?  I dunno.  I've also read that the soils in the area are clayey, making them more difficult to drain effectively, as well as making the friction angle lower than most other soils.  Can't say whether those two effects were involved, but they would sure be on my list of suspects and unindicted co-conspirators.  They also have swelling clay in that area.  Co-conspirator, or an innocent bystander (this time)?
 

RE: slope failure endangers homes in San Antonio Texas subdivision

I would not be surprised if there was no design.  I have seen Pulte do the same around here.

Sad thing is that you can typically get a pretty cheap design if you have soil data in the surrounding area.

RE: slope failure endangers homes in San Antonio Texas subdivision

OK you experts.  Do you guys design walls to hold back super-saturated clay?  Kinda asking a lot of any wall I think.

If clay is all you have to work with, do you install drains to keep it from turning to a semi-liquid?  Is that feasible?

Isn't the whole idea of any wall here and the use of clay fill rather dumb when you know what rainy weather and clays do there?

Would not the city be at fault for letting this overall development to be done in the first place?  That's what Plan Commissions are for.

The poor wall happened to be in the wrong place at the wrong time.  It can't be all the wall's fault.   As to a permit, did those folks get a permit for their fences there?  Since when does a wall have to have a permit?  

In summary, it's a lot more complicated than just a wall.  

RE: slope failure endangers homes in San Antonio Texas subdivision

Where I live (and coincidentally just built in my back yard a wall with lots of rebar, a drain, and a key, on swelling clay) a permit is required for any retaining wall over 4 feet from bottom of base slab to top of wall.  With or without the drain, "supersaturated" is implausible at my site due to topography (runoff --> v. little infiltration), and I'll be surprised if I ever get a drop out the end of the pipe.  The drain is there "just because," and its cost was quite small compared to the concrete and excavation.

And as of a few months ago, the county zoning people claim authority over colors and textures - earth tones and natural-looking textures.  Apparently "concrete" is not an earth tone.  Makes me want to paint the thing amethyst purple and see if they come after me.  Amethyst is a naturally occurring mineral; therefore, its color is an earth tone, like turquoise, jet black, ruby red, sapphire blue, etc., right?

Permits are required for fences here also.

RE: slope failure endangers homes in San Antonio Texas subdivision

Clay soil is nothing new to Texas, and a retaining wall that hieght is well within the capabilities of a retaining wall if designed and constructed properly.

Soil saturation can be an issue, but walls can be designed for them as well by factoring in the additional hydrostatic pressure.

From the pictures it didn't appear the me the wall was constructed with any geogrid either.  Adding a well drained backfill against the wall with weepholes could have also prevented hydrostatic buildup.

Here in Dallas any wall over 4-feet in height needs a building permit, which in turn requires engineer stamped drawings.

I have the feeling the city of San Antonio is going to be on the hook here as well.  Or at least not get off scott free.

I'll bet the City issued C.O.'s (certificates of occupancy) for those houses.  If the City required a permit for the wall, but then gave C.O.'s without issuing that permit, they are going to be liable as well.  

There is an inferred responsibility when giving C.O.'s that city requirements have been met.

If I were those residents, I would hire the most bulldog onery lawyer I could find go after the city, developer, contractor and the inspection company all at the same time.  If engineered drawings for the wall are discovered, have them peer reviewed by a separate civil engineer.  

Even those away from the collapse will have their property values adversely affected, just by being in the same subdivision.

RE: slope failure endangers homes in San Antonio Texas subdivision

There are weeps; you can see them some photos by enlarging the photo.

 

RE: slope failure endangers homes in San Antonio Texas subdivision

Over here in LA a wall over 2.5' and not having a flat slope behind is basically considered retaining and needs permits and can fall under lots of restrictions/variances.

For those working in Texas on 6'+ walls, do you use conventional walls or pile supported more often for clay soils???

Seems like everything new out here in SoCal is built on slopes and/or on landslide areas now, so piles are just so common.

 

Civil Development Group, LLC
Los Angeles Civil Engineering specializing in Hillside Grading
http://www.civildevelopmentgroup.com

RE: slope failure endangers homes in San Antonio Texas subdivision

I found this passage from the news article amusing:

Travis Watson, 36, said he noticed the ground behind his 3-month-old home in the 8300 block of Valley Well had shifted down several inches Friday, and he called the home builder.

"I was told that I wasn't watering enough," he said.

 

RE: slope failure endangers homes in San Antonio Texas subdivision

"Darnell conceded Centex had no city permit to build the retaining wall, but he said he thought the company followed city regulations and standard industry practices in its construction. He disputed the city's allegation that the wall was improperly built."

http://www.star-telegram.com/462/story/1924034.html

Maybe it was properly built, but improperly designed?
 

RE: slope failure endangers homes in San Antonio Texas subdivision

oldest guy

The MSE or Modular Block walls I have been associated with included select fill or aggregate. (low to no PI stuff) along with the geo-mat or strap type anchor systems, and utilized underdrain systems when necessary.

Many have been constructed adjacent to high PI (35+)fill material. Too my knowledge none have came close too this scope of failure.

I think I get your point but, are you saying a wall could not have been designed and constructed here that would have met the needs of these site conditions?

RE: slope failure endangers homes in San Antonio Texas subdivision

Oh it's Centex and not a Pulte built home?.?.?

Civil Development Group, LLC
Los Angeles Civil Engineering specializing in Hillside Grading
http://www.civildevelopmentgroup.com

RE: slope failure endangers homes in San Antonio Texas subdivision

Pulte owns Centex.  Two peas in a pod.

RE: slope failure endangers homes in San Antonio Texas subdivision

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&amp;page=1
http://www.eng-tips.com/viewthread.cfm?qid=263740&amp;page=1
other discussions going on multiple threads

Ryan Coggins, P.E., S.I.
http://www.linkedin.com/in/ryancoggins

RE: slope failure endangers homes in San Antonio Texas subdivision

More info here:
http://www.mysanantonio.com/news/local_news/Collapsed_wall_wasnt_built_according_to_specs.html

Quote:

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.

RE: slope failure endangers homes in San Antonio Texas subdivision

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.

RE: slope failure endangers homes in San Antonio Texas subdivision

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?

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