Global Stability

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Special Retaining Wall Conditions

Thursday, July 30th, 2009

I just wrapped up helping an engineer with his design and thought to myself “this is a great wall to blog about.”  The wall was nothing spectacular, in fact, it was quite ordinary.  All too often we get excited about those monster walls and forget that most of the walls built are less than 10 ft (3 m) tall.  What made this wall interesting was the slope below the wall, a parking lot above the wall, and a very tight jobsite.  For a detailed look at some of the conditions you need to watch for, see the Conditions page on the Allan Block website.

The first thing we addressed was the slope below the wall.  A general rule of thumb is to bury additional block when there is a slope below the wall so you have a minimum level area, or bench, of 5 ft (1.5 m) extending from the toe of the wall horizontally to daylight. 

In relation to this slope below the wall, we discussed the potential for a global stability problem.  A global stability failure can be defined as the entire hillside slipping and sliding to the bottom of the slope which would obviously take the Allan Block wall with it.  Global stability can also be a concern when building terraced walls, working in poor soil conditions, or building near water.

 

Wall Setback

Wall Setback

Next, we discussed the location of the wall on the jobsite.  The wall’s setback was a critical factor in determining what type of block would be used.  Segmental walls have a setback, or batter, and as you can see in this picture, as the wall height increases the wall setback from vertical also increases.  We determined that the AB Stones, with an approximate 12-degree setback, would not fit in the proposed location.  Because of this, the engineer chose to use the AB Classic block with a rough setback of 6-degrees.

Finally we discussed the surcharge from the parking lot.  A surcharge can range from a sidewalk for a walking path, to a major freeway carrying thousands of vehicles per day.  The wall design will be influenced by the expected load of this surcharge and its location in respect to the wall.  When the engineer designed the wall in AB Walls 2007, he was sure to include the load from the commercial parking lot on the retaining wall.

Once the local engineer had the wall designed, he used the new “Send Info” button in AB Walls 2007 to send a preliminary material estimate to his local AB Sales Representative.  In no time, the sales rep contacted the engineer with a couple AB Certified Contractors that were interested in bidding the job.

As I mentioned, this was by no means a monster wall, but it did demonstrate a few of the conditions that need to be considered when designing your retaining wall.  For additional information, please visit allanblock.com or your local engineering professional.

Tom

Posted in Commercial Retaining Walls, Landscape Retaining Walls, Reinforced Walls | No Responses »
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The Hidden Condition: Global Stability

Friday, February 27th, 2009

I recently assisted one of our local sales representatives to develop a retaining wall preliminary estimate for a site with very poor silty-clay soils. This time I was fortunate enough to have a soils report for the site prepared by a local geotechnical engineer. As I read the report and looked at the site plan my concern was not for the integrity of the Allan Block retaining wall, but the stability of the overall site. On this particular site, the propose wall would have a steep slope below the wall. Steep slopes below the wall are always a sign of potential global stability issues.

Global Stability

Global Stability

After reviewing all the information, I called the AB local sales representative and told him that I was concerned about the global stability of the site. We ended up proceeding with the preliminary material estimate so that the contractor would have a rough set of numbers to discuss with the homeowner. Prior to sending the information, I was able to talk to the contractor to discuss global stability and recommended that a local geotechnical engineer do a global stability analysis and determine the actual wall design and geogrid lengths.

The contractor went to the homeowner armed with a better understanding of the challenges their job may be hiding, a pending material estimate that he developed, and the AB Commercial Wall Manual open to the page discussing global stability. After sitting with the owners, he was awarded the job because he presented himself as an industry expert selling solutions instead of simply a contractor there to build a wall.

Later that week, the homeowners and contractor met with a local engineer to discuss the job. After a global stability analysis, the engineer provided a detailed construction plan so they could start building the retaining wall. Soon, the owners will be able to enjoy their remodeled backyard with the added confidence that the slope was properly analyzed and stabilized.

Tom

Posted in Commercial Retaining Walls | No Responses »
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