April 8, 2016

Special Retaining Wall Conditions

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 job site.  For a detailed look at some of the conditions you need to watch for, see the Conditions page on the Allan Block website. 

Slope Below a Wall
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. 

Global Stability
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.

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 Design Software, 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 Design Software 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.