Yesterday I received a call from a contractor wondering why a chart on the Allan Block website limits the height of a gravity wall. After talking to them for a while I had a sketch in front of me of the wall they wanted to build.
The contractor sounded like he had a good head on his shoulders, so I started explaining the concepts of external stability. Since he mentioned the website, I directed him to the page that shows external stability so that he could see pictures and the recommended factors of safety for each of the design criteria.
External stability can be broken down into 4 main components:
- Sliding: The wall is pushed forward
- Overturning: The wall tips or leans forward typically pivoting on the toe of the base course.
- Bearing: The retaining wall settles due to the weight of the retaining wall itself.
- Global Stability: The site the retaining wall is built on has soils that are not stable. This analysis should always be done by a local geotechnical professional, especially when dealing with a site where you will have a slope above or below the retaining wall.
After our lengthy discussion on external stability, and looking at the website, the contractor had a firm grasp of why gravity walls are limited in height. In the end, he decided that geogrid reinforcement would be necessary for his wall.
Tom
All the more reason for Good contractor certification training. AB’s Cert training touches on all the basic engineering / construction / design elements giving the installer all the tools they need to not only build a wall w/ appropriate design ( after an Eng Stamps that design)but also the ability to intelligently talk to building officials about wall design criteria. All leading to avoiding building code fines for no permits and wall failures. AB Engineering Dept. – keep up the good work of instructing us on how to avoid these issues w/ safe and profitable wall execution