- Wall Height: What is the height of your wall? If you are only going to have 2 or 3 courses of block then geogrid is likely not needed, but as your wall starts getting taller, geogrid may be required.
- Soil Type: What type of dirt do you have? Different soils have different strength characteristics. What an engineer looks at is the shear capacity of the soil (also known as the friction angle). In common terms this is an approximation of how easily the individual particles of soil will slide past each other. This value helps an engineer determine how much pressure the soil will apply to the back of the wall. Unfortunately this is where things start to get less clear. Soils can vary greatly in their friction angle and therefore the pressures can also vary by a large margin. In general, clays are typically the weakest soil and exert more pressure on a wall than gravel would.
- Surcharges/Load above the Wall: Is there anything extra the wall needs to support? Loads above the wall can be sidewalks, roadways or a steep slope above the wall. These can also apply pressure that the wall must support.
- Other Things to Consider: wall set back, block weight, and soil weight that are also important and play a role in the answer.
So what is the answer then? Because all of the above variables come into play, there isn’t a single definitive answer for every job site. But there are some tables and charts to help you get to the answers you need. You can find these charts in the Allan Block Installation Manuals (walls OVER 6 ft. 1.8m or walls UNDER 6 ft. 1.8 m) or at www.allanblock.com.
These charts can be used as a reference chart but it is not intended for use for final design purposes. If the city is requiring a design for your permit you should always be working with a qualified design professional. This design professional can take a look at the particular conditions for your wall and provide you with the final design.