Design concrete footings for residential houses according to the 2018 IRC even if its on solid rock.
The footing transmits the loads from above to the soil. So when an aggregate such as gravel , and the soil beneath it is compromised, then it will fail. This is why the many concrete design variations are covered in detail in the 2018 IRC.
For a standard designed footing follow these steps
- Erect batter boards to ensure a proper lay out.
- Use a quality level such as an optical level (transit), or a laser level to dig the trenches.
- Dig the ditch with a backhoe equipped with a 24″ bucket.
- Make the depth 10″ and be sure the bottom of the ditch is below the frost line unless it is on solid rock.
- Use 1/2″ re bar sawed to 16″ for grade stakes every 6′ apart.
- Use two runs of 1/2″ re bar on top of cradles.
- If the slope of the site exceeds 8″, then use step downs to maximize efficiency.
If the path of the footing is disrupted by solid rock, then there is a simple fix. Build forms to allow it to rest rest on top of the rock even if it is above the frost line. Check out this video to learn more. VIDEO
If the ground is shifting, or its soft, then the bad ground should be removed down to a level below the bad soil and replaced with a more stable solution. Driving piles down to bedrock or using extra large footings is just not economical. It is more practical to simply change out the soft ground for some harder stuff.