0

Should I be concerned about the added weight of 3/4" river rock in a french drain 12 inches from my foundation? If there is already a crack in the foundation and a slight lean, will the weight of the gravel add pressure? I've seen that river rock displaces weight horizontally better than sediment or other gravel.

My trench is already wider than my 4 inch pipe so I'm thinking of digging the gravel out and putting dirt behind the fabric to narrow the trench and reduce the amount of gravel.

Any thoughts?

1

Sure, everything affects the load on your footing. However, eliminating moisture (water) on your footing is more important.

There are three types of loads that affect the displacement of your footing: 1) Dead Loads, 2) Live Loads, and 3) Superimposed Loads.

1) Dead Loads include the weight of the footing, building loads on the footing, and soil on and around the footing. Depending on the “angle of repose” of your soil conditions, all the soil or gravel next to your footing can add a load to the existing soil bearing.

Gravel can weigh about 90 lbs. per cubic foot (dry) and soil is about 100 lbs. per cubic foot (dry and depending on the type) and water is about 62 lbs. per cubic foot. Wet soil and water soaked gravel is considerably heavier.

As you can see, gravel and soil has similar weight characteristics. So, when you replace the soil with a 4” perf pipe and gravel, the weight is probably less, due to the void in the pipe.

2) Live Loads of people, benches, landscaping, etc. also adds weight and should be considered.

3) Superimposed Loads such as a parked car next to the footing also adds weight and should be eliminated from the area by the footing.

Summary:

Many things contribute to loading weight to the soil under the footing and can contribute to settlement. Just because the soil or gravel is not directly on top of the footing, it can still add “load” to the soil under the footing due to the angle of repose of the soil. A safe rule of thumb is anything (including Live Loads and Superimposed Loads) within a 45 degree angle from the edge of the footing will contribute “load” and cause settlement.

BTW, don’t install the gravel below the existing footing within the angle of repose. Otherwise you could undermine the bearing of the footing.

| improve this answer | |
  • Wow, this is a very educated and thought out response. You obviously have a wealth of knowledge to share, thank you for your contribution. This actually answers some questions that I have not even thought to have yet! – timz08 Apr 1 at 14:14
  • Forgot to mention that I do not have a footing. I've thought about digging one small section at a time out from under the foundation and pouring concrete. Not underneath the compromised wall of course, atleast until it's stabilized. – timz08 Apr 1 at 14:16
  • As noted in the answer, the gravel will weigh about the same as the dirt it replaces. However, since you're creating a french drain, you're actually helping to reduce the duration of weight resting on the wall, so that weight will be there for a shorter period of time. – FreeMan Apr 2 at 17:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.