Backfilling basement retaining walls

[BruceWard]

I am too far along to change plans but thought I would share some pictures.

The backfill consists of a 4" drain pipe at the bottom followed by a 2' to 6' wide section of 36 yards of 1 & 1/4 grave (sewer rock)l in 2' to 4' of depth. That is covered by landscape cloth and then city put (crusher run). At the top will be another french drain with pipe and gravel. Seventy two yards of gravel and two other french drains were placed under the slab and footers. The walls have two coats of foundation coating. Rain water and a few thousand gallons of drain clearing city water ran out through the french drains under the slab without leaving any puddles.

So far we have only applied the city pit to one corner than we need to get a concrete truck to. The remainder will be back filled after the floor trusses are installed.

Before the concrete

The drain pipe

Gravel going in

Covered with 6' wide landscape fabric

City pit going in

Corner filled from the East

From the South

From the West

 

[2manyrocks]

Some folks will install blueboard or something over the waterproofing to try to keep the rock from scratching a hole in their waterproofing.

 

[moored4]

Looks like you put in a walk in safe in the basement!:thumbsup:

 

[BruceWard]

Thanks guys.

I considered several products designed to keep the rock from scratching the waterproofing or to drain water down the wall. I want to use the coolness coming through the walls to reduce the need for airconditioning. I have no plans to heat the basement. So I decided not to use anything on the outside that would insulate the walls. I know the footer and base of the wall is very well drained. The white city pit will setup like concrete once it is wet. After it dries it repels water very well. I will be installing a second french drain above the city pit. My goal is to keep the water away so that waterproofing is never needed. I did check as we were backfilling and did not find any damage to the waterproofing.

 

[Darren]

Did you place the geotextile so that it was at the bottom of the excavation and extended up the sides so you can fold it over the top of the gravel to prevent soil from migrating into the gravel over the years?

What type of soil do you have? The drain pipe should have been placed with the holes down to prevent water from needing to build up to the top of the pipe before getting into the pipe to drain. Does the gravel under the slab meet the gravel used for backfill?

I would have placed some sort of board against the waterproofing since you can count on rock moving over time. Was the gravel compacted as it was placed?

If not, I'd be careful about placing concrete trucks on that gravel to prevent damage to the waterproofing in any case. Make sure they know they need to bring extra chute sections if they need to keep the trucks off the gravel.

 

[BruceWard]

I did not fold the fabric over from under the gravel. I may do that as a I backfill the remaining sections.

The soil type is solid rock. The excavation is 10' deep. The top 2' feet contain small amounts of soil. The rest is rock.

The backfill gravel meets the gravel under the footers which meets the gravel under the slab. All of which are sitting on solid rock.

The gravel was not compacted. It does not compact. The city pit was compacted to within 3 feet of the wall. Last nights storm should have compacted the rest. Once it dries it will be a solid chunk.

The concrete truck has a 26 foot chute. It will be able to sit back 8' or so from the retaining wall.

 

[JB4310]

I am too far along to change plans but thought I would share some pictures.

The backfill consists of a 4" drain pipe at the bottom followed by a 2' to 6' wide section of 36 yards of 1 & 1/4 grave (sewer rock)l in 2' to 4' of depth. That is covered by landscape cloth and then city put (crusher run). At the top will be another french drain with pipe and gravel. Seventy two yards of gravel and two other french drains were placed under the slab and footers. The walls have two coats of foundation coating. Rain water and a few thousand gallons of drain clearing city water ran out through the french drains under the slab without leaving any puddles.

Do not put another perforated drainage system near the top, it will work in reverse/ against you. Never install a perforated drainage system above the floor level. You can install surface swales and surface drains/ catch basins, but use solid pipe.

JB.

 

[BruceWard]

Do not put another perforated drainage system near the top, it will work in reverse/ against you. Never install a perforated drainage system above the floor level. You can install surface swales and surface drains/ catch basins, but use solid pipe.

JB.

I am curious about why it would be a bad idea to use perforated pipe to gather water near the top above a close to waterproof layer. I expect to trap the water above the city pit layer and for it to run off through the gravel. Some of that water might run through the pipe. Certainly I would use solid pipe where I want to move water instead of catching it. I have looked at surface drain options which would catch water and route them into the pipe. Their surface area is much smaller than the gravel area I plan to have along the front and side of the house. I can not see how they would be more effective than the gravel.

 

[JB4310]

I am curious about why it would be a bad idea to use perforated pipe to gather water near the top above a close to waterproof layer. I expect to trap the water above the city pit layer and for it to run off through the gravel. Some of that water might run through the pipe. Certainly I would use solid pipe where I want to move water instead of catching it. I have looked at surface drain options which would catch water and route them into the pipe. Their surface area is much smaller than the gravel area I plan to have along the front and side of the house. I can not see how they would be more effective than the gravel.

It's good that you have an impervious fill in there, why would you want to cut a hole in it and encourage water to TRY to percolate into the soil. The first path the water is going to follow is down (gravity). By cutting a trench you are increasing the surface area for potential percolation, by filling the trench with stone you are slowing the water flow down, basically creating a reservoir to collect water, I know that's not what you want.

I would suggest a surface swale about 4-8 feet away from the foundation, draining down hill with the natural topography. If natural flow is not possible, a good sized catch basin in the swale, with pipe to daylight. Sounds like you have some good material to work with and it looks like the topography is favorable for good storm water control.

If you are determined to do the stone trap drain, do it 8-10 ft from the foundation, create as much slope/pitch as possible and line the bottom and sides of the trench with an impervious liner.
I would just warn you, I have seen many shallow, stone trap/perforated pipe trench drains do more harm than good when installed outside a foundation with basement
.
Good luck, JB.

 

[Duffster]

JB offers excellent advice here. :thumbsup: