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:

 

[AGGIE00]

I wish I had that much fall on my property to have a walk out basement. As it stands now I am going to have to have a 2' step up out of the basement to finish grade outside the basement. Looks like you have an awesome view.

 

[BruceWard]

Thanks Aggie, my wife fell in love with the view. I bet you have less rock to deal with

Unfortunately I excavated too deeply. Actually I failed to consider that I was on the down hill slope when excavating. Much of what is between the top of the hill and the retaining walls is solid rock and would have to be jack hammered to create a swale. I can create the swale in the backfill area or add a french drain to the backfill area away from the house.

This picture shows the slope toward the house.

 

[2manyrocks]

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.

Since you brought this up..... I've got a house with a crawl space where the ground slopes into the back wall. I'm seeing wet block in the crawl space--it's dark from moisture. I suspect there is no footer drain at all.
I want to get this corrected.

Do you think I'd better off installing a footer drain in crushed gravel wrapped in geotextile fabric or should I come out further from the house and dig a french drain or swale?

 

[JB4310]

Since you brought this up..... I've got a house with a crawl space where the ground slopes into the back wall. I'm seeing wet block in the crawl space--it's dark from moisture. I suspect there is no footer drain at all.
I want to get this corrected.

Do you think I'd better off installing a footer drain in crushed gravel wrapped in geotextile fabric or should I come out further from the house and dig a french drain or swale?

Many variables I don't know about your situation. how old, how deep, what soil type, are you getting water ponding in the crawl space etc. etc.

But in general neither drains or swales will prevent block from showing wet on the inside, especially if you have poorly drained soil. You wouldn't necessarily need a footing drain (we call them footings) if you were not getting water in the crawl space, actual standing water, not moisture.

Around me we have alot of clay in the soil, so it does not drain well, every structure either has a footing drain or needs one. We only have concrete foundations, with footing drainage systems inside under the floor, 6-8 ft below grade.

With pervious block, if you really want to correct the problem, you may have to excavate around the foundation and apply a waterproofing/damp proofing material (like Bruce did) to the exterior of the foundation. If you went through all that work, then you would install a footing drain system outside, as long as it could be discharged to daylight. In other words, to somewhere on your property where the grade is lower than the crawlspace floor.

We may be blowing this out of proportion though. If you're just getting a little moisture staining on the blocks, a dehumidifier might be all you need.

JB.

 

[955Lincoln]

Unfortunately I excavated too deeply. Actually I failed to consider that I was on the down hill slope when excavating.

Hi Bruce, I'm going to be starting a foundation soon and my property has the same fall as yours so I'm very interested in your progress.

Could you please expand on these two sentences, especially the second one. Did you hire an excavator or do the digging yourself?

 

[2manyrocks]

We may be blowing this out of proportion though. If you're just getting a little moisture staining on the blocks, a dehumidifier might be all you need.

JB.

Thanks.

Before I installed a vapor barrier, I had water droplets on the joists in the crawl space from condensation. I see droplets on the other side of the clear plastic now, but at least they are not on the joists.

The prior owner must have had actual water in the crawl space because they dug a small pit at one corner.

We have a mixed clay soil.

Part of the problem is that the grade slopes directly to the back of the house. It should have been graded differently when the house was built or at least they should have put in some drains. Right now, the house is in the plane where the water will naturally drain off the site.

I can drain it to daylight whether I do a footer drain or a french drain a few feet away from the house.

If they had put in as much gravel as the OP, probably wouldn't be the problem I'm having.

 

[JB4310]

Thanks.

Before I installed a vapor barrier, I had water droplets on the joists in the crawl space from condensation. I see droplets on the other side of the clear plastic now, but at least they are not on the joists.

The prior owner must have had actual water in the crawl space because they dug a small pit at one corner.

We have a mixed clay soil.

Part of the problem is that the grade slopes directly to the back of the house. It should have been graded differently when the house was built or at least they should have put in some drains. Right now, the house is in the plane where the water will naturally drain off the site.

I can drain it to daylight whether I do a footer drain or a french drain a few feet away from the house.

If they had put in as much gravel as the OP, probably wouldn't be the problem I'm having.

If you are getting that much condensation you need a dehumidifier. Most basements need a dehumidifier in the humid months, even with a perfectly functioning drainage system. Of course ground water would exacerbate the problem, but the water you see on the plastic comes from the air.

If you are not seeing water running down the walls or across the floor, you should start with the dehumidifier. It's great that you have the topography that would allow it but even a well designed gravity drain system would be no guarantee against humidity and condensation.

JB.

 

[crbr]

yes...

 

[BruceWard]

Hi Bruce, I'm going to be starting a foundation soon and my property has the same fall as yours so I'm very interested in your progress.

Could you please expand on these two sentences, especially the second one. Did you hire an excavator or do the digging yourself?

Sure. I hired two excavation companies. I supervised and ran some of the equipment. Really I ran around with a builders level, grade rod and laser marking corners most of the time. We started with a goal of digging down 9' from where the uphill concrete wall sits. 9' was chosen since it would allow the entire foundation to sit on undisturbed material and I thought it would work well with the 10' concrete walls. The initial plan was to dig down to grade then dig footers.

This is what it looked like before excavation.

We started with a 90,000 pound Komatsu 300

We ran into rocks like this

When the 300 left it looked like this

The hill on the far side of the excavation and the flat area that exists beyond the current slab was built up by cutting benches in the hill below where large rocks were placed as a retaining wall and then smaller material filled in behind.

This is taken from below, and is where my gravel and pipes drain to.

Then we brought in these guys to get down to grade

Starting from far to near, the Komatsu 150 is around 40,000 pounds and outfitted with a 2' bucket to clean up rocks that are broken free. The Case is a lightweight at 20,000 pounds with I believe a 1,200 pound hammer. You can probably guess what happened when we finished drilling. The Case hammered for 5.5 days. About 60 holes were drilled. We had planned to use a 60,000 pound Komatsu 220 with an 8,000 pound hammer but we were finished before it made it out of the shop.

This guy did a lot of grading

This is what it looked like before we put gravel in. The low areas have frozen water in them. That area is primarily footer now. Additional footers were hammered out later.

Then we put gravel in

We dug / hammered out additional footers and built the center of the slab up about 8"

Water runs through the gravel, below the slab and above the solid rock below.

If I were staring over again I would excavate 8' instead of 9 so I could slope up a little leading toward the house.

 

[JB4310]

Much of what is between the top of the hill and the retaining walls is solid rock and would have to be jack hammered to create a swale. I can create the swale in the backfill area or add a french drain to the backfill area away from the house.

This picture shows the slope toward the house.

Looks like enough room for a swale, and close to 20% grade on the North side (looks like the far wall is the north side) the water is gonna rip through there, may have to put heavy stone in there to slow the water and erosion.

It's a little trickier on the west side (I'm assuming west is facing up hill) that water has to come around the front of the house some how. Try to get as much water as you can to go around that back side. You may need a large catch basin in front of the corner closet in this picture, then piped under the level ground in front of the house to empty down hill. To avoid having a storm run off river right outside your front door.

I still think you are better off trying to manage the storm water at the surface where possible. Best thing to do is observe where the water wants to go during a storm and try to improve that path around and away from the house. I would put a couple of feet of fill over those parts of the footing drain system that isn't covered yet. A heavy storm will wash alot of sediment in there like that.

I could be all wrong, trying to evaluate your situation from a picture, but that's how I see it from here. Just trying to be helpful, at least give you something to think about. When I go to see potential basement waterproofing jobs, I tell little old ladies they know their home's issues better than I do, and I have to take their intuitions seriously.

Edit: Looking at those pictures in post 19, Man that was quite the undertaking, getting that site ready!
Don't know how high that hill goes up above you, but if it's huge, might want to get those machines back there and do some serious storm water control up above the house.

JB.