Basement flooding

[geostruct]

We have a house where the basement floods during heavy rain events. In the back of the house, the basement has a door that walks out to the back yard. The door is not down steps, rather the grading of the yard slopes down towards the door and into the center of the yard like a valley. You basically walk out through the valley with a small retaining wall on both sides for the first 10', then just sloped grading of a yard. This is all underneath a deck from the floor above.

We have a sump pump on the same side of the house as the walk out basement door, but I think sometimes the basement will flood when the water level rises up high enough from puddling up outside the door. There is a large concrete slab (5'x3') there that I think is taking away a lot of the drainage capability between the two small retaining walls (4' high).

What I was thinking of doing is demo'ing the concrete slab, excavating down maybe 6-10 inches, and placing a high void ratio type of stone/rock (maybe 2b washed? or AASHTO #57).

Do you think that the combination of that fix and the existing sump pump would be sufficient to keep out the water? I know its hard to tell without seeing it.

What type of stone would you recommend? Would the sump pump prevent any possible water seeping up through between the slab and basement walls on that side of the basement? (20' long wall)?

Thanks

 

[msquared48]

If this is an older house, I think that there is a very likely additional problem to remedy - the drain tile at the base of the concrete foundation wall may be plugged too, contributing further to the flooding problem. It may need to be replaced as in $$$$$$$$$$$$$$$$$$$$$$.

Where do your foundation drains drain to? Do you know?

Mike McCann
MMC Engineering

 

[geostruct]

I am a junior engineer and not very familiar with house foundations...I dont know where the foundation drains drain to, or where they are located and how they work. Can you explain for a typical case?

Is there an easy way to check foundation drains and how they are working? I would need to figure out where they are as well.

It is my opinion that the flooding is mainly from the poor drainage outside the back door. I may be wrong, but it doesent flood often, only when there is a large storm lasting a day or two or longer with heavy rain and/or melting snow concurrently, etc.

 

[msquared48]

Foundation drains are placed at the basement slab level and should be backfilled over with porous drain rock. The foundation drain system sand downspout drainline system should not be linked - they should be two separate systems. The downspout lines (if present) could go into the storm sewer, or into an infiltration system. The foundation drain may be daylighted to the surface somewhere on the property, grade permitting, and may never see any water except in times of high flows. It may also drain to a dry well.

If either system gets clogged, or the water table gets to high for gravity flow to work properly, you will have problems in the circumstance you describe.

Whatever the problem, whatever the system you have, it cannot handle the flow you describe in it's current state by gravity alone.

Sounds like you need to do a little investigation.

Mike McCann
MMC Engineering

 

[tnboy82]

It seems that if it were coming through the door, it would be easy enough to spot. As you would be able to go down to that door and watch the water come in, or you would be able to see the rot and water damage around the bottom of that particular door.

My grandfather had the same problem on his split level ranch home, and it took him years to figure out the issue. Turns out that after his home was built, he had put an underground drain in for his gutter downspouts and it had formed a small tunnel into the crawlspace under the upper end of his house. He kicked the the downspouts out and filled the line with as much dirt as he could pack into them all around the house. Then he ran a piece of downspout about 6 foot out from his house into the yard (which I wouldn't recommend as it looks terrible, but you know how old guys are). He hasn't had any water issues since.

Later my uncle had the same issue at his house, however, the flooding occurred slower and several days after siginificant rain. It turns out that his foundation drain system had collapsed, and he end up digging out around the entire foundation, and replacing the drain tiles and sealing the outside of basement walls. Then he sealed the inside of the walls as well.

 

[msquared48]

Ten years ago, my father-in-law had a similar problem with the basement flooding and it was due to two things - the drain tile system being clogged, and the downspout system plugged - he had connected these lines to his foundation drains. We also installed a french drain (cutoff trench) on the backside of the foundation wall to catch surface water. His flooding too only happened in periods of heavy rains.

We installed a french drain (cutoff trench) on the backside of the foundation wall to catch surface water. Then we routed all the downspout lines away from the structure and he has not had a problem since.

Mike McCann
MMC Engineering