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Old 11-25-2011, 10:01 AM   #1
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crawlspace slab voids, basement water


hello,
i have a ranch style house that has a basement that is 50% full and 50% crawlspace. the foundation walls are poured concrete in both areas, full height on the full basement side and around 3-4' on the crawlspace side.
the crawlspace is open to the full basement, and there is a 5 course tall masonry block retaining wall. the crawlspace has a 'rat slab' poured rough concrete floor.
water has been seeping out the masonry block retaining wall since before i bought the house 6 years ago. recently i had the backyard regraded, all the downspouts connected to piping that leads to a drywell downgrade and away from the house, and finally a patio poured on the back of the house that runs the entire length of the crawlspace section of house and goes about 20-25' into the back yard. everything is sloped away from the house.
a few days ago we had a good amount of rain and my basement flooded. i drilled a hole in the base of the masonry retaining wall a few inches from where the wall meets the foundation wall in order to relieve the pressure and the water was streaming in full blast (like a hose) for hours, and continued until well after the rain stopped. i knocked a hole in the top course of block to get a look behind the retaining wall and found that there was a rather large expanse of empty space under the crawlspace slab where the soil had either settled or been washed away over the years.
i think im going to have a sump pump installed in the corner and that will keep my basement dry, but im concerned that the void beneath the crawlspace slab will get bigger and may compromise the integrity of my foundation wall.
the volume of water was more this last time than all previous episodes before - combined. obviously, more water is somehow being directed towards my foundation. the confusing thing is that all the water that sheds from my roof, which used to be deposited onto a backyard that had poor grading, is now being taken far away and downgrade, yet i am getting more water than before.
the soil in the backyard is mainly clay, and the patio is constructed of 6-10" of stone with a 4-6" stamped pad poured on top. could this be acting like a giant french drain, collecting water from the soil and directing it towards my foundation?
if i have a french drain installed on the perimeter of the patio and somehow solve the water from getting into the house, should i be concerned about the voids that are below the crawlspace slab? how could they possibly be filled?

here is a rough layout
crawlspace slab voids, basement water-layout.jpg

picture below from me reaching in the hole at the top of the retaining wall and snapping a picture towards the back of the house. the 'ceiling' is the bottom of the concrete slab in the crawlspace.
Name:  crawlspace void.jpg
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any thoughts are greatly appreciated!

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