Pit to capture water in house

OK - so following on from my first question (see thread about absorption pit) we have a problem under our house to fix.

We are on a sloping block and the previous owners excavated at the back to put an extention on. The room at the back is lower than both the street level and the rest of the house.

Unfortunately for some reason they backfilled the wall so the subfloor level is about 1m higher than the concrete slab for the new part. And they did not put ag line behind the wall.

So we have water leeching through the wall.
So far we have
* put a new damp course in
* dug out the soil behind the wall lower than the concrete slab

We are planning to water proof the wall, put an ag line and blue metal in and dig a pit for the water to drain into (lower than the slab level) and maybe put a pump in to drain the pit.

Any suggestions on what we should use for waterproofing and how we should build the pit - ie should we line the base with concrete and should we line the rest of the pit to stop dirt getting in

Jeffbo01,

Got a bit of an idea what you are going through. When we moved into this house about 20 years ago, some bloke would only have to piddle in the gutter 3 doors up and under our house would flood, and if it rained .............well.

We do not have any concrete retaining walls, but the whole of the under house area is concreted as the house is a Queenslander - up on stumps, and water would come up through the joints in the concrete and god knows where else and take forever to go away.

So here is our solution and it works like a dream in, and we have never, I repeat never had water under the house since, even in the most severe storms.

A trench, that extended some 600 mm below the level of the slab was dug around the perimeter of the house, all falling to a single low point. 100mm poly ag pipe was laid in the bottom and then backfilled with 'pea' gravel to within about 100mm of the top of the trench, then normal soil that would support the growth of grass.

Where the low point is I set into the ground a 900mm diameter [ID] concrete pipe, Where the ag pipe comes up to the concrete pipe a hole was knocked into the concrete pipe to allow the ag pipe entrance. Placed into the bottom of the concrete pipe was a grundfos submersible pump with a float switch. This was plumbed into a stormwater drain.

So what happens is any groundwater finds its way into the ag pipe, which drains into the concrete pipe, as the water level rises the float switch comes on and empties the drain [you can adjust the level of the float swith]. The concrete pipe is capped with a removeable cover.

The first pump lasted 18 years.

The system has not failed!:2tsup:

I noticed this thread and I have a similar problem which I need some advice on how to fix.

My house is on a concrete slab where the ground level is split into two levels. The front part of the ground level slab is approx. 300mm lower than the back part of the concrete slab. The block slopes downwards towards the street.

Lately we have noticed yellow and black stains on the wall where the slab steps indicating that moisture is coming through the concrete slab as behind the stepped level is backfill.

During rainy periods the water seems to pool at the back of the concrete patio.

I have had recommendations such as:
- ag pipes to go around the back patio and drain into storm water pipes or street, however this may require cutting concrete and footings
- waterproof the stepped down part between concrete and brick inside the house (not sure if this will entirely solve the problems as there is backfill behind the stepped down wall part of the wall)

I would appreciate any suggestions to rectify the problem.

jeffbo, sounds like your on the right track, use a plastic pit (450mm should do it), line the trench with geo fabric, then your ag drain then backfill with blue metal and wrap the fabric around the lot before backfilling with dirt.
Before this all goes on however you need to waterproof the wall, first clean it with a gerni and let it dry completely. I recently used a product from bunnings called Bondall Waterbased Bitumen, around $45 for a 4L tub and it works a treat. The only downside was you cant backfill for 7 days after you have applied it but well worth it. Hint, after waterproofing has cured and before installing ag drain etc. I like to use some folded geo cloth to cover the water proofing just to protect it a bit from being pierced by a rogue bit of blue metal or similar. Some people use plastic for this but plastic sweats and you will probably find it will for ever be damp. Good luck.