HELP! – Advice on water table with footings.

2 Oct 2007
Reaction score
United Kingdom
:( Hi

I am asking for help here because as you will see, I'd prefer not to contact our building inspector
for advice because I have made an official complaint to the council which is on going.

They are waiting for me to let them know that a "raft" has been constructed.

Extension to front of property with 3m wide x 1.5m deep porch - and the roof
(whole width of property) to be extend to the front.


So, following incorrect advice from our building inspector last September regarding footings
for an extension to the front of our property on "reclaimed beach land", we have been left with
a huge hole at the front, after the land just collapsed under the digger.

The building inspector insisted that we had to employ a structural engineer to determine
a safe way to continue.

We had 3.5 cubic metres of C20 delivered as a base for a reinforced raft to sit on.
The base was tapped perfectly flat - exactly 225mm below the original footings.
This perfectly flat base has caused a problem because there is no run off.

The raft is to be made of shuttering with A142 reinforced mesh sitting 40 mm up from ground level
and A142 mesh positioned 40 mm below top raft concrete level
(to be tucked under existing exposed foundations).

Because the C20 base has basically turned the large hole into a DEEP pool of water because we
have a very high water table (we are 4 inches below sea level) and it really hasn't stopped
raining since October for any decent length of time we cannot get the raft started.

We thought that the C20 base would stop the water table rising from underneath, but the water
seeps out from the sidewalls. So every day since early October the float pump is running for at
least 2 hours every day - the pump removes 144 litres per minute - and it takes 2 hours to
remove the water that rises to just under the existing foundations. I do not let it go above the
bottom the foundations because when it rains I run the risk of it coming into the property.

So the pump is removing around 17,280 litres most days.

The pump has to be turned on manually because the 'float switch' does not turn on/off the pump.
Brought another pump - same problem with float switch.


We have attempted to construct the raft on a few occasions, but because the pump drains off the
water down to a depth of 10 mm, and then the reminder of the water has to be scooped up and
removed. This can take over an hour, but you are fighting a losing battle because the water table
is seeping out of the sides of the excavation.

The raft has to be built in situ - as it has to be tucked under the original footings.

Once constructed with the A142 mesh and covered with a removable section so that rain cannot
penetrate - I will not be able to get the pump in the enclosed raft section because of the mesh.
But I still have the problem of the water seeping under the shuttering. So it will fill up the enclosed
shuttering, which will not be OK for the C35 to be delivered.

Once the shuttering is complete, I have to wait for the building inspector - then arrange for C35 to
be delivered and fill the raft. All this could take around two weeks. Plenty of time for the raft to
become not fit to have the C35 delivered.

Have you any suggestions how I might get around this problem.

I have included images of the pond problem.

Many thanks in advance to anyone who has a suggestion how I might overcome this problem.

Regards, Stephen
Sponsored Links
This all seems a bit late in the day.
There are Civil Engineering solutions, but surely you dont want to go down that route?
Perhaps you are familiar with: ref. TCC/05/19
A further thought: how was your house constructed, and any recent nearby structures?

Water will float damn near anything unless the thing is deep "anchored." Presumably you've already come up with some tie-down solution(s)?

Brick & Block - All the floors are concrete. There is no problem with the rest of the property - just this big hole.
Sponsored Links
noseall, with respect,
the house is 100mm below sea level and situated, from what i can make out, by the seaside. Any attempted, drainage channel would only bring further water into the excavation.
Unless I am missing something ... form a sump hole or channel for the water to drain in to, and then be pumped out by your pump, or extra pumps of higher capacity to keep the trench empty

Either way, 10mm of water is not going to matter much when concrete is poured

Or, bottom the trench a bit deeper than required, and lay a 50 or 100mm layer of concrete, and then form the sump hole in that. Then you have a smoother layer to work from and it prevents the bottom of the trench from softening under constant water
I'm wrong, i apologise.

I agree, a drainage channel, and stronger pumps, or better, the conc.layer and inset sump should do the trick.
Thanks once again.

I cannot dig out the trench as it is 9 inches deep of C20 - part of the structural engineers stipulations - this is to support the raft - and then the blocks/bricks go on top.

The problem is that the enclosed raft will hold 9 inches of water - as it will seep under the ply shuttering.

I had thought of sand bags - I think that is going to be my only option now.

Along with a roof mastic on bottom of ply to help stop water from getting in.

Thought I'd pick your brains.

Hopefully mid-June might improve things weather wise.

What a nightmare.

All properties around here are built on shallow wide footings to prevent this problem from happening. Trust me to get a BI who didn't know the area, and said it had to be 1m deep x 600mm wide (size of bucket).

Thanks anyway.
Trust me to get a BI who didn't know the area, and said it had to be 1m deep x 600mm wide (size of bucket).

The lesson of the day is .... building control are not there to design your project. They are a "checking service" only.

Employ professionals to do the design work, and the BCO can't argue with them if the work is designed properly
Izzy As per norm, you are going about things ar*e about face.
Woody is about right, but will polish his post up a bit for you.
Your pump is only fit for emptying a bird bath. From your figures it is only discharging 8.6m3 per hour. A new born baby drinks a bottle faster than that.
The smallest pump to suit your purpose will be a 2 inch (50mm) diesel open set with a 2m lift and a discharge rate of 55m3 per hour. Try Speedy Tool Hire, they have a couple of hire centres in Sussex. Hire rate about £80 plus vat a week
Dig a sump hole big enough to take a builders bucket outside of where your shuttering will be with the top of the bucket 150mm below existing slab. Stick your pump inlet hose into bucket and pump her out, take about 19 minutes. The weight of hose will hold bucket in place and bucket will stop a load of cr*p blocking up hose filter. You want a long enough discharge hose to stop water running back in on her self..
Shutter your base, cut a slot 200mm long x75mm high bottom of shutter where sump hole is, turn your pump of and lay on B.I.
We use Tarmac Ready Mix for our concrete and when working in heavy water a mix called Toproc UW, with a tight a slump as possible This is a mix that can be poured into either fresh or sea water with a 50% less wash out and high water resistance.
How ever we never pour direct into water, as we always have a pump big enough to deal with the water ingress, before pouring, whilst pouring and after pour finished
If you have not got Tarmac Ready Mix in your area, phone your local outfit, explain what you want to do and they will design a mix for you.
Pump out before you pour, don’t worry about 10mm or so of standing water on top of existing sub base, sweep it into the slot that you have left in shutter. Start pour farthest point away from slot and sump and pour towards slot and sump driving any standing water into sump. When you come to slot stand short length of 25x100 up against slot to stop concrete going into sump.
Leave pump running for at least 6 hours after pour complete If 50mm pump to fierce, then drop your electric in just to keep sump hole dry.
Izzy, Make 100% job on your shutter, because if she goes you are really in the sh*t.
Regards oldun
oldun - thanks for your detailed reply. This info has been really helpful.
And it makes sense.


DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links