crawl space to basement

[derwall]

hi i have a lower ground level at my home, half is a fullsize basement kitchen the other half is a crawlspace, 4ft plus clearance.  i want to dig down another 3.5 foot but without going so close to the foundations that i need to underpin.  i have heavy clay soil and was thinking i could go down 3.5 t foot as long as i was 2 foot away from the foundations and then cover the clay with concrete back to the wall and lay a new floor slab.  I have an appointment with a structural engineer tomorrow, but was wondering if anyone on here has done this too. Thank you

[Mr Punter]

Good that you have the engineer visiting.  Have you dug a trial hole to expose the existing foundation right to the base?

[derwall]

i dug a trail hole down to about a foot below the ground level, it's 2meters away from any foundations and about 10 x 12 inches wide.  There was no signs of water as i dug the hole (even as  i was leveling out the the very bottom of the hole.  I was surprised the next day to see about 2 inches of very clear water in the hole. ( not muddy at all, and the water never came from above).   yes looking forward to hearing what the SE says about our plans, i know hiring professionals is a must with this type of protect. thanks. 

[Ferdinand]

Here's hoping that your wall goes down all the way.

[Gone West]

3 minutes ago, derwall said:

I was surprised the next day to see about 2 inches of very clear water in the hole.

That is the water table. When you dig the hole and remove the soil hydrostatic pressure pushes water into the hole in the same way that a spring finds a weak point and comes out.

[derwall]

13 minutes ago, PeterStarck said:

That is the water table. When you dig the hole and remove the soil hydrostatic pressure pushes water into the hole in the same way that a spring finds a weak point and comes out.

thanks for the information very helpfull,,  does finding this water about 2 foot below my intended new concrete slab floor mean i cannot now plan to dig down and lay a slab in the basement at ground level.  if so glad i dug the plot hole down that far.    thanks

[derwall]

23 minutes ago, Ferdinand said:

Here's hoping that your wall goes down all the way.

the foundations don't go as far down in the crawlspace to ground level unfortunately,  they are about 3 foot above ground level. thanks,  there is a brick pile or tower fundation also in the center of the crawlspace, the SE going o look at a design to replace this with a heavy steel post.  I'm hoping if i don't dig to close to the wall foundations i can avoid underpinning and then concrete over the soil clay that remains around the room and back to walls.  thanks

[Gone West]

1 hour ago, derwall said:

thanks for the information very helpfull,,  does finding this water about 2 foot below my intended new concrete slab floor mean i cannot now plan to dig down and lay a slab in the basement at ground level.  if so glad i dug the plot hole down that far.    thanks

I don't know much about building basements but it would be useful to know whether the water table height varies over the year. If it drops a lot in the summer, as it does around here, then it's much easier doing that type of work in the summer.

[derwall]

thanks for the info cheers