Ground floor slab in area with High water table

[voi]

We have a site where up to 5 m the ground is silty clay. We removed the entire clay and reached rock. The building is founded on rock at about 5 m below ground with columns/ pad footings and the walls are supported on strip footings also on the rock. We are near an area with high water table so we have water entering inside the building up to 3 m below ground level. So we are worried that our ground floor slab will be compromised by this water entering the building. If we backfill the entire 5 m excavated area with rocks and concrete and use french drains on the outside will that stop the water entering the building. We have no basement and so the walls below the ground is not reinforced concrete. Our major issue is how to stop water entering the 5 m excavated hole

 

[SlideRuleEra]

1) If we backfill the entire 5 m excavated area with rocks and concrete and use french drains on the outside will that stop the water entering the building.

2) Our major issue is how to stop water entering the 5 m excavated hole

1) No. French drains work by gravity (that is, water flows downhill to a "dryer" area). What you describe has no "downhill" and is no "dryer" area at a lower elevation.

2) Well point dewatering surrounding the hole, preferably beginning before start of excavation and continuing until completion of subsurface construction.

 

[HTURKAK]

If you do not have basement storey , what is the reason for founding the footings 5 m below and constructing perimeter wall? moreover you would have bouyancy and uplift problems If you provide basement with sog.

I would prefer ( if the 5 m uppermost 5 m the ground is silty clay is weak )soil improvement such as ; soil exchange with gravel material , jet grout or piled found. and construct the ftgs above GWL ..








Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

 

[Ron]

Have you considered placing a lean concrete "mud slab", applying coal tar waterproofing and then waterproofing the exterior walls of the below grade sections?

Agree with SRE...french drain won't work

 

[voi]

We founded the footings at 5 m because that is where we encountered firm material. I like the idea of Ron of applying coal tar and placing lean concrete. The only concern is the cost. Backfilling entire 5 m with lean concrete will increase the cost.

 

[SlideRuleEra]

Backfilling entire 5 m with lean concrete...

That is not a mud slab. A mud slab that Ron mentioned is concrete a few centimeters thick that is used as a stable work surface for construction of structural concrete.

In either case (mud slab or backfill full depth with lean concrete), if the concrete is placed in an excavation containing a significant depth of ground water, the result will be a worthless mix of segregated sand and concrete aggregate (ground water washes out the cement). There are ways to place to place a rich concrete mix thru standing water (tremie), but that is outside the capability of many Contractors. In both cases the best method for 5 meter deep excavation is to dewater... most likely with well points before excavation begins. This is normal construction practice in areas with high water table, it's just that you may not be familiar with it.