Small Structure Construction in Water Table

[Redacted]

Hi there,

I designed a square manhole 8' wide and total depth including the footing, top slab and grade rings is about 7' max excavation down.

I managed to convince the client to move the manhole further uphill where the water table is not as high, however it may still be present to some degree.

Although its not really my decision on the contractors means and methods, I would like to have an idea in my head how this can play out.

If the contractor excavates down and finds that the water table is about 2' high in the new excavation location, what would be the construction process for :

1) How would you compact the subgrade when constructing in the water table? Instead of trying to compact, would it be better to just excavate down another 1' or 2' and backfill with a 1' or 2' layer of aggregates? I already specified a 4" concrete blinding in the drawings but could use the aggregate as a firm base instead?

2) As for constructing in the water table for a small structure like this, it was mentioned to use diaphragm pumps, which is an option. I was thinking also, what if the contractor built the rebar cage for the foundation on site (where the water table wasn't an issue) and just lifted it in the excavation hole and poured the concrete after? The concrete would displace the water; if the water table is only 2' or so, this may also work?

If the water table is only 1' or 2' up into an excavation do you have any tips or advice to make the construction process easier/simpler?

 

[HTURKAK]

I will suggest,

1- Dewater the excavation pit and excavate 100-150 mm more and use 100-150 mm compacted ballast layer then 100 mm blinding..
2- Check the manhole for bouyancy and use waterstop at cold joints ( btw base slab and walls ) and use exterior water proofing

 

[Redacted]

@HTURKAK

Thanks, do you have any other recommendations for dewatering other than using diaphragm pumps?

How should the contractor go about compacting the ballast layer if it is also in the water?

From the design side, I checked the manhole for buoyancy and included exterior waterproofing as a bituminous coating and specified waterstops/gaskets seals for pipes as well, so that is all covered.

 

[HTURKAK]

You did not provide any info. for the soil conditions and the utility line.. If this is a single manhole , the use of portable submersible pump may also work .. If this is a network const. , the use of well point is an option.. Consult with local geotechnical people.


The ballast layer SHALL be compacted after lowering the GWL with dewatering..

 

[Redacted]

@HTURKAK Ok thanks that seems logical to me.

This is only a single manhole, so I will try the submersible pumps.

Noted about the ballast layer, thanks.