Waterproofing leaking basement 2

[hnguyen8]

Hi guys, just want to get some ideas on how to waterproof a leaking concrete basement.

I have a concrete basement consisting of 350-400mm thick slab and 200mm thick precast retaining walls. The slab was originally built with cold joints with inadequate shear transfers. The basement was damaged by earthquake and there are a lot of cracks in slab and retaining walls letting water in.

Waterproofing from inside doesn't seem very reliable. The water table is high, people have tried epoxy injection but failed. One potential solution I've heard was to drill the slab and walls and inject bentonte to seal the basement from outside. Not sure if anyone has had any experience with this or has any other ideas? This problem is similar to waterproofing an existing tunnel where access to the other side is not easy.

Any input or idea is greatly appreciate.

 

[CELinOttawa]

With respect, hire a local engineer qualified to undertake this work to help you.

 

[graybeach]

I think this was done in my house by a previous owner. There are 1" diameter grouted holes at about 1 foot spacing all around the perimeter of my basement. I don't have any issues with water except in one corner on one occasion some water came in. It was during a tropical storm so I think this was related to overwhelmed gutter leaders. My basement is finished and, when I pulled up the wet carpet, I saw the grouted holes. To be on the safe side, I replaced the carpet with "flor" carpet tiles. If any water gets in again, I can just pick up the affected carpet tiles and let them dry in the sun.

I don't have any cracks like you do. You will probably want to inject those before grouting.

 

[jike]

Determine FIRST, if the basement walls and slab were designed and built to retain water. You will need a structural engineer to do this.

 

[oldestguy]

This subject has been discussed on forums here before. If at all possible, collect the water outside with a suitably filtered collection system. Sealing cracks may result in sufficient pressure build up outside so as to lift the floor or cave in the walls. Be careful with bentonite, since it will swell and may add to existing pressures.

If it is surface water that dissipates and not a ground water thing, consider changing outside grades and waterproofing the ground surface. For that option look up my comments under the "Soil/ground water environmental engineering" room.

 

[hnguyen8]

Hey guys, thanks for the responses.

I am the structural engineer hired for this job . It's a multistorey apartment complex damaged by the Christchurch earthquakes. Waterproofing the basement is among the other works I'm putting together schematically for this building to see whether we can even fix it.

The basement walls and slab were indeed designed as a water retaining structure, but quite poorly detailed, especially at the slab joints.

It is a ground water issue. The water table is high and what happened was that the slab and the joints could not take the uplift hydrostatic pressure so when the earthquakes happened they cracked and ruptured. The structural solution I'm thinking of is using micropiles to hold the slab down at critical locations and/or improving the shear connection at the cold joints. This will help with the uplift pressure. But at the same time I need to incorporate a waterproofing solution at the cracks in the slab and walls. In my mind we would need to somehow seal the slab and walls from outside with some kind of injection, but I haven't come across any products for this.

The existing water collection system outside is poor, but even if we can improve it, we can't lower the water table without constantly pumping water out, which is not viable in the situation I have.

 

[CELinOttawa]

This is not a problem to guess at or handle alone for your first time. In Christchurch I would heartly recommend Jan Kupec at Aurecon as the local support.

 

[hnguyen8]

Hey CELinOttawa, thanks for the tip. I'm part of a team working working together and also seeking professional external expert help for this problem. Eng-tips is just one of the sources of help that I thought I'd tap into. Hopefully I didn't come across as someone just trying to find shortcuts and solve problems beyond their capability.

 

[CELinOttawa]

Hi hnguyen8; To be honest my first reply was purely because it was your first post. Very judgmental of me, but a lot of first posts turn out not to be engineers at all.

Frankly from your posts this is likely something that you could handle, but very difficult works are often best handled with external expert consultation. I would encourage you to try to handle all the design work, but use an expert to help determine the approach.

Dr. Kupec is an excellent engineer and a personal friend. You would be in very good hands if able to interest him in the project. Whether you use him or someone else, I think you should have some help. I certainly wouldn't be undertaking this without some very capable Geotech/Earth retension expertise on the team.

 

[oldestguy]

If your codes allow it, one form of treatment is to let the water seep in, but collect it and send to the local sanitary sewerage system. A small channel is cut in the floor at the walls and that is the collection "device". Also some have then hidden that channel at the wall by a new wall on the inside. Quite obviously that has to be resistant to any dampness.

 

[hnguyen8]

Hey oldestguy, thanks! That's one of the solutions we were thinking of too, let the existing walls be wet walls and build new walls adjacent to them. A couple of problems with this are maintenance and the fact that we'll need to constantly pump water out and discharge it. Also the current slab distortion and cracking make it hard to collect water seepage from the slab. Nevertheless this is one of the more viable solutions.

 

[RHTPE]

Belated thoughts on this issue:

Sometimes the most practical solution is found by observing the past. In New England many homes & buildings were constructed on dry-laid stone foundations which survive today. Rather than worry about water getting through the foundation and into a cellar or basement, they simply anticipated it and built accordingly. Floors (or the dirt surface) were configured to drain water, and the foundations were sited such that the water could be conveyed to daylight.

Of course, subsequent alterations to the surrounding terrain has been known to cause new problems in the cellar or basement (Duh!). But I diverge a bit.

The point is, preventing the water intrusion, while desirable, is probably impossible. The not-so-good aspects of preventing the intrusion may be:
- Hydrostatic & buoyancy forces on a foundation & floor slab not designed for that (in this case they were).
- The cost of competent means to prevent intrusion.

The 'practical' solution is to control the water internally. If it is possible to remove the existing floor slab, even better.

Then, plan the best routing of water from the walls to a sump pit based on the use of the space. If the floor slab can be removed and replaced, strip enough base material (if any) to encourage the surface flow to the sump. Replace the base material with 1/2"-3/4" stone over filter fabric. Cover with a layer of poly over filter fabric. Replace the slab, holding it back from the perimeter foundation walls perhaps 4-6" to provide an open trench for the wall water. One can stop here and put removable grate over the trench.

Now, if wet walls are not acceptable, consider the following: Install metal lath 2" away from the face of the existing wall using concrete screws w/washers. Fill the space between with pea stone. Anticipate and install ties for later formwork. Cover the lath with filter fabric, then a layer of poly. Install a layer of WWF using 2" spacers. Install formwork and cast a 4" minimum thick layer of concrete. Use stainless steel or galvanized formwork ties that will engage the new concrete as this will be its only connection to the existing walls. The top edge of the new concrete, well above the detected intruding water elevation, could be made to wrap over the top of the lath & stone to provide a finished appearance.

This would be a non-maintainable solution, so quality construction is essential. If silt infiltration is likely, this may not be a good solution, and the wet wall/grated perimeter trough solution may be wiser.

However you approach it, if there's no practical gravity drain run to daylight, a sump pit & pumps is a reality. Deal with it and size the pit for an appropriate volume of water.

All of this presumes that the structural integrity has been checked for all aspects of the remedial measures. If the floor slab braces the bottom of the perimeter foundation wall, then the existing slab should be taken out in sections, leaving alternate sections in place to brace the wall. The new slab would need to provide "stubs" that bear against the wall, with sections of grated trough in between.

Just a thought on how to deal with it from a New England Yankee.


Ralph
Structures Consulting
Northeast USA

 

[hokie66]

hnguyen8,
CEL has given you some good advice. Without a detailed assessment of your structure, further comment won't help.

I assume the 350 to 400 thick slab is a mat foundation, or a slab designed to span between columns to resist hydrostatic uplift. This type slab is very difficult to waterproof after the fact, as are precast wall panels. What is the use of the basement? Car parking, I suppose. Has any thought been given to abandoning the basement as occupied space?

 

[hnguyen8]

RHTPE, thanks for your input. Very helpful indeed.
hokie66, you are correct about the slab. There are some wing walls to help support the slab, but the slab issue is exacerbated by the fact that it was constructed in separate sections joined together by dowels that aren't very strong.
The basement is a car park. If the car park can't be saved, it will be a demolish and rebuild.

 

[darthsoilsguy2]

2 things to add to the list to look into
1-polyurethene injection (google deneef)
2-blind-side waterproof on interior sides of walls and slabs and place a waterproofed inner building.

 

[hokie66]

hnguyen8,
Are you saying that the slabs are structurally inadequate to resist uplift because the reinforcement at construction joints is discontinuous? That would be very strange indeed.

 

[hnguyen8]

darthsoilsguy2m
Thanks for the suggestions!

hokie66,
That is correct. There are plate shear dowels at the construction joints and no continuous reinforcement. Very unconventional construction for a water retaining structure. That's why we're thinking of using micropiles to hold the slab down and reduce the stress in the joints.

 

[structuralprashant]

If walls and the floor slab are structuraly in good condition then to grout the cracks and providing a ferocement jacket monolithic for floor and walls with use of SBR and shear studs at suitable spacing will be an economical and effective solution. Regards.