Diaphragm wall used as a temporary structure

[geo321]

I have been assigned a new site where they constructing a tower with 4 basements.
A diaphragm wall is bring used as a temporary structure to support the excavation during construction. A basement wall will be constructed afterwards.
Water table is at -3 meters from the ground.
My question is what are the reasons for not using a diaphragm wall a permanent basement wall ?
Is it the high watertable and the fear of having water infiltrating in the basments through the joint in the pannels of the D-wall ?
Thanks

 

[KootK]

I think that it's:

1) waterproofing as you mentioned.
2) the tie backs relaxing over time if not designed for permanence.
3) needing to destress the tie backs depending on indemnity agreements etc.
4) somewhat rough interior surface provided by the diaphragm wall.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Retrograde]

There is no reason not to use the diaphragm wall as the permanent wall - it has been done successfully many times.

 

[geo321]

Ok but if a D-wall is used as a permanent wall, how can we ensure its waterproofing ?
Even if the watertable is not present, all the walls in contact with soil should be protected by some bituminous painting.
Since it is a buried wall, it can not be accessed to ensure that (from the face of soil).
Thinking loudly, Many types of waterproofing can be added to the concrete mix (crystalline waterproofing) so at the end we will have a concrete which has been mixed with waterproofing material which will ensure a good barrier against the soil and water.

 

[Guest262653]

The best concrete in the world won't stop the water seeping in if the joints between panels aren't properly waterproofed and there will be water in your basement. I've seen some double waterproof joint details that might be worth a taking a look at but their effectiveness is always somewhat questionabe. If a high water table exists, most likely there will always be some water seeping into your building. If that is only a mild concern and if you'd like to eliminate the rc walls, you could recommend an additional interior masonry wall, separated 10 or 15 cm from the diaphragm wall in order to install a half-pipe to collect the seeping water. This, however, reduces the available space and constrains future inspections of the wall.

Regarding KootK's point on the relaxing of the ground anchors if not designed as permanent, I'd comment that in basements it is not common to use permanent ground anchors as the floor slabs usually provide enough lateral restraint for the at-rest pressure that will develop on the walls. Temporary anchors are usually deactivated after the construction of the floor slabs. This is, of course, if there is a uniform soil level around the basement and if there is a clear load path for the floor system to carry the horizontal loads.

 

[geo321]

The contractor is proposing a circular joint between the panels since, according to him, this will prevent water.
In my case, the diaphragm wall is just temporary. However, the basement wall which will be constructed adjacent to him will not be able to receive a waterproofing membrane.
Is what I said in the above post can be applicable ? Mix waterproofing with ready mixed concrete before it is discharged can be an option right ?

 

[BigHarvey]

Diaphragm walls are often used as a permanent structure. Another possibility used in some subway stations like in Lille (France) is to have a temporary wall and a classical basement wall designed to withstand only the water pressure, soil pressure being taken care by the diaphragm wall.
Regarding watertightness of a diaphragm wall, there are two aspects :
- one is the permeability of the concrete, normally if you respect a minimum quantity of 400 kg of cement per m3, the concrete is dense enough to be watertight. Nevertheless you can add silica fume to the concrete that will produce a self Healing concrete in case some leakage happens.
- the second is the watertightness of the joint between panels. It mainly dépends on the quality of the excavation supporting fluid (bentonite or polymer). Good mud = wtatertight joint, bad mud = leak whatever waterstop bar you put. In case a joint is leaking you need to grout it with a two components resin on the full height (even if the leak is localized otherwise the leak will migrate)

 

[KootK]

Interesting. It seems that diaphragm walls as permanent structure is much more common that I`d imagined. As a building designer, here are two more concerns that I would have:

1) Normally, basement walls will be load bearing walls supporting the adjacent slabs. Would the slabs then be doweled into the D-wall and supported via shear friction? Would there be an eccentric strip footing installed at the bottom of the D-wall to support the imposed axial load?

2) Normally, for a tower, we consider the perimeter basement walls to form a wickedly strong at stiff network of shear walls. Can the diaphragm walls be used in that way? Again, would it require the installation of strip footings at the bottoms of the walls?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Retrograde]

Westminster Station in London uses a diaphragm wall as the permanent structure. The wall had a grillage of concrete beams cast in front and then massive steel props installed. This was required as there are no floor slabs in the escalator zones to provide lateral restraint.

A well constructed wall should be waterproof (but may be damp). Grout injection through the wall can be used to patch up any local leaks.

 

[BigHarvey]

The large bearing area of diaphragm walls and the large friction area in the embedment depth allow to support easily the axial loads.

Intermediate slabs are linked to the wall via two solutions : one is to have crossed bars with couplers concreted in the wall which is ok if you can accept a 5 cm vertical tolerance for the couplers. Th second one is to drill in the wall and seal bars, in this case you don't need any vertical tolerance.
As mentioned by Retrograde a good wall should be almost watertight.

 

[KootK]

Thanks for that BigHarvey. Is there any way to have the horizontal rebar lapped between the panels?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BigHarvey]

It can be done but it's very difficult (and therefore very expensive). It is generally not needed since you have a capping beam at the top of the wall and panels behave like vertical beams. The only situation where you might need it is for seismic design.

 

[KootK]

you have a capping beam at the top of the wall and panels behave like vertical beams.

For a multi story basement and typical panel widths, I have a hard time imagining that the panels would be adequate as independent, vertical, cantilevered beams. Perhaps designers count on a mechanism other than rebar for vertical shear transfer between panels? Seismic or wind, a big force is a big force.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[geo321]

Regarding the waterproofing of the diaphragm, i was reading an interesting documemt where they state that the slurry used to stabilize the excavation will penetrate the granular soils and membrane is created by formation of a mud cake in which the void space of the granular soils filled with the slurry will provide a very good waterpeoofing barrier.