6m Deep Basement 2

[clarke1973]

I have a 6m deep single level basement to design for a three story concrete framed building. basement footprint is 11,000 m[sup]2[/sup]. The ground is sand, water table is at approx 4m.
The wall will be built on the plot boundary so no external excavation or ground tie backs will be possible. For the retention system my initial thought is a secant pile wall. A steel sheet piled wall is also an option, but waterproofing this is a concern. I assume this would have to be done by welding the joints?
For a secant wall, i assume there are options for the water proofing - admixtures in either the piles or the cast in place internal wall. could tanking be applied to the face of the piles prior to pouring the wall as another option?
Also have a concern regarding constructing foundations for the temporary props in the excavation with ground water present.

Does anyone have any experience with this and can offer some thoughts or advice? would be much appreciated.

Thanks in advance

 

[Guest262653]

Regarding waterproofing I'm sure that other members of this forum can provide better advice than me, but the water pressures won't be very high as you only have a 2m water level, so any of the options you mentioned, if detailed and executed properly, seems fine to me. I'll only throw some ideas regarding the support system.

For your case, in my area a steel sheet pile would be uneconomical due to the high retained height, and a diaphragm wall would probably be an overkill for that low water pressure of only 2m (this is also much more expensive in my area) so I'd also go for a secant pile wall (80 or 100cm diameter piles), as you suggested. Additionally, if you have pile foundations for the building, the pile equipment is already there. For lateral support, depending of course on plot boundaries and on the position of building foundations, I suggest the use of either the inverted method (executing floor slab bands first and excavate underneath) or propping the walls to the foundations executed in the central area of the plot. If toe passive pressure for the wall is a concern, a thick jet-grouting watertight slab may be executed to form a strut on the wall toe, keeping the internal forces on the wall low, providing a more or less watertight base to work on and reducing water flow to the excavation.

I was once involved in a preliminary design for a 10m excavation for a large plot basement for a 25 storeys tower where the water table was near the surface and there was also a 6m difference between the ground and the -2 floor. The geotechnical conditions were pretty bad at the top, with landfill, alluvial soils and thick mud deposits overlying cdg and bedrock layers. At the time, we proposed thick diaphragm walls penetrating all the way down to the bedrock to resist the water and soil pressures and to provide adequate foundation for the bottom levels' slabs. There was also present, of course, a thick watertight bottom slab.

As we couldn't use tiebacks either since the plot was a reclamation area in a lake, the lateral support for the excavation was to be provided with the concrete slabs that would constitute the ground and -2 levels (we call it the inverted method). After the installation of the diaphragm wall and tower columns pile foundations, part of the ground floor slab was concreted leaving large areas near the centre of the plot unconcreted to allow for excavation equipment to dig under the completed slab. The slab bands adjacent to the wall were supported vertically by the wall and by temporary steel columns inserted into the pile foundations for the tower. These were later encapsulated with concrete to form the tower columns.

I enclose a picture of the construction sequence we proposed at the time (sorry for the captions in Portuguese).

 

[clarke1973]

Thanks for taking the time to reply. Some excellent advice there for me to consider.

 

[CBEngi]

With regards to waterproofing steel sheet pile wall it is pretty common practice in Atlantic Canada for large excavations. They use some type of gasket between the sheets as they are being driven. Most steel sheet pile suppliers also offer a buy back program where they will purchased the second hand sheets back once the project is complete, if they are not abandoned in place, they will often even sell you second hand sheets if you don't mind at the onset of a project. Skyline would be a company to contact about sheet piles in Canada, I believe they are also fairly large through out the USA.

 

[GeoPaveTraffic]

What types of walls did the geotechnical engineer recommend?

Mike Lambert

 

[LonnieP]

Clarke, don't forget to review the floor slab for boyant uplift and provide adequate slope and a pump sump for the incidental leaks that are certain to happen. Take care with the floor to wall joints. Like it or not, you're now a marine engineer. Don't count on the 4m water depth either. Once had a soils report say "no ground water within the project depth". Soils report was done in a dry year. Two years later we were having to dewater the elevator pits during construction. Nature and water is so very aggrivating.

 

[SlideRuleEra]

The ground is sand...

LonnieP is right.

After wall construction and excavation has begun, can the 11,000 m[sup]2[/sup] area inside the wall be continuously dewatered to an elevation below the bottom of the excavation? Without dewatering, in sand you could get groundwater flow beneath the wall then up into the excavation.

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[oldestguy]

Add to Sliderule era: That movement toward the excavation will loosen deeper ground and, under some circumstances, would affect support of the whole structure.

 

[csd72]

6m is quite tall for a single floor so you would probably need 600mm diameter secant or quite chunky sheet piles.

Also look at the available space you have to surrounding structures and the required clearance you need for each.

Both options would need to be propped during construction though I would expect the secant would need one level and the sheet piles could need 2 levels of props until the basement is installed.

In the uk sheet piles are almost always cheaper to install but they are not as good at taking vertical loads as secant pile walls and may need intumescent paint if the they are supporting structure. Welding the clutches after the basement slab is poured is the usual method.

If you are in the uk then you should look at the basement waterproofing code as the starting point as this will often dictate your options for the particular basement use.

There is alsways the option of pouring a waterproof concrete liner wall to the inside of the sheet piles as a structural waterproofing layer.

Steel intensive basements is a good guide for sheet piled basement walls in the uk.

Comments above regarding groundwater are very good ones, if you have clay not too far down it may be worth seeing if you can sheet down to it and provide a 'cut off' wall keeping most of the water out.

 

[racookpe1978]

But, if the sheet around the basement walls prevents/reduces water coming inward - at least until it rusts out, what does he use to prevent the remaining water pressure from coming up vertically through the floor of the basement? Simple plastic wrap doesn't seen sufficient against the steady water pressure + time.

 

[hetgen]

Secant piles are not cheap; if possible you should look into getting permission to install temporary anchors and utilize anchored soldier piles with an arched gunite wall in between. This is assuming that the ground floor slab could be used to prop the solider piles as a final solution.

 

[Retrograde]

utilize anchored soldier piles with an arched gunite wall in between

I do not think that a soldier pile wall would be feasible when excavating below the water table in sand.