Basement under sea level

[DNC Consulting Engineers]

Hi all,

I am currently in the process of design of a full tanking basement just under sea level (-0.02) and it is very close to the sea, within 300m.

Geotechnical engineer found water at -2m but I do expect lots of ground water during excavation.

Basement wall: We are planning to install 300mm dia. RC piles @ 600mm spacing (as per recommendation from geotech engineer), 150mm shotcrete walls (all have xypex) for the external basement wall. Hydrotite to all joints.

Basement slab: We are planning to install 100mm of bulk concrete then 250mm RC suspended slab above (all have Xypex).

Waterproofing: After installation of shotcrete wall and the bottom 100mm slab, they can install a full torch on bitument lining continue through from wall to floor. Then install the suspended slab.

Is there any way I can improve my construction method to minimize the risk of water enter the building?

Thanks to all the experienced engineer out there !

SD

 

[HTURKAK]

It will be better if you provide some descriptive sketches.. some of the questions;

- The foundation type ( probably mat ) supported on piles ?
- what is 0.02 ? basement level ?
- no. of basement storeys and no. of above ground storeys ?
- dewatering during construction ?
- soil type?

Good luck ..

 

[EireChch]

Yes, use a secant wall instead of contiguous wall.

 

[DNC Consulting Engineers]

@HTURKAK

Ground NSL is approx. 2.20 above datum.
Since the water was found -3.8 to -4.5 below the NSL, which is approx. 1.4m below basement FFL, the geotechnical engineer suggested contiguous piles with capping beams and suspended slab for ground floor slab.
-0.02 is the FFL of basement level.
The building only has 3 levels above the ground floor (light weight structure)
Since the water is below the basement level, we will have to use CFA method to install the piles. Once the piles are installed, excavation then shotcrete the wall every 1m.
Silty sand was found there all the way down 8m.

My biggest concern is waterproofing the place. Xypex will be added to the concrete mix and hydrotite will be installed to all cold joints however, is it enough for habitable area?

I am thinking to install a torch on layer over the concrete continuous through to floor to seal it then plater over it. However, I can't find any proper recommended standards for waterproofing basement like this. What is your opinion on this?

Thank you,

SD

 

[hokie66]

300 piles at 600 spacing is not a contiguous pile wall.

 

[SlideRuleEra]

...design of a full tanking basement just under sea level (-0.02) and it is very close to the sea, within 300m.

Is the basement floor being designed to resist hydrostatic uplift if (when) ground water level is higher than the basement floor?

http://www.SlideRuleEra.net

 

[HTURKAK]

Dear DNC Consulting Engineers ,

As per your description , i don see any reason for water proofing..

If i ware in your shoes,

- I would wait for the excavation to see the necessity of water proofing and xypex before deciding..
- I would design the found. with thin slab with increased thk. under the perimeter walls and column. Provide crashed stone layer under the slab ( to prevent the capillary raise ), Provide hydrotite to all cold joints and bitumen paint of exterior surfaces to protect concrete.

-I do not see the reason for full water proofing and xypex .. You may repair the concrete and seeping joints with XYPEX after construction if necessary ..

If you provide some sketches showing the proposed found. etc you may get better responds..

 

[DNC Consulting Engineers]

@Hturkak,

I agree with you waiting for excavation to confirm the waterproofing requirements.

I have increased the thickness of the slab to resist the hydrostatic pressure.

@hokie66 - you are correct , i meant CFA actually.

@slideruleera - I have designed the slab to resist 1m of hydrostatic pressure in case the water table level raised.


Thanks again for your inputs.

SD

 

[hokie66]

How deep is your excavation? How many floor levels below ground? 300 CFA piles don't sound familiar for any deep excavation walls I have been associated with.

 

[EireChch]

@hokie, why are 300mm piles at 600mmc/c not contig?

I also agree with 300mm dia piles sounding small for a basement retaining wall. I infer that the retained height is at least 4.6m.

 

[DNC Consulting Engineers]

It only retains 2.4m actually.

 

[hokie66]

EireChch,

Think about it. To be contiguous, 300 piles have to be at 300 centres.

 

[EireChch]

Contiguous just means they are not connected. 300 dia at 600mm cc, is a 300mm gap from edge of pile to edge of pile. This is contig to me...

 

[MotorCity]

EireChch - according to the dictionary, contiguous means just the opposite. It means they are in contact.

 

[EireChch]

Well in the world of construction, contiguous piled wall is a wall that is not "secanted" (i.e. over lapping piles). They can be touching but most typically have a 100-150mm gap between piles.

 

[hokie66]

Maybe there is a bit of space, but 300 piles at 600 centres does not qualify as a contiguous or tangent piling wall.

https://www.berkelandcompany.com/sheeting-and-shoring/secant-and-tangent-walls/