Basement Construction Methods - Basement Design

[db725]

Hi all,

I am working a design project for a geotechnical engineering subject and it goes as follows:

A 6 storey building requires a 12m deep basement comprising 3 levels of car park with plan dimensions of 50m by 50m and is to be constructed in a medium clay. The water table maximum level is 1m below the surface. At the depth of 30m there is impermeable basalt. the clay has the following properties:

Su (undrained shear srength) = 55 kPa
m= 1
k (permeability) = 1.1 x 10^(-8) m/s
γ (in-situ density) = 20 kN/m^3

A construction foreman has suggested anchors to be places at a grid of 3 m (vertical spacing) by 2 m (horizontal spacing). Calculate both the vertical (uplift) and horizontal (lateral Earth and water pressure) force on the each anchor; you may assume that the total lateral and vertical force is evenly distributed between all anchors. (Moments on the basement wall can be ignored)

The anchors are arranged so they each support an equal area of basement wall so therefore 4 anchors are needed.
From the bottom basement bottom to first anchor and from surface to anchor is 1.5 m and the others are 3 m apart. So from bottom up 1.5 m, 3 m, 3 m, 3 m, 1.5 m.

Horizontal Water Pressure
Area = (1.5 + 1.5) x 2 = 6m^2
Max Water Pressure = 9.8 x 11 = 107. 8 kPa
Pav = 107.8/2 = 53.9 kPa
Force per anchor = 53.9 x 6 = 323.4 kN
Earth Pressure
Force per anchor = 20 x 6 = 120 kN
∴ Total Lateral Force per anchor = Earth Pressure + Horizontal Water Pressure
= 120 + 323.4
= 443.4 kN
Vertical (Uplift) Pressure
Area of basement = 50 x 50 = 2500 m2
K= 1-m(4SU/γH) - this equation was given in the question = 1-1[(4 x 55)/(20 x 12)] = 0.0833
1.0KγH (this was given in the question) = 1 x 0.0833 x 20 x12 = 20 kPa
Perimeter = 50 x 4 = 200m
∴ Number of Anchors = (200 / 2) x 4 = 400 anchors
Uplift force per anchor = (area of basement x uplift pressure) / total # anchors
= (2500 x 107.8) / 400
= 673.75 kN
Critical Length
• Grouted Diameter = d = 200 mm
• Installation angle = 20°
• Su = 55 kPa
• Factor of Safety = 2
• Critical Length = Ftotal /( π x d x Su)

Critical Length (horizontal component) = 471.86 / (π x 0.2 x 55)
= 13.654 m
∴ 13.654 x 2 (FoS) = 27.309 m
Critical Length (vertical component) = 230.44 / (π x 0.2 x 55)
= 6.668 m
∴ 6.668 x FoS = 13.337 m

These lengths are not reasonable as they are too large. So there needs to be an alternative design.

1) Without draining the basement, what method/design alternative could we use to get a reasonable anchor length?
2) Assuming that the anchorage length issue is solved recommend a retaining wall type for your alternate design solution including the method of joint and concrete waterproofing.
3) Represent your alternative design solution sketching and indicating basement depth, retaining wall type, anchors (without dimensions)

As seen from above I have done some calculations but I am stuck on these 3 aspects on the improvement and need some guidance as I have not been able to even find any resources to read on anchors and such to be able to improve the design..

Any help would be appreciated as I am really stuck on this!

Thanks in advance! :)

 

[Studiot]

I presume the problem with the anchors is that the thirteen and a half metres will take them outside the property bounday?

So what retaining wall design types do you know of?

For one common type you have 18 metres between the bedrock and the basement ground slab.
Is this enough?

For another type never mind how you would construct it for the moment, think of what sort of basement walls are required and how you would make them self retaining?

Edit
This may sound like a daft question but what are you studying?

 

[db725]

Hi Studiot,

The basement wall types that I know of are :

1- contiguous piles ( bored of CFA) : piles almost touching
2- secant hard-soft piles (bored or CFA): piles overlapping
3- Diaphragm wall (d-wall)
4- Soldier pile walls

I looked up all these wall types and seems like the diaphragm wall is a good option as it suits any ground condition , waterstops at panel to panel interface improve water tightness. I think using this type of wall means that we are able to apply concrete waterproofing?

I just can't think of a alternative design solution to give a more reasonable anchor length. The parameters I think of including in this new alternative design are:
Grouted diameter, installation angle, factor of safety but I'm not sure how to link them all together or how I should approach it. We are expected to come up with design solutions thinking out of the square.

I am studying civil engineering and this is like a foundation engineering component of a geotechnical engineering course.

Thanks for all help!

 

[Studiot]

Yes since it is clay I would certainly look at a bored pile wall.
Off the top of my head I would think 18m penetration would cantilever 12 m retained height but you obviously need to check. You could also go for a temporary reduction of the retained height around the back of the wall and back fill later.

Alternatives might be a back counterforted diaphragm wall.
I remember doing one of these by excavating a slit trench in bentonite before the main bulk excavation was commenced.

Your condition states that you want to create the retaining structure before the main excavation takes place to avoid major pumping. Both these will allow this.

Oh and good luck with your course.

Edit.

Ground freezing is another (expensive) option in clay.

 

[db725]

Thanks very much for the suggestions, have been researching some other methods and I think this is the best one so far.

I know it's a pain but would you mind further explaining '18m penetration would cantilever 12 m retained height but you obviously need to check. You could also go for a temporary reduction of the retained height around the back of the wall and back fill later'. I am still in the process of learning and how foundation engineering works so haven't really got my head around with all the methods available and adjustments that can be made to design.

Thanks again

 

[Studiot]

Let us assume you have sensibly level ground.
You say there is 30m of clay overlying bedrock.
If you dig a hole it will soon fill with water and any substantial exposed face will be unstable.
Certainly a 12m face would collapse pretty quickly.
If you drive or bore piles and partially excavate one side of the piles you can dig a hole, but you will still have a small water ingress problem. This can be coped with by slightly sloping the excavation and digging a sump and pumping from the sump.
Any piling system you employ will go in at most 30m. You want 12m depth of excavation. That leaves a max of 18m of pile set into undisturbed ground below basement level.
For (wet) clay you will need to assume full active pressure acting on the exposed 12m face of the piling.
If you could dig a trench along the outside of the piles as well of depth 1 to 3 m this would reduce the retained height from 12 m to 9 or so.
A contiguous bored pile wall will still leak slightly.
A facing wall would be constructed against the bored piles to provide both the cosmetic finish and watersealing/proofing.