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Basement Construction Methods - Anchor Length Calculation

Basement Construction Methods - Anchor Length Calculation

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.

I was thinking of using a diaphragm wall as its not dependent on the type of ground conditions and the this will allow concrete waterproofing as the waterstops at panel to panel interface improve water tightness. What other explanation might be valid?

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! :)

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