A sewage pump is to be located in the proposed underground vault shown in Figure 8.26. The site will be temporarily dewatered during construction, but afterwards the groundwater table will be allowed to return to its natural level. The highest probable level during the life of the project is as

Question

A sewage pump is to be located in the proposed underground vault shown in Figure 8.26. The site will be temporarily dewatered during construction, but afterwards the groundwater table will be allowed to return to its natural level. The highest probable level during the life of the project is as shown.
The vault will be made of reinforced concrete and waterproofed so very little groundwater will enter. The vault and overlying soil has a mass of 30,000 kg, exclusive of the floor. A sump pump will eject any water that might accumulate inside.
The weight of the vault and the overlying soil must be sufficient to provide a factor of safety of at least 1.5 against buoyant uplift (i.e. these weights must be at least 1.5 times the total uplift force). Write an equation for the buoyant uplift force, then compute the required base thickness, [latex]t[/latex]. Neglect the weight of the pump and neglect any sliding friction between the sides of the vault and the soil.

Accepted Answer

Uplift pressure

[latex]u=\gamma _{w}z_{w}=\left(9.81 kN/m^{3}\right) \left(2.40 + t m\right) =23.5 + 9.81 t kPa[/latex]

Uplift force

[latex]P= u A= (23.5 + 9.81 t)(3^{2})= 212 + 88.3 t kN[/latex]

Weight

Use [latex]ρ[/latex] concrete [latex]=2400 kg/m^{3}[/latex]

[latex]W=Mg=\left[30,000 kg+\left(2400 kg/m^{3}\right) \left(3^{2} m\right) t\right] \left(9.81 m/s^{2}\right) \left\lgroup\frac{1 kN}{1000 N} \right\rgroup =294+212t kN[/latex]

For factor of safety = 1.5

[latex]1.5 (212 + 88.3t) = 294 + 212t[/latex]

t = 200 mm

Question 8.5: A sewage pump is to be located in the proposed underground v...

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