Practice Question: Evaluating Factor of Safety Against Heave in Clayey Soil

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Solution

Length of Excavtion, \(L = 500\ ft\)
Width of Excavation, \(B = 50\ ft\)
Height of Excavtion, \(H = 25\ ft\)
Depth to stiff layer beneath excavation base, \(D = 20\ ft.\)

\[\frac{B}{L}=\frac{50}{500}=0.1\]

\[\frac{H}{B}=\frac{25}{50}=0.5\]

From the graph of bearing capacity factor read value of Nc for H/B=0.5 and B/L = 0. 
(Civil PE Reference Handbook V2.0, Evaluation of Bottom Heave Potential in Soft to Medium Clays)

 

\[N_c=5.8\]

 

\[S_u=1,200\frac{lb}{ft^2}\]

 

\[\gamma =125\frac{lb}{ft^3}\]

The Factor of Safety against heave failure is given by:

\[FS=\frac{N_cS_u}{H\left(\gamma -\frac{S_u}{D}\right)}\]

\[FS=\frac{5.8\times 1,200}{25\left(125-\frac{1,200}{20}\right)}=4.28\]
 

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