Vertical wall adhesion in undrained loading
Short term total stress conditions (i.e. undrained loading) represent the state in the soil before the pore water pressures have had time to dissipate i.e. immediately after construction in a cohesive soil. For total stress the horizontal active and passive pressures are calculated using the following equations:
pa = Ka ( γ z+q) – Su Kac
pp = Kp( γ z+q) + Su Kpc
Where:
(γ z+q) represents the total overburden pressure
Ka = Kp = 1.0 for cohesive soils.
Design su= sud= sumc/ FSsu where Fssu is typically 1.5. The earth pressure coefficients, Kac and Kpc, make an allowance for wall /soil adhesion and are derived as follows:
Kac= Kpc= 2√(1+Swmax /Sud)0.5
According to the Piling Handbook by Arcelor (2005), the limiting value of wall adhesion Swmax at the soil/sheet pile interface is generally taken to be smaller than the design undrained shear strength of the soil, sud, by a factor of 2 for stiff clays. i.e. Sw max = α x Sud, where α = 0.5. Lower values of wall adhesion, however, may be realized in soft clays. In any case, the designer should refer to the design code they are working to for advice on the maximum value of wall adhesion they may use. Currently, these modifications can be used only in conventional limit equilibrium analyses.
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