I only computed it with a finite element mesh for a liquefaction assessment and it varied throughout the slope and depth. The 2008 Soil Liquefaction during earthquakes by Idriss and Boulanger explains it as equaling the static horizontal shear stress divided by the vertical effective consolidation stress.
The first stage of site investigation is desktop and it informs the engineer of the anticipated subsurface conditions. By precluding the site investigation the design engineer cannot accept any responsibility for providing a safe and economical design.
thanks for the response! A lot of paper have been published regarding the effect of alpha on Cyclic Direct simple shear test results but I could not find one to explain how to obtain the alpha as the initial static shear stress is unknown under a sloping ground. The answer will be used to determine what static shear bias will be applied to the CDSS sample for testing.
In my case the value of alpha nearest the base of a trapezoidal slide plane varied between 0.06 to 0.1, in the upstream portion of the slide plane the value varied between 0 to 0.04, presenting the results I showed both the influence with an alpha of 0.1 and alpha of zero. For my liquefaction assessment the alpha just made loose materials indicate they will have less cyclical resistance and dense materials have more cyclical resistance. You could also use a finite element program to determine contours of alpha below the slope, then you would need to determine where the likely failure plane is and figure out which alpha range to use.
The first stage of site investigation is desktop and it informs the engineer of the anticipated subsurface conditions. By precluding the site investigation the design engineer cannot accept any responsibility for providing a safe and economical design.
