## Construction pore pressures in an earth dam

A knowledge of the induced pore pressures occurring during the construction of an earth dam or embankment is necessary so that stability analyses can be carried out and a suitable construction rate determined. Such a problem is best solved by numerical methods. During the construction of an earth dam (or an embankment) the placing of material above that already in position increases the pore water pressure whilst consolidation has the effect of decreasing it: the problem is one of a layer of soil that is consolidating as it is increasing in thickness. Gibson (1958) examined this condition. If it is assumed that the water in the soil will experience vertical drainage only, the finite difference equation becomes:

U0,k+1 = r(u2,k + U4,k - 2u0,k) + U0,k + B7AZ where

Az = the grid spacing, and also the increment of dam thickness placed in time At

7 = unit weight of dam material B = pore pressure coefficient cvAt r —

In order that Az is constant throughout the full height of the dam, all construction periods must be approximated to the same linear relationship and then transformed into a series of steps. The formula can only be applied to a layer that has some finite thickness, and as the layer does not exist initially it is necessary to obtain a solution by some other method for the early stages of construction when the dam is insufficiently thick for the formula to be applicable. Smith (1968b) has shown how a relaxation procedure can be used for this initial stage.

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