Page - 743 - in Book of Full Papers - Symposium Hydro Engineering

Fig. 6. Maximum allowable pore pressure assumed for accident loads. Pore pressure distribution for normal design loads is shown as a dotted line. The criteria for pore pressure distribution provides a logical correlation between the load effects from the dam and the resulting pore pressure for normal design loads. When there is pressure in the entire foundation, the bond between the concrete and the foundation can be assumed to be intact. Thereby, a linearly decreasing pore pressure under the dam will probably be a conservative assumption and generally contribute to a high safety level. The additional check for accident loads provides an extra safety in case the pore pressure should be greater than assumed for normal design loads. If the maximum permissible pore pressure for accident loads represents the uncertainty in the pore pressure distribution, the difference of pore pressure between design load situation and accident load situation may be defined as the corresponding load factor. This difference represents an increase in pore pressures of 43%, or a load factor of 1.43. The correlation between the FoS for design loads and accident loads can thus be expressed as shown in the following figure, where FoS = 1 represent a linearly decreasing pore pressure distribution under the dam. Fig. 7. Computed correlation between load factor and FoS with changing pore pressure (blue line = sliding; orange line = overturning). 743