Vízügyi Közlemények, 1969 (51. évfolyam)

4. füzet - Rövidebb közlemények és beszámolók

(13) HYDRAULIC PROBLEMS IN SLOPE STABILITY By Nagy, László, Dr. Engr. (For the Hungarian text see pp. 22) Based on a study of bank development on rivers embedded in granular alluvial soils a method is given for estimating the stability of the slopes under a variety of hydraulic forces. The shape and geometrical dimensions of banks in granular materials are controlled —beyond the inherent properties of individual soil fractions— primarily by flows of different direction and velocity. The hydraulic factors influencing bank stability are considered in three groups, namely — flow parallel to the bank, — flow perpendicular, or almost perpendicular to the bank, and — flow emerging from the slope. A great number of tests lias shown the bed material of the Danube over I lie Hungarian reach to lie in the range D

k = (mean diameter) = 20 to 0.2 mm. The basic properties of the soil material, such as grain-size distribution, density and underwater angle of friction should be known reliably and determined by repeated tests. Flow parallel to the banks is taken into consideration mainly when studying sediment transportation. It should be remembered, however, that the formulae published in the literature for the computation of the critical- or mean velocity are valid only in the case of flow which is horizontal, or in the range corresponding to the bottom slope. In view of the fact that over the bank slope —depending on its steepness (inclination) —the particles tend to move already when they are at rest at the bottom, the critical velocity is given in the form v

c

r = 8.25 cotan Q + v

n where о is the slope inclination at which the particles are in the boundary condition between movement and rest, while v

a is a metrial property having the dimension of velocity. Hydraulic forces acting perpendicular to the bank and flow emerging from the slope are always associated with a change of waterlevel in the hed. Under critical conditions this change is always a negative one (a drop), which is either caused by a hydraulic factor, or seepage flow starts to emerge from the slope as a consequence of a level drop. These hydraulic effects, which act on the slope either from the waterside, or from the bank in a direction closely perpendicular, are associated with a change of waterlevel and are related positively to the rate of recession, dh/dt. Considering the influence of the' rate of recession on slope stability it was found that the observation points for a particular material plot in terms of the cotangent of slope inclination and the rate of stage recession along a straight line, the analytical expression of which is given as dh cotan o = d,

—- + cotan <p . dt During I lie drawdown tests a relationship has been established between the slope inclination and the underwater angle of friction : 9min = 0.75?>'. With this relationship the expression dh cotan (0.75?>cr) = cotan (0.7ü№

0) - 0.02 — ot has been derived, using which the straighl line expressing hydraulic properlies can be produced for any granular material