Use of a filterless antiwave revetment

Abstract

The problem of constructing a concrete revetment on the upstream slopes of earth dams without placing a reverse filter under it, which was raised by Lubochkov [i], deserves great attention, since the wide introduction of this revetment into practice would effect a considerable economy. Unfortunately, the problem raised has been covered insufficiently from the viewpoint of the physics of the processes occurring, although a revetment on constructed experimental sections operated completely satisfactorily.As shown by investigations and full-scale observations of the operation of reinforcedconcrete revetments on the upstream slopes of earth dams, the main cause of the disturbance and destruction of such revetments is the formation of cavities under the slabs on the wavebreaking section from the settlement of the rubble filter and transformation of the slab from one supported on an elastic foundation, as specified by the calculation, to one with two supports, or operating as a cantilever. E. A. Lubochkov is absolutely right in this regard, which is indicated also by certain other investigators [2,3,4]. Let us examine the cause of such cavity formation. In investigations [2, 3, 4] the authors were inclined to regard the case of cavity formation under the slab due to liquefaction and sliding of the earth in the slope on the wave-breaking section due to the dynamic wave loads, which cause in the under-slab space pressure pulses with high accelerations. The graph of the maximum accelerations =max occurring in the under-slab space upon impact of a wave on the slab as a function of the wave height and size of the slab is shown in Fig. 1 according to the investigations [4].As is easy to see from Fig. i, for slabs 0.2-0.25 m thick and large planar dimensions with a side of 15-20 m, at wave heights of 2-3 m we can expect ~max = 500-1000 mm/sec 2, i.e., accelerations that are equivalent in magnitude to accelerations during intensi~y 8-9 earthquakes. It would seem, actually, that there are grounds to fear for the dynamic stability of the earth embankment. To Judge this, we turn to Fig. 2, where, according to the data of the author and Sukhorukov [3, 6], the graph acr = f(D) is plotted for sandy soils of different types and grain size. Here ~cr is the critical acceleration at which liquefaction of the sand occurs and D is the coefficient of its relative density.It follows from the graph that all sands, from coarse-grained to flne-gralned rounded, at a relative density D > 0.4-0.6, i.e., a density easily attained when constructing an embankment by the hydraulic or dry method, cannot be liquefied at The values of Umax indicated above. The exceptions are the fine-grained silty sands, distinguished by pronounced thixotrophy, which are easily liquefied even in the most dense structure. But such soils should, as a rule, never be placed on a dam slope. It follows from the above that the conditions for formation of cavities under slabs due to liquefaction of the embankment earth are generally absent and, in any event, they are easily eli...