In connection with intense underground mining operations, not only industrial and civil buildings and structures, railroads, and pipelines but hydraulic structures as well --earth dams, reservoirs, and ash and sludge storage ponds --fall into the zone of influence of undermining.According to the existing standards all undermined structures should be protected from the harmful effect of undermining. To provide reliable operation of structures it is necessary to know the permissible values of the parameters of deformation of the foundation and in the case of exceeding the permissible deformations, to take various steps to eliminate possible failure of the structure.Failures of earth dams, as shown by the considerable experience of operating these structures, are associated with considerable material damage in connection with emptying the reservoir and flooding of nearby territories and with loss of life, and failures of dams of sludge storage ponds can lead to pollution of the environment [1][2][3]. it is impossible to forbid undermining of earth dams and dikes of reservoirs and ash and sludge storage ponds, since in this case it will be required to leave untouched large mineral resources located under them. This problem is completely new. It is of interest to examine the conditions under which hydraulic structures fail.Underground mining has one common important characteristic consisting of as a result of extracting minerals, cavities are formed in the rock mass which disturb the equilibrium of the surrounding rocks. This disturbance leads to their subsidence starting directly from the adjacent lower beds with subsequent development right up to the surface, which also experiences vertical and horizontal displacements and deformations. In this case the part of the surface subjected to the effect of mining operations is usually called the subsidence trough~ In the general case points of the surface and underlying layers have both vertical absolute movements (subsidences) and horizontal displacements (offsets) directed from the margins of the trough toward its center. The subsidence trough is usually also characterized by relative quantities, whereupon by horizontal elongation--compression strains e is meant the same as in hydraulic engineering, i.e., Sx = Al/l (Al is the increment (shortening) of the investigated area with length l). Nonuniform subsidences are denoted in terms of the slope i and curvature K = I/R = Ai/lav (Ai is the difference of slopes of two adjacent areas; lay is the average length of these same two areas). The vertical sections of the trough passing through points with maximum subsidences of the surface are called its principal sections. Central, flat, and marginal parts of the trough are distinguished according to the character of the distribution of the principal parameters. The character and regularity of the distribution of the principal parameters of subsidences along the length of the marginal part of the trough L were established on the basis of long-term on-site instrumental observations...
with the creation of a berm skeleton from concrete blocks with simultaneous dumping of a gravel filter and intense hydraulic filling of the underwater part of the channel dam without lagging behind the berm could be technically possible and economically expedient.3. In the construction of hydrostations where the cost of stone is high it is necessary to provide for the possibility of using methods of damming with the maximum use of hydraulicking. The flexible scheme of organizing damming used at the Cheboksary hydrostation completely meets this requirement.4. Damming of large rivers with the use of hydraulicking is possible also in the period of intense ice formation with a sufficient guarantee of providing a good quality of construction of the structures.5. In the case of an erodible channel it is necessary to dump the berm from both banks simultaneously to prevent deep scouring of the bottom. To reduce the consumption of stone materials, it is expedient to carry out hydraulic filling of the underwater part of the dam simultaneously with dumping the berm without lagging behind it.6. In the case of pioneer damming of rivers with an erodible channel, reliable protection of the bottom of the gap is necessary. Protection by a train of stone and rubble with a thickness of 1 m is no guarantee against scouring. The size of the stone of the train should correspond to the velocity in the gap. It should be dumped as an even layer without the formation of mounds. The need for good clearing away of the cofferdams acquires particular importance in this case.7. In the case of dumping a berm skeleton composed of large concrete blocks and elimination of the construction of a filter on its upstream slope, the formation of individual washouts at the site of closing the gap, threatening the integrity of the berm, is possible after completing the works.
At present many dams and embankments of reservoirs and slime ponds are located in zones of underground mining. To ensure reliability of these structures it is necessary to predict the changes in their stresses upon displacement of the ground surface from undermining causing relative horizontal compressive and tensile deformations in the surface layer of the rock mass. Especially dangerous for earth dams are deformations due to spreading (extension) of the ground surface, which are characteristic for the marginal part of a subsidence trough [i, 2]. In the first approximation these deformations can be taken on the basis of the results of actual measurements and in calculations can be regarded as the initial data independent of the parameters of the dams.Deformations of the foundations underlying earth fill (dam, embankment) inevitably cause the occurrence in it of regions of a limit stressed state, the dimensions of which are determined by the magnitudes of the deformations and strength parameters of the fill soils. In [3,4] the stressed state of fill on spreading foundation is assumed determined according to V. V. Sokolovskii's theory of a granular mass on the assumption of a limit (plastic) state of the entire fill. In this case practically important problems of the development of the zone of limit equilibrium and determination of the magnitudes of deformations of the foundation converting the fill completely to a plastic state remain unsolved. Calculations of the state of stress and strain of earth fill under any loads up to breaking on it becomes possible on solving the mixed problem of the theories of elasticity and plasticity, which takes into account the occur=ence and development of regions of a limit stressed state of soil, and taking for it a unified model of the elastoplastic medium at all loading stages. The formulation of this problem within the scope of the theory of plastic flow with the use of the associated and unassociated laws for plastic deformations developing in regions of limit equilibrium is e~_m~ned in [5, 6] for variants of plane and volume strains of a soil medium. In the adopted model the soil is conceived as a continuous medium in a prelimlt (elastic) state, linearly or nonllnearly deformable and passing into a llmlt (plastlc) state in conformity with the strength condition used. In this case the physical relations between the stress and total strain vectors are taken in incrementsis a physical matrix equal to [D e ] in zones of the prelimit and to [DP] in zones of the limit state of the soil. The expressions for [D e] and [DP] are given in [5,6]. The algorithm developed there for numerical solution of the mixed problem on an electronic computer calls for using step (graduated) loading and the finlte-elementmethod (FEM), and as applied to problems of plane strain is realized in a program for a BESM-6 computer.It calls for using in [D e] variable shear moduli G and volume strains K (or modulus of deformation E and poisson's ratio ~) at the loading steps and a variable (depending on densi...
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