INVESTIGATION OF THE INFLUENCE OF STRATIFICATION AND FRACTURING OF ROCKS ON THE STABILITY OF SLOPES

ABSTRACT

In this paper, the occurrence of cracks on the surface of rotation in an anisotropic ally deformed mass as a result of massive explosions in a block depends on their direction relative to the principal stress.

АННОТАЦИЯ

В данной статье рассмотрено возникновение трещин на поверхности вращения в анизотропно деформированной массе в результате массовых взрывов в блоке. Возникновение трещин зависит от их направления относительно главного напряжения.

Keywords: crack, deformation, stresses, orientation, stability, anisotropic.

Ключевые слова: трещина, деформация, напряжения, ориентация, стабильность, анизотроп.

It is known [1] that cracks in the mountain massif are formed from endogenous processes: at the first stage of rock formation, endoclivage, cracks of separations and cracks of shrinkage occur; then cracks caused by exogenous processes because of tectonic disturbances; and after that, weathering cracks and cracks formed during mining operations appear.

Thus, the rock mass is broken up by a network of cracks of various origins, and the cracks have different lengths, widths and ages. Fracturing of rocks is associated with the strength and deformation characteristics of rocks, and, consequently, with the stability of rocks in outcrops.

The realization of cracks in the sliding surface in an anisotropic ally deformable array depends on their orientation relative to the main stresses. It was found [2, 3] that larger but rarer cracks correspond, as a rule, to lower values of bond strength. Consequently, the geometric parameters of the cracks (length and to a lesser extent width) allow us to speak about the perfection of the cracks: the more perfectly formed the crack in the array, the lower the value of the main strength characteristics we have.

The perfection of a crack is its qualitative characteristic, which is a manifestation of the anisotropy of the array and is determined by their orientation relative to the main stresses, geometric parameters and strength characteristics of the cracks.

The analysis of the displacement and deformation of anisotropic fractured arrays allows us to represent cracks according to their degree of perfection as follows:

1. Ideal cracks are cracks of unlimited extent or not interrupted within a mineral deposit with smooth edges and approximately constant width. The main strength characteristics: adhesion is zero or close to it, the angle of internal friction is significantly less than the angle of internal friction of the host rocks. Ideal cracks include cracks created artificially with strength properties specified by them. An example is deep cracks formed by the energy of an explosion or by mechanical means. In nature, such cracks can include flexural zones and large tectonic disturbances of great extent with clearly defined and smooth «shores» filled with infiltration materials or «lapping» clay, which form sliding planes under conditions of flooding and soaking of clay products.
2. The perfect cracks are cracks with a length of 50 to 500 m. These include tectonic and disjunctive disturbances traced by exploration wells. The strength properties are as follows: the angle of internal friction is less than the angle of internal friction of the host rocks; the adhesion is hundreds of times less than the adhesion in the monolith.
3. Perfect cracks – cracks with a length of 30-50 m, which, under certain conditions, are realized in the sliding surface. The adhesion along the cracks is tens and hundreds of times less than the adhesion in the monolith, the angle of internal friction is equal to or less than the angle of internal friction of the host rocks. These include lithogenetic and contraction cracks and cracks of tectonic origin.
4. Weakly imperfect cracks are cracks of small extent (less than 30 m), the adhesion along them is several times less than the adhesion in the monolith, and the angle of internal friction does not differ from the angle of internal friction of the host rocks. As a rule, these are lithogenetic and contraction cracks or weak cracks of any genesis.
5. Imperfect cracks – «healed» cracks filled with a material whose strength characteristics are higher or equal to the strength of the host rocks.

When studying the structural features of the massif, cracks of groups 2-4 are more common. A prerequisite for the formation of structural blocks in the array is the presence of at least three systems of cracks in it.

In studies [3, 4], rocks with horizontal layering (the angle of incidence of layers b=0÷5º) are considered as a homogeneous medium. The presence of weak contacts between the layers does not affect the stability of the ledge. The parameters of the ledge are determined from the total strength of the layered thickness and the duration of its standing (fig. 1, a).

When the layers are flat (b=6÷15º), the potential sliding surface at the base of the ledge may coincide with weak interlayers or contacts with low shear resistance characteristics.

Arrows – the direction of the shift, dotted lines – the weakening surfaces that contribute to deformation

Figure 1. Schemes of the influence of rock stratification on the deformability of ledges

In the presence of «untreated» systems of cracks associated with layering, this can lead to deformation of the ledge (fig. 1, b).

The hollow-inclined occurrence (b=16÷30º) leads to the cutting of contacts between layers in the non-working ledges of the recumbent side. Therefore, the stability of ledges depends on the presence of weak contacts or interlayers of clay rocks with low shear resistance. At the height of the working ledges of 10-15m, deformations in the form of shear along the contacts do not affect the technological processes. The stability of the ledges of the hanging side and at the ends of the quarry is not affected by layering (fig. 1, c).

When the layers are inclined (b=31÷45º), it is recommended to cut the ledges of the recumbent side in parallel with the stratification of the layers. The exception is ledges composed of rocks that allow pruning of contacts with angles of incidence up to 35^о. At slope angles equal to the angle of rock fall a=b, the deformation of the ledge is possible only in the case of insufficient resistance of rocks to shear across the stratification (fig. 1, d). Such layering does not affect the stability of the ledges in the hanging side. At the ends of the quarry, deformations are possible along grooved sliding surfaces in the presence of slanting cracks.

The steeply inclined occurrence of layers (b=46÷65^о) is favorable for the ledges of the recumbent side when they are mowed down on the surfaces of the stratification or with the incision of layers (fig. 1, e), however, it contributes to the stratification of rocks in the ledges of the hanging side (fig. 1, e). The process is activated in the presence of conjugate systems of cracks forming the second a family of sliding surfaces, which leads to significant deformations of the upper platform of the ledge, without, however, causing a loss of the overall stability of the ledge.

Their fracturing has a significant impact on the stability of rocks. The effect of cracks on the stability of ledges is twofold. Intensive uniform fracturing reduces the overall shear resistance of rocks and promotes scree formation. The array is considered as a quasi-homogeneous medium (Fig. 1, g).

The greatest potential danger is represented by cracks oriented parallel (sub-parallel) to the slope strike and falling towards the recess at an angle of 30¸50º       (fig. 1, h). In clay rocks, deformations are also possible at angles of incidence of 20÷25º. When pruning such cracks, the ledge may collapse along the surface of the cracks. Combined sliding surfaces are formed because of the intersection of crack systems, or cracks and weak contacts between layers oriented parallel to the slope strike. The two main attenuation surfaces are most characteristic, the angles of inclination of which represent the mechanism of destruction of the ledge: a shift along an inclined and a separation along a steep surface (fig. 1, i); a shift along two inclined surfaces with the formation of a curved surface (fig. 1, k); a shift along an inclined and flat surfaces with a chipped prism of active pressure and formation of a polyline surface.

As can be seen from the described mechanisms of the influence of stratification and fracturing of rocks on the stability of ledges, deformations of ledges can be diverse. However, the main attenuation surfaces that cause deformation of the ledges can be grouped into four groups:

• weak contacts between rock layers;
• cracks whose prostration coincides with the prostration of the ledge;
• tectonic disturbances and fault zones;
• weak layers and interlayers of rocks.

A layered and fractured massif is considered as a quasi-homogeneous medium (isotropic) if the existing attenuation surfaces are oriented in such a way that they cannot participate in the formation of the sliding surface [3-6].

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