The civil engineer and the geologist are very much concerned with rock
slopes because
they may be called upon to build and maintain safe, economical and useful
structures
even on hasardous ground. Such a task requires not only proficiency and a
thorough
understanding of the factors involved, but also intuition and experience.
It should not be
thought, however, that there is always a feasible solution or if there is
one it should be pushed through at all cost

Gálos, Miklós

English

Periodica Polytechnica (Budapest University of Technology and Economics)

PERIODICA POLYTECHNICA SER. CIV. ENG. VOL. 43, NO. 2, PP. 153–161(1999)ROCK SLOPES AS ENGINEERING CONSTRUCTIONSMiklós GÁLOSDepartment of Engineering GeologyTechnical University of BudapestH-1521 Budapest, HungaryReceived: December 20, 1997AbstractThe civil engineer and the geologist are very much concerned with rock slopes because they may becalled upon to build and maintain safe, economical and useful structures even on hasardous ground.Such a task requires not only proficiency and a thorough understanding of the factors involved, butalso intuition and experience. It should not be thought, however, that there is always a feasible solutionor if there is one it should be pushed through at all cost.Keywords:engineering geology, rock engineering.1. IntroductionDuring the performance of construction engineering activities in a consolidatedsurrounding rock, the question of the solidity of rock slopes very often emerges.Generally speaking, rock slopes appear as a necessary evil beside our constructionscausing additional troubles in the periods of design, construction and, later on,operation as well. This additional trouble manifests itself in additional expenses.Basically, rock slopes can be classified into two groups. From the point ofview of their formation, they can be either– natural rock slopes, formed as a result of tectonic (geological) processes or– artificial ones.This grouping is well represented byTable 1.Table 1. Classification of rock slopesRock slopesNatural ArtificalMotive:structural quarry bench railway, road, etc.strength deformation open pit cutglacier grounderosion recultivation154 M. GÁLOSA rock slope may become an engineering construction in either group iftheir solidity is provided for by conscious engineering activities. They differ fromother engineering constructions in their structural material because the engineeringmaterial is not chosen by the engineer here, but he has to accept the block ofrock formed as a result of tectonic processes as structural material. In the caseof concrete, reinforced concrete and steel structures, the engineer can influence thecharacteristics of the engineering material to a great extent (by choosing the categoryof solidity, the method of reinforcing, the thickness of structures and supportingwalls, etc.). However, in the case of rock slopes, modification possibilities arerestricted (to injection, solidification, placement of supporting structures).The first stepof dealing with rockslopes is alreadya task requiring engineeringwork: from the surrounding rock, the block of rock has to be marked to which itseffect extends. This activity has a decisive and determining effect on the entiresubsequent work. Marking the block of rock must be based on preliminary designwork and the results of surveys provided for by different methods of excavation.In itself, the block of rock is still a very heterogeneous space element. Forthis reason, the block of rock has to be divided into bodies of rock, having thesame geological structure and/or joint system. The heterogeneity of the geologicalcomposition would generally require the isolation of quite a lot of smaller bodies ofrock. On the contrary, the local nature of preparatory work (may be tied to drilling)results in a greater extent of generalisation. Therefore, at the marking of bodiesof rock, an engineering geological approach structure taking technical aims intoconsideration is necessary. This approach creates an optimal balance between thetwo opposing trends from the point of view of the construction.Within the body of rock, the featureless blocks of rock and fissure are equiv-alent space elements. We mention slopes of rock using their geological name: e.g.limestone or dolomite slope. On the other hand, slopes of rock are also charac-terised by their fissuration, as well as their fissuration system and its direction: e.g.a strongly mashed and banked slope of rock.The process of the construction of geological engineering models, the inter-action of model elements and the issues concerning homogeneity and isotropy aredealt with in GÁLOS – KERTÉSZ (1981). Fig. 1 summarises the usual names forthe sake of quick and easy survey.2. Deterioration of Rock SlopesIn the case of surface forms, deterioration manifests itself in mass restructuringattached to different forms of movement. Forms of surface movements are groupedby the engineering literature in different ways. Basically, there are three forms ofmovement (SZILÁGYI – HORVÁTH, 1980):– avalanche,– foundering and– flowing.ROCK SLOPES 155Fig. 1. Discontinuities in rocksIn the case of rock slopes, only the first two forms of deterioration occur(Fig. 2). We can speak about rock avalanche and peeling when rock masses or bodiesof rock having lost their toughness disintegrate, as well as sliding and slumpingbelonging to the second category. Although they are the characteristic forms ofdeterioration friable rock, but they can also be interpreted to rock slopes as well.Fig. 2. Damaged typesSliding is the rapid movement of bodies of rock having lost their toughnesson a surface possible to determine in advance. Slumping is the large-scale rockavalanche along the surface of deterioration when bodies of rock having lost theirtoughness cause the movement of the block of rock.Slow deformation, belonging to the category of foundering, may indirectlycause deterioration if the surfaces of articulation of bodies of rock open wide as156 M. GÁLOSa result of the loose sediment under the consolidated block of rock, susceptible toslow deformation (Fig. 3).Fig. 3. Instability in creepingThe deterioration of rock slopes results in a large-scale mass movement, de-serving increased attention from the point of view of both personal and propertysafety. It can be prevented only if tectonic (geological) conditions are known.3. Natural Rock Slopes (Rock Splays)Concerning the slopes consisting of a single type of rock, VENDL (1953) states,referring to Cholnoky, that they are concave on the bottom and convex on the topand it is on the inflexion line that slope is the most steep. As a result of weatheringprocesses, the concave top is continuously deteriorating and the convex bottom isfilled with pelt and fallen pieces of rock.Depending on whether the concave or convex part is the larger, we talk aboutconcave and convex slopes.If weathering products are transported rapidly from the bottom of the slope,a straight slope will be formed. This is the steepest slope that can be formed fromthe given rock (Fig. 4).Different geological and geological engineering handbooks (VENDL, 1953;MOSONYI – PAPP, 1959) tell slope angle values based on empirical data, sum-marised for individual rock groups byFig. 5.It can already be seen from the outline ofFig. 5 that this grouping is stronglyinformative and cannot be used in engineering work. Its danger is that creates abasis for considering friction angle in the case of rock slopes as well, resulting inerroneous conclusions in this case.Fissuration has a determining role already in the formation of a single-typerock slope. This statement is increasingly valid if the rock material of the slope isnot homogeneous, that is made heterogeneous by the settlement of other types ofrock or layers of the same rock with different quality.Basic cases are shown inFig. 6.ROCK SLOPES 157Fig. 4. Types of rock slopesFig. 5. Natural rock slopesFig. 6. Variations in rock stabilityIf the direction of the slope is at an oblique angle with the strike, humps andhollows appear on the surface of the slope depending on local rock quality (Fig. 7).158 M. GÁLOSFig. 7. Stability of jointed rockNatural rock slopes become the indirect subject of engineering activities, sincetheir deterioration should be dealt with only if they threaten the stability of otherestablishments, as well as personal or material safety.Beginning from their formation, rock slopes are continuously exposed todeterioration. When the need for intervention arises that is to be regarded as thebasic state of our survey, that is our activities connected to the given stage of thedevelopment of rock slopes. Its purpose is to restore the stability of rock mass withour modest means compared to the forces of nature.4. Artificial Rock Slopes (Rock Cuttings)Compared to natural rock splays, artificial rock slopes are engineering constructionsregarding the entire process of their formation. From the construction need to theoperation, they are under engineering supervision both from technological andeconomical aspects.We differentiate artificial rock slopes as constructions from the point of viewof their planned lifetime. Their formation and method of protection depend onwhether they are temporary or final constructions.Temporary rock slopes are the walls of productive mine plants, continuouslychanging during the course of production, as well as the side walls of excavation pitsand working areas necessary for the construction of other establishments, borderingthe area during the time of construction.Final rock slopes are the accompanying cuttings of linear establishments andthe ones bordering the area of establishments settled among variable ground con-figuration conditions.A natural rock slope can become an artificial one if its solidity is to be pro-tected, that is dealing with it goes beyond the scope of solidity survey.Natural rock slopes become the indirect subject of engineering activities, sincetheir deterioration should be dealt with only if they threaten the stability of otherestablishments, as well as personal or material safety.Beginning from their formation, rock slopes are continuously exposed todeterioration. When the need for intervention arises that is to be regarded as thebasic state of our survey, that is our activities connected to the given stage of thedevelopment of rock slopes. Its purpose is to restore the stability of rock mass withour modest means compared to the forces of nature.ROCK SLOPES 1595. Artificial Rock Slopes (Rock Cuttings)Compared to natural rock splays, artificial rock slopes are engineering constructionsregarding the entire process of their formation. From the construction need to theoperation, they are under engineering supervision both from technological andeconomical aspects.We differentiate artificial rock slopes as constructions from the point of viewof their planned lifetime. Their formation and method of protection depend onwhether they are temporary or final constructions.Temporary rock slopes are the walls of productive mine plants, continuouslychanging during the course of production, as well as the side walls of excavation pitsand working areas necessary for the construction of other establishments, borderingthe area during the time of construction.Final rock slopes are the accompanying cuttings of linear establishments andthe ones bordering the area of establishments settled among variable ground con-figuration conditions.A natural rock slope can become an artificial one if its solidity is to be pro-tected, that is dealing with it goes beyond the scope of solidity survey.Natural rock slopes become the indirect subject of engineering activities, sincetheir deterioration should be dealt with only if they threaten the stability of otherestablishments, as well as personal or material safety.Beginning from their formation, rock slopes are continuously exposed todeterioration. When the need for intervention arises that is to be regarded as thebasic state of our survey, that is our activities connected to the given stage of thedevelopment of rock slopes. Its purpose is to restore the stability of rock mass withour modest means compared to the forces of nature.6. Artificial Rock Slopes (Rock Cuttings)Compared to natural rock splays, artificial rock slopes are engineering constructionsregarding the entire process of their formation. From the construction need to theoperation, they are under engineering supervision both from technological andeconomical aspects.We differentiate artificial rock slopes as constructions from the point of viewof their planned lifetime. Their formation and method of protection depend onwhether they are temporary or final constructions.Temporary rock slopes are the walls of productive mine plants, continuouslychanging during the course of production, as well as the side walls of excavation pitsand working areas necessary for the construction of other establishments, borderingthe area during the time of construction.Final rock slopes are the accompanying cuttings of linear establishments andthe ones bordering the area of establishments settled among variable ground con-figuration conditions.160 M. GÁLOSA natural rock slope can become an artificial one if its solidity is to be pro-tected, that is dealing with it goes beyond the scope of solidity survey.Fig. 8. Stability of hard rockIf the cutting to be formed in a consolidated surrounding rock were made inan heterogeneous surrounding rock, that is block of rock, our business would bevery simple. In this case, rock mechanics characteristics determined by laboratorysurveys would be the determining factors of the structural formation and geometricaldimensions of the cutting. The cutting could be constructed with high vertical walls.This geological situation is approximated by the bordering walls of the quarry ofFert̋orákos stone pit, formed in a Miocene limestone.The situation is more complicated if the surrounding rock consists of bodiesof discontinuous rock but having the same structure. Cases to be considered are byFig. 9.Fissure may be undirected, having a so-colled homogeneous joint system,where surfaces of sliding might be formed through the fusion of articulation surfaces(Fig. 9a). The hazard of peeling is the largest in this geological situation.In the case of directional joint systems, i.e. where there is bedding, there arethree basic cases, each of which can be subdivided into two parts depending onwhether the internal fissure of layers are of the same degree or not. Bedding can behorizontal or nearly horizontal and it can have the same or the opposite dip directionas the slope.In the case of horizontal bedding (Fig. 9b), steep slopes can be formed, thesolidity of which is determined by the internal joint system of the layers. Morestrongly articulated intermediate layers can make this situation critical (Fig. 9c).Their unfavourable solidity and strain characteristics might make them places caus-ing deterioration, since their primary state of stress changes with the formation ofthe slope.The worst case is when the layers are of the same dip direction as the slope tobe formed (Figs 9dand9e). When identifying the blocks of rock with a movementhazard, it is on the one hand the relationship of the layers and the angle of theROCK SLOPES 161Fig. 9. Artificial cutsslope and on the other hand the internal joint system which has to be taken intoconsideration. The angle of the slope might be lower, higher than and equal to thedip direction of the layers.If the dipdirection of the layers is opposite to the angle of the slope (Figs 9f and9g), a steep slope can be formed so long as sharp intersections, already susceptibleto movement, do not occur in the case of possible steep slopes.Unworthy of detailing is the series of cases when the geological environmentconsists of layers or bodies of rock with different lithological structure. Beingdangerous it must, however, be stressed especially when they have a low solidity,are susceptible to compression and sensitive against water, etc. Preparatory phasesbefore starting planning work have a definitive importance here.References[1] GÁLOS, M. – KERTÉSZ, P. (1981): A mérnöki munkák környezetének modellezése – a mérnök-geológiai k̋ozetmodell.Mélyépítéstudományi Szemle,Vol. XXXI. No. 12. pp. 540–545.[2] M OSONYI, E. – PAPP, F. (1959): M̋uszaki Földtan. M̋uszaki Könyvkiadó, Budapest, p. 534.[3] SZILVÁGYI , I. – HORVÁTH, ZS. (1980): Magyarország csúszásveszélyes területeinek katasztere.Előtervezés-Mélyépítés,FTV Kiadvány, pp. 252–256.[4] V ENDL, A. (1953): Geológia I. Tankönyvkiadó, Budapest, p. 655.

Rock Slopes as Engineering Constructions

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Rock Slopes as Engineering Constructions

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