Jovanovski Milorad1 Blasko Dimitrov

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EXPERIENCES, METHODOLOGY AND PRINCIPLES FOR ROCK SLOPE STABILISATION, CASE FOR A ROAD TO ARCH DAM “SVETA PETKA”

Summary: Rock slope engineering and design of rock cuts is extremely complex task that involves the

collection of geotechnical data, the use of appropriate design methods, implementation of excavation methods and stabilization/protection measures suitable for the specific area of interest. The methodology of working during the design of slope protection measures for the access road for the arch dam Sveta Petka is analyzed in the frame of this article. The used approach is given briefly, with overview on the data from geological and geotechnical investigations. Data about analytical and numerical methods used in analyses and probabilistic theory approach during the design of support measures, are also given. The slope protection methods and support types are explained in the text. Some typical drawings and figures are shown, in order to illustrate the used approach. The methodology of cost estimation for slope protection types is explained, as well as the basics of the used excavation techniques in a blasting procedure for the zone of arch dam foundations. The methodology is illustrative and gives the procedures which can be explained as a highest level of the state of the art in this very important scientific and practical engineering field. The paper can serve as an example for further analyses in this field for similar cases.

Key words: Analyses, Arch Dam, Design method, Excavation principles, Probability of failure, Rock

Slope Engineering, Safety factor, Slope stability, Stabilization measures

1. INTRODUCTION

The arc dam “Sveta Petka” in Republic of Macedonia is one of the three dams placed along the Treska River It is situated in the vicinity of the capital Skopje, spaced about 20 km SW from Skopje (Figure 1).

a

b Figure 1. Key map: a- position of R. Macedonia in a Balkan Peninsula and Europe;

b-location of arch dam “Sveta Petka”

The dam is in a phase of construction, and its main elements are following: - Height of the dam 64 m - Length in the crown 118 m - Dam volume (concrete part) 30 689 m

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- Volume of reservoir area 9,10 х 106 m

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1 Professor, Civil Engineering Faculty, Partizanski odredi 24, 1000, Skopje, R.Macedonia, +398(0)70236962, jovanovski@gf.ukim.edu.mk

2 Graduated Civil Engineer, Public Enterprise ELEM, HEC Treska, 27 Mart 9, 1000, Skopje, R.Macedonia

Whole Hydro Electric System Treska, beside the dam, is composed of several important elements. Among the other, the acces road is a vital and very important part, constructed in heavy morphological and geological conditions. The lentgh of the acces road is aboutr 11 km, but a special problem from stability aspect is a part of the road with a lentgh of about 6 km. This section is in fact analysed in a design documents for slope protection, becose, during the constructional and exploitation phase, numerous stability problems with local rockfalls, local slidings as well as some injuries of the working staff was recordet. The slopes are prepared with extremely heights and high slope gradients (Figure 2).

Figure 2. Illustration of the slope heights along the access road to the zone of arch dam “Sveta Petka”

The reason for occurences of stability problems, sometimes is connected with blasting procedures, that are

not always suited to the geological and geotechnical conbditions. In order to minimise the costs for cleaning of the road, and to insure safe traffic and working conditions, the responsible persons form Public Electicity Entrprise „ELEM“ from Skopje, went in the procedure of preparation of Design Documents for Slope Protection. During the design, the complex procedures are used. Authors believes that the experienes, aplied methodology and suggested mesaures applied in the Project can be interesting for the scientific persons. Having this in mind, this article presents basic elements and approach in the design. The gathered experiences in the blasting procedures during excavations at the zone of foundation for the arch dam, are also present. This is a typical case, where it is very obvoious that depending on the blasting tehniques, the results in the disturbance in the rock masses can vary a lot.

1.1. Main geological and geotechnical conditions alon acces road route

The area along road route is composed mainly of foliated or masive Rifeum-Cambrian marls, jointed and tectoniccaly affected. Local occurences of caves and karst phenomenon are also present. On specific sections, the talus (scree) deposits with quarterian geological age are present, which is very important from stability aspects. The talus zones are mainly suported by gabion walls but the overturning and rolling of the talus blocks across the gabions is still a problem (Figure 3).

Figure 3. Typical talus zone supported with gabion wall at the bottom of the slope

Typical geotechnical parameters, assumed from the well known empirical methods, as well as with a direct

mesaurements in a large scal for a zone of arcm dam are given in a table 1, as well as on the Figures 4, 5 and Figure 6.

Table 1. Typical Rock Mass parameters estimated with Hoek-Brown emprical strentgh criteria

Zone

Rock Mass Cohesion (kPa)

Internal friction

angle m [ ° ]

Uniaxial compressive

strength c [MPa]

Emprical

constant m

Emprical

constant s

1 50-100 28-32 15-25 0,4-0,5 0.00033– 0.00042 2 100-200 32-36 25-30 0,5-0,8 0.00042 – 0.004 3 200-400 36-40 30-40 0,8-1,0 0.004 – 0.007 4 400-800 40-42 40-45 1,0-1,5 0.007 – 0.008

5 800-1000 42-44 45-50 1,6-2,3 0.008 – 0.03 6 1000-1200 44-46 50-70 >2,3 0.03

Figure 4. Results from Point Load Strentgh index tests for the intact rocks,

Jovanovski, Ilijovski, 2005

Figure 5. Results from Uniaxial Compresssive Strength, Jovanovski, Ilijovski 2005

Figure 6. Histogram with overview of categories of rock mass by RMR system, Jovanovski,

Ilijovski, 2005

All geological and geotechnical data, somehow are incorporated in a procedure of defining of kinemantic modes of failure, analythical as well as numerical procedures.

2. METHODOLOGY AND USED APPROACH

Methodology of analyses and used aproach was based on the fact that the slopes were already prepared earlier, and that is the greater amount of information avallable on the faces of existing slopes. The exposed slope faces usually gives excellent information on geological conditions and study of past failures. Back-analysis of these failures were the most reliable parameters for prognosis of future modes of failure, estimation of rock strength although joints etc.. In general, the main procedure for a case for the acces road to arch dam Sveta Petka, is similar like a suggested procedure in a Figure 7.

Figure 7. General Slope Stability Program (3)

Some steps in the applied methodology are modified, based on the field conditions. Having this in mind, this

section summarizes some elements and basic data that is required for slope stability analyse. A basic feature of all slope design methods is that shear takes place along either a discrete sliding joint surfaces, or within fractured zones, behind the face. Instability could take the form of displacement that may or may not be tolerable, or the slope may collapse either suddenly or progressively. Beside ussuаl used techniques and methods, in a frame of project analyses, a special rockfall analyses are prepared. The aim of such analyses was to predict possible rockfall trajectories and kinetic energy from rockfall, as one element that can produce problems in the working area. Based upon these concepts of slope stability, the stability is analysed using following aproaches:

(a) Defining of Factor of safety (FS) with known limit equilibrium methods applicable for soils (talus zones), plane or wedge failure (for jointed hard rock masses). (b) Defining of probability of failure, which are expressed as probability distributions of safety factors. (c) Numerical analyse with Finite Element Method for some cases. (d) Analyses of rockfall hazard and rockfall trajectories. Some typical examples (computer outputs) for each used method are given on a Figure 8, Figure 9, Figure

10, Figure 11, Figure 12 and Figure 13.

Figure 8. One case of stability analyses for one talus zone using software Slide 5.01

Figure 9. Illustration of planar failure mode for one profile zone using software ROCPLANE

Figure 10. Illustration of probability of failure for a case of natural slope exposed to infiltration of water into the joints with

relative high safety middle factor Fs=1, 44, but also with high probability of failure (PF=35, 8 %)

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b

Figure 11. Illustration of one case of supported wedge with low probability of failure PF=3, 2 % (A-presentation of joint surfaces: b-probability distribution histogram)

Figure 12. Case od rockfall hazard analyses for one slope along acces road using sofware ROCFALL

(the upper left section is the zone of the road, while the lower right section is the working zone for excavations for the arch dam foundations)

Figure 13. Ilustration of Finite Element Analysis usig PLAXIS software

(case of support with reinforced raft structure with rock bolts)

All figures are in fact a clear illustration, that, in every analysed case, depending on local geological, tectonical and structural geological conditons, it is necessary to define possible modes of failure, and after that to apply some analythical or numerical tehnique. This is of special importance during the defining of realistic models for slope protection analyses.

3. SUGGESTED DESIGHN TYPES FOR SLOPE PROTECTION ALONG ROAD ROUTE

Based on the detailed analyses in the desigh documents, several possible slope-protection methods and solutions are accepted at a first level of analyses. They are dividet in several so-called Slope Protection Types. Without going into details, basic elements for each solution, are ilustrated on a Figure 14, Figure 15, Figure 16 and Figure 17.

А- Slope Protection Type 1, Reinforced Rockshed. On some specific section, the reinforced rock shed is designed with elements of a solution given on a Figure 14.

Figure 14. Typical sections for Slope Protection Type 1 (Reinforced Rockshed covered wit gravel or talus)

This solution can be aplied on the sections with very high slopes, where the access to the zone of slopes is

very heavy, but from technical-economical point of view is one of the most expensive solutions. B- Slope Protection Type 2, Reinforced Raft Structure with Rock Bolts On some sections with dominant larger potential unstable rock block, the solution with reinforced raft

structure was analyzed. The distances between columns of the raft is about 2, 5 meters, and in all nodes, the reinforcement with rock-bolts is assumed (Figure 15).

Figure 15. Typical sections for Slope Protection Type 2 (Reinforced Raft Structure)

C- Slope Protection Type 3, rock bolts combined with wire mesh and shotcrete Probably the most effective permanent protective system for this case is the protection as a combination of

rock bolts, comined with wire mesh and shotcrete. The thicknes of the shotcrete, as well as lentgh of rockbolts varies from a case to case, depending on structural elements of the joints and volumes of potential unstable blocks. The elements of the solution is given on a Figure 16.

Figure 16. Typical section for Slope Protection Type 3

The wire mesh shall be of type Q188. The limitations for aplication of such solution lies in a fact that the

slopes are already constructed, so a special methodology of working is necessary.

D- Slope Protection Type 4, Chainlink mesh, local removal of unstable blocks and non-systematic application of rock bolts

This is typical and usual way of protection for all slopes, where we have smaller block dimensions. During the working, the removal of unstablel blocks shall be prepared, in a combination with local instalation of rock bolts.

E- Slope Protection Type 5, Treatment of talus sections Talus sections are problematic areas with permanent occurrences of local instabilities during exploitation of

the access road. This leads to the increased costs and investments in a road cleaning, and it is a permanent danger on a safe driving. It should be mentioned that in a first phase, the talus sections are supported with flexible retaining gabion walls, but experience shows that this not sufficient measure. Having this in mind, in the design documentations, the solutions with partial removal (unloading) of the upper sections, installing of fence barriers on several levels and gabion walls is suggested. If necessary application of shotcrete combined with wire or chain link mesh will be additional measures. The quasi-static analyses indicate that if a measure with shotcrete is applied, there is some positive effect, but during earthquake conditions, it is possible to have reactivation of talus sections. So, here, it is necessary to prepare a combination of measures, in order to insure as much as possible safe solutions. One case is given on a Figure 17.

Figure 17. Typical section for Slope Protection Type 5: M-marbles; T-Talus

What can be concludet about all suggested solutions?. All of them has its own limitations as well as positive

effects. The main limitation for application of the solutions is connected with the fact that the slopes are earlier prepared with extremely heights and high slope gradients, the road shall be under traffic conditions, and in the mean time the safety of the worknig staff must be insured. The way of instalation overcomes the frame of this article, but some reccomendations are given in design documents.

4. METHODOLOGY OF COST ESTIMATION FOR ALL SLOPE PROTECTION TYPES

In order to have fast way for cost estimation for each solution, and for comparations between costs, in a frame of the analyses, some typical diagrams are prepared. Namely, the Figure 18, shows the cost estimation for Slope Protection Solution 1. The estimation is presented with regresion line.

Figure 18. Cost estimation for Slope Protection Type 1 for different lentgh of Rockshed

The diagram is constructed according to the detailed bill of quantities for typical lentgh of rockshed of 10

meters, with all necesary concrete, reinforcement, earth works etc. After that, the extrapolation for other lentgh is necessary.

Similar approach is used for cost estimation for other Slope Protection Types. Some typical diagrams are given on Figures 19 and 20.

Figure19. Cost estimation for Slope Protection Type 3 for different rock bolt length

Figure 20. Cost estimation for Slope Protection Type 5 (talus zones)

Using similar approach, all access road is divided in several sub-sections, and according to the types of

protection, a detailed analyses of the costs is prepared. Without going into details, the estimation for all works on slope protection is about 5.000.000 Euros. Compared with the costs in an annual level for cleaning of the road, it comes out that the annual present costs is about 110 000 Euros. The extrapolation of the costs is given on a Figure 21. It is clear that, the costs for cleaning of the road are relatively high, so during 20-25 years operational period, larger part of the investments for slope protection shall be returned. Of course, the risks on the security of the peoples cannot be transferred into money directly, so it is clear that the slope protection program shall be applied in several phases.

At a first phase, the most critical sections can be protected, while after that all slopes shall be insured with optimal protective measures.

Figure 21. Estimation of costs for road cleaning during the operational phase

This short and “simple” analyses, gives a clear view on the complexity of the problem, and necessity of slope protection imediately after the excavation with a so-called top to down approach. Later it is very heavy to apply some mesaures when the slopes are prepared with such heights as for a case od road to dam Sveta Petka.

5. MAIN DATA ABOUT BLASTING PROCEDURES AT THE ZONE OF DAM FOUNDATIONS

During rock excavation for a foundations it is often necessary to use controlled blasting procedures that limit damage to the rock in the bearing surface and any surrounding rock cuts. On an oposite from experiences during road preparation, at the zone of dam foundation excavations, a very precise and controled tehnique of blasting is applied. The principle of controlled blasting are given on a Figure 22.

Figure 22. Scheme for preparation of contour holes and blast holes used for foundation media of arch dam Sveta Petka

The drilling of the blast holes is in etages with a height of 1,0m to 1,67m on the right dam bank, and 2,0 to

2,50m on a left dam side. According to the reccomendations of Langefors, using the known formulas, the parametrs of radius of dangerous zones are calculated, and other parameters for blasting are defined, but this overcomes the frame of this article. The main blasting and drilling parameters are given in a table 2.

Table 2. Basic drilling and blasting parameters for difefrent etages

Basic drilling and blasting parameters Unit Right dam side Left dam side

Height of etages for blasting m 1,00 1,67 2,00 2,50 Diameter for drilling holes mm 33 44 44 44

Lentgh of drilling holes m 1,20 2,00 2,30 2,90 Distance between blasting holes in a row m 0,70 0,80 1,10 1,20

Distance between rows of Blasting holes (BH) m 0,60 0,70 1,10 1,20 Quantity of explosive in blasting hole kg/BH 1,00 1,30 1,56 2,21

Maxinal value of explosive in countour holes gr/BH 110 180 340 420 Line of minimal resistance m 0,60 1,00 1,10 1,20

Specific quantity of explosive kg/m3 0,79 0,78 0,65 0,64

The practice shows, that, a poorly designed blast can induce cracks several metres behind the last row of

blastholes. Clearly, if such damage has already been inflicted on the rock, it is far too late to attempt to remedy the situation by using controled blasting to trim the last few metres of excavation. On the other hand, if the entire blast has been correctly designed and executed, smooth blasting can be very beneficial in trimming the final

excavation face. Figure 23 and Figure 24 are good examples for comparison between the results achieved by a normal blast and a face created by presplit blasting in a jointed marbles. It is also not difficult to imagine that the pre-split face is more stable than the section which has been blasted without special attention to the final wall condition.

Figure 23. Illustration of blasting effects with minimal rock mass disturbance in foundation media of arch dam Sveta Petka

. Figure 24. Illustration of blasting effects with high rock mass disturbance in some slopes at the access road

to arch dam Sveta Petka

It must be stated that the experiences with applied blasting method at the zone of arch dam foundations are

very positive, and authors can wormy recommend such working technology.

6. CONCLUSIONS

From the given analyses, it is clear that each design is unique and the acceptability of the structure has to be considered in terms of the particular set of circumstances, rock types, design loads and end uses for which it is intended. The project realisation shall follow some rules and stages in construction, in order to minimise unpredicted conditions during excavations. On the oposite, the cost of the project can be much higher than expected, and this is ussualy connected with a delay in construction.

The responsibility of the geotechnical engineer is to find a safe and economical solution which is compatible with all the constraints which apply to the project. Solutions should be based upon detailed analyses, but also on engineering judgement guided by practical and theoretical studies such as stability or deformation analyses. There is still large space for development, innovation and improvement in rock slope engineering, but there are also a tools and techniques available to minimise the damages into the rock mass and surrounding media. Unfortanetly, they are not being applied widely in the mining or civil engineering projects because of a lack of awareness of the benefits to be gained, and a fear of the costs involved in applying controlled blasting techniques. The presented experiences are good illustration, that, knowledge of geological, tectonical and structural geological conditons is the basis for all analythical and numerical analyses and supporting mesures design.

The main conclusion is, there are an urgent need for improved communications between the geotechnical, traffic, blasting specialists and project managers who are competent to design optimum design systems in the design phase.

7. REFERENCES

1. Angelkovic V., Model deformabilnosti krečnjačkih stenskih masa u funkciji statičkih i dinamičkih uticaja, Geotehnički aspekti gradjevinsrstva, Kopaonik, 24-27 oktobar 2005

2. Barton N.: Physical and discrete element model of excavation and failure in jointed rock, NGI, Oslo, 1994. 3. Federal Highway Administration (US) (1989) Rock Slopes: Design, Excavation, Stabilization. FHWA, US

Department of Transportation. 4. Hoek E.: Practical Rock Engineering, http://www.rocscience.com, 2000 5. Hoek, E., Carranza C., Corcum B., 2002. Hoek-Brown failure criterion-2002 Edition, Manual from program

ROCKLAB, Rocscience Inc., Toronto, Canada 6. Hoek, E. and Bray, J. W., Rock slope engineering, 402 pages. Institution of Mining and Metallurgy, London,1997 7. Hudson, J. A. and Harrison, J. P., Engineering rock mechanics, 444 pages. Pergamon, Amsterdam, 1997. 8. Jovanovski M., Ilijovski Z., Velevski A., Geotehnicko modeliranje terena brane „SVETA PETKA“ primenom

geofizickih i geothickih metoda, Kopaonik, 2005 9. Jovanovski M., Gjorgevski S., Main design book for Slope protection along road to arch dam Sveta petka, Skopje,

2009. 10. Langefors, U. and Kihlstrom, B. (1967) The Modern Technique of Rock Blasting, Wiley, New York. 11. Wyllie, D.C. (1991) Rock slopes stabilization and protection measures. 34th Ann. M. AEG, Chicago, October. 12. Wyllie, D.C. (1995) Stability of foundations on jointed rock—case studies. Proc. Int. Workshop on RockFoundations,

Japan, A.A.Balkema, pp. 253–8.