ALTERNATIVE FOR LARGE SCALE TESTING OF INTERFACE SHEAR STRENGTH

8/12/2019 ALTERNATIVE FOR LARGE SCALE TESTING OF INTERFACE SHEAR STRENGTH

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J. Br. Papi, M. Jovanovski, I. Peevski, V. Vitanov, Sp. Gjorgjevski, J. Josifovski

Chair of Geotechnics, Faculty of Civil Engineering Skopje, R. Macedonia;[email protected]

ALTERNATIVE FOR LARGE SCALE TESTING OF INTERFACE SHEAR

STRENGTH

SUMMARY

This paper presents a specific methodology for testing shear strength of interfacesimplemented when investigating and designing the rockfill dam Kozyak in the Republic ofMacedonia. There are details about applied apparatus for testing, as well as the results fromshearing along interfaces bedrock-rockfill material, bedrock-filter zone, bedrock-clay andconcrete-clay. The testing is performed in shear box with size 100x100x60 cm. The benefitsof this unique large scale methodology are underlined, as well as the possibilities for dataextrapolation.

1. INTRODUCTION

For designing purposes of large rockfill dams, it is from great importance the shearingstrength parameters at the interface of the bedrock and the other materials which are used fortheir construction. This is especially emphasized during the numerical modelling of theartificial structure (the dam) and the foundation media (the geological setting), respectively,their consideration as a whole in mutual synergy, when the alleged parameters are necessaryand from cruciall importance for the correct interpretation of the behavior of this system.

On the current level of development of the geotechnical science, few rare examplesfrom testing of these parameters on a small scale models or as in-situ tests are known. During

analysis it is very usual to assume them, and very often this problem is not even treated.Along with this, it is very difficult to conclude how close is the prognosis of the parameters tothe actual conditions which are expected in the phase of exploitation of the dam. From presentknowledge for analysis of this problem, it can be stressed that in the rock mechanics verygood experimental and analytical methods are developed, when behavior of thediscontinuities as a specific type of interface is of interest. Also, there are knownmethodologies for testing of the interface concrete-bedrock, for the purposes of designing ofconcrete dams.

On the other hand, the shearing strength parameters along the interface of differentmaterials which are composed during the construction of the rockfill dams, very rare aretreated in the scientific literature, even thou certain data for these problems can be met. In thiscontext, the authors goal is to stress out the advantages of the methodology which theyimplemented during the solving of a specific practical problem in the phase of investigationand designing of the rockfill dam Kozyak on the river Treska. However, the goal is not to

present the possible models with which the stress-strain relations on the contacts are defined,but to present a specific methodology of investigation which was implemented in this case.This methodology arises from the age-long experience of the authors during the solving ofdifferent practical problems connected with phases of investigation and designing of rockfilland concrete dams. Its uniqueness is in the physical modeling of the problem, where a largescale model for shearing was made. With the conducted investigations it enabled gathering ofnumber of data related with the mechanical behavior of the interface between the bedrock

with the rockfill material, the filter zones with the bedrock, as well as the interfaces clay-bedrock and clay-concrete. Some of the obtained conclusions are very interesting, so,


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recommendations that can help in eventual implementation for similar structures and thepossible conditions for exploitation are given.

2. APPLIED APARATURE AND METHODOLOGY OF INVESTIGATION

The idea for defining the contacts properties came out in the Chair of Geotechnics atthe Faculty of Civil Engineering in Skopje, R. Macedonia, where all the testing wasconducted. For that goal, direct shear-box (figure 1) for coarse-sized material with knownsurface of shearing and restrained side spreading of the material was used. The originalversion is with dimensions 1,50x1,50x0,60 m, but it was modified to dimensions1,00x1,00x0,60 m in order to make conditions for appliance of higher normal stresses.

Figure 1. Direct shear box 1,00x1,00x0,60 m applied during the testing

The shear surface is horizontal (it is positioned in the middle between the lower andthe upper frame of the shear box), the normal stress is achieved with 4 vertical presses from1000 kN, and the horizontal load (shear stress) is achieved with two inclined hydraulical

presses mounted under an angle of around 11in relation to the horizontal plain. The lowerframe is static, while the upper frame is moving over it with the help of rollers which help to

restrict unwanted resistance from any kind. Usually, coarse-sized material which is used inembankment bodies is compacted under certain conditions in the whole height of the box,after which a procedure of testing with know methodologies for shearing is applied.

In this case, a specific treatment of the contacts is applied, where modelling of thebedrock is done in the lower frame of the shear box (figure 2). For the modelling, data for theconditions of the rock from geological mapping is used. The most present scale and type offracturing in the diversion tunnel is adopted, given through the number of monoliths on 1 m2.

The space between the monoliths is filled with concrete. The surface part of themodeled base is with local irregularities which are in the order of 5-6 mm, with which theroughness of the bedrock under the dam is simulated. It was intended that the modelledsurface of the bedrock is as close as possible to the lower part from the upper frame, in order

to secure failure along the contact surface rather than in the materials which are compacted inthe upper frame.

Measuring of verticaldisplacements

Measuring of horizontaldisplacements


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On this model, along with the direct shearing between the interface surfaces, continualmeasuring of the vertical displacements was also performed. With such disposition the nexttypes of interfaces were investigated

-bedrock (marbleized limestone) - rockfill;

-bedrock - filter material;-bedrock - clay.

After the performed tests with the bedrock, it was removed and changed with concreteslab to conduct the testing of the interface concrete-clay.

Figure 2. Illustration of modelled bedrock in the lower frame of the shear box

3. METHODOLOGY OF TESTING

During the testing of the rockfill material, granulometric composition with mix ofgrains with d15cm was adopted, coefficient of uniformity Cu=d60/d10=11-14 and maximalcontent of fine fraction under 0.6 cm around 8-11%, which corresponds with the confirmedassumptions in the practice of similar materials. The filter material is filled as an averagesample of granulometric content from the filter zone II of the dam.

The testing is conducted in the given aparature with dimensions 110.6 m. Itsconstruction in the current state allows appliance of maximal vertical stress of around 1 MPa.The materials are installed in the aparature in two layers with height of the individual layersof around 0.15 m. The physical and mechanical parameters obtained during the compactionare shown in the next table.

Table 1 Physical and mechanical parameters of the compacted materials

MaterialVolumetric weight in

natural conditionVolumetric weight in

dry conditionWater

contentLiquidlimit

Plasticitylimit

Plasticityindex

[kN/m3] d[kN/m3] w [%] wL[%] wP[%] IP[%]

Rockfill 20,45-21,50 19,70-20,93 2,60-3,74Filter 23,28 22,28 4,00

Clay 17,93-20,22 14,52-16,18 21,46-26,40

43,30 23,30 20,00

rock

concrete

force/displacement


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After installation, the material is subducted under a vertical laod, with which normalstress is applied over the surface of shareing. This was done in 4 stages, until consolidation ofthe vertical displacements (Uv) on every stage, as well as during max, after which the verticalload was maintained constantly for 24 hours. The selection of the intensity of the loadingdepends from the characteristics of the aparature i.e. its maximal value. For the rockfill and

the filter it was adopted max=0.9 MPa, while the others are interpolated on 0.30 MPa and0.60 MPa. Because of the expected greater displacements, for the clay it was adopted =0.20,0.40 and 0.60 MPa, where at certain points, for control, it was continued until max= 1.0,relatively 0.3 MPa.

After the performed consolidation, a horizontal load was applied, also in stages(r=1/20i,max), up until the provoking of failure along the contact surface (bedrock, orconcrete-properly installed material). During that, the horizontal displacements (Uh) are

permanently registered for every step of stress to their consolidation, apropos until failure.Also, during the whole process of shearing, the vertical displacements are also controlled, inorder to get the complete picture for the testing and the behavior of the material. One exampleof the adopted stress pattern in the phase of shearing is shown on the next picture.

bedrock-filter zone interface (seriae I)

failure

max=0.90 MPa

max=0.72 MPa

Uh

=f(t); Uh=f(t) [MPa]

Time t [min]

0 50 100 150 200

5

4

3

2

1

0

0.9

0.7

0.5

0.3

0.1

0

Uh[cm]

Figure 3. Example of stress pattern in the phase of shearing

One experiment in one series consists of three points with proper maximal vertical loading.For every material a total of two series of examinations over the bedrock, and two series of clay overconcrete base were conducted.

4. REVIEW ON SOME OF THE RESULTS

For illustration of the obtained results, few typical diagrams are shown on the nextfigures. On figure 4 the diagrams of normal stress and vertical displacement (= f(Uv)), forall the types of contacts are shown. On figure 5 summarized diagrams for the relation of shearstress and horizontal displacement (= f(Uh)) are given, where a example of same level ofvertical stress (=0.6 MPa) is chosen, in order to get an insight in the differences in theachieved displacements until the moment of failure. On figure 6, classic diagrams for therelation of tangential and normal stress (= f()) are presented.

All these diagrams show very clear the difference in the mechanical behaviour of thedifferent contacts, from which it can be concluded that the contacts of the filter material and

the rockfill with the base give similar relations, while the contacts clay-bedrock and clay-concrete show specific behaviour.


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concrete - clay

bedrock - rockfill

bedrock - clay

bedrock - filter

Vertical displacement [mm]

Normalstress[

MPa]

0 10 20 30 40 50

1.0

0.8

0.6

0.4

0.2

0.0

Figure 4. Summarized diagram of the relation normal stress-vertical displacement

Horizontal displacements Uh[mm]0 10 20 30 40 50 60

6

5

4

3

2

1

0

Shearstress[MPa]

bedrock-filter

bedrock-rockfill

bedrock-clay

concrete-clay

=600 kPa

Figure 5. Summarized diagram of the relation shear stress-horizontal displacement under same level of

vertical stress

It is very obvious that failures in rockfill and filter material occur under largedisplacements, while for the clay under much lower. It is very typical that the time for failureis shortest for the clay, and for same value of max,which is very logical and consistent withthe properties of the clay in relation to the other materials.


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concrete-clay

Normal stress [MPa]

0 0.2 0.4 0.6 0.8 1.0

1.0

0.8

0.6

0.4

0.2

0.0

Shearstress[MPa]

bedrock-clay

bedrock-filter

bedrock-rockfill

Figure 6. Summarized diagram of the relation shear stress-normal stress

In relation to the shearing strength parameters at the contacts, in all the cases certain non-linearity is noticed, which is illustrated in diagrams of the type /=f(). These diagrams show thatthe influence of the amount of the normal effective stress () is very important, especially forthe interface rockfill-bedrock.

Bedrock-rockfill

/= 0,8776e (-0,146)R 2 = 0,6498

0,74

0,76

0,780,8

0,82

0,84

0,86

0 0,2 0,4 0,6 0,8 1

Normal stress[MPa]

Stressratio(/)

Figure 7. Relation /=f() for interaction bedrock-rock fill (summarized diagram for two series)

Bedrock-filter

/= 0,7592 (-0,1894)R 2 = 0,928

0

0,2

0,4

0,6

0,8

1

1,2

0 0,2 0,4 0,6 0,8 1

Normal stress[MPa]

Stressratio(/)

Figure 8. Relation /=f() for contact bedrock-filter (summarized diagram for two series)


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Bedrock-clay

/= 0,3186 (-0,081)R 2 = 0,6854

0

0,1

0,2

0,3

0,4

0,5

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7

Normal stress[MPa]

Stressratio(/)

Figure 9. Relation /=f() for contact bedrock-clay (summarized diagram for two series)

concrete-clay

/= 0,2417 (-0,2355)R 2 = 0,6062

0

0,1

0,2

0,3

0,4

0 0,2 0,4 0,6 0,8

Normal stress[MPa]

Stressratio(/)

Figure 10. Relation /=f() for interface concrete-clay (summarized diagram for two series)

Analyzing the obtained data, the authors think that the applied methodology has manyadvantages, considering that it enables obtaining of the needed input data for stress-strainanalysis for rockfill dams. With careful additional analysis, authors had concluded that thereare possibilities for comparison of the obtained results and extrapolation of the parameterswith the method which was first introduced by Barton and Kjaersli (1981), which is notsubject of this paper.

5. CONCLUSION

The knowledge of the shear strength parameters and deformability of the interfaces ofthe bedrock with the other materials which are used for the construction of large rockfill damsare from great importance, so the development of methodologies for investigation of theinterfaces presents a great challenge for scientific research. Authors stress out that for allsignificant structures, examination on physical models always should be conducted, whichfurther will be analyzed numerically in order to get a real picture of the behavior of the systemin interaction artificial construction (dam)-foundation (geological setting). Having in mind theobtained results, authors think that the applied methodology of direct shearing in large scale isvery convenient for implementation. In combination with well known established methods,there are possibilities for comparison of the obtained results and extrapolation of the

parameters for areas which are in same range of size as is the structure. In this way a


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prerequisites for real stress-deformation analyses and successful designing of large rockfilldams are created.

REFERENCES1. Anelkovi, Vl.: Analysis of shearing modulus on bedrock-concrete interface, Monography:

Managing water resources of Serbia, 2001. (in Serbian)2. Anelkovi, Vl., okovi, Ks., umarac, Vl.: Modelling of granulometric content influence on therockfills shearing strength, Second practical and scientific meeting Geotechnical aspects ofCivil Engineering, Soko Banja, 2007, pp.461-466 (in Serbian)

3. Barton N., Kjaernsli B.: Shear strentgh of rockfill, Journal of the geotechnical engineeringdivision, Vol.107, N0GT7, July 1981

4. Barton N., Chobey, V.: The shear strength of Rock Joints in Theory and practice, RockMechanics, Austria, Vol.10, N01/2, 1977, pp.1-54

5. Gapkovski, N., Jovanovski M., Vitanov V.: Elaborate from performed geotechnical investigationsof direct shearing between bedrock and materials intended to be built in rockfill damKozyak in large shear box, Documentation found of the Chair of Geotechnics, Faculty ofCivil Engineering - Skopje (in Macedonian)

6. Jovanovski M., Gapkovski N., AnelkoviVl., PetroviLj.: Some possibilities for determinationof bedrock-concrete interface shearing strength in Hoeks box, Proceedings from the Firstsymposium of Macedonian Association for Geotechnics, Ohrid, 2002, pp.78-86 (inMacedonian)

7. Jovanovski M., Gjogjevski Sp., Papi Br. J., Josifovski J., Peevski I.: Laboratory geotechnicaltests of shearing strength of rockfill for the Rovni dam in the Republic of Serbia, Proceedingsfrom the Second congress on dams, Macedonian Committee of Large Dams, Struga, 2009,

pp.55-64 (in Macedonian)8. Papi Br. J., Vitanov V., Jovanov Z.: Shearing strength parameters of rockfill for the Kneevo

dam, Proceedings from the Third symposium of MAG, Struga, 2010, pp.53-60 (inMacedonian)

9. PapiBr. J., Jovanovski M., Vitanov V., Peevski I.: Analysis of failure envelope of rockfill for the

Rovni dam, Theoretical and experimental research on constructions and applications in civilengineering, Vol.3, Ni, 2010, pp.D-1-D8 (in Serbian)

10. Tanev Lj.: Statical analysis of rockfill dams, Studentski zbor, Skopje, 1989. (in Macedonian)

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ALTERNATIVE FOR LARGE SCALE TESTING OF INTERFACE SHEAR STRENGTH