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REVISTA BRASILEIRA DE

COMITÊ BRASILEIRO DE BARRAGENSANO V Nº 06 JULHO 2018

REVISTA BRASILEIRA DEREVISTA BRASILEIRA DE ISSN: 2594-7451

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STRUCTURAL BEHAVIOUR OF ROCKFILL DAMS

António VEIGA PINTO | Civil Engineer – Veiga Pinto Consulting

Rita AFONSO | Civil Engineer – Veiga Pinto Consulting

Construction of rockfill dams with vibratory rollers is still recent. Knowledge of the stress–strain behaviour of the rockfill materials is quite recent, as well.In this work some physical-mechanical properties of rockfills are discussed, namely the blocks breakage caused by the stress state, collapse and size of rock fragments. Behaviour of rockfill materials in laboratory and in prototypes (dams) is discussed. The main types of incidents in rockfill dams and their causes are analysed.

ABSTRACT

BARRAGENS DE TERRA E ENROCAMENTO

29REVISTA BRASILEIRA DE ENGENHARIA DE BARRAGENS

1. INTRODUCTION

Because of their height, embankment dams and in particular rockfill dams, belong to the most daring civil engineering structures in view of their size, cost, benefit,

and also of the potential risk of accidents.Rockfill Mechanics is a recent science under 60 years old.

In this paper, we aim to reflect on several issues related to the behaviour of rockfill dams of interest in the design, construction, and safety evaluation.

2. CHALLENGE, BACKGROUND, AND PURPOSE

The motivation to write this paper appeared when the authors took notice of 3 papers from China researchers on the interest to build super high rockfill dams [1], [2] and [3].

In Portugal, the construction of the Paradela Dam, in 1958, of dumped rockfill, 110 m height, was at the time of its construction the highest rockfill dam in the world with the upstream curtain (Concrete Face Rockfill Dam; CFRD),. In view of the large settlements occurred in the first filling, the upstream face concrete facing has suffered significant damages, leading to high water leakages that keep happening today [4].

In view of that incident, Paradela dam designers expressed the need to acquire better knowledge of the rheological behaviour of rockfills before new designs of this type of dams would be made. This was the challenge launched at LNEC to conduct studies on rockfill materials to improve the design tools of such structures. In a first study, modelling of rockfills, namely the scale effect of stress-strain behaviour between samples tested in the laboratory and in-situ was analysed [5]. In a subsequent study, the structural behaviour of a rockfill dam was modelled prior to construction. This study allowed the development of more reliable technologies to support the project of rockfill dams [6].

In the studies carried out at LNEC, the physico-mechanical characterization of rockfill materials was particularly taken into account. Beliche Dam, which served as a prototype in the prediction type A in the mentioned study, had evidenced higher than expected deformations, a fact that has raised the interest of the national and international community in such studies, leading, among other things, to the elaboration of 13 PhD theses around the world.

The lessons learnt from the study of rockfill materials and that led to the technological innovation of this type of structures showed that laboratory tests, behaviour observation in prototypes, construction techniques, and mathematical modelling are the key-issues for success.

3. DISCRETE MODELS IN ROCKFILS

3.1 INTRODUCTION

In an approach used with rockfill materials it is intended to reconcile micro-rheology, that is models of individualized particles, and the influence of external factors, with the macro-rheology that is treated as a continuous model.

3.2 FACTORS INFLUENCING STRESS-STRAIN BEHAVIOUR OF ROCKFILL MATERIALS

Several factors influence the stress-strain behaviour of rockfill materials, the most important being the state of stress and the strength of the blocks to breakage [7], although not all authors subscribe this paradigm [8].

3.2.1 STRESS STATE

The stress state is undoubtedly the most relevant factor in the stress-strain behaviour of rockfill dams.

This behaviour results from the sharp increase of contact force between rock blocks with increase in the stress state. The contact forces are directly proportional to the applied external pressures, and it has been verified in laboratory that, with the increase of the tension state, fracturing of blocks increases, which leads to the reduction of shear strength and modulus of deformability in the rockfill body. This fact has high relevance in the increase of installed stresses with increasing height of dams.8

3.2.2 BREAKAGE STRENGTH

Blocks breakage strength depends on several factors, and therefore it is essential to test representative rock fragments to estimate their strength and to predict its evolution in time, that is to say, their weatherability. In latu sensum, high weatherability may signify increased fragmentation and larger settlement in a long-term run. This subject is tackled in Chap. 6.

3.2.2.1 INDEX PROPERTIES

In micro-rheological terms, stress-strain characteristics of rockfill materials depend largely on the properties of the rock blocks. In this line, several tests, designated here as index-properties, have been carried out in Portugal on rock fragments

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extrapolation accuracy of the stress-strain characteristics from laboratory to the prototype. This problem with the techniques inherent to it is called modelling.

As mentioned in (3.2.1), fracturing of blocks is the most important factor in the stress-strain behaviour of rockfill bodies, which increases with the block size. In this line, a study was carried out at the LNEC where triaxial compression and one-dimensional compression tests were performed with specimens under similar conditions, except for different sizes of the rock fragments [6].

Following studies of Marsal [10], a large number of tests on rock fragments were carried out in which the contact forces of a particulate medium under an external pressure of 1 kgf/cm2 were proved to follow a normal law, in which the average rupture force, , of particles is proportional to the mean diameter, , of particles [5]. The correlation is given by Equation 1, where is expressed in gf and in mm:

of rockfill materials whose results have been used in the design of rockfill embankments for dams and expressways [9].

The tests allow to characterize texture, compressive strength, durability and sensitivity to water. Over 200 tests on samples of rock fragments from all rock types existing in Portugal have been carried out; carbonate rocks, granites, metagreywackes and basalts were studied. The results of these tests allowed to characterize, for example, whether the tested materials are of in a class of high or low strength [9].

4. CONTINUOUS MODELS IN ROCKFILLS

The continuous models correspond to the macro-rheology, that is to say, it is disregraded that the rock embankment is constituted of particles and accepts that their stress-strain behaviour is analysed as a continuous body.

In the laboratory the testing chambers have dimensions of the order of 0.50 m in diameter, which already allows extrapolation to prototype values, i.e., the real embankment, with reasonable accuracy.

Fracturing of rock elements has a preponderant action on the stress-strain behaviour of the rockfill embankment. For hard rockfill materials, this phenomenon leads to a marked curvature of the Mohr-Coulomb envelope.

Fracturing of the blocks by the effect of water is another important factor, as can be seen in one-dimensional compression tests with and without the presence of water (Figure 1) [6]. The tested material, a weathered metageywacke rock, was used in the upstream shoulder of Beliche Dam, where a high collapse supervened, as discussed in (6.3.2).

Another aspect to be taken into account, with influence on the structural behaviour of rockfill bodies, is the variability of the oedometrical modules of rockfills tested in the laboratory (LNEC), where variations between 24 and 110 MN/m2 have identified, which therefore can lead to highly different settlements.

From the analysis of laboratory tests, an inverse relation between the oedometric module and the K0 was verified [6].

5. ROCKFILL MODELLING

Due to the blocks size, rockfill embankments cannot be tested in the natural scale. It will be necessary to conduct tests with smaller rock fragments, which raises the question of the

STRUCTURAL BEHAVIOUR OF ROCKFILL DAMS

Figure 1 – One-dimensional compression tests DU50. Relation stress-strain. Weathered rockfill material. Water effect [6]

[1]

[2]

On the other hand, through laboratory tests, it was verified that crushing strength of blocks of a particulate medium also obeys to a normal law, Equation 2, where is expressed in kgf and in cm [5]:

Figure 2 - Comparison of the decrease in shear strength and increase in deformability with increasing size of rock fragments

Figure 2 shows, on the left, the Gauss curves of contact forces and crushing strength values of laboratory tests. On the right, the corresponding values from samples of the prototype. Considering equations 1 and 2, with increasing particle size (D2 > D1), there is an increase in the overlaping zone (shaded area). Thus, there is a decrease in shear strength and increased deformability in the prototype samples compared to those tested both obtained in the laboratory.

6. BEHAVIOR OF ROCKFIL IN FIRST FILLING AND THE STRESS EFFECT. COLLAPSE AND CREEP

6.1 INTRODUCTION

Rockfill dams have generally exhibited satisfactory structural behaviour. There has been, however, incidents and the need for rehabilitation works. Such incidents are mainly associated to excessive displacement in dams, especially in the first filling stage.

Cracking of upstream reinforced concrete facing and the need to increase the height of clayey core dams to restore the freeboard to acceptable levels after excessive settlements are examples of typical incidents. Prediction of displacements when applying hydrostatic pressure is therefore an important requirement at the design stage of rockfill dams.

These displacements result from the rheological characteristics of the rockfill materials and are mainly associated to the effect of the stress state, collapse, and creep. The following sections describe how these factors can be characterized in the laboratory and how they have been evidenced in prototypes.

6.2 EFFECT OF STRESS STATUS

6.2.1 IN THE LABORATORY

In (4) the rheology of the rockfill was approached as a continuous body, and the increase of the stress state was considered that the factor of larger influence on the stress-strain behavior. In fact, with increasing stress state in the particulate media, fracture of blocks increases, leading to lower shear strength and deformability modulus.

6.2.2 IN DAMS

An anomalous effect occurred with CFRD, when height reached an appreciable value. An example is the case of Paradela dam, the highest (110 m) CFRD in the world, at the time of its construction (1958), which exhibited high settlements and cracking of the curtain and quite significant infiltrations [4].

In 2004, settlements at the upstream crest berm, still reached a rate of 4.8 mm/year, and the infiltration rate was of the order of 734 l/s [4].

At the time of construction of Paradela Dam, no laboratory testing equipment for rockfill samples was available, and fracturing due to the effect of the stress state and to the increase in the size of the blocks was not yet known. Although the dam was constructed with high strength granite blocks, fracturing should have been appreciable. In addition to the stress state, fracturing should have been influenced by the very high void ratios caused by the construction procedure, the dumped rockfill process. For this reason, a new construction technology of rockfill dams, with the introduction of compacting with

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vibrating cylinders was adopted, thus allowing to obtaining embankments with lower void ratios.

In 2005 and 2006, the effect of fracturing of blocks, due to the effect of the stress state, may also have been the reason for the decrease in the CFRD deformability modulus, when the dams reached a height record. As an example, the cracking of the reinforced concrete curtain upstream of Barra Grande (H = 185 m), Campos Novos (H = 202 m), and Mohale (H = 145 m) dams should be underlined.

From a posteriori analysis of the behavior observed in prototypes, it was concluded that the deformability modules of rockfills were very reduced, varying between 50 and 70 MPa. According to Pinto [11], one of the most effective corrective measures to reduce compression deformation in upstream concrete facings in super-high CFRD will be the increase in stiffness of the rockfill embankment.

These accidents have sometimes been associated to the use of low-strength rockfill materials [1]. However, another possible cause, as expressed in this work, can result from even high hard rockfills when subjected to high stresses, in increasing height dams, to be able to exhibit reduced deformability modules.

Therefore, it is considered that super high dams should be constructed with only high strength rock blocks and for the characterization of rock material, index-property tests should be carried out (see 3.2.2.1).

6.3 COLLAPSE AND CREEP

6.3.1 IN THE LABORATORY

Laboratory tests have shown that instantaneous volumetric deformations in rockfill samples occurs when a sudden increase in water content happens. This phenomenon is called collapse. It is considered to be due to increased breakage of blocks (Figure 1). The most plausible justification for the loss of resistance of these

blocks is the existence of physico-chemical reactions of water with the mineral components of the rockfill blocks.

In order to estimate the order of magnitude of the collapse of rockfill samples, index property tests such as insoluble residue, swelling strain, and compressive strength under dry and saturated conditions in representative rock samples should be performed.

Tests of this type have been performed in the laboratory, but the results have not allowed satisfactory extrapolations for predicting the behavior of prototypes [6].

6.3.2 IN DAMS

One case in which significant collapse settlements occurred in the Beliche Dam, built in Portugal, in 1985.

Figure 3 refers to the maximum cross-sectional profile of the dam. It is a rockfill dam with a central clayey core. Both stabilizers are rockfills. ECRD, near the core, is a low-resistance rockfill and the one located in the outer zone is a high-strength rockfill.

In Portugal, geodetic measurement systems in dams to measure settlements after construction have been used, which are not yet fully replaced by satellite observations. Figure 4 refers the system used in Beliche Dam [12].

During the construction phase, results of inclinometric tubes presented high settlements, especially in times of larger rainfall values. In this way, it was decided to carry out a large-scale test, by creating an excavation zone on the surface of the embankment where leveling plugs were placed. The test zone was abundantly saturated. The results led to settlements in the transition material (of low resistance rockfill), about 10 times higher than that of the rockfill of the outer stabilizer mass. For this reason, instructions were given for a more efficient compaction in the transition material, by placing abundant water during the spreading and compaction phases, by checking the number of cylinder passages and the height of layers and a

Figure 3 - Beliche Dam type profile

STRUCTURAL BEHAVIOUR OF ROCKFILL DAMS

33REVISTA BRASILEIRA DE ENGENHARIA DE BARRAGENS

greater accuracy in the grain size, so as to avoid the placement of fine fraction material.

It was also decided to construct an upstream berm at a height of less than 9 m in relation to the crown, to place leveling marks in order to measure the displacements on the surface of the upstream surface, at the time of the first reservoir filling.

This decision was a pertinent one, because in the next winter, with the dam still under construction, a partial filling of the reservoir up to the quota 31.0 m (60% of NPA) occurred. During this period, between December 1984 and March 1985, a collapse of about 0.48 m in the mark of the upward bank of the central dam area (see Figure 5) occurred, resulting certainly from the collapse of the lower resistance rockfill material of the upstream stabilizer mass [12].

Figure 4 - System of Geodetic Measurements of the Beliche Dam

Figure 5 - Beliche Dam. Vertical displacement measured in the central upstream dam zone, by geometric leveling

The Ataturk dam in Turkey, 184 m high, was built in 1990. It is also an ECRD. The dam showed high settlements, which led to the need, in 2000, to carry out rehabilitation works to maintain the clearance of 7 m and to continue safely operating.

The high settlements of the Ataturk Dam should have resulted from a combination of unfavourable mechanical conditions, both in part of the clayey core material and due to the rockfill of the stabilizing mass [13].

The rockfill materials of the Ataturk Dam were submitted to index-property tests. Two rock varieties were used: a sound aphanitic and an altered vesicular basalt. The first material was found to have a tensile strength 10 times higher than the second. Vesicular basalts, because they contain montmorillonite and secondary calcite lenses filling voids, easily crumble and may have been the main cause of high collapse settlements.

Creep in rockfill dams has not led to excessive settlements. However, when collapse of rockfill occurs, the rate of deformation over time may be more relevant, and creep more pronounced. This is what happened at the Beliche Dam, where, at the end of the first fill, 5 years after construction, the monthly settlement rate at the crown level was still about 5 mm/month.

7. FINAL CONSIDERATIONS

The stress-deformation characteristics of rockfill materials depend mainly on the fracture of the individual rock blocks. This fracturing is increasing with the state of tension. Thus, with increasing dam height, rockfill materials should exhibit lower angles of internal friction and deformability modulus.

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Also the laboratory tests showed that with the increase of the blocks dimension, the mechanical properties are reduced, reason why the embankments, made of larger blocks, present smaller shear resistance and stiffness when compared to values measured in laboratory tests.

As mentioned before, physical and mechanical characteristics of the rock fragments may have a marked effect on the rheology of rockfill bodies. In this way, tests of index properties must be carried out to support the design of rockfill dams.

The collapse of rockfill materials, resulting from physico-chemical reactions in the mineral matrix in the presence of water, has been recorded in the laboratory. However, extrapolation for prototypes has not been sufficiently reliable. In fact, settlements, caused by the increase in water content in dam rockfills have led to greater displacements than those estimated from laboratory tests.

Creep in rockfill material is not relevant, but it can be enhanced by the effect of the collapse of rock blocks.

8. REFERENCES

[1] ZHOU, J.; YANG, Z.; WANG, F. and LIU, C. (2017) – “Research on Key

Technologies for Super-high Concrete Face Rockfill Dams”, Revista Brasileira de

Engenharia de Barragens, Nº 05, Rio de Janeiro, 36-42.

[2] XU, Z. and JIANG G. (2017) – “Technologies on Construction of Earth

Core Rockfill Dams”, Revista Brasileira de Engenharia de Barragens, Nº 05, Rio

de Janeiro, 43-54.

[3] XU, Z. (2017) – “Discussions on the Technical Issues of the Construction of

300m High CFRD”, Revista Brasileira de Engenharia de Barragens, Nº 05, Rio de

Janeiro, 55-59.

[4] CRUZ, P. and VEIGA PINTO, A. (2007) – “Paradela and Campos Novos

Dams”, III Symposium on CFRD-DAMS, Florianópolis, 217-227.

[5] VEIGA PINTO, A. (1986) – “Rockfill Modelling, Workshop on Rockfill

Technology for High Fill Dams”, Central Board of Irrigation & Power, New

Delhi, 81-112.

[6] VEIGA PINTO, A. (1983) – “Previsão do Comportamento Estrutural de

Barragens de Enrocamento” (Prediction of Structural Behavior of Rockfill Dams),

(Specialist thesis presented to obtain the position of Assistant Researcher, LNEC),

Lisbon, 1-157.

[7] MARSAL, R. J. (1973) Mechanical properties of rockfill, in Embankment-

Dam Engineering, Casagrande Volume, Jonh Wiley & Sons Pub.

[7] VEIGA PINTO, A. (1986) – “Mechanical Properties of Rockfill Materials”,

Workshop on Rockfill Technology for High Fill Dams, Central Board of Irrigation &

Power, New Delhi, 113-146.

[8] T. CRUZ, P.; MATERÓN B. and FREITAS M. (2009) – “Barragens de

Concrete Face. Enrocamento. Rockfill Dams com Face de Concreto”, S. Paulo, 1- 448.

[9] VEIGA PINTO, A. (2007) – “Influence of the Type of Rock on Mechanical

Properties of Rockfill”, 5th Int. Conference on Dam Engineering, February,

Lisbon, 1-11.

[10] MARSAL, R. J. (1973) Mechanical properties of rockfill, in Embankment-

Dam Engineering, Casagrande Volume, Jonh Wiley & Sons Pub.

[11] PINTO, N. L. (2007) – “Very high CFRD Dams – Behaviour and Design

Features”, III Symposium on CFRD-DAMS, Florianópolis, 3-16.

[12] VEIGA PINTO, A. and HENRIQUES, M. J. (2008) – “Contribution of

Geodetic Positioning to Preventing Incidents in Fill Works and Mitigation of Their

Effects”, 4th Symposium on Geodesy for Geotechnical and Structural Engineering,

Lisbon, 1-10.

[13] CETIN, H.; LAMAN, M. and ERTUNÇ, A. (2000) – “Settlement and

Slaking Problems in the World´s Fourth Largest Rock-fill Dam, the Ataturk Dam in

Turkey”, Engineering Geology, 57, May, 225-242.

António VEIGA PINTOResearcher in LNEC (http://www.lnec.pt/pt/) during 35 years. In 1983 his LNEC thesis was presented about a type A structural prediction of a rockfill dam. With this work, in 1985 was awarded with Gulbenkian Prize for contribution for economic progress in his country. The laboratory test results of the thesis were carried out 13th PhD thesis. 7 abroad.

He has been involved in studies for almost every rockfill dams built in Portugal. In the safety of existing 4 rockfill dams built before 1965 and in 10 rockfill dams built after his thesis. He was the coordination of the laboratory tests, quality control of the embankment and for the conception and placing of instrumentation. He has been consultant to United Nations to India (1984 and 1986) and UAE (1989). Invited consultant to PWRI, Japan (1994 and 1996). Consultant of CH2M (USA; 2014). Consultant of World Bank (since 2016).Presently he has CEO of Veiga Pinto Consulting (http://veigapintoconsulting.com/).

Rita Tavares AFONSORita Afonso concluded in 2011 the Integrated Master in Civil Engineering at the University of Aveiro.She has developed her professional activity, in national and Spanish companies, in the area of Geotechnics, namely applied to transportation networks.Rita Afonso has recently had an active participation as an advisor in support of the Technical Assistance to the in Pico da

Urze Dam Project, under construction in the Autonomous Region of Madeira.Amongst the aspects he has collaborated on are: the analysis and interpretation of the results of test rockfills, index properties of rock fragments, the analysis and interpretation of structural behavior of the embankment and excavations, as well as support for the elaboration of construction specifications and control of quality of rockfill materials and the installation of observation devices.

STRUCTURAL BEHAVIOUR OF ROCKFILL DAMS