Relevant Aspects of the Geotechnical Design and Behavior of La Yesca Dam....

RD 2011 CFRD 16

The Second International Symposium on Rockfill Dams 1

RELEVANT ASPECTS OF THE GEOTECHNICAL DESIGN FOR ‘LA YESCA’ HYDROELECTRIC PROJECT AND OF ITS BEHAVIOR

DURING THE CONSTRUCTION STAGE: THE MEXICAN EXPERIENCE IN CONCRETE FACE

ROCKFILL DAMS

Juan de Dios Alemán-Velásquez1, Humberto Marengo-Mogollón1, Rigoberto Rivera-Constantino2, Armando Pantoja-Sánchez 1 and Amós Francisco Díaz-Barriga1

1, Comisión Federal de Electricidad, [email protected], [email protected]

2, Facultad de Ingeniería, Universidad Nacional Autónoma de México

ABSTRACT This paper presents the geotechnical design of “La Yesca” concrete face rockfill dam (CFRD), currently under construction that takes into account the index and mechanical properties obtained from laboratory test results and from rockfill test embankments using the materials with which the dam is being built. Description is made of the procedure applied to achieve the compaction determined from the rockfill embankments and the behavior of the dam is analyzed based on the results of the instrumentation installed. Finally, its behavior is compared with other dams of the same type built in Mexico, namely, Aguamilpa and El Cajón. The behavior of the dams referred to is analyzed in terms of settlements for the following conditions: end of construction, first filling, and long-term performance using for this purpose the information resulting from the instrumentation installed at the embankment and with special emphasis paid to the physical and mechanical characteristics of the geotechnical zoning of each of the embankment dams as well as to the compaction procedure used in the field, being these factors directly related to the recorded behavior Key words: CFRD, Dam, Behavior, Settlements, Rockfill properties 1. BACKGROUND 1.1 Overview This paper describes some of the most relevant aspects related to the geotechnical design of “La Yesca” Hydroelectric Project currently under construction (progress achieved to date, may 2011, amounts to 82% in the placement of materials in the embankment) and an account of its behavior during the construction stage is also presented. Brief mention is made too for comparison purposes of the behavior of two other large concrete face rockfill dam completed in our country, i.e. Aguamilpa and El Cajón, both located downstream from the hydrological system of the Santiago River in the state of Nayarit, Mexico. 1.2 Factors affecting the deformability of the CFRDs Because the materials used in the different zones of a CFRD have in general very good quality, the slope stability of the dam is no longer a relevant problem but the deformations that can be experienced by the concrete slab during the construction stage, first filling and accidental loads (earthquakes) are of serious concern; therefore, the compressibility properties of the different zones, particularly of material 3B that acts as a support to the concrete face, should be carefully analyzed because the


objective is to achieve an embankment with low compressibility that ensures a suitable behavior of this element.. The major factors affecting the characteristics of compressibility of the CFRDs are related to the physical and mechanical properties of the constitutive materials of the various zones of the embankment, namely: grain size distribution, grain shape, size and mineralogy of the particles (hardness), relative density (represented by the void ratio or the dry unit weight), tightness or interlocking, the state of confining stresses, the degree of saturation and the rate of application of the load. In addition, it is a known fact that the geometry of the valley also affects importantly the deformations likely to be experienced by the dam because of the arching phenomenon. Mention should be made that the compaction process used during construction is fundamental in the values of the module of compressibility that are obtained in order to achieve a suitable behavior of the embankment and consequently of the concrete face. The more important issues in this process are related to the type of compaction equipment (usually a vibratory roller compactor, static weight of the drum´s compactor, centrifugal force, number of passes, thickness of the layer to be compacted and, for rockfill, addition of water during compaction (spraying). 1.3 Geometry and zoning of the high CFRD used in México Cross sections of embankments for concrete face rockfill dams from a stability point of view, pose by no means a relevant problem and slopes of 1 vertical to 1.3 - 1.5 horizontal have been adopted (Cooke, 1984, 1998; Sherard, 1995). The distribution of the materials within the cross section of the embankment takes into account the deformability and permeability. In general terms, material 2 with permeability of about 1x10-3 cm/s, directly supports the concrete slabs, having maximum particle size from 5 to 7.5 cm, well graded, with percentage of non plastic fines ranging from 7 to 12%, and sand content of 35% or higher. Immediately adjacent, material (3A) can be found acting as a filter and separating the former from the main rockfill (3B). Toward downstream material (3B) is also placed, and it governs the settlements of the concrete slab and it is typically well graded, with maximum particle sized of 60 cm for alluvium and 80 cm for rockfill. To complete the cross section, sound coarser granular materials are placed (T and 3C), generally well graded and with maximum particle size from 80 to 120 cm. The downstream slope of the embankment is protected with rock fragments exceeding 100 cm in size (material 4). The three large dams presented in this paper were designed and constructed with this criterion. See cross sections in figures 1 to 3.

Fig. 1 Maximum cross section of Aguamilpa Dam


Fig. 2 Maximum cross section of El Cajón Dam

Fig. 3 Maximum cross section of La Yesca Dam

2. GEOTECHNICAL DESIGN OF LA YESCA HYDROELECTRIC PROJECT La Yesca hydroelectric project is located on the Santiago River, 105 km NW of the city of Guadalajara and 22 km NW of the small town of Hostotipaquillo in the state of Jalisco, Mexico. The dam site is located 4 km downstream of the confluence of the Bolaños and Santiago rivers, upstream of El Cajón and downstream from Santa Rosa hydroelectric power plants. This project considers a power plant equipped with two vertical Francis turbo generators of 375 MW each, allowing a total annual average generation of 1210 GHz with a plant factor of 0.19. The reservoir will also help regulate the river runoff from its own basin and contribute to optimize the electric generation at El Cajón power plant. In the first part of this article, a brief description of the site’s geological conditions and of the main project structures is presented. The second part details the work done for the geotechnical design of the dam. 2.1 Project description The general layout of La Yesca hydroelectric project is presented in Fig. 4.


Fig. 4 General layout of the project: 1, Diversion tunnels, 2, Cofferdam, 3, Dam, 4, Underground powerhouse, 5, Open channel spillway

With respect to the geology, the canyon of La Yesca dam site was carved by the Santiago River on a group of Cenozoic volcanic rocks, including andesitic rocks, crystal litchi hyalite tuffs highly silicified (Title), rhyodacitic ignimbrite with fluidal texture (Tmird) and porphyritic dacite ignimbrites (Tmid), affected by various intrusive bodies. All this lithological variety is partly covered by lacustrine deposits and pumice (Qlp), alluvial terraces (Qta), talus (Qdt) and recent alluvial deposits (Qal). The detailed description of this structure, as well as the study and geotechnical design made for the project are presented in the following section. 2.2 Dam design 2.2.1 Selection of the type of dam After economic and technical evaluations of various types of dams, a concrete face rockfill dam was chosen for La Yesca project, with gravel and rockfill zoning for the embankment. The rockfill is obtained from the existing ignimbrites on the site, while the gravel is obtained from the alluvial deposits in the area close to the dam site Extensive field studies and laboratory tests to determine the geomechanical properties of the selected construction materials for the dam were carried out; in addition, several stress-strain analysis of the structure were performed to forecast its expected behavior by using finite element method techniques. The results of these works are described later on.

2.2.1 Field works Alluvial and rockfill test embankments Gravel and rockfill test embankments were constructed to a height of 10 m. Three areas were managed with different layer thicknesses (0.6 m, 0.8 m and 1.0 m in the gravel embankment, and 0.8, 1.0 and 1.2 m for the rockfill embankment). The layers were compacted with a 12.2-ton vibratory roller by varying the number of passes and determining the void ratio every two passes. In the rockfill test embankment 200 L/m3 of water were added before compaction of each layer. It was observed that the addition of water improved significantly the compactness of the rockfill. The gravel


test embankment was compacted practically dry, after realizing that the addition of water did not improve its mechanical properties. The results of these tests are summarized in the table 1.

Layer thickness

M

Gravel void ratio Layer

thickness m

Rockfill void ratio*

Number of passes Fluidal dacite

Porphyritic dacite

4 6 8

0.60 0.258 0.247 0.233 0.80 0.336 0.361

0.80 0.250 0.25 0.239 1.0 0.331 0.410

1.0 -- 0.257 0.292 1.2 0.368 0.422

* For 8 passes of 12.2 ton roller Table 1 Void ratio variation in gravel and rockfill test embankments

Deformability The deformability measurement of the materials in the test embankments was done in two ways: through plate tests and from the instrumentation results. The plate test results were strongly influenced by scale effects and the effect of the light confinement, especially in the alluvial test embankment, so their results were not taken into account. On the other hand, the results obtained from the instrumentation installed (measuring the settlement of the test embankments due to its own weight), gave more reliable values. The average results of modulus of deformability in each layer are shown in table 2.

.Layer thickness m

Average deformability modulus E, in MPa

Gravel Rockfill 1 Rockfill 2

0.60 277 0.80 256 148 174 1.00 246 135 158 1.20 126 150 1: Fluidal dacite rockfill; 2: Porphyritic dacite rockfill

Table 2 Average deformability modulus for materials used in the test embankments 2.2.2 Laboratory tests Index tests Some of the results of tests done on the construction materials are listed in table 3. The failure load values obtained from the particle rupture test appear in table 4, where values of El Cajón dam materials have also been included as a reference.


Test Material Material Rupture load, in kN

Gravel Rockfill 1 Rockfill 2 Dry Saturated

Absorption < 2.25% <2.2% <2.3% Gravel La Yesca 12 10 Accelerated weathering < 1% <9% <5.3% Fluidal dacite La Yesca 5 3.5

Abrasion (Los Angeles)

<12.5% <14% <17% Porphyritic dacite La Yesca 4.8 4

Ignimbrite El Cajón 1.9 to 3.3 1.7 to 2.6 1: Fluidal dacite rockfill 2: Porphyritic dacite rockfill Compression strength of 5-cm diameter particles on the average

Table 3 Values of several index properties Table 4 Particle rupture load (Pa) results These results, reflecting the influence of the shape and the hardness of the particles in the deformability parameters, confirm that the alluvium is much less deformable than the rockfill embankment, which is consistent with both the experience observed in other dams and the parameter values measured in the test embankments. Shear strength and deformability in giant triaxial tests Consolidated drained (CD) triaxial tests were done in giant rockfill test samples, 30 cm in diameter and 70-cm high. The results are shown in figure 5. The tests were executed on samples with a maximum particle size of 38 mm, coefficient of uniformity similar to that on site, and void ratio of about 0.30.

Fig. 5 Effect of the confining stress on the friction angle and confining stress effect on initial

tangent deformability modulus (Eti) 2.3 Analysis and design 2.3.1 Overview La Yesca Dam will be the highest of its kind in America and one of the highest in the world. Recently some high dams of this kind have endured serious cracking in the concrete face, which in turn has led to large leaks of the order of several cubic meters per second. It appears that the origin of these cracks is the high deformability of the embankment rockfill placed in those dams so, in order to avoid these effects at La Yesca Dam, special emphasis was put on the strain-stress analyses to estimate the displacement and the compressive stress on the concrete face induced by the embankment deformation. In this way it was possible to define the most adequate construction specifications.


Therefore, the zoning design of La Yesca Dam was made to meet the following criteria: � Achieve low deformability of the 3B zone in the rockfill embankment � Get a transition zone between the 3B zone material and the 3C zone rockfill at the embankment. � Keep the deformability modulus ratio among adjacent zones in the embankment smaller than 2

in order to avoid stress concentrations. � Attain effectiveness and efficiency of construction materials this is, proper behavior at a

reasonable cost. It was also considered that the peripheral and tension joints will have barriers against leakage similar to those used at El Cajón Dam (copper seal both in the lower and upper face of the joint as well as self-sealing material covering them), installed strictly following the corresponding specifications. 2.3.3 Material properties for numerical analyses Based on the above information, 3D finite element analyses were carried out to assess the magnitude of the settlements in the dam. The deformability modulus values were defined according to the following criteria: The deformability modulus of a rockfill or gravel-sand embankments depends mainly on the following factors (Alberro, 2000, Marsal, 1972): x Hardness and shape of grains (measured by the Grain Rupture Load, in kN). x Compactness or degree of arrangement achieved (measured by the Void Ratio). x Grain size distribution (Measured by the Coefficient of Uniformity, Cu = d60/d10). x Maximum particle size. x Wetting of the rockfill before compacting. x Weight of roller used in compaction, layer thickness and the number of passes. In general, the deformation modulus, Ec, value increases when the grain rupture load of the materials and the coefficient of uniformity gets larger and both the void ratio and the maximum particle size decreases. Figure 6 shows a graph that contains the total population of deformability modulus calculated with the different field and laboratory tests for the alluvial and rockfill test embankments (Pantoja, 2006 y 2007), as well as the exponential trend line for alluvial, fluidal and porphyritic dacite materials. Additionally, the vertical, horizontal and estimated octahedral stresses were calculated for dam materials 3B, T and 3C, by using FLAC 3D software at one third of its total height. With these stresses and by using the corresponding trend line graph, the corresponding deformability modulus to perform the 3D the finite element analyses of the dam were determined. Table 6 presents a summary of index and mechanical properties expected for materials 3B, T and 3C.

Fig. 6 Total population deformation modules obtained in the studies, and trend lines


Material

Unit

Weight Poisson’s

ratio Ec

Ell

kN/m3 MPa MPa

3B, Gravel

1*

20.7 0.2

171 226

2* 240 480

3* 171 342

T, Rockfill

1* 19.5 0.2

153 169 2* 130 260

3* 115 230

3C, Rockfill

1* 18.5 0.25

119 129 2* 85 170

3* 85 170 * Parameters used in the analysis: 1, Trend line; 2, Most probable behavior considering Aguamilpa and El Cajón dams experience; 3,

Conservative viewpoint

Table 5 Parameters used in La Yesca analyses 2.3.4 Analysis of stresses and strains in three dimensions (3D) The numerical analysis was performed using the FLAC 3D (Itasca Consulting Group), which allows to run three dimensional numerical modelling of advanced geotechnical problems. Basically the capabilities FLAC 3D were used to model two aspects: a) the process of building the dam, which must be modelled as a layer sequence, and b) first filling of the reservoir, applying the hydrostatic loading of the water on the concrete face. In this analysis, the elastic linear model was considered as the constitutive model for earth materials, due to the excellent concordance observed between the measured and calculated values by using this model in other dams built in Mexico. The three dimensional model of the embankment for the construction sequence is shown in Fig. 7. During construction of the model, the geometry of the dam and its contact with the walls of the canyon, were reproduced as close as possible. The results presented below are in terms of displacement contours and stresses at the embankment and concrete face. In the analysis of the first filling of the reservoir, the slab/embankment contact was modelled considering that there would be no relative displacements between them. This is a simplified approach, which is not far from reality because of the high shear strength generated for the range of normal stresses that occurs in a slab-embankment contact, virtually preventing the occurrence of a failure in this interface.

Fig. 7 Finite element mesh used in the analyses


In the analysis presented in this report, the existing vertical joints in the concrete face were not included, resulting in extreme conditions in some aspects, such as, the horizontal tensile stress in the lateral side of the concrete face and at the joints. In spite of this, based on the experience with other dams (Alemán et al., 2005, 2007) the results obtained define quite precisely the zones of compression and tension on the concrete face as well as the order of magnitude of these values. A linear elastic model for the materials of the dam and the concrete face was used, taking into account, the differences that arise between the vertical modulus used during construction and the modulus used during first filling. Calculated dam behaviour at the end of construction The maximum vertical stress at Zone T at end of construction was in the order of 3.5 MPa. The maximum vertical settlement calculated in the embankment was of 0.80 m, approximately at the center of a longitudinal section. The maximum vertical displacement in a cross-section of the dam was equal to 0.81 m (see figure 8).

Fig. 8 Total displacement contours at the end of construction

The maximum horizontal displacement was variable between 0.04 m in the area of material 3B (upstream) and 0.11 m at the 3C material (downstream). Calculated embankment and concrete face behaviour upon reservoir filling For first filling a maximum total displacement of the concrete face of 0.17 m was calculated, approximately, at the center of its height and length (see figure 9). The maximum horizontal displacement in the concrete face, along the dam axis was of 0.03 m.

0.175

Fig. 9 Total displacement contours due to first filling of the reservoir (in m)


The compression and horizontal stresses generated in the concrete slabs due to first filling of the reservoir were equal to 4.8 MPa or smaller and were located at the center of the slab. The maximum tensile stress was of 9.8 MPa and occurred in small concentrated areas located at the contact edges of the slab with the abutments, with a maximum average tensile strength amounts to 3 MPa. The stresses generated in the slabs due to filling of the reservoir were calculated in the direction of the dam slope, obtaining a maximum compression of 1.43 MPa at the center of the height and length, and a maximum tensile stress of 5.47 MPa acting on small concentrated areas near the contact edges of the slabs with both abutments. Taking into account the average maximum stress in the area near the abutments, the tensile stress becomes equal to 2.5 MPa. The horizontal compression stress and tensile zones of the concrete face, where the construction joints are located, were defined with this information and can be observed in figure 10.

Fig. 10 Areas of tensile and compressive stresses at the concrete face

3. BEHAVIOR OF THE DAM 3.1 Behavior during construction and comparison with other dams constructed in Mexico Finally, studies and analyses made it possible to define the specifications of construction materials to be used in La Yesca Dam as displayed in table 6. The table also shows information about Aguamilpa and El Cajón for comparison (Montañez, 1993; Aleman, 2006, 2007). In figure 11 the mean grain size distribution curve for 3B material used in these three dams are showed. It may note that in Aguamilpa a compactor lighter than in El Cajón and La Yesca was used, and that the rockfills were compacted with thicker layers and without adding water. On the other hand, the gravel used in the 3B zone in Aguamilpa was finer than those used in La Yesca. All this was reflected in lower deformation of Aguamilpa gravels and a large deformation of its rockfills.

Zone

Type of material and layer thickness

m

Compaction Type

Number

of passes

Aguamilpa El Cajón

La Yesca Aguamilpa El Cajón and La Yesca Aguamilpa

El Cajón and La Yesca

2 Alluvial 0.3

Alluvial 0.30

Alluvial 0.3 10.6 ton VR/ 10 ton NPK plate 10.6 ton VR/ 10 ton NPK plate --- ---

3B Gravel 0.6

Rockfill 0.8

Gravel 0.6 10 ton VR Dry

12.2 ton VR adding 200 l/m3 of water in El Cajón and dry in La

Yesca 4 6

T Rockfill 0.6

Rockfill 1.0

Rockfill 0.8 10 ton VR Dry 12.2 ton VR adding 200 l/m3 of

water 4 6

3C Rockfill 1.2

Rockfill 1.2

Rockfill 1.0 10 ton VR Dry 12.2 ton VR adding 200 l/m3 of

water 4 6

4 NA NA NA Placed by backhoe Placed by backhoe NA NA

Table 6. Compaction characteristics of materials placed at embankments


Fig. 11 Grain size distribution curves used in 3B materials in several dams Indeed, in the figures 12 a 14 the contours of settlement of the three dams are showed. We can see that the settlement at the end of construction was around 1.7 m in Aguamilpa, 0.8 m in El Cajón, and in La Yesca, with 82% constructed, there is a settlement of 0.45 m and it is not expected at the end of the construction exceeds 0.75 m.

Fig. 12 Contours of settlements and modules of deformability determined with hydraulic levels in La Yesca Dam (April 2011) Courtesy of Structural Safety Department, CFE

Fig.13 Contours of settlements and modules of deformability determined with hydraulic levels at

the end of construction in El Cajón Dam (courtesy of Structural Safety Department, CFE)

0

10

20

30

40

50

60

70

80

90

100

0.010.11101001000

%�Passing

�(weight)�

Size�particule,�in�mm

Xibeiku(95m)

El�Cajón�(187�m)

La�Yesca�(207�m)

Aguamilpa�(185m)

Foz�do�areia�(160m)

BOULDERS GRAVEL SAND SILT�AND�CLAY||

95 MPa 140 MPa 70 MPa


Fig. 14 Contours of settlements and modules of deformability determined with hydraulic levels at the end of construction in Aguamilpa Dam (Courtesy of Structural Safety Department, CFE) In Table 7 the modulus of deformability and maximum settlement measured in the three dams are showed. We can see that modulus in the gravel of Aguamilpa and La Yesca dams are very similar, and that the confinement of the material T reduces its deformability. With respect to material 3C, in Aguamilpa had a high deformability y due to three factors, poor graduation, using layers of 1.2 m and the dry compaction. In El Cajón and La Yesca, gradation of these materials was improved and compaction was performed by adding water and using a heavier vibratory roller. This produced a less deformable rockfill. The extreme case was La Yesca, where for construction reasons the contractor used the same gradation for T and 3C, making way for modules in the order of 131 MPa in the latter material, 4 times higher than Aguamilpa.

DAM

Height H, m

A/H2

Mean void ratio and deformability modulus,

Maximum settlement at

the end of construction,

in m

3B T 3C

e E

MPa e

E MPa

e E

MPa

AGUAMILPA

185

3.92

0.19

250

0.29

118

>0.40

35

1.7

CAJON(1)

186

3.21 0.32

95 0.36

140 0.39

70

0.8

LA YESCA

207

2.38 0.18

240 0.28

176 0.30

131

0.75(2)

1, void ratio are estimated 2, estimated according with current measurements

Table 7 Geometric characteristics, void ratio and deformability modulus measured in the three dams

260 MPa 129 MPa 49 MPa


0

10

20

30

40

50

60

70

200 400 600 800 1000 1200 1400 1600 1800

depth,�m

Dynamic�Elasticity�modulus�(crossͲhole�test),�MPa

Material�3C

Material�T

Material�3B

0

50000

100000

150000

200000

250000

300000

350000

400000

450000

500000

0 100 200 300 400 500 600

Dyn

amic

She

ar m

odul

us, i

n kP

a

Mean confining stress, kPa

Material�3C

Material�T

Material�3B

a) b)

Fig. 15 a) Dynamic Elasticity modulus; b) Dynamic shear modulus measured in materials of La Yesca dam

For La Yesca, the dynamic modulus of elasticity (Ed) and dynamic shear modulus (Gmax) were measured for the materials of the dam, by means of cross hole test. The results are shown in Figure 15. It can be noted that Ed has values around 300 MPa for low confining stress, and grows up to 1200 MPa for mean confining stress of 0.55 MPa, while, Gmax presented values among 80 and 440 MPa. Using the Seed and Idriss equation, we could represent the variation of the Gmax with the mean confining stress (Vc) as: Gmax = 1000 K2 Vc

0.5

With K2 between 20 to 34, and values of Gmax and Vc in kPa. We can see that the value of K2 are smaller than the reported in the technical literature for rockfills, and are near to values for dense sands. 3.2 Behavior during first filling Currently, La Yesca Dam is in under construction, and is expected to begin first filling in second quarter of 2012. However, based in its behavior during construction, it is considered that the displacement of the concrete face will be those predicted in the analyses made (less than 20 cm), so it will not present cracking problems.


3.3 Long term behavior Considering the behavior of La Yesca dam during construction, and the fact that water was added during the compaction process, do not expect long term settlements larger than 30 cm, similar to settlements measured in El Cajón, and almost a half of the settlement in Aguamilpa. Indeed, monitoring of the embankment deformations in Aguamilpa and El Cajón has been an on-going process, i.e. it has covered from the construction stage to date. Figures 16 and 17 shows the settlement contours of Aguamilpa and El Cajón dams as of October 2010, where it can be observed that settlements have evolved since first filling of the reservoir of the embankment. The current rate of settlements has been estimated of about 0.9 cm /year, which decrease year by year. In Aguamilpa dam, the long term settlement have occurred mainly in T and 3C material, with a maximum value of 48 cm near to the crest of the dam, but this have influenced the 3B zone. Measurements indicate that the rate of these settlements increase during the rainy season, due to the moistening of the rockfill that favors the rupture of the fragments of rocks. Note that these settlements have resulted in deflections and cracks in the concrete face of the dam to the elev. +180 (50 m below the crown of the dam), without major consequences for its behavior.

Fig.16 Settlement contours at Aguamilpa Dam as of October 2010 (first filling is included)

Courtesy of Structural Safety Department, CFE With respect to El Cajón, the maximum long term settlement has occurred in the crown of the dam with a value of 25 cm (see figure 17). Mention should be made that the highest rates of deformation were recorded at the time period covering from the starting date of the first filling to February 2007 when the rate of deformation began to decrease. Currently the rate of settlement is very slow, which indicates that this phenomenon has practically stopped.

Fig.17 Settlement contours as of November 2010 measured at El Cajón Dam (first filling is

included). Courtesy of Structural Safety Department, CFE


4. CONCLUSIONS Geotechnical studies and designs allowed to define the site requirements and the use of materials for the dam, as well as to obtain information about the strength and deformability parameters of the gravels and rockfill used to construct the embankment. The results of the analyses made it possible to establish the importance of using transitions in the zoning of the dam to prevent sudden changes in the module of deformability of materials that could induce undesirable stress concentrations and lead to tensions and cracking in the concrete face. It was found that, in high dams, the use in the 3B zone of very well compacted and well graded gravel or rockfill, with layer thickness less than 60-80 cm It will ensure a proper behavior of the concrete face by keeping the maximum displacements in values less than 30 cm, and maximum compression stresses at the central concrete face at lower than permissible values. There is no doubt that the characteristics of compressibility of the materials constituting La Yesca dam, their geometry and the compaction procedures adopted in the field have been determinant in the magnitudes of the settlements measured as of to date and of those to be expected in the long term. To get a rockfill with low deformability, we need to use a sound rocks, well-graded grain size distribution curves, addition of water during compaction, layer thickness less than 0.8 m in 3B zone and less than 1.2 m in 3C zone, and a heavy vibratory roller compactor (12.2 t of mass in the drum). Addition of water during rockfill compaction also reduces the long term settlement of the embankment. It seems that the K2 constant used to calculate dynamic shear modulus in seed and Idriss equation has values between 20 to 34, substantially lower than those recommended in the literature.

ACKNOWLEDGEMENTS The authors thanks Ing. Reginaldo Hernández for allowing us to use the figures of the contours of displacements developed in the Department of Safety of Structures of CFE. REFERENCES [1] Alberro, J, 1998, “Agrietamiento de presas de enrocamiento con cara de concreto”, Memorias

de la Conferencia Internacional sobre Presas de Almacenamiento, SMMS [2] Alemán, JD et al, 2006, “Studies and geotechnical design of El Cajón Dam”. Proceedings of the

International Symposium of Dams, ICOLD-SPANCOLD Barcelona, Spain, Vol. 1 [3] Alemán, JD et al, 2007, “El Cajón Dam. Analysis of its behavior during construction and first

filling”. Proceedings of 5th International Conference of Dam Engineering, LNAE, Lisbon, Portugal, 2007

[4] Cooke, JB 1984, Progress in Rockfill Dams, Journal of Geotechnical Engineering, ASCE,

October 1984. [5] Cooke, JB 1998, “Empirical Design of CFRD”, Hydropower & Dam, Issue six, 1998


[6] Sherard, JL October 1985.“The Upstream Zone in Concrete-Face Rockfill Dams”, ASCE Symposium on Concrete Face Rockfill Dams, Detroit, USA

[7] Pantoja, A, et al, 2006, Informe No. 06-62-SGM/S. “P.H. La Yesca. Informe del pedraplén de

aluvión”. Diciembre de 2006. Subgerencia de Geotecnia y Materiales, GEIC, CFE. [8] Pantoja, A, et al, 2007, Informe No. 07-05-SGM/S. “P.H. La Yesca. Informe de los pedraplenes

de enrocamiento”. Marzo de 2006. Subgerencia de Geotecnia y Materiales, GEIC, CFE. [9] Marsal, RJ, 1972, “Resistencia y deformabilidad de enrocamientos y gravas”, Informe No. 306,

Instituto de Ingeniería, UNAM, Mexico. [10] Alberro, J y Gaziev, E, 2000, “Resistencia y compresibilidad de los enrocamientos”, Instituto de

Ingeniería, UNAM. [11] Montañez, L, Hacelas, J y Castro, J. , 1993 “Design of Aguamilpa Dam” [12] Itasca Consulting Group, Inc. Minnesota, USA, 2002. Fast Lagrangian Analysis of Continua in

3 Dimensions (FLAC 3D).

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