003 Construction Evaluation of A Sand Dam of Great Height PDF [PDF]

Zitiervorschau

CONSTRUCTION EVALUATION OF A SAND DAM OF GREAT HEIGHT Sergio Barrera Luis Valenzuela

Arcadis Geotecnica, Chile Arcadis Geotecnica, Chile

Abstract The Quillayes tailings impoundment of Los Pelambres Mining Company started its operation in 1999. The impoundment is located in a highly seismic area of geological and topographical complexity with a snow-rain watershed surface of 200 km2 that required a relatively high rate of the heightening of the tailings dam during the first two years of operation. These specific characteristics presented a significant challenge in the design of a sand dam, initially designed for a height of 175m. The design of a dam with these characteristics required very well specified construction materials as well as precise constructiont specifications and a strong basal drainage system. Due to the great height of the dam, the verification of the stability of the dam and sand behaviour under seimic loads received special atention. After three years of operations,a detailed geotechnical investigation was carried out in the tailings sand dam aiming to confirm the in situ characteristics of the placed sand as well as the position of water level in the dam. The article presents a summary of the main design aspects of a high sand dam, the most interesting operational aspects encountered, and the results of the geotechnical investigation recently carried out. Resumé Le barrage de stériles Los Quillayes appartenant à la Compagnie Minière Los Pelambres a commencé son opération en 1999. Le site d’implantation se situe dans une zone à haute sismicité, de grande complexité géologique et topographique. La ligne de partage neige - pluie couvre une surface de 200 km2, ce qui a exigé un taux de croissance relativement haut du mur du barrage pendant les deux premières années d’opération. Ces caractéristiques spécifiques ont présenté un défi significatif pour la conception du mur de sable, initialement prévu pour une hauteur de 175 m. La conception d'un barrage avec ces caractéristiques exigé des matériaux de construction très bien spécifiés aussi bien que des spécifications de construction précises et un système de drainage de base puissant. En raison de la grande hauteur du mur, la vérification de la stabilité du barrage et du comportement du sable, sous charges séismiques, ont reçu une attention particulière. Après trois ans d'opérations, une recherche géotechnique détaillée a été menée dans le mur de sable visant à confirmer les caractéristiques in situ du sable placé aussi bien que la position du niveau d'eau dans le mur. L'article présente un résumé des principaux aspects de la conception d'un haut barrage de sable, les aspects opérationnels les plus intéressants rencontrés, et des résultats de la recherche géotechnique récemment effectuée.

1

LOS QUILLAYES TAILINGS DEPOSIT AND DAM

Los Quillayes is the current tailings deposit of Los Pelambres copper mine in Chile, located about 200 km north of Santiago in Los Andes mountains. The mine is owned by Compañía Minera Los Pelambres. Although the mine facilities were originally designed to process 85,000 tons of mineral per day, the average current processing rate is 114,000 tons per day. The operation of the tailings deposit started in 1999 with a planned capacity of 257 millions tons of dry tailings. Recently Minera Los Pelambres got the approval to raise the Dam to 198 meters height , what is under design ., increasing A possible expansion to a the total capacity to of 360 millions of tons is now being studied. The deposit is formed by two dams, a main tailings sand dam and a 50 m high embankment dam, closing a narrow valley at an average elevation of 2,000 m over sea level, in a highly seismic area.

1.1 Tailings dam The main dam is a sand tailings dam originally designed to have a maximum height of 175 meters after 7 years of operation. The initial starter dam is a 70 meters high compacted embankment. The dam has been daily heightened through hidraulic deposition of relatively clean sands, with no more than 18% of fines (material passing ASTM 200 mesh) and compaction of the inclined deposition surfaces with bulldozers and smooth vibratory rollers, using the downstream method of construction with 1(V):3.5(H) to 1(V):4(H) downstream slopes, following the general chilean practice of the last 20 years (Valenzuela and Barrera,1995; Valenzuela,1996). The sand is obtained by cycloning the tailings in two cycloning stations with 22 cyclones each. Figure 1 shows a typical cross section of Los Quillayes tailings dam. River alluvial Granodiorite bedrock Tailings

STARTER DAM

TAILING SAND

TAILINGS

Figure 1: Typical Cross Section of Los Quillayes Dam

1.2 Initial geotechnical characterization of tailings sand Being the sand the main construction material and also the most critical one from the seismic perfomance point of view, most of the engineering effort during the design stage for the original 175 m high dam was focussed precisely in the determination of the main geotechnical characteristics, both static and dynamic, of the sand to be used in the dam. The sand samples used in all laboratory tests were obtained from the El Chinche tailings dam, an old dam located only 500 m from the site of the present Los Quillayes dam. El Chinche was the tailings deposit used by Los Pelambres mine up to 1998, when the original mine was processing of the order of 5,000 tons of mineral per day. The index properties of the El Chinche sand are shown in Table 1.

Table 1 Index Properties of Tailings Sands Parameter

Unit

El Chinche sand Max size mm 0.66-0.70 D50 mm 0.14-0.16 D10 mm 0.033-0.035 Fines(200mesh % 14-18 G 2.65 to 2.71 3 t/m 1.77-1.79 γmax t/m3 1.28-1.30 γmin t/m3 1.65-1.69 γdStandard Proctor Permeability cm/s 1x10-3 to 1x10-4 Fines(200mesh % 14-18 Classification USC SM S

Quillayes sand 0.6-0.7 0.2-0.3 0.06-0.04 12-16 2.67 to 2.70 1.77-1.79 1.28-1.31 1.60-1.65 1x10-3 to 1x10-4 12-16 SM

It should be noticed that for a density of 1.60 t/m3 the permeability of the sand varies from about 1x10-4 for 25% of fines to about 5x10-4 cm/s for 15% of fines (Barrera and Lara,1998).

Consolidated isotropicaly undrained (CIU) triaxial tests on 2” samples prepared to a density of 1,50 t/m3 were carried out on El Chinche tailings sand. The test were performed for effective confining stresses of 0.1; 0.2; 0.4; 0.8; 1.6 and 2.4 Mpa in order to cover most of the stress range that would be present in the dam during its operation, according to the original design of a 175m high dam.

q (0.1 MPa)

σ3'=2.4 MPa

q (0.1 MPa)

25

σ3'=1.6 MPa

25

20

σ3'=0.2 MPa

20

15

σ3'=0.1 MPa

15

σ3'=0.8 MPa

10

10

5

5

0

q = σ1-σ3 p'= (σ'1+2σ'3)/3 M=1.43

0 0

5

10

15

20

ε1(%)

a) Stress-strain curve

0

5

10 15 20 25

p'(0.1MPa)

b) Stress paths

Fig. 2: Stress-strain curve and stress paths of El Chinche sand. Figures 2a and 2b show the deformation and stress path characteristics for El Chinche sand. From these tests an average frictional angle of 35° with cero cohesion was assumed for stability analysis purpose.

1.3 Cyclic shear strength of El Chinche tailings sand Cyclic shear triaxial tests with controlled stresses were carried out to evaluate the liquefaction potential of the tailings sands, using 2” samples of sand from El Chinche dam, prepared by wet compaction (7 to 8%) to a density of 1,50 t/m3. Confining stresses of 0.2; 0.4 and 0.8 Mpa were applied as well as a cyclic stress ratio of 0.05 to 0.23 at a frecuency of 0.1 to 2 Hz. (A)El Chinchesand(1997) 19%F, Kc=1.0 (B)Verdugo-30%F, Kc=1.0 (C)Verdugo-22%F, Kc=1.0 (D)Verdugo-15%F, Kc=1.0 (E)Verdugo-0%F, Kc=1.0 (F)Obilinovic&Barrera(1982), 10-15%F, Kc=1.5, σ3C'=0.2Mpa (G)Quillayessand(2001), 14-18%F

0.5 ' σo 2/ σd ht gn er tS ci lc yC

0.4 0.3 0.2 0.1 0 1

10

100 1000 Number of Cycles N

10000

Fig. 3: Cyclic strength vs Number of Cycles. Curve A in figure 3 shows the cyclic strength of El Chinche sand for different number of cycles of applied load. Curves B to E represent the same type of relationship for other hard rock tailings sands as reported by Verdugo (1983). Curve F shows a reinterpretation of this relationship for Talabre tailings sand as reported by Obilinovic and Barrera (1982). Curve G is the relationship obtained for the Los Quillayes tailings sand as it will be discussed later on in this paper.

In general it can be concluded that all the tailings sands present similar pattern, showing a reduction of cyclic shear strength with the number of cycles. Also the cyclic shear strength increases significantly with the reduction of the fines content.

1.4 Seismic risk analysis From the seismic risk analysis that was carried out specifically for this project the main earthquake source identified was the subduction of Nazca plate, where most of the high magnitud earthquakes are produced ( Ms> 7.5), being the best example one earthquake ocurred in April 1943 (Ms= 8.3) with a focal depth of 23 km and only 123 km away from the dam site. Main characteristics of the design earthquake are indicated in Table 2. Table 2. Design Earthquake Characteristics Characteristics Max Acceleration (g) Duration of Strong Movement (s) Total Duration (s) Dominant Frecuency (Hz)

EQ Horizontal 0.37 27 80 3

EQ Vertical 0.25 27 80 3

1.5 Seismic stability analysis at design stage The basis for the seismic design of Los Quillayes tailings dam relied mainly in the specification of the most convenient characteristics of the sand to be obtained economically through the cycloning of the tailings and also in the specification of relatively conservative construction procedures including some compaction effort, all this in order to guarantee an adequate behaviour of sands under extreme seismic loads. But the main feature of the design is related to the relatively high permeability of sands due to a low percentage of fines (less than 18%), allowing a rapid drainage of the sand dam body during construction and operation, limiting in this way the potential liquefaction of sands under the action of very strong earthquakes to very minor and isolated sectors within the dam thus not affecting its overall stability. Great attention was given then to the design of drains under the downstream part of the dam to adequately take care of the seeping water. Static stability analysis using limit equilibrium methods and drained shear strength parameters for static analysis and undrained parameters for post-seismic analysis resulted in factors of safety of 2.57 and 1.0 respectively. Pseudostatic analysis with undrained shear strength parameters and a seismic coefficient of 0.15 resulted in a factor of safety 1.34 for abandonment stage. The pseudo-dynamic analysis proposed by Makdisi and Seed (1978), together with Pelambres seismic response spectrum were used to estimate a maximum horizontal deformation of 2.60 m, quite acceptable for a dam of this type and height. With the described design features there was no practical concern on either static or seismic stability of such a dam, consideration that was confirmed by the results of the stability analysis just commented. Nevertheless there were some factors considered to be unique for this dam, mainly: the high raising rate of more than 20 m per year in the first two years; the geometry of the relatively narrow valley where the dam had to be build and the height of the dam, the highest of its type in the country at that time. These factors supported the decision to carry out a dynamic stability analysis of the tailings dam, using the finite differences FLAC3D code. The model used for this dynamic stability analysis considered a Mohr-Coulomb failure criteria including the estimate of increments in pore water pressure according to Martin and Finn (1975). Although seepage calculations through the dam as well as previous experiences with similar dams showed that the free pore water pressure line would not be over 4 m above foundation level, a conservative assumption with the free pore water pressure line located up to 10 m

above basal drains level all over the downsream body of the dam was adopted as the initial situation before the earthquake load was applied. In order to consider the effect of the geometry of the narrow valley a 3D model was used, to complement the results of the 2D analysis. The analysis were carried out for a total dam height of 180 m. The main results of this dynamic stability analysis were: • • • •

2

Maximum deformation of the order of 2.85 m near the toe of the dam, in the slope direction. Potential liquefaction within the dam is restricted to small zones near the toe of the dam. Excess pore water pressure in this area does not exceed 50% of effective confining pressure. The impact on the dam behaviour of the geometry of the narrow valley is only reflected in the slightly bigger deformation at the toe of the dam when comparing 2D and 3D results. The higher accelerations occur at the crest of the dam: 0.6 g, larger by a factor of 1.62 than the 0.37 g considered at the base of the dam at foundation level.

CONSTRUCTION AND OPERATION OF LOS QUILLAYES DAM

The construction of the 70 m high starter embankment dam and of the basal drains of the main sand dam were completed in 1999 (see Fig. 4 and Photo 1). Main findings during the construction of these works were: •

•

Open long run fractures in the rock outcrop that formed part of the left abutment of the dam were identified. These fractures were treated with extensive concrete infilling and dental concrete. In the starter dam area where water could be in direct contact with the abutment before the tailings slimes would push it back in the deposit, some concrete grouting was also carried out. During the construction of the 70 m starter dam most of the alluvial foundation soils were removed, resting only some minor sectors with 3 to 5 m of very dense alluvial clayey gravels. In the foundation of the sand dam these alluvial clayey gravel soils less than 5 m in thickness are also quite dense.

Fig. 4: Starter dam cross section

Photo 1: Starter dam just buit

At present about 1,000,000 m3 of sand are deposited and compacted monthly in the dam. Figure 5 shows the raising curve for the dam that in June 2003 would have a height of about 150 m. During the first two years the dam grew more than 30 and 20 m respectively. This is about 60 and 40 cm per week, requiring monthly relocations of the sand slurry distribution system.

1420

Elevation (m.a.s.l.)

1410 1400

Dam Tailings

1390 1380 1370 1360 1350 1340 1330

05-01-03

05-11-02

05-09-02

05-07-02

05-05-02

05-03-02

03-01-02

03-11-01

02-09-01

03-07-01

03-05-01

03-03-01

01-01-01

31-10-00

31-08-00

01-07-00

01-05-00

01-03-00

1320

Time

Fig. 5: Raising Curve of Los Quillayes Tailings Dam After the sand slurry is placed, with a solids content in weight of 70 %, on the downstream slope of the dam in lifts of variable thickness, intensive spreading, reshaping and compaction of the sand is carried out with a D8 bulldozer and

a 10 ton smooth vibratory roller travelling along the slope (see Photo 2). Densities of more than 95% of Standard Proctor density are obtained. These densities are checked at depths of 30, 60 and 90 cm. During the first year of operation the main challenge was the placement of the sand according to the design geometry of the downstream slope i.e. 1 to 4 (H:V). The sand was discharged from the crest of the dam and this was particularly difficult due to the high raising rate and the initially reduced area of the sand dam footprint. In order to overcome this, aditional discharge points were implemented in order to increase the placement surface (dam footprint) and to make more efficient the drainage of the wet sand. This problem was reduced at the same time the dam was heightened and it doesn’t constitute a concern any more. At present, with almost 30 millions of m3 of dried sand already in place a new modification to the construction procedure will be necessary: to build an intermediate berm in the dowmstream slope, in order to allow the discharged sand to reach the toe of the dam without excesive use of earth moving equipment. Actually from the stability point of view it could be perfectly possible to have a much steeper downstream slope of the order of 1 to 3 or 3.5 (H:V). The unit cost of the sand placement (including tailings pipes relocation) is about 0.25 US$/m3.

Photo 2: Sand tailings dam

3 RECENT GEOTECHNICAL INVESTIGATIONS IN THE SAND DAM During 2001 the mining company retained SRK Consultores to carry out a soil investigation in the sand tailings dam in order to evaluate in-situ geotechnical properties of the sand as placed, to determine if conditions meet pre-

construction assumptions and to re-assess the stability of Quillayes dam and to define parameters for use in the design of a future tailings dam with the same sand. The investigation considered the following elements: • • • • •

Two rotatory drill holes 52 and 54 m deep, SPT and shelby sampling of sands Four seismic cone penetrometer (SCPT) boreholes 53-66 m deep Down the hole shear wave surveys Seismic refraction surveys, two profiles Three sampling test pits 2 m deep

The results of this investigation has been presented by Swaisgood (2002) in a recent international seminar on tailings. The basic sand properties obtained are also shown in Table 1, as those corresponding to Los Quillayes sand. The results of the cyclic shear strength tests are included in Figure 3. The main conclusions of this investigation confirmed a layered embankment without weak zones, with absence of water.The investigation also concluded that the existing geotechnical properties meet pre-design assumptions, that the dam is stable and that the same construction method and general design can be applied in the new dam to be designed. Table 3 shows a comparison between the basic design parameters of the original design and those recomended for the new tailings dam to be built with the same tailings sand.

Table 3. Comparison of Key Geotechnical Properties Parameter Original Revision Design Relative Density (%) 59 60 Friction Angle (º) 35-35.7 35 CRR (*) 0.21 0.24 (*) Cyclic Resistance Ratio

4 HEIGHTENING OF LOS QUILLAYES TAILINGS DAM The original design considered a final maximum height of the dam of 175 m corresponding to a capacity of 257 millions tons of dry tailings and approximately 7 years of operation at an average production of 114,000 tons of mineral per day. Considering the satisfactory results of the investigation of Los Quillayes and adequate performance of the dam and in order to gain time to optimize the location and design of the second tailings deposit, Minera los Pelambres decided to study the feasibility of increasing the capacity of Los Quillayes tailings deposit to 360 millions tons i.e. heightening the dam to a maximum height of 198 m, thus deferring the change to a new deposit in approximate 3 years. For this heightening in general the same design has been considered but adapting the basal drains to the new production level. The axis of the dam has been slightly shifted near the right abutment in the downstream direction in order to avoid a small depression inside the deposit area. Preliminary pseudo-dynamic stability analysis have confirmed the feasibility of this heightening but nevertheless cyclic shear strength tests now on Los Quillayes tailings sands are currently under way at higher confining stresses to

check sand behaviour at those stresses although no significant differences with the existing data is expected. On the other hand (if necessary) there is room for many adaptations considering the conservative present design of Los Quillayes dam. From the environmental point of view this heightening of the existing dam is quite positive in the sense that there is not significant new impact on the area of the deposit, the water diversion scheme is mantained but the regulatory capacity of the impoundment to control extreme floods has been increased significantly as well as the capacity of the structures for seepage control (aditional cut off and higher capacity of the seepage water collection pond).

5 CONCLUSIONS The perfomance of Los Quillayes tailings dam after 3 years of operation has been quite satisfactory with geotechnical properties of the placed sand meeting quite closely pre-design assumptions and without showing weak zones in the layered embankment for a high volume of 1,000,000 m3 of tailings sand deposited and compacted monthly,. The very low levels of pore water pressure registered up to now, practically at the level of the alluvial soil foundation, complete this positive picture. Despite the unprecedent high dam raising rate, the difficulties in the sand placement at the beginning of the operation were overcome at very low cost through additional discharge points and the enlargement of the area of sand placement complying with the geotechnical properties of the design. The experience with Los Quillayes tailings dam shows that hydraulic sand dam adequately designed following the general concepts associated to the practice of the large mines in the last two decades and properly operated could be easily adapted to accommodate constraints or height increase.

ACKNOWLEDGMENTS The authors thanks Compañía Minera los Pelambres for their support and for permitting the publication of the information on Los Quillayes tailings deposit of their property. The authors are very gratefull to engineers José Luis Lara and Edgar Bard of Arcadis Geotecnica who helped with the collection of information and review of the paper.

REFERENCES Barrera, S., Lara, J.L. (1998) “Geotechnical Characterization of Cycloned Sands for the Seismic Design of Tailings Deposit”, 3rd International Congress on Environmental Geotechnics, Lisboa, Portugal Garga, V. K., Castilla-Camacho, J. E. (1995) “Embankments Dams”, X Panamerican Conference of Soil Mechanics and Foundation Engineering, General Report Theme V, Guadalajara, Mexico Lara, J. L.; Gómez, P. y Barrera, S., (1999) “Análisis Dinámico Tridimensional de la Presa de Arena de Relaves Quillayes Pelambres-Chile”, XI Conferencia Panamericana sobre Mecánica de los Suelos e Ingeniería Geotécnica (XI CPMSIG), Foz do Iguaςu, Brasil Makdisi, F.I., Seed, H.B.(1978) “Simplified Procedure for Estimating Dam and Embankment Earthquake-Induced Deformations”, Journal of the Geotechnical Engineering Division, ASCE, Vol. 104 Martin, G.R., Finn, L. And Seed, H.B. (1975) “Fundamentals of Liquefaction under Cyclic Loading”, Journal of the Geotechnical Engineering Division, May, pp. 423-438. Obilinovic, H., Barrera, S. (1982) “Dynamic Analysis of the North Talabre Tailings Dam”, First Chilean Conference of Geotechnical Engineering, Santiago, Chile Swaisgood, J. (2002) The Investigation and Evaluation of Quillayes Dam”, Technical Visit, High Density & Paste 2002, Santiago, Chile Valenzuela, L. (1996) “Characteristics of Chilean Tailings Dams”, Large Dams in Chile. Edited by ICOLD.

Valenzuela, L., Barrera, S. (1995) “Large Tailings Dams Current Practice in Chile”, X Panamerican Conference of Soil Mechanics and Foundation Engineering, Guadalajara, Mexico Verdugo, R., (1983) “Influencia del Porcentaje de Finos en la Resistencia Cíclica de Arenas de Relaves”, Civil Engineer Thesis, Catholic University, Chile.