Santa Maria tailings dam

A continuous improvement process.Luis Haro Chavez *PROESMIN S.A.C., Lima, Peru.

ABSTRACTDue to the impossibility of having an area authorized for a new tailings deposit, it was decided to run a current deposit rising building a dike in section such caissons, used loan material would mean the tenth part of what originally had been calculated for an Earth Dam. This difference is based on gabion dam is an armed structure and non-gravity.Then, a second extension, which should ensure an additional capacity of 2 years, time that it takes us the empowerment of a new deposit was conceived. Conditions of design that ensure us the stability of the deposit, we decided to apply the theory of reinforced soil, which allows us to use the tailings as material for the creation of the dam.As all soil, the tailings has compressive strength but not to shear stress, then adding an element capable of absorbing these efforts, both in the tough area of the deposit, were used with geotextile lined gabions, filled with compacted tailings on geogrid. This feature gives the structure the typology of "Soil or reinforced tailing". This method has allowed us a savings of US$ 400,000, reducing costs of provision of US$ 4.00/ton to US$ 0.95/ton, also high initial investment is not required because the costs are distributed throughout the life of the project, is not disturbed the environment as is no longer necessary for loan material quarrying, and the non-use of equipment and machinery to minimize negative impacts on the environment.           

 * Engineering Manager PROESMIN S.A.C., Calle 24 No. 141-301 URB Corpac, Lima, Perú. Phone: + 51 1 2244071. Email: lharoch@tse.pe INTRODUCTIONOne of the activities that became important in our operations, with the aim of lowering costs and be more competitive, it was the disposal of tailings from the Plant Santa Maria, for the following reasons:

It should be possible to build a regrowth of the tank, because the current was saturated, i.e. the time factor was paramount.

The original design of the reservoir was with the method of downstream, i.e. with an earth dam of seriousness which should overlay under this same modality where the constraint against was the impossibility of obtaining quarry for borrow material (70.000 m3), the factor materials was critical.

The availability of materials and machinery for the construction of this regrowth was low; the cost factor also was enforceable.

Then of techno-economic appraisals, it was decided to run the tank lift on  building a dam in gabions of 5.00 m in height and of section such that the borrow material (in this case 7.000 m3 of stone for the filling of the gabions) meant the tenth part of what was originally calculated for the earth dam. The reason for this difference is based on the dike of gabions is an armed structure and not gravity, in addition to its use is immediately and lets us grow the deposit under  remade of sludge.Then, and given the need for a new tank of tailings, and that, because of financial problems prevented the empowerment to make a timely new area for the construction of this work, it was decided to build a second regrowth of the deposit, which should ensure a capacity of 4 years, time that we delay the empowerment of a new tank. This regrowth with a dike would be 20.00 m high, executed with gabions filled with sludge on reinforcement of geonets. (See Illustration 2).For constraints on design and have secured the obtaining of safety factors that support us the stability pseudo-static and post-liquefaction of the structure, we decided to apply the theory of soil armed, i.e. using the same sludge as material that formed the rugged area of the tank, what was I going to allow us not rely on equipment, machinery or borrow material for the formation of the perimeter levee. It was decided to incorporate within the tailing a material capable of absorbing the efforts to drive or to the court that is going to be subjected this deposit, which was a geogrid high resistance (200 kN/m2) as well as in the area of the dike resistant use gabions, which allow for an input necessary cohesion. The gabions, previously lined with geotextile, were filled with the same material compacted sludge. These features provide the structure the typology of "Soil Armed". 

Figure 1: Schematic placement gabions and geogrid

We can conclude that this system of disposal of sludge has allowed us a savings on the actual execution of the work of US$ 400.000 , lowering our costs of disposal of US$ 4.00 /ton to US$ 0.95 /ton, in addition they are not running high initial investment because the costs are spread out throughout the life of the project, will not disturb the environment because it is no longer necessary quarrying for borrow material, and the non-use of equipment and machinery minimizes the adverse impacts on the environment.

METHODOLOGYDESIGN OF THE REGROWTH OF THE SANTA MARIA TAILINGS DAM.

LOCATION OF THE PROJECTThe project area is located in a place called Santa Maria, in the district and province of Pataz, department of La Libertad.The tailings deposit is located at the foot of a hillside on a terrace fluvio-alluvial and colluvial, it is of the type downstream and is formed by a dam of land of 1500 m of height, occupies approximately 3 hectares and is 70.00 m downstream of the plant, which is in the dimension 2.500 masl; and the tailings deposit develops between dimensions 2.425 and 2.470 meters above sea level.

SEISMIC RISK ASSESSMENTWe evaluated the seismic risk using statistical and probabilistic methods. The maximum accelerations, have been calculated using the formulas of attenuation of Casaverde and Vargas (1980) for earthquakes whose origin is the subduction, while for earthquakes that are associated with continental failures we used the formula of McGuire (1974). As a result of these analyzes were found peak accelerations for different periods of return with a probability to exceed 5 %.In the case of a pseudo-static analysis, the use of a maximum horizontal acceleration, is too conservative, since its occurrence is timely, in addition the slopes of soil or land

are not rigid elements, so the acceleration to be used should be much lower. On this particular the accelerations of basic design, should be applied coefficients between 1/3 and 1/2 of the values of the maximum acceleration.According to this, the design acceleration for a return period of 500 years, will be:

a design = 0.14 g

GEOTECHNICAL INVESTIGATIONS IN THE AREA OF THE PROJECTThe geotechnical investigations carried out in the field and laboratory have as objective to determine the parameters of resistance from the tailings fine to evaluate the possibility of occurrence of the phenomenon of liquefaction in the tank for a return period of 150 years, and defined within the framework of the existing devices, the type and geometry of the regrowth that ensures the stability of the tank in static and dynamic conditions or seismic.The sludge is transported from the plant to the tank in the form of pulp with density 1620 kg/m³ with 59.6 % of percentage of solids in average.

The flow of the pulp is 2.4 lts/sec of which 0.83 lts/sec correspond to the solid phase. That is, the flow of water that enters the tank along with the sludge is about 1.57 lts/sec. The treatment plant Santa Maria has a capacity of 200 ton/day.

GEOTECHNICAL INVESTIGATIONS PROGRAMThe geotechnical investigations consisted of three perforations executed through the procedure "wash boring", in each one of which is made to its time of Standard Penetration Tests (SPT), according to the Standard ASTMD-1586 to depth intervals of 1.00 m. In each drilling was used to install a piezometer open tube of PVC of Ø 1 ½ ".

LABORATORY TESTSWith the disturbed and undisturbed samples extracted from the soundings were investigated the following tests:

Determination of the moisture content of ASTM D 2216 Liquid Limit and plastic ASTM 423 and ASTM D 424 Sieve Analysis ASTM D 421 and ASTM 422 Specific gravity of the solid ASTM D 854 Consolidation test ASTM D 2435 Testing of unconfined compressive ASTM 2166 Direct cut test ASTM D 3080

The results of sieve analysis of the sludge correspond to silt and sands with variable content of fine between 64.96 % and 86.94 % by what the sludge could be described as silt.The results of trials by Atterberg limit determined that more than 70 per cent have no plasticity (IP = 0) while the rest can be classified as low plasticity (IP < 30).The results of the tests of particle size distribution and Atterberg limits allow to classify this sludge as a limo of low plasticity (ML).The specific gravity tests indicate that a large part are within the range of 2.77 to 3.07 so that 2.87 is the most representative.

The testing of moisture content values that are within the range of 18.61 % to 23.07 % which are erratic, possibly caused by the method of fitting the tailings in the tank.The testing of direct cut was made for degrees of saturation of 70 % to 80 %. The results indicate resistance values not drained: friction angle Ø = 14.27° and cohesion c = 0,195 kg/cm². There was also a compression test not confined to that on the basis of the results of compression and friction angle is determined by the cohesion not drained and non-consolidated basis.Test the consolidation is to estimate the parameters of compressibility of the sludge caused by the load of on tank lift. The trial was conducted in a sample pre-shaped with 75 per cent of saturation and 1.86 g/cm³ density. The results indicate load values for consolidation, compression rate and rate of expansion.

REGROWTH OF THE TAILINGS DAMIn accordance with the requirements of the Poderosa Mining Company S. A., you need the current overlay to a tank capacity of tailings storage for 48 months.Information with regard to the production of tailings from the Plant Santa Maria is the following:

Production of tailings: 6,000 Ton/month Tailings Density  1.62 Ton/m 3. Minimum reservoir capacity : 48 Months

Tank Capacity required: Production of tailings: 6,000 Ton/month Volume of tailings: 6.000 /1.62 =3.704 m3/month Volume of deposition in 48 months:  48 * 3, 704 =177.792 m3

This volume is related to the appropriate dimensions with the can be projected the regrowth; without affecting its stability by sliding and differential settlements.The alternative,  consider increasing the volume of deposition by a dam upstream of gabions, whose shaft alignment is located approximately 5 m away from the face of the current wall of gabions. This alternative additionally considers the implementation of stabilization work shaped by drain secondary vertical and horizontal; where the first would be located at 12 m upstream of the dam of gabions and the horizontal would be connected to the vertical drains and spaced 3.0 m along the entire length of the alignment. The horizontal drains are connected to a main drain, which vacated the waters outside the tailings deposit. Also in the project have been considered employ geosynthetics, to maintain the stability of the present wall of gabions, and ensure the stability of the tailings deposit that is planned for the new wall of gabions, but filled with sludge. (See Illustration 3)This regrowth involves different activities, the first is the placement of a biaxial geomalla of 200 kN /m to double layer, which will be installed in a width of 28.00 m in the Nv. MASL 2455.50 sector where it is intended to begin on raise the tank using four blocks of 5.0 m high each (I, II, III and IV), with perimeter dikes of 1.0 m high filled with compacted sludge. On the geomalla will be built, the first block of gabions (I) with the same structural fill material  compacted sludge dried up to a height of 5.0 m. This wall will be installed separately to 6.00 m of the gabion placed last in the current repository of tailings (II). The level at which you can reach this first block will be the 2460 for disposal of sludge and the 2460.50 as the crown of this first block, with a length of 224.60 m. 

Then, and with a gap of 6.0 m will begin to lift the second block (II) with similar characteristics, but prior to its execution must be placed a layer of uniaxial geomalla of 200 kN /m in a length of 21.0 m, as shown in Figure 3. This second block will reach the level 2464.50 as level of readiness and the level 2465 as the crown. The length of this block will be 216.94 m.Then, recreciendo will continue the tank using the blocks III and IV in the same provision of gabions of blocks I and II. See pictures 6 and 7.

Figure 2: Typical Section Santa Maria tailings Dam

RESULTS AND DISCUSSIONANALYSIS OF STABILITY Analyzes were performed to verify the internal stability (efforts on the dyke of gabions and geonets) and to verify the external stability (sliding surfaces deep). You get FS > 1.5 for the static conditions and FS > 1.1 for the seismic condition.  

Table 3 Physical Parameters of resistance

MaterialNatural Density (KN/m3)

Saturated

Density (KN/m3)

Cohesion

(KN/m2)

Angle of friction (

º)

Soft sludge (CL-ML) 16 17 15 20.5

Sludge released (silt with sand) 17 18 0 23

Sludge Firm (silt with sand) 18 19 0 30

Material of earth dam (GC/GM) 20 21 30 40

Alluvial Soil (GC/GM) 20 21 20 40

Gabion Wall. 19 20 20 40

Gabions with structural fill material of sludge. 19 20 40 37

The parameters of resistance was been selected based on information from the field tests, laboratory, and the experience in similar projects.The stability analysis was been developed for the section 0+140, the current axis of prey.The stability analysis was been developed using the Slide program v. 6,020. The method of Modified Bishop, was used in the respective calculations, generating up to 400 sliding surfaces, selecting the 10 results with fewer safety factor (see Figure 5)The following tables present a summary of the results of the analysis of stability.

EXTERNAL STABILITY GLOBAL: LONG-TERMTable 1 Global stability analysis results

SECTION SAFETY FACTOR

STATIC SEISMIC0+140 1,687 1,131

Figure 3: Internal Stability Analysis post-liquefaction

As you can appreciate this alternative, has safety factors eligible for external stability in the long term, both static condition as in post-liquefaction.

The results of the stability analysis indicate that the elevation of the tailings deposit Santa Maria with the gabion wall reinforced with geogrids, is stable, both in static conditions and post-liquefaction. In addition, the internal stability complies with the stresses of load analyzed.  

CONCLUSIONThe results of the stability analysis indicate that the elevation of the tailings deposit Santa Maria in its second phase with the wall of gabions filled with sludge, is stable, both in static conditions and post-liquefaction. In addition, the internal stability complies with the stresses of load analyzed.  In consequence this alternative defines to the wall of gabions which will allow the elevation of the tailings deposit in its second phase for an additional period of 24 months.The alternative has also been analyzed considering the wall of gabions with additional height 1.00 m to increase the time of deposition of the tailings. The results obtained indicate that even with the maximum reinforcement with additional layers to the current wall of gabions, obtained safety factors indicate that the overall structure is stable.With respect to the drainage system of the tank, is important a proper management to depress the phreatic level what will allow you to maintain a stable tank during the stage of operation.Finally, this second stage which considers the above lifting of the deposit of tailings through a dam of gabions previously covered with geotextile and filled with tailings, located upstream and built on the fine tailings from the tank, reinforced with high-strength geogrid (200 kN/m) in two layers; they confer resistance by which the respective analysis shed safety factors indicate that the structure is stable, by which such alternative was chosen.

Figure 4: Detail gabions filled with compacted dry tailings.

Figure 5: Santa Maria Tailings Dam

Table 2 Comparative Table costs

STAGEBUDGET

EXECUTEDUS$

CAPACITYSTORAGE

Ton

COSTSTORAGEUS$ /ton

LIFEPROJECT

YearsFIRST

(Original Design) 423.790 95.795 4.33 1.50

SECOND regrowth 107.761 153.414 0.91 2.00THIRD

Regrowth 135.920 97.324 0.94 1.60

REFERENCES "Stability analysis of slopes and embankments", by Sarada K. Sarma.

"Stability coefficients for earth slopes", by Bishop, A. W. and Morgensen, 1960

"Design with geosynthetics", by Robert M. Koerner, 4th Edition.

"Use of geosynthetics to stabilize Powerful Mine tailings", by Dennis Grubb, PhD, Georgia Institute of Technology, August 3, 1998.

Re-evaluation of the physical stability of the tailings deposit Cia. Minera Poderosa", by Association Klohn Crippen-SVS Ingenieros S.A., Main Report and Annexs I-XI, May 1998.

"Design and evaluation of tailings dam", technical report, U.S. Environmental Protection Agency, 1994.

"Designing for Soil Reinforcement", Dr. R. A. Jewell, University of Oxford, 2000.