10 criterios de diseño.pdf

Dr Ing Antonio Karzulovic

CRITERIOS DE DISEO 1

CRITERIOS DE DISEO


Julio 2006



Geometra de los Taludes en Minas a

Rajo Abierto



GEOMETRIA DE LOS TALUDES DE UNA MINA A RAJO ABIERTO

r

r o

ho

hr

r

b

h



erh

s

q

b

p

= inclinacin de la cara del banco

= inclinacin media de la cara del banco doble

= ngulo interrampa (de pata a pata)

= altura del banco simple

= salto por perforacin (igual a 0 en el caso de bancos simples)

= quebradura

= ancho de berma (nominal)

= distancia horizontal pata a pata

Geometra del Sistema Banco-Berma




b

h

qp

rGeometra Banco-Berma

(bancos simples)




Geometra Banco-Berma(bancos dobles)

2h

qp

r

b

s

e




h q tan=

Banco Simple:

q b p +=

+= q b h tana r

Banco Doble:

+

= tan s 2h

tan2h atan e

h2 q

etan=

q b p +=

+= q b h tana r

Relaciones Geomtricas




Consideraciones Generales



Brown (2004)

Evolucin de la profundidad en minas a rajo abierto

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1880 1900 1920 1940 1960 1980 2000 2020 2040

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m

)

Chuquicamata

Palabora

Escondida

Collahuasi

Grasberg

Bingham

QuestaKoffiefonteinPremier

Stobie

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F

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Kidd CreekAtalayaKiruna

Mount IsaPyhasalmi

Craigmont



The size of the mining production equipment has also increased. In 1970 the capacity of shovels and front loaders was 15 and 10 yd3, and the truck payload was 50 tons; while today there are 80 yd3 shovels, 37 yd3 front loaders and 360 ton trucks.

Also the production drilling has evolved from using a 61/2 in diameter, in the seventies, to the 105/8 and 121/4 in diameters commonly used today in large open pits. Larger mining equipment allows larger production blasts, but it makes difficult to achieve well-groomed slopes and increase the blast-induced damage in the rock mass. All this and a decreasing trend in the price of most metals, makes the slope design a key factor for the open pit mining business.

Hence, any optimization is related to the feasibility of achieving steeper slopes in pits whose depth in many cases will exceed 800 m. This can be achieved only by sound geological and geotechnical engineering, a team work focused on slope monitoring and management, and accepting the occurrence of some failures through the open pit operating period.

Karzulovic (2004)



Bench height defined by the characteristics of drilling and loading equipment.

Bench face inclination depends on the structural pattern and the quality of the blasting.

Berm width defined to contain the material from bench-scale failures.

This bench-berm system defines the maximum possible interramp angle, which for the purpose of mine planning is defined from bench toe to bench toe and does not depend on the number of benches.

Ramp width usually defined considering the characteristics of the trucks.

Interramp slope stability is evaluated considering the maximum interramp height to define the interramp angle. If this value is lower than the one resulting from the bench-berm geometry it becomes necessary to increase the berm width and/or reduce the maximum interramp height.

Once the interramp slopes have been defined the overall slope is evaluated. If required, the overall angle is reduced by increasing the ramp width and/or decreasing some interramp angles and/or adding ramps.

Karzulovic (2004)



There are two main factors for designing an open pit slope: the bench-berm geometry, and the interramp slopes. The bench-berm geometry defines the interramp angle; and it should be such that the mine operation can achieve the required bench face inclination, and the berm width should be wide enough to contain the debris from bench-scale failures, guarantying a safe mine operation.

The interramp slopes not only define the stability of the overall slope, but also the availability of ramps. Indeed, loosing a ramp due to an interramp slope failure can have a larger impact on the mine operation than an overall slope failure in a sector without ramps.

In addition to a good characterization of the rock mass and its geological structures, the geotechnical design of a pit slope also requires: an assessment of the quality of the mine operation in connection with blasting (blast induced damage) and loading (shovel induced damage), the analysis of different types of failure, the evaluation of their possible consequences and impact on the mine plan, and some acceptability criteria defining which is permissible regarding the degree of stability of the different pit slopes, commonly defined in terms of the minimum permissible factor of safety, FS, and/or the maximum permissible probability of failure, PF.

Karzulovic (2004)



Consideraciones Operacionales



LO BUENO....



LO MALO....



Y LO FEO.



Criterios de Aceptabilidad



To define if a slope design is acceptable an acceptability criteria is required. In civil engineering slopes it is common to consider a minimum permissible value of 1.5 for the factor of safety, and in many pit slopes a value of 1.3 has been considered the minimum acceptable. This numbers seem to have worked in the past, but they give no indication on the uncertainties in the input data, the reliability of the design or the possible consequences of slope failure, making very difficult to decide rationally which is the best option for the open pit mining business from a series of pit slope designs.

Open pit mining business looks for steeper slopes, but at the same time it requires a safe operation and that the consequences of an eventual pit slope failure do not exceed the possibilities of the mining business. Hence, the questions are:

What types of slope failures can affect the pit?

Which is the probability of occurrence of a certain type of pit slope failure?

What are the possible consequences of such pit slope failure?

Can the mine afford those consequences?

Karzulovic (2004)



( )1yProbabilit = FSPF( )esConsequencCostPFR == Risk

Accidents with time loss, ATLEconomical losses, ELUnacceptable economical losses, UEL

and defines maximum acceptable probabilities for each one of these classes. In the case of accidents with time loss, the probability should low enough to fulfill the best international standards on safe mine operation. The definition of the maximum permissible probabilities for economical losses and unacceptable economical losses is not a geotechnical but a corporative business decision.

Karzulovic (2004)



Once these maximum acceptable probabilities are defined, the model uses fault and event trees to determine the resulting probabilities of suffering accidents with time loss, economical losses and unacceptable economical losses for a pit slope failure with a given failure probability of occurrence. Hence, if PFATL, PFEL and PFUEL are the values of PF such that results in the maximum acceptable probabilities for ATL, EL and UEL, the maximum permissible value of PF is given by:

{ }UELELATL PFPFPFPF , , min =

Karzulovic (2004)



Optimizacin del Negocio Minero



The concepts previously discussed have been successfully applied to optimize the mining business of some open pit operations. A good example is the case of Collahuasis Ujina pit in northern Chile, where the pit slope design of the feasibility study was optimized by defining an acceptability criterion that accepted the occurrence of slope failures, and redesigning the pit slopes according to these criteria. The acceptability criteria were defined considering that:

There are two access ramps to the pit bottom.

The final pit wall would be achieved using controlled blasting techniques, including pre-splitting, in order to minimize the blast induced damage.

A slope drainage and depressurization system would be implemented, in accordance with the recommendations of the hydrogeological study.

There are no in pit infrastructure, and all surface infrastructure is far away from the pit.

The mine will maintain an ore stockpile of at least 1,000,000 tons.

The mine operation will include 56 to 73 yd3 shovels, 28 yd3 front loaders, and 240 to 360 ton trucks.

The debris resulting from an eventual pit slope failure should be removed in no more than one week.

Karzulovic (2004)



Considering all this, the maximum permissible failure tonnage was defined as 800,000 tons.

For failures affecting tonnage or smaller ones, a probability of failure of 30% was considered as the maximum acceptable.

For larger failures the following acceptability criteria were defined:

If the failure does not occur in a critical location, then:FS 1.25 and PF 12%.

If the failure does occur in a critical location, like a ramp switch back, then:FS 1.45 and PF 4%.

Karzulovic (2004)



Using these criteria the pit slopes were redesigned, which resulted in steepening the interramp angles from 6o to 15o in respect to the design of the feasibility study. This steepening improved the net present value of Ujina in about US$ 100,000,000, considering that the mine operation would last 8 years and taking into account the costs of implementing the controlled blasting and slope dewatering and depressurization programs.

Ujina pit began its operation in June 1996, and its operation ended in August 2004. During these years there have been several slope failures at Ujina:

Karzulovic (2004)



January 1998: 3 benches (45 m) in the SW wall suffered a slope failure affecting about 80,000 tons. The probable cause was the presence of a geological fault.

April 1998: 4 benches (60 m) in the SW wall suffered a slope failure affecting about 120,000 tons. The probable cause was the presence of argillic and poor quality rock at the slope toe.

July 1998: 5 benches (75 m) in the SW wall suffered a slope failure affecting about 800,000 tons. The probable cause was the presence of a geological fault.

Karzulovic (2004)



Failure of 5 benches (75 m) in the SW wall of Ujina pit, affecting about 800,000 tons.July 1998

Karzulovic (2004)



December 1998: 5 benches (75 m) of the West wall suffered a slope failure affecting about 180,000 tons. The probable cause was the presence of a geological fault.

July 1999: 3 benches (45 m) in the SE wall suffered a slope failure affecting about 80,000 tons. The probable cause was the presence of a geological fault, and poor quality rock at the slope toe.

July 1999: 2 benches (30 m) in the SW wall suffered a slope failure affecting about 30,000 tons. The probable cause was the presence of a geological fault, and poor quality rock at the slope toe.

July 1999: 2 benches (30 m) in the SE wall suffered a slope failure affecting about 80,000 tons. The probable cause was the presence of a geological fault, and poor quality rock at the slope toe.

February 2000: 6 benches (90 m) in the West wall suffered a slope failure involving about 800,000 tons. The probable cause was the presence of poor quality ignimbrite dipping towards the pit, and this was aggravated by intense rains and an earthquake.

Karzulovic (2004)



400 m

120 m

400 m

120 m

Sector affected by cracks in the upper part of the West wall of Ujina pit.August 1999.

Karzulovic (2004)



Failure of 6 benches (90 m) in the West wall involving about 800,000 tons. The probable cause of this failure was the presence of poor quality ignimbrite dipping towards the pit, and this was aggravated by intense rains and an earthquake.February 1999

Karzulovic (2004)



Mining equipment working on the removal of the broken rock, and changing the slope geometry in the sector affected by the slope failure.

Karzulovic (2004)



West wall of Ujina pit after the stabilization of the sector affected by the slope failure of February 2000.

Karzulovic (2004)



March 2000: The South wall showed signs of instability, and a failure could affect the access ramp. The remedial measures included the removal of 500,000 tons (Pushback 3) and 100,000 tons (Pushback 4).

May-October 2002: The nose defined by the Phases East and South of Ujina pit (which has been analyzed using Flac3D) showed cracking and signs of instability. This behavior was expected and, after a careful geotechnical inspection, it was decided that any failure will be too slow to affect the mine operation and remedial measures were taken, except the periodic removal of spilled material.

Karzulovic (2004)



Three-dimensional model of the nose defined by the Phases East and South of Ujina pit, developed using Flac3D by Mendez & Lorig (1999).

Karzulovic (2004)



Plan view showing the displacement contours resulting from the Flac3D model. The nose height is 240 m, and the maximum displacement is 1.25 m (from Mendez & Lorig (1999)).

Karzulovic (2004)



S

E

BENCH 4270

Overall view of the nose defined by the Phases East and South of Ujina pit, northern Chile, in 2002.

Karzulovic (2004)



Cracking and loosening of the nose defined by the Phases East and South of Ujina pit (which has been analyzed using the Flac3D model. This behavior was expected and, after a careful geotechnical inspection, it was decided that any failure will be too slow to affect the mine operation.

Karzulovic (2004)



March 2003: 2 benches (30 m) in the East Wall suffered a slope failure involving about 40,000 tons. The probable cause was the presence of poor quality paleogravels overlying argillic rocks.

October 2003: 3 benches (45 m) in the East Wall suffered a slope failure involving about 140,000 tons. The probable cause was the presence of structures and poor quality argillic rocks.

Karzulovic (2004)



In all these cases instability signs were detected before the failure, and the measures required to maintain a safe mine operation were opportunely implemented. These good results have been achieved thanks to a team work including the geotechnical, mine planning and mine operation groups.

The cumulative cost of all these failures is about US$ 7,000,000. Therefore, rock slope engineering allowed in an eight-year period savings of more than US$ 90,000,000 at Ujina, optimizing Collahuasis mining business.

Karzulovic (2004)



FIN

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