Presentacion 022.pdf

7/25/2019 Presentacion 022.pdf

1/52

GEOMECNICA APLICADA EN MINERA

1CONCEPTOS GEOMECNICOS FUNDAMENTALES

DETERMINACION DE LOSMODULOS ELASTICOS.

Lambe & Whitman (1969)SOIL MECHANICS

J. Wiley & Sons


2/52



RELACIONES ENTRE LOSMODULOS ELASTICOS.

Hunt (1984)


3/52



MODULOS DINAMICOS

Hunt (1984)


4/52



Goodman (1989)Lambe & Whitman (1969)


5/52



Calculo de las Propiedades de la Roca Intacta:

(1) Realizar ensayos de compresinm uniaxial (5 a 10) para

determinar UCS y los mdulos elsticos E y .

(2) Realizar ensayos triaxiales para un mnimo de 5 presiones de

confinamiento, y de modo que se alcance ewl 40% al 50% de

UCS. Se recomienda repetir a lo menos una vez cada ensayo

(o sea 2 ensayos x cada presin de confinamiento).

(3) Utilizar estos resultados para determinar los parmetros del

criterio de Hoek-Bown. Se recomienda emplear el software

ROCDATA y usar el mtodo simplex. Deber verificarse que

los resultados son razonables (e.g. mi < 36).


6/52



ESTRUCTURASY SUS

PROPIEDADES


7/52



PARAMETROS GEOMETRICOS

MANTEO

DIRECCION DE MANTEO

TRAZA O EXTENSINESPACIAMIENTO

GAP


8/52



Mquina de corte directo fija en laboratorio (tomada

de Franklin & Dusseault (1989)).

Mquina de corte directo porttil (tipo Hoek, tomada de

Franklin & Dusseault (1989)).

Ensayo de corte directo in situ sobre planos de

estratificacin, en un talud de reservorio en Grecia

(tomada de Franklin & Dusseault 1989)). Esquema del montaje tpico de un ensayo de corte

directo in situ (tomada de Franklin & Dusseault (1989)).


9/52



Montaje para la ejecucin

de ensayos de cortedirecto sobre estructurascon un rea expuesta deunos 400 cm2.


10/52



Estructura despus del ensayo.Estructura antes del ensayo.


11/52



RESIS

TENCIA

RESID

UAL

RESISTE

NCIA

PEAK

cpeak

peak

res

n

CONDICION PEAK

CONDICION RESIDUAL

Curva carga-deformacin

para un valor dado del es-fuerzo normal efectivo.

u

cres

RESIS

TENCIA

RESID

UAL

RESISTE

NCIA

PEAK

cpeak

peak

res

n

CONDICION PEAK

CONDICION RESIDUAL

Curva carga-deformacin

para un valor dado del es-fuerzo normal efectivo.

u

cres

RESISTENCIA


12/52



MTODO DE BARTON-BANDIS:

MAX = tan( b + JRClog(JCS/))

MAX = tan( equiv )

MAX RESISTENCIA AL CORTE ESFUERZO NORMAL EFECTIVOb ANGULO BASICO DE FRICCION (b r)

JRC COEFICIENTE DE RUGOSIDADJCS RESISTENCIA EN COMPRESION UNIAXIAL

DE LA PARED DE LA ESTRUCTURA


13/52




14/52



MTODO DE BARTON-BANDIS:

equiv 70

0.01 /JCS 0.30

ESTRUCTURAS SIN RELLENO

ESTRUCTURAS SIN DESPLAZAMIENTO PREVIO


15/52



EFECTO DE ESCALA EN LA RESISTENCIA AL CORTE DELAS ESTRUCTURAS.


16/52



EL AUMENTO DE LA EXTENSIN DE LA ESTRUCTURA PRODUCE TRES EFECTOS

PRINCIPALES: REDUCE LA RUGOSIDAD, REDUCE LA DILATANCIA, E INCREMENTA ELDESPLAZAMIENTO NECESARIO PARA MOVILIZAR LA RESISTENCIA PEAK.


17/52



EFECTO DE ESCALA EN EL PARMETRO JRC


18/52



EFECTO DE ESCALA EN EL PARMETRO JCS


19/52



0.1 1 10 100 1000 10000 100000

EXTENSION DE LA DISCONTINUIDAD, L (m)

15

20

25

30

35

40

45

50

55

ANGULOD

E

FRICCION

(grados)

LA SALBANDA ARCILLOSASE HACE MUY IMPORTANTELA SALBANDA ARCILLOSASE HACE MUY IMPORTANTE

Efecto de escala en el valor peak del ngulo de friccin de estructuras de distinta extensin,conforme con lo valores reseados por Pusch (1997).


20/52



PROPIEDADES TIPICAS

JointsJoints c = 75 a 150 kPac = 75 a 150 kPa = 30= 30oo a 35a 35

Joints en Roca ArgilizadaJoints en Roca Argilizada c = 25 a 100 kPac = 25 a 100 kPa = 22= 22oo a 30a 30

Fallas con Salbanda ArcillosaFallas con Salbanda Arcillosa c = 0 a 50 kPac = 0 a 50 kPa = 18= 18oo a 25a 25

Zonas de FallaZonas de Falla con Salbandacon Salbanda c = 25 a 75 kPac = 25 a 75 kPa = 20= 20oo

a 30a 30y Rocay Roca BrechizadaBrechizada


21/52



CARACTERIZACIN& PROPIEDADES

DEL MACIZO ROCOSO


22/52



EL PROBLEMA ES DEFINIR UNA CALIFICACINDE LA COMPETENCIA DEL MACIZO ROCOSO QUEPERMITA EL ESCALAMIENTO:

Prop. Macizo Rocoso = Fact. Escala Prop. R. I.

RQD

FFRMR (Bieniawski)

Factor de EscalaRMR (Laubscher)QGSI


23/52



Ejemplo 04

Modo de Clculo del RQD(Deere (1989))


24/52



Indice RMRBieniawski (1989)


25/52



Indice RMRLaubscher (1996)


26/52



GEOLOGICAL STRENGTH INDEX

The strength of a jointed rock mass depends on the properties of theintact rock pieces and also upon the freedom of these pieces to slide

and rotate under different stress conditions. This freedom is controlledby the geometrical shape of the intact rock pieces as well as thecondition of the surfaces separating the pieces. Angular rock pieceswith clean, rough discontinuity surfaces will result in a much strongerrock mass than one which contains rounded particles surrounded by

weathered and altered material.

The Geological Strength Index (GSI), introduced by Hoek (1994) andHoek et al. (1995) provides a system for estimating the reduction inrock mass strength for different geological conditions.

This system is presented in Table 3, for blocky rock masses, and Table4 for schistose metamorphic rocks.


27/52



Table 3:Characterisation of a blocky rock masseson the basis of particle interlocking anddiscontinuity condition.After Hoek, Marinos and Benissi (1998).


28/52



Table 4:Characterisation of a schistose metamorphicrock masses on the basis of foliation anddiscontinuity condition.(After M. Truzman, 1999).


29/52



AL CALIFICAR LA COMPETENAL CALIFICAR LA COMPETEN--

CIA DEL MACIZO ROCOSO ESCIA DEL MACIZO ROCOSO ESPRECISO CONSIDERAR UNPRECISO CONSIDERAR UN RANRAN--GOGO DE VALORES, YA QUEDE VALORES, YA QUE DIFIDIFI--CILMENTECILMENTE ESTAESTA CORRESPONCORRESPON--DERADERA A UN SOLO VALOR.A UN SOLO VALOR.


30/52



GENERALIZED HOEK-BROWN CRITERION

, are the maximum and minimum efective stresses atfailure

is the value of the Hoek-Brown parameter m for therock mass

, are constants which depend upon the rock mass cha-

racteristics

is the uniaxial compressive strength of the intact rockpieces

a

ci

bci sm

++=

''' 331

'1

'3

ci

bm

a s

(1)


31/52



Eq. (1) can be used to generate a series of triaxial test values,simulating full-scale field tests, and a curve fitting process can be usedto derive an equivalent Mohr envelope given by:

, are material constants

is the normal effective stress

is the tensile strength of the rock mass

'

n

tm

A B

B

ci

tmn

ciA

=

'(2)


32/52



In order to use the Hoek-Brown criterion for estimating the strength ofjointed rock masses, three properties of the rock mass have to beestimated:

(1) The uniaxial compressive strength of the intact rockpieces

(2) The value of the Hoek-Brown constant for these intactrock pieces

(3) The value of the Geological Strength Index GSI for therock mass

ci

im


33/52



The Hoek-Brown failure criterion, which assumes isotropic rock androck mass behaviour, should only be applied to those rock masses inwhich there are a sufficient number of closely spaced discontinuities,with similar surface characteristics, that isotropic behaviour involving

failure on multiple discontinuities can be assumed. When the structurebeing analysed is large and the block size small in comparison, therock mass can be treated as a Hoek-Brown material.

Where the block size is of the same order as that of the structure being

analysed or when one of the discontinuity sets is significantly weakerthan the others, the Hoek-Brown criterion should not be used.

In these cases, the stability of the structure should be analysed byconsidering failure mechanisms involving the sliding or rotation of

blocks and wedges defined by intersecting structural features. Figure2 summarises these statements in a graphical form.



34/52



Intact RockSpecimensUSE EQ. 3

One Joint Set

DO NOT USEHB CRITERION

Many JointsUSE EQ. 1

WITH CAUTION

Heavily Jointed Rock MassUSE EQ. 1

Two Joint Sets

DO NOT USEHB CRITERION

Figure 2:Idealised diagram showing thetransition from intact to a heavily

jointed rock mass with increasingsample size.



35/52



Once the Geological Strength Index has been estimated, theparameters that describe the rock mass strength characteristics, arecalculated as follows:

= 2814

100

exp a

GSI

mm ib

= 96

100

expo0 a

GSI

s

200

65.0o0.5

GSI

a =



36/52



For better quality rock masses (GSI > 25), the value of GSI can beestimated directly from the 1976 version of Bieniawskis RMR, with the

groundwater rating set to 10 (dry) and the adjustment for joint orientationset to 0 (very favourable). If the 1989 version of Bieniawskis classificationis used, then GSI = RMR89 - 5 where RMR89 has the groundwater rating setto 15 and the adjustment for joint orientation set to zero.

For very poor quality rock masses the value of RMR is very difficult toestimate and the balance between the ratings no longer gives a reliablebasis for estimating rock mass strength. Consequently, Bieniawskis RMRclassification should not be used for estimating the GSI values for poor

quality rock masses (RMR < 25) and the GSI charts should be useddirectly.



37/52



DEFORMATION MODULUSSerafim and Pereira (1983) proposed a relationship between the in situmodulus of deformation and Bieniawskis RMR. This relationship is basedupon back analysis of dam foundation deformations and it has been found

to work well for better quality rocks. However, for many of the poor qualityrocks it appears to predict deformation modulus values that are too high.

Based upon practical observations and back analysis of excavationbehaviour in poor quality rock masses, the following modification to

Serafim and Pereiras equation is proposed for:

= 40

10

10100

GSI

ci

mE

(12)



38/52



Figure 5: Deformation modulus versus Geological Strength Index GSI.

Geological Strength Index GSI

0 10 20 30 40 50 60 70 80 90 100

DeformationmodulusE-GPa

0

20

40

60

80

100

120

140

160

180 ci = 100 MPa

ci = 50 MPa

ci = 30MPa

ci = 15 MPa

ci = 10 MPa

ci = 5 MPa

ci = 1MPa



39/52


Note that GSI has been substituted for RMR in this equation and that themodulus E

mis reduced progressively as the value of falls below 100.

This reduction is based upon the reasoning that the deformation of betterquality rock masses is controlled by the discontinuities while, for poorerquality rock masses, the deformation of the intact rock pieces contributesto the overall deformation process.

Based upon measured deformations, eq. 12 appears to work reasonablywell in those cases where it has been applied. However, as more fieldevidence is gathered it may be necessary to modify this relationship.



40/52


MODULO DE DEFORMABILIDAD:

E = ESEISMIC (Deere et al. (1967)).

E = 2RMR 100 (RMR > 50, Bieniawski (1978)

E = 10((RMR 10)/40) (Serafim & Pereira (1983))

EMIN = 10log(Q)

EMEAN = 25log(Q) (Barton (1983))

EMAX = 40log(Q)



41/52


1.0

0.8

0.6

0.4

0.2

0.01.00.80.60.40.20.0

VFIELD/ VLAB , RQD



42/52


STRESS RELAXATIONWhen the rock mass adjacent to a tunnel wall or a slope is excavated, arelaxation of the confining stresses occurs and the remaining materialis allowed to expand in volume or to dilate.

This has a profound influence on the strength of the rock mass since,in jointed rocks, this strength is strongly dependent upon theinterlocking between the intact rock particles that make up the rockmass.

As far as the authors are aware, there is very little research evidencerelating the amount of dilation to the strength of a rock mass. One setof observations that gives an indication of the loss of strengthassociated with dilation is derived from the support required to

stabilize tunnels. Sakurai (1983) suggested that tunnels in which thestrain, defined as the ratio of tunnel closure to tunnel diameter,exceeds 1% are likely to suffer significant instability unless adequatelysupported.



43/52


This suggestion was confirmed in observations by Chern et al. (1998)who recorded the behavior of a number of tunnels excavated inTaiwan.

They found that all of those tunnels that exhibited strains of greaterthan 1 to 2% required significant support. Tunnels exhibiting strainsas high as 10% were successfully stabilized but the amount of effortrequired to achieve this stability increased in proportion to the amountof strain.

While it is not possible to derive a direct relationship between rockmass strength and dilation from these observations, it is possible toconclude that the strength loss is significant.



44/52


An unconfined surface that has deformed more than 1 or 2% (basedupon Sakurais definition of strain) has probably reached residualstrength in which all of the effective cohesive strength of the rockmass has been lost.

While there are no similar observations for rock slopes, it is reasonableto assume that a similar loss of strength occurs as a result of dilation.

Hence, a 100 m high slope which has suffered a total crest displace-

ment of more than 1 m (i.e. more than 1% strain) may start to exhibitsignificant signs of instability as a result of loss of strength of the rockmass.



45/52


BLAST DAMAGE

Blast damage results in a loss of rock mass strength due to thecreation of new fractures and the wedging open of existing fractures by

the penetration of explosive gasses.

In the case of very large open pit mine blasts, this damage can extendas much as 100 m behind the final row of blast holes.

In contrast to the strength loss due to stress relaxation or dilation,discussed in the previous section, it is possible to arrive at anapproximate quantification of the strength loss due to blast damage.

This is because the blast is designed to achieve a specific purpose

which is generally to produce a fractured rock mass that can beexcavated by means of a given piece of equipment.



46/52


Figure 6 presents a plot of 23 case histories of excavation by digging,ripping and blasting published by Abdullatif and Cruden (1983). Thesecase histories are summarised in Table 5. The values of GSI areestimated from the data contained in the paper by Abdullatif and

Cruden while the rock mass strength values were calculated assumingan average slope height of 15 m.

These examples shows that rock masses can be dug, obviously withincreasing difficulty, up to GSI values of about 40 and rock mass

strength values of about 1 MPa.

Ripping can be used up to GSI values of about 60 and rock massstrength values of about 10 MPa, with two exceptions where heavyequipment was used to rip strong rock masses.

Blasting was used for GSI values of more than 60 and rock massstrengths of more than about 15 MPa.



47/52


Blasting8685

Blasting11785

Blasting6477

Blasting13577Blasting8477

Blasting5476

Blasting3571

Blasting1569

Blasting1768

Blasting3068

Ripping by D9L bulldozer4267Ripping by D9L bulldozer3367

Ripping by track loader2.458

Ripping by 977L track loader9.557

Ripping by track loader0.851

Digging by 977L track loader1.242

Digging by wheel loader0.540 Digging by hydraulic face shovel0.534


Digging by wheel loader0.225

Digging by hydraulic backhoe0.224

Digging by D9 bulldozer0.119


Excavation MethodRock Mass Strength, CM

( MPa )GSI

Table 5:Summary of methods used to excavate rock masses with a range of uniaxial compressive strength values,based on data published by Abdullatif and Cruden (1983).



48/52


Figure 6: Plot of rock mass strength versus GSI for different excavation methods, afterAbdullatif and Cruden (1983).

Geological Strength Index GSI

0 10 20 30 40 50 60 70 80 90 100

Rockmassstrength

ci-MPa

0.1

1

10

100

Excavation method

Dig

Rip

Blast



49/52


Figure 7 summarizes the conditions for a muckpile that can be dug

efficiently and the blast damaged rock mass that lies between thedigging limit and the in situ rock mass. The properties of this blastdamaged rock mass will control the stability of the slope that remainsafter digging of the muckpile has been completed.

Figure 7: Diagrammatic representation of the transition between the in siturock mass and blasted rock that is suitable for digging.



50/52


The thickness D of the blast damaged zone will depend upon the designof the blast. Based upon experience, the authors suggest that thefollowing approximate relationships can be used as a starting point in

judging the extent of the blast damaged zone resulting from open pitmine production blasting:

D = 0.3 to 0.5 H Carefully controlled poduction blast with a free face

D = 0.5 to 1.0 H Production blast with some control, e.g. one or more buffer rows,

and blasting to a free face

D = 1.0 to 1.2 H Production blast, confined but with some control, e.g. one or morebuffer rows

D = 1.0 to 1.5 H Production blast with control but blasting to a free face

D = 2.0 to 2.5 H Large production blast, confined and with litle or no control



51/52


EN LA PRACTICA SE ESTA UTILIZANDO CADA VEZ MAS EL MTODO DEHOEK & BROWN, CON LAS CONSIDERACIONES SIGUIENTES:

SE DETERMINAN LOS PARAMETROS mi Y ci EN BASE A UNACUIDADOSA INTERPRETACION DE LOS RESULTADOS DE ENSAYOS

TRIAXIALES SOBRE TESTIGOS DE ROCA INTACTA (USUALMENTEUTILIZANDO ROCKDATA).

SE DETERMINA EL RANGO DE VALORES PROBABLES PARA EL INDICEGSI (USUALMENTE 15 A 20 PUNTOS).

SE DETERMINA EL RANGO DE PRESIONES DE CONFINAMIENTO Y SI SETRATA DE UN MACIZO BIEN TRABADO O NO.

SE ESTIMA LA INCERTEZA ASOCIADA A CADA PARAMETRO Y SUPOSIBLE FUNCION DE DISTRIBUCION.

SE EVALUAN LAS PROPIEDADES DEL MACIZO ROCOSO UTILIZANDO LAMETODOLOGIA PROPUESTA POR HOEK (1998,99).



52/52


PROBLEMAS :

EL METODO NO SIEMPRE ES APLICABLE.

SE DEFINE UNA RESISTENCIA ISOTROPICA.

PARA MACIZOS MASIVOS Y COMPETENTES EL METODODEBE APLICARSE EN FORMA FLEXIBLE.

PARA MACIZOS DE MALA CALIDAD GEOTECNICA, POBRE-MENTE TRABADOS Y POCO CONFINADOS EL METODOPUEDE SOBREVALUAR LA RESISTENCIA.

EN EL CASO DE ROCAS ESQUISTOSAS O FOLIADAS EL

METODO DEBE APLICARSE MUY CUIDADOSAMENTE.

Documents

Presentacion 022.pdf