J. SE Asian Appl. Geol., Jul–Dec 2011, Vol. 3(2), pp. 93-102

GROUND MOVEMENT PREDICTION DUE TOBLOCK CAVING MINING GEOMETRY USINGGIS

Agung Setianto∗1 and Eman Widijanto2

1Geological Engineering Department, Faculty of Engineering, Gadjah Mada University2Rock Mechanics Engineer PT Freeport Indonesia, Timika

Abstract

Large scale block cave mining has been operated forover 30 years in the Erstberg Mining District inthe province of Papua, Indonesia. The ore body isdivided into four vertically stacked ore bodies: Gu-nung Bijih Timur (GBT), Intermediate Ore Zone(IOZ), Deep Ore Zone (DOZ), and Deep Mill LevelZone (DMLZ). The GBT and IOZ mines were closedon 1993 and 2003, DOZ mine is in its peak produc-tion performance 80 ktpd, and DMLZ mine is stillin the development stage to prepare mine infrastruc-tures. This situation generates gradual downwardsettling of the surface or subsidence. Significantdeformation changes at the surface by block cavingsubsidence could damage the mine’s infrastructuresin surface and underground and also affect geologi-cal structures overlying the mining areas which mayresult in surface impacts on the natural geomorphol-ogy and land use.

In this paper, integrated system based on Geo-graphic Information System (GIS) platform appliedto predict ground movements due to undergroundmining. Deep Ore Zone (DOZ) block cave mine isstudied for subsidence prediction. The mining ex-traction thickness model is obtained from height ofdraw (HOD) observed data. Subsidence Engineer-ing Handbook (SEH) of empirical model and mea-sured data from mining fields is used for subsidencecalculation parameters. The calculations were per-

∗Corresponding author: A. SETIANTO, Geological En-gineering Department, Faculty of Engineering, GadjahMada University. Jl. Grafika No. 2 Yogyakarta 55281 IN-DONESIA. E-mail: agung.setianto@gmail.com

formed in GIS. The maximum vertical displacementhas been predicted about 12m by means of full cavingmining method.Keywords: Ground movements, block caving, GIS,underground mining, and subsidence

1 Introduction

Subsidence can be defined as the sudden sink-ing or gradual downward settling of the earth’ssurface with little or no horizontal motion. Themovement is not restricted in rate, magnitude,or area involved. Subsidence may be caused bynatural geological processes, such as solution,thawing, compaction, slow crustal warping, orwithdrawal of fluid lava from beneath a solidcrust; or by human activity, such as subsurfacemining or the pumping of oil or groundwater(American Geological Institute, 1996).

The phenomena of ground movement dueto block caving is an important issue since itwill impact the existing critical mine infras-tructures for both surface and undergroundareas, i.e. electrical poles, orepasses, venti-lation shafts, and surface roads. The Erts-berg East Skarn System (EESS) is a calcium-magnesium silicate skarn (Coutts et al., 1999)which is divided into different extraction levels:Gunung Bijih Timur (GBT), Intermediate OreZone (IOZ), Deep Ore Zone (DOZ), and DeepMill Level Zone (DMLZ). However, the GBTand IOZ block cave was depleted in 1993 and in2003 (Szwedzicki et al, 2004) respectively. Cur-rently, the DOZ block cave is the largest sin-

93

SETIANTO and WIDIJANTO

gle underground block cave mine activity in theworld.

The production level of the DOZ block cave isat 3126 level lies at a depth of about 1200 metersbelow the surface. In DOZ block caving, the fullorebody or an approximately equidimensionalblock of ore is undercut fully to initiate caving.The undercut zone is drilled and blasted pro-gressively and some broken ore is drawn off tocreate a void into which initial caving of theoverlying ore can take place. As more brokenore is drawn progressively following cave initi-ation, the cave propagates upwards through theorebody or block until the overlying rock alsocaves and surface subsidence occurs.

2 DOZ block caving

2.1 Geological information in the vicinity of

DOZ

The geological information map such as faultlines and mineral distributions in vicinity ofDOZ is shown in Figure 1. The sedimen-tary rocks in the subsurface that hosted theDOZ deposit have been hydrothermally alteredinto various calc-silicate mineral assemblages.The main structural features in the vicinityof DOZ area are the west-northwest trending,steeply dipping Ertsberg 1 fault, Ertsberg 2fault, Ertsberg 3 fault and the Ertsberg 4 faultto the south. These faults have kilometer-scale, reverse-sense offsets. This contact strikesroughly north/south, has vertical dip, and cutsthe WNW trending Ertsberg Fault system. Bed-ding, preserved in some alteration assemblages,dips 50 – 55 degrees to the northeast.

2.2 Current subsidence monitoring

Basically, there are several existing monitor-ing to determine changes or deformation in PTFreeport Indonesia subsidence area (Figure 2).The changes of cave profile in subsurface isidentified using regular cave inspection in un-derground drift or tunnel, Time Domain Reflec-tometry (TDR) cables which can identify crackor cave location based on cable fault/break po-sition, and microseismic monitoring which candifferentiate seismogenic and massive zone sur-

Figure 1: Sub-surface geological map, lithol-ogy distribution and the geological structure atDOZ deposit

rounding the cave area. For the changes at thesurface subsidence is monitored using prismpoint survey to identify position or coordinatechange (x, y, z) and aerial photo mapping togenerate topography surface (Figure 3 and 4).

The current monitoring has limitations, espe-cially the surface monitoring, since it dependson weather and field condition; survey pick-upand aerial photo have to be conducted in goodweather.

2.3 Extraction panels and its caving model

The DOZ mine initiated caving in November2000. Figure 4 shows the sequences of extrac-tion area from 2000 to 2009. Starting from 2000,the extraction was progressed to the east to re-duce impact of the DOZ caving on the opera-tions of the IOZ block cave mine. After the IOZwas exhausted in 2004, caving was started to-wards the west. Figure 3 shows the 3D viewof extraction points and its calculated extractionheights. The extraction height is obtained fromheight of draw (HOD) data. The HOD is the to-tal amount of material taken during the process

94 c© 2011 Department of Geological Engineering, Gadjah Mada University

GROUND MOVEMENT PREDICTION DUE TO BLOCK CAVING MINING GEOMETRY USING GIS

Figure 3: The subsidence aerial photo mapping which taken on 2010

Figure 4: View of extraction area and the schedule of extraction from 2000- 2009

c© 2011 Department of Geological Engineering, Gadjah Mada University 95

SETIANTO and WIDIJANTO

Figure 2: Prism monitoring at the surface subsi-dence

of extraction. Maximum height of extraction atpoints from 2000 to 2008 is about 459 m whilethe minimum is about 4 m.

3 Methodology of ground movementprediction

3.1 Basic Influence function model for hori-

zontal extraction panel

According to the principle of superposition,the consequence for the extraction panel wouldbe equal to the sum of the effects caused bythose infinitesimal extraction areas. Based onthe stochastic theory, the occurrence of a rock-mass movement over the extraction elementmay be a random event that takes place witha certain probability. An extraction with aninfinitesimal unit width, length and thickness(∂w∂l∂m) is called the extraction element. Thevertical displacement of any point in the subsi-dence trough is called the basic influence func-tion of vertical displacement (Se). The occur-rence of the event that surface movements inan infinitesimal area; dA = dxdy, at horizon z,with point P(x, y, z) at its center, is equivalentto the simultaneous occurrence of two eventscomposed of a movement in the horizontal stripdx and the horizontal strip dy through P. Fun-damentally, the probability can be written sep-arately for these two events by C(x2)dx andC(y2)dy, respectively, where C is the subsidence

trough function. The probability for a simulta-neous occurrence of these two events is:

P(dA) = C(x2)dx × C(y2)dy = C(x2)C(y2)dA(1)

The stochastic influence coefficient governsthe geometric rule for distribution of subsi-dence owing to the extraction element. Con-sider a polygon extraction that both the widthw and the length l of extraction and then the ex-traction volume is given as w× l ×m (Figure 5).

The final surface subsidence due to an ex-traction of finite width w and infinite length(l → ∞) will be:

S(x) = Smax

[

1

2er f (

√π

x

ru)− 1

2er f (

√π

x − w

ru)

]

(2)Where: ru = (dhu − H)/ tan γ is radius of the

influence circle which controlled by the angle ofdraw and distance from surface point to the ex-traction level; and H = depth of extraction panelto surface point.

The horizontal displacement of the groundsurface will be

v(x) = bSmax

[

er f (− π

r2u

x2)− er f

[

− π

r2u

(x − w)2

]]

(3)Differentiating equations of horizontal dis-

placement produces the horizontal strain due toan extraction of finite width w :

e(x) =dv(x)

dx=

2πbSmax

ru×

{

− x

ruexp(−π

x2

r2u

)+

(x − w

ru) exp

[

−π

(

x − w

ru

)2]}

(4)

3.2 Algorithm Implementation of prediction

model in GIS

Figure 6 shows the flow chart for the algo-rithm of prediction analysis using GIS func-tions in order to calculate 3D surface move-ments. In the first step of GIS computational

96 c© 2011 Department of Geological Engineering, Gadjah Mada University

GROUND MOVEMENT PREDICTION DUE TO BLOCK CAVING MINING GEOMETRY USING GIS

Figure 5: 3D view of a horizontal extraction and terrain surface for predicting subsidence at a sur-face point

process, the panel on global coordinate sys-tem is transformed into local coordinate sys-tem. The transformation coordinate panel isperformed to obtain the angle direction of zoneradius of the main subsidence influences. Then,a prediction of subsidence distribution is cal-culated in the stochastic-prediction procedures.Finally, the calculated results of subsidence areobtained according to the working panel num-ber. All entered data (such as panels caving, cal-culation points, and subsidence parameters) forsubsidence prediction are available by using thefunctions of GIS. Meanwhile, the input parame-ters for subsidence calculations are divided intotwo categories. The first parameter is the gen-eration of surface grid points for providing cal-culation points at the surface, including the dis-tance of each grid point in the x and y direc-tions in which the number of points along thegrid lines, the number of grid lines, point inter-vals along the grid lines, and the grid-line direc-tions need to be established. The second param-eter is entering the subsidence-calculation data,including the panel caving (panel ID), extrac-tion thickness, subsidence factor, horizontal-movement factor, and angle of draw. By enter-ing those data into the prediction system, the

3D movements are calculated with respect tothe polygon panels.

4 Prediction of ground movement due toDOZ

4.1 Constructing 3D discrete polygons and

3D grid calculation points

There are three main processes in GIS for con-structing 3D geometrical models: extractionboundary projection, cut-fill spatial analysis,and the extraction value of the raster to the vec-tor polygon layer. Figure 7 shows the flow chartfor constructing 3D extraction model which isbased on the HOD data from 2000 to 2008. En-tirely, there are 275 extraction points with theminimum elevation is 3,132 m and the maxi-mum elevation is 3,580 m. In order to obtainoptimum boundary model and accurate subsi-dence calculation by using influence functionmodel, polygon extraction model in z directionis divided with interval 10 m.

However, small interval of polygon increasesnumber of the total polygon and then requiresmore time to calculate. As a result, a 3D viewof the constructed 3D discrete polygons for the

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SETIANTO and WIDIJANTO

Figure 6: Flowchart improved GIS prediction model to calculate ground movement on terrain sur-face

98 c© 2011 Department of Geological Engineering, Gadjah Mada University

GROUND MOVEMENT PREDICTION DUE TO BLOCK CAVING MINING GEOMETRY USING GIS

Figure 7: Flow chart of GIS data processing and analyzing for develop 3D extraction model

c© 2011 Department of Geological Engineering, Gadjah Mada University 99

SETIANTO and WIDIJANTO

geometry model of block caving at DOZ and the3D grid calculation points with interval 20m inx and y direction that have different elevationdata is shown in Figure 8 and 9, respectively.

4.2 Parameter estimation using empirical

and observed data

Ground movement calculation, simulating theextraction process between 2000 and 2008, werecarried out by employing parameters of subsi-dence factor (a) = 0.1, horizontal movement fac-tor (b) = 0.025, and tangent of draw angle (γ) =2.14. Those parameters were obtained from theSubsidence Engineering Handbook (SEH 1975)and some measurements by previous study.The effect of the time factor, which expressestime-delayed ground deformation, is not con-sidered in this calculation.

4.3 Prediction of vertical displacement and

horizontal strain using GIS

Prediction models calculate the movements ofa surface point for an extraction panel in threedimensions. Because an individual point mayconsists of difference in ground elevation, theuse of the 3D prediction model for obtainingthe spatial distribution of subsidence is verytime-consuming without the GIS as each sur-face point has to be calculated separately. Toovercome the problem of complex geometricaldata conversion, the model of subsidence calcu-lation points can be performed within the GIS.

All modules are related to the GIS spatial-analysis function, which is implemented by aGIS component. Figure 9 shows the 3D viewof subsidence distribution and horizontal straindistribution resulting from past DOZ mining instages including from 2000 to 2008. For thesespatial simulations, all calculation points wereinterpolated by using the Kriging interpolationmethod.

5 Result and discussions

According to the simulation results, the signifi-cant influence of horizontal tensile strain at theeast side of surface subsidence area grew be-cause of extensive extraction ore was done early

in stages including 2001, 2002, and 2003. At thesame time, the concentration of tensile strainwas identified as the same as the location wherehuge rock slope failure was occurred at the eastside of subsidence area (Figure 9). Moreover,because the distribution of vertical displace-ment is developed at the terrain slope, the de-gree of slope becomes steeper than the originalslope value in particular at the upper part ofslope. Thus, the increasing slope degree maybe contributed to the slope instability factor aswell. The direct observation for ground eleva-tion change is not yet performed in the site loca-tion because of dangerous slope, high elevation(about 4000 m) and hazardous area. Compar-ing to the result from monitoring topographicchange in the vicinity of DOZ using aerial pho-togrammetry (Setianto et al., 2008), the predic-tion method using GIS indicates a high valueof subsidence. This may be attributed to thefact that, in this model, the effects of miningmethod, time dependent factor and the condi-tion of caving boundary such as the existence ofmajor faults intercepting the cave surface nearthe DOZ have not been taken into considera-tion.

6 Conclusion

Using a GIS prediction model, this paper hascalculated ground movement under irregularmining geometry and complex topographiccondition. Differential-ground movement char-acteristics, such as vertical displacement, andhorizontal strain, can be computed using GISeffectively. Ground movement in the study areawas successfully simulated using empirical andobserved data. The calculated result has beenused to investigate subsidence-induced rockslope failure resulting from block caving atDOZ.

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100 c© 2011 Department of Geological Engineering, Gadjah Mada University

GROUND MOVEMENT PREDICTION DUE TO BLOCK CAVING MINING GEOMETRY USING GIS

Figure 8: The generated 3D grid points by extracting Digital Elevation Model (DEM) data

Figure 9: (a) The 3D view of the location of huge rock slope failure of subsidence area which inducedby ground movement and the calculated maximum ground movement due DOZ extraction upto 2008 by means of complete extraction: (b) The vertical displacement; (c) The horizontal straindistribution

c© 2011 Department of Geological Engineering, Gadjah Mada University 101

SETIANTO and WIDIJANTO

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102 c© 2011 Department of Geological Engineering, Gadjah Mada University