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A METHODOLOGY FOR GEOTECHNICAL DESIGN- With Special Reference to CO FD ha!e " Slope De!i#n

Samuel Banda – Geotechnical Engineer, NOP

ABSTRACT

The steeply dipping Chingola Open Pits (COP) F and D orebodies; an even steeper foliation in the Basement Schist on the footall; the

geological str!ct!ral infl!ence of the Chingola "nticline# a comple$ thr!st related northest trending ridge of the %!f!b! &neiss and

Schist; and the presence of gro!ndater; ere among the diffic!lties that have been enco!ntered in designing slopes for COP FD'

"ltho!gh m!ch e$perience in design is available from changa Open Pit (OP) and other satellite pits# it as felt that d!e to the different geological environment of COP FD# design parameters co!ld not simply be transferred from OP and or other satellite pits to COP FD'

To overcome these diffic!lties# a design methodology as developed incorporating data gathering# roc* mass rating classifications#

geostatistical modelling and stability analysis techni+!es' The methodology aims at obtaining a better geotechnical !nderstanding of the

str!ct!ral and geological implications as ell as the spatial variability of the geotechnical parameters of the roc* mass conditions and

ho these ill be affected by creation of an e$cavation' The methodology is both comprehensive in addressing pertinent iss!es and

a!ditable'

"ltho!gh the design methodology has been developed to address the specific challenges enco!ntered in the COP FD# an open pit mine#

its applicability is not restricted to open pit design'

Intro$%ction

The COP ‘F’ and ‘D’ orebodie are hallo! l"ingorebodie !hich are to be mined b" o#en #it method$

The" are ituated on !hat traditionall" ha beendecribed a the outh!et limb o% the Chingola&nticline, a com#le' north!et trending ridge o% (u%ubuGnei and Schit locall" re%erred to a Baement Schit,B&S )Figure *+$

Fi#%re "& S%rface #eolo#' (ap of the Chin#ola Open it! area)

& tee#er di##ing orebod" )u# to -.+, #reence o% ground!ater and !ea/er roc/ ma condition com#aredto the Nchanga O#en Pit, !ere ome o% the %actor thathad to be incor#orated into the deign #roce$ Toaddre thee di%%icultie, a deign methodolog", aimed at

obtaining a better geotechnical undertanding o% thetructural and geological im#lication a !ell a the#atial 0ariabilit" o% the geotechnical #arameter o% theroc/ ma condition and ho! thee !ill be a%%ected b"creation o% an e'ca0ation, !a de0elo#ed$

Three main #hae are in0ol0ed in the deign #roce,!hich can be ubdi0ided into ten indi0idual te#1

&$ Data Gathering

,' -evie of Previo!s Design -eports

.' -evie of the &eological Plans and Sections

/' &eotechnical %ogging

0' Field 1apping

B$ Data Proceing2' -oc* 1ass Classification

3' &eostatistical "nalysis

4' -oc* 1ass 1odel "nalysis

C$ Deign Formulation and &nal"i

5' Determination of 6$cavation Parameters

7' 8inematic "nalysis for Probable Fail!re 1odes

,9' Stability "nalysis for B!l* Fail!re

Figure 2 outline the deign #roce o% the ten te#methodolog", !hich i dicued in detail in thi #a#er$

Fi#%re *& roce!! $ia#ra( of the $e!i#n (etho$olo#'

STE "& Re+ie, of re+io%! De!i#n Report!

The #ur#oe o% re0ie!ing the #re0iou deign re#ort ito re0iit the bai %or electing the #ro#oed e'ca0ationdeign #arameter and to con%irm that the" are

ade3uatel" u##orted b" data$ 4aluable in%ormation can be gathered %rom an earl" tage, !hich can be ued toidenti%" the trength and hortcoming %or %urther conideration in the deign #roce$

5n the COP F and D area, %our #re0iou deign re#orthad been #re#ared$ T!o o% thee re#ort, &6C (,772)and 7e#!orth (,774), #ro#oed lo#e angle baed on0er" limited drilling in%ormation$ One borehole, !ageotechnicall" logged !hile ground!ater condition!ere baed on in%ormation obtained %rom t!o otherhole$

The third re#ort, (eech (,773), relied on geotechnicallog %rom one borehole and geotechnical #arameter %romabout 89 geological borehole log, to come u# !ith the #it!all #arameter$ The %inal re#ort, Tunono and 6ur#h"(.99,), utilied e'teni0e geotechnical logging data )i$e$: borehole+ to deri0e #it !all #arameter$

STE *& Re+ie, of Geolo#ical lan! an$ Section!

The geological tructure %orm the bai %or an"geotechnical deign, hence, it i im#erati0e to obtain the

correct inter#retation$ The geological tructure ha atrong in%luence on the roc/ ma condition$ The COPFD orebod" lie on the outh;!etern limb o% theChingola &nticline, a com#le', thrut related, north!ettrending ridge o% the (u%ubu Gnei and Schit )B&S+!hich %orm the baement on !hich the ediment !erede#oited )Figure


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Since the %oot!all o% the COP FD Phae * #it i to bee'ca0ated in the B&S, an undertanding o% the in%luence o% the thrut and the %oliation in the B&S on the %oot!alllo#e tabilit" become im#ortant$ The inter#reted thrutlie %ar bac/ into the lo#e and a the" do not da"light onthe lo#e %ace, the" are unli/el" to ha0e an" igni%icantin%luence on the tabilit" o% the %oot!all$ The %oliation inthe B&S i li/el" to ha0e a ma=or in%luence on the %oot!alltabilit"$

Fi#%re & NW-SE !ection orientation !ho,in# the !tr%ct%ral

relation!hip of CO FD. CO E an$ CO C)

STE & Geotechnical Lo##in#

Diamond drilled core are critical in that the" #ro0idethe onl" !indo! to ee the nature o% the roc/ ma #rior to e'ca0ation$ 7ence, it i im#erati0e to enure that thediamond drill core are correctl" logged, geotechnicall"$The %ollo!ing #arameter are aeed duringgeotechnical drilling and logging o% core1 ;

5ntact >oc/ Strength )5>S+, ?oint@%racture #acing,

Dicontinuit" condition and

Ground!ater condition$The data collected %orm the bai %or roc/ ma

clai%ication, !hich i an im#ortant tool in e0aluatingthe #robable roc/ ma condition$ 5n the cae o% orientedcore, dicontinuit" trend can be obtained, !hich arecritical in tructural and dicontinuit" anal"i$

5t i eential %or an a##ro#riate deign to etablih ale0el o% con%idence in the #arameter obtained %romgeotechnical log$ For COP FD, the drilled core, !hich

had been /e#t, had u%%ered ome deterioration due tohandling$ 7ence, to etablih con%idence, the #arameter%rom geotechnical log !ere com#ared !ith thoeobtained %rom geological log !here thee !erea0ailable$ 6ore recent geotechnical log !ere com#ared!ith the core to identi%" and recti%" an" inconitencie$

To %urther im#ro0e the con%idence le0el in the log, oldgeological log containing geotechnical #arameter !ereincor#orated in the geotechnical #arameter databae$

STE /& Fiel$ Mappin#

The #ur#oe o% %ield ma##ing i to collect data that can

be utiliAed to con%irm the general trend o% ma=or dicontinuitie identi%ied in Ste# 2 and oc/ ma clai%ication "tem #ro0ide ue%ultool in the 3uanti%ication, deri0ation and election o%roc/ ma #ro#ertie and e'ca0ation #arameter$

The COP FD geotechnical data !a #roceed into t!oclai%ication "tem Bienia!/i’ )*+ >oc/ 6a>ating )>6> B+ and (aubcher’ )*::+ >oc/ 6a>ating )>6> (+$ Since the ground!ater en0ironment %orthe COP FD area i not "et %ull" de%ined, aum#tion %orground!ater !ere made in both clai%ication "tem$Thee aum#tion !ere baed on the a0ailable

in%ormation$ The >6> B and the >6> ( clai%ication"tem !ere ued to ae the roc/ ma condition o%COP FD$ (aubcher’ >6> ( clai%ication !a %urtherad=uted %or !eathering, orientation and blating e%%ect toobtain the 6ining >oc/ 6a >ating )6>6>+, !hich!a ultimatel" ued in aeing the initial lo#e angle o% COP FD Phae * )ee Table *+$

STE 7& Geo!tati!tical Anal'!i!

The !hole #ur#oe o% roc/ ma clai%ication "temi to obtain roc/ ma condition o% the entire area in!hich the e'ca0ation i to be made$ Geotatitical tooluch a thoe ued %or orebod" modelling can beem#lo"ed to model the roc/ ma condition o% an areautiliAing #oint and@or line data obtained %rom drill coreand@or ma##ing$

The roc/ ma clai%ication data )%rom Ste# -+ %or COPFD !a modelled and the reulting >6> B and >6> (6odel )Figure + !ere ued to anal"e the roc/ macondition and to obtain the geotechnical Aone #re0ailingin the COP FD roc/ ma$

130


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Fi#%re /& Section "8N 4W-E orientation5 !ho,in# the La%1!cher2!

RMR L cla!!ification (o$el an$ the $eter(ine$ #eotechnical 9one! of

CO F roc3 (a!!

STE :& Roc3 Ma!! Mo$el Anal'!i!

The aim o% underta/ing an anal"i o% the roc/ ma ito ditinguih the roc/ ma into geotechnical Aone$ &geotechnical Aone i de%ined a an area in !hich imilar

roc/ ma #arameter #re0ail$ E'ca0ation deign can then be modi%ied to uit #re0ailing condition$

&nal"i o% the COP FD roc/ ma model reulted inthe ditinction o% the COP FD roc/ ma into %our geotechnical Aone )Figure +1

1. The !pper hanging all stac* : com#riing the (aterite

and ##er >oan Dolomite )>D+ !ith >6> ( !ith0alue a0eraging 2 and 6> 0alue o% *-and *+, re#ecti0el",

2. The intermediate hanging all stac* : com#riing the

Shale !ith Grit )SG+ and the Chingola Dolomite)CDO(+ !ith >6> ( 0alue a0eraging 0alue o% 2 and 40

35>45

45>60


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acce#table criterion %or the a%et" %actor %or #it lo#e i*$2$

Fi#%re :& ropo!e$ CO FD ha!e " pit !ho,in# the !tr%ct%ral

re#ion!. the !tereo#raphic plot! an$ !%((arie! of anticipate$

fail%re (echani!(! in each re#ion

Fi#%re ;& Section GTF on the foot,all 4ea!t ,all5 of the CO FD

ha!e " pit. anal'!e$ %!in# Sli$e /)8 !oft,are. !ho,in# a Glo1al

Mini(%( !afet' factor of ")67

Fi#%re =& Section GTF7 on the han#in# ,all of CO FD ha!e "

pit. anal'!e$ %!in# Sli$e /)8 !oft,are. !ho,in# a Glo1al Mini(%(

!afet' factor of ")*;

Ta1le & S%((ar' of the Sli$e /)8 anal'!i! re!%lt! on !electe$

critical !lope !ection! of the CO FD ha!e "

Monitorin# the I(ple(entation of the De!i#nThe #ur#oe o% monitoring i t!o %old$ Firtl", to enure

that mining o% the e'ca0ation com#lie !ith the deign #arameter and that an" 0ariation are identi%ied ande'#lained$ Secondl", to con%irm that the #er%ormance o% the e'ca0ation i according to the deign e'#ectation$

Concl%!ion

To underta/e a com#reheni0e geotechnical deign #roce o% COP FD Phae *, that addreed the #ertinentiue, re3uired the %ormulation o% a deign methodolog"$

&lthough the deign methodolog" ha been de0elo#ed toaddre the challenge encountered in the COP FD, !hichi an o#en #it mine, it a##licabilit" i not retricted too#en #it deign alone$ Since the methodolog" aim atobtaining a better geotechnical undertanding o% thetructural and geological im#lication and #atial0ariabilit" o% the geotechnical #arameter o% the roc/ macondition and ho! thee !ill be a%%ected b" creation o% ane'ca0ation, the methodolog" hould be a##licable to botho#en #it and underground geotechnical deigning$6oreo0er, bet #ractice call %or a deign #roce that inot onl" com#reheni0e in addreing #ertinent iue, butauditable a !ell )#endi' *+$ Thi methodolog" aim toaddre thi challenge$

The e%%ecti0ene and robutne o% the methodolog"thu %ormulated, remain to be teted, ince COP FD Phae* mining i currentl" in #rogre$

Reference!

1. -oc* Slope 6ngineering (revised second edition) b" E$

7oe/ and ?$$ Bra" – The 5ntitution o% 6ining and

6etallurg", (ondon )*::+

2. " geomechanics classification system for the rating of roc*

mass in mine design b" D$7$ (aubcher – ?ournal o% the

South &%rican 5ntitute o% 6ining and 6etallurg", 0ol$ 9,

no$ *9, ## 2-:;2:< )October *9+

3. -oc* 1ass Classification System ("fter Bienias*i ,757) :

obtained from Chapter / -oc* 1ass Classifications of

-oc* 6ngineering Co!rse otes b" E0ert 7oe/ 4. Preliminary estimation of roc* slope stability !sing roc*

mass classification systems b" &$ 7aine and P$?$ Terbrugge

– S> Conulting Engineer and Scientit, ?ohanneburg,

South &%rica

5. >e#ort No$

2:*8 )December 299*+

6. COP FD Phase , Slope Design -eport b" Samuel Banda –

C6 5nternal >e#ort )?anuar" 299+

7. =nvestigation for Determination of Preliminary Slope

"ngles for Proposed COP>s D and F b" &%rican 6ining

Conultant (td )ril *-+

8. "ssessment of COP D and F Slope "ngles b" N$ 7e#!orth – C6 5nternal >e#ort )?ul" *:+

9. -ecommended final pit all design for COP D ,,9m

=nterim Pit b" Simon ?$ (eech – C6 5nternal >e#ort )?ul"

*8+

10. Proposed Slope angles for Satellite Pits : COP F b" &$B$

Tunono and B$ 6ur#h" – C6 5nternal >e#ort )299*+

Acknowledgements

5 !ould li/e to than/ &lan Naimith, C6 Grou# Geotechnical

Engineer %or the technical u##ort and %or editing the #a#er$

132

?i!hop 4!i(plifie$5 Safet' Factor!

Section No) it Slope Area @pper

Slope

Lo,er

Slope

O+erall

Slope

O+erall

Slope

Hei#ht

O+erall

Slope

An#le

GTF* Foot!all ; *$C8 *$C8 :2 ,D

GTF2 Foot!all *$:C *$C: *$8< *-C


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Appen$i< "& S%##e!te$ A%$it Chec3li!t for the Ten Step De!i#n Metho$olo#'

A) Data Gatherin#

1. Previous Design Reorts

i$ &re the" a0ailableJ Ke No

ii$ 5% "e to *, ha0e the" being re0ie!edJ Ke No

!. "eological Plans and Sections i$ &re the" a0ailableJ Ke No

ii$ 5% "e to abo0e, ha0e the" being re0ie!edJ Ke No

iii$ 5% "e to abo0e, ha0e tructural and dicontinuit" trend being identi%iedJ Ke No#. Core $ogging

i$ Doe drill core %or the area e'itJ Ke No

ii$ 5% "e to abo0e, ha it being logged geotechnicall"J Ke No

iii$ &re geological log a0ailable %or the ame coreJ Ke No

i0$ 5% "e to abo0e, are the geotechnical log conitent !ith geological logJ Ke No

0$ 7a0e com#arion !ith core being done to identi%" and rati%" inconitencie

in the logJ

Ke No

%. &ield 'aing

i$ &re there e'#oed area in the #ro'imit" o% the #lanned e'ca0ationJ Ke No

ii$ 5% "e to abo0e, ha0e the" being ma##ed %or tructural and dicontinuit"

trendJ

Ke No

iii$ 7a0e the condition o% the e'#oed area being recordedJ Ke No

?) Data roce!!in#

(. Rock 'ass Classification i$ 7a the logging and@or ma##ing data being #roceed into roc/ ma

clai%icationJ

Ke No

ii$ 7a an" aum#tion being made in the clai%ication "temJ Ke No

iii$ 5% "e to abo0e, tate the aum#tion1LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL

). "eostatistical Anal*sis i$ 7a the roc/ ma rating data being modelledJ Ke No

+. Rock 'ass 'odel Anal*sis i$ 7a0e geotechnical Aone being identi%ied %rom the modelJ Ke No

ii$ 5% "e to abo0e, are the" conitent !ith geological boundarieJ Ke No

C) De!i#n For(%lation an$ Anal'!i!

,. Determination of -cavation Parameters i$ &re the e'ca0ation #arameter obtained b" ue o% a deign chartJ Ke No

ii$ 5% "e to abo0e, tate the deign chart ued1LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL/. 0inematic Anal*sis

i$ 7a a tructural and dicontinuit" anal"i o% the #lanned e'ca0ation being

underta/enJ

Ke No

ii$ 5% "e to abo0e, ha0e #robable %ailure mode being identi%iedJ Ke No

iii$ 7a0e the identi%ied %ailure mode being ran/ed in term o% li/elihood o%

occurrenceJ

Ke No

1. Stabilit* Anal*sis for Bulk &ailure

i$ 7a a correct inter#retation o% the tructural geological condition in the roc/

ma being obtainedJ

Ke No

ii$ 7a0e the bloc/ and !edge that can be releaed b" the creation o% the

e'ca0ation being identi%iedJ

Ke No

iii$ 7a the tabilit" o% the bloc/ and !edge being anal"ed uing an

a##ro#riate modelJ

Ke No

i0$ 7a the deign being e0aluated in term o% a%et" %actor and #robabilit" o%

%ailureJ

Ke No

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