PIT SLOPE DESIGN CHALLENGES IN RESIDUAL SOILS AND WEATHERED ROCK: BACKGROUND AND ACASE STUDY
H W. Newcomen and B. T. BurtonBGC Engrg Inc.
Vancouver, BC, Canada
J. GeyerGold Reserve CorpSpoken, WA, USA
ABSTRACTFor open pits developed in tropical climates the
upper portions of the pit walls are often located inresidual soils and weathered rock. Due to thepresence of relict structures, and the relatively lowstrength of the residual soils and weathered rock,design slope angles in these materials have to bedeveloped by blending the results of kinematicassessments of geologic structures with rock massstability analyses and traditional soil mechanics.Background information on the engineering consid-erations of residual soils are discussed, and theresults of geotechnical assessments for proposed pitdesign of the Brisas del Cuyuni Project, located insoutheastern Venezuela, are presented.
RESIDUAL SOILS
Introduction
Excavations in tropical climates often encounterresidual soils and weathered rock. A residual soilcan be described as a soil-like material derived fromthe weathering and decomposition of rock which hasnot been transported from its original location (Blight,1997). This general definition is a broad term thatincludes saprolites, mature soils and laterites. Theterm residual soil is sometimes used in more specificterms to describe mature soil alone.
Saprolites are materials that have soil-like strengthor consistency, but retain recognizable relicts of thestructure and fabric of the parent rock (Blight, 1997).As an example, a saprolite derived from a lava maycontain flow bands, amygdules, and joints. Relictstructures often constitute planes of weakness andzones of higher permeability within a soil mass.
Mature soil is that part of the soil profile which hasundergone physical and chemical weathering to theextent that no evidence of the parent rock’s fabric orstructures remain.
Laterites are highly altered residual soils that havehad the silica leached out and have some degree ofcementation by sesquioxides (Blight, 1997), givingthese soils a granular or nodular appearance.Laterites are typically rich in hematite and boehmiteand their high iron content gives them a deep red-dish color. The term laterite, however, is used veryloosely and is sometimes applied to soils with littleinfluence from sesquioxides. Laterization may occurin ancient transported soils as well as residual soils.Due to the nature of their formation, laterites tend tooccur near the surface and extend to limited depths.Laterites are often excellent construction materialsand may be a source of aggregate.
The progression with chemical weathering of a soilfrom saprolite to mature soil, to laterite will onlyoccur in a favorable chemical environment and isdescribed in greater detail below.
Distribution
Residual soils can be found throughout the world,but primarily in the tropics, as shown in Figure 1.Broad classes of residual soils can be seen toextend beyond the tropics where favourablecircumstances permit. Conversely, numerous areasexist in the tropics where residual soils are overlainby more recent alluvial and aeolian deposits.
Structure, Fabric and Discontinuities
The following definitions are proposed for use(Fookes, 1997) when discussing the characteristicsand properties of residual soils:
SME Annual MeetingFeb. 28-Mar. 1, Salt Lake City, Utah
• Structure – the fabric, texture and discontinuitypatterns making up the soil-rock material,mass or unit;
• Fabric – the spatial arrangement of componentparticles;
• Discontinuities – the nature and distribution ofsurfaces separating elements of fabric, mate-rial or soil-rock mass.
Figure 1. Simplified world distribution of the principal types of residual soils; based on F.A.O. World Soil Map(after Fookes, 1997).
The structure of tropical residual soils includesmacroscopic features such as relict discontinuities,and microstructure or fabric. Although microstruc-ture is important to understanding the engineeringbehavior of soils, particularly partially remolded soilssuch as compacted fills, the behavior of the in-situsoil mass is frequently more influenced by macro-scopic features. The structural orientation ofmacroscopic features such as schistosity, fissures,veins, joints, faults, and voids can have a significantinfluence on the behavior of the soil mass.
Mineralogy
The mineralogical composition of residual soils isdependent on the composition of the parent rockand the climatic conditions. The mineralogy oftropical soils has engineering significance in theaggregation and cementation of soils (Burton, 1998).The mineralogy can have a significant affect onindex properties such as moisture content, and hasbeen observed to affect field instruments such as
nuclear densometers. Swelling or collapsing soilscan have a significant effect on slope stability.
Figure 2 shows that, in a general sense, clay min-eralogy varies in relation to the distance from theequator. In the tropics, feldspar minerals (alumino-silicates) weather initially to kaolinite, hydrated ironand aluminum oxides, such as goethite (Fe2O3•H2O)and gibbsite (Al2O3•3H2O ), also referred to assesquioxides. Other minerals which are moreresistant to weathering, such as quartz (SiO4) andmica (Kal3Si3O10(OH)2 - muscovite) may persist, oftenas individual sand grains in a clayey matrix. Withfurther weathering the kaolinite content may de-crease and the sesquioxides progressively alter tohematite (Fe2O3) and boehmite (Al2O3•H2O) (Mitchelland Sitar, 1982). Further chemical action maycement these materials and produce lateritic gravels.
Weathering Profile
The engineering properties of a soil will vary withdepth even if it has developed from a uniform parent
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material. Near the surface, soil layers are affectedby humus, seasonal wetting and drying, and biotur-bation. At greater depth, the moisture contentfluctuates less with the seasons, and less organicmatter is present. Also, at greater depth thegroundwater movement is often slower, and soilparticles and solutes are less likely to be trans-ported.
Figure 2. The effect of climate on frequency of clay mineraloccurrence with climate zones represented in a simplifiedmanner as distance from the equator (from Millot, 1979; seeUehara, 1982).
The transition from saprolite to fresh rock may besudden or may occur over tens of meters. Crystal-line rocks, such as granites, granodiorites,migmatitic gneisses, and some metavolcanics, tendto have a fairly sharp boundary. More fissile rocks,such as schists and phyllites, tend to have grada-tional transitions over many tens of meters. Well-jointed igneous rocks will have an increasing abun-dance of corestones with depth.
A knowledge of typical weathering profiles for dif-ferent parent rocks can help the slope designerpredict how geomechanic properties will vary withdepth, allowing a rational approach to be taken todividing up slopes for design. An example of atypical weathering profiles for metamorphic andigneous rocks are shown in Figure 3. This figurealso shows a classification system for weatheringgrades. Many classifications have been proposed;however, it is out of the scope of this paper to com-pare them.
Selected Residual Soil Properties
It is difficult to relate the properties of a residualsoil directly to the parent rock because of the com-plex superposition of effects from climate,topography, geologic age, and structure. For exam-
ple, weathered granite from the warm, humidMalaysian peninsular has quite different propertiesfrom weathered granite from cooler, semi-arid SouthAfrica (Blight, 1997). However, some general trendsexist for residual soils, depending on their depth ofoccurrence and geologic age. Figure 4 shows howsoil properties tend to vary with depth due to theeffects of weathering.
Figure 3. Weathering classification system proposed byDeere and Patton (1971) with typical weathering profiles formetamorphic and intrusive igneous rocks (after Deere andPatton, 1971).
Figure 4. Changes occurring with depth in a weatheringprofile (adapted from Tuncer and Lohnes, 1977, and Sueoka,1988; after Blight, 1997).
Grain Size: Experience obtained in residual soilsfrom Brazil (Mori, 1979) indicates that for thoseclimatic conditions the three main types of bedrock
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commonly found consistently weather to differentgrain sizes. The corresponding rock types and grainsizes of the residual soil are:
• basalt – clayey
• gneiss – silty (often micaceous)
• granite – (sandy)
The resultant grain sizes are generally a function ofthe mineralogy of the various parent rock types.
Plasticity: A correlation between plasticity indexand parent rock type is shown in Figure 5. Relation-ships between the index properties of plastic limitand liquid limit of a soil and shear strength are wellaccepted for temperate soils. In the authors’ experi-ence, at the Brisas property and elsewhere, the localvariability in plasticity for residual soils is muchgreater than the variability in shear strength. Therelationships developed for temperate soils areconsidered appropriate as a first approximation ofshear strength; however, they should be used withcaution.
Figure 5. Plasticity Index Results for Various Residual SoilParent Rock Types (after Blight, 1997).
Site Characterization
Investigating the properties of a saprolite differsfrom investigations of temperate soils in three gen-eral ways:
• relict structures must be characterized
• relict structures must be accounted for in per-meability testing
• weathering distributions should be mapped.
Relict structures can be difficult to characterize,often they are not readily apparent in drill core dueto smearing from the drilling process. Mapping oftrench and test pit walls is generally more successfulbut often these are also obscured by smearedsurfaces resulting from the excavation process. This
often requires that the faces be washed or exposedto rainfall to enhance the visibility of the structures.
Testing the hydraulic conductivity of saprolite musttake into account scale effects. The in-situ perme-ability is often controlled by relict joints and thuslaboratory testing can underestimate the conductivityby orders of magnitude. The success of openborehole testing can be affected by smearing of thesidewalls, and packer testing often has limitedsuccess due the difficulties associated with obtaininga proper seal for the packers. Larger scale pumptesting can be performed to overcome these difficul-ties; however, the results obtained will incursignificant additional costs.
Mapping the base of weathering requires carefulconsideration. Slope instability has occurred whereinterpreted bedrock was in fact a very large boulderfloating in a weathered rock matrix. In some areas,fresh outcrops often give way to troughs of weath-ering 15 to 20 m deep over horizontal distances of100 to 200m, and quite sudden depth increases of30 to 50m (Fookes, 1997). Visual mapping is not areliable method, because what appears to be rockfrom a distance may break down to clay and siltunder relatively light pressures.
CASE HISTORY- BRISAS DEL CUYUNIPROJECT
Regional Setting and Project History
Over 100 years ago, the KM 88 Mining District wasone of the richest gold-producing areas in the world.This area is located approximately 375km south ofCiudad Guyana (locally known as Puerto Ordaz)near the village of Las Claritas, (Figure 6) in Vene-zuela’s Bolivar State. The area is underlain by thePrecambrian Guyana shield which extends intoBrazil, Suriname, Guyana and French Guyana.Most of the shield is covered in dense rainforest, andmining has traditionally been carried out in the highlyweathered saprolitic bedrock. Considerable mineralwealth has also been discovered more recently inthe underlying hard rock.
The Brisas property is located in the KM 88 miningdistrict. The “property” refers to the Brisas alluvialgold concession and the Brisas hard rock conces-sion for the gold, copper and molybdenum containedbelow the alluvial. The property, through a whollyowned Venezuelan subsidiary, was acquired byGold Reserve Corporation (GRC) in 1992.
In February 1998, a detailed pre-feasibility studywas completed by Jacobs Engineering of Denver,CO. In addition, a supplemental study was com-pleted in August 1998 on potential improvements tothe milling using the Cominco CESL group milling
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process, to facilitate on-site treatment of copperconcentrates.
GRC continues to conduct work on updating themineable reserves, optimizing cutoff grades, and iscarrying out additional metallurgical testing andtailings characterization. Slope stability assess-ments for a deeper and larger open pit are beingcarried out on an ongoing basis by BGC Engineeringof Vancouver, BC (Newcomen, et al, 1997; 1999).
Figure 6. Location Map
Engineering Geology
Four horizons have been identified at the propertyby GRC geologists. They consist of:
• saprolite soils
• a mixed zone
• weathered rock
• fresh rock
The saprolites are formed by the complete weath-ering of bedrock, and are distinguished by theabsence of any significant rock content. The top ofthe saprolite is generally oxidized, with an orange orreddish-brown color, to a depth of between 15 and20m. The base of the oxidized saprolite is denotedin the drillhole logs by the marker “BOS” (Figure 7).
The oxidized saprolites generally overly a green-ish-grey, sulfide-stable saprolite. The marker “BAS”is assigned to the base of the saprolite. The depthto the BAS marker is variable across the site, buthas been logged to depths of up to 100m.
Relict structures, such as joints and beddingplanes are common in the saprolite. At Brisas, thesaprolites generally consist of silt-size particles. Theoxidized saprolite often contains zones of white,chalky low-plasticity kaolin.
The mixed zone is a gradational horizon betweenthe overlying saprolite and the underlying bedrock.This zone has layers of saprolite mixed with layers ofhighly weathered rock. The thickness of this zone isvariable, ranging from 5m to greater than 30m. Themarker “BZM” has been assigned to the base of themixed zone.
Figure 7. Typical Section Through the Proposed Pit
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Bedrock at the Brisas del Cuyuni site consists of asequence of metavolcanics composed of andesites,tuffs and volcanic sediments which have beenintruded by dykes and stocks of diorite, quartzporphyry, and gabbro. The average depth to theweathered rock is about 50 m across the site. Theweathered rock is generally identified by carbonateleaching. Rust staining on the fractures and jointssuggests that groundwater flow occurs predomi-nately within this horizon.
The fresh rock corresponds to unweathered ver-sions of the andesite, tuffs and volcanics, asdescribed above.
The dominant structural features throughout thesite are bedding and foliation. Bedding and foliationin the rocks both dip to the west at similar orienta-tions and are difficult to distinguish from on another.No significant faults have been identified within theproject area. Up to four additional joint sets havebeen identified from surface outcrop/trench mapping.
Hydrogeology
The phreatic surface at the project site fluctuatesseasonally between 2 to 5 m below the groundsurface, and generally conforms to the existingsurficial topography. The principal aquifer identifiedon the property is the weathered rock. Groundwaterflow in this semi-confined aquifer will likely be pre-dominantly controlled by fissures and joints.Recharge is derived from gravity leakage out of theoverlying saprolite and regional sources. The freshrock is believed to act as an impermeable lowerboundary.
Hydraulic conductivities in the weathered rock areestimated to be between 9x10-5 and 1x10-3 cm/s, withcalculated transmissivities between 3x10-4 and 2x10-5
m2/s (Hydro-Triad, 1996). Conductivities in thesaprolite are estimated to be approximately threeorders of magnitude lower. These values fall belowthe threshold of what would be considered produc-tive for water well installations. However, it is ouropinion that the actual values are likely higher thanthose calculated, as they were determined usingpacker tests which are often found to be unreliablein ground conditions such as those found at Brisas.
Geotechnical Investigations
BGC Engineering has been carrying out office andfield studies since 1997 to refine pit slope angles forvarious designs proposed by GRC. A comprehen-sive approach to slope stability has been taken,incorporating engineering geology and rock me-chanics information collected during GRC’sexploration program.
Mapping: Surface outcrops, trenches and mostsmall pits within the concession have been structur-ally mapped. All of the exposures in the outcropsconsist of saprolite or heavily weathered rock, mak-ing identification and mapping of geologic structuresa challenge. Several small open pits excavated byprevious mining activities are located throughout theproperty. The pits are often filled with water; how-ever, lower water tables during the dry season haveallowed structural mapping of the pits to be carriedout on a limited basis.
Five test pits were excavated around the propertyto depths of about 5m during the geotechnicalinvestigations. The test pits were logged for stratig-raphy and engineering properties. The undrainedstrength of the soil was measured in the test pit wallsusing a Geonor hand vane. Disturbed saprolitesamples were collected at regular intervals in thetest pits. One undisturbed block sample of saprolitewas collected from the test pits.
Core Logging: GRC’s exploration program in-cludes diamond drilling using single-tube diamondcoring techniques. To date, GRC has drilled 763holes on the property, for a total of over 165,000 mof drilling. The drill holes are typically HQ sizeinitially, reducing to NQ when competent rock isreached. The majority of the drill holes are inclinedto the east to intersect the mineralization across itsdip. Two vertical drill holes were drilled specificallyfor the geotechnical program, in an area where thebedding dip was expected to be critical to the pit walldesign.
Engineering logging has typically been carried outby GRC personnel before the drill core is split.Consequently, accurate estimates of RQD, corerecovery and hardness have been determined. Thelogging is generally carried out in material below thesaprolite zone, in the weathered and fresh rock.
Additional detailed logging of over 3,500 m of corefrom eleven selected drillholes was undertaken byBGC to determine the properties of the saprolite andthe rock. The following geomechanical parameterswere assessed:
• rock hardness (as per ISRM standards)
• core recovery
• RQD
• fracture frequency
• joint condition
• degree of breakage
• degree of weathering/alteration
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The geomechanics information was recorded in aformat amenable to entry into a computer databasefor statistical analysis and subsequent estimation ofRock Mass Rating (Bieniawski, 1976) and rock massstrength (Hoek, 1994).
Point load testing was also undertaken on selectedrepresentative core using a Roctest point-loadtesting machine. Diametral and axial testing alongand across the foliation were carried out on split andun-split core, to determine if any strength anisotropyexists in the rock mass. Point load strengths wereconverted to unconfined compressive strengths forsubsequent use in rock mass strength assessments.
Laboratory Testing: Extensive triaxial testing wascarried out by others (Abel, 1997) prior to BGC’sinvestigations. This information was assessed andused, in conjunction with field vane testing results, toestimate undrained strengths for the saprolite (Fig-ure 8).
Su vs DepthSaprolite Samples
0
20
40
60
80
100
120
140
160
180
200
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Shear Strength (kPa)
Dep
th (
m)
Undrained Strengths Determined by BGC with Hand Vane in Test PitsInterpreted Undrained Shear Strength of Oxide Saprolite from triaxial testing undertaken by Abel (1997)Interpreted Undrained Shear Strength of Sulfide Saprolite from triaxial testing undertaken by Abel (1997)
During the field investigations, disturbed samplesof oxidized and sulfide stable saprolites were ob-tained from exploration drill core and from trenches.The saprolite samples were sealed in plastic bagsand containers and shipped to a Venezuelan labo-ratory for standard index and soil classificationtesting. The index testing consisted of:
• liquid and plastic limits
• moisture content
• grain size distribution
One undisturbed sample was transported to NorthAmerica for direct shear testing. The results ofdrained shear testing on the saprolite indicated apeak strength of ϕ=25°, c’=50 kPa, and a residualstrength of ϕ=21.5°, c’=0 kPa.
Six core samples with natural fractures from theweathered rock and fresh rock units were collectedand transported to North America. Direct sheartesting was carried out on two of these samples.The testing indicated a shear strength (peak andresidual) of ϕ= 35°, c'=0 kPa for discontinuities inboth the weathered rock and the fresh rock.
Geotechnical Units
Based on the engineering geology, trench map-ping, geomechanics assessments and laboratorytesting results, three geotechnical units have beendefined according to similarities in engineeringproperties. The distribution of the various geotechni-cal units on a typical section through the proposedpit is shown in Figure 7.
For geotechnical purposes and slope stability as-sessments, the “saprolite” unit has been defined asthe material above the BZM marker, which includesboth the saprolite and the mixed zone horizons.Although the mixed zone often contains a significantamount of weathered rock, it was considered appro-priate to combine the two horizons. The strength ofthe saprolite has been assigned to this unit, sincethe weaker materials will dictate the stability.
The strength of the saprolite is highly variable, butit generally has the consistency of a firm to very stiffsoil. The design strength curve for the saprolite,based on drained direct shear testing and indextesting, is shown in Figure 9.
The “weathered rock” geotechnical unit has beendefined as the zone between the BZM and the baseof weathering (BDM). The weathered rock has anISRM hardness of R1 to R2, placing it in the cate-gory of “very weak to weak” rock. It has a moderatedegree of fracturing and the joints are generallyopen, highly weathered and have little infilling.Based on point load testing and the rock mass ratingconducted on the weathered rock, it has an averageunconfined compressive strength of 20 MPa, and aRMR of 49, classifying it as a “fair” quality rockmass. The rock mass strength curve for the weath-ered rock is shown in Figure 9.
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The “fresh rock” geotechnical unit is generally un-weathered, and is defined as the material below theBDM. The fresh rock is classified as a mediumstrong to very strong rock, with an ISRM hardness ofR3 to R5. It generally has few fractures, and thejoints are infilled with calcite. Varying degrees ofsericitic alteration are present in the fresh rock.Based on the point load testing and the rock massrating conducted on the fresh rocks, it has an aver-age unconfined compressive strength of between115 and 183 MPa (from the southeast and northwestsides of the proposed pits, respectively), and a RMRof between 70 and 73. In terms of rock mass qual-ity, the fresh rock is classified as “good”. The rockmass strength curves for the fresh rocks are shownin Figure 9.
Note the difference in the estimated rock massstrength of the fresh rock between the NW and theSE sides of the pit. This difference is due to theanisotropy of the rock mass strength parallel to andacross bedding.
Structural Domains and Design Sectors
Areas with similar geologic structure, based onstructural mapping information collected on theBrisas property, were grouped into structural do-mains. Two structural domains were identified in thepit area. Domain 1 encompasses the southwesternone-third of the currently proposed pit (Figure 10).The dominant structures in this domain are thefoliations, which have an average dip of about 42°
towards 288°. A bedding joint strikes sub-parallel tothe foliation and dips approximately 60° to thenorthwest. Three other weaker sets of cross joints,dipping east and south were also identified.
Structures mapped in Domain 2 have been used torepresent the anticipated structure for most of theproposed pit, particularly the central and northeast-ern portions. The foliations in Domain 2 have a peakdip of about 55°, dipping towards 298°. Several ofthe discontinuity sets observed in Domain 1 wereabsent or very weakly represented in Domain 2.
Design sectors for the open pit were derived bycombining structural domains and pit wall orienta-tions. A total of sixteen design sectors weredeveloped for the proposed open pit, as shown inFigure 10.
Design Method
Potential Modes of Instability. Instability in exca-vated rock slopes is commonly initiated alongstructural discontinuities in the slope. Potentialfailure modes can be predicted by comparing theorientation of the discontinuities to the orientation ofthe proposed excavation. Certain failure modes arekinematically possible when the bench face, inter-ramp or the overall slope undercuts geologicstructure (i.e. planar) or the line of intersection of acombination of the discontinuities (i.e. wedge). Toassess these types of instabilities, the shear strengthof the discontinuities are of primary importance.
If a rock mass is highly fractured or of poor quality,such as the saprolite and the weathered rock, failurecan occur along a rotational surface or through therock mass. To assess these types of instability, theshear strength of the saprolite or the rock mass areof primary importance.
Approach: Structural mapping information wasplotted on lower hemisphere stereonets. The plotshave been used to determine kinematically feasiblefailure mechanisms in each design sector of theproposed pit. Limit equilibrium stability analysistechniques were then used to estimate a Factor ofSafety (FS) for each potential mechanism. Potentialplanar modes of instability were by assuming asimple sliding block of unit width. The weight of theblock and the resisting forces along the slidingplane, along with any water pressures were calcu-lated to determine a FS of any potential planarfailures. The stability of potential wedges identifiedwas analyzed using the commercially availablecomputer program SWEDGE. The calculated FSvalues were then used to select allowable slopeangles based on kinematics.
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Figure 10. Conceptual Pit Design and Design Sectors
The potential for rotational failures in saprolite andweathered rock was examined using chart solutions(Hoek and Bray, 1981) and the slope stability soft-ware SLOPE/W, respectively. Due to the highlyvariable thickness of the saprolite and the weatheredrock throughout the proposed pit area, a genericapproach to the analysis was chosen. Curves ofslope angle vs. slope height for a specified (allow-able) FS of 1.2 were developed to assist in design(Figure 11), using the drained strength of the sapro-lite and the rock mass strength curve developed forthe weathered rock (Figure 9). The average depthof the saprolites and/or weathered rock for eachdesign sector was plotted on the design curvesshown in Figure 11 to determine an allowable slopeangle.
The potential for both rotational/rock mass failureand instability due to kinematic controls was as-sessed for each design sector. The allowable anglebased on kinematic controls was compared to theallowable angle based on rotational/rock massfailure, and the lesser of the two angles was used fordesign.
Figure 11. Design Curves for Saprolite and WeatheredRock (FS=1.2).
RESULTS AND DESIGN ANGLES
Saprolite
In terms of kinematic analysis, numerous potentialwedge and planar modes of instability were identi-fied within the proposed pit area, for several slopeorientations. Most of the potential wedges identified
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in the saprolite have a FS <1.0 under completely dryconditions. This indicates that wedges formed byrelict structures in the saprolite will be unstable ifthey are undercut.
In terms of rotational failure or shearing throughthe soil mass, effective stress analyses using chartsolutions for two groundwater conditions (dry andpartially dewatered) indicate that, for slope heightsover about 20m in the saprolite, the allowable slopeangle is significantly reduced if dewatering of theslope is not completely achieved.
Based on these assessments, with the exceptionof two structural domains (1B and 2I), the mainmode of potential instability in the saprolites, par-ticularly for inter ramp slopes, is anticipated to berotational failure. Due to the presence of relictstructures, it is likely that some element of structuralcontrol will exist in these rotational failures. Thiscould result in complex modes of instability occur-ring; however, these are beyond the current scopeof this project.
For preliminary planning and economic analysis,40° is considered to be a reasonable angle for pre-feasibility level design in the saprolites. This angle isbased on an average depth to the weathered rock of50m, and the assumption that the saprolites can befully dewatered. If they cannot be dewatered, slopeangles of less than 30° will be required.
Weathered Rock
Similar discontinuity and pit wall orientations tothose used for the saprolite were used to assess thestability of the pit wall in weathered rock, albeit withstrength parameters for the discontinuities modifiedaccordingly. A majority of the potential wedgesidentified in the weathered rock have a FS <1.0,even under completely dry conditions. This indi-cates that wedges formed by discontinuities in theweathered rock will likely be unstable.
The anticipated modes of instability in the weath-ered rock in the southeast and southwest walls ofthe proposed pit (i.e. Domain 1) are predominantlykinematic (i.e. planar and wedge) failures. Based onthe current structural geologic information fromDomain 2, which indicate that fewer kinematiccontrols exist, the anticipated mode of potentialinstability along the northeast and northwest walls isrock mass failure. The rock mass failure analysisconducted indicated that the potential for rock massor rotational failure in the weathered rock can besignificantly reduced if the weathered rock can bepartially dewatered.
The recommended average design angle for theweathered rock is 45o. The actual angle that can beachieved will also depend on the dominant influenceof the overlying and underlying geotechnical units.Achieving these angles will require successfuldewatering of the pit wall in the upper slopes.
Fresh Rock
Due to the limited structural data available for thefresh rock, in which kinematic constraints are ex-pected to control the slope stability, design angles insome design sectors within Domain 2 cannot beaccurately defined. The high competency of thefresh rock, and the apparent absence of any unfa-vorably oriented discontinuities, indicate thatinterramp pit wall angles in excess of 55° may bepossible. However, 55° is currently considered areasonable upper bound for feasibility level planning.Justification of steeper angles for a pit of the cur-rently proposed depth (>350m) will require additionalgeotechnical information at greater depth, and moresophisticated analyses techniques where stressesand slope deformations are assessed (Newcomen,et al, 1999).
SUMMARY AND CONCLUSIONSIn summary, the key features of residual soils and
weathered rock, and their impact on pit wall stabilityare:
• Weathering profiles are influenced by climateconditions, parent rock type, structure, topog-raphy, and geologic age;
• Weathering profiles have a large influence onthe strength and permeability of a rock;
• Permeability of intact soil may be considerablyless than the permeability of the in situ soilmass;
• Relict structures often act as planes of weak-ness;
• Pit slope designers are familiar with groupingpit slope sections into design sectors as part ofa rational approach to design. Pit design inareas of residual soils should incorporate thepotential variability in geomechanical proper-ties due to weathering into the designapproach; and
• Geotechnical engineers must collaborateclosely with mine planners to produce a work-able and practical mine plan where greatvariations in geomechanical properties are ob-served at a property.
At the Brisas del Cuyuni Project, the results ofengineering geology and kinematic assessments of
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the structural geology have been blended with rockmass stability analyses and soil mechanics analysesto develop design angles for proposed pit wallslopes in saprolites and weathered rock.
The steepness of a significant portion of the lowerpit wall, which is located in relatively strong, freshrock, does not currently have any kinematic or rockmass constraints. However, additional structuralgeologic information is required at depth to confirmthat kinematic controls are not a concern. Imple-mentation of the design angles presented in thispaper will require a concerted effort to dewater thesaprolites and weathered rock.
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4. Burton, B.T., “Earthworks with wet, finegrained tropical residual soils., “ UnpublishedMaster of Engineering Report, University of Al-berta.
5. Deere, D.U., and Patton, F.D., 1971. “Slopestability in residual soils,” Proceedings of the 4thPanamerican Conference on Soil Mechanics andFoundation Engineering, San Juan, Puerto Rico,Vol. 1, pp. 87-170.
6. Fookes, P.G., editor, 1997. “Tropical residualsoils - A geological society engineering groupworking party revised report,” The Geological So-ciety, London, 184 p.
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