Prospecting & Surveying

TheQuarry Regulations 1999 state that allexcavations and tips should be designed,appraised and, if significant hazards

are identified, subjected to a geotechnicalassessment (Regulations 30, 32 and 33). Thecontents of a geotechnical assessment are setout in Schedule 1 of the Quarries RegulationsACoP, of which the following are pertinent tothis paper:1. Site survey2. Site investigation6. Findings of analysis8. Requirements during and after

constructionUntil recently, geological and geotechnical

data were difficult to obtain safely and thus,in some situations, the analysis andsubsequent design criteria could be flawed.This paper outlines an integrated approach

to geotechnical data collection using long-range, high-definition LiDAR (light-detectionand ranging) surveying equipment anduniquely specialized geotechnical analysissoftware to acquire and interpret rock-massdata from a ‘safe’ distance from the quarryfaces. Indeed, in some cases the analyses canbe undertaken from as far away as the quarryperiphery.The approach outlined in this paper not only

provides for remote geotechnical datacollection, but also allows a significantlymore detailed ‘survey’ of the faces to beobtained. It also provides a permanent recordof the condition of the quarry faces on the dayof the assessment, which can be referencedat a later date if stability issues arise.

SSiittee ssuurrvveeyyTraditionally, a quarry survey would be createdusing either dGPS (differential GlobalPositioning System) or by total-stationsurveying instruments. The resultant surveywould comprise a series of break-line data (topof face, bottom of face etc) and spot levels uponwhich the ground contours would be derived.For the purposes of providing a topographicalsurvey for quarry plan production, setting outor producing a reserve calculation, thismethodology is perfectly adequate.In situations where greater detail is

required upon a quarry face (eg changes ingeology, fault planes, back scars fromrockfalls, or simply more data points for amore accurate rockfall analysis), the surveyormay obtain a series of spot levels on thequarry face using direct-reflective (DR)techniques. These data are relatively slow toobtain as each spot level has to be aimed from

A ‘Hands-off’ ApproachGeological and geotechnical mapping and analysis using long-rangehigh-density LiDAR surveying equipment

By A.P. Wilkinson, QuarryDesign Ltd

March 2011 www.Agg-Net.com 21

the instrument.LiDAR surveying has improved dramatically

in recent years, and with faster acquisitionspeeds and ranges in the order of 3,000m, itis now possible to effectively scan quarry facesfrom outside the working area (eg from anobservation platform or screening bund on theouter edge of the quarry). It should be notedthat not all LiDAR scanning equipment is thesame. Exceptionally high-resolution, short-range scanners are used in Formula 1 motorracing (to obtain 3D models of cars forcomputer wind-tunnel simulations), whilemedium-range scanners are used for internalbuilding and factory surveys. Long-rangescanners are often employed to undertakecoastal erosion monitoring and landsliderisk analyses for strategic infrastructure inmountainous areas. It is this latter approach,using Optech’s ILRIS-3D ER scanner, thatQuarryDesign Ltd have adopted for thequarrying industry.LiDAR scanning produces a ‘point cloud’ (a

series of very closely spaced points witheither an RGB colour value (from a digitalcamera) or a greyscale ‘intensity’ value (fromthe amount of returned light being recordedback at the scanner), as shown in figures 1,2 & 3.

Fig. 3. Scan of Tarmac’s Stancombe Quarry processing plant against a backdrop of heavily vegetatedslopes and clean quarry faces. In this example greyscale ‘intensity values’ are shown

of software for the production of plans or forfurther geological and geotechnicalinvestigations or analyses.

SSiittee iinnvveessttiiggaattiioonnIt is possible to extract both geological andgeotechnical information regarding the rockmass from these point clouds, and thus theuse of LiDAR scanning can contribute greatlyto the site investigation element of theRegulation 33 Geotechnical Assessment.In terms of geological mapping, it has

been found that different strata returndifferent quantities of the originally

transmitted light (ie they have differentrecorded ‘intensity’ values). For example,clays, shales and vegetation exhibit low-intensity values (with a greater amount of thelight being absorbed than being reflected) andare initially processed as dark grey points,whereas granites exhibit high-intensity values(with more light being reflected thanabsorbed) and are initially processed as palegrey or white points.In a similar manner, different grades of

weathering of the same material can returndifferent intensity values; with more-weathered material returning lower-intensity values than less-weatheredmaterial. ‘Groups’ of similar intensity pointsrepresenting similar grades of weatheringcan be separated and coloured, makingthe visual analysis easier (fig. 4). Thenumber of points within each group ofsimilar coloured bands can also be summedand expressed as a relative percentage of thetotal number of points (in effect, producinga point-sampling method based upon theexposed surface of each grade of weatheredmaterial).In addition to geological mapping of the

strata, geotechnical data can be obtained fromthe LiDAR scans in the form of discontinuitydata (dip, dip direction, spacing, persistenceand roughness). Although these data areobtained from the processed scans back in theoffice, their creation does, in effect, stillform part of the site investigation, as it

22 www.Agg-Net.com March 2011

Fig. 1. Scan of a coastal rock face where each point has an RGB (red green blue) value allocated to itfrom the associated digital photograph (note: figure 1 is not the photograph but an actual screen shotfrom the 3D model)

Fig. 2. Close-up of a section of figure 1 whereeach individual point can be seen (in this casewith a point spacing of 25mm)

The resultant point cloud can be eitherprocessed on a ‘local’ co-ordinate system or geo-referenced to the national co-ordinate system in the same manner as traditional surveys. It can also beconverted into a triangulated mesh orwireframe digital surface model (DSM),which may include vegetation and buildings,or a digital terrain model (DTM) wherepoints above an interpolated ground surfaceare removed.From these DSMs or DTMs the relevant

data (break lines, cross-sections, meshes orxyz points) can be exported into other suites

Fig. 4. Coloured groups of points representingpoints with similar intensity valuescorresponding to different grades of weathering(both along the orthogonal joints and as anexfoliation surface of a block of granite)

March 2011 www.Agg-Net.com 23

Prospecting & Surveying

replaces traditional methods of discontinuitydata collection using a compass-clinometer(fig. 5).Discontinuity data can be obtained by

either manually digitizing each joint plane andrecording its dip and dip direction (fig. 6) or byutilizing automated proprietary softwarespecifically written to obtain geotechnical datafrom collected point clouds (figs 7 & 8).There are advantages and disadvantages toboth methodologies and a sensible approachis to adopt an automated analysis reinforcedby manual analysis.With a manual point-cloud analysis, the

engineer has the same control that he wouldhave had in the field had he been collecting

the data with a compass-clinometer, butwith the added benefit that the readings willnot be restricted to low ‘safe’ faces or to theheight of the engineer.However, this manual process can be slow

to undertake and, as is human nature, can beinfluenced by what the engineer thinks are thejoint sets (ie if he sees three sets he willpreferentially record the dip and dip directionon the joints that match those sets).One of the many advantages of an

automated method is that it produces far moredata and can reduce the potential ‘human’influence of the engineer. In figure 8, a‘shotgun scatter’ of readings has been plottedon the stereonet for every visible joint planerecorded within a certain tolerance. Thismeans that blast-induced joints are alsorecorded, which widens the distribution of thedocumented joint orientations. If these datawere to be exported into DIPS (where each

joint reading is equally weighted), thesubsequent kinematic analysis would bedifficult to undertake. However, the automatedsoftware also expresses the area of eachmeasured joint, with larger circlesrepresenting larger exposed joints (as shownin fig. 8). In this example, a series of largejoints forming Joint Set 1 can be observeddipping steeply at 83° towards 065° (EastNorth-East). What is also apparent is that oneof the joints in that region is very persistentand at a slightly different orientation(65°/048°); this actually represents a localizedminor fault and is not part of the joint set.

FFiinnddiinnggss ooff aannaallyyssiissAs shown in the section above, data obtainedby the LiDAR survey can be used togeologically map and geotechnicallycharacterize quarry faces. The collected datacan then be used to determine the potential

Fig. 5 A potentially unsafe and possiblyredundant method of obtaining discontinuitydata

Fig. 6. Manual digitizing of discontinuity data

Fig. 7. Automated discontinuity data

Fig. 8. Stereographic interpretation of automated discontinuity data

24 www.Agg-Net.com March 2011

modes of failure (kinematical analysis) and,moreover, to calculate ‘factors of safety’and/or ‘probabilities of failure’ for a givenfailure mechanism. For example, circularfailures in weak rock masses, soil slopes,embankments and lagoons; and planar,wedge, and toppling failures in rock masses(figs 9 & 10).Furthermore, the enhanced survey detail

obtained by LiDAR surveying also greatlyincreases the engineer’s ability to analyse thepotential trajectories of rockfalls. Figure 11shows Rockscience’s 2D RocFall softwarebeing used with an ‘older’ cross-sectionobtained by direct-reflective (DR) surveyingtechniques. This cross-section compareswith figure 12 which shows a more recentRocFall analysis based upon a high-definitionLiDAR face survey.The overhangs and ledges on the detailed

LiDAR survey can clearly be seen to beplaying a major part in the potentialtrajectories of rockfalls. These could easilyhave been missed on ‘simpler’ cross-sectionsand the resultant remediation measures (egrock traps) under-designed.Recent software developments (notably

in the US) have used long-range, high-definition LiDAR surveys to assess thepotential locations and hazards associated

with landslides and rockfalls on to publichighways, railways and other infrastructure.QuarryDesign are working with a US-

based company to provide 3D simulations ofthe potential trajectories of rockfalls fromquarry faces (fig. 13).One of the exciting potentials of this new

software is that it can account for the breakingup of larger blocks into smaller fragments andproject their potential trajectories as well asfor the whole block. It also demonstrates themitigation effect of rock traps and fences.Moreover, it is influenced by whole sectionsof the quarry face and not just single cross-section locations. In figure 13, the path of thetrajectory is clearly oblique to the quarry faceand would not have been predicted in a 2Danalysis. With the new 3D approach, slopingledges are accounted for and can be shownto cause rockfall material to bouncetangentially across as well as down a quarryface.

RReeqquuiirreemmeennttss dduurriinngg aannddaafftteerr ccoonnssttrruuccttiioonnAs LIDAR scanning produces a rapid detailedsurvey of a given quarry face or slope, it canbe undertaken on a periodic basis toaccurately measure and record potential

changes in slope geometry, and can satisfy(where required) a need for monitoring.Using LiDAR surveying techniques it is

possible to monitor the performance ofslopes, tips and lagoon walls, and to calculatethe rates of any developing circular failures,wind erosion of sand faces etc.QuarryDesign are currently trialling

seasonal monitoring of several natural andquarried faces to ascertain if small changesbetween successive surveys might allow themeasurement of potential rock-massdisplacement, due to either repeatedfreeze/thaw cycles or a reduction in normalstress due to ‘unloading’. It is hoped that suchdisplacement measurements might be usedto predict the location of future rockfallevents before they occur (fig. 14).Moreover, with the average spacing of the

fractures being obtained from the point-cloud analysis described above in the SiteInvestigation section, the correct rockfallseeding location and block size can bedetermined and used in the 3D rockfallsoftware shown in figure 13.

CCoonncclluussiioonnLong-range, high-definition LiDAR surveyingtechniques can be used as part of anintegrated approach to geological and

Fig. 9. Slope stability analysis Fig. 10. Planar failure analysis

Fig. 11. DR survey and 2D rockfall analysis Fig. 12. LiDAR survey and 2D rockfall analysis

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Prospecting & Surveying

geotechnical mapping, and to allow moreaccurate data to be collected significantlyquicker and more safely.Furthermore, advances in both computer

processing power and software engineering

are allowing more complex (and realistic)simulations to be undertaken.All of this means that, as well as removing

the potential risk to the engineer, the quarrydesign criteria or remediation advice being

offered by the engineer should be moreaccurate.For further information contact

QuarryDesign Ltd on tel: (0121) 288 3228 oremail: info@quarrydesign.com

Fig. 14. Displacement measurements between successive rock facemonitoring surveys showing expansion of the rock mass (green) androckfall location (blue and pink)

Fig. 13. LiDAR survey and 3D rockfall analysis