Survey from the sky
Adrian Wilkinson and Rupert Wilks, QuarryDesign, UK, explore the use of drones in quarry surveys.
Modern surveying techniques produce exceptionally high quality rock-face and soil-slope surveys. The interrogation of these, using specialist software, enables a better understanding of the geological and geotechnical properties of a wide variety of industrial mineral and aggregate quarries.
The technologies of long-range high-resolution light detection and ranging (LiDAR) scanning and unmanned aerial vehicle (UAV) derived photogrammetric models allow the production of a point-cloud survey – a survey frequently comprising millions of points rather than the hundreds or thousands obtained from conventional survey methods.
Both LiDAR and photogrammetric – multiple overlapping photograph – surveys produce an initial point-cloud. For topographical survey uses, these can be subsequently post processed to produce the more typical break-line surveys – used to define features such as walls and spot level, a reduced point on the surface – most quarry managers will be familiar with. Both systems can be used from the ground or mounted on aircraft. Increasingly, the aircraft mounted options are now referred to as unmanned aerial vehicle (UAV), unmanned aerial system (UAS), small-unmanned aerial system (sUAS), or remote piloted aircraft system (RPAS) – or simply as a drone.
Photogrammetry relies on two or more (now frequently hundreds or even thousands) of overlapping photographs, some scaling or survey control, and a photogrammetric software package to produce a 3D model consisting of coloured points. One advantage of the photogrammetric technique is that high quality aerial photography can also be obtained for a site, as in Figure 2 on page 14, which shows an oblique aerial image of the Kingsbury works taken by the UAV during the survey.
LiDAR is similar to a traditional total station – an electronic/optical instrument used for surveying – but makes numerous shots on a grid pattern. High-resolution long-range LiDAR scanners can take shots with micro-radian spacing. As well as flight time, shot bearing, and angle, which is subsequently converted to an XYZ coordinate, many LiDAR scanners record the percentage of the light reflected back, measured as intensity, and can colour the point-cloud from a calibrated digital camera. In general, buildings and strong, bright rock types – un-weathered granites and limestones – produce high reflectance points and vegetation and weak dark rock-types – shale, coal, clay, and soil – produce low reflectance points in the point-cloud.
In addition to the collection of ground survey data, these technologies have had a beneficial impact on rock-mass classification, quarry face stability assessments, and slope failure monitoring.
Both photogrammetric and LiDAR derived point-cloud surveys have been used at numerous quarries throughout the UK to help the geotechnical engineer better understand the rock-mass characteristics – joint orientation and persistence, kinematical analyses and projection of potential failure planes.
The two systems produce an exceptionally high quality survey of the quarry faces and, being co-ordinated to the relevant ordnance grid and data centre, the joint sets and fracture orientations can be measured. The advantage of using point-clouds is that fractures out of reach of the geotechnical engineer can safely be measured in the office rather than by compass clinometer in the quarry, and can be done using an automatic joint detection software such as Split-FX, or manually as shown opposite. A second advantage is that a permanent record of the faces at the time the data was collected is retained. In many instances, the ability to cross reference newer surveys with older surveys is useful in building up a 4D picture of the changing geotechnical conditions – being 3D plus time as the 4th dimension.
As well as being able to measure the dip and dip-direction of a joint plane, surveying software enables the extrapolation of an exposed joint plane upwards through previously excavated rock or back beyond the quarry boundary and downwards into the remaining rock. The extrapolation of a given joint plane downwards ensures that the geotechnical engineer/geologist producing the quarry design can predict and ameliorate against future potential planar or wedge failure.
Extrapolation of joint data in this manner has been used to determine the likely break-back distance for a historic – pre-Quarry Regulations 1999 – but still active, large-scale wedge failure in Northern Ireland. In this instance, unstable land in the fields behind the quarry face – lying within the projection of the two intersecting joint planes constituting the wedge failure zone – was accurately determined. This ensured that the operator only fenced the minimal land-take required for increased additional perimeter safety.
Slope failure monitoring
Comparisons between successive LiDAR and, to some extent, photogrammetric point-cloud surveys can be used to monitor rock-mass and slope failure. For centimetre displacement monitoring, LiDAR is the better technology. However, for larger scale movements (decimetre or metre displacements) photogrammetric surveys can be used. The advantage of both systems is that far more data points are monitored compared with a total station and reflector, thus producing a wider understanding of the nature of the movement. That is not to say that total station and reflector survey monitoring is inadequate, as that is the far more accurate system, but rather that these relatively newer survey techniques provide additional data.
In a quarry investigated by QuarryDesign, a steeply inclined veneer of limestone at the quarry’s periphery failed when the soils behind it became oversaturated during a very wet winter. Quarterly monitoring by LiDAR from fixed survey stations has enabled the rates of movement to be calculated. It has been determined that, with rates of movement moderating, the failure may well be reaching equilibrium and be wholly contained within the easement to the site’s boundary.
All quarry managers will realise the health and safety benefits of removing personnel on foot from operational areas wherever possible. As remote surveying technologies have developed over the years, this has enabled occasional visitors such as geologists, geotechnical engineers, and surveyors to spend less time in areas subject to plant movements or with other hazards such as quarry edges/faces or water/silt lagoons.
While it is usually necessary to set out ground control points (GCPs) with traditional survey techniques prior to remote surveying with a drone, GCPs can often be surveyed in safe locations away from plant movements around the quarry perimeter. Where it is necessary to survey a small number of GCPs in operational areas, this can be managed safely and quickly by the surveyor being accompanied by a member of quarry staff – in radio contact with the other plant – usually in a light quarry vehicle with appropriate safety features.
The Kingsbury aerial photo and subsequently produced point cloud model shown in the images above and opposite, were obtained using a GeoX-8000 octocopter, one of QuarryDesign’s first UAVs. QuarryDesign expanded its fleet during 2016 and 2017 with fixed wing UAV systems, which have proven reliable for obtaining point-cloud data for general surveys. The benefit of fixed wing technology is a longer flight time than previously possible with copter type UAVs.
QuarryDesign’s most recent acquisition is the fixed wing UAV boasting vertical take-off and landing technology, using three rotors for initial take-off but changing to a glide mode upon reaching a suitable altitude. This provides the best aspects of both copters and fixed wing aerial systems enabling longer flight times before requiring a landing and battery change while retaining the ability for the pilot to fine control take-off and landing for a greater selection of safe landing areas.
Point-cloud surveys are invaluable in characterising rock-mass failure potential and providing information on joints beyond the reach of the geotechnical engineer. The attitude of these joint planes can be projected back through the quarry boundary to assess any potential failure beyond the quarry and downwards to ensure that a safe and effective design is adopted. The high level of detail offered by these types of surveys ensures that larger failures can be monitored and their movement rates assessed. Regardless of the technology used – terrestrial or aerial, photogrammetric, or LiDAR – this can often be done from the safety of the perimeter or even from the air, which significantly reduces the residence time of the surveyor and geotechnical engineer within the operational area of the quarry.