Notice: This page was translated automatically. If anything seems unclear, please refer to the German version.

The use of drones and photogrammetry in geotechnical engineering

Abstract

Due to rapid advances in computing technology, it is now possible to process extremely large amounts of data in a short period of time. Accordingly, it is now feasible to use photogrammetric methods, with the aid of drones, to solve geotechnical engineering problems and open up new fields of application for practical use, or to create new solutions for problems that have not yet been solved. This publication describes the use of drones in special civil engineering and their application in geotechnical engineering. Photogrammetric methods are used to answer specific questions quickly and efficiently. It describes the use of drones and photogrammetry for structural monitoring, movement monitoring and deformation observation of retaining structures made of plastic-reinforced earth/gabions, for quantity and mass checks during installation, for documenting construction processes and for special tasks. The use of these methods for quality assurance of KBE structures is presented, specifically addressing the question of whether the completed structures meet the quality criteria desired by the client in terms of deformation, position and tolerances. The method and its possible applications are explained using a series of practical examples. Accuracy, limits of application and boundary conditions are also covered in the publication.

Introduction

Increasingly complex construction processes, faster sequences and interlocking construction concepts require the use of new technologies in all areas of construction. These technologies also include photogrammetry and drones. The current state of technology enables efficient use and opens up new fields of application. This publication presents the basics for the application of photogrammetry and drones in

geotechnical engineering, the fields of application, boundary conditions and fundamentals. Information on the technical

procedures and practical examples of use are presented here. In conjunction with photogrammetric methods, drone technology can be used efficiently for quantity and mass determination, for building surveillance and monitoring, and for special applications. The starting point and topic is the increasingly widespread use of support structures made of gabions and KBE structures, either in combination or individually. If these structures are used in the field of public road construction management, the regulations for engineering structures in accordance with ZTV-ING /1/ apply to these structures.

Fig. 1-1: BAB A6 Nuremberg

In order to be able to regularly assess the durability and condition of the structures, these constructions are subject to mandatory testing in accordance with DIN 1076 and the RI-EBW-Prüf /02/ test manual. Structures are subject to regular inspection and must be checked for deformation and structural condition. This regularly leads to problems, especially when the structures are located in public road networks, as this must be done while traffic is flowing. Contactless methods that do not interfere with traffic, such as photogrammetry in conjunction with drones, are ideal for this purpose. This is generally permitted in accordance with the test manual /2/. Annex 1 of the test manual expressly recommends and approves the use of drones and photogrammetric evaluation.

Table 1-1: Extract from the gabion test manual

In this respect, the legal and regulatory requirements for use in structural monitoring are met. Regardless of this, as we will explain below, there are other areas of application that will be considered here as follows:

• Application for quantity and mass determination
• Application for structural monitoring and structural surveillance
• Deformation monitoring
• Special applications

Photogrammetry and drone technology

Photogrammetry

Photogrammetric methods refer to the contactless capture of object data using optical

methods, usually with photographs. This usually requires a large number of images in digital form. The position data is recorded at the time the images are taken. Since the position of the images in space is known, a three-dimensional object model can be calculated from these boundary conditions. Triangulation methods are usually used to calculate the 3D models. The following figure illustrates the principle.

Fig. 2.1-1: Principle of photogrammetry

The advantages and disadvantages of photogrammetric methods are already apparent from this. These will be briefly explained below:

Advantages:

• Contactless acquisition of surface and geometry data
• Possibility to check the data (visual inspection via image)
• Large amounts of data can be captured in a short time
• Data capture is automated or can be carried out with minimal personnel
• The process allows extremely large amounts of data to be mapped, including large models, structures, etc.

Disadvantages:

• Good lighting conditions are required (no shadows / limited or not applicable at night) • Requires a lot of computing power and software
• Requires large storage volumes for raw data and evaluation data
• Measurement accuracy and thus model and evaluation accuracy are highly dependent on hardware and software and, to some extent, on weather conditions
• Requires calibration of 3D models

A detailed examination of the methods and explanations can be found in /3/. With regard to the accuracy of photogrammetric methods, these depend largely on the hardware and software technologies used. In principle, accuracy depends on the area of application and the size of the object.

Table 2.1-2: Photogrammetric areas of application depending on object size and accuracy

For photogrammetric methods in architecture and construction in conjunction with drones, the following approximate relationship can be used for the achievable accuracy:

Accuracy: g ≤ f (fill height H)

g ≤ 0.001 x H [mm]

Drone technology

Drone technology is generally used to enable the fastest possible, large-scale capture of objects with photos with as little contact as possible. This technology is defined as "UAV photogrammetry" (UAV = unmanned aerial vehicles).

Fig. 2.2-1: Device technology

Drone technology, originally developed for military purposes, has now become so powerful and affordable that its widespread use in the civilian sector is also conceivable. The standard configuration of a commercially available drone system consists of a drone (quadcopter or hexacopter), which is usually equipped with a high-resolution (4K) camera. To achieve sufficient stability when taking photographs during flight, the camera is supported by an automatic positioning system (gimbal). The drones are remotely controlled. They can be controlled via laptop, PC, tablet or mobile phone. Commercially available drones can fly for between 30 and 60 minutes. Drones come in different weight and size classes. All have the following advantages and disadvantages:

Advantages:

• Easy to operate
• High manoeuvrability and flight stability
• Depending on the system, very precise and low-noise images
• Extremely easy to operate and inexpensive Disadvantages:
• Flight conditions depend on weather conditions, high susceptibility to wind
• For larger construction projects, the current flight times of 30 to 60 minutes possible with electrically powered drones are usually insufficient due to the necessary batteries. Spare batteries are necessary.
• Depending on the object and location, drone flights are subject to authorisation and, in some cases, impossible under the Drone Regulation.

Special rules apply to the use of drones in the civil sector in the Federal Republic of Germany. The Drone Regulation /4/ must be observed in this regard. The Drone Regulation distinguishes between aircraft according to weight. The following applies:

> 0.25 kg: Labelling requirement

> 2.00 kg: Proof of knowledge

> 5.00 kg: Permit required

In general, a special permit is required for flights above 100 m. As in normal "air traffic", no-fly zones apply in the vicinity of industrial facilities, nature reserves, crowds of people and airfields, federal authorities, etc. As a rule, a corresponding flight permit should be obtained before flying over an object. This can be obtained from the relevant aviation authority within 2 to 3 working days for a fee. The application and approval process is usually straightforward. The following figure shows the boundary conditions again.

Fig. 2.2-2: Boundary conditions according to /4/

Software and hardware requirements

Digital systems are generally advantageous for taking the necessary photos. These record a large number of high-quality images. It is generally recommended to work with a resolution of at least 4K in order to achieve the appropriate point density and accuracy during evaluation. This can already be achieved with commercially available cameras today. However, special computing methods are required to process the photos, which can create 3D point clouds from the geotagged images and in turn generate 3D models from these point clouds. A relatively wide range of both commercial and non-commercial programmes are available on the market for this purpose. In the commercial sector, PointCab, AgiSoft and Pix4D are worth mentioning. A large number of shareware programmes are also available. However, these differ in terms of their range of functions and compatibility. For professional use, it is important that these programmes have a functional interface to common CAD programmes (dxf, dwg, etc.), as the photos are basically only used to generate a 3D point cloud. A 3D surface model with meshing must be created from this point cloud.

meshing. 2D and 3D data can then be read from this meshed 3D model. The superordinate model for CAD programmes is usually the 3D meshed model.

Basic procedure

If projects involving photogrammetric methods and the use of drones in any form are to be planned, a general procedure as described below must be taken into account.

Preparatory work:

The preparatory work includes the necessary examination of the boundary conditions, obtaining the necessary flight permits, selecting the appropriate flight system and considerations regarding the accuracy of the evaluation. In any case, it is advisable to consider calibrating the system during the preparation phase. With regard to calibration, please refer to section 2.5.

Data acquisition:

Once all requirements have been clarified, data acquisition can be carried out by means of aerial photography. In doing so, it must be ensured that sufficient image coverage is maintained. As a rule, in order to achieve sufficient point and object density and photo overlap for subsequent point cloud generation, at least 40-60% of the photos must overlap. This requires sufficient longitudinal and transverse coverage and the creation of a flight plan. A basic distinction must be made here between linear and circular flights. Circular flights are suitable for point objects, while linear flights, if possible with a return leg, are suitable for elongated objects. The following figure shows possible coverage models.

Fig. 2.3-1: Coverage models

Fig. 2.3-2: Flight paths

Calibration:

During the recording of the object and the creation of the base photos, it must be possible to calibrate the entire model. Since dimensions cannot be read directly from the photos themselves, calibration points must be provided for each object. To do this, it is advisable to set fixed points at the outer edges of the field of view before starting the recordings. These can be marked in colour so that they are clearly identifiable in the photographs. In order to then be able to calibrate the point cloud models, these points must be recorded conventionally in terms of position and height so that the 3D model can be assigned in space. Conventional GPS surveying methods are suitable for this purpose. Once all field data has been determined, the photographs are processed using the methods described in section 3.3.

. This involves creating 3D point clouds from the photos and the geotagging information recorded during the recording. These are then meshed and processed into 3D models so that they can be edited in CAD systems. The aim of image processing and evaluation is to obtain true-to-scale 3D models that can be used for further planning in standard CAD systems, either in 2D or 3D. Surface and edge models are possible. The advantage here is that the point clouds remain parallel as a database for the 3D models used, meaning that visual information is available during further processing.

Fig. 2.3-3: Point cloud

Results:

Any information (longitudinal/cross-sections, quantities, volumes, etc.) can then be determined from the digital data. This is usually done using the aforementioned program systems or alternative methods. The accuracy depends on the boundary points and boundary conditions mentioned in the previous sections.

Application

Application for volume determination

One of the main applications of the described method in geotechnical engineering is volume and mass determination. In the following example, a direct comparison was made between a terrestrial survey and a photogrammetric survey of a waste dump. Often, the problem here is that the data must be collected quickly and in a timely manner, but at the same time, as in the present case, it is difficult or impossible to access the landfill site in order to comply with occupational health and safety requirements. The task here was to determine the volume of waste to be disposed of in the landfill. In a first step, a conventional terrestrial survey was carried out, followed by a drone flight in a second step. Figure 3.1-1 below shows the result of the terrestrial survey with meshing. Figure 3.1-2 shows the result of the drone flight.

Fig. 3.1-1: Result of terrestrial survey

Fig. 3.1-2: Result of drone flight

Table 3.1-3: Results of volume comparisons

Table 3.1-1 compares the results of the volume comparisons. As can be seen, the terrestrial and photogrammetric methods are almost identical, with the photogrammetric methods showing slightly higher volumes. This is due to the higher point density and accuracy. These methods are suitable for volume determination because they provide much more accurate volume information due to the higher point density and the resulting higher network density. Another advantage here is that, in contrast to terrestrial methods, optical information is retained in addition to geometric information.

Deformation measurements

For special tasks, it is often necessary to carry out deformation observations. Photogrammetric methods in conjunction with drone technology can also be used advantageously here. The following example shows the monitoring of an excavation pit shoring and its deformations and the deformations determined. In the case of the object mentioned, an excavation pit with a depth of approx. 8.00 m was to be constructed with simple back anchoring. Here, the excavation pit was completely recorded and flown over at regular intervals. On the one hand, as explained in section 3.1, the excavation volumes could be determined very accurately over time, and on the other hand, this also enabled parallel deformation monitoring of the shoring. The following figures show a section of the shoring after completion.

Fig. 3.2-1: Shoring process

Fig. 3.2-2: Development of shoring

Fig. 3.2-3: Deformation pattern (green v ≤ 2.0 mm)

The deformation is determined using the generated volume models and their calibration by calculating the differences. At time T1, a volume model of the excavation pit or the object to be observed is created, and at a later time (T2), the same model is created again. The models are then superimposed and the differences are shown. The differences determined are the deformations sought. However, this requires that both models are calibrated accordingly. The advantage of using this method is obvious. On the one hand, data can be collected without contact or touch, and on the other hand, data can be collected across the entire object, rather than just at specific points, as is usually the case. This means that the entire object can be monitored using a single method. The time and monetary advantages are clear.

Structural monitoring of gabion/KBE constructions

As explained at the beginning, it is essential for the structural inspection of gabion and KBE constructions that deformation monitoring and visual inspections are carried out at regular intervals. Photogrammetry with drones can be used effectively for this purpose. The following figure shows a construction that has been monitored at regular intervals.

Fig. 3.3-1: Deformation detection

This is a KBE structure with a gabion facing shell. Deformations were detected in one section during regular aerial surveys. The basic principle here is similar to the previous example. Flights are carried out at regular intervals and a calibrated 3D model is created. Deformations can be determined by calculating the differences between the 3D models. The following figure shows the 3D model with the calibration points.

Fig. 3.3-2: 3D model with calibration points

After superimposing the models or point clouds with the design drawings (see Figure 3.3-3), deviations can be easily identified.

Fig. 3.3-3: Superimposition of design plans with determined deformations

This enables comprehensive, non-destructive and highly accurate inventory recording.

Data on irregular objects

Geotechnical issues often require the determination of geometrically difficult objects. For example, if stability assessments of rock slopes, embankments or rock overhangs with or without crevices are to be determined, this is not possible using normal methods. However, this data must be obtained for the planning of safety measures (e.g. meshing, nailing). For safety reasons, it is usually not possible to walk or climb on these structures, and even if it were, the data yield would be very low. Therefore, the use of drone technology with photogrammetry is an ideal application for such situations. The following example illustrates such a situation. A dangerous rock slope had to be secured. In order to obtain sufficient data density and basic information for the tender, quantity and mass determination, and planning, the situation had to be recorded using measurement technology. Determining the data and creating a 3D model using conventional methods would have been very costly and insufficient. Clefts, fissures, etc. would have had to be recorded separately, and the geometric data via GPS. Using photogrammetry and drones, it is possible to create a 3D model with a single flight.

Fig. 3.4-1: 3D model

Fig. 3.4-2: Meshing

All geometric specifications can be determined precisely from the model. The planning of safety structures, such as anchors and nets, can be carried out with sufficient accuracy on this basis. Another advantage here is that, thanks to the photos used in the planning process, fissures and fissure structures can be taken into account directly in the planning, and anchors/nails as well as meshing and safety devices can be positioned accordingly without any subsequent problems (additional work) arising on the construction site.

Special applications:

The methods can also be used effectively for special tasks. A direct comparison with density determination using a densitometer was checked below. Volume determination is usually carried out with a densitometer to determine the compaction achieved in soils. Alternatively, however, volume determination can be carried out using photogrammetric methods. The following figure shows the method.

Fig. 3.4-3: Volume determination using a densitometer

Fig. 3.4-4: 3D model

Table 3.4-4: Results

The results in Table 3.4.1 show that volume determination generally produces an analogous result. The aim of the investigation was to assess whether adequate results are possible, which was confirmed. This method is definitely suitable for larger fillings (stone and rock fillings). Here, volume determination can be carried out using this method, so that densitometer methods or methods that are not suitable for loose rocks with a maximum diameter > 63 mm can be replaced.

Quality assurance for gabions

Another field of application is the determination of the quality of filled wire gabions. There is often discussion here as to whether and how the filling was carried out. Was it low in voids and uniform, and does it comply with the static specifications or not? Photogrammetry can be used analogously for this purpose.

Fig. 3.5-1: Examination of cavity uniformity

Fig. 3.5-2: Examination of cavity uniformity

When using photogrammetry for this purpose, the procedure is similar. High-resolution photos are taken of the entire surface of the gabions, a 3D model is created and the void content is determined from the 3D model. In addition to the cavity content, deformations of the wall surface, stone sizes, wire spacing, use of fasteners and missing areas can be recorded across the entire surface.

Summary

The previous sections explained the boundary conditions for the use of drone technology and photogrammetry as well as their fields of application using practical examples. The following conclusions can be drawn from the investigations:

• Photogrammetry in conjunction with drone technology is a simple, fast and cost-effective method for determining volumes, deformations and special tasks in geotechnical engineering.
• Contactless measurement in conjunction with the stored photos allows for highly precise and accurate planning. The density of information that can be obtained with the stored photos is significantly higher than with conventional surveying methods.
• Planning is always based on a 3D model. This allows for more efficient variant analyses and considerations than with conventional 2D planning. Quantities and masses can be determined more accurately and implemented more precisely in tenders and planning.
• There is a need to calibrate the models and systems. Georeferencing is absolutely essential here.
• The observation method according to DIN 1054 can also be used for larger measures and permanent use and can be recommended for this purpose.
• For structural monitoring in accordance with DIN 1076, especially for gabion and KBE constructions, the method is an optimal method for comprehensive deformation determination and for verifying the quality of the workmanship achieved.

LITERATURE

/1/ Federal Highway Research Institute (2013). ZTV-ING – Additional Technical Contract Conditions and Guidelines for Engineering Structures, FGSV Verlag;

/2/ Federal Ministry of Transport, Building and Urban Development (2017). Feedback-EBW-PRÜF – Guideline for the uniform recording, evaluation, documentation and analysis of building inspection results in accordance with DIN 1076;

/3/ Čuboň, P. (2017). Master's thesis – Possible uses and areas of application of photogrammetry/drones in geotechnical engineering;

/4/ Federal Ministry of Transport and Digital Infrastructure (2017). The new drone regulation, available: http://www.bmwi.de/SharedDocs/DE/Publikationen/LF/flyer die-neue-drohnen-verordnung.pdf