Lidar is a high definition, 3-dimensional remote sensing method, which uses a pulse of light emitted from a laser scanner and reflected from an object to determine range. Using this technique, millions of points (x, y, and z) form a high-density point cloud, which can be manipulated by the end user to create a plethora of surface models, volumetric reports and surface characterisation reviews.

Where total stations use prisms to calculate distance, LiDAR systems use pulses from an onboard laser. The laser scanner fires a ray of pulsed light and then calculates how long it takes for that pulse to reflect from a surface back to the system. Mobile mapping systems are the latest iteration of the traditional laser scanner and are capable of capturing over one-million measurements per second.

Mobile Laser Scanning (MLS) systems are becoming prevalent in the Civil Engineering and Construction sector thanks to their highly portable and compact nature and their remote capture capabilities.

A DIFFICULT ENVIRONMENT

Deployment of LiDAR in tunnelling environments is increasing significantly, but this has not always been the case. Previously, the challenging conditions associated with tunnelling, such as dust, light, damp and space, restricted the development of this technology in these areas, with other remote sensing techniques such as total stations or digital photogrammetry taking precedent. However, as technology advances and the need for real time, automated systems increase, we are now seeing LiDAR becoming the go-to solution for large-scale remote sensing and mapping

Systems such as Process Monitor Live (by the author’s company) have utilised high speed, highly accurate scan-to-scan registrations without the need for targets to develop 3D point clouds that do not impact excavation workflows. Laser scanners adhere to an Ingress Protection (IP) rating, which classifies the degrees of protection from intrusion of dust and water. Most scanners will now register towards the top of the rating system, meaning no ingress of dust or water will affect the internal mechanisms.

Short range laser scanners deployed in tunnels are becoming smaller, lighter and ruggedized to deal with the restricted space available in busy tunnels. For instance, the scanner used by the author’s company weighs 3.7kg, is 180mm wide and IP65 rated. The 3DLM-SR80 is a short-range laser measurement system specifically designed to operate in unfavourable environmental conditions meaning that the data collected is still dense and accurate.

LiDAR solutions use the laser pulses as their own source of light, so can operate in limited visibility or complete darkness. This is obviously ideal for underground or tunnel applications where natural light is non-existent.

The focus is also on the software capabilities as much as the hardware capabilities. It is crucial that the accompanying point cloud software is specifically designed for the end user. In this case, two modules are usually developed: acquisition and analysis.

The software modules that are used to acquire scans are designed to basic, clear and designed for touch screen commands. These modules will normally be installed on a laptop or tablet, which will be used at the heading of the tunnel. Typically, the analysis software modules are designed to view, align and model point cloud data, a function of data quality and computing power, so require a large 3D viewer and a more detailed interface than the acquisition software. By adapting the software with the end user in the tunnel in mind, the process of integrating LiDAR in to an excavation workflow becomes rather simple.

By reducing scan times, eliminating target registration and developing advanced hardware and software capabilities, LiDAR is now a greatly effective tool for the tunnelling industry.

APPLICATIONS OF LIDAR IN TUNNELS

One of the key benefits of LiDAR in a tunnelling environment is its versatility. Many applications exist that can benefit contractors, engineers or geologists to enhance safety, quality control or financial productivity.

For contractors, tunnel engineers and quality control officers, applications such as tunnel profile mapping and automated shotcrete thickness reports give a level of accuracy that cannot be obtained and sustained by human measurements. Data collected during construction phases can also be assessed and interpreted to determine where areas have been over profiled or areas which require strengthening. This helps save time on repeated manual inspections and also means that valuable materials and resources are only deployed where necessary.

The benefits also extend to geologists, structural geologists and engineering geologists. Surface characterisation is a commonly used application of LiDAR due to the high-density point cloud that is achieved, which can determine joints, faults and centimetre scale fractures. By completing multiple scans during the excavation cycle, feature mapping becomes a significant tool that can be used with LiDAR and can lead to structural discontinuity geometry mapping, structural overbreak analysis and topographical mapping.

The future looks bright, too. Research and development of underground LiDAR scanners is ongoing and will inevitably overtake current methods which rely heavily on human input. Regardless of the amount of progress made in developing and streamlining LiDAR into excavation workflows, there will always need to be a significant interpretative input for constant quality control. In the near future, the author believes LiDAR will be a common tool in a variety of tunnelling environments for geotechnical monitoring, enhancing safety and productivity