ROCK TUNNELS ARE USED EXTENSIVELY for conveying water in industrial and municipal applications, such as hydro and nuclear power, water for drinking, sewage water, raw water supply to industrial plants, and other instances. Hydropower is a major application of this infrastructure: tunnels in hydropower plants are usually a few kilometres in length with some even being tens of kilometres long.
Although such tunnels are constantly flooded, they still need monitoring and maintenance inspections at regular intervals. In addition to rock fallouts, sediment collection can also impede water flow and cause blockages. Besides blockages in waterways, tunnel instability can in addition affect soil stability on top of the tunnel and cause gullies, with the potential risk of damage to third-party property and life.
For cost and operational reasons, tunnel inspections should affect operations as little as possible. This can be achieved by performing inspection and maintenance with a remote-controlled underwater robot, a so-called ROV (remotely operated vehicle) equipped with sonar, cameras, and instruments for measuring and sampling. An ROV can operate in a tunnel without the need for dewatering it and under safe working conditions.
DEGRADATION OF ROCK TUNNEL’s STRUCTURAL INTEGRITY
The structural integrity of rock tunnels relies on the rock mass itself in combination with reinforcement comprising rock bolts and shotcrete/lining, steel structures or cast concrete vaults. The tunnel structure must be designed, dimensioned and constructed so that harmful degradation is prevented during the technical lifetime. Inspections and continuous maintenance of rock tunnels are necessary to ensure tunnel stability during the planned period of service.
Degraded tunnel structures share many similarities, regardless of whether this occurs in dry or flooded tunnels. In a flooded state, the hydrostatic pressure and its variations, as well as abrasive water flow, are added as possible influencing factors. The main features affecting the structural integrity of a flooded tunnel can be summarised as:
¦ Degradation of the rock mass (e.g. due to poor rock quality, swelling clays, blasting damage and rock stresses).
¦ Degradation of rock bolts (e.g. corrosion of non-grouted bolts, leaching of cement mortar, chemical degradation of cement mortar)
¦ Degradation of shotcrete/cast concrete (e.g. carbonation, leaching, erosion, corrosion of fibres/mesh).
¦ Variations in hydrostatic water pressure (e.g. pressure stroke, swelling or negative pressure during load changes or shutdowns).
REMOTELY OPERATED (UNDERWATER) VEHICLES
ROV equipment was developed in the 1950s – mostly for military and research purposes. Since 1970, the equipment has been developed for commercial use, mainly due to the boom in oil and gas exploration. Today, there are numerous types and sizes of ROVs that are tailored for a specific purpose. For tunnel inspections, so-called ‘inspection’ or ‘mid-sized ROVs’ are most often used. They usually weigh between 100kg and 1,000kg and are optimised for medium depth and long range.
The ROV can be adapted to specific needs and tunnel dimensions. Factors to be considered in the choice of ROV are tunnel length, size and layout; the size of the inspection hatch; and possible obstacles in the tunnel. The challenge is to achieve an optimal balance between the size and thrust power to be able to navigate in close spaces, and at the same time, be able to pull the tether containing data and power-supply for long distances out from the access point.
The access point to the water surface can be an open water basin, swell tower, inspection hatch or access tunnel. At the water surface, the ROV is released and moves submerged in the water using thrusters. The tether for power and data transmission has the same density as water, providing neutral buoyancy and permitting long-distance runs. ROVs typically move at an average speed of around 1km/h.
The position of the ROV in the tunnel being inspected is monitored by readings from a cable counter, the DVL (Doppler Velocity Log), depth sensors and compasses. Positioning sensors can generate a degree of length deviation (up to one metre) which increases further away from the access point. However, this problem can be easily addressed if length measurement is calibrated against known fixed points, for example, at intersecting tunnels or fixed installations with known positions.
SONAR TECHNOLOGY
Equipping an ROV with sonar makes it possible to scan the tunnel geometry and block or sediment masses even in poor or zero-visibility conditions.
Before 2016, a single beam-scanning sonar was used to provide line profiles at periodic intervals. This required the ROV to be held in position for the conduction of every single scan. It generated limited coverage, limited pure data traceability and slow inspection speed.
In 2016, Teledyne Marine developed the BlueViews T2250 multibeam sonar that proved to be ideal for tunnel inspections. Loxus 3D Tunnel Inspections was the first ROV operator to equip its ROVs with this new 3D multibeam sonar. The BlueViews T2250 is a stateof- the-art, compact system, specifically designed to produce 3D data suitable for the inspection of tunnels with centimetre precision. The system features high-frequency, low-power acoustic multi-beam technology and uses 2,100 overlapping narrow ‘beams’ to create a continuous 360° profile. The multibeam sonar operates at a frequency of 20Hz and creates a dense 3D point cloud. BlueViews T2250 sonar is suitable for use in tunnels with diameters in the 2m-15m range.
Tunnel condition is assessed by a rock/tunnel engineer via live streaming of 3D scanning and video. The 3D sonar data can later be used to create a full 3D model of the whole inspection interval from the point-cloud data.
Known deficiencies in data from multibeam sonar can be caused by gas/air bubbles, vegetation or ice. Disturbance and reflection can also be created towards the water surface if air pockets are present.
TECHNICAL ASSESSMENT OF INSPECTION DATA
Assessing the structural integrity of a tunnel during ongoing ROV inspection requires preparation that includes an analysis of the geological data, as-built documentation and documentation of the previous inspection(s), if available.
During the inspection, the tunnel engineer has the opportunity to follow the ROV inspection through streaming video received from several cameras. The video streams are usually directed towards the tunnel bottom, walls and roof. In good visibility, these different streams can be important tools to evaluate geological parameters (e.g. lithology, blockiness etc) and the status of the reinforcement (e.g. exposed parts on bolts, shotcrete, and concrete/steel elements). Video inspection, however, may be impossible to undertake in poor visibility.
The biggest advantage with sonar data is that the technology works in poor visibility conditions and is independent of lighting and water quality. Accuracy within the range of some centimetres that is achievable with 3D multibeam sonar is sufficient to document the tunnel geometry, the reinforcing elements of concrete/steel, and identify and quantify rock burst, tunnel collapses, sediment accumulation and other phenomena. However, if a detailed analysis of individual objects in millimetre scale is required, only visual data through the camera can be used e.g. photogrammetry and its limitations depending on visibility.
Judgment on the structural integrity of the tunnel based on sonar and visual data has nonetheless its limitations. It must be acknowledged that this is a remote sensing method not comparable to physical inspection of drained tunnels. Important inspection observations such as crack properties, detached blocks, degree of weathering in rock and concrete are difficult or sometimes impossible to identify with remote methods. Therefore, access to an experienced and trained rock/structural engineer and a careful analysis of all available data before the inspection are required in order to be able to make an overall assessment of the structural integrity of the tunnel.
ROVs FOR MAINTENANCE
Apart from inspection, the ROV can also be used for the maintenance of flooded tunnels and can be equipped with a manipulator arm, drilling equipment for sampling and other tools. Accumulations of sediment and sludge are a common cause of waterway blockage at low-water flows or in a tunnel with a complex geometry. An ROV equipped with a nozzle and connected to a vacuum pump outside the tunnel can enable targeted and monitored emptying of the tunnel at several hundred meters distance from the entry point without the need to dewater.
CONCLUSIONS AND DISCUSSION
An ROV equipped with a 3D multibeam sonar is currently the most advanced tool for inspection of flooded tunnels in an efficient and HSE-safe manner. An optimised size of ROV and length of tether allow tunnel inspections to distances of over 10km from the access point. The combination of the sonar and video data provides an opportunity for a trained tunnel engineer to make an overall assessment of the structural integrity of the tunnel without the extremely costly and time-consuming process of dewatering and executing a physical inspection.
Do we need common play rules?
At present, to our knowledge, there are no industry practices, manuals/ guidelines or international/national regulatory requirements for the inspection of flooded tunnels. Tunnel owners handle the maintenance based on their own best practice.
The significant effects of possible tunnel collapse for plant owners as well as liability for damage caused to third parties demand common industry practice and guidelines for the safe and long-term operation of this crucial infrastructure.
