Geotechnical research in universities and also development of equipment and systems for the drill-and- blast method of excavation are both enjoying further advances.
Norwegian research has produced a new approach to rock stress measurement that can be in-tunnel, tight-behind- the-face and undertaken far more frequently than before but would be easily done with existing equipment. The benefit of the new controlled hydraulic jacking test system would be to gain far more in-situ data and do so as tunnelling progresses, then helping to better decide any local lining support needs and potentially also grouting pressure.
Henki Ødegaard, senior engineering geologist at Multiconsult, tells T&T: “The Rapid Step-Rate Test (RSRT) approach can obtain stress data faster and without hindering the ordinary drill-and-blast excavation cycle much, thus enabling the execution of more measurements than is current practice.”
He adds that this ‘more distributed approach’ to rock stress testing does not demand many new types of equipment to secure the practical and economic benefits of gaining much more real data. Typically, the equipment mainly used on drill-and-blast tunnel projects is needed, adds Ødegaard, whether for new tunnels, or the work in existing tunnels for maintenance or rehabilitation.
New equipment continues to be introduced by industry in support of drill-and-blast, such as Sandvik’s new DT923i jumbo rig and the latest version of the company’s iSURE excavation process control software. Epiroc too has a new boomer, drill and semi-automated explosives charging system.
While government policy initiatives and industrial investments look at ways to help mitigate climate concerns, and new technologies are starting to emerge at mass-scale more economically, such as large electric battery technologies, the switchover to adopting such systems underground is still getting underway, especially in tunnels compared to mines.
Meanwhile, one could be forgiven for thinking it was all about TBMs but in fact, there are plenty of tunnel projects on the go using drill-and-blast. Fleets of new and owned jumbos are at work globally on multiple large and smaller project challenges, bringing trademark flexibility and adaptability to geological conditions. While TBMs get increasing attention on projects, drill and blast, and other traditional ways of excavation, are being employed to perform cumulatively large sections of major projects; one example is on the Lyon- Turin Base Tunnel currently underway on the Italian- French border.
ROCK STRESS MEASUREMENT
“I believe we have to abandon the idea that rock stress can be predicted a priori,” says Ødegaard, “at least to the level of accuracy required for detailed design purposes of unlined pressure tunnels.”
His PhD research, recently completed at the Norwegian University of Science and Technology’s (NTNU) Research Centre for Hydropower Technology (HydroCen), and supervised by Prof Bjørn Nilsen, looked primarily at in-situ rock-stress measurements at specific areas of tunnels, where the headrace lining changes from high-pressure steel-lined penstock to, just upstream, unlined rock tunnels. With the lining transition zone having to be judged where best to be, its location affects the construction choices, as well as the cost and performance of waterways, and as such the PhD research project was requested by HydroCen industry partners.
“For projects adopting an unlined design concept, the final design can only be settled during the construction stages, from stress measurements inside the tunnels,” says Ødegaard. “Hence the focus on the stress measurement techniques in such settings.”
But the principles apply not only to hydraulic pressure tunnel linings but indeed to all types of project, tunnels and excavation methods, as well as tunnel depths, he tells T&T, for the central idea is to measure stresses at far more locations.
“Relying on overburden as an indicator of minimum principal stress has – over and over – proven to be inadequate for pressure tunnel design – he says. “I have developed the RSRT in an effort to facilitate greater numbers of tests, such that we can measure stress, rather than guess it.”
The key, says Ødegaard, is to have many more sampled data points distributed in the rock mass along the alignment axes. The result is improved interpolations and interpretations of the stress state between the data points, from borehole tests, he adds.
Typical test regimes presently use more advanced test protocols, with relatively few testing locations, or alternatively a variety of informal hydraulic jacking test protocols to estimate the minimal principal stress. These supplement the broad estimates of rock stress based on overburden depth.
The RSRT method, instead, uses an ordinary jumbo to drill 1 – 3 boreholes at regular intervals along the tunnel and extending at least two times the tunnel diameter to get beyond the near-field stress caused by tunnelling to reach unaffected rock mass. Testing can be close behind the advancing face, some 20m back, and even at the face, “without much hindrance to the excavation cycle,” Ødegaard explains.
RSRT measures the magnitude of normal stress action across the fracture being stimulated, or jacked open, during tests and, in principle, provides “an estimate of how much pressure the rock mass can sustain without failing,” he adds.
The testing system was developed with, first, laboratory experiments using a custom-built true-triaxial test rig to allow controlled hydraulic jacking experiments on a granite test specimen. The rock specimen was hydraulically fractured to create a planar fracture for later testing. Acoustic emission monitoring was used to map fracture geometry and investigate fracture behaviour.
With known geometry and controlled stresses, the test rig enabled development of a testing regime to have the flow increased over a series of equal periods, and then decreased – a controlled and graduated ramping up, and then down. The time-flow steps are to be adapted to local site conditions and then the resulting development of hydraulic pressure measured to map rock-stress behaviour, followed by interpretation of the pressure curve based on previous research (Hayashi and Haimson, 1991; Raaen et al, 2001).
“I believe the RSRT to a large extent can replace more advanced stress measurement techniques when it comes to pressure tunnel design investigations, where we only need to measure the minimum principal stress,” he explains.
“Interpretation can be made visually by detection of a characteristic behaviour of the pressure development during fracture closure. No need for advanced pressure transient analysis.”
Ødegaard explains that the method has been developed with a robust, simple and semi-automated test protocol and there would be less requirement for specialised testing crews. But any testing should be planned and overseen by qualified personnel, he adds.
The RSRT method is new and has had one full-scale field trial for verification, with 29 test cycles over seven boreholes at Løkjelsvatn hydro plant, Norway. The test sites were measured by the standard methods beforehand and RSRT results correlated well. It is believed many tests down boreholes may be leading to initial rock fracturing and may not only be the controlled jacking open of existing slight cracks. More tests should be made of the system, though, he says.
Companies involved with RSRT include Injeksjonsteknikk, which developed an injection pump for routine execution of the tests, and, says Ødegaard, plans to use it for stress measurements on drill-and- blast tunnels in hydropower projects. The RSRT system he adds is to be among the stress measurement provided by Stantroll, which has established offices in The Philippines.
Next steps also include supporting better grouting operations. The idea, he tells T&T, is to look at grouting where the emphasis in recent years has been to keep pressures as high as possible while trying not to locally or unintentionally jack open the rock – enlarging or creating more minor cracks. Research would measure rock stress before each round of grouting and, it is hoped, quickly assess the maximum allowable grouting pressure to reduce the risk of hydraulic failure.
“I am advocating a change from the current approach of performing relatively few, and targeted, measurements,” he says, “over to an investigation strategy with a more distributed approach,” – and able to take measurements along the entire length of a tunnel. Using existing and not specialised or costly equipment, plus gaining more data to better refine designs, is a combination Ødegaard believes can help tunnelling performance and economics without causing impact to the drill-and-blast work at the face.
TRENDS AND EQUIPMENT
Technology is advancing in drill-and-blast as well as equipment for other traditional methods of excavations. The trend is towards more instrumented and intelligent tunnelling drills, Sandvik’s product manager of tunnelling drill products Tommi Salo tells T&T.
Salo says: “Customers are also asking for more multi-purpose equipment” – such as for injection drilling, pipe umbrella drilling, and hollow bar anchors – “in addition to traditional face drilling.”
Sandvik’s latest launch is the DT923i with the improved boom design and rod handling system SRH, and which completes the i-series following the earlier launch of DT1132i. “Both drills have been used in tunnel projects in Europe and we have had good feedback,” adds Salo. “Some of the new drill rigs have also been sold to Asia.”
The rigs are working on Stockholm subway, where the rock is relatively hard and work includes a lot of injection drilling using the SRH rod-handling system, adds Salo.
The DT923i has two electro-hydraulic booms with automatic computer-controlled drilling functions. The rig also uses the recently introduced RD5 series high-frequency rock drills.
An additional product for the company’s offering is the AT umbrella pipe system, from its recently acquired subsidiary DSI Underground.
Last year, Sandvik introduced the iSURE 8.1 version of its excavation process control information management software aimed at helping to improve drill-and-blast performance. The new version enables more advanced metrics analysis and produces Key Performance Indicators (KPIs) based on data collected, round by round, onboard the i-series drill rigs.
Recorded data include location of holes, recording every 20mm using the Measure While Drilling (MWD) method, and can include geological interpretation of the rock and scanned point clouds of the tunnel profile achieved. Sandvik says the data can be studied per round or from the perspective of progress on each tunnel on a project.
The iSURE 8.1’s approach with KPIs is to help dive into analyses of production data (drilled volume and total length achieved, number of different types of drilled holes, realised blasting pull-out factor, linear advance of the tunnel, and changes in rock drillability) and realised schedule, including cycle time. Results can be compared across rigs and tunnels, and by time.
Salo notes a rise in popularity for training simulators, supporting remote training which has been important during Covid travel restrictions. Sandvik has its Digital Driller package to meet the needs, and plans to add a module matched to the DT923i rig this year.
Among recent innovations from Epiroc are: introduction of its Boomer M20 face drill rig with internal hydraulics; the COP MD20 rock drill designed to have longer service intervals; and the Avatel mechanised, semi-automated explosives charging system through its collaboration with Orica. Epiroc also notes increasing demand for automation and digitalisation, and it has lifted its stake to full ownership of underground positioning systems business Mobilaris.
Among other 1business moves, Orica announced two months ago a deal to sell its ground engineering products subsidiary, Minova, to European investment firm Aurelius Group.
