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Slower impacts for improved rock sealing

As much as 11 times more effective than current methods; researchers at RISE are developing a new method of injecting grout into rock cavities as small as 70 micrometres or even less. This can lead to reduced costs and safer tunnels and underground storage spaces.
“At first, I couldn’t believe the results. We were forced to go back and retest several times. It was fantastic,” says RISE researcher Ali Nejad Ghafar.

Almost every subsurface structure we build requires sealing; tunnels, shelters, storage areas for spent nuclear fuels. The structure must be protected from penetration by water, as well as from anything leaking out.

“This is generally an expensive, timeconsuming process,” says Ali Nejad Ghafar. “Take the Stockholm Bypass, where something in the region of 20-30% of the excavation budget is set aside for insulation.”

Tunnels and similar structures are sealed to create watertight zones. This is to ensure reliable sealing for a long period of time. Urban road tunnels are a good example – there it is desirable to have a 5-10 metre watertight zone around the tunnel.

Thin cracks hard to penetrate

Current grout injection methods begin at low pressure, which is then gradually increased. This is an effective method for filling large cavities but it is ineffective in penetrating deeply into small cracks, which can be smaller than 70 micrometres, or narrower than a human hair.

Previous studies into possible solutions for filling these extremely small cracks have focused on the application of high frequency oscillating pressure. Although these were successful in reducing the viscosity and friction of the cement mixture, they did not solve the problem of the rapid loss of effect as the grout penetrated further into cracks.

“Instead, our method uses lower frequency impulses,” explains Ali Nejad Ghafar. This can be compared with striking a wall; If you employ multiple extremely rapid strikes, eventually the wall will no longer register these as separate blows – instead, it will register as a constant force that the wall is able to resist. If, however, you strike hard, wait for a moment, and then strike hard again and repeat the process over time, the wall finds it harder to withstand the impact.”

Field experience

Ali Nejad Ghafar explains that he had a great deal of practical field work under his belt prior to commencing work on his doctorate at KTH. As an engineer and foreman on rural building projects, his work was often very hands-on and he learned to improvise practical solutions on the spot. This experience formed the basis for developing the concept for the new low-frequency method.

“Another thing that makes it interesting is that we use standard cementbased grout and normal pumps. These are already available on the market.”

So far, work has been limited to multiple tests conducted in test environments. The next step will involve testing in actual working structures in order to verify and demonstrate the results obtained. The hope is that testing will be conducted at the Swedish Nuclear Fuel and Waste Management Company’s Äspö Hard Rock Laboratory, where it is possible to test injecting grout to a depth of 450 metres into the Swedish bedrock.

“I am convinced that this method will have a major impact,” says Ali Nejad Ghafar.