The Effect of Blasting On The Grout-Paper

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The study examines whether blasting damages freshly injected cement grout during Scandinavian tunnel excavation, where pre-grouting is commonly followed by a prescribed waiting period before drilling and blasting.<br /><br />Researchers measured grout stiffness and strength as it hardened using rheometer tests, then compared these properties with strains estimated from blast vibrations in a small field trial. The trial involved connected boreholes in fractured rock, grouting with ordinary micro-cement, and a blast approximately three hours after grouting.<br /><br />The measured blast-induced strain was about 300 microstrain at one metre from the blast hole. Even estimates closer to the hole—around 1,000 microstrain—were substantially below the strain needed to make the grout flow. The results therefore indicate that, for a blast charge comparable to that used in contour holes, vibrations are unlikely to harm grout located at least one metre from the detonation. Laboratory tests also showed that grout behavior changes as it hardens, underscoring the value of assessing its shear properties rather than relying on a fixed waiting time.<br /><br />The authors suggest that drilling and blasting may be able to begin soon after grouting, without an arbitrary general delay. However, the conclusion is limited to the tested conditions and charge size. Water flushing during drilling, hydraulic testing, and pressure in boreholes connected to grouted fractures may still disturb the grout. The study recommends characterizing grout shear modulus and tailoring grouting and excavation schedules to measured material properties and site conditions.

Keywords

grout damage

blast vibrations

tunnel excavation

pre-grouting

micro-cement

shear modulus

rheometer tests

drilling and blasting

fractured rock

Scandinavian tunneling