Challenges of Grouting for groundwater control in Hard Rock Tunneling

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Abstract: Reasons for challenges in groundwater ingress control are highly variable ground conditions, numerous alternative grouting methods, and materials properties. Consequently, strategies abound, and opinions are quite divergent. This paper covers strict ingress limits for systematic pre-excavation grouting (PEG). The paper points to a disconnect between theoretical models and the demands of at-the-face decision making. The focus should be on the lowest possible PEG cost and execution time. It is highly beneficial to utilize information from drilling ahead and pumping of grout. No theoretical model will replace this. Inject only the necessary volume of grout. For an example PEG fan in crystalline hard rock, the targeted rock volume of 7465 m3 contains 22 L of fracture volume. The relevant empirical grout volume would be 1000 times this fracture volume. To properly seal off such a rock volume, 22 L of grout is way too little, but it does indicate that the practical volume can be reduced. Huge conductivity contrasts have significant effects on PEG, which needs more attention. Water and grout take the path of least resistance and grouting simultaneously into highly conductive channels and smaller joints will not work. This was well established decades ago for dam foundations, limiting hole-sections for grouting to 3-5 m length. The paper continues to discuss maximum pumping pressure, stop criteria, the frequent priority on stop-on-pressure, attempts at saving time by one-round PEG and the benefits of the two-round strategy. The paper explains the controlled setting of cement grout by adding accelerator and benefits identified. Process management at the face requires a fully implemented living document method statement. Finally, find a new method for grout spread control presented as a challenge for on-site testing.

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