High-pH water from shotcrete, segment grout and ground treatment is the most common tunnelling consent breach. Automated CO² or acid dosing brings it back into the consented band before discharge.
Cementitious works — shotcrete lining, segment grouting, rock grouting and ground treatment — lift the pH of contacted water to 11–12.5. That is well outside the typical pH 6–9 discharge band, and it is the single most frequent reason tunnel water fails consent. Neutralisation looks simple but the high buffering of cement water and its variability make closed-loop control essential. We design dosing systems — usually CO² for safety, or mineral acid where required — that hold pH in band across the full flow and load range.
Reliable pH Control on Buffered Water
Cement-affected water resists pH change, so a fixed dose never works. We use flow-paced, pH-trimmed closed-loop control with correctly sited electrodes and adequate mixing and contact time. CO² dosing is inherently self-limiting and cannot overshoot below neutral, making it the safe default; concentrated acid is used where space or cost dictates, with the interlocks that demands. Either way the loop is tuned for the buffering so the discharge stays in band.
CO² for safe, self-limiting correction; mineral acid where duty requires, with interlocks.
Flow-paced, pH-trimmed dosing holds the consented band as load varies.
Loop and contact time sized for the high buffering of cement water.
Self-limiting CO² and interlocked acid handling protect operators and the receiving water.
The Control Problem
High pH is easy to measure but hard to correct reliably on tunnel water that is both strongly buffered and highly variable.
Cement water resists pH change, so dose demand is large and non-linear.
Grout and shotcrete cycles send pH and flow up and down through the day.
Acid overdose can drive pH too low — CO² or good control prevents it.
How Neutralisation Is Controlled
Sited electrodes and a flow signal feed the control loop.
Dose is paced to flow for the bulk correction.
Adequate mixing and contact time let the reaction complete before measurement.
Final pH is logged against the consent before discharge.
Out-of-band water is recirculated, not discharged.
What drives the dose demand
| Source | Typical pH | Neutralisation |
|---|---|---|
| Shotcrete contact water | 11–12 | CO² / acid |
| Segment grout return | 11–12.5 | CO² / acid |
| Rock / ground grouting | 10–12 | CO² / acid |
| Natural fissure water | 6.5–8 | None usually |
| Discharge consent | 6–9 (typical) | Target band |
Plant We Build Into This Stage
Automated acid or CO² dosing neutralises high-pH cement-affected water.
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