Water-rich sand cross passage: 3 mm settlement, 6 days ahead
A metro cross passage in East China crossed water-rich fine silty sand. With high permeability, plain bentonite slurry lost over 15 L/min; the failed wall triggered surface settlement and a 5-day stoppage on a schedule-critical section.
Project overview
- Region
- East
- Project type
- Metro / shield
- Project scale
- One metro cross passage on the line's critical path, beneath a city road
Construction challenges
The main construction challenges encountered on this project.
Challenge 1Heavy loss
Permeable silty sand pushed plain slurry loss beyond 15 L/min, consuming material fast.
Challenge 2Settlement sensitive
A city road sat overhead with a 10 mm settlement control line.
Challenge 3Tight schedule
The cross passage governed the whole line — breakthrough could not slip.
Solution & materials
Materials and mix ratios used step by step on this project, for reference by similar works.
| Stage | Material | Function | Ratio / dosage |
|---|---|---|---|
| Mix design | PAM-S | Film-forming main material | 1:400 |
| Crossing section | PAM-S concentrated slurry | Reinforced protection | 1:250 |
| Base slurry | BENT-N | Base make-up | 8% |
| Maintenance | Compound top-up | Re-viscosify by return tests | As required |
Implementation notes
PAM-S film-forming slurry replaced the original bentonite mix, with a 1:250 concentrated section through the crossing. The film formed quickly on the sand wall, cutting loss from above 15 L/min to under 2 L/min. With staged top-ups and monitoring, peak settlement reached just 3 mm and breakthrough came 6 days early with 100% hole acceptance.
Delivered results
Site photos
Related products
NPT-PAM-S- Solids content ≥ 98%
- Loss rate < 2 L/min
- For Φ800–Φ2400 boreholes
NPT-BENT-N- Yield ≥ 18 times its mass
- Low iron and sulphur
- Consistent batch to batch
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