Chapter 6 — Piezometers and Pore-Pressure Monitoring¶
6.1 Why pore pressure is central¶
Pore pressure — the pressure of water within soil, rock, or concrete — drives seepage, slope stability, and uplift. It is among the most important quantities monitored on dams. A rise in pore pressure can precede both internal erosion and slope failure.
6.2 Types of piezometers¶
| Type | Principle | Best for |
|---|---|---|
| Vibrating-wire (VW) | Frequency change of a tensioned wire | Long-term automated monitoring; robust |
| Pneumatic | Gas pressure balances water pressure | Remote/ungaged locations, no power |
| Standpipe (open) | Water level in a pipe | Simple, direct, but slow to equalize |
| Hydraulic (twin-tube) | Liquid-column balance | Multiple points, established method |
Saturation is everything
An unsaturated VW piezometer reads nonsense. The saturation procedure — removing air from the porous tip and connecting tube — must be done carefully at installation. A piezometer that was never properly saturated is a silent gap in the program. (See Dunnicliff Ch. 9.)
6.3 Where to place them¶
Piezometer location follows the failure modes:
- Downstream of cores and cutoffs to detect seepage buildup.
- In the foundation beneath and downstream of the dam.
- At the phreatic surface to track the seepage line through the embankment.
- At the base of concrete dams to monitor uplift.
Multiple depths at a single borehole (multi-level or MPBX-style) reveal the vertical pore-pressure profile.
6.4 Installation essentials¶
- Drill and case the borehole; avoid smearing the surrounding soil.
- Place the tip in the stratum of interest with proper filter packs.
- Backfill and seal to prevent short-circuiting of water along the borehole.
- Saturate VW and pneumatic tips fully.
- Record installation details — depth, stratum, orientation, date — in the permanent file.
6.5 Reading and interpreting¶
Readings are typically converted to piezometric head (elevation of the equivalent water surface). Key interpretations:
- Steady rise without a reservoir rise → possible blockage, seepage increase, or sensor fault.
- Pore pressure lag relative to reservoir → indicates drainage path and permeability.
- Exceedance of the phreatic line used in design → stability concern.
6.6 Common problems¶
- Air entry / desaturation → drifting or unresponsive readings.
- Borehole short-circuit → reads the wrong zone.
- Freeze damage → lost winter data in cold climates.
- Electrical/cable failure → most common automated-instrument failure.
6.7 Key takeaways¶
- Pore pressure drives seepage, slope stability, and uplift — monitor it carefully.
- Saturate VW and pneumatic tips properly; an unsaturated piezometer is useless.
- Place instruments where failure modes manifest; use multi-level installations.
- Interpret as piezometric head and compare against the design phreatic surface.