C20 Station
Tuesday 22 December 2009 by Wanda BEROLO

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C20 station location

Instrument specifications:

  • Acquisition mode:
- Piezometer HITEC CP5210 and thermometer in drilling

- Bi-axial tiltmeter 815-L/T on the surface

  • Measurement frequency: 10 minutes time lapse
  • Transfer mode: daily recovery by modem from the recording station
  • Representation: daily or hourly (averages)

Example of C20 station raw data and conversion formulas:

Date and time Recording Piezometry (mV) Temperature (°) Inclinometry (mA)
13/10/2009 16:40 27322 7,331 14,83 -241,7 -259,3
13/10/2009 16:50 27323 7,331 14,83 -241,7 -259,3
13/10/2009 17:00 27324 7,331 14,83 -241,7 -259,3
13/10/2009 17:10 27325 7,331 14,83 -243,1 -259,3
Piezometry data conversion: Data = (Piez_raw-4)*15/16-6,7128125
Inclinometry data conversion: Data = Incl_raw*0,58791*π/180-calibration constant

Piezometry recordings:

C20 piezometer technical note

Piezometry data, compared to pluviometry, enable to show changes in water table levels due to rainfall. The fluctuation is caused by two distinct phenomena:

  • a general trend in water table discharge, well identified during the low water period (from June on the graph),
  • a recharge of the water table at each rainfall event, keeping the water level by the surface during the rainy period.

Temporal series, on the long term, will enable to constrain accurately the hydrological responses of the slope. One can notice that water level variations in the massif are significant over a few months period (nearby the surface during the wet season and more than 3 m deep during the dry season).

C20 piezometry and temperature data from November 2008 to October 2009. Comparison with rainfall over the landslide (start times of rainfall events in dashed lines)

Comparison between inclinometry, piezometry and rainfall data in C20. The start times of the recorded rainfall events are plotted with dashed lines.

A relationship between pluviometry, the pressure head of the water table and deformation can be observed.

The slope piezometry is controlled by pluviometry, as described above. Nevertheless, rainfall affects the water table level more significantly during the wet season than the dry season when rainfall events of low or medium importance do not play upon the slope piezometry.

Paradoxically, the opposite effect can be noted regarding the deformation measurement which indicates more important variations during the dry season than the rainy season, taking into account the same rainfall events.

 
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