Sensors
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Photograph of January 2006 soil pit near B3, water has perched on a dense layer about 0.5 m above bottom of hole.
Precipitation gauges
The site is equipped with two 8-inch-diameter tipping-bucket rain gauges that are calibrated to 0.01 inch of rainfall per tip. The field data loggers record and transmit 15-minute precipitation.Tensiometers
Since late October 2006, the Portland Hills site has been equipped with seven tensiometers arranged in two arrays or nests. The tensiometers (UMS Corp. Model T-8)* are equipped with current transducers that can measure pressures in the range from +850 hPa to -1000 hPa (suction) and accuracy of ±5 hPa are installed in boreholes that are inclined from 20° to 35° from the vertical, to facilitate refilling. The porous tips are at depths ranging from 73 to 220 cm below the ground surface. Pore pressures are reported in hectoPacsals (hPa). One hectoPascal is roughly equivalent to the pressure exerted by a 1-cm-high column of water. Porous tips of the tensiometers are in direct contact with native soil.
Photograph showing installation of a tensiometer at the Portland monitoring site.
| Tensiometer Number | Depth below surface, cm | Inclined distance from tensiometer tip to ground surface, cm |
|---|---|---|
| I-1 | 206 | 178 |
| I-2 | 155 | 134 |
| I-3 | 91 | 79 |
| I-4 | 164 | 143 |
| I-5 | 92 | 80 |
| I-6 | 220 | 194 |
| I-7 | 73 | 63 |
Piezometers
The site has seven piezometers. Four of the piezometers in hole B-1are embedded in a grout consisting of bentonite and Portland cement and are capable of measuring negative pressures when the surrounding soil is partially saturated or unsaturated. The three piezometers at sites B-3, 4 and 5 are each embedded in a sand pack at the bottom of the borehole which was then filled with grout. Pore pressures are reported in hectoPacsals (hPa). One hectoPascal is roughly equivalent to the pressure exerted by a 1-cm-deep column of water. Six of the piezometers are temperature-compensated vibrating-wire transducers (Slope Indicator, model 52611024)* that have a range of -345 to +3448 hPa (or -352 to +3515 cm of water), resolution of ±0.025%FS about (±0.95 hPa or ±0.96 cm of water) and accuracy of ±0.1% (±3.8 hPa or ±3.9 cm of water). One of the piezometers is a temperature compensated vibrating strip transducer (Slope Indicator)* with similar range and resolution.
Soil Temperature sensors
Each tensiometer and piezometer has a built in temperature sensor, either a thermistor or RTD (resistive temperature device). Soil temperature measurements are used in correcting measurements by the tensiometers and piezometers and in detecting when the ground is frozen.
Field Layout
Schematic cross section, looking south, of Portland instrument site. Cross section shows relative locations of vertical and inclined boreholes. All of the vertical holes except B2 have been instrumented with piezometers. Tensiometers which have been installed in inclined (approximately slope normal) holes. One line represents each group of tesiometers.
