Monitoring System

In the following, the main geotechnical and geophysical measurements of the monitoring system are described. The main technical features of all the probes used in the monitoring program (e.g., the manufacturer, accuracy, and operational range) are given in Table 4.7.

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Table 4.7. Main Technical Features of Probes Used for Monitoring

Device Product

name

Manufacturer Accuracy and

operational range Moisture sensors SM300 Delta-T Devices (http://www.delta-t.co.uk/) ± 0.025m3_.m-3 (2.5%) - Full accuracy over 0 to 0.5m3_.m-3 Moisture sensors Vegetronix VH400 Vegetronix (http://www.vegetronix.com/) 2% at 25°C

Tensiometers SWT-5x Delta-T Devices

(http://www.delta-t.co.uk/)

±0,5 kPa

From -160kPa to 160kPa

Soil Moisture Sensors

To record the changes in the volumetric water content of the soil by time, six Vegetronix VH400 soil moisture sensors were initially buried horizontally in one side of the slope model. The location of the soil moisture sensors is shown in Figure 4.13. The accuracy of these sensors is 2% and they measure volumetric soil moisture content, using transmission line techniques to detect the dielectric constant of the soil. The size of the sensors is very small, 9.40cm long and 0.70cm thick, thus causing minimal disturbance to the surrounding soil.

The water content of the soil is recorded at frequency 1/s. Since reliable measurements are very important while performing experiments, two SM300 soil moisture and temperature sensors were also obtained later on during the tests and were also buried inside the slope model for independent measurements. These sensors are even smaller than the VH400 sensors causing least disturbance to the soil, their accuracy is 2.5% for the moisture content and 0.5°C for the temperature and they give continuous readings. The

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112 SM300 sensors are used in combination with the others to ensure correct readings.

Vegetronix VH400 soil moisture sensors were connected with an Arduino microcontroller and a code was written to obtain readings every second while SM300 sensors were connected to a GP2 Delta-T devices Data Logger obtaining continuous measurements.

Soil moisture content tests took place in the laboratory to calibrate the output voltages of the sensors prior to the main tests. Moreover, some preliminary tests for the response time of these sensors were conducted, using small samples of the same density as that of the slope model and with known water contents which showed that the sensors responded accurately in less than 1s of contact with the soil.

Tensiometers

The suction of the soil can have a considerable influence on the slope stability (Fredlund et al., 1978); therefore it is essential to determine the suction of the slope during the test to evaluate the likelihood of a slope failure. In order to capture the changes in the soil suction during the tests, two tensiometers (SWT-5x) were employed. The accuracy of these tensiometers is ± 0.5 kPa while the small size of the shaft (5mm diameter and 5mm length) causes very small disturbance to the soil. These sensors were also connected to the GP2 Delta T devices Data Logger to obtain the readings.

GeoPIV analysis

The displacement vectors were obtained using GeoPIV. GeoPIV is a Matlab module which combines the technologies of digital imaging, close range photogrammetry and the image-processing technique of Particle Image Velocimetry (PIV) in a manner suited to geotechnical testing (White and Take,

113 2002, White et al., 2003, White et al., 2005). The analysis technique used is based on the principles of PIV, which is a digital image-based surface displacement measurement method that compares a reference image to a series of deformed images. It measures whole velocity fields by taking two images in successive time instants and calculating the distance individual particles travel within these instants. Since the accuracy of displacement measurements using particle image analysis is strongly dependent on the surface contrast of the soil, some of the glass beads were painted blue and white to be used as markers, in order to have unique and easy to track patches (Figure 4.17) for the GeoPIV analysis.

Figure 4.17. Soil texture after using blue and white paint for some glass beads

During the event, two high resolution cameras were aimed at both sides of the container, to record the process of the test and the movement of the soil. These cameras have a video resolution of 1920 x 1080 pixels and a maximum frame rate of 25 fps. There were two cameras to verify that the wetting front is moving homogeneously through the model at both sides; the displacement vectors were obtained by analysing the images of the video only at one side via Particle Image Velocimetry (PIV). Frames were extracted from the video and were analysed to measure the displacement vectors of the soil

Chapter 4: Experimental methodology

114 and the velocity of the wetting front, without causing any disturbance to the slope model.

Matlab Image Processing

When the experiment starts, the readings of every sensor are steady and after some time the moisture sensors readings start to increase due to the infiltration of the water. This is an indication that the wetting front has reached to the corresponding location of the sensor. To validate the accuracy of the sensors, the idea was to use red coloured water for the rainfall and the movement of the red colour could then be tracked through Matlab image processing, thus giving the movement of the wetting front and then compare these results were with those obtained from the soil moisture sensors, regarding the movement of the wetting front.