MONITORING TECHNIQUES

The monitoring of cooling systems is essential for a successive and economic ground freezing project. Mainly the frost expansion from the borehole into the ground is monitored, and therefore different instruments have been proved very useful. There are three basic techniques of monitoring, for the determination of borehole deviation, for measurement of the temperature field and for the assessment of the degree of freezing with ultrasonic waves.

Borehole inclination method

To choose the right distance between the different boreholes is a very important part of the ground freezing project. If they are placed to far away from each other it may result I poorly frozen zones or even in frost gaps. This is the reason why the determination of the borehole traces by using borehole inclinometers is fairly important. These inclinometers can be used to trace the freezing holes and to measure the uniformity of the frost-body and the displacements due to freezing and consecutive thawing by comparing the results of repeated measurements. Not only stationary gauges but as well movable probes can be used for these intensions.  To describe the trace of a vertical borehole the coordinates of the borehole collar and the deviation of the borehole axis from the vertical as well as the azimuth have to be known. These values can be received by the use of measuring devices that are positioned in the hole with centralizing instruments located at both ends of the instruments. These devices have a measuring base of 1 m and by moving the instrument down the borehole in increments of 1 m, a vector sequence is obtained that shows the trace as an open polygon. To control the direction of the deviation there are several different possibilities (Jessberger 1979).

Temperature measurements

The easiest way to control the progress of the freezing of the soil is to measure the temperature. There is a huge variety of techniques that differ in price and accuracy.  The sensors can be stationary or movable and they may have a direct or remote readout.  They can be used to monitor the cooling liquid or to control the soil temperatures in specific observation boreholes. Usually stationary sensors are more accurate than movable but in the case of inhomogeneous soil or where local heat patterns modify the normal pattern, movable sensors will bring a more reliable result (Jessberger 1979).

Ultrasonic measurements

Ultrasonic measurements allow the assessment of the actual state of the frost-body in two or even three dimensions. This kind of measurement can be used to control the frost progress in water bearing soil or rock by means of the change of the velocity of an ultrasonic wave when water transforms to ice.  The velocity of such a wave differs a lot when travelling through ice (about 4000 m/s) and travelling through water (about 1500 m/s).  Thus it is possible to control the actual state of the frost-body from liquid to solid.  An easy way to control the advance of frost is to set the ultrasonic submitter in one borehole and the receiver into the neighbouring and then comparing the travel time needed. First a so called zero-measurement is done when the temperature is still above zero and then repeated measurements are made after freezing has started. The amount of the measured velocity indicates the percentage of water that is in the soil. The faster the velocity the more water has been transformed into ice. While freezing it is obvious that the compressive strength of the soil increases. Thus the ultrasonic velocity indicates as well the existing of compressive strength in the soil (Jessberger 1979).