Key to accurate and representative subsurface instrumentation and/or monitoring is the installation of appropriately designed and constructed instrumentation casing and/or monitoring well casing. These casings/wells serve as conduits into the subsurface to allow for the proper installation and retrieval of various instruments and/or sampling devices. How the casing/well is constructed during installation will determine if the casing/well will serve as a useful tool for future monitoring efforts or whether it will be a waste of time and money. We have extensive experience with several common instrumentation casing and monitoring well installation procedures and we have summarized the general purpose and procedure for each method below.
Groundwater Level Monitoring
Groundwater level monitoring wells are a very common geotechnical exploration and monitoring tool which allow for the periodic and/or continual measurement of the groundwater table elevation at a given location using a water level meter and/or piezometer. The well casing is most often constructed of small-diameter PVC pipe, which is subsequently slotted/perforated and/or open at its bottom end to allow groundwater to flow freely through the well casing. The depth and length of slotting/perforations is predetermined by the engineer/geologist on-site and usually corresponds to a depth interval where the groundwater table occurs. The well casing is inserted into an open borehole, corehole, or excavation and then the annulus/excavation is backfilled with various materials, including (but not limited to): drill cuttings/spoils, sand, gravel, bentonite, grout, etc. The well casing typically extends above-grade (also known as a stick-up or wellhead) and can (if need be) be protected by a security vault constructed of steel, wood, or some other protective material. Sometimes, the well casing is cut-off below grade to protect the monitoring well from surface impacts (i.e. vehicles, equipment, animals, vandals, etc.). In this scenario, a subterranean vault (such as a manhole) is often employed to protect the well casing and allow for casing access.
Thermistor Casing
Thermistor casing issued to provide a vertical conduit into the subsurface to allow for the insertion of ground temperature monitoring equipment (i.e., thermistor strings), which are used to record the temperature of the surrounding soil/bedrock. Thermistor casing typically consists of small-diameter PVC pipe, which is sealed at each pipe joint and at its ends in an effort to prevent any groundwater from infiltrating into the casing. Groundwater which seeps into a thermistor casing is subject to freezing; which can plug the inside of the thermistor casing and obstruct instrumentation and/or can freeze instrumentation into the casing, preventing its removal. In areas where casing leakage is expected, seamless tubing is sometimes placed inside of the casing to add an additional level of groundwater protection and help ensure proper instrumentation. The thermistor casing is inserted into an open borehole, corehole, or excavation and then the annulus/excavation is backfilled with various materials, including (but not limited to): drill cuttings/spoils, sand slurry, grout, etc.
Inclinometer Casing
Thermistor casing are specialized plastic pipes that are used to take inclinometer measurements. The casings, which come in different diameters, typically have two sets of precision machined grooves which serve as guides for various instruments (e.g.,inclinometers, downhole seismic geophones, etc.) to follow during measurements and which keep the instruments oriented in a specific direction. Inclinometer casings are typically installed for slope stability monitoring in natural slopes and/or constructed fill/cut slopes. The casings are grouted into place using a grout mixture designed for the specific subsurface conditions at a given project. We have successfully installed inclinometer casing for a number of projects with varying subsurface and surface conditions including within the intertidal zone.
