Construction of a piezometric well
Groundwater is one of the most important natural resources. In order to gain insight into the quality and quantity of groundwater we use piezometric wells.
Piezometric wells are used to monitor groundwater levels and determine aquifer parameters such as transmissibility, hydraulic conductivity and storage, and they also give us the ability to sample groundwater for chemical and microbiological analysis. All these parameters later serve as a basis for:
- well design for water supply, irrigation or industrial needs;
- environmental studies;
- groundwater protection and remediation designs.
Documentation required for the construction of piezometric wells
Before starting the construction of piezometric wells, it is necessary to prepare a work program for the piezometric wells to meet the water legislation requirements from Hrvatske vode.
The work program must contain the following:
- location data and location description;
- geological characteristics of the investigated area;
- hydrogeological characteristics of the investigated area;
- drilling technology;
- technical construction;
- quantities and characteristics of backfill and tampons;
- development of water wells;
- test and trial pumping program;
- parameters for the chemical analysis of water;
- special construction conditions.
Drilling technology
Piezometric wells can be built using drilling methods such as continuous coring, direct drilling, and reverse drilling, whereas the rotary drilling method with a DTH hammer with simultaneous compressed air material exhausting can be used in rock mass.
The most reliable data on the lithological material are obtained by continuous coring, because the drilled core is stored in core crates 1 m long, so that the depth equivalent of 1 m of core is in one section of the box. With other methods, only a representative sample of drilled material (core) is taken for each meter of drilling, and therefore we do not have a detailed insight into the lithological structure of the site.
Technical construction
It is necessary to position the technical construction, i.e., pipes, into the drilled well in order to enable the use of a piezometer. The technical construction can be made of steel, iron with anti-corrosion coating, or PVC, and the material is chosen depending on the purpose and depth of the piezometer. The diameter of the piezometer technical construction ranges from 114.00 mm to 21.10 mm, depending on the type of testing and monitoring to be conducted.
There are three types of pipes: solid pipes, precipitator and screen pipes. The precipitator is installed at the bottom of the piezometer and is used for the deposition of small particles, if any. Filter pipes are installed in the aquifer or in the layers that are of interest for conducting tests. There are several designs of filter pipes such as slotted, bridged and continuous-spiral screen pipes; with slotted pipes being the most commonly used for piezometers. The opening of slotted pipes ranges from 0.20 mm to 3.00 mm and depends on the lithological structure and granulometric characteristics of the material. Solid pipes are installed in horizons of little interest or in horizons with low hydraulic conductivity (k <10-6 m / s) such as silt and clay horizons.
Backfill and tampons
After the installation of the piezometric structure in the interspace between the piezometric structure and the well, it is necessary to lay granular gravel to a depth of at least 2 meters above the filter section. The gravel granulation composition is determined based on the granulometric characteristics of the aquifer and the slot opening of the filter section.
Above the gravel, it is necessary to install a clay-bentonite plug made of quality clay up to the surface of the terrain to prevent precipitation and potential pollutants infiltrating into the aquifer.
Cleaning and development
After the installation of the technical structure and backfill, it is necessary to clean it and remove small particles and sand that could affect the operation of the pump. Cleaning and development are performed by compressor and air-lift. The air-lift construction consists of pressure pipes with a diameter of e.g., φ 3” and air pipes with a diameter of e.g., φ ¾” to which the compressor is connected.
Air-lift has two modes of operation, the quiet mode and the standard mode, which are combined. In quiet mode, the compressor provides a constant volume of air and water continuously flows out of the air-lift structure. Standard mode is the sudden opening and closing of the compressor (approx. 10 times) followed by the quiet mode. In standard mode, a larger number of small particles and sand from the filter backfill is introduced into the technical construction. The piezometer is cleaned and developed until sand removal stops and until the water clears.
Conclusion
Piezometric wells give us an insight into the state of groundwater and enable us to perform testing, analysis and monitoring of groundwater levels and quality. The construction of piezometric wells begins with the development of the design, i.e., the work program, and the fulfilling of the water legislation requirements from Hrvatske vode. Drilling technology, technical construction and types of testing and analysis are determined according to the lithological structure of the terrain and the type of project.
In geotechnics, piezometers are used to determine the hydraulic conductivity of the foundation soil and monitor groundwater levels, which serve as an input parameter for selecting the type of open pit protection and in geotechnical calculations. Piezometers in hydrogeology and ecology provide us with the ability to monitor the quality and conditions of groundwater over time and enable us to develop a strategy for groundwater management and protection.
Read more: Geotechnical investigation and testing
