Terminal Sisak
Located in the southern industrial zone of the city of Sisak, near the settlement of Crnac, an exploratory piezometric borehole needed to be drilled as an alternative for the water supply of the fire protection system. Additionally, a test pumping had to be conducted to determine the anticipated exploitable quantities of groundwater. For the purpose of executing the exploratory piezometric borehole, according to the applicable Water Act and the Ordinance on the Issuance of Water Rights Permits, it was necessary to prepare a Work Program for obtaining water rights conditions from Croatian Waters before commencing the works.
Work Program for obtaining water rights conditions:
- Introduction, aim and purpose of the research
- Technical conditions for performance of works
- Location information and location description
- Technology of drilling an exploratory piezometric well
- Technical construction and characteristics of backfill and buffer
- The technology of cleaning and conquering the well by experimental pumping
- Chemical analysis of water and provision of an exploratory piezometer well
- Technical report
- Special performance conditions
Drilling technology for exploratory piezometric borehole
The drilling of the exploratory borehole is carried out using the direct drilling method with continuous coring, utilizing a core barrel with a diameter of ∅131 mm, along with the use of drilling fluid as needed and the installation of protective casings. The drilled material (core) is placed in designated core boxes of 1.0-meter length, ensuring that the 1.0-meter depth equivalent of the core corresponds to 1.0 meter along the length of the box, with markings for the borehole and depth interval. Using other methods, representative samples of the drilled material (core) from each lithological unit of the exploratory piezometric borehole will be collected. Laboratory tests were performed on the specified samples.
Technical construction and backfilling
It was necessary to install a technical construction in the drilled exploratory piezometric borehole. The technical construction of the exploratory piezometric borehole was implemented using standard plastic pipes with threaded walls, with a diameter of 125.0 mm (4½'') and a wall thickness of 5.0 mm. The filter (screen) of the piezometer is made of perforated pipe with a minimum permeability of 10%. Below the screen, a sump was to be installed, consisting of a full plastic pipe with a diameter of 125 mm and a conical bottom. The exact placement and depth of the screen (filter) are determined by the supervising engineer based on the analysis of the drilled material (core).
After the installation of the piezometric construction, the annular space between the piezometric construction and the borehole was filled with granular gravel to a depth of at least 2 meters above the filter section. Above the gravel, a clay-bentonite plug was required to be installed up to the ground surface to prevent the infiltration of rainfall and potential contaminants into the aquifer.
Cleaning and development of piezometers through test pumping
After the installation of the technical construction, cleaning and removal of fine particles and sand that could affect the pump's operation were carried out. The cleaning and development of the piezometer were performed using a compressor with the air-lift method, which involved alternating between steady and variable operation (pulsing) of the compressor. The piezometer was cleaned until sand discharge ceased and the water became clear.
The testing of the exploratory piezometric construction is conducted in two parts. In the first part, a step test is performed with three different discharge rates using a submersible pump, and the recovery of the groundwater level is measured. Based on the results of the step test, the operating capacity for the submersible pump is determined for the constant rate test. In the second part of the test, a constant discharge rate is maintained, and the recovery of the groundwater level is also measured. During the test pumping, it was necessary to measure the drawdown of the groundwater level in the piezometric borehole and the discharge rate.
Securing the exploratory piezometric borehole
Securing the exploratory piezometric borehole, as requested by the client, was carried out by constructing a concrete block with dimensions 1.0 x 1.0 x 0.6 meters. The piezometer is secured with a cover and a padlock.