In the period from September 13 to October 6, 2023, the field team of Component 2 of the CRONOS project, asst. Ph.D. Mario Gazdek and Bruno Mravlja, carried out preparatory work for the installation of borehole accelerograph systems at locations in Sinj, Vrlika and Imotski, supervised drilling at all locations and installed the borehole accelerograph system at the location in Imotski as part of field education.
Following the drilling plan for the installation of borehole accelerograph systems (borehole and surface accelerograph) at the locations in Vrlika, Sinj and Imotski, parallel to the drilling works, preparatory actions for the installation of protective systems around the drilled boreholes were also planned. Therefore, before going to the locations, we acquired and prepared for installation – the protective cabinets, frame material for concreting the concrete steps for the fence, formwork for concreting the plates to protect the borehole mouths and the protective fence.
On September 15, we arrived at the first drilling location in Vrlika. The borehole was drilled by continuous rotary drilling with coring to a depth of 17 m, with the use of protective columns. The next day we continued with cleaning the bottom of the borehole and preparation for cementing the bottom, about 1 m of concrete plug. In addition to cementing the bottom, the space between the borehole wall and the pipe was also cemented, in order to stabilize and transfer the stress of the soil and rock mass to the walls of the borehole. The water from the plastic pipe was pumped out. On the last day at the location, the cementation and stability of the inserted plastic pipe was checked, as well as the water level in the pipe. It was concluded that the pipe is very well fixed, stable and that there is no penetration of underground water. In the meantime, the excavation for the foundation feet was carried out, the formwork for concreting the feet of the protective fence and the panels for the protective cabinet were placed, and everything was concreted. Project associate Mario Gazdek attended the mentioned field works as a drilling and construction works supervisor.
The borehole in Sinj was drilled using the same technique, with works beginning on September 18. Upon encountering the layer speed inversion, the borehole was subsequently deepened and completed at a depth of 25 m, in compact deposits of solid marine limestones. After the stability of the cemented pipe was checked and confirmed on September 20, the water was pumped out. As in Vrlika, construction works related to the protection system of the accelerograph were carried out parallel to the drilling works.
Before drilling at the last location in Imotski, we prepared the rest of the equipment for the borehole protection system, primarily the protective fence. The drilling in Imotski was carried out in the same way as the previous two locations, this time with a drilling depth of 40 m. The works lasted from September 25 to 27, 2023.
In the following phases of field work related to borehole accelerograph systems, in the period from October 1 to 6, associate Bruno Mravlja joined. Preparatory work was carried out for the installation of borehole systems at all three locations (Imotski, Sinj and Vrlika). Specifically, in Imotski, the foundations for the borehole protection system and the solar panel support were concreted. In Imotski and Sinj, the protective system (fences and cabinets) was placed. Finally, at all three locations, the installed pipe was inspected by lowering the borehole camera to the bottom. The pipe was dry, clean and vertical at all locations, ready for the installation of borehole sensors.
For the first borehole accelerograph system at the location in Imotski to be properly installed and put into operation, and to ensure professional independent installation of the remaining accelerograph systems, a field education was organized by the team from Kinemetrics, whose instruments were purchased. The field education lasted from October 4 to 6, 2023. On the first day, the inspection of the patency and safety of the pipe was repeated in Imotski using a borehole camera, and it was concluded that the conditions for installing the sensor were excellent. A cable was prepared for lowering the sensor into the borehole. The next day, the entire borehole accelerograph system was installed – borehole, surface sensor, A/D converter, data log system and other related equipment. At the end, the solar panel was mounted and finally the protective fence was installed. On Friday, October 6, we made a final check of the system, determining that the borehole accelerograph system is fully functional (sensors and solar power supply work correctly, and the data transfer is in order), which ended the field education.
The following segment of this phase of field work will be the installation and start-up of borehole accelerograph systems at the remaining two locations – in Sinj and Vrlika.
Prepared by Bruno Mravlja