Open pit protection and foundation design for the New School in Kastav
Because of the increasing number of students, Milan Brozović Elementary School in Kastav, which currently has over 800 students, is facing the need to address the overburdening of its capacity and improve working conditions. In response to these challenges, the construction of a new branch school is planned. The architectural design was developed by 3LHD, while our part of the work includes geotechnical investigation and testing and producing a geotechnical report as well as an open pit protection and foundation design.
The site is located in the southern part of the town of Kastav and features a wooded area covered with tall vegetation, full of small depressions typical for this region, with an elevation difference of 29 meters.
The overall concept of the school is designed to harmoniously fit into the existing context, allowing for the harmonious integration of the structure with the natural environment. The two-story structure with classrooms on the south side is to have an underground connection to the school’s gym. Above this underground section, there is to be a public pedestrian pathway running east to west, while the sports field is to be located on the roof of the gym, forming a covered school square and simultaneously marking the main entrance. The space between the school building and the public pathway is to contain accompanying facilities that are to form atria.
Geotechnical investigation and testing
To prepare the geotechnical report and construction design, engineering and geological as well as geotechnical investigation and testing were conducted, which included:
- the geological and geotechnical prospecting of the wider area;
- dynamic penetration with a light probe (DPL);
- the engineering and geological mapping of the terrain and the determination of probing pits; and
- the processing of data from previous investigations and correlation with the new data.
Open pit protection
The geotechnical review of the site and the conducted geotechnical investigation and testing determined that the geotechnical profile of the site consists of three geotechnical units: made ground, red clay and medium to poorly weathered limestone. The open pit is to be created by forming a wide excavation with a slope of 3H:1V and locally of 5H:1V in the rock mass, as well as a slope of 1H:1V in the layer of made ground and red clay. The open pit is to be carried out on the northern, western and eastern sides of the site.
Excavations outside the fault zone are to be protected by a protective 8 × 10 hexagonal mesh, while the fault zone itself needs additional support with bar anchors with a minimum diameter of 32.0 mm, lengths of 3.0 m and 4.5 m on a general grid of 3.0 × 3.0 m (H × V). In areas with larger irregularities and cavities, the faces of the excavation are to be filled with reinforced concrete cushion.
Foundation and foundation soil improvement
Below the base of the foundation slab and strip foundations of the planned gym and school structures, foundation soil improvement is planned. The excavation of the open pit is to be performed to a depth of 30.0 cm below the bottom level of the foundations, and material replacement for foundation soil improvement is to be carried out with a layer thickness of 30.0 cm. The installation of mechanically stabilized granular stone material with a granulation diameter of 0–63 mm is planned, with a layer thickness of 30.0 cm.
Conclusion
After the construction of the planned structure, all horizontal loads, both static and dynamic, will be transferred to it. This project considers the protection measures of the open pit as temporary, since after the completion of the structure, the space between it and the walls of the open pit will be filled with stone material. The construction of the planned structure must also be capable of bearing all loads that come from the surrounding soil, both under normal and seismic conditions during use, as this geotechnical project analyzed the state of protection of the temporary open pit.