Open pit protection design for the building in Stenjevac

In the northern part of Zagreb, in the Gornji Stenjevec district, the construction of a residential building with three separate housing units is planned. For the execution of the project, it is necessary to cut into the existing terrain to a maximum depth of approximately 9.50 meters and provide temporary open pit protection.

Geotechnical investigation and testing

For the purpose of creating the open pit protection design, we conducted engineering and geological as well as geotechnical investigation and testing. This included the geological and geotechnical prospecting of the wider area, engineering and geological mapping of the terrain, CPTU testing as well as the analysis of previous investigation data and correlation with new data.

geotehnički profil — *Figure 1 Geotechnical profile*

The geotechnical investigation and testing determined that the geotechnical profile at the site consists of a single geotechnical unit, which, based on its physical and mechanical properties, is comprised of silty sediments. The overburden layer identified at the site is disregarded in geostatic analyses because of its minimal thickness.

A stability analysis of the area was also conducted, classifying the site as Category II in terms of slope stability. Category II denotes conditionally stable slopes – naturally stable under existing conditions but potentially unstable during construction in the event of improper execution.

Open pit protection

For the construction of the residential building and the associated retaining walls on the plot, it is necessary to excavate into the existing terrain to a maximum depth of 9.50 m and implement a temporary open pit. The excavation is to take place within silty sediments (Geotechnical Unit 2).

To adapt to the geometry of the structure and the spatial constraints, including the property boundary, the open pit is to be made with slopes of 3V:1H, 2V:1H and 1.5V:1H. The protection system is divided into three zones.

tlocrt jame — *Figure 2 Floor plan of the open pit with protection zones*

Within Zone 1 (northern part of the site), excavation is planned with a slope of 3V:1H in one or two levels, with additional stabilization of all excavation slopes using a soil nailing system.

Zone 2 covers the eastern part of the site (towards Dominik Mandić Street), where excavation is planned with slopes of 3V:1H and 2V:1H in three levels, with additional protection of all excavation slopes, also using a soil nailing system.

In the southern part of the site, excavation is planned with slopes of 3V:1H and 1.5V:1H in two levels, with additional protection of the lower excavation level at a 3V:1H slope, using a soil nailing system (Zone 3). The upper excavation level at a 1.5V:1H slope will require temporary access to the currently undeveloped neighboring plot for the construction of retaining walls at the plot boundary. The soil nailing system is to consist of a layer of shotcrete measuring 10.0 cm in thickness, reinforced with one layer of Q-257 mesh, combined with self-drilling anchors measuring 38.0 mm in diameter and with lengths of 3.0 m, 4.0 m and 6.0 m.

Because of the complexity of the excavation geometry, a three-dimensional model of the terrain and the open pit has been created for the project.

Conclusion

After the construction of the future building, all horizontal loads, both static and dynamic, will be transferred to it. This project considers the open pit protection measures as temporary, as the space between the building and the excavation walls will be filled with crushed stone material upon completion. The structure of the future building must also be capable of withstanding all the loads from the surrounding soil, both under normal conditions and during seismic events, throughout its service life. This geotechnical project specifically analyzes the temporary open pit protection system.