kustošija

Open pit

Open Pit Protection — Gornja Kustošija Branch School

In the western part of the City of Zagreb, in the borough of Gornja Kustošija, the construction of the Gornja Kustošija Branch School is planned. For the construction of the structure, it is necessary to conduct an excavation into the existing terrain to a total depth of approximately 10.0 m. The open pit is temporary in one portion of the southern side and permanent on the remaining sides (future parking area and playground).

Geotechnical investigation and testing

For the purpose of the open pit protection design, engineering geological and geotechnical investigation and testing were carried out, consisting of:

  • the geological and geotechnical prospecting of the wider area;
  • exploratory rotary drilling;
  • standard penetration testing (SPT);
  • the engineering geological mapping of the terrain and core logging;
  • laboratory testing; and
  • the analysis of previous investigation data and correlation with new data.
geotehnički profil
Figure 1. Geotechnical profile

The investigation determined that the site is composed of two lithostratigraphic units: clay and silty deposits overlain by fill material (humus soil as well as brown clay with roots, brick fragments, and pieces of plastic).

Open pit protection

The excavation of the open pit is to be carried out within both geotechnical units using a combination of vertical excavation and sloped cuts at gradients of 1V:1H and 1V:1.5H. The selected slopes and corresponding protection measures were adapted to the excavation zones depending on the excavation depth, permanence, geotechnical characteristics, and spatial constraints.

tglocrt građevne jame
Figure 2 – Plan view of the open pit with indicated protection measures

bored pile walls are planned, consisting of piles with a nominal diameter of 400 mm and lengths of 4.50 m, 7.0 m, 7.50 m, 8.50 m, 10.50 m, and 11.50 m. The piles are to be installed with a center-to-center spacing of 0.80 m.
The top of the pile wall is to be connected using a reinforced concrete capping beam (with a width of 0.50 m and a height of 0.50 m).

The open pit is to be protected (depending on the zone) by the installation of self-drilling bar anchors with a minimum diameter of 38.0 mm, arranged in one, two, three, or four rows, with a horizontal spacing of 1.60 m.

In the temporary section of the open pit, the self-drilling bar anchors are to be installed through a horizontal beam constructed using two UPN120 steel profiles made of S355 steel.
In the permanent section, the self-drilling bar anchors are to be installed through a reinforced concrete horizontal beam (with a width of 0.30 m and a height of 0.50 m).

poprečni presjek
Figure 3 – Cross-section 2-2
poprečni presjek
Figure 4 – Cross-section 4-4

Conclusion

During the excavation and protection of the open pit for the construction of the Gornja Kustošija Branch School, continuous geotechnical supervision and displacement monitoring of the open pit are required.
The monitoring equipment will consist of twelve geodetic measuring points and three vertical inclinometers.

Because of spatial constraints (proximity to the plot boundary) and the requirement to keep the existing retaining wall on the northern part of the site, the design of the open pit protection presented a challenge. However, with the selected protection methods, its global stability will be ensured.

Škola Kustošija by Geotech Rijeka on Sketchfab

PRead more: Open pit protection design

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