Open pit protection for the building in Zagreb

Open Pit Protection – Office Building in Zagreb

In the City of Zagreb, at Ulica grada Vukovara 49, cadastral municipality Trnje, reconstruction and extension works are being carried out on an office building. For the purposes of performing the works on the building, a main design for the protection of the temporary open pit has been prepared. The protection of the open pit is to be implemented by installing driven steel profiles, anchors, and jet-grouted columns.

Geotehnical investigation and testing

The open pit protection design was made based on previously conducted site investigations. Three exploratory boreholes were executed: borehole B-1, depth 20.0 m; borehole B-2, depth 15.0 m; and borehole B-3, depth 15.0 m.

The foundation soil consists of five basic geotechnical units:

Geotechnical Unit 1 — Fill;
Geotechnical Unit 2 — Clay (CL);
Geotechnical Unit 3 — Sand (SM);
Geotechnical Unit 4 — Gravel (GW); and
Geotechnical Unit 5 — Clay (CL).

Open pit protection

For the protection of the temporary open pit, side walls are to be formed by driving IPE240 steel profiles made of S235 steel at a center-to-center spacing of 0.70 m. In addition, vertical and inclined secant jet-grouted columns with an expected diameter of 800.0 mm are to be installed at a center-to-center spacing of 0.60 m.

The open pit is protected (depending on the specific zone) by installing self-drilling bar anchors with a diameter of 51.0 mm in one, two, or three rows at a horizontal spacing of 2.10 m. The anchor pre-tensioning (locking) force is 150.0 kN.

The open pit protection includes the following works:

excavation to the level required for driving the steel profiles;
driving IPE240 steel profiles made of S235 steel, with a length of 12.0 m, at a center-to-center spacing of 0.70 m;
installation of a capping beam made of UPN200 sections, steel grade S355, connecting the IPE240 steel profiles;
pilot drilling for the execution of vertical secant jet-grouted columns;
construction of vertical and inclined secant jet-grouted columns along the perimeter of the open pit, with a diameter of 800.0 mm, at a center-to-center spacing of 0.60 m, where the top of the jet-grouted column is at an elevation of +108.70 m a.s.l. and its bottom at +100.70 m a.s.l.;
phased excavation of material for the installation of the first row of bar anchors;
installation of the first row of self-drilling bar anchors and the steel horizontal beam, where the self-drilling bar anchors have a diameter of 51.0 mm and are to be installed at a horizontal spacing of 2.10 m, while the horizontal beam is to be constructed by installing two UPN200 sections made of S355 steel;
phased excavation of material for the installation of the second row of self-drilling bar anchors (where required, depending on the excavation depth);
installation of the second row of self-drilling bar anchors and the steel horizontal beam (where required, depending on the excavation depth), where the self-drilling bar anchors have a diameter of 51.0 mm and are to be installed at a horizontal spacing of 2.10 m, while the horizontal beam is to be constructed by installing two UPN200 sections made of S355 steel;
phased excavation of material for the installation of the third row of self-drilling bar anchors (where required, depending on the excavation depth);
installation of the third row of self-drilling bar anchors and the steel horizontal beam (where required, depending on the excavation depth), where the self-drilling bar anchors have a diameter of 51.0 mm and are to be installed at a horizontal spacing of 2.10 m, while the horizontal beam is to be constructed by installing two UPN200 sections made of S355 steel;
installation of Q-188 reinforcement mesh and geotextile material on the front face of the open pit to prevent soil raveling;
loading of surplus excavated material onto transport vehicles and disposal at the nearest appropriate facility or site in relation to the location of waste generation, taking into account economic efficiency and environmental acceptability;
continuous pumping of groundwater from the open pit during excavation works; and
installation of monitoring equipment and the implementation of monitoring.

cross-section — *Photo 2 Cross-section along Ulica grada Vukovara*

Conclusion

During the excavation and support installation works for the open pit, continuous geotechnical supervision and monitoring of open pit movement are carried out. The verification of the effectiveness of the designed measures is performed through systematic observation during construction. The monitoring of the measuring equipment is performed during the excavation of the open pit as well as during the construction of the building within the pit. It can be concluded that sealing the sides of the open pit with jet-grouted columns has reduced water inflow into the pit, thereby enabling successful pumping of groundwater from the open pit.