Open pit protection design – House of Senj Writers
In the old town of Senj, the reconstruction and repurposing of a building adjacent to the city museum is planned. As part of this reconstruction and repurposing, we participated in the development of the open pit protection design.
Existing condition of the building
The load-bearing walls of the building are made of stone and solid brick, while the floor and roof structures are made of wood. In 1991, the building was heavily damaged in a fire, when the floor and roof structures completely burned down, allowing significant weathering impact and further damage to the building.
Due to the severe deterioration of all parts of the building and the problematic remediation and reconstruction, all load-bearing and partition structures of the existing building were removed, and the necessary excavations and the planned open pit protection were carried out.
Intended purpose of the building
In the scope of the planned renovation, the building will be repurposed into a museum space. The vaulted basement room and the entire basement level will become a part of the expanded City Museum complex. The ground and above floors will become an independent unit which will function as the cultural center of the city of Senj. A special section in the center will be an exhibition space dedicated to the Senj writers active in the 19th and 20th centuries.
Open pit protection design
For the construction of the new basement level of the building, it is necessary to excavate the existing terrain to a variable depth of approximately 0.50 m to 3.30 m (measured from the existing ground level). In accordance with the results of these exploratory excavations, the final excavation of the open pit will primarily be within the cover layer (fill and colluvium), with potential finds of bedrock.
Given the spatial constraints at the site, including the proximity of the neighboring buildings and streets within the old town, a vertical excavation with additional protection measures along most of the excavation area is planned. A wide excavation with stable geometry (3V:1H) is planned along a smaller part of the excavation area.
With regards to the planned protection measures for the vertical excavation, the open pit is divided into three zones.
Zone 1 extends along the eastern part of the site with an average excavation depth of approximately 3.30 m.
Within zone 1, the vertical excavation is secured by constructing a retaining wall of micropiles with a diameter of D=178.0/152.0 mm and a length of L=4.0 m. The micropiles are installed at a spacing of s=0.50 m, with alternating vertical piles and piles with an incline of 20°. Steel pipe profiles (D=101.6 mm, t=4.0 mm) made of S 355 steel are installed in the previously drilled micropiles, and the boreholes are filled with a cement-based grout. The top of the micropiles are connected by a reinforced concrete capping beam measuring 30.0 x 40.0 cm, made of concrete with a compressive strength class of C 30/37.
A shotcrete layer is applied to the surface of the retaining wall in two layers, each 5.0 cm thick, along with a reinforced mesh Q-188 to prevent the material from spalling between the micropiles.
Zone 2 of the open pit is located in the southern part of the site (towards Milan Ogrizović street) with an average excavation depth of approximately 1.30 – 1.75 m.
Along the specified zone, the vertical excavation is protected by installing a shotcrete in two layers, each 5.0 cm thick, along with a reinforced mesh Q-188 to stiffen the vertical face of the excavation.
Within zone 3 of the open pit (northern and northwestern part of the site), the adjacent wall of the existing building (City Museum) is protected by underpinning to enable vertical excavation immediately next to it.
Given the assumed elevation of the base of the adjacent building’s wall, the wall is underpinned to a height of approximately h= 1.50 – 2.0 m. The underpinning is performed in horizontal layers, each 1.0 m wide and 50.0 cm thick, made of concrete with a compressive strength class of C 25/30. Self-drilling anchors with a minimum diameter of 32.0 mm and a length of L=2.0 m are installed through each horizontal layer at a horizontal spacing of s=1.0 m to provide additional support of the underpinned wall.
Conclusion
The renovation and repurposing of the House of the Senj Writers is a an exceptionally challenging project due to its position within the old urban core, immediately adjacent to existing buildings. It is important to highlight that the average width of the streets within the old urban core is approximately 2.0 m. Selecting the solution for the open pit protection was complex due to the limited space for the access of the required machinery and the intention to minimize damage to the surrounding historic streets during the excavation process.
