Open pit protection and foundation design – UPOV Požega
In the eastern part of the city of Požega, the reconstruction and extension of the Požega agglomeration wastewater treatment plant (UPOV Požega) is planned. For the construction project, we made the main and detailed temporary open pit protection and foundation design for the following facilities:
– facilities of the biological block (floor plan dimensions approx. 100.0 x 50.0 m);
– septic reception facility (floor plan dimensions approx. 10.0 x 6.0 m); and
– the added part of the building next to the administrative building (floor plan dimensions approx. 10.0 x 5.0 m).
Geotechnical investigation and testing
Prior to the creation of the main and detailed designs, the following geotechnical investigations and testing were carried out:
– exploration drilling;
– dynamic tests;
– geophysical tests;
– engineering geological mapping of terrain and determination of drill cores; and
– laboratory sample testing.
Our geotechnical investigation and testing have found that the location of the future construction is characterized by alluvial deposits of material in the form of sandy clay, dusty sand, and sandy-dusty gravel, which are all materials with unfavorable characteristics in terms of load-bearing capacity and deformability.
Temporary open pit protection (biological block and septic tank reception facilities)
The temporary open pits for the biological block facilities and the septic tank reception facility are planned through a combination of a wide excavation at a slope of 1V:1.50H and a vertical excavation with additional protection. The expected cutting depth for both pits is up to 6.0 m in relation to the level of the existing terrain.
The vertical excavation with additional protection is planned because of the immediate proximity of the sewage pipes, which must remain functioning, and in relation to the face of the open pits. Along with the sewage pipes, prior to the execution of the vertical excavation, the formation of a plateau at their bottom elevations is planned to reduce the total height of the excavation along the pipes.
The protection of all vertical excavations along the intervention is provided in the form of a temporary retaining structure made of driven steel and profiles. Between them, parallel to the progress of the excavation, successive wooden plates (timber) are placed horizontally. This type of protection is often called the “Berlin Wall” among practicing professionals.
In the parts of the open pit for the biological block facilities, the driving of IPE270 steel profiles (length L=7.50 m) at an axial distance of 1.0 m is planned. In the part of the pit for the septic reception facility, the driving of the same profiles at the same axial distance, only shorter in length (L=3.0 m), is planned.
On the temporary wide excavations along the intervention in the slope of 1V:1.50H, the installation of construction PVC film is planned in order to prevent erosion of the surface parts of the slope due to the effects of weather.
The foundation of the biological block facilities, the septic reception facility, and the added part of the building next to the administrative building
The foundation of the future biological block facilities and the septic reception system is carried out on foundation slabs. In order to unify the conditions of the foundation soil with regard to the variable directions of layers of different deformability along the project, soil arrangement under the foundation slabs is carried out by replacing the material. For the replacement, stone material with a thickness of 50.0 cm was installed.
In order to separate the soil layers with significantly different characteristics, geotextiles were installed on the bottom of the open pit excavation, before replacing the material.
The foundation of the new (added) part of the building next to the administration building is carried out on foundation strips with a width of 0.80 and 1.0 m and a thickness of 30.0 cm. Prior to the execution of the foundation strips, the foundation soil is arranged by mechanical compaction.
The mechanical compaction and material replacement are carried out in order to reduce the total and differential settlement of the structures in the future.
Conslusion
In spite of the unfavorable conditions at the construction site of the UPOV structures – the spatial limitations (proximity to existing buildings and sewage pipes that must remain in operation) and the geotechnical characteristics of the location (materials of high deformability and low load-bearing capacity) – this demanding project was brought to a satisfactory conclusion. Works on the excavation and open pit protection have been completed, and the construction of future structures is currently underway.
