Deep foundation for a new school building in Osijek
The construction for a new school building of the Prva gimnazija high school is planned in the northwestern part of Osijek. The site is located between Strossmayerova street to the south, opposite the Pejačević Castle, with Podravska street to the east and the river Drava to the north.
Planned works
The new building will cover 7758.0 square meters and will have 3 floors. Along with the new building, a new schoolyard, access road and parking lot are planned. The works also include the leveling of the northern part of the facility intended for the sports gym. The main architectural project is developed by STUDIO MAPA d.o.o. from Rijeka.
Given the existing terrain characteristics, it is necessary to transfer the load of the building to deeper and more favorable foundation layers in order to reduce differential settlements. Based on the conducted geotechnical exploration and investigation works, we opted for foundation soil improvement by stone columns and deep foundation in the form of jet grouting columns.
Geotechnical Features of the location
For the purpose of foundation design, we have carried out engineering-geological and geotechnical exploration and investigation works. These works consisted of exploratory rotary drilling, engineering-geological mapping and determination of drilling cores, laboratory testing of samples from exploratory boreholes and processing data from previous research.
Based on the geotechnical report, we have determined that there is made ground of discontinuous cover on the southern part of the scope, which is a result of uncontrolled and long-term deposition of building material up to 8 meters high on the existing terrain meters high.
The geotechnical exploration and investigation works found that the geotechnical profile of the location consists of four geotechnical units with characteristic geomechanical features (made ground, inorganic clay, organic clay and silts and silty sand).
The conducted investigation works have identified the occurrence of groundwater in the wider area, while locally it has not been determined. The change in groundwater level at the location is conditioned by the increased inflow from the hinterland, intense rainfall and fluctuations in the water level of the river Drava. Drainage water during and shortly after rainfall periods tends to pass through made ground deposits, while clay and silts deposits represent a local hydrogeological barrier for groundwater and surface water movement.
Deep foundation
For the northern part of the building, intended for the school gym and associated facilities, a basement foundation level is planned. At the basement foundation level of the building, the foundation soil improvement by stone columns is planned. At the ground floor level of the building, a deep foundation with jet grouting columns is planned.
Plans include deep foundation and foundation soil improvement with jet grouting columns of D = 600.0 mm in and L = 10.0 and 12.0 m. At the ground floor level, jet grouting columns of L = 10.0 m below the base plate are planned. At the position of reinforced concrete strip footing, jet grouting columns of L = 8.0 m are also planned.
The execution of the jet grouting columns starts by drilling to the top of the planned jet grouting column. Drilling is done from the top of the future foundation structure. After drilling up to the level of the planned column, jet grouting starts. An injection is performed using a single-fluid system (CCP).
The deep foundation and foundation soil improvement are used at the basement foundation level of the building.
The foundation soil improvement at the basement level with stone columns of D = 800.0 mm and L = 10.0 m is planned. Stone columns execution (deep vibratory filling) is a method of improving the mechanical characteristics of the foundation soil and the drainage conditions. The installation of stone material (gravel or crushed stone) improves the mechanical properties of the foundation soil (overall strength parameters and average modulus of soil compressibility).
The execution of the stone columns is carried out from the bottom of the future foundation structure. Prior to the column design, a working plateau – crushed stone layer of granular material Ø0-63 mm, 30.0 cm thick, is constructed beneath the future foundation structure. After the execution of the stone columns, it is necessary to compact the crushed stone until the measurement determines the minimum compressibility modulus Ms = 60.0 MN / m2.
Conclusion
The construction of a new school building such as this one is an extremely complex and demanding project, but one that will surely provide Osijek high school students and teachers with more adequate space and better working conditions. We are extremely pleased to be part of a project that promotes higher quality education and ultimately a more promising future.
To ensure the highest possible reliability and clarity when designing, the deep foundation and foundation soil improvement is made with BIM technology.
A model of deep foundation (stone and jet grouting columns) for foundation soil improvement of the Prva gimnazija high school in Osijek is shown below:
