Open pit protection design for the Health center in Opatija
An expansion of the existing Health Center in the center of Opatija has been planned. The architectural design is by Athitektonsko-građevinski atelje d.o.o. To facilitate the execution of this construction project, we have developed the main and detailed temporary open pit protection design. The open pit protection is divided into three types, depending on the geological characteristics and external influences at the location.
Geotechnical investigation and testing
Prior to developing the open pit protection design of the planned structure, we conducted the following geotechnical investigation and testing:
- geological and geotechnical surveying of the broader area;
- exploratory rotary drilling;
- excavation of probing pits;
- engineering and geological mapping of the terrain and core sample identification;
- standard penetration testing (SPT);
- laboratory testing;
- geophysical testing; and
- analysis of previous investigations and correlation with new data.
The conducted investigations determined that the project site is composed of two lithostratigraphic units: a limestone breccia bedrock overlain by fill deposits.
Open pit protection
The excavation of the open pit will be carried out within both geotechnical units. Because of external factors (such as the proximity of adjacent buildings, the nearby parking lot and the plot boundary), the excavation is to be conducted vertically and at an incline. Given these spatial constraints, the open pit protection has been divided into three zones, as illustrated in the following figure.
In zone 1 of the open pit (northwestern part), the installation of a pile wall with drilled piles of varying diameter is planned (Ø640.0 mm within the fill and Ø520.0 mm within the bedrock) over a length of 9.10 m, with a spacing of 1.50 m between piles. On top of the pile wall, a reinforced concrete capping beam with dimensions H × V = 0.72 m × 1.0 m will be constructed. Prior to the installation of the pile wall, excavation in Zone 1 will be carried out at a slope of 1V:1H down to the platform level for pile installation.
After the pile wall is completed, vertical excavation in front of the wall is to be initiated. The vertical excavation will be carried out in stages, corresponding to the anchor positions. Self-drilling bar anchors will be installed in three rows, with lengths of 11.0 m, 9.0 m and 6.0 m, at an axial spacing of 1.50 m between anchors.
To prevent material from moving between the piles, geotextile is to be installed on the face of the pile wall, combined with a Q-188 reinforcement mesh and a layer of shotcrete with a thickness of 10.0 cm.
In Zone 2 of the open pit (eastern and southern parts), a temporary excavation is planned with a slope ratio of 1V:1H (for overburden material) and 2V:1H (for the rock mass), down to the final excavation level. In the northeastern section, a berm will be constructed at an elevation of 47.0 m above sea level, with a width of 1.50 m and a slope of 3% towards the open pit. For all excavations in the overburden layer, the installation of construction-grade PVC sheeting is planned to ensure erosion stability and protection against erosion.
In Zone 3 of the open pit (western part), a vertical excavation within the rock mass is planned. The specificity of Zone 3 is the presence of an existing underground shelter and another auxiliary structure, which must be removed before starting the vertical excavation. Protecting the pit in its western portion will be carried out using self-drilling rod anchors 4.0 m long at a distance of 1.50 m in combination with shotcrete 10.0 cm thick. In addition to cutting, additional protection of the existing inclined stone retaining wall using self-drilling anchors is also planned.
After the excavation of the open pit has been carried out to its bottom and after the installation of all the planned protection measures, the extension of the Health Center and the filling of the space between the face of the excavation and the structure will be carried out.
Conclusion
After the construction of the future structure, all horizontal loads (static and dynamic) are transferred to it. This design takes the open pit protection measures only as temporary. The future structure must be able to bear all the loads from the surrounding soil (both in a standard and seismic activity calculation situation) during use because, in this geotechnical design, only the temporary open pit protection was analyzed.
