Open pit protection at KBC Zagreb

Open pit protection at KBC Zagreb – Rebro

From Site Investigation to BIM-Based Design

The reconstruction of KBC Zagreb – Rebro represents a demanding project within the existing hospital complex. Geotech’s scope included the open pit protection for two temporary excavations: one for the new hospital building and one for the open parking garage. The hospital building has an average excavation depth of approximately 11 m, while the open parking garage has an average excavation depth of approximately 6 m, with local deepening to about 10 m.

Although both open pits form part of the same development, they are characterised by significantly different spatial constraints. At the open parking garage, the available space allowed a greater proportion of the excavation to be formed as an open cut with sloped sides, whereas the hospital building excavation is constrained along almost its entire perimeter by existing structures and infrastructure. The design therefore required a detailed assessment of the spatial relationships between the existing conditions, the future structures and the excavation support elements.

Geotechnical Site Investigation and Ground Model Development

The geotechnical site investigation identified predominantly cohesive deposits of stiff clay and silt, overlain by a surficial layer of anthropogenic fill. The recorded groundwater level was below the design excavation level of both open pits. The investigation data formed the basis for development of the geotechnical ground model and open pit protection.

*Figure 1 Locations of site investigation points*

*Figure 2 Log of a representative borehole*

BIM Model as a Common Design Platform

For the project, a BIM model of open pits was developed by integrating survey data of the existing conditions, the results of the geotechnical investigations and the IFC models of the proposed structures. The borehole locations were linked to the survey base, while the design models were aligned with the absolute site elevations. This created a common 3D environment in which the proposed structures, open pits, existing buildings, service tunnels and utilities could be assessed simultaneously.

*Figure 3 Development of the 3D/BIM geotechnical model*

Spatial Constraints as a Key Design Challenge

The BIM model was used not only to visualise the final solution, but as an active design tool. This was particularly important for the hospital building excavation, which is located immediately adjacent to the operating block, service tunnels, a church and an underground energy-plant service duct. Some of the existing structures and service routes had to remain operational throughout construction, while an additional constraint was the connecting tunnel to the operating block, as much of which as possible had to be retained. The spatial model was particularly useful for developing ground-anchor layouts with different lengths, inclinations and elevations, and for checking potential clashes.

*Figure 4 Spatial constraints around the hospital building excavation*

Open pit protection for the Hospital Building

Most of the open pit was designed as a vertical cut supported by bored-pile walls. The primary support system consists of reinforced-concrete bored piles with diameters of 800 mm, or 600 mm in spatially constrained areas, connected by capping beams and supported by bar ground anchors whose length and inclination were adapted to the spatial constraints along the excavation. In the area with the maximum excavation height of approximately 12.6 m, four rows of anchors were used along one section, while an adjacent section required a system of steel tubular struts due to spatial conflicts.

*Figure 5 Final BIM model of the hospital building open pit*

*Figure 6 Plan layout of the hospital building open pit*

Open pit protection for the open Parking Garage

For the open parking garage, most of the open pit could be formed as an open cut with side slopes of 1.5V:1H, or 1V:1H for greater excavation heights. Vertical excavation support was required only along the north-western site boundary and the existing road on the southern side, where bored-pile walls comprising 600 mm diameter piles were designed and additionally supported by 25 mm diameter bar ground anchors.

Comparison of the two open pits demonstrates that the value of the BIM approach does not lie in creating a more complex support system, but in enabling accurate identification of spatial constraints and selection of an appropriate solution for each part of the project.

*Figure 7 Final BIM model of the open parking garage open pit*

*Figure 8 Plan layout of the open parking garage open pit*

Numerical Verification of the Design Solutions

The stability and deformation response of characteristic sections were analysed using the finite element method in PLAXIS 2D, with staged modelling of pile installation, excavation, anchors and struts. The design solutions were thereby additionally verified through geotechnical numerical analyses.

Figure 9 *Displacement contours for a representative section of the hospital building open pit*

From Design to Construction

On the KBC Zagreb – Rebro project, BIM proved to be far more than a three-dimensional visualisation tool. Its value lay in integrating investigation data, survey information and design models into a common basis for geotechnical design decisions.

On complex projects of this type, however, design does not end with the issue of construction documentation. Actual site conditions, design adaptations during construction, and the role of designer’s site supervision and monitoring are the subject of Part 2 of this blog series.