Long term stability of an abandoned construction pit in Eocene flysch rock mass: case study of Bracka street construction site (Split, Croatia)

Excavations in soft rocks usually have to be performed by blasting with explosives or with heavy pneumatic hammers. However, in a certain period after excavation, their physical and mechanical properties begin to change to a level where even manual excavation can be used. These changes can be significant during the building design life, where the initial design solution of the slope cut may prove inappropriate, sometimes resulting in collapse. In this context, it is necessary to define the causes of changes in the soft rock physical and mechanical properties, and determine all the necessary parameters (primarily strength parameters, but also all others relevant to describe the change in rock properties over time) in all phases of expected change during construction or other applications (such as use of slope area, in case of abandoning the site in certain time period, etc.).

Furthermore, when preparing project documentation for construction, in the part where the calculations of the global stability of the building on the slope are performed, the possibility of significant changes in the shape of the slope during the structure/building design life are usually neglected. Therefore, this paper also presents the Fisher Lehmann model of the change of slope geometry during the period of construction use, and explains the influences of weathering factors on parameters of the soft rock over time by using laboratory simulation of weathering.

Combined changing the geometry of the slope and the properties of the rock can have a negative impact on the safety of the structure, which is explained and shown through an example of an abandoned construction pit at Bračka Street in Split, where the stability of neighboring residential houses is endangered. By using appropriate mathematical models of the slope morphology change, results of long term slope monitoring by TLS and appropriate software for slope stability analysis (Slide 2, RocScience), the time span in which the instability can occur for Bračka Street case study is determined for multiple possible future intervention scenarios.