Restoration and Strengthening of Historical Buildings: The Example of Minceta Fortress in Dubrovnik

Firstly, some important aspects of the restoration and structural strengthening of historical masonry buildings are briefly discussed. Particularly, as an illustration of the appropriate approach to solving such an issue, an example of the restoration of the Minceta fortress in Dubrovnik has been described. Based on the results of the extensive field and laboratory works with defined damages and cracking state in the fortress, as well as located sliding surfaces in foundation rock near deep excavation, the static and dynamic analysis of stress-strain states and safety was performed at the characteristic time point in the history of the fortress. Different geometry models for discretization of the fortress and the foundation rock, as well as different constitutive models for masonry and rock, were considered. Based on the results of the case study, the solutions were proposed for the strengthening of the foundation rock, as well as for the restoration and the limited allowed strengthening of the fortress. It is believed that this study may be helpful in the restoration and strengthening of similar historical masonry structures.

Case Study: In Situ Experimental Investigation on Overburden Consolidation Grouting for Columnar Jointed Basalt Dam Foundation

The dam foundation rock mass, at the Baihetan hydropower station on the Jinsha River, is mainly columnar jointed basalt, with faults and fissures developed. Considering adverse factors such as the unloading relaxation or the opening of the fissures due to excavation blasting, consolidation grouting is needed to improve the integrity of the dam foundation rock mass. According to the physical and mechanical properties of columnar jointed basalt and the continuity of construction, the effectiveness of overburden consolidation grouting is experimentally studied. The results show that this grouting technology can obviously improve the integrity and uniformity of a dam foundation rock mass and reduce the permeability of the rock mass. After grouting, the average increase in the wave velocity of the rock mass is 7.3%. The average improvement in the deformation modulus after grouting is 13.5%. After grouting, the permeability of 99% of the inspection holes in the Lugeon test section had Lugeon values of no more than 3 LU. This improvement is considerable and provides a case to engineering application.

A Vibration-Isolating Blast Technique with Shock-Reflection Device for Dam Foundation Excavation in Complicated Geological Conditions

Under complicated geological conditions, the vibration in the dam foundation caused by blasting can lead to further deterioration of the foundation rock mass and adversely affect the safety of foundation. In order to effectively control the vibration in dam foundation rock mass, a new bench blasting technique with shock-reflection device is proposed. It introduces a shock-reflection device consisting of high wave impedance block and cushion material, which is placed at the bottom of vertical borehole. This shock-reflection device can effectively reflect the explosion shock wave from vertical direction to horizontal direction after detonation, which can make blasting energy concentrated on the rock mass above dam foundation, so the vibration in the foundation can be controlled. Field blasting experiment was carried out to contrast the blasting induced vibration in foundation rock by bench blasting with shock-reflection device and conventional bench blasting. The results indicate that the vibration in the foundation rock can be reduced by 30%~57%. In addition, the vibration at the bottom of the borehole is also demonstrated by numerical simulation, with results similar to the field experiments. The production experiment results show that the new blasting technique can replace the conventional excavation method of dam foundation in complicated geological conditions, and the new blasting technique has been successfully applied to the Baihetan dam foundation excavation.

Qualitative Rock Wedge Stability Evaluation Performed For Foundation of Green Peter Dam, Oregon

Green Peter Dam is a concrete gravity structure located in west-central Oregon on the Middle Santiam River within the Willamette Valley Basin. A risk assessment for the project identified continuous and adversely oriented low-angled shear zones underlying portions of the foundation that could potentially facilitate sliding instability of one or more monoliths during earthquake loading. Conceptually, a potential foundation rock wedge could be formed with a shear zone as its sliding surface and joints as the side planes. This wedge, which would otherwise be stable under static conditions, could feasibly be displaced and/or shifted during seismic ground shaking, resulting in significant structural damage and/or breach of the dam. A qualitative evaluation was performed to characterize the geomechanical conditions and geometry of movement (i.e., kinematics) of the dam-foundation system associated with rock wedges. The study revealed that wedges could indeed be formed by adversely oriented and intersecting rock mass discontinuities. The qualitative evaluation concluded that the displacement geometry and geologic conditions in the foundation collectively suggest that the wedges would likely be stable under even large probabilistic seismic loading. While no concrete dams are known to have failed due to seismic loading, an increased knowledge of higher seismicity in the Pacific Northwest region warranted a careful evaluation to ensure that the risks of foundation rock wedge deformation are well characterized, and that our level of confidence in the available data is acceptable to better constrain the potential risk posed by this failure mode. This paper summarizes the background, findings, and results of the preliminary and qualitative dam-foundation system stability evaluation that was performed for Green Peter Dam.