Material Characterization, Dynamic Identification and Mechanical Modelling of the Fifth Minaret of Herat, Afghanistan

Abstract

The Fifth Minaret of Heart, part of the “Musalla” complex, 15^th century, is the only one still standing in the Gawhar Shad area. The 42 m tower, whose external surfaces were completely covered by glazed tiles, used to corner the Madrasa, which was demolished at the end of the 19^th Cent. This fact, coupled with several low-intensity earthquakes and recursive floods, has induced increasing leaning for over a decade starting from the end of the 80s, and the formation of an extensive crack at the base of the shaft. An emergency intervention carried out in 2003 prevented the collapse.

The study pointed at a broad, multi-discipline knowledge path to systematize available information and determine the intrinsic material characteristics, the building technology solutions, the damage and vulnerability sources, the behavior under dynamic loading and the structural response in the current configuration.

To determine the material characteristics, the following tests were carried out: uniaxial compression and indirect tension tests on bricks, penetrometer tests on mortar joints on site, porosity tests on bricks, X-ray diffraction tests and observation of thin sections at the optical microscope in polarized light on bricks and mortars and clay mineral composition. The building technology solutions were inspected at the site and supported by the analysis of borescope images. A seismic network composed of five seismometers was installed to implement Operational Modal Analysis. Results were exploited to calibrate the mechanical properties of the 3D solid mesh of the Minaret, retrieved by the Terrestrial Laser Scanner survey, and a set of modal pushover analyses were employed to determine the expected damage areas in the current configuration.