from [Report on the Conservation and Restoration Work of the Prasat Suor Prat Tower]
Micro-Tremor Measurement Survey of the Prasat Suor Prat Tower
In order to examine the effects of structural mechanics on the process of its collapse, the monument’s deformative movement must be empirically examined. Past studies have attributed the collapse of the monument to climatic factors such as weathering and temperature changes, biological factors associated to plants and animals, and ground-related factors caused by uneven subsidence. In particular, they suggested the constant vibrations caused by strong winds of nearly 20 m/s observed during the rainy season as the potential cause of its collapse.
This paper has been compiled from the results of a micro-tremor measurement survey conducted at the Royal Plaza in August 2003 and September 2004.
We used five GPL-6A3P portable seismographs for micro-tremor measurement. They were set to a 100Hz sampling and 30 Hz low-pass filter. Micro-tremors of each structure were measured for thirty minutes, and of each ground array, for ten minutes. We placed seismographs at the measurement points of each of the measurement cases, and conducted a multi-point simultaneous measurement using GPS.
We conducted micro-tremor measurements at the Bayon central tower and sub towers and Prasat Suor Prat towers, and estimated the vibration characteristics of each of the monuments. We also conducted a micro-tremor array investigation at the Royal Plaza to estimate the ground structure.
The basic horizontal frequency of the Prasat Suor Prat tovers is around 3 Hz. Because it is about 15 meters form the surface of the platform, their predominant frequency is similar to a regular four-story building.
Comparing the measured natural frequencies with their analytic counterparts, we assed the equivalent Young’s modulus of the entire structure to about 1/15 of the physical property of stone materials.
We were able to estimate ground structures corresponding to the N value trend by simulating the results of the micro-tremor array investigation conducted at the Royal Plaza with a higher first mode Rayleigh wave. The reasons for the predominance of the first mode must be pursued in the future, but these examination results can serve as valuable reference material, as there is extremely little in information on the ground structure of the Royal Plaza.
The first frequency of the Prasat Suor Prat towers corresponded to a translational first mode. We also confirmed that the Prasat Suor Prat tower targeted for mode measurement had a torsion mode at 6.1 Hz, respectively.
The horizontal first and second soil-coupled predominant frequencies of the Prasat Suro Prat towers were around #Hz and 9 Hz, respectively. Its base-fixed natural frequencies were 5 ～15% higher than the soil-coupled predominant frequencies. In regard to the damping ratio, it was around 2% for both the first and second frequencies.
Additionally, the predominant frequencies of the direction in which other structures are attached the antechambers in the case of Prasta Suor Prat were 10% or so higher than the predominant frequencies of the other components. Judging from this, we can assume that attached structures have the effect of increasing structural stiffness.
We can match measured natural frequencies with their analytic counterparts, with the equivalent Young’s modulus for the entire structure of about 1/18 ～1/14 the physical property of stone materials (sandstone in the case of Bayon Temple and laterite in the case of Prasat Sur Prat). However, in regard to the vertical component, our measurement results indicated frequencies 30% ～ 90% higher than the analysis results. To rationally explain the natural frequencies of both the horizontal and vertical components, further examination is necessary, including their analysis as discontinuum.