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Noninvasive geotechnical site investigation for stability of Cheomseongdae
Journal of Cultural Heritage 13 (2012) 98–102
Case study
Noninvasive geotechnical site investigation for stability of Cheomseongdae Park Heon-Joon a , Kim Derk-Moon b , Kim Ki-Seog c , Ahn Hee-Yoon c , Kim Dong-Soo a,∗ a
Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291, Daehak-ro, Yuseong-gu, Daejeon, Republic of Korea Research Division of Architectural Heritage, National Research Institute of Cultural Heritage, Republic of Korea c Heesong Geotek Co., Ltd., Republic of Korea b
a r t i c l e
i n f o
Article history: Received 10 October 2010 Accepted 24 May 2011 Available online 1 July 2011 Keywords: Architectural heritage Geotechnical approach Noninvasive site investigation Cheomseongdae
a b s t r a c t Architectural heritages are exposed to natural and man-made disasters so that the need for research to prevent disasters has arisen. Cheomseongdae, known as the oldest astronomical observatory in East Asia, displays separation between the members and is tilted to the north-northeast. To diagnose the present conditions and to resolve problems, site investigations are performed. Boring is conducted in the surrounding ground of Cheomseongdae, and noninvasive investigations such as seismic tests and electrical resistivity surveys are conducted as well. The present study proposes a geotechnical engineering approach focused on noninvasive site surveys, and through this, relates current tilted condition of structure and provides information on the stability of Cheomseongdae. © 2011 Elsevier Masson SAS. All rights reserved.
1. Introduction Architectural heritages reflect their temporal backgrounds and history in their entirety, and we thus have a responsibility to preserve them safely and to transmit them to posterity. In particular, architectural artifacts are exposed to natural and man-made disasters so that the need for systematic safety management and research to prevent disasters and to preserve the structures has arisen. To preserve cultural heritages, it is necessary to establish an interdisciplinary research system for comprehensive cultural heritage disaster prevention that encompasses not only fields in the humanities and the social sciences such as history, folkloristics, art history, archaeology, and policy studies but also scientific and engineering approaches such as conservation science, natural disaster science, architectural engineering, and geotechnical engineering. Cheomseongdae, a star-gazing platform, has buttressed the sky above the ancient city of Gyeongju in Korea. Known as the oldest astronomical observatory in East Asia, this structure boasts its magnificent and beautiful form, still shrouded in mystery. Although there are few historical records on Cheomseongdae, simply the view of the structure, which has stood for over 1,300 years, leads us to feel the thousand-year-long history of Silla. Fig. 1 shows past and present photographic records of Cheomseongdae [1,2]. In particular, as in the photograph from the 1950s in Fig. 1(b), in the past, there existed a thoroughfare to the imme-
∗ Corresponding author. Tel.: +82 42 350 3619; fax: +82 42 350 7200. E-mail addresses: [email protected] (P. Heon-Joon), [email protected] (K. Derk-Moon), [email protected] (K. Ki-Seog), [email protected] (A. Hee-Yoon), [email protected] (K. Dong-Soo). 1296-2074/$ – see front matter © 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2011.05.008
diate north of Cheomseongdae so that there seems to have been a possibility of differential settlement in the ground due to the effect of ground shaking. Currently, Cheomseongdae displays separation between the body and the lower stone in the southwest and the north as well as cracks in the #-shaped stone in the top part. In addition, Cheomseongdae is tilted at an angle, at most, of 3◦ to the north direction as shown in Fig. 1(c) according to previous study [3]. Based on the current observation, Cheomseongdae is certainly suffering differential settlement of the ground. The settlement of foundation in the north direction is about 117 mm larger than the settlement in the south direction. The structure’s crack and separation can be related to the differential settlement. To identify the geotechnical threats to architectural heritage, site investigations have to be performed [4]. Because the differential settlement comes mostly from the underground condition, it is essential to grasp the composition and material property distribution beneath and surrounding ground of Cheomseongdae through site investigations. However, the practical difficulty lies in utilizing boring and intrusive method in the site investigation near the architectural heritage like Cheomseongdae. To resolve the problem, noninvasive site survey together with minimal borings a little way off the artifact should be adopted and the current tilted condition is related with the subsoil conditions. In this study, two borings, 20 m away from Cheomseongdae stereobate, were conducted to the bedrock and standard penetration test and downhole seismic test were performed to identify the subsoil profile and the stiffness in general. Due to the difficulties in drilling very near the Cheomseongdae, the linear offset seismic test and 3-dimensional (3D) DC resistivity survey were performed and the subsoil conditions beneath the Cheomseongdae were assessed
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99
Fig. 1. The past and present of Cheomseongdae [1,2].
noninvasively. Finally, the current tilt condition was related with the subsoil conditions. 2. Cheomseongdae, the oldest astronomical observatory in Asia The Gyeongju Historic Areas (designated UNESCO World Heritage Site as Kyongju Historic Areas in 2000 [5]) contain a remarkable concentration of outstanding examples of Korean Buddhist art, in the form of sculptures, reliefs, pagodas, and the remains of temples and palaces from the flowering, between the seventh and the tenth centuries, of this form of unique artistic expression. A representative stone structure from the Silla dynasty in Gyeongju Historic Areas, Cheomseongdae (National Treasure No. 31) is an architectural artifact that harmoniously combined straight lines and curves. Constructed under Queen Seondeok’s rule during the Silla dynasty, Cheomseongdae is known as the oldest astronomical observatory in Asia. On a square stereobate made of processed granite, 27 layers of stones are piled up in a bottle-shaped curve, placed on which is a #-shaped top part consisting of parallel rectangular stones, and the height of the entire structure is 9.17 m. The cylindrical part consists of fan-shaped stones piled up in 27 layers, and, unlike the smoothly faced exterior, the interior is uneven because of jags. With a window facing the southeast at the centre, the lower half is filled with rubble stones while the upper half is hollow to the top. According to old records, “people were to ascend (Cheomseongdae) in the centre”. As such, this is a structure where one presumably observed the sky after putting up a ladder outside, entering the building through the window, and climbing to the top using the same ladder.
property distribution of the beneath ground and the surrounding ground of architectural artifacts. Therefore, the geotechnical surveys were conducted on the subsurface and the surrounding ground of Cheomseongdae as shown in Table 1. Boring, sample extraction and downhole seismic test were conducted in the surrounding ground of Cheomseongdae, and noninvasive site investigations such as seismic tests and electrical resistivity surveys were conducted as well. Fig. 2 shows the location of two boreholes and survey lines for linear offset seismic test and DC resistivity survey. 3.1. Boring and downhole seismic test The practical difficulty lied in boring near architectural heritage. Therefore, boring was conducted down to the bedrock at two
3. Site investigations All architectural structures are based on land, and the stability of the base ground is directly linked to the stability of structures. To diagnose the present conditions of architectural artifacts and to conduct engineering research for the resolution of their problems and historical questions regarding them, the starting point is none other than to understand the composition and the material
Fig. 2. The location of two boreholes and survey lines around Cheomseongdae ground.
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Table 1 Site investigation methods and objectives around Cheomseongdae ground. Site investigation method
Location
Quantity
Objectives
Boring Sample extraction
20 m away from Cheomseongdae
2 boreholes (South: to the depth 19 m, North: to the depth 16 m)
20 m × 20 m range with Cheomseongdae as the centre 40 m × 40 m range with Cheomseongdae as the centre
8 fan-shaped survey lines
Soil stratigraphy, ground water table Soil stratigraphy, ground classification, disturbed sample SPT-N value Soil stratigraphy, shear wave velocity profile for 2 boreholes, dynamic soil property The geologically anomalous zone from the apparent velocity of seismic waves Detecting geologically anomalous zone
Standard penetration test (SPT) Downhole seismic test
Linear offset seismic test 3-D DC resistivity survey
spots, approximately 20 m to the north and to the south, respectively, of Cheomseongdae stereobate. Boring and downhole seismic test were performed in order to identify soil strata and stiffness profile. According to the results of the boring, a thick alluvial layer existed in the ground surrounding Cheomseongdae, and it consisted mostly of gravel with sand and gravelly silty sand. Fig. 3 shows the core boxes and boring logs for two boreholes, soil classification and shear wave velocity profiles. In the case of borehole 1, the alluvial layer was to 17.2 m depth. In borehole 2, the alluvial layer was to 15.0 m depth. The Gyerim area, where Cheomseongdae is located, is a typical sedimentary basin where the streams such as Namcheon and Bukcheon are developed so that, due to phenomena such as floods, a thick alluvial layer is distributed at the top. Through boring, it was confirmed that the alluvial layer consisting of gravel with sand were deeply formed. The noninvasive site survey together with minimal borings a little way off the artifact should be adopted. Linear offset seismic test and 3D DC resistivity survey were conducted for identifying the subsurface and surrounding ground and compared with the current tilt condition of Cheomseongdae. 3.2. Linear offset seismic test To understand the geology around Cheomseongdae, linear offset seismic tests, which make use of differences in the arrival time, were conducted. A way promptly to understand the general geology of the broadband, the linear offset seismic test uses a geophone array different from that of the general offset seismic test. In other words, geophones are arrayed in a fan shape at spots away from the shot points, which are the control points. According to the purposes of the investigations, the intervals between the receiving point and the geophones are established, the geophones are installed in a fan shape, and transmission is conducted. Here, because the distance from the receiving point is identical for all geophones, when there are no changes to the ground under the receiving point and the receiver points, the initial arrival time recorded at each geophone is recorded as being identical. However, when zones of anomalous velocity such as underground rock caverns and geologically anomalous zones are passed through, using the time lead recorded at each geophone, the zones of anomalous velocity for each propagation path come to be known. According to the results of first arrival picking, when compared to the central part, which was the closest to Cheomseongdae, the arrival time was analysed to be more prompt for the south direction than for the north direction for the in-line, and the arrival time was analysed to be more prompt for the north direction than for the south direction for the out-line because the locations of the geophones installed were affected by mutually opposite ground characteristics, with Cheomseongdae.
17 survey lines
Fig. 4(a) shows an image of the results of investigations of the surface of the ground surrounding Cheomseongdae expressed as 2-dimensional (2-D) images based on the data processed. As for data analysis, data from each receiving point per direction were used, and, in particular, the data from the east-west direction were analysed using remote receiving points. As in the figure, overall, with Cheomseongdae as the centre, high velocity values were distributed around Cheomseongdae, and this seemed to be due to Cheomseongdae plat, which is made of granite. The apparent velocity of seismic waves decreased increasingly from the south direction to the north direction. This means that the time taken to reach the receiver points from the actual receiving points was greater for the north direction than for the south direction, which in turn strongly implies that a geologically anomalous zone may exist in the north direction. This is area A in Fig. 4(a). This result can match the status of Cheomseongdae, which is tilted to the north as shown in Fig. 1. As for area B in Fig. 4(a), it seems to be an inversion error due to deficient data from the area. 3.3. 3-dimensional DC resistivity survey As the boring results show, a geological structure exhibiting the coexistence of a deep gravel layer and of a partly weathered silty sand bed at the lower part of the gravel layer led to expectations of 3-D development. Consequently, to express changes in the overall geological structure in 3-D images, 3-D DC resistivity surveys were conducted. As for the contents of the investigations per survey line, with Cheomseongdae and within the 40 m by 40 m range, a total of seventeen survey lines – twelve survey lines in the east-west direction and five survey lines in the north-south direction – were designed. To image the underground structure at the lower part of Cheomseongdae in 3-D, the survey lines were formed in grids, and dipole-dipole array, modified pole-pole array, and electrical resistivity surveys were conducted on these survey lines. Fig. 4(b) shows images from 3-D DC resistivity surveys. The figure expresses the locations and directions of the overall geological distribution and geologically anomalous zones around Cheomseongdae, with the X-Y plane. With the 4.5 m depth, low resistivity anomalous zones began to appear. A low resistivity anomalous zone existed around the artifact at a depth 8 m. Another low resistivity anomalous zone were distributed around the artifact at a depth 4 to 10 m in the northwest and northeast directions. Overall, with Cheomseongdae as the centre, electrical resistivity distribution differed for the north and the south. Generally, electrical resistivity values of approximately 300 ohm-m were distributed in the south while those of approximately 50 to 150 ohm-m were distributed in the north. In addition, relatively high resistivity was distributed in the , and low resistivity part immediately below Cheomseongdae anomalous zones were distributed in the lower part surrounding Cheomseongdae . Because the stereobate, which can be seen as
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Fig. 3. Boring and downhole seismic test results for Cheomseongdae ground: a thick alluvial layer existed in the ground surrounding Cheomseongdae, and it consisted mostly of gravel with sand (GP).
the foundation of Cheomseongdae, consisted of granite, anomalous zone consequently was expected to react relatively more strongly than the electrical resistivity values of the surrounding area. Surrounded by granite with relatively higher electrical resiscould be expected to exhibit a tivity values, anomalous zone distribution of low resistivity anomalous zones, as in the figure, because of high porosity or moisture content caused by the prolonged weathering of the alluvial layer. Like anomalous zones and , anomalous zone in the lower part of Cheomseongdae in
the northwest direction could be expected to exhibit a distribution of electrical resistivity values lower than those for the surrounding area due to partial weathering over a long period. As for the electrical resistivity values of the lower ground of Cheomseongdae, while the east-west direction exhibited relatively homogeneous rheological changes, in the north-south direction, it was confirmed that low electrical resistivity values and resistivity zones emerged earlier in the north than in the south. Although the electrical resistivity of the alluvial layer differed according to the
Fig. 4. (a) The 2-D image for linear offset seismic test: the difference of apparent velocity implies that a geologically anomalous zone may exist in the north direction, (b) in the northwest direction of Cheomseongdae subsurface could be expected to exhibit a distribution of electrical resistivity The results of 3-D DC resistivity survey: zone values lower than those for the surrounding area.
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moisture content, overall, it was low. The alluvial layer of the area investigated could be classified largely into a gravel layer and a sand bed, and, generally, gravel layers have higher electrical resistivity values than do sand beds. When the results of the 3-D DC resistivity surveys were synthesized and analysed, it was possible to expect that the number of boulders in the north alluvial layer would be smaller than in the south side. 3.4. Discussion As aforementioned, the apparent velocity of Cheomseongdae ground decreased from the south direction to the north direction in the linear offset seismic test results. And the low electrical resistivity values and resistivity zones emerged earlier in the north than in the south, while the east-west direction exhibited relatively homogeneous rheological changes, as for the electrical resistivity values of the lower ground of Cheomseongdae. When the results of the linear offset seismic test and 3-D DC resistivity survey were synthesized and analysed, it was possible to expect that a geologically anomalous zone exists in the north direction. Through two noninvasive tests, linear offset seismic test and 3D DC resistivity survey, the remarkable differences of stiffness and resistivity were detected between the north and south of Cheomseongdae. These test results are related with the current tilted condition of Cheomseongdae. The differential settlement stems from differences in the properties of the ground to the north and south. Additional investigations can be needed in order to analyse the cause of differential settlement for Cheomseongdae. 4. Conclusion The study proposed a geophysical approach focused on noninvasive site surveys, and through this, relates the current tilted condition and provides information on the stability of Cheomseongdae. Cheomseongdae is a stone architectural heritage presumed to have been constructed during the Silla dynasty, approximately 1,300 years ago. With the passage of considerable time, the structure’s deformation and degradation are judged to appear due to diverse reasons, and the structure suffers the differ-
ential settlement due to the passage of time or the accumulation of ground displacement factors. Through the results of the boring, the ground surrounding Cheomseongdae consisted mostly of gravel with sand and undisturbed sampling was not possible. In addition, through seismic tests and electrical resistivity surveys, test data were collected to assess the soil stratigraphy, geometry, stiffness difference, and the presence of anomalous zones, among other things. There are the quantifiable differences of stiffness (or seismic wave velocity) and resistivity between the north and south of the artifact. Based on the analysis of such data, it is likely that the tilt to the north and the differential settlement of Cheomseongdae stems from differences in the properties of the ground to the north and south of Cheomseongdae. However, these noninvasive approaches have the limits in identifying time dependent differential settlement and strength of foundation soil. Acknowledgements This study, which forms a part of the project, has been achieved with the support of national R&D project, which has been hosted by National Research Institute of Cultural Heritage of Cultural Heritage Administration. We express our gratitude to it. References [1] Gyeongju City, “Gyeongju-Pungmul-Jiriji (The scenery, customs and geography of Gyeongju),” 2006 (In Korean). [2] Cultural Heritage Administration of Korea Home Page, Retrieved Sep 1, 2009 from http://www.cha.go.kr/unisearch/imagefiles/national treasure/ a0031000037001.jpg. [3] H. Shon, S.B. Kim, Studies on the characteristics of stone structures by shape reversal, geotechnical and dynamic structural engineerings Proceedings of conference on Korean Society of Earth and Exploration Geophysicists, 2004, pp.25–48, (In Korean). [4] F.T. Gizzi, Identifying geological and geotechnical influences that threaten historical sites: a method to evaluate the usefulness of data already available, J. Cult. Herit. 9 (3) (2008) 302–310. [5] World Heritage Committee, UNESCO (United Nations Educational, Scientific and Cultural Organization), “Convention concerning the protection of the world cultural and natural heritage”, Cairns, Australia, 2000, pp. 47.
Case study
Noninvasive geotechnical site investigation for stability of Cheomseongdae Park Heon-Joon a , Kim Derk-Moon b , Kim Ki-Seog c , Ahn Hee-Yoon c , Kim Dong-Soo a,∗ a
Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291, Daehak-ro, Yuseong-gu, Daejeon, Republic of Korea Research Division of Architectural Heritage, National Research Institute of Cultural Heritage, Republic of Korea c Heesong Geotek Co., Ltd., Republic of Korea b
a r t i c l e
i n f o
Article history: Received 10 October 2010 Accepted 24 May 2011 Available online 1 July 2011 Keywords: Architectural heritage Geotechnical approach Noninvasive site investigation Cheomseongdae
a b s t r a c t Architectural heritages are exposed to natural and man-made disasters so that the need for research to prevent disasters has arisen. Cheomseongdae, known as the oldest astronomical observatory in East Asia, displays separation between the members and is tilted to the north-northeast. To diagnose the present conditions and to resolve problems, site investigations are performed. Boring is conducted in the surrounding ground of Cheomseongdae, and noninvasive investigations such as seismic tests and electrical resistivity surveys are conducted as well. The present study proposes a geotechnical engineering approach focused on noninvasive site surveys, and through this, relates current tilted condition of structure and provides information on the stability of Cheomseongdae. © 2011 Elsevier Masson SAS. All rights reserved.
1. Introduction Architectural heritages reflect their temporal backgrounds and history in their entirety, and we thus have a responsibility to preserve them safely and to transmit them to posterity. In particular, architectural artifacts are exposed to natural and man-made disasters so that the need for systematic safety management and research to prevent disasters and to preserve the structures has arisen. To preserve cultural heritages, it is necessary to establish an interdisciplinary research system for comprehensive cultural heritage disaster prevention that encompasses not only fields in the humanities and the social sciences such as history, folkloristics, art history, archaeology, and policy studies but also scientific and engineering approaches such as conservation science, natural disaster science, architectural engineering, and geotechnical engineering. Cheomseongdae, a star-gazing platform, has buttressed the sky above the ancient city of Gyeongju in Korea. Known as the oldest astronomical observatory in East Asia, this structure boasts its magnificent and beautiful form, still shrouded in mystery. Although there are few historical records on Cheomseongdae, simply the view of the structure, which has stood for over 1,300 years, leads us to feel the thousand-year-long history of Silla. Fig. 1 shows past and present photographic records of Cheomseongdae [1,2]. In particular, as in the photograph from the 1950s in Fig. 1(b), in the past, there existed a thoroughfare to the imme-
∗ Corresponding author. Tel.: +82 42 350 3619; fax: +82 42 350 7200. E-mail addresses: [email protected] (P. Heon-Joon), [email protected] (K. Derk-Moon), [email protected] (K. Ki-Seog), [email protected] (A. Hee-Yoon), [email protected] (K. Dong-Soo). 1296-2074/$ – see front matter © 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2011.05.008
diate north of Cheomseongdae so that there seems to have been a possibility of differential settlement in the ground due to the effect of ground shaking. Currently, Cheomseongdae displays separation between the body and the lower stone in the southwest and the north as well as cracks in the #-shaped stone in the top part. In addition, Cheomseongdae is tilted at an angle, at most, of 3◦ to the north direction as shown in Fig. 1(c) according to previous study [3]. Based on the current observation, Cheomseongdae is certainly suffering differential settlement of the ground. The settlement of foundation in the north direction is about 117 mm larger than the settlement in the south direction. The structure’s crack and separation can be related to the differential settlement. To identify the geotechnical threats to architectural heritage, site investigations have to be performed [4]. Because the differential settlement comes mostly from the underground condition, it is essential to grasp the composition and material property distribution beneath and surrounding ground of Cheomseongdae through site investigations. However, the practical difficulty lies in utilizing boring and intrusive method in the site investigation near the architectural heritage like Cheomseongdae. To resolve the problem, noninvasive site survey together with minimal borings a little way off the artifact should be adopted and the current tilted condition is related with the subsoil conditions. In this study, two borings, 20 m away from Cheomseongdae stereobate, were conducted to the bedrock and standard penetration test and downhole seismic test were performed to identify the subsoil profile and the stiffness in general. Due to the difficulties in drilling very near the Cheomseongdae, the linear offset seismic test and 3-dimensional (3D) DC resistivity survey were performed and the subsoil conditions beneath the Cheomseongdae were assessed
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99
Fig. 1. The past and present of Cheomseongdae [1,2].
noninvasively. Finally, the current tilt condition was related with the subsoil conditions. 2. Cheomseongdae, the oldest astronomical observatory in Asia The Gyeongju Historic Areas (designated UNESCO World Heritage Site as Kyongju Historic Areas in 2000 [5]) contain a remarkable concentration of outstanding examples of Korean Buddhist art, in the form of sculptures, reliefs, pagodas, and the remains of temples and palaces from the flowering, between the seventh and the tenth centuries, of this form of unique artistic expression. A representative stone structure from the Silla dynasty in Gyeongju Historic Areas, Cheomseongdae (National Treasure No. 31) is an architectural artifact that harmoniously combined straight lines and curves. Constructed under Queen Seondeok’s rule during the Silla dynasty, Cheomseongdae is known as the oldest astronomical observatory in Asia. On a square stereobate made of processed granite, 27 layers of stones are piled up in a bottle-shaped curve, placed on which is a #-shaped top part consisting of parallel rectangular stones, and the height of the entire structure is 9.17 m. The cylindrical part consists of fan-shaped stones piled up in 27 layers, and, unlike the smoothly faced exterior, the interior is uneven because of jags. With a window facing the southeast at the centre, the lower half is filled with rubble stones while the upper half is hollow to the top. According to old records, “people were to ascend (Cheomseongdae) in the centre”. As such, this is a structure where one presumably observed the sky after putting up a ladder outside, entering the building through the window, and climbing to the top using the same ladder.
property distribution of the beneath ground and the surrounding ground of architectural artifacts. Therefore, the geotechnical surveys were conducted on the subsurface and the surrounding ground of Cheomseongdae as shown in Table 1. Boring, sample extraction and downhole seismic test were conducted in the surrounding ground of Cheomseongdae, and noninvasive site investigations such as seismic tests and electrical resistivity surveys were conducted as well. Fig. 2 shows the location of two boreholes and survey lines for linear offset seismic test and DC resistivity survey. 3.1. Boring and downhole seismic test The practical difficulty lied in boring near architectural heritage. Therefore, boring was conducted down to the bedrock at two
3. Site investigations All architectural structures are based on land, and the stability of the base ground is directly linked to the stability of structures. To diagnose the present conditions of architectural artifacts and to conduct engineering research for the resolution of their problems and historical questions regarding them, the starting point is none other than to understand the composition and the material
Fig. 2. The location of two boreholes and survey lines around Cheomseongdae ground.
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Table 1 Site investigation methods and objectives around Cheomseongdae ground. Site investigation method
Location
Quantity
Objectives
Boring Sample extraction
20 m away from Cheomseongdae
2 boreholes (South: to the depth 19 m, North: to the depth 16 m)
20 m × 20 m range with Cheomseongdae as the centre 40 m × 40 m range with Cheomseongdae as the centre
8 fan-shaped survey lines
Soil stratigraphy, ground water table Soil stratigraphy, ground classification, disturbed sample SPT-N value Soil stratigraphy, shear wave velocity profile for 2 boreholes, dynamic soil property The geologically anomalous zone from the apparent velocity of seismic waves Detecting geologically anomalous zone
Standard penetration test (SPT) Downhole seismic test
Linear offset seismic test 3-D DC resistivity survey
spots, approximately 20 m to the north and to the south, respectively, of Cheomseongdae stereobate. Boring and downhole seismic test were performed in order to identify soil strata and stiffness profile. According to the results of the boring, a thick alluvial layer existed in the ground surrounding Cheomseongdae, and it consisted mostly of gravel with sand and gravelly silty sand. Fig. 3 shows the core boxes and boring logs for two boreholes, soil classification and shear wave velocity profiles. In the case of borehole 1, the alluvial layer was to 17.2 m depth. In borehole 2, the alluvial layer was to 15.0 m depth. The Gyerim area, where Cheomseongdae is located, is a typical sedimentary basin where the streams such as Namcheon and Bukcheon are developed so that, due to phenomena such as floods, a thick alluvial layer is distributed at the top. Through boring, it was confirmed that the alluvial layer consisting of gravel with sand were deeply formed. The noninvasive site survey together with minimal borings a little way off the artifact should be adopted. Linear offset seismic test and 3D DC resistivity survey were conducted for identifying the subsurface and surrounding ground and compared with the current tilt condition of Cheomseongdae. 3.2. Linear offset seismic test To understand the geology around Cheomseongdae, linear offset seismic tests, which make use of differences in the arrival time, were conducted. A way promptly to understand the general geology of the broadband, the linear offset seismic test uses a geophone array different from that of the general offset seismic test. In other words, geophones are arrayed in a fan shape at spots away from the shot points, which are the control points. According to the purposes of the investigations, the intervals between the receiving point and the geophones are established, the geophones are installed in a fan shape, and transmission is conducted. Here, because the distance from the receiving point is identical for all geophones, when there are no changes to the ground under the receiving point and the receiver points, the initial arrival time recorded at each geophone is recorded as being identical. However, when zones of anomalous velocity such as underground rock caverns and geologically anomalous zones are passed through, using the time lead recorded at each geophone, the zones of anomalous velocity for each propagation path come to be known. According to the results of first arrival picking, when compared to the central part, which was the closest to Cheomseongdae, the arrival time was analysed to be more prompt for the south direction than for the north direction for the in-line, and the arrival time was analysed to be more prompt for the north direction than for the south direction for the out-line because the locations of the geophones installed were affected by mutually opposite ground characteristics, with Cheomseongdae.
17 survey lines
Fig. 4(a) shows an image of the results of investigations of the surface of the ground surrounding Cheomseongdae expressed as 2-dimensional (2-D) images based on the data processed. As for data analysis, data from each receiving point per direction were used, and, in particular, the data from the east-west direction were analysed using remote receiving points. As in the figure, overall, with Cheomseongdae as the centre, high velocity values were distributed around Cheomseongdae, and this seemed to be due to Cheomseongdae plat, which is made of granite. The apparent velocity of seismic waves decreased increasingly from the south direction to the north direction. This means that the time taken to reach the receiver points from the actual receiving points was greater for the north direction than for the south direction, which in turn strongly implies that a geologically anomalous zone may exist in the north direction. This is area A in Fig. 4(a). This result can match the status of Cheomseongdae, which is tilted to the north as shown in Fig. 1. As for area B in Fig. 4(a), it seems to be an inversion error due to deficient data from the area. 3.3. 3-dimensional DC resistivity survey As the boring results show, a geological structure exhibiting the coexistence of a deep gravel layer and of a partly weathered silty sand bed at the lower part of the gravel layer led to expectations of 3-D development. Consequently, to express changes in the overall geological structure in 3-D images, 3-D DC resistivity surveys were conducted. As for the contents of the investigations per survey line, with Cheomseongdae and within the 40 m by 40 m range, a total of seventeen survey lines – twelve survey lines in the east-west direction and five survey lines in the north-south direction – were designed. To image the underground structure at the lower part of Cheomseongdae in 3-D, the survey lines were formed in grids, and dipole-dipole array, modified pole-pole array, and electrical resistivity surveys were conducted on these survey lines. Fig. 4(b) shows images from 3-D DC resistivity surveys. The figure expresses the locations and directions of the overall geological distribution and geologically anomalous zones around Cheomseongdae, with the X-Y plane. With the 4.5 m depth, low resistivity anomalous zones began to appear. A low resistivity anomalous zone existed around the artifact at a depth 8 m. Another low resistivity anomalous zone were distributed around the artifact at a depth 4 to 10 m in the northwest and northeast directions. Overall, with Cheomseongdae as the centre, electrical resistivity distribution differed for the north and the south. Generally, electrical resistivity values of approximately 300 ohm-m were distributed in the south while those of approximately 50 to 150 ohm-m were distributed in the north. In addition, relatively high resistivity was distributed in the , and low resistivity part immediately below Cheomseongdae anomalous zones were distributed in the lower part surrounding Cheomseongdae . Because the stereobate, which can be seen as
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Fig. 3. Boring and downhole seismic test results for Cheomseongdae ground: a thick alluvial layer existed in the ground surrounding Cheomseongdae, and it consisted mostly of gravel with sand (GP).
the foundation of Cheomseongdae, consisted of granite, anomalous zone consequently was expected to react relatively more strongly than the electrical resistivity values of the surrounding area. Surrounded by granite with relatively higher electrical resiscould be expected to exhibit a tivity values, anomalous zone distribution of low resistivity anomalous zones, as in the figure, because of high porosity or moisture content caused by the prolonged weathering of the alluvial layer. Like anomalous zones and , anomalous zone in the lower part of Cheomseongdae in
the northwest direction could be expected to exhibit a distribution of electrical resistivity values lower than those for the surrounding area due to partial weathering over a long period. As for the electrical resistivity values of the lower ground of Cheomseongdae, while the east-west direction exhibited relatively homogeneous rheological changes, in the north-south direction, it was confirmed that low electrical resistivity values and resistivity zones emerged earlier in the north than in the south. Although the electrical resistivity of the alluvial layer differed according to the
Fig. 4. (a) The 2-D image for linear offset seismic test: the difference of apparent velocity implies that a geologically anomalous zone may exist in the north direction, (b) in the northwest direction of Cheomseongdae subsurface could be expected to exhibit a distribution of electrical resistivity The results of 3-D DC resistivity survey: zone values lower than those for the surrounding area.
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moisture content, overall, it was low. The alluvial layer of the area investigated could be classified largely into a gravel layer and a sand bed, and, generally, gravel layers have higher electrical resistivity values than do sand beds. When the results of the 3-D DC resistivity surveys were synthesized and analysed, it was possible to expect that the number of boulders in the north alluvial layer would be smaller than in the south side. 3.4. Discussion As aforementioned, the apparent velocity of Cheomseongdae ground decreased from the south direction to the north direction in the linear offset seismic test results. And the low electrical resistivity values and resistivity zones emerged earlier in the north than in the south, while the east-west direction exhibited relatively homogeneous rheological changes, as for the electrical resistivity values of the lower ground of Cheomseongdae. When the results of the linear offset seismic test and 3-D DC resistivity survey were synthesized and analysed, it was possible to expect that a geologically anomalous zone exists in the north direction. Through two noninvasive tests, linear offset seismic test and 3D DC resistivity survey, the remarkable differences of stiffness and resistivity were detected between the north and south of Cheomseongdae. These test results are related with the current tilted condition of Cheomseongdae. The differential settlement stems from differences in the properties of the ground to the north and south. Additional investigations can be needed in order to analyse the cause of differential settlement for Cheomseongdae. 4. Conclusion The study proposed a geophysical approach focused on noninvasive site surveys, and through this, relates the current tilted condition and provides information on the stability of Cheomseongdae. Cheomseongdae is a stone architectural heritage presumed to have been constructed during the Silla dynasty, approximately 1,300 years ago. With the passage of considerable time, the structure’s deformation and degradation are judged to appear due to diverse reasons, and the structure suffers the differ-
ential settlement due to the passage of time or the accumulation of ground displacement factors. Through the results of the boring, the ground surrounding Cheomseongdae consisted mostly of gravel with sand and undisturbed sampling was not possible. In addition, through seismic tests and electrical resistivity surveys, test data were collected to assess the soil stratigraphy, geometry, stiffness difference, and the presence of anomalous zones, among other things. There are the quantifiable differences of stiffness (or seismic wave velocity) and resistivity between the north and south of the artifact. Based on the analysis of such data, it is likely that the tilt to the north and the differential settlement of Cheomseongdae stems from differences in the properties of the ground to the north and south of Cheomseongdae. However, these noninvasive approaches have the limits in identifying time dependent differential settlement and strength of foundation soil. Acknowledgements This study, which forms a part of the project, has been achieved with the support of national R&D project, which has been hosted by National Research Institute of Cultural Heritage of Cultural Heritage Administration. We express our gratitude to it. References [1] Gyeongju City, “Gyeongju-Pungmul-Jiriji (The scenery, customs and geography of Gyeongju),” 2006 (In Korean). [2] Cultural Heritage Administration of Korea Home Page, Retrieved Sep 1, 2009 from http://www.cha.go.kr/unisearch/imagefiles/national treasure/ a0031000037001.jpg. [3] H. Shon, S.B. Kim, Studies on the characteristics of stone structures by shape reversal, geotechnical and dynamic structural engineerings Proceedings of conference on Korean Society of Earth and Exploration Geophysicists, 2004, pp.25–48, (In Korean). [4] F.T. Gizzi, Identifying geological and geotechnical influences that threaten historical sites: a method to evaluate the usefulness of data already available, J. Cult. Herit. 9 (3) (2008) 302–310. [5] World Heritage Committee, UNESCO (United Nations Educational, Scientific and Cultural Organization), “Convention concerning the protection of the world cultural and natural heritage”, Cairns, Australia, 2000, pp. 47.