INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING

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This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. Geotechnical Issues and Preservation of Korean Heritage Sites

Problèmes géotechniques et conservation des sites du patrimoine coréen

Yeon-Soo Jang Department of Civil Engineering, Dongguk University, , [email protected]

ABSTRACT: From 1995 to 2016, Korea has 12 properties registered on the UNESCO World Heritage List. They include Buddhist temples, royal palaces, castles and tombs, and Backjae historic areas as well as Jeju natural historic site. This study would reveal beauties and spirits of heritage structures of Korea by showing various figures and telling the background stories. The design of the foundations and walls of the historic palaces and castles would be discussed. Some recent works of geotechnical scholars for preserving the historic pagodas and the restoration work of ancient demolished bridge located at Wolseong in Gyeongju historic area are explained. Jeju island, a natural geological museum, which contains 368 parasitic cones and lava tubes formed by explosive eruptions of basaltic lava is introduced.

RÉSUMÉ : De 1995 à 2016, la Corée a 12 biens inscrits sur la Liste du patrimoine mondial de l'UNESCO. Ils comprennent des temples bouddhistes, des palais royaux, des châteaux et des tombes, les zones historiques de Gyeongju et Backjae ainsi que le site historique naturel de Jeju. Cette étude présentera les beautés et les esprits des structures patrimoniales de la Corée en montrant diverses images et en retraçant leurs histoires. La conception des fondations, des murs des palais et des châteaux historiques sera discutée. Certains travaux de chercheurs en géotechnique récents portant sur la préservation des pagodes historiques et les travaux de restauration de l'ancien pont démoli situé à Wolseong dans la zone historique de Gyeongju seront montrés. Cette étude présentera aussi l'île de Jeju, musée géologique naturel, qui contient 368 cônes volcaniques parasites et des tubes de lave formés par des éruptions explosives de lave basaltique. KEYWORDS: UNESCO world heritage, Buddhist, palace, Geyongju, Jeju, Preservation, Geotechnical issue. Mots Clés: Patrimoine mondial de l’UNESCO, Bouddhiste, palais, Geyongju, Jeju, Conservation, Problèmes Géotechniques.

1 INTRODUCTION. The subject of geotechncial engineering taught as a Table 1. UNESCO World heritage list and the registered year subdivision of civil engineering in the universities of Korea was No Heritage list Description Year regis started in 1960s. At that time geotechnical engineering was tered taught as soil and foundation engineering. Geotechnical 1 Seokguram Grotto The works of and 1995 engineering became flourished after Korean geotechnical and architecture in Unified period Temple (AD 676 – 915). society(KGS) was launched in 1984. Eventhough such a late start 2 Shrine Royal Ancestral Shrine in of the geotechnical engineering in Korea, Korean ancesters Dynasty (AD 1392-1910) produced the tangible cultural artifiacts such as Korean palaces, 3 Depository for the Tripitaka castles, shrines and temples etc. using the technologies of Temple : Koreana Wooden blocks produced Janggyeong in Goyreo period (AD. 918-1392) geotechnics from very early times. Panjeon Traditional Korean structures which are recently registered in 4 Hwaseong Korea’s first planned city made 1997 UNESCO World Heritage List are summarized in Table 1. Fortress from 1794 to 1796. The rampart of The asset No. 1 and 3 are related to Buddhist arts, No. 2, 5 and 9 (Castle) castle runs for 5.7km. 5 Renowned for architecture and are shrine, palace and tombs of Kings and Queens in Joseon Palace Complex garden design in Eastern Asia Dynasty. No 11 and 4 are the castles in the mid and later periods palace. Buildings are harmonized of Joseon dynasty. No. 7 and 12 show the group of remains in into nature and topography. 6 Dolmen Sites Archeological remains in Bronze 2000 Gyeongju and Backjae of Ancient Korea (Table 2). Besides, No. in Gochang, Age Culture. Korea has 40% of all 6 is ancient stone tombs, No. 10 is Confucian village in Joseon Hwasun and Dolmens worldwide. dynasty, and No. 8 is Jeju volcanic island located in southern sea Ganghwa of Korea. The distributions of each heritage is shown in Fig 1. 7 Gyeongju Old capital of the Silla Kingdom Historic Areas (57 B.C. – A.D. 935). Contain a Among the structures sited, the temples and palaces have total of 52 officially recognized wooden structures on the stone stylobate and compacted soils. cultural properties of Korea Most of Korean historic structures are wooden architectures 8 Jeju Volcanic Natural heritage site; Spectacular 2007 island and Lava geological museum in Korea rather than the stone ones, because the regimes of Korean Tubes famous for parasitic cones and dynasty had chosen the caring policy of the people rather than lava tube systems. the oppressing policy. Conventional architectures in Korea were 9 Royal Tombs of 40 tombs are registered as the 2009 Joseon Dynasty UNESCO national heritages. constructed complying with the surrounding nature and showing Famous for architecture of arts and the artistic beauty, which could not be found in the gigantic stone ecological preservation structures in other places of the world. 10 HistoricVillages Traditional villages which has 2010 : Hahoe and Confucian order. Queen Elizabeth In this study, the cultural meanings of the historic heritages are Yangdong II, Queen of UK visited in 1999. enlightened and the characteristics of the foundations and the 11 Namhansan- Emergency capital in Joseon 2014 walls of the heritages cited are analyzed from the geotechnical seong (Castle) dynasty. Shows the technology of th th points of view. Recent research of the heritage structures related castle construction from 7 to 19 century. to the earthquake occurred in Korea and the case of rehabilitating 12 Baekje Historic Eight places of castles, temples, 2015 an ancient bridge in Gyeongju historic area are introduced. Areas tombs, architectures and pagodas Geological significance of Jeju volcanic island, the only natural in Baekje period (18 B.C. - A.D. 660) UNESCO registered heritage in Korea up to now, is reported.

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Figure 1. Location of UNESCO world heritages in Korea They stand out as the ingenious works of Buddhist architecture 2. Historical Order of Korean UNESCO world heritages and art with few peers in Asia. The times of construction for historic structures in Table 1 are Goryo (9l8-1392) was a devout Buddha State and ruled over shown in Table 2 by the chronological order of Korean historic nearly five centuries. The worst national crisis occur was the regimes and listed in parallel with the Roman and the invasions of the Mongol and, in 1235, the nation faced with the Renaissance periods in Europe. third invasion of Mongol. As a means of soothing public anxiety The old Korean peninsula about 2000 years ago was divided and beseeching the benevolence of Buddha, publishing of the into three countries as Goguryo, Baekje and Silla. They are Buddhist canon was done at that time. The cannon was preserved unified to Silla in AD 668 and called as Unified Silla. Bulguksa, in Haeinsa temple in very technically developed fashion even in the Buddhist temple, and Seokguram, a stone statue of Buddha, nowadays. Joseon dynasty was started in AD 1392 and continued both located in Gyeongju historic site, were built in the Unified about 500 years. Six UNESCO world heritages were produced in Silla. this period.

Table 2. Chronological chart of Korean dynasty for construction of Korean UNESCO world heritages

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In the next sections, the cultural meaning and the geotechnical issues are explained for the heritage structures which have geotechnical significance.

3. PALACE and SHRINE in JOSEON DYNASTY

In Joseon Dynasty, the main palace was Gyeongbokgung, gung means palace, built in 1395. Changdeokgung was built in 1405 as a secondary palace of Joseon Dynasty by King Taejong, the third sovereign of the Dynasty. Although the Changdeok palace was originally intended to be used as a separate palace, when the king wanted to reside outside of the main palace, the palace served as the official residence of the largest number of kings of Joseon during the five hundred year rule of the Dynasty. Jongmyo was founded by King Taejo in 1392. It is the royal ancestral shrine, which houses the spirit tablets of 18 past kings Figure 3. Night view of Buyongji (pond) in Biwon, a back yard garden and queens of Joseon Dynasty (1392-1910). It is a place that of royal family which is fully harmonized with surrounding nature reflect about the time passed and the eternity, and was registered as UNESCO memorial heritage in 1995. Changdeok palace was listed on the UNESCO World heritage in 1997, saying that “This palace is an outstanding example of 3.1 Changdeok Palace Far Eastern palace architecture and garden design, exceptional Changdeok palace was built blending with topography, by for the way in which the buildings are integrated into and following the Pungsu (geomantic, the theory of divination based harmonized with the natural setting, adapting to the topography on topography) principle of ideal home, i.e. the mountain at the and retaining indigenous tree cover.”(CHA, 2002, 2011) back and the water in the front. It also followed the rule of palatial construction, i.e. the government offices should be placed in the 3.2 Jongmyo outer court and the private residence of the royal family in the Construction of Jongmyo had begun in 1394, when the capital inner quarters behind. The king’s quarters are surrounded by was moved from Gaeseong to Hanyang, and had completed next many layers of buildings and courtyards for the sake of security. year. Jongmyo has two halls, Jeongjeon (main hall) which keeps The grounds of Changdeok Palace are largely divided into four 34 tablets of kings and queens of Joseon Dynasty, and Hall of areas: the entrance area, the office area, the royal residence and eternal peace which has tablets of less worthy rulers and their rear garden. The Hall of Benevolent Governance (Injeongjeon) wives. The main hall is the largest among contemporary wooden is the throne hall in Changdeok Palace (Figure 2). Many official structures in the world, the front length is 101 meters along 25 events and ceremonies took place at this hall throughout the bays (Figure 4). Joseon Dynasty. The broad stone platform with 110 meters wide and 70 meters long is constructed using the roughly dressed granite blocks. Jongmyo is an architectural monument built by top-class architects and engineers of the past dynasty.

Figure 4. View of main hall shrine called Jeongjeon in Jongmyo. The broad stone platform was used for ancestral rites and constructed using granite blocks

With filial duty, Korean royal families performed rites 4 times Figure 2. Injeongjeon in Changdeok Palace, office area which is used to in a year to venerate their ancestors. Nowadays, the rites are accept the retainer’s felicitation or to receive audiences of foreign performed at Saturday of the first week in May and November ambassadors under the auspices of Jeonju Yi Clan, the association consisting of descendants of Joseon royal family (Figure 5). Veneration rite The rear garden in the north direction, called Biwon, is the and ceremonial performance called Jongmyo ritual music were largest and most beautiful royal garden of the Joseon period conducted in the Confucian cultural sphere at the royal memorial (Figure 3). It was a place for outdoor activities for the royal shrine. The ritual music was composed by King Sejong who had family. The Biwon shows the traditional Korean landscape created Hangeul, the Korean Language, and has been played at architecture, following the natural contours of hills and valleys. the ceremony for five hundred years. Waterfalls, ponds and woods are conserved in their pristine condition by minimizing man made elements.

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sunshine during daytimes. The drainage system of the Weoldae was made by mixing clay and sand to the upper stone pavement and discharged the rain drops from the roof of the architectural structures. Schematic sections and explanations are given in Figure 7. Rainwater dropped from the roof infiltrated into the soil and flew under the stylobate.

a) Ritual officials in the Main hall, where the royal spirit tablets are ensconced in cubicle like spirit chambers behind the yellow curtains

Figure 6. Weoldae, a flat extension of stone stylobate, at Geunjeongjeon in Gyeongbokgung, main official Palace of initial Joseon Dynasty.

b) Ritual officiants return to their positions after completing ceremonies at the Hall of eternal peace

Figure 5. Ritual ceremony under the auspices of Jeonju Yi Clan, Figure 7. Description of schematic section of the traditional architecture descendants of Joseon royal family (CHA. 2011

Hwagae is a retaining wall that resist the horizontal soil

pressure from the slope. It is used for the rear gardens of Palace 3.3 Description of stone stylobate and wall at palace and shine (Figure 8). Hwagae was constructed with different heights in Joseon Dynasty depending on the inclination angles. High stylobates were

installed at low points of the slope to provide the stability of the Most of Korean heritage architectures were built using timber slope. Corner log stone methods is unique traditional work of old with granite rock foundation, contrary to the old structural Korea, which is composed of dislocation, interlocking and head heritages of Europe and Asia which were built using limestones, locking. Basic type of the stone structures of Hwagae is the marbles and sandstones. The stylobates of the main palace and gravity retaining wall, which was constructed above stable shrine are composed of granite since the granite rocks are ground to withstand its own weight and pressure. abundant and distributed widely in Korea. The sedimentary rocks including limestones and marbles are partially distributed in the central and the eastern part of Korea. To prevent the collapse of the wall and the foundation of the wooden upper structures from soil pressure by piling ground soil. Stones were cut into rectangular shape and finished with chiseled lines on the horizontal surfaces of up-down and left-right. The rear was displayed as wedged lozenge. The stones were piled naturally around the outskirts of stylobate. This stonework was used for construction of Weoldae, the foundation of the palace and shrine, and Hwagae, the retaining wall of soil slopes. Weoldae is the platform and the expanded parts of the stylobate, composed of stonewall outside and crushed stone with soil inside. It is used as the place of various ceremonies in the palace and the shrine for kings and queens in Joseon Dynasty. Weoldae is constructed mostly to the height that can be dealt with at the eye level using three story stones (Figure 6). The front of Figure 8. Hwagae at Ami hill in Gyeongbok Palace, a retaining wall made most structures orient to south or east direction to allow long of multi stone stages in the rear garden of the Palace

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The drainage system of Hwagae adopted the clay-sand filler to Recently, lime mixed soil method is actively used for making minimize the infiltration of the surface water into the stone wall the traditional stonewalls of ordinary houses in Bukchon, a and to prevent the accumulation of the pore pressure in Hwagae. popular touring course near Changdeok Palace in Seoul. Vegetation like shrubs and trees were planted to filter the initial Bukchon was a village of nobles in Joseon dynasty. infiltrated rainwater, which cleaned stylobate surfaces and to Traditional lime mixed soil was called as Ganghoe, which was produce aesthetically stabilized space (Figure 9). a mortar composed by the quick-lime with Kaolinite clay. Kaolinite was used as the source of silicate, which is an essence material for pozzolanic reaction. Seaweeds and kelps were also added to strengthen the cohesion of the mixed soils. Quicklime (CaO) was produced by heating the limestone powder (CaCO3) in the temperature about 800 to 1,100 °C.

Add 800 °C∼ 1,100 °C heat ↓ CaCO3 → CaO+ CO2 -2.65 kcal/mol (1)

For repairing of cultural architectures, quick lime of 90% CaO produced in the temperature of 850 to 900 oC is currently used in the field. Kim et al. (2002) suggested several quality control points for active use of lime mixed soils for traditional stone walls in modern times: 1) To diminish quality variation which is caused by slaking the quick-lime at the field, that is formed in the

Figure 9. A typical section of Hwagae and drainage in the backfill process of formula (2), it is desirable to use the pre-slaked-lime (Ca(OH)2) for lime-soil consolidation of the heritage structures,

3.4 Use of lime with soil as mortar for the stone wall CaO+ H2O → Ca(OH)2 + 15.59 kcal/mol (2)

Lime mixed soil was used as the major bonding, waterproof and 2) For uniform construction of lime-soil consolidation, and finishing material for traditional architectures in Korea. Part builders must use mixer for uniform mixing, i.e. use premixed of Weoldae and Hwagae stone wall were finished using lime type materials, and compact the materials in the field. mixed soils as the waterproof material. It was also used as a water proofs for bottom of water ponds like Biwon, the rear garden of Changdeok palace (Fig 3). 4. CASTLES in Joseon Dynasty There is an old record for use of lime mixed soil from early in Three Kingdom period (Table 2). Lime was very scarce material During Three Kingdom’s period, i.e. first century, B.C. to in ancient Korea and there was a limitation for general use of seventh century, AD., many fortresses were built including lime for general houses. In Goryo and Joseon dynasty, it was , which later provided the protection against the used for the foundations, walls and roofs of public architects, e.g. invaders in several serious national crises during Goryo (918- palaces, temples, shrines and castles, etc. 1392) and Joseon (1392-19010) dynasties. Figure 10 shows an example of using lime mixed soil for bearing corner stones as well as for preventing humidity intrusion 4.1 upward from subsurface. Construction was progressed in the Hwaseong Fortress was completed in two and half years from order such as compaction of crushed stone, compaction of lime 1794 to 1796 under King Jeongjo’s reign (1776-1880). mixed soil and compaction of soil fills. Lime mixed soil Hwaseong refers to the rampart that runs 5.7km and the attached compaction was also applied for stylobate to prevent the loss of facilities of the area 120 ha. It also refers to the entire city within the paved soil and stone by the falling rainwater from the roof. the castle which was the Korea’s first planned city. King Jeongjo stationed 5,000 troops as the outer unit of royal guard garrison in Hwaseong, displaying his military power to the possible enemies in and out of the country. Construction of Hwaseong Fortress was the largest public work since the transfer of the capital from Gaeseong to Hanyang exactly 400 years earlier in 1394. A most prominent young officer, Jeong,Yak-Yong at the age of 31, received a royal order to take charge of the entire construction of the castle. He dedicated himself to build the fortress using the best technologies available at that time. For instance, he produced the crane called Geojunggi adopting the design chart used at Portugal in Europe. The fortress had four main gate, Janghan Gate to the north, Paldal Gate to the south, Hwaseo Gate to the west and Changyong Gate to the east. Each gate is protected by another layer of curved wall called “Wongseong” attached to outer side of the gate (Figure 11). Both the north and the south gates have guard platforms at the sides to observe enemy movements and attack the flank of enemies approaching the wall.

Figure 10. An example of cornerstone foundation with lime soil improvement (Kim et al. 2002)

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This castle was made to commemorate the Prince Sado who was the father of Jeongjo and was dead in the small wooden box by the King Youngjo, who was the grandfather of King Jeongjo. King Jeongjo had held the castle visit festival every year from 1785 to 1800 for 13 times. Figure 12 shows the scene of River crossing participating 1,900 people in 1796, when King Jeongjo returned from Hwaseong. The process of Hwaseong Fortress construction is well written in “Hwaseong Euigwyae” which is now possessed in French national library. This book published in 1801 had described the plan of castle construction, the related institution and law, the calculation of budget and the wage of workmen involved, the construction machinery, the production of construction materials, and the daily record of construction progress using many figures. a) Janghan Gate located at the north of Hwaseong

4.2 Namhansanseong. Namhansanseong is located 25km away to the southeast direction from central part of Seoul and its elevation is EL. 480m on the summit of the rugged mountain called Namhansan. The circumference of the castle is 12km and the wide flat land is located within the castle (Figure 13). This castle was chosen as the temporary capital of Joseon dynasty when the invasion of Qing, , occurred between 1672 and 1673.

b) Hwaseo Gate located at the west of Hwaseong

Figure 11. Gate of Hwaseong with full or semi-circular wall in Suwon

At the west side of the castle, Paldal mountain is located and, at the east, low hilly plane is located. This type of castle is called Pyeongsan Castle (平平平). This castle was built using the state of the art technologies and is a representative civil structure in th 18 century. Figure 13. Old map of Namhansanseong which marks various public houses and structures inside the castle

Namhansanseong had existed from 7th century in Silla period about 1,300 years and called “Jujangseong”. Thus Namhan- sanseong shows the development stages of castle wall construction technology well from 7th century. Joseon experienced two major invasions: Imjimwaeran by Japanese troops in 1592-1598 and Byeongjahoran by Qing, China, in 1672-1673 (Table 2). After experiencing these two major invasions, Joseon adopted the western style weapons and the defense system of the castle was changed. The inner wall of the castle is shallow and low areas, but outer wall is high to defend the enemies who climb up the mountain (Figure 14). Crenels were made in the parapet of the castle wall (Figure 15) and Bastions were also made at the back side of the castle, since the major weapon at the castle war was changed from arrows to guns and artilleries. Part of historical development of military civil engineering in far-east Asian countries can be seen in the castle walls, since the construction of the castle was the result of the synthesis of the castle construction technology in far-east countries. This castle was registered as 11th UNESCO National heritage in June, 2014.

4.3 Construction of Castle walls Figure 12. Scene of Han river crossing in 1796 when King Jeongjo There were two types of castle wall construction: dry masonry returned to Hanyang from Hwaseong. A temporary bridge was made work and wet (mortar) masonry work. The former method piles connecting boats by participating the 1,900 people. The place of upper up Kenchi (cog tooth) stones without mortar. This method was side of the river is currently Noryangjin in Seoul.

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used mostly in the castles of ancient Korea (Figure 14). Wet masonry work used the clay and lime mortars among the stones 5. GYEONGJU HISTORIC SITE as mortars and can be found in the recent works of castle construction, e.g. Hwaseong in Figure 11, in Joseon Dynasty. Gyeongju is the ancient capital city of Silla (57 BC-AD 935) Compacted lime mixed soil was also used for backfill and which has history of the millennium-long kingdom. At its peak embankment construction of castle walls in order to increase the in eighth century, Gyeongju had 180,000 households with strength of the backgrounds of the castle wall (Figure 15). 900,000 residents, although this city now has the population of 270,000 as of 2015. It was very prosperous like Constantinople (present day Istanbul) of the Byzantine (Eastern Roman) Empire, Bagdad (present day Iraq) of Islam Empire and Changan (Xian) of the Tang Dynasty, China. The city has numerous historical monuments scattered all around the area. Archeologists still dig Wolseong area in search of cultural remains buried underground. The areas include Mt. Nam (Namsan), Moon Fortress area (Wolseong, a royal palace site), the great Tumuli park, a cluster of burial mounds, and the ruins of Hwangnyongsa, a state temple. Detailed locations of the historical artifacts of Gyeongju are shown in Figure 16 (Park et al., 2013). Namsan is considered as an outdoor museum, which contains hundreds of Buddhist treasures including stone pagodas, lanterns and lotus pedestals during the seventh to the tenth Figure 14. Castle wall of Namhansanseong. Middle and lower part of century. The images of Buddha are carved into rocks instead of wall was constructed using dry masonry work and the upper parapet wall was made using wet masonry work man-made foundation. Gyeongju is located in a relatively high earthquake prone region near fault and has historical records of frequent seismic damage. The historical data of earthquakes in the Gyeongju area are tabulated in NRICH (2009). Park et al. (2013) performed a seismic risk assessment and an extensive geotechnical survey including series of in situ tests at the historical areas shown in Figure 16.

5.1 Bulguksa Temple Bulguksa has the meaning “the temple for the land of Buddha” and aimed to create an architectural realization of the Budha’s teachings in the present world. The construction of the temple had begun by Daseong Kim in 751 of King Gyeongduk and completed in 774 of King Haegong. The elevated sanctum stands on a series of beautiful stone terrace stretching 100 meters across the front façade (Figure 17). The temple compounds on the terraces symbolize the world of Buddhas, while the ground of Figure 15. Example section of castle wall. Backfilled by the crushed down below symbolize the world of humans. stones inside and compacted lime mixed soils outside (Park et al., 2012)

Figure 16. Satellite picture of Gyeongju historical areas, the location of typical cultural heritages (Park et al., 2013)

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earth. The centerpiece of Seokguram is the Buda in the circular Two stone stairways called the bridges of Blue cloud and main hall (Figure 19). A 3.5m high Buda is seated cross legged White cloud on two levels connected the two worlds. The terrace on lotus throne. There are 38 images of bodhisattvas and was built with carefully dressed granite slabs and natural stones disciples who show faithfulness and gather to hear the Buddaha’s of various sizes. words. No mortar was used and the stones are held together only by stone rivets. The process of constructing the grotto is: (1) construction of the stone Buda, (2) Lay end to end the stone sculpture of Bodhisattva (Buddist saint), and (3) Ceiling is made with many stone nails and (4) the earth is covered on the ceiling. When a Buddhist monk goes into the gallery and looks at the Budha, the face of Budha is seen in the center of circular luminance at the back. The grotto had the natural ventilation system in which the chilly groundwater flowing underneath the floor controlled the temperature and humidity inside the cave to prevent the due formation. But the drainage pipes were installed under the cave to let the groundwater to flow out from the grotto during the colonial period (1910-1945) and the natural ventilation system was lost. Nowadays artificial ventilation system is used to control temperature and humidity.

Figure 17. Bulguksa Temple; Lower side is made of white granitic rocks in 8th century (CHA, 2002, 2011)

The temple area is divided into three main courtyards, each containing a worship hall, the Hall of the Great Hero, or Daeungjeon, dedicated to the historic Buddha, the Hall of Vairocana, or Birojeon, dedicated to the Buddha of Great Illumination, and the Paradise Hall, or Geungrakjeon dedicated to Amitabha, the Buddha of Western Paradise. These three areas stand for the Land of Sakyamuni Buddha, the Lotus Land of Vairocana, and the Pure Land Paradise of Amitabba, respectively. Various stone structures, e.g. long slim stone, arch stone, column and guard rail stones, show the technology of stone arts in 8th century. The original wooden architectures were burnt down by fire of wars and rebuilt in 19th century. Two pagodas, the Pagoda of many treasures (Dabotap), and the Sakyamuni Pagoda (Seokgatap). The terraces and stairs were made by the skills of Silla masons, who handled solid granite. Figure 19. Buddha in the center main hall of Seokguram Most temples built during the Unified Silla period (AD 676-935) have two stone pagodas with identical shapes rising side by side 5.3 Cheomseongdae before the main hall. The Pagoda of Many Treasures is known Cheomseongdae is a star-gazing platform constructed at for its ornate decorative style, while the Sakyamuni Pagoda is ancient city of Gyeongju during Silla period of Queen Seonduk. widely known for its princely dignity and simplicity. The heights It is an oldest astronomical observatory, which has stood for over of two pagodas are 10.63 meters and 10.34 meters, respectively. 1,300 years. Cheomseongdae is currently registered as the Korean National Treasure No.31. Its dimensions are: the height, 9.17m, and the widths, 2.85m and 4.93m at the top and the bottom (Figure 20). Straight lines and curves are harmoniously combined in the structure.

Figure 18. Two pagodas, the Pagoda of many treasures (Dabotap) left, and the Sakyamuni Pagoda (Seokgatap) right

5.2 Seokguram Grotto Seokguram literally means “stone cave hermitage” and is a man-made grotto built of 360 granite pieces and covered with Figure 20. Dimensions and tilting status of Cheomseongdae (CHA, 2009)

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Recently, it is observed that Cheomseongdae is suffering earthquake (Choseon Il Bo, 2016). Urgent preparation of differential settlement of the ground. The settlement of developing the proper reinforcement methods seems to be foundation in the north direction is about 117 mm larger than the necessary for the heritage structures in Gyeongju area for settlement in the south direction. Several studies were performed possible earthquakes at the scales of 6.2-6.4 in the future. by the team of Prof. Kim in KAIST to prevent the damage of the ancient heritage including Cheomseongdae (Park 2012; Park et al. 2012, 2013, 2014). Park et al. (2012) performed a noninvasive site survey, i.e. two noninvasive tests, linear offset seismic test and 3-D DC resistivity survey together with minimal borings a little way off the artifact. They thought that the current tilted condition is related with the subsoil conditions and identified that a geologically anomalous zone exists in the north direction. Remarkable differences of stiffness and resistivity were detected between the north and south of Cheomseongdae (Figure 21).

Figure 23. Appearance of the upper head stone had fallen out and photo capture using high speed camera – model 1 (upper) and model 2 (lower): (a) before testing, (b) after testing, (c) capture of high speed camera video (Park 2012)

5.4 Restoration of Woljeonggyo Bridge Woljeonggyo, gyo means bridge, is located at Wolseong, meaning moon castle, which was the one thousand year’s capital city of Silla (Figure 24). Excavations of the ruins of ancient city is going on to identify the status of the legacy. Namcheon, i.e. Figure 21. 3-D image results of DC resistivity survey: At Zone 4 in the Nam-river in Korean name, flows at the southern part of northwest direction, subsurface has lower electrical resistivity values Wolseong, Old Woljeonggyo was the bridge, which went across than those of the surrounding area (Park et al. 2012). the Namcheon at the west entrance of Wolseong. Illjeonggyo (bridge) was located at the eastern outlet of Namcheon. Prof. Kim in KAIST performed the centrifuge tests to check the stability of Cheomseongdae from the possible earthquakes of nearby Yangsan fault and the seismic analysis of the structure. Figure 22 shows the two models used in the centrifuge tests, i.e. with (model 1) and without (model 2) the head stone at the lower part of the 28th layer (Park 2012).

Figure 22 Differences of model 1 (upper) and model 2 (lower): (a) 19th th th Figure 24. Location of Woljeonggyo (bridge) restoration site layer, (b) 25 layer, (c) 28 layer. Lower part of the head stone, (d) (KICT 2010) view of model in the centrifuge box (Park 2012)

Woljeonggyo of 60.57m long was constructed at AD 760 in For model 1, the head stones were observed to move slightly Unified Silla of King Gyeongdeok. According to the excavation from the seismic class I structures in Korean earthquake code work, many pieces of burnt wood and roof tile were found (return period 1000 years = 0.14g), and for model 2, the lower between the abutments of the bridge. It was thought that the part of the head stone and other lower parts fell off at the same upper side of the bridge piers was formed by a house type bridge excitation level (Figure 23). The seismic role of the head stone to with roof. Trace of 4 piers was found when the excavation work endure the medium scale earthquakes was recognized from the was progressed. The distance between the piers was 12.6m. tests. The foundation of No. 1 pier located at the north side of the Also, it was found from the seismic analysis that the old bridge was made using the cube stones of both the size of Cheomseongdae can move 21mm and 55mm, respectively, for the two models after exerting earthquake of return period 500 1.8～2m(W)×1.8m～2m(L)×0.6～0.7m(H) and of 0.6m(W)× years (0.11g) with 15 second duration. At 3:00 A.M. September 2m (L)×0.6m(H). The spread footings of the 4 piers by the size 14, 2016, Gyeongju experienced earthquake two times at scale of of 12.9m long and 3.9m wide, were located to the direction of 5.4 and 5.8 (average acceleration = 0.12~0.14g). It was reported east and west (Figure 25). The edges of the piers has the that the Cheomseongdae was tilted 2 cm more after the triangular shape to prevent the erosion of the foundation from

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strong stream forces of Namcheon. the rehabilitation work. Slope stability analysis was performed to check the stability of the abutments. It was found that soil reinforcement of abutment is needed to do safe rehabilitation work. Reinforcement of 6-7 layers of geogrid was suggested and installed for the abutment after surface backfill of 1m thick was removed (KICT 2010). The abutment stone wall remaining at the site was kept when the abutment was restored. Besides, hydraulic model experiment was performed using the completed gallery type bridge model as well as the comparison work to select the proper timber materials for the wooden upper structure. Woljeonggyo during restoration work of the bridge in April, 2014, is shown in Figure 27.

Figure 25. Photo of uncovered Woljeong bridge piers: 3 piers were buried in the soil cover of 1.5 to 2.4m thick before excavation work started (Lee and Cho 2016)

Scour protection work, made for the purpose of preventing the erosion of the river bottom, is still left between the No. 4 pier and south abutment (Figure 26). Latticed block frame was made using timber and stones were filled into the lattice space. The number of lattice space was 24 and the size of each block was 2.6m×2.7m. The height of the upper face of the dug timber was similar to the foundation stones of the No.4 pier. This scour protection work was seemed to be made after the piers were Figure 27 View of Woljeonggyo bridge during construction (2014.4) constructed.

6. TRIPITAKA DEPOSITORY in HAEINSA TEMPLE

Haeinsa Temple was built in AD 802 during Unified Silla period, on Mt. Gaya in Hapcheon county, South Gyeongsang Province. Haeinsa has been recognized as one of Korea’s leading Buddhist monasteries and it houses the Tripitaka, a collection of the Buddha’s teachings. It is called as the Temple of Dharma. Goryo (AD 918-1392) was a devoted Buddha State and the worst national crisis in Goryo was the invasion of Mongol over (a) Scour protection work (b) Detail shape of the lattice nearly five centuries of Goryo’s period. A common belief in Buddhist States was that compiling and keeping Tripitaka would Figure 26. Remnants of the erosion prevention work of stream bottom help to ensure peace and prosperity. Goryo had compiled the

Tripitaka when the Khitans invaded in 1010. The Khitans In order to restore the bridge in this site, the preliminary work withdraw when the Tripitaka was about to be completed. performed for the ground was the following:

1) Assessment of the stone remnants of the Woljeonggyo and the identification of the geology and lithology of the site. 2) Integrity test of the stone remnants and the engineering property test of the subsurface of the piers and abutments using non-destructive test, e.g. elastic wave method. 3) Soil boring and standard penetration test to estimate the detailed status of the subsurface of the piers and abutments. Naked eye inspection of the excavated pits. 4) Estimation of the bearing capacity of the soil using the data both from laboratory and in-situ tests. 5) Assessment of the behavior of the stone remnants and the foundation ground during the rehabilitation work using numerical models of structure and ground. 6) Load test and the related analysis for foundation of piers. 7) Field monitoring work for stability of structure and ground during both the periods of the construction and the operation of the bridge Figure 28. Haeinsa Temple; Tripitaka depository is located on upper two long buildings The results of geotechnical survey showed that the soils were composed of 2.0～2.8m thick sand layer, 3.0～5.0m thick gravel The storage buildings for the are located on layer and weak rock layer below. The soil under the foundation the highest temple grounds at 655 meters above the sea level on of the piers of Woljeonggyo was sandy layer of N value 25, the middle of the slope of Mt. Gaya which is 1,430 meters high which showed that the ground condition was good enough to do (Figure 28). The elongated wooden pavilions face the southwest

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so as to receive sunlight for long hours, i.e. 12 hours a day in the summer, 9 hours in the spring and the autumn, and 7 hours in the winter. No parts of the buildings are in permanent shade and the printing blocks are kept in perfect condition. Although the wood may be vulnerable to fire, moisture and insects, all these blocks have been successfully preserved, without a single piece showing signs of decay or warping, for the past seven and a half centuries (Figure 29).

Figure 31. Ground treatment of depository buildings to keep Tropitaka panels intact

7. JEJU VOLCANIC ISLAND and LAVA TUNNELS

7.1 Jeju Island Jeju island was created by the “hot spot” type volcanic activities between 1.2 million to 250,000 years ago. This island is composed by Mt. Halla and its 368 parasitic cones, and lava tunnels formed when eruptions of basaltic lava occurred. The number of parasitic cones derived from a single volcano is largest in the world. Mt. Halla is risen 1,950m above sea level at the center of the island (Figure 32). It is a shield volcano, i.e. a volcano with Figure 29. Tripitaka Depository which preserves the 81,258 panels of gradual slopes formed by the fluid lava flew across a broad area Tripitaka Koreana of land, except for the steep lava dome at the top. This shield volcano occurred in the oceanic plate and over 90 percent of its The perfect preservation of the Tripitaka Koreana was due to land is covered with basalt, a dark-colored rock of volcanic origin. scientific design utilizing the natural condition. The depository buildings were designed to take the advantage of the topography of the terrain as well as the technology of building the structure. The windows have different sizes which is the essential for efficient adjustment of air flow and sunlight and ensuring maximum ventilation, temperature and humidity control (Figure 30). For instance, fresh air is brought in through the large windows to circulate around the hall before escaping through the windows on the opposite side. The small windows keep moisture from seeping in from the ground out side.

Figure 32. Landform of Halla mountain in Jeju island

The UNESCO World Heritage committee selected Jeju island as a World Natural Heritage in 2007. The basis of the its selection was the site’s natural beauty, the properties as a habitat for endangered species and its geological features Figure 33 shows one of parasitic volcano called Seongsan Sunrise Peak and basaltic columnar joints formed after the volcanic eruption and flow. Figure 30. Window design of Tripitaka depository buildings to the directions of (a) south west and (b) north east 7.2 Lava Tunnels in Geomunoreum Lava tube system

The Geomunoreum volcano, which is a parasite cone of Mt. The depository halls are built upon thin granite foundation that Halla was erupted repeatedly between 300,000 and 100,000 facilitate smooth drainage. These buildings have clay floors with years ago and the lava flow from these eruptions flew down to thick layer of salt, charcoal and lime with clay underneath, which northeastern directions to the coastline. The flow created over 20 absorb the excess moisture during the summer monsoon season caves along the course and is called Geomunoreum Lava Tube and maintain an optimum humidity level by letting out the System. moisture during the dry winter months (Figure 31) Among them, five lava tubes, i.e. Manjang Cave, Gimyeong The temperature of the depository halls can be maintained Cave, Bengdui Cave, Dangchaemul Cave and Yongcheon Cave about 0.5 to 2oC lower than the temperature outside. And the have exceptional qualities and have been listed in the UNESCO humidity can be 5 to 10% lower than the humidity outside. World Heritage. Manjang Cave runs 7.4 km without stopping and

Gimnyeong Cave has called as the Snake Cave for its

meandering passage. Bangdui Cave is famous for its habitat of endemic species like spotty belly greenlings and spiders, and for its complicated labyrinthine structure.

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a) Seongsan Sunrise Peak, a parasitic cone created by an underwater eruption

Figure 35. Speleothems in Yeongcheon cave. Rainwater dissolved carbonate and infiltrated into the lava tube, producing a variety of speleothems. Clockwise from top left; white cave corals, star-shaped cave popcorn, cave pearl and yellow cave corals.

Some 30,000 dolmens were found in South Kora and 10,000

to 15,000 in North Korea, which count for around 40% of all b) Basaltic columnar joints in Jeju island dolmens identified in the world. Figure 33. Parasitic cone called Seongsan Sunrise Park and Columnar Examples of similar ancient structures are: Stonehenge in joint in Jeju island England and Pyramid in Egypt. Stonehenge structure is presumed to be used for the ancient memorial rites. However, Dangcheomul Cave and Yeongcheon Cave feature a wide Korean dolmens are thought to be used as funeral structures. array of speleothems and limestone caves creating beautiful The sites, where Dolmens are located, registered as the World landscape (Figure 34). Heritage are Gochang, Hwasun and Gangwha. Korean dolmens serve as clues for studying the formation and evolution of prehistoric megalithic structures.

a) Table-type dolmen at Bugeun-ri in Ganghwa Island, 2.45m tall Figure 34. Inside scene of Yeongcheon Cave propping stones with capstones measuring 6.4m by 5.23m, 1.12m thick and weighing 50 tons

The speleothems found in limestone caves in these areas are: soda straw, stalactite, stalagmite, cave coral, cave pearl and cave flower, etc. These cave features were formed by rainwater mixed with carbonate sand on the ground, which infiltrated into the caves and deposited the calcium carbonate in various forms (Figure 35)

8. DOLMENS in

Dolmens are the megalithic barrows that ancient human civilization had left. In Korea, they are called as Goindol which b) Checker board dolmen, i.e. Go-table dolmen in Gochang county literally means the “propped stone” and served as burial sites in Figure 36. Two representative types of Dolmens in Korea Bronze Age around 1,000 BC. Dolmens are found all over the world and concentrated in the Northeast Asian countries like Dolmens are divided into two types: Table Dolmen and Korea, China and . Checkerboard Dolmen depending on the shape and position of

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the burial chambers. A table dolmen is built with two to four still have their own beauties conforming to surrounding natures. stone slabs to surround an above-ground burial cist and a large As already seen in the palaces and temples of Korean heritages, flat capstone that serves as the roof (Figure 36a). The checker the upper structures were made of wood and founded on the stone board dolmen, i.e. Go-table dolmen, is an underground burial stylobates. Using the rubble stone and compacted lime mixed chamber made by erecting stone slabs or piling up broken stone soils, individual footings were constructed on each column fragments. A flat capstone laid on low propping stones covers the points. The walls of castles were made either by dry masonry chambers (Figure 36b). work, particularly in the ancient period, or wet masonry work in Process of constructing dolmens are predicted by the mid to later period of Joseon dynasty. The castle built in 17th archeologists from civil engineering point of view as follows. century, i.e. Hwaseong in Suwon, reflected the state of the art The first task to make dolmen, was making stones into technologies of civil engineering in Far East Asia and the part of appropriate size and shape. In order to break off a stone piece Europe from a rock base or boulder, a deep groove was made on the rock In recent years, researches are progressed related to surface and a wooden stake was wedged in and moistened with preservation of cultural legacy in Korea. For example, the pre- water. As the wood swelled, the rock would crack off (Figure safety checks of the traditional architectures were performed to 37a). earthquakes possible to occur in Korea. It is an assignment to Korean engineering community to find out the sustainable ways to reinforce and preserve the cultural structures not detracting the traditional maturity. Woljeonggyo was restored in Gyeongju historical site based on the field remains of stone piers and roof tiles. Nevertheless, there are scholastic inquiries of phased restoration. Long term investigation of the site and more detailed documental archeology was demanded for the restoration of the sites. Systematic efforts are necessary preserving and restoring the heritage structures and the development of the related technology in the future. More opportunities should come to geotechnical engineers to expand the contributions in this area. a) Make hole or groove into the rock, put wood piece and swell using water 10. ACKNOWLEDGEMENTS The author would like to express deep thanks to his graduate student, Y.S. Jo in Dongguk University for searching the literature and drawing figures.

11. REFERENCES

Choseon Il Bo 2016. Earthquake emergency in Korean peninsula, September 14. Cultural Heritage Administration (CHA) 2002 World heritage in Korea

Cultural Heritage Administration (CHA) 2009. Retrieved Sep 1, from b) Move a stone slab using rolling logs and ropes. http://www.cha.go.kr/unisearch/imagefiles/national treasure a0031 Figure 37. Assumed method to crack and move a large slab of rock to 000037001.jpg. build a dolmen (Cultural Heritage Administration of Korea, EP08: Cultural Heritage Administration (CHA) 2011. Nulwa-World heritage in Dolmen) Korea (1995-2011) Jang I.S. 2010. A study on the interpretation of stonewall techniques in the traditional landscape space, Master’s thesis, Seoul City Stone slabs produced in this method were moved on rolling University, p.174. logs (Figure 37b). Empirically 10 to 20 men are needed to move Lee K.W. and Cho S.D. 2016 A study on provenance of the stone relics a 1-ton capstone over a distance of 1.5 kilometers using round of WoljeongGyo Bridge built in Silla Kingdom based on geological logs and ropes. 200 men were required to move about 32 ton properties, J. Korean Geosynthetics Society Vol.15 No.4 79-88. capstone. With propping stones erected in pits, dirt was piled KICT 2010. A study on stability management of pier and abutment in the over the stones until a gentle sloped dune was formed. The Woljeonggyo-bridge rehabilitation capstone was moved up along the slope and then the dirt Kim J.M., Kwak E.G., Seo M.C. and Cho H.Y. 2002. Questions and underneath was removed. solutions on repair of lime-soil consolidation in traditional buildings, 15th National Meeting of the Korean Society of Conservation Science for Cultural Properties, February 22, 21-31 (in Korean) 9. SUMMARY AND OPINION NRICH (National Research Institute of Cultural Heritage) 2007. Stone stupa of Gyeongsangbuk-do I (in Korean) In this study, major heritages of Korea, particulary the ones NRICH (National Research Institute of Cultural Heritage) 2007. Assessment of damage for Gyeongju Cheomseongdae (in Korean). registered in UNESCO world heritage lists, were reviewed based Park H.J., Kim D.M, Kim K.S., Ahn H.Y., Kim D.S. 2012. Noninvasive on the historical and cultural respectives and the geotechnical geotechnical site investigation for stability of Cheomseongdae, issues contained in the heritage strcutures. Journal of Cultural Heritage 13 98–102.. There has been a following administrative code in “Gyeonguk Park H.J. 2012. Seismic risk assessment for architecture heritages in Daejeon”, which was made in King Seongjong’s period in Joseon historic areas using field and centrifugal tests, Ph D. Thesis, dynasty (CHA 2012), “If a palace is ostentatious, it will cause KAIST. trouble for the people; if it is shabby, it will not show the dignity Par, H-J., Kim D.S., and Choo Y.W. 2014. Evaluation of the seismic of the government. Frugal but not shabby, and splendid but not response of stone pagodas using centrifuge model tests, Bulletine of Earthquake Eng. ostentatious, this must be what is beautiful.” Based on the spirits Park H.J., Kim D.S., and Kim D.M. 2013 Seismic risk assessment of written in the administrative code above, the traditional architectural heritages in Gyeongju considering local site effects, architectures of Korea were built relatively simple and frugal in Nat. Hazards Earth Syst. Sci., 13, 251–262. comparison with the gigantic heritages found in the world. They

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