The first record on the reinforcement of the Gongsanseong Fortress can be found in a Korean history book called the Chronicles of the Three States, and the accounts stating that the fortress was repaired in AD 526 prove that the ramparts were built before this point in history. Although records on the reinforcement of the ramparts no longer remain in literature materials, the fortress of the periods of Unified Silla and Joseon Dynasty have been confirmed by archaeological discoveries on the ramparts and differences in the fortress construction technology, which reveals that collapses and reinforcements of the ramparts repeatedly took place for a long period of time. Full-scale reinforcement and repair of the fortress began in the 1960s, and records on the locations and details of reinforced sections remain relatively intact. Looking at these records for the locations of the ramparts where reinforcements took place, we can see that most of the ramparts have been reinforced.
Records of the collapse of the Gongsanseong fortress wall are confirmed in more than 6 instances but all of them have been completely reinforced today. Though the ramparts collapsed in a number of occasions, attempts to understand the status of damages on the ramparts were never made. In this research, visual inspections were conducted along the outer face of the Gongsanseong Fortress wall, and various damages and deformations created on the ramparts were discovered. A variety of damage types observed on the ramparts can be classified largely into damages on individual rocks and structural deformations on the ramparts.
Damage types on individual rocks are manifested in the form of cracks, protrusion and loss while structural deformations of the ramparts are further categorized into the structural relaxation, inflating phenomenon and unstable foundation (Fig. 2). Only a single type of damage may be observed on the fortress wall but, from time to time, a number of damage types occur complexly. Also, it is possible to observe damages on individual rocks without any structural deformation but, on the ramparts that have experienced a structural deformation, damages are often identified on individual rocks.
Results of inspections confirmed the 41 spots of the ramparts with damages and, in case several different types of damages are complexly observed in a single section, all of the types were recorded (Fig. 3). Looking at the types of damages that occurred on the fortress, damages on individual rocks are observed with any structural deformation on 7 spots. The 24 of the 34 sections where structural deformation spots occurred only displayed structural deformations without any damage on individual rocks and, at the remaining 10 spots, a variety of damage types were shown in a complex manner. Looking at the percentages of different damage types, inflating phenomenon and unstable foundation occurred at 24 (38.1%) and 16 (25.4%) spots, respectively, which tells us that structural deformations have happened on more than half of the ramparts (Fig. 4). Other damage types shown were losses at 12 (19.0%), cracks at 5 (7.9%), protrusion at 2 (3.2%) and structural relaxation at 1 (1.6%) spots, respectively.
Looking at the distribution of sections where damages occurred, we can see that 23 of the total 41 sports with damages are concentrated in the section between Geumseoru and Yeonji along the Geumgang River (Fig. 3). On the west and east sides, an extremely small number of sections have damages and, in contrast, damaged sections are shown to be concentrated on the ramparts near Ssangsujeong Pavilion on the south side. The root-cause investigation of the collapse of the ramparts reported that the incident was comprehensively affected by slopes of the surrounding topography, rampart construction techniques, surface water, underground water flows, conditions of the ground, and the distribution pattern of damages on the ramparts is also believed to have been influenced by such environmental factors [18, 19].
Classification and morphological characteristics
Construction materials
The construction of the Gongsanseong Fortress used igneous, metamorphic and sedimentary rocks, which are further subdivided into over 10 different types of rocks. Among them, gneiss and leucocratic granite are observed the most dominantly, and they show signs of quartzite and miscellaneous rock types mixed together (Fig. 5A to 5C). Leucocratic granite was dominantly used on most of the ramparts while a great amount of gneiss is observed on northwest ramparts of the Gongsanseong Fortress from Geumseoru to Gongbukru.
The northwest ramparts of the Gongsanseong are a section where full-scale reinforcements and repairs were performed in the 1970s and we can see that the material primarily used to construct the ramparts during the period was gneiss. There are certain sections where miscellaneous rocks show over 70% share, and most of them are those that have been restored after the 20th century by using sandstone and conglomerate. Quartzite is observed in nearly all ramparts but, as it has the form of filling stones inserted in between the rocks, exists in small sizes compared to other rocks.
Another construction material in addition to the rocks is internal fillers. Internal fillers refer to gravel, sand, clay and cement mortal. used to fill the interior of the ramparts in order to help transfer loads, and, although only one of these materials is sometimes used for an application, a number of them are mixed together for use in other cases. An endoscope camera was utilized to find out the types of internal fillers of the Gongsanseong Fortress, and natural materials such as gravel, sand and clay were mainly identified (Fig. 5D and 5E). However, some of the restored sections were confirmed to have used cement mortar on the inside (Fig. 5F) and considering the differences in the type and mixture ratio of the aggregates depending on the location, it is understood that they were arbitrarily manufactured without any particular specification.
Stone processing
Stones that constitute of the Gongsanseong Fortress have different processed shapes depending on the location, and some of the sections used unprocessed stones. Also, it is possible to observe sections constructed by mixing original stones from the past and new stones processed when the ramparts were repaired after a collapse. Looking at the lengths of the stones, the horizontal lengths somewhat vary but the vertical lengths are mostly between 15 and 30cm. Stones are normally longer in the horizontal direction than the vertical direction, and they vary in size from square stones with the 1:1 ratio to rectangular rocks with the 1:4 ratio. Stones become smaller in the ramparts set up on a steep slope, which is presumed to be intentional in consideration of transport efficiency during the process of constructing the ramparts.
Different construction techniques are applied depending on the curves of the ramparts, angles of outer ramparts, construction methods, and the Gongsanseong Fortress shows clear discrepancies from one section to the next because of different construction methods. A construction method is based on continuity of joints and standardization of stones, and three construction techniques, which are straight stones piling; uneven stones layer piling; and uneven stones piling, are applied to the Gongsanseong Fortress. Straight rock piling is a technique in which rocks that are standardized and, thus, nearly uniform in size are built to fit the joints (Fig. 6A) whereas uneven stones layer piling is a technique of constructing stones of atypical shapes in line with the joints (Fig. 6B).
Uneven stones piling is found in the ramparts to which no particular construction technique is applied and refers to the form in which stones of atypical shapes are piled up by simply matching the external surfaces (Fig. 6C). After classifying the ramparts based on the shapes of the stones and construction techniques, we were able to see that most of the Gongsanseong Fortress are built by the uneven stones layer piling method. Straight stones piling is applied mainly in restored sections while the section restored in the 1970s on the northwestern side of the fortress is mostly built by the uneven stones piling method.
Construction types
After comprehensively considering the materials used to construct the ramparts, shapes and construction techniques of the stones, we specified construction types to be able to group the ramparts that possess identical characteristics. Construction types are largely divided into Type I, II and III, and these can further subdivided into nine types (Table 1 and Fig. 7). Type I is the section using unprocessed stones and constructed by the uneven stones piling method. Gneiss was primarily used while certain sections used granite. Type II mainly used granite and applied the uneven stones layer piling method. Lastly, Type III was mostly constructed by the straight stones piling method and mainly applied igneous and sedimentary rocks. All of the ramparts that fall under Type III are those that were restored through reinforcements and repairs since the 1970s.
Table 1
Description of construction types for the fortress in this study.
Construction types
|
Descriptions
|
Type I
|
I - A
|
- Uneven stones piling using the unprocessed stone
- Having migmatitic gneiss construction parts over 70% shares
- Internal filling using natural materials
- Using stones with 18 to 26cm length and ratios in 2.0 to 2.6
|
I – B
|
- Uneven stones pilling using the unprocessed stone
- Using leucocratic granite and aplite predominantly
- Internal filling using natural materials
|
Type IIs
|
II – A
|
- Uneven stones layer piling mainly using leucocratic granite
- Variation in form and size as their location
|
II – B
|
- Straight stones pilling using mixed rock with rectangular shape
- Variation rock type for construction as the location
|
Type III
|
III – A
|
- Having sandstone construction parts over 40% shares
- Two construction type of straight stones piling with processed stone and uneven stones layer piling with unprocessed stone
|
III – B
|
- Using the processed stones with pinkish granite and biotite granite and unprocessed stones which has diverse rock type
- Variation of construction type as the location
|
III – C
|
- Straight stones piling using processed stone
- Having granite construction parts over 70% shares
- Internal filling with natural materials
|
III – D
|
- Internal filing with cement mortar
|
III – E
|
- Uneven stones layer piling using mainly leucocratic granite
- Maintenance in 1987
|
Looking at the locations for each type (Fig. 7), Type I is concentrated on the northwestern side of the Gongsanseong Fortress and shown to be distributed along the Geumgang River. Type II is observed on some parts of the northeastern and southern sides of the fortress and normally distributed along slopes. Type III has the highest share and is extensively distributed along the southern ridge of the Gongsanseong. Therefore, we can see that most of the Gongsanseong Fortress have been affected by repairs and reinforcements and the ramparts with no such records remaining are mostly located along the Geumgang river to the north of the fortress.
Behavior monitoring
Automatic monitoring system
The variety of damages exist on the ramparts, and structural deformations are observed throughout the Gongsanseong Fortress. In order to quantitatively detect and analyze such changes monitoring devices must be utilized to measure the changes in physical quantity. The types of a physical quantity that can be obtained through measurements are deformation, displacement, slope, load, pressure, velocity and acceleration, and a suitable sensing element needs to be used for the physical quantity we want to measure. In this research, potentiometer-type displacement sensors and conductive liquid-type clinometer sensors were attached to check the movement and behavior changes of the ramparts.
These sensors were installed at 20 locations with high degrees of risk among the points where structural deformations, confirmed through visual inspections, have occurred. After projecting the original forms prior to the occurrence of structural deformations and considering the progress until now, we selected locations where we believed changes in physical quantity would manifest the most definitely and attached measuring sensors. We also installed environment monitoring devices in order to measure the changes in microclimate such as the temperature, humidity, rainfall, direction and speed of the wind in the area surrounding the Gongsanseong Fortress (Fig. 8).
The monitoring system consists of sensors that measure changes in the ramparts and a data logger that stores the measured values and sends them via communication. First, considering the fortress' locational characteristics, it is difficult to supply electricity so solar panels were installed, also, storage batteries were utilized to supply electricity to the sensors both during the day and at night. Structural changes of the ramparts affect the physical quantities measured by the sensors, which detect such changes by differences in electrical signals.
The data is then stored in the logger as digital signals, and a wireless communication device is utilized to send the data to the server computer in the lab. Once the data in the digital format is enabled to infer the form of the analog signal through the process of inverse transformation, we can visually check the behavior of the ramparts easily. Several sensors can be connected to a logger but, because the direct connection with the sensors must be formed by cables, 4 locations were designated considering the distribution of damaged sections in the Gongsanseong Fortress to establish the monitoring system. The measurement cycle of individual sensors was set as minimum of 10 minutes and maximum of 30 minutes.
Daily behavior
Through the measuring sensors installed on the Gongsanseong Fortress, daily changes of the behavior were examined in comparison to the changes in temperature and, in order to minimize the effect of the amount of sunshine. We used measurement results from spring and fall, two seasons with identical lengths of day and night (Fig. 9). Looking at the temperature change first, the temperature begins to increase at about 6 AM when the sun rises and reaches the maximum around 2 PM. The maximum temperature is maintained until about 4 PM, from which the temperature starts to drop and continues to fall until sunrise on the following day.
Most of the sensors observed gradual movements as the temperature began to go up after sunrise and, after 4 PM, showed a patterns in which they return to the state prior to the behavior change starts to occur. Although the temperature continued to drop until sunrise on the next day, the ramparts showed almost no movement after 8 PM and maintained a stable state.
However, the breadths of change and the main hours of movement slightly vary depending on the direction ramparts are facing. The monitoring sensors currently installed to face west, south and north, and those that face north and south demonstrate movements that are extremely similar to the temperature change. The ramparts that face west display stable behavior changes until the noon but, after this point until sunset, drastic behavior changes are observed. The fine movements detected by monitoring sensors attached on the ramparts can be interpreted to be mainly driven by expansion and contraction of the stones due to the thermoelastic coefficient depending on the temperature change and, subsequently, influenced by direct sunlight, which changes according to the sun's diurnal motion.
Behavior changes by the time series can be clearly seen once they are shown on a one-dimensional graph but difficulties exist in visually checking real behavior changes of the ramparts. If the results of monitoring from clinometer sensors are displayed on a two-dimensional coordinate system, the route of movement can be tracked, and there are cases of applying this method to, for instance, the Leaning Tower of Pisa in Italy and Big Ben of England in order to analyze their movements [20–23].
Showing the movements of the Gongsanseong Fortress on a two-dimensional plane, we see that movements as minutes occur at sunrise and behavior changes progress in a rotating form (Fig. 10). But it is also confirmed that the ramparts do not perfectly return their original positions after 24 hours, which means that slight differences occur. Such differences accumulate every day at all measuring points to make the rocks lean to a certain direction or produce gaps in between, and huge changes are detected in some sections. The direction to which stones move varies depending on the ramparts where sensors are attached, and some of them rotate in the clockwise direction while others rotate counterclockwise. To explore the factors that influence the direction of rotation, the data was compared with various morphological characteristics of the ramparts but no particular correlation was found.
Annual changes
Changes in minute daily movements produced by the sun's diurnal motion and temperature changes are repeated in a 24-hour cycle and, as the minute behavior changes are accumulated, a structural deformation occurs throughout the ramparts. To check the ramparts' movements and behavior changes over a one-year cycle, three to five years of measurement data was shown in the form of a distribution graph on a two-dimensional plane. The number of measured data sets accumulated in a single sensor over a year is over 50,000 and, as it may be difficult to accurately understand the behavior due to an excessive volume of data, movements during a month were shown on a graph together at 12 AM on the first of every month to check the general trend.
As a result, we were able to classify behavior changes of the Gongsanseong Fortress into reversible changes and irreversible changes based on the reversibility of the change. Irreversible changes can then be subdivided into the predictable type and unpredictable type depending on the degree of predictability (Fig. 11). First, the reversible change type vibrates with periodicity centered on the origin of measurement and forms points of inflection during summer and winter each year.
This is the outcome to which the ramparts' thermoelastic behavior due to annual temperature changes is reflected, and it can be interpreted as that the ramparts retain structural restoring force. However, the ramparts do not perfectly return to the original state after a one-year period and, instead, their slopes or distances slightly change to a certain direction. The magnitudes of changes generated are very similar from one year to the next, and the changes are continuously repeated without any change in the direction.
Among the irreversible change types, the predictable type has points of inflection during summer and winter, and allows to observe the progress of behavior changes with periodicity but also shows a pattern of slowly drifting away from the origin of measurement, causing the ramparts to move. The general form of behavior changes is very much standardized, and the same pattern repeatedly occurs every year so future trends of the changes can be projected approximately.
The direction to which an irreversible behavior proceeds does not change but, rather, continues to be maintained but the amount of change varies depending on the year measured. It was confirmed that periods during which characteristically significant changes occurred vary from one sensor to another, which may possibly be an effect of external factors such as the ground, structure and underground water near the ramparts rather than yearly differences in the weather environment.
Lastly, the unpredictable type of irreversible changes continuously produces irreversible structural deformations on the ramparts and no specific periodicity and regularity are observed, which makes it impossible to predict future behavior trends. This type ramprats tend to exhibit stable and predictable movements in spring, summer and fall but, during winter, these ramparts commonly show abnormal behaviors as the direction and amount of increase of behaviors fluctuate frequently. The fact that abnormal behaviors only occur in the wintertime is believed to be due to moisture remaining inside the ramparts that, as it goes through a process of freezing and melting and changes the volume, therefore, directly affects the structural deformation.
Effect on environment
Most monitoring sensors were affected by temperature changes the most, and the effect of direct sunlight was detected principally by the direction. To check the effect of humidity, rainfall, wind direction and speed in addition to temperature, a comparative analysis was conducted with environmental measurement results, and it has been confirmed that the occurrence of rainfall and behavior changes coincide on some sensors (Fig. 12). However, not all the ramparts exhibited movements during rainfall, and behavior changes were only displayed in case of precipitation greater than a certain level.
The amount of precipitation needed for the behavior to occur varies from one rampart to the next and, on some sensors, movements of ramparts were observed even for a small amount of rainfall that lasts a long period of time. This points to the possibility of the behaviors affected not only by the rainfall taking place in an instant but also by the percentage of water content in the soil, which supports the ramparts.
Moreover, in order to check the effect of the vibrations caused by an earthquake, the time at which the earthquake took place and the changes in minute behaviors were examined. Large-scale earthquakes happened in Gyeongju and Pohang, cities about 200km away from the Gongsanseong Fortress in 2016 and 2017 in the period during the monitoring. Both earthquakes were in the magnitude of 5.0 with the maximum earthquake intensity over 8, and large vibrations in the earthquake intensity size of 5 were detected in the Gongsanseong area as well. Behavior changes of the ramparts before and after the time at which the earthquakes occurred were compared by referring to the Korean earthquake observation data but no specific changes were identified.
This may have stemmed from the fact that the epicenters were located too far away and, thus, the energy delivered to the ramparts was not enough to cause any structural deformation. Also, the measurement cycles are set at 10- to 30-minute intervals so, if the ramparts are restored immediately after behaviors caused by the earthquakes occur, it is impossible to detect them through sensors. Therefore, it has been revealed that the instantaneous movements caused by the earthquakes failed to reach over the structural critical point of the ramparts and, thus, no permanent deformations took place.