Correlation Between Occurrence Environment and the Cause of Deterioration at Ancient Earthen Ruins: A Case Study at Shenna Ruins, Qinghai, China

The earthen ruins are precious historical imprints left over from ancient human life and production, and have important cultural values. Chinese ancient ruins are widely distributed, large in number, and numerous in types. Most of the unearthed ruins are large in scale, immovable, and closely related to the surrounding environment. This paper takes the Shenna ruins as the research object, realizes the investigation of the occurrence environment of the ruins through geotechnical survey technology, and explores the reasons for the deterioration of the soil at the ruins of Shenna through the research of the correlation between the occurrence environment and typical diseases. On the basis of traditional cultural relics survey, the geological, environmental characteristics and geotechnical engineering conditions of the ruins were identied, analyzed, and evaluated through survey methods. Combined with indoor experimental analysis, it was found that the main reason for the deterioration of Shenna ruins was the migration of water and salt in the soil caused by seasonal precipitation, and combined with the geotechnical investigation results, the possibility of groundwater damage in this area is eliminated, which provides a research basis for the preventive treatment of water environment in the future protection and restoration of the Shenna ruins, and provided very useful technical application reference and research idea for such earthen ruins protection in Northwest China.


Introduction
The earthen ruins are precious historical imprints left over from ancient human life and production, and have important cultural values. Chinese ancient ruins are widely distributed, large in number, and numerous in types. Most of the unearthed ruins are large in scale, immovable, and closely related to the surrounding environment [1][2][3][4][5][6][7] .
Through a large number of eld investigations and literature reports, it is found that soil sites are threatened by erosion, alkali, wind erosion, surface weathering, rain erosion, cracking and collapse, and salt damage plays an important role in these diseases. The occurrence of soil salt damage is related to many factors, such as salt content, type of salt, water content, soil environment and so on. In long-term practice, researchers have found a close relationship between salt and water in soil sites. Salt only dissolves in water and moves with water migration, resulting in void structure destruction in the process of recrystallization, so that various types of salt diseases can occur [20][21][22][23] . Salt migration caused by water migration is the fundamental cause of salt damage.
Salt damage is a very important reason for earthen sites deterioration. However, there are many reasons for salt damage in soil sites. What we pay attention to is the fundamental cause that causes the development of soil salt damage and accelerates the damage of soil salt damage. The water factor in the site environment is the fundamental reason that causes the soluble salt in soil to harm the safety of the site. At the same time, the sources of water factors in the occurrence environment are extremely complex, therefore, the diseases of earthen ruins are greatly affected by the geological, especially hydrological and environmental factors in which they are located, and the protection of the ruins shall fundamentally nd scienti c solutions to the relationship between the occurrence environment and the causes of the deterioration. The protection of earthen ruins is one of the most di cult in the protection of cultural relics.
The Shenna ruin is located in the Chengbei District, Xining City, Qinghai Province, China. In 2006, the State Council announced it as the sixth batch of national key cultural relics protection units. The ruin is located on a secondary terrace at the intersection of Huangshui and its tributary Beichuan River. The terrace is narrow from east to west and long from north to south. It is a long strip with low south and high north, covering an area of about 100,000 square meters. Its archaeological value is rstly manifested in that it was a hub on the corridor of cultural exchange between the Hexi Corridor and the Western Regions 4000 years ago, names the Qijia culture, which is an archaeological culture of special value in the upper reaches of the Yellow River and an important source of Chinese civilization. It is mainly distributed in the eastern part of Gansu to the west to zhangye and Qinghai Lake area within a range of nearly 1000 kilometers, across Gansu, Ningxia, Qinghai, Inner Mongolia and other four provinces and the communication function of this route during the Qijia cultural period was the main road for the spread of Eastern and Western culture in that period; secondly, the Qijia culture where the Shenna ruin is located is an era of transformation and revolution, mainly based on Qijia culture, with a small amount of Majiayao type, Banshan type and Kayue culture, therefore, it is a representative settlement in this era of transformation and revolution. In 1992 and 2016, the Qinghai Provincial Institute of Cultural Relics and Archaeology conducted archaeological exploration and excavation of 2000 square meters of it, and the excavated area was 2 meters thick, and discovered 171 houses, 358 ash pits, and 19 tombs, it is the site of a Qiang settlement about 3500 years ago, and the relics are very rich.
Geotechnical engineering survey [8][9][10] is the foundation of the earthen ruins protection engineering design. It is mainly used to identify, analyze, and evaluate the geological, environmental characteristics and geotechnical conditions of the ruins based on the requirements of the earthen ruins protection project and on the basis of traditional cultural relics surveys through a combination of innovative survey and methods. Therefore, this paper takes the Shenna ruins as the research object, and realizes the investigation of the occurrence environment of the ruins through geotechnical survey technology.
Through the correlation research of the occurrence environment and typical diseases, the reasons for the soil deterioration [11][12][13] of the Shenna site are explored.
1 Survey Area As Fig. 1 is shown, the survey scope of this project is the Shenna Ruins Exposure Zone, which covers an area of 1620 square meters. It is divided into Zone I and Zone II according to the location of the site. Zone I mainly includes 5 excavation pits (No. 1~5). Zone I includes 1 excavation pit (No. 6).

Survey And Research Methods
Geotechnical engineering survey is the foundation of the earthen ruins protection engineering design, and the description of the methods mentioned in "Geotechnical and mechanical parameter statistics and values" are carried out in accordance with the provisions of "Code for Geotechnical Engineering Investigation" GB50021-2001, "Technical Code for Geotechnical Engineering Investigation" (YS5202-2004) and "Standard for Geotechnical Test Method" (GB/T50123-2019).
Base on the fully collecting and investigating archaeological excavation data, combined with the geographical characteristics of the occurrence environment of the Shenna ruins, it is carried out using a combination of drone aerial photography, on-site measurement, disease surveying and mapping, sampling and indoor testing. It is mainly drilling, including engineering geological survey [14] and mapping, in-situ testing, sample collection, indoor test survey and other exploration and testing methods combined with comprehensive survey methods to expose the adverse geological effects of the Shenna ruins and assess the stability of the ruins, detailed surveys and investigations are carried out on the types, scales and scope of the existing diseases. The research on the relationship between the occurrence environment and the causes of the typical diseases of the ruins and the evaluation of the development trend of the diseases have laid a research foundation for the research on the governance and protection of the ruins diseases.

Sampling and Testing Methods for Geotechnical Investigation
The principle of minimum intervention should be adopted, the exploration holes for taking soil samples and conducting in-situ tests along the ruins area shall not be less than 1/2 of the total number of exploration holes, the number of sampling holes shall not be less than 1/3, and a certain number of exploration drilling holes shall be arranged. According to the above principles, the 4 exploration points (4 boreholes) are arranged in a grid according to the surrounding lines and corners of the greenhouse. The number of effective mechanical parameters that meet the coe cient of variation of the main engineering geological layers within the survey depth is not less than 4 groups, for the drill sampling, the mud wall is used for drilling below the water level, the loess thin-walled soil extractor is used for static pressure soil extraction above the water level, and the original sample below the water level is taken from the drilling tool. The in-situ exploration test of the ruins during the investigation can complete the standard penetration test and heavy dynamic cone penetration test in situ, as well as the conventional test of soil samples, collapsibility test, self-weight collapsibility test, permeability test, soil corrosion analysis of the ruins, and rock compressive strength analysis test etc. This time, the ve locations were selected for the double-loop method to determine the permeability coe cient of the vadose zone. The test points were all located at the periphery of the Shenna ruins. The test method was to clear the test points and press the concentric iron rings with the outer ring diameter of 50 cm and the inner ring diameter of 25 cm into the soil body 10cm, the ring wall is in close contact with the soil layer, and the inner ring is covered with 2~3cm lter coarse sand. During the test, water is added to the ring space between the inner ring and the outer ring to keep the water level at 10cm. The in ltration water volume of the inner ring is used as the ow rate for calculating the permeability coe cient, and the permeability coe cient of the vadose zone is calculated by the water seepage formula.
Formula: K=QL/{F H k +Z+l } Among them: K-permeability coe cient m/d, Q-stable in ltration water volume m³, F-inner ring water area m 2 , H k =-capillary pressure, L-in ltration depth.
when the water column is equal to 10cm, the head gradient can be considered close to 1. (K=V)

Statistics and Value of Geotechnical Physical and Mechanical Parameters
The physical and mechanical properties of each layer are counted according to the layered rock and soil, and the obvious discrete data caused by the unevenness or interlayer of the rock and soil layer is eliminated.

Indoor Test Analysis
The samples from the east side (SN-A), west side (SN-B), south side (SN-C), and north side (SN-D) of the excavated area were taken from the bottom, middle and upper parts of the excavated area. The moisture content, element composition and salinity of the ruins soil body were analyzed and tested.
pH meter: select three paintings with different sunlight intensity, water erosion degree, and different locations to use a non-destructive acid tester (Sartorious PB-10, Sartorious Scienti c Instruments Co., Ltd., China) to test the pH of the paper. The samples are all measured at three different points. XRF: use XRF-1800 X-ray uorescence spectrometer produced by Shimadzu Corporation of Japan for analysis, X-ray tube target: rhodium target (Rh), X-ray tube voltage: 40KV, X-ray tube current: 95mA, power: 3KW. Grind the sample soil in a mortar and fully dry it to a constant quality in a vacuum drying oven at 105 degrees Celsius, and it is used for analysis after sieving the sample soil.

Salt Analysis
Researching the types and content of soil salt is of great signi cance to understanding the occurrence of diseases in soil ruins. The primary minerals in the soil undergo slow weathering under the action of the external environment to generate a large number of secondary minerals, and there are a large amount of soluble salt in the generated secondary minerals, which crystallize under the action of soil moisture. The dynamic changes of crystallization-dissolution-crystallization, along with such changes, the soil has produced a series of changes in physical properties, and the effects of different contents and different types of salt are very different.
Ion chromatographic analysis: ICS-1500 ion chromatograph (Dion Corporation, USA), the collected samples are dried at 105 degrees Celsius after the impurities are removed, ground and sieved, and 0.1000g soil sample is accurately weighed to dissolve in 10mL ultrapure water (18.2mΩ/cm), put the beaker in an ultrasonic cleaner and sonicate for 30min, then pour the soil solution into a 25mL centrifuge tube, centrifuge at 8000r/min for 10min, the ion chromatography test solution was prepared by ltration with the 45μm lter head. Anion analysis: Dionex IonPac AG9 guard column and IonPacAS9-HC anion separation column, eluent is 12.0 mmol/L Na2CO 3 , ow rate is 1.0 mL/min, AMMS anion suppressor, injection volume is 10 µL. Cation analysis: Dionex IonPac CG12 guard column and IonPac CS12A-HC cation separation column, eluent is 20.0 mmol/L methanesulfonic acid, ow rate is 1.0 mL/min, CSRS-300 electrochemical suppressor, injection volume is 10 µL.
Conductivity analysis: Dry the soil sample at 105 degrees Celsius and pass it through a 20-mesh sieve. Weigh 10g of the soil sample into a 100 ml beaker, and then add 50 ml of ultrapure water to the beaker (water-soil ratio is 5:1). Oscillate on a shaker for 3 minutes and let it stand for 30 minutes, put a small amount of soil suspension in a small beaker, and conduct a conductivity test according to the conductivity meter operating procedure.

Results And Discussion
Through investigation and research on the topography, stratum and lithology characteristics of the ruins, geological structure, groundwater and surface water, and unfavorable geological phenomena, the results show as follows:

Topography, Geomorphology and Geological Structure
The Shenna Ruins are located in the north of Xiaoqiao Village, Chengbei District, Xining City, Qinghai Province, they start from a shady slope in the north, to Fenmugou in the south, to Luangou in the west, and the Ningzhang Highway underneath the eastern platform. The entire ruins are long from north to south and narrow from east to west, the terrain in the area has small undulations. The geomorphic unit is the second terrace at the intersection of Huangshui and its tributary Beichuan River. According to the research results of "Northwestern Region Stability Evaluation Map" ("Northwestern Region Engineering Geology Map Manual"), the area where Shenna Ruins are located has no signs of structural development, and it is a relatively stable block with no active faults in the area, and the strata of the ruins is stable.

Meteorology and Hydrology
The ruins are within the range of the Huangshui River, the distance is 0.6km away from the Huangshui River. The ood season of the Huangshui River is from May to September, and the main ood season is from July to August. The survey period is a dry period, according to the survey data of the area where the ruins are located, the aquifer is the unpressurized pore water in the Quaternary medium dense round gravel layer, the absolute elevation of the stable groundwater level is 2338.01~2338.79m, the average elevation is 2338.4m, and the stable water level buried depth is 32.8 ~33.2m, with an average depth of 33.0m, the amount of groundwater is large, and it is mainly recharged by atmospheric precipitation and upstream ow, the ow direction is from southwest to northeast, and groundwater replenishes river water. According to the data, the water level varies between 0.50-1.00m during the wet and dry seasons. The permeability coe cient of the aquifer is 30.0m/d. Within the depth of 25m in the Shenna ruins area, the in uence of groundwater may not be considered.
In addition, the area where the Shenna Ruins are located has a continental climate of arid and semi-arid plateau, with high altitude, low air pressure, small precipitation, large evaporation, long freezing period, short frost-free period, large daily temperature difference, strong ultraviolet rays. The annual average temperature is 2.6~5.3℃, the annual frost-free period is 140~150 days, and the annual average precipitation is 527.6mm (as is shown in Fig.2). The maximum rainfall per hour is 47.7mm, the average evaporation for many years is 900~3000mm, and the average humidity for many years is 61%. Therefore, it can be inferred that the precipitation factor of the site's occurrence environment will be the key water environment source.

Soil Geotechnical Characteristics and Permeability
Within the survey depth of this ruins area, it is Quaternary Upper Pleistocene (Q3al-pl) alluvial deposits.
The foundation soil is divided into 3 layers from top to bottom: the surface is plain ll, the upper part is collapsible loess, and the lower part is non-collapsible loess, and the lithological characteristics of each layer are as follows: Plain ll (Q4ml): earthy yellow, yellow-brown. The main ingredient is silt. It contains plant roots and is widely distributed in the eld. The thickness is 1.10 1.50m, and the average is 1.33m, the bottom elevation is 2272.85 2273.20m, and the average is 2273.08m, the depth of the bottom is 1.10 1.50m, and the average is 1.33m.
Collapsible loess (Q32al-pl): yellowish brown, brownish yellow. It is mainly in brown-yellow tones, the soil is mainly composed of powder particles, and the content of powder particles accounts for more than 60%. The root system of the upper plant is well-developed, with wormholes and large pores. The 3mm soil strip is easy to break when rubbed by hand, and it has a slightly sandy texture when pinched by hand. The soil is relatively uniform, slightly dense to medium dense, slightly wet to wet. The shaking response is moderate, the dry strength is low, and the toughness is low. The layer is distributed stably and the eld is generally distributed, with a thickness of 16.00 17.95m, an average of 17.50m, an elevation of the bottom of the layer 2255.25 2257.05m, an average of 2256.40m, and a buried depth of 17.50 19.25m, an average of 18.00m.
Non-collapsible loess (Q32al-pl): brownish yellow. The particle size component is mainly composed of powder particles, which are easily broken by hand rubbing into 3mm soil strips. There is a sticky feeling of sand when pinched by hand. The powder particles account for more than 50%. It is slightly wet to wet, the density is slightly dense and medium dense, the soil is relatively uniform, the lower part contains sand and gravel, and the structure is relatively loose. The shaking response is moderate, the dry strength is low, and the toughness is low. The eld is generally distributed, and the maximum exposure depth is 2.75m, which is not exposed. As the list of permeability coe cient of the Shenna ruins indicted that the ruins are located on the secondary terrace, and the soil permeability coe cient of the soil at the ruins is found to be weak, the groundwater has a negligible impact on the ruins and combined with the hydrological data, we can preliminarily exclude the possibility of groundwater damage in the area where the site is located, and speculate that the wet environment and water damage of the site are mainly from seasonal precipitation. It provides a research basis for the preventive management of water environment in the later period of the site.

Evaluation of Soil Corrosion and Soluble Salt
According to the exploration hole in the eld area, each section of the soil is taken to analyze the soluble salt, according to the evaluation clause of the ruins soil corrosion in the "Geotechnical Engineering Survey Code" (GB50021-2001) (2009 Edition). According to the environmental conditions of the ruins, the corrosiveness of ruins is evaluated.
As the Fig. 4 is shown, the pH value of the soil is 8.4-8.5, the content of SO 4 2 in the soil is 360.00-600.00mg/kg, the content of HCO 3in the soil is 360.00-430.00 mg/kg, the content of Cl in the soil is 230.00-400.00mg/kg. It can be seen from the contents of various soluble salts in the gure 4 that there are a lot of soluble salts in different sampling locations of the site, and the total salt content is as high as 17%-23%. The environmental type of the ruins is considered as Category III, and the soil on the ruins is slightly corrosive.

Microstructure of the Ruins and Soluble Salt Analysis
From the SEM morphology (Fig.5), it can be clearly observed that there is a large amount of soluble salt in the interstitial soil particles on the surface of the crisp powder. When a large amount of soluble salt precipitates on the surface of the ruins, the ruins will have whitening and lose its original appearance and cause alkali disease [15][16][17] . These slightly soluble and soluble salts lled between the soil particles, under the in uence of the external temperature and humidity and the moisture inside the earthen ruins, on the one hand, through physical action, that is, the soluble salt in the pores can repeatedly crystallize-dissolverecrystallize, the process of continuous squeezing of the pores of the surrounding soil can result in the destruction of the surface structure of the earthen ruins and the shedding of crisp powder.
On the other hand, through chemical action, that is, water and carbon dioxide in the air and some acid gases have complex chemical reactions with the salt in the soil and soil minerals, which change the original stable material composition, resulting in changes and reductions in the soil structure, and the agglomeration force of the soil causes the crumbs to fall off the surface under the action of air ow and gravity. However, salt is a very important reason for earthen sites deterioration which is only dissolves in water and moves with water migration, resulting in the process of recrystallization. Salt migration caused by water migration is the fundamental cause of salt damage. Therefore, it is of great signi cance and value to explore the water factors in the environment of ruins. when combined with the small radius cations Ca 2+ (1.05Å) and Na + (0.98 Å), it is easy to surround the cation with a layer of water molecules to form a more stable water-containing sulfate. When they precipitate from the interstitial solution of the earthen ruins, the large gaps become smaller, and the small gaps expand and squeeze the adjacent particles, and the void structure is destroyed. For example: gypsum (CaSO4•2H 2 O) not only makes the soil soft, but also anhydrite (CaSO 4 ) will form gypsum after hydration, which expands by 60% during the hydration process, which is a kind of the important exothermic reaction causing volume change, which results in soil loosening and crispy powder. The salt content of the soil in different areas of the ruins was compared (as Tab. 3 is shown), the research found that the salt content of the exfoliated soil in the crispy powder area was much greater than that in the non-crispy powder area, which indicates that under the combined action of evaporation and capillary action, the salt migrates from the inside of the ruins to the surface of the ruins and continuously accumulates on the surface of the ruins. The surface salt is periodically dissolved and crystallized under the combined action of environmental temperature and humidity and moisture in the ruins. It continuously exerts force on the soil, and the long-term force leads to the destruction of the soil skeleton, weakening the agglomeration force of the soil, and nally falling off the surface of the ruins under the dual effects of air ow and gravity attraction.   More importantly, the Shenna ruins are in a special environment, and shallow surface water is still active in the ruins. Therefore, the soluble salt in the soil continuously migrates and accumulates to the surface soil layer of the ruins along with the movement of shallow water [19] . In fact, the harm of soluble salt to the ruins is obvious, especially in winter, the annual average temperature of the area is 2.6~5.3℃, the temperature difference between day and night is large, and the annual minimum temperature is around minus 20 degrees, the freezing and thawing expansion is severe in winter, when the water content of the soil is high, the freezing and thawing expansion will inevitably occur, which will also cause the expansion of the soil and make the surface powdery [22] . At the same time, due to the differences in the mineral components contained in the soil itself, different minerals have different thermal expansion coe cients, under the conditions of rapid temperature changes, differential expansion occurs, which makes the structure of the soil loose and its strength decreases.
Among them, the main reasons for the shedding of pit wall pieces of the ruins are preliminarily inferred as follows: The excavation of the pit resulted in the destruction of the pit wall structure, the stability and balance being broken, and the partial shedding of the pit wall soil, The upper soil of the pit wall is mainly lled with soil and the structure is loose. When the excavation pit is excavated, this part of the soil is very easy to fall off or even collapse, The splitting action of the plant root system accelerates the fall of the pit wall ll. On the other hand, the ruins survey results found that there were many cracks in the soil at the ruins, including vertical penetration cracks, zigzag diagonal cracks, and longitudinal cracks on the top surface. The length of the crack is generally 0.5~4.1m, the opening is 1~40mm, and there is no lling inside. The cracks cut the ruins and partially penetrated each other, reducing the overall strength of the excavated pit wall, and even led to the collapse of the pit wall.
The cracks are the conditions for the occurrence of collapse, the main reasons for the cracks are as follows: Unloading in uence: During the archaeological excavation, the balance of the preservation of the site was broken. From the excavation to the construction and protection of the exhibition hall, due to the unloading effect, multiple cracks were formed in the pit wall. Shrinkage and cracking: After the excavation of the soil beam, the internal water evaporates and loses continuously, causing the soil to shrink and crack. Cracking rst produces tiny cracks, under the in uence of various factors such as external man-made disturbances and salt content, the tiny cracks continue to develop into wide cracks, the further extension of wide cracks creates local dangerous soils, in the absence of timely measures, it can easily cause diseases such as soil collapse. The effect of temperature difference: the temperature is an indispensable factor for the occurrence, development and aggravation of various diseases. Although there is a protection hall, the indoor temperature difference still exists, which provides extremely favorable conditions for the occurrence, development and aggravation of various diseases, the cracks and aky peeling are inseparable from the effect of temperature. During the development of cracks, the temperature difference plays an indispensable role, in the development of aky denudation, it is the difference in temperature that causes the shrinkage of the mud skin, which lays a good material foundation for the later denudation, the difference caused the migration of salt and the uneven shrinkage and looseness of the rammed soil at the ruins, which provided a prerequisite for the further development of this disease. Man-made disturbance: The ruins survey found that the top of the soil barrier beam was the working channel for the on-site archaeological staff, walking back and forth caused the load on the top of the soil barrier beam to increase, and at the same time caused certain disturbances, which made the soil barrier beam easier to produce smoothness, in addition, man-made disturbances also have a certain in uence on the development of the cracks in the soil beam crack, the combined action of multiple factors results in the development and extension of the cracks. The role of herb vegetation: the splitting effect of plant growth on the wall of the pit will also accelerate the development of the original cracks or produce new cracks. The existence of various cracks on the wall of the pit provides a good channel for the leakage of water. Due to the lack of drainage system in the eld, rainwater seeps along the cracks, increasing the static earth pressure of the rock and soil on the wall of the pit, causing cracks, Various cracks reduced the overall stability of the pit wall, and reduced the ability of the pit wall to withstand dynamic loads such as earthquakes and strong wind, The staggered combination of various cracks on the pit wall develops, cutting the pit wall to form a dangerous block, which accelerates the destruction of the pit wall.
To sum up, the main reason for the salt damage of the Shenna ruins to cause the soil crispy powder is due to the volume change caused by the repeated dissolution shrinkage-crystal expansion-dissolution shrinkage process of the soluble salts CaSO 4 2and Na 2 SO 4 2in the soil of the ruins body, and the crystallization pressure directly destroys the aggregate structure of the soil, increases the distance between soil particles, reduces the agglomeration of the soil, and causes the phenomenon of soil crispy powder. In view of the weak in uence of groundwater at the Shenna ruins on the ruins, the excavation area and other key protected areas were in a semi-enclosed "indoor" environment, the high salt content soil has the strong hydrophilicity and has a strong ability to absorb moisture in the air and the seasonal precipitation caused the water movement in the ruins, the soluble salt migrate with the water is the main reason that caused the salt damage and crisp powder in the soil. The borehole locations in the ruins (a) and the Simpli ed geological stratigraphy observed (b) in the borehole(1-4).