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:
3.1 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.
3.2 Meteorology and Hydrology
The ruins are within the range of the Huangshui River, the distance is 0.6km away from the Huangshui River. The flood season of the Huangshui River is from May to September, and the main flood 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 flow, the flow 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 coefficient of the aquifer is 30.0m/d. Within the depth of 25m in the Shenna ruins area, the influence 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.
3.3 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 fill, the upper part is collapsible loess, and the lower part is non-collapsible loess, and the lithological characteristics of each layer are as follows:
① Plain fill (Q4ml): earthy yellow, yellow-brown. The main ingredient is silt. It contains plant roots and is widely distributed in the field. 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.
② 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 field 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.
③ 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 field is generally distributed, and the maximum exposure depth is 2.75m, which is not exposed.
Tab. 2 List of Permeability Coefficient of the Shenna ruins
Test location
|
Permeability coefficient (cm/s)
|
Test section lithology
|
Permeability evaluation
|
4m to the south Area
|
6.60×10-4
|
Fill soil
|
Weakly
|
10m to the south Area
|
4.37×10-4
|
Fill soil
|
Weakly
|
3m to the north Area
|
6.39×10-4
|
Fill soil
|
Weakly
|
10m to the north Area
|
3.08×10-4
|
Fill soil
|
Weakly
|
5m to the west Area
|
4.17×10-4
|
Fill soil
|
Weakly
|
As the list of permeability coefficient of the Shenna ruins indicted that the ruins are located on the secondary terrace, and the soil permeability coefficient 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.
3.4 Evaluation of Soil Corrosion and Soluble Salt
According to the exploration hole in the field 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 SO42 in the soil is 360.00-600.00mg/kg, the content of HCO3- in 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 figure 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.
3.5 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 [15-17]. These slightly soluble and soluble salts filled between the soil particles, under the influence 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-dissolve-recrystallize, 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 flow 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 significance and value to explore the water factors in the environment of ruins.
Tab. 2 the mineral composition analysis results of the soil samples at Shenna ruin
XRF sample
|
SiO2%
|
CaO%
|
Al2O3%
|
K2O%
|
SO3%
|
Na2O%
|
SN-A
|
46.82
|
19.13
|
10.35
|
5.23
|
2.83
|
3.14
|
SN-B
|
51.01
|
18.23
|
10.16
|
5.03
|
2.42
|
2.84
|
SN-C
|
50.62
|
17.63
|
9.83
|
3.98
|
3.05
|
2.53
|
SN-D
|
49.18
|
16.75
|
10.18
|
4.23
|
2.52
|
2.34
|
EDX sample
|
Si%
|
Ca%
|
AI%
|
K%
|
S%
|
Na%
|
SN-A
|
48.02
|
19.45
|
11.45
|
7.63
|
3.06
|
2.75
|
SN-B
|
53.30
|
17.32
|
11.23
|
7.32
|
2.87
|
2.53
|
SN-C
|
51.89
|
17.62
|
10.52
|
4.82
|
3.13
|
2.21
|
SN-D
|
52.76
|
17.93
|
11.28
|
5.84
|
2.93
|
2.07
|
In addition, from the comparison of the mineral composition analysis results of the soil samples (as Tab.2 is shown), it shows that the crispy soil sample on the surface of the Shenna ruins contains gypsum, which further shows that the soluble salt on the surface of the soil at the Shenna ruins is mainly CaSO4 [18-20]. It and NaCl are the main salts that cause soil salinization and will destroy the structure of the soil. Among them, the sulfate in the soil expands with the change of temperature, which makes the soil soft. Gypsum not only makes the soil soft, but also forms gypsum (CaSO4•2H2O) after hydration of anhydrite (CaSO4), which expands in volume during the hydration process, causing soil loosening and into crisp powder. In the sulfate crystal structure, the complex anion SO42- has a large radius (2.95Å), when combined with the small radius cations Ca2+ (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•2H2O) not only makes the soil soft, but also anhydrite (CaSO4) 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. Thenardite (Na2SO4) becomes mirabilite (Na2SO4•10H2O) after hydration and crystallization, and its volume expansion is 3.1 times of the original.
Tab. 3 Ions content of the soluble salt and conductivity in different sampling location at Shenna ruins
Sampling location
|
Ions content of the soluble salt (g∙kg-1)
|
conductivity(ms/cm)
|
crisp powder area
|
uncrisp powder area
|
crisp powder area
|
uncrisp powder area
|
SN-A
|
17.41
|
4.54
|
5.82
|
1.43
|
SN-B
|
11.77
|
4.21
|
5.28
|
1.74
|
SN-C
|
14.23
|
3.17
|
5.23
|
1.52
|
SN-D
|
18.14
|
3.62
|
6.85
|
1.63
|
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 finally falling off the surface of the ruins under the dual effects of air flow and gravity attraction.
The ion content of soil in different sampling location at Shenna ruins (Tab.3) shows that the highest conductivity of the soil in the crispy powder area reached 6.85 ms/cm, while the highest conductivity in the non-crispy powder position was only 1.74 ms/cm, indicating that the leaching solution of the soil of the crispy powder area contained a large amount of soluble salt ions, but there are few free ions in the leaching solution of the non-crispy powder soil, and the test results of electrical conductivity are consistent with the test results of soluble salt content, which once again shows that there is a large amount of soluble salt in the soil in the area where the crisp powder occurs at the ruins.
Tab. 4 Ion content of soil in different sampling location at Shenna ruins(mg·L-1)
Sampling location
|
Anion
|
Cation
|
CI-
|
NO3-
|
SO42-
|
Na+
|
K+
|
Mg2+
|
Ca2+
|
SN-A
|
11.0287
|
11.2244
|
157.9967
|
41.0894
|
1.1840
|
35.0987
|
158.3572
|
SN-B
|
19.8219
|
12.5318
|
129.5034
|
50.3222
|
1.2381
|
34.5045
|
192.5225
|
SN-C
|
19.5310
|
8.4453
|
187.5635
|
67.5365
|
1.0326
|
29.5300
|
224.2424
|
SN-D
|
25.3951
|
9.4610
|
184.6944
|
48.9909
|
0.9904
|
26.9119
|
142.8778
|
It can be seen from the Tab.4, it shows that the content of SO42- in the anions in each test area of the ruins is relatively high, and the content of Ca2+ and Na+ in the cations is relatively high. This is mutually corroborated by the elemental analysis results of soluble salts. It can be inferred that the soluble salts CaSO42- and Na2SO42- caused the crispy powder and salt precipitation of the soil at the ruins. In addition, a certain amount of NO3- in the ruins may be related to the high content of organic matter in the ruins. It is speculated that the organic nitrogen in the soil is converted into inorganic salt form through the nitrification of microorganisms. The chloride and magnesium salts obtained by ion chromatography also proved our inference.
3.6 Correlation Between Occurrence Environment and the Cause of Deterioration
In 1995, a temporary protection shed was built in the excavation area of the Shenna ruins. The protection shed covered the important foundations of the first excavated ruins and played an important role in protecting cultural relics. The key protected areas such as the Shenna ruins excavation area are in a semi-enclosed "indoor" environment. Because the ruins are located in northwestern China, the Xining City has more rain in summer than in other seasons, and water vapor constantly gushes out of the ground, as a result, the internal humidity of the ruins is relatively high in summer and the ventilation facilities are simple, they only rely on the top window for ventilation, and the ventilation facilities cannot effectively improve the ventilation conditions in the ruins. Therefore, the occurrence environment of the ruins in the temporary protection greenhouse is characterized by high humidity and heat in summer, and dry in autumn and winter, and the soluble salt in the soil moves with the water to cause salt precipitation, crispy powder and other diseases of the soil ruins.
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 coefficients, 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: ①The 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, ②The upper soil of the pit wall is mainly filled 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, ③ The splitting action of the plant root system accelerates the fall of the pit wall fill. 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 filling 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: ①Unloading influence: 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. ②Shrinkage and cracking: After the excavation of the soil beam, the internal water evaporates and loses continuously, causing the soil to shrink and crack. Cracking first produces tiny cracks, under the influence 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. ③The 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 flaky peeling are inseparable from the effect of temperature. During the development of cracks, the temperature difference plays an indispensable role, in the development of flaky 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. ④ 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 influence 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. ⑤The 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 field, rainwater seeps along the cracks, increasing the static earth pressure of the rock and soil on the wall of the pit, causing cracks, ②Various 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, ③The 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 CaSO42- and Na2SO42- in 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 influence 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.