Introduction

In recent years much attention has been given to the preventive conservation of the cultural-relic buildings. Many international organizations and scholars, such as the World Heritage Committee, International Center for the Study of the Preservation and Restoration of Cultural Property, and International Council on Monuments and Sites, are committed to adhering the concepts of prioritizing conservation and further exploring how to conduct inheritance and preservation of historic buildings using scientific methods [1,2,3,4,5].

Historic buildings are mostly composed of brick-stone-wood-soil materials. Due to the long history of the buildings and the influence of the change in the environment, some deterioration problems arise, such as water erosion, peeling off, crack and lower stability [6,7,8,9]. Changes in the environmental are one of the most important factors that directly influence the deterioration problems. Excessive humidity in historic buildings can cause material corrosion and other deterioration problems [10, 11], while low humidity can cause cracking and deformation [12]. Temperature changes in the environment lead to shrinkage, deformation, cracking, and chemical deterioration of the cultural relics [13,14,15,16]. These deterioration problems can have a significant impact on the durability and heritage value of the historic buildings. Therefore, to protect historic buildings, it is crucial to identify the causes of deterioration. The Venice Charter (International Charter for the Conservation and Restoration of Monuments and Sites) and related protection principles [17, 18] stipulate that any research work should not damage the original structure of the cultural heritage, which makes it difficult to study the causes of deterioration and preventive protection.

The science and technology provide important support for the protection and study of historic buildings. In recent years, scholars have widely used indoor tests, field monitoring, and numerical simulation and analysis to explore and study the causes of historic buildings deterioration [19,20,21], which have significant research value for the understanding of the deterioration mechanisms and the provision of protective measures. Aiming at the influence of environmental changes, scholars have adopted non-destructive detection methods to study the structural damage characteristics of historic buildings from different theoretical levels [22, 23], and to establish safety-assessment methods.

For historic buildings, it is essential to scientifically assess the state of the environment in which the buildings are located, determine the deterioration causes, and use scientific monitoring techniques [24,25,26,27]. Research and practical experience in regulating the upper building environment in order to achieve optimal preservation has been relatively mature, but there is fewer research on the water-holding properties of the soil under the historic buildings. Applying science and technology to reflect the changes in the water environment of the soil under buildings, so that the non-destructive tests from qualitative determination gradually changed to quantitative characterization, and quickly determine the conditions of the soil are prerequisite and necessary link to enrich the safety assessment methods of historic buildings.

Therefore, in order to better understand the direct influence of change in the water environment of the soil under historic buildings, and based on the need for the protection and development of the historic buildings, this study investigates the water-holding properties and electrochemical characteristics of the soil under the Grand Deliverance Hall of the Chongshan Temple, Taiyuan city, China. The study summarizes the main deterioration problems of the Grand Deliverance Hall and the cultural relics in the hall, analyzes and concludes that the main factor affecting the generation and development of deterioration is groundwater erosion through environmental monitoring and geotechnical engineering investigation. The impact of changes in moisture content on the water-holding properties and pore distribution of the soil under the cultural-relics buildings was further assessed. Then, this study provides a novel and effective way to monitor and reflect the water-holding properties of the soil using electrochemical detection. Finally, based on the research of this study and the conservation ideology of cultural relics that respects the original and minimizes intervention, protection recommendations to prevent groundwater erosion are proposed in order to provide guidance for future research.

Case study

Overview of the Chongshan Temple, Taiyuan City, China

The Chongshan Temple (Fig. 1) is located in Taiyuan City, Shanxi Province, and represents the Baima Temple of the Tang Dynasty and the Chongshan Buddhist Temple of the Ming Dynasty in China. The temple is 550 m long from north to south by 250 m wide from east to west, with a total area of 140,000 m2, and its layout includesthe bell tower, the wing room, and the Grand Deliverance Hall. The Grand Deliverance Hall is official-style wooden building of the early Ming Dynasty in China, which is highly reflecting the development of ancient culture and the rise and fall of religions. It was identified as a national key cultural relics protection unit in 2013. The Chongshan Temple also preserves many historical relics for the study of ancient Chinese culture, including the Buddhist sutras reflecting the history of Chinese woodblock printing, the copy of a mural map** the history of Chinese painting, and three clay statues of bodhisattvas embodying the art of Chinese sculptures.

Fig. 1
figure 1

San** (Shanxi) ancient temple: the Chongshan Temple

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Preliminary analysis of the deterioration problems

Deterioration problems

Precious relics such as ancient wooden buildings and statues, are sensitive to the preservation environment due to the particularities of the materials and structures [28, 29]. Different types and degrees of deterioration problems have occurred in the Grand Deliverance Hall, significantly affecting the value and stability. The main deterioration problems are as follows:

  • The walls and floors of the hall have suffered from salt efflorescence and crack damage (Fig. 2), while the walls are tilted with a high front and a low back, or with a low center and two high ends.

  • Uneven settlement and decay have occurred in the interior of the hall and the columns in the wall, and the decay of the columns in the wall is particularly severe (Fig. 2).

  • Statues in the hall have suffered from the loss of pigment layer and fracture damage (Fig. 2). And the deterioration problem of the bottom part of the statues, which is near the floor, is more serious.

Fig. 2
figure 2

Deterioration problems of cultural relics in the Great Deliverance Hall

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Preliminary analysis of causes

Environment monitoring

Environmental factors influencing the stability of cultural relics mainly included: moisture uptake, temperature variation and wind erosion. Among them, the change of temperature and humidity is one of the most important factors affecting the preservation of cultural relics [30, 31]. Therefore, the seven temperature and humidity data loggers were set in the Grand Deliverance Hall to record the hall temperature and humidity in real-time. Monitoring in accordance with the Law of the Cultural Relics Protection Law of the People’s Republic of China and the Protection Guidelines of the Cultural Relics in China. The data collection period was set from 1 January 2023 to 31 December 2023. The seven sets of data are basically consistent. The temperature and humidity monitoring data for the Grand Deliverance Hall are shown in Fig. 3.

Fig. 3
figure 3

Temperature and humidity data of the Grand Deliverance Hall

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As shown in Fig. 3, the trends in temperature and humidity changes in the hall are more consistent. The temperature inside the hall was at its lowest in December, and then slowly increased to reach a maximum temperature of 27.15 °C in July. The relative humidity in the hall reaches 50% or more most of the year, with lower humidity of about 40% in April through May relative to the other months.

Geotechnical investigation

Exploring the characteristics of the soil under the historic buildings is a prerequisite for further clarifying the causes of the deterioration problem. Geotechnical investigation of the soil under the Grand Deliverance Hall in accordance with the relevant provisions of the Code for Investigation of Geotechnical Engineering and Code for Investigation of Earthen Heritage Sites Conservation Works. The investigation concluded that the soil texture is mainly silty soil with high moisture content. The soil contained more chloride and sulfate ions. With the increase in investigation depth, the moisture content and the pore ratio of the soil increases, the density decreases.

Causes analysis

The Chongshan Temple of Taiyuan City is located in the temperate continental monsoon climate area, which has the characteristics of four distinct seasons and the same period of rain and heat. In the spring (March to May), as the temperature rises, the water in the soil gradually evaporates, resulting in a lower humidity and lower moisture content. Precipitation is highest during the summer (June to August), and the humidity in the hall and soil are at the highest. In the autumn (September to November), as the temperature drops, precipitation and the humidity in the hall decreases. In the winter (December to February), as precipitation continues to decrease, temperature and humidity first continue to fall and then rise. It can be preliminarily concluded that the temperature and humidity changes in the hall have significant correlated with the temperature and humidity changes in the soil under the hall. The caused for the deterioration of the hall and the cultural relics may include the following aspects.

  1. (1)

    The main building materials for the Grand Deliverance Hall and the cultural relics are brick, block and wood, which showed highly water absorption. Under the action of groundwater erosion, capillary pores pressure is formed inside the material pores. At the same time, capillarity effect causes internal expansion stress on the material pores, and the building materials have flaking and cracks due to internal stress changes [32]. The schematic diagram of groundwater erosion is shown in Fig. 4.

  2. (2)

    Soluble salts in the soil under the influence of moisture migration. Can cause material cracking, severe corrosion occurred along the bottom of the column and other problems, thus reducing the mechanical properties of the material and affecting the overall stability of the structure.

  3. (3)

    External temperature changes could aggravate the stress formed by the internal moisture of the material on the pore walls, which in turn damages the microstructure. The wind erosion could lead to the coursing of the material surface texture and increase the surface roughness by forming the wind erosion dent, which also affects the further groundwater erosion.

Fig. 4
figure 4

Schematic diagram of groundwater erosion in the Grand Deliverance Hall

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Through the above comprehensive analysis, we can conclude that the main factor affecting the generation and development of deterioration problems of the Grand Deliverance Hall and the cultural relics in the hall is groundwater erosion. Moreover, climatic factors such as temperature changes and wind erosion can exacerbate the severity of deterioration. Therefore, in order to comply with the specificity of cultural relics conservation, it is necessary to use a non-destructive monitoring method to reflect the water-holding properties and changes in the moisture content of the soil under the hall.

Research aims

At present, the protection of cultural relics is develo** from salvage protection to preventive protection. The main factor affecting the deterioration problem of the Grand Deliverance Hall is groundwater erosion, therefore, the use of non-destructive methods to reflect the water-holding properties and moisture changes of the soil under the hall is a necessary preparation for the proposal of targeted repair measures as well as the formation of a comprehensive conservation ideas.

The research objectives of this paper are:

  1. (1)

    Clarifying the water-holding properties and pore water distribution of the soils under the hall.

  2. (2)

    Utilizing non-destructive detection methods to reflect changes in the moisture content and water-holding properties of the soils under the hall.

  3. (3)

    Providing the scientific basis for monitoring and conservation of the cultural-relic buildings are at serious risk due to changes in the water environment of the soils under the historic buildings.

Materials and methods

Test materials

According to the geotechnical investigation, the soil texture of the investigation range is mainly silty soil. In view of the protection of cultural relics, a large number of soil samples under the Grand Deliverance Hall cannot be removed for testing, so after comparing the soil texture in the surrounding range, choose the 2 # soil samples as the test materials. The schematic diagram of sampling locations based on the actual location of the Grand Deliverance Hall is shown in Fig. 5, and the basic properties of the soils are shown in Table 1.

Fig. 5
figure 5

Schematic diagram of sampling locations

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Table 1 The basic properties of soils
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The X-ray Diffraction analysis of the soil samples under the Grand Deliverance Hall and the soils soil samples from the sampling site 2 showed that the minerals in soils were mainly comprises of quartz, anorthosite, calcite and clay minerals (chlorite and illite), and the mineral contents of these two soil samples were relatively consistent. Figure 6 shows the XRD pattern of the soil samples used for testing, the content of minerals is presented in a tabular form. Among them, quartz and calcite are weakly hydrophilic minerals, which have little effect on the adsorbed water in the soil. Chlorite and illite are both hydrophilic clay minerals, which have a large effect on the adsorbed water in the soil, and are the main source of soil suction [33].

Fig. 6
figure 6

The XRD pattern of the soil used for testing

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Test methods

Sample preparation

The soil samples were naturally air-dried, crushed, and sieved through a 2 mm soil sieve before being placed in a blast drying oven to dry the soil at 105 ℃ for 12 h. To clarify the effect of varying moisture contents on soil properties, and in conjunction with the actual situation, the initial dry density of the soil samples was determined to be 1.55 g/cm3 and five different initial gravimetric moisture contents were selected as test variables (12, 14, 16, 18, and 20%). The dried soil samples were re-mixed with the target moisture and kept in an airtight container at constant temperature and humidity for 24 h to ensure uniform distribution of water in the soil pore.

Water-holding property tests

The water-holding properties of soils are commonly described by curves of the volumetric moisture content versus matrix suction. Therefore, in this study, the total suction and conductivity of soils were tested using a Dewpoint Potential Meter (WP4C, Pullman, WA, USA) and a Conductivity Meter (DDS-307A, Shanghai, China). The WP4C meters was turned on for 30 min before the test, calibrated to ensure the test accuracy, and selected the test mode as Continuous Mode. The soil samples for suction test were cylinders with a diameter of 38 mm and a height of 10 mm.

Electrochemical test

The CorrTest CS350 Electrochemical Workstation (CorrTest Instruments, Wuhan, China) was used for electrochemical impedance spectroscopy (EIS) test of the soil. The electrochemical behavior of soil was investigated by using a three-electrode system, a copper plate electrode as the working electrode, platinum filament as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The working electrode was in advance sealed with epoxy to ensure the tested area of 1.0 cm2 and ground with SiC abrasive paper, and then ultrasonically cleaned with ethyl alcohol, dried in the air. To minimize the iR-drop at the soil-electrode interface, the distance between the electrodes was set close to each other. Before the test, the instrument needed to be turned on for at least 20 min to minimize the temperature drift, and the test was started with the open-circuit potential stabilized to ensure accuracy and reliability of test data.

The amplitude for EIS test was 10 mV sine wave voltage, and the scanning frequency range was 105–10–1 Hz. Experimental data of the EIS were analyzed by Zview2 software. The reliability of the impedance spectra was verified by checking the linearity condition and determining the causality condition by Kramers–Kronig transforms.

Sample preparation and testing methods refer to the Standard Geotechnical Test Method and the Technical Specification for Conservation Tests on Earthen Sites. The electrochemical testing was performed according to literature concerned [34,35,36,37]. All tests were repeated 3 times to increase the scientific consistency of the results.

Results and discussion

Water-holding properties test

Calculation principles

Calculation of total suction

The saturated vapor pressure at the dew point temperature of the soil was measurable by the Dewpoint Potential Meter. And the total suction (Ψ) of the soil can be calculated from the Kelvin equation (Eq. 1), where the total suction, is the sum of the matrix suction and osmotic suction.

$$\Psi \,{ = }\,\frac{RT}{{M_{W} }}In\frac{{e_{s} \left( {T_{d} } \right)}}{{e_{s} \left( {T_{s} } \right)}}$$
(1)

where, \(e_{s} \left( {T_{d} } \right)\) is the saturated vapor pressure at the dew point temperature, kPa; \(e_{s} \left( {T_{s} } \right)\) is the saturated vapor pressure at the sample temperature, kPa; R is the universal gas constant, 8.314 J/(mol∙K); T is the absolute temperature, ℃; \(M_{W}\) is the molar mass of water, g/mol.

Calculation of osmotic suction

The osmotic suction of soil saturated extracts (\(\Psi_{os}\)) was calculated using Eq. 2 after measuring the electrical conductivity of the soil saturation extract (\({\text{E}}_{{\text{c}}}\)), and the actual osmotic suction of the soil (\(\Psi_{\theta }\)) was then calculated using Eq. 3. The volumetric moisture content (\(\theta\)) in Eq. 3 can be calculated by using Eq. 4.

$$\Psi_{os} { \,= - 0}{\text{.036E}}_{{\text{c}}}$$
(2)
$$\Psi_{\theta } \,{ = }\,\Psi_{os} \frac{{\theta_{{\text{s}}} }}{\theta }$$
(3)
$$\theta \,{ = }\,\frac{{\omega \rho_{d} }}{{\rho_{\omega } }} = \frac{{V_{W} }}{V}$$
(4)

where, \(\theta_{{\text{s}}}\) is the saturated volumetric moisture content of the soil%; \(\omega\) is the gravimetric moisture contents of the soil, %; \(\rho_{\omega }\) is the density of distilled water, 1.0 g cm3.

Effect of changes in moisture content on matrix suction

Due to the small size of the soil samples, the moisture content of the soil will change during the testing process. Therefore, at the end of the measurement of the total suction, the soil samples were dried again and the actual gravimetric moisture contents was calculated to obtain. The result of mean electrical conductivity (\(\overline{{{\text{E}}_{{\text{c}}} }}\)) of the saturated extract is 158.4 μS/cm, so the \(\Psi_{os}\) is 5.70 × 10–5 MPa. The volumetric moisture contents of the soil samples calculated according to Eq. 1 to Eq. 3 are shown in Table 2, and the total suction is shown in Fig. 7.

Table 2 Moisture contents of soil samples (%)
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Fig. 7
figure 7

Total suction, osmotic suction and matric suction of soil samples

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The two indexes of the volumetric moisture content (θ) and saturation (Sr) are usually used to reflect the state of water in the soil and the filling situation. Where the θ is the ratio of the volume of pore water (Vw) to the total volume of the soil (V), and the Sr is the ratio of the volume of water to the volume of pore in the soil.

According to Eq. 4 to Eq. 6, it can be seen that the increase of \(\omega\) means the increase of θ and Sr.

$$S_{r} \,{ = }\,\frac{{d_{{\text{s}}} \omega }}{e}$$
(5)
$$\rho_{d} \,{ = }\,\frac{{d_{{\text{s}}} \rho_{\omega } }}{1 + e}$$
(6)

where, \(d_{s}\) represents the relative density of the soil, 2.7; \(e\) represents the void ratio of the soil.

According to the principle that the that matrix suction is equal to the difference between total and osmotic suction, the relationships between the suction and volumetric water content of soil samples with different initial water contents were calculated and shown in Fig. 7, where the volumetric water content is calculated from Eq. 4 to Eq. 6. It can be seen that the volumetric water content has a significant effect on the total suction and matrix suction of the soil. When the volumetric water content was lower, the total suction and matrix suction were greater.

Soil–water characteristic curves (SWCC)

Using the classical van-Genuchten model, the SWCC of soil samples with different moisture contents were obtained (Fig. 8) to visualize the variation of matrix suction with the volumetric moisture content. Where, θ, θr, and θs are the volumetric moisture content, residual moisture content, and minimum suction moisture content, respectively. The correlation coefficient R2 of the van-Genuchten model was 0.987.

Fig. 8
figure 8

SWCC after van-Genuchten model fitting

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The SWCC can be categorized into saturated, transitional, and residual zones, which reflect the water-holding properties of the soil [38]. When the volumetric moisture content is low and approaches the residual volumetric moisture content, the soil is close to being completely dry and smaller changes in the moisture content cause more sensitive changes in matrix suction. This is because there are many large pores in the soil with good pore connectivity, the particles do not easily come into contact each other. Clay minerals such as quartz, feldspar and illite in the soil particle component, have strong surface energy. The electric field generated by its charge adsorbed on the surface acts in conjunction with the van der Waals force to make the adsorption of the adsorbed water on the soil surface extremely strong and has a large effect on the matrix suction with the change of the moisture content, forming an adsorbed water layer with good water-holding property [39].

As the volumetric moisture content increases, the SWCC in the transitional zone becomes steeper and the change in matrix suction is smaller. This is due to the emergence of a bound water layer, which together with the adsorbed water layer constitutes membrane water. The bound water is subjected to an attractive force that decreases with increasing distance from the soil surface, so it can be divided into the strong bound water layer and the weak bound water layer [40, 41]. When the moisture content continues to increase to the extent that the free water appearing in the soil, the SWCC reaches the saturated zone and became flatter. Matrix suction continues to decrease until the soil is completely saturated, at which time the volumetric moisture content is the saturated volumetric moisture content.

Electrochemical test

Test principle

Electrochemical Impedance Spectroscopy (EIS) can characterize the electrochemical systems by applying a variable frequency sine electric potential perturbation to the study the current response [42, 43]. The microscopic pore structure within the soil is closely related to its water-holding properties. According to the EIS data of the soil, combined with the conductive characteristics, the circuit elements inside the soil and the connection between the elements can be predicted, and the ideal equivalent circuit model can be established. Further the circuit elements values are obtained to characterise information such as water content and pore structure to understand the internal structural changes of the soil [34, 37, 44].

Electrochemical impedance spectroscopy (EIS)

Figure 9 shows the EIS of soils with different moisture contents. The Lin-KK degree was used for analyzing the data validity obtained from electrochemical test. The results of the Kramers–Kronig transformation are given in the form of residuals, which the data are valid if they are less than 0.5 [45]. The residuals plot of the electrochemical data of the soil samples are shown in Fig. 10. It can be seen that the residuals of the electrochemical data measured in this study after the Kramers–Kronig transformation are less than 0.5, which indicates that the data are reliable and reasonable.

Fig. 9
figure 9

Electrochemical impedance spectra of soil samples with different moisture contents

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Fig. 10
figure 10

Residuals plot of the electrochemical data of the soil samples

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The Nyquist plot shows capacitive impedance arc in the high-frequency region, the center of the arc is below the abscissa, which corresponds to the non-homogeneous, porous nature of the soil. It is often assumed that the radius of the capacitive arc is related to the charge transfer resistance occurring at the soil-electrode interface, and the value at the intersection of the capacitive arc in high-frequency zone and the abscissa can represent the resistance of the soil [38, 46]. With the increase of the moisture content, the value at the intersection of the Nyquist decreases, which indicates that the impedance of the soil follows a decreasing trend, and at the same time, the radius of the arc in the high-frequency zone is also decreasing, which is in line with the change of the electrical resistivity of the soil.

The Bode plot suggests that the impedance modulus (|Z|) of soil samples in the |Z|—log frequency plot show continuous decreasing trend with increasing frequency and then stabilizes. It can be seen specifically from the Bode plot that the impedance values in the low-frequency region (10–1–103 Hz) decreases more, while the impedance values in the middle-frequency region (103–104 Hz) and the high-frequency region (104–105 Hz) remain almost unchanged. With increasing moisture content, the starting point in the low-frequency region of the |Z|-log frequency plot is similar, while the horizontal line in the mid- and high-frequency regions shows a significant downward translation. It shows that the impedance value decreases and the electrical conductivity of the soil increases, which is because pore water is the main factor affecting the electrical conductivity of the soil. Also, the increase of moisture content will make it easier to form connected pore channels in the soil, which in turn affects the electrical conductivity [Correlations analysis between the index of water-holding properties and electrochemical parameters

The solid, liquid, and gas phases of the soil each have their own characteristics, and the phases interact with each other, causing changes in the pore characteristics of the soil and making the soil structure more complex. This is the fundamental factor that affects the physical and mechanical properties of the soil and contributes to its water-holding properties and electrochemical characterization.

Figure 11 shows the model of water film on the surface of soil particles. The electrical conductivity of the soil depends mainly on the moisture content, the pore solution and the double electric layer of the soil. Among them, the conductivity of the pore solution comes from the free mobile ions in the solution, and the conductivity of the double electric layer comes from the ion migration.

Fig. 11
figure 11

Modelling of water film on the surface of soil particles

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Since the test soil were prepared using deionized water to obtain the target water content, the effect of ionic conductivity on soil conductivity could be ruled out. It can be inferred that the pore saturation and the state of the adsorbed water layer are the main factors affecting change in the soil conductivity, so the pore resistance was chosen to characterize these factors.

At the same time, the conductive channels within the soil were characterized by electrochemical impedance spectroscopy tests, and the conductive channels were classified into connected pore channel and unconnected pores channel. We find that connected pore resistance (Rcpp) is always larger than unconnected pore resistance (Rpp) for soils with different moisture contents, indicating that the electrical conductivity of soil is mainly determined by the connected pores.

By comprehensive consideration, the Rcpp was selected as the vital parameter that directly influences the electrochemical responses for the correlation analysis between water-holding indicators and electrochemical parameters. Therefore, the volumetric moisture content of the test soil was fitted to the electrochemical parameter Rcpp, and the fitting relationship is shown in Fig. 12. The value of Rcpp of the soil exhibits a negative power function correlation with the volumetric moisture content.

Fig. 12
figure 12

The correlation of matrix suction and Rcpp with the volumetric moisture content

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Soil with low moisture content has large intergranular pores and more interconnected pores, and the surface of the clay particles forms adsorbed water layer and thin strongly bound water layer. These cause the higher capillary increase and higher matric suction in the soil, which in turn cause high pore resistance and poor electrical conductivity of the soil [41]. With increasing moisture content, the thickness of the bound water layer and electrical conductivity of the soil increases, while the adsorption force and matrix suction of soil decreases. At the same time, the connectivity increases and the bonding strength between the soil particles decreases due to thickening of the water film [53].

The existence of an intrinsic relationship between the volumetric moisture content of the soil and the pore resistance and matrix suction further demonstrates that the electrical conductivity occurs mainly through the double electric layer in the connected pore conductive path. The higher the moisture content of the soil led to the lower the connected pore resistance, the higher the electrical conductivity of the connected pore paths, the lower the matrix suction, and the weaker the water-holding capacity of the soil. The connected pore resistance can characterize the changes in the electrical conductivity of the soil as well as the changes in the water-holding capacity of the soil, so the electrochemical detection can be applied to monitor the changes in the water-holding properties of the soil under the cultural relics.

Conclusion

The study summarizes the main deterioration problems of the cultural relics in the Grand Deliverance Hall and the cultural relics in the hall, and concludes that the main factor affecting the generation and development of deterioration is groundwater erosion through environmental monitoring and geotechnical engineering investigation. The impact of changes in moisture content on the water-holding properties and pore distribution of the soil under the cultural-relics buildings was further assessed. Then, combined with the soil property, this study provides a novel way to reflect the water-holding properties of the soil using electrochemical detection. The specific conclusions reached are as follows.

  1. (1)

    The moisture content has a significant effect on the water-holding properties of the soil. The matrix suction of the soil tended to decrease with increasing moisture content and the rate of decrease gradually slowed down.

  2. (2)

    Pore structure and moisture content are the main factors affecting the electrochemical characteristics of the soil. With the increase of moisture content, the connected pore resistance decreases and the electrical conductivity of the connected pore paths increases.

  3. (3)

    The connected pore resistance and matrix suction decrease with the increase in the moisture content, both showing a power function law. Changes in moisture content cause changes in the thickness of the double electric layer, which in turn affects the connected pore resistance and matrix suction of soil. The greater the connected pore resistance, the greater the matrix suction and the water-holding capacity. The electrochemical parameter can be used to characterize the water-holding capacity of the soil, which means that the electrochemical detection can be applied to monitor and reflect the water-holding properties of the soil under the cultural-relics buildings.

Recommendations and inspirations for the prevention of groundwater erosion in historic buildings

Cultural-relic buildings are susceptible to deterioration under the influence of groundwater erosion, and it is crucial to propose protective measures for preventing deterioration. Based on the research of this study and the conservation ideology of cultural relics that respects the original and minimizes intervention, the proposed measures to prevent groundwater erosion include the following aspects.

  1. (1)

    Electrochemical detection is a novel and effective way to monitor and reflect the water-holding properties of the soil under the cultural-relic buildings. Periodically recording and evaluating the electrochemical characteristics of the soil under the buildings, and further characterizing and reflecting the water-holding properties of the soil. In this way, it is possible to detect changes in the water-holding properties of the soil in time for protective measures to be carried out in a scientific and targeted manner. For example, when the hidden danger of groundwater erosion is found, adequate ventilation and air circulation measures can be provided to ensure that the cultural-relic are dry, to inhibit deterioration due to excessive humidity inside the heritage buildings, and to prevent the occurrence of further structural problems.

  2. (2)

    For the groundwater erosion caused by cultural-relic have appeared structural damage and strength deterioration, it is recommended that the use of suitable materials combined with the traditional technology for emergency repair. Our research group is collecting further data for construct evaluation model to comprehensive prediction and evaluation of the deterioration degree under groundwater erosion, and carry out targeted research on modified materials to improve the water resistance and durability of the cultural-relic buildings. This study can provide preliminary data and a research basis for the test considering the structural characteristics of cultural relics under the influence of groundwater erosion.