Considering the scale of earthen sites, preservation environment, weathering characteristics and soil properties, penetrating consolidation application at earthen sites is currently a popular research direction (Pan et al. 2020, Li et al. 2011, Wei et al. 2012, Chen et al. 2018, Zhang et al. 2021). Penetrating consolidation improves the weather-resistant ability of soil by penetrating the weathered layer at the surface of sites to increase the mechanical strength. Researchers have developed a wide variety of consolidants, which can be divided into inorganic materials (Li et al. 2011, Wei et al. 2012), organic materials (Zhang et al. 2021), and composite materials (Chen et al. 2018) according to their properties. These consolidants are typically adsorbed onto the surface of soil particles, thus strengthening the connection between soil particles (La Russa et al. 2019). The soil in Suoyang city, a typical site with a low clay content in Northwest China, was selected as a sample. Potassium silicate (PS [Li et al. 2011], inorganic material), ethyl orthosilicate (organic material) and nano-SiO2 sol (composite material), which have been widely used as reinforcement materials, were employed to observe the bonding relationship between soil particles before and after reinforcement. The following conclusions could be drawn from this experiment: before reinforcement, soil particles occurred in the form of dangling or granular contact structures, and cement was observed in the pores. The skeleton particles were mainly single particles with lamellar forms, and most particles occurred in point contact (Fig. 5a). After consolidation treatment, soil particles were adsorbed or coated by consolidants. Point contact between particles was gradually transformed into surface contact. With increasing contact area, a dense network structure was formed via particle agglomeration and cementation (Fig. 5b-d) to increase the soil mechanical strength. In most cases, during weathering layer reinforcement, consolidants are adsorbed on the surface of clay mineral particles. The reinforcement process generates adhesive films that adsorb or encapsulate clay mineral particles without affecting the skeleton and pores between the clay particles. Pores remain connected, so the soil permeability is almost unaffected. At the same time, these materials tend to provide a very good stability and weather-resistant ability. Under certain conditions, consolidation treatment is an ideal weather-resistant reinforcement measure. High-modulus K2SiO3 (PS) provides a satisfactory treatment effect in the arid region of Northwest China (Li and Wang 1997).
However, penetrating consolidation also suffers certain limitations. Due to the adsorption of clay minerals, the reinforcement material is adsorbed by surface clay minerals in the penetration process from the surface to the interior, which leads to colloidal agglomeration. During penetration, the solute and solvent are separated. The solute becomes increasingly enriched in the surface layer, while the solvent continues to deeply penetrate, resulting in a smaller thickness of penetrating reinforcement. The higher the clay content is, the more obvious the above phenomenon. Therefore, penetrating consolidation cannot be used to reinforce relatively thick weathering layers. When the weathering layer is thick, consolidants cannot completely penetrate, and a layered interface with different mechanical properties can be formed in the weathering layer. The external environment can easily affect the generated interface and cause the reinforcement layer to detach. In terms of temperature, the thermal expansion coefficient of soil generally ranges from 6 to 12E-6/K. Even at a temperature difference of 60 °C, the thermal expansion rate is lower than 1‰. In addition, the employed reinforcement materials effectively connect soil skeleton particles in different ways, which results in directional expansion of mineral components, thus limiting irregular expansion and deformation of soil particles. The magnitude of differential expansion and contraction caused by temperature is very small. Therefore, the influence of temperature on the reinforcement layer is not obvious. In the case of water, soil expands and contracts to different degrees under the action of water. The soil expansion and contraction rates across different sites vary between 1 and 10%. While the applied reinforcement material solidifies the treated soil, the activity of clay minerals is also constrained. The capacity of soil to expand and contract under water can be inhibited after reinforcement (Elert et al. 2008). Differential expansion and contraction on both sides of the reinforcement layer is the main reason for failure of the reinforcement layer. The experiment indicated that soil expansion and contraction mainly occurred at the capillary water stage. Whether or not capillary water crosses the interface of the reinforcement layer is the critical condition for the application of penetrating consolidation (Zhang 2021). Under certain conditions, penetrating consolidation represents an ideal weather-resistant reinforcement measure. PS materials have also been verified to provide satisfactory weather-resistant effects in arid areas of Northwest China (Li et al. 2008). Penetrating consolidation materials develop similar consolidation and failure mechanisms. By simulating rainfall conditions in Northwest China (Fig. 6), combining rainfall and failure mechanisms, slope design at different sites, and monitoring the preservation status of the reinforcement layer and layered interface during rainfall cycles, the application scope of penetrating consolidation materials could be preliminarily obtained.
Considering the weathering characteristics of sites, soil properties and spatial–temporal distribution characteristics of precipitation, penetrating consolidation exhibits a higher probability as a suitable choice for consolidation interventions at earthen sites in extremely arid areas of China (Fig. 7). There also exists a correlation between the slope of earthen sites and precipitation. The impact of slope on rainfall must be considered. The higher the slope of the site surface is, the lower the rainfall per unit area of the slope and the lower the penetration amount. Under the precipitation intensity in arid areas, the precipitation penetrating depth is smaller than the thickness of the reinforcement layer, and the dry–wet change process slightly influences the interface of the reinforcement layer. Therefore, penetrating consolidation provides a better durability. However, the applicability should also be determined after evaluation and field experiments based on the catchment characteristics of the site surface. In an indoor environment or unaffected by water conditions, this measure could also be considered after experimentation. The selection of appropriate protection measures for a specific site must be evaluated after determining the site cultural significance and site, environmental, and material conditions.
The weather-resistant mechanism of the sacrificial layer aims to cover the site surface with a mud layer to ensure that exogenic forces first destroy this mud layer to protect the site. The sacrificial layer method can reduce the influence of weathering factors. To prolong the life of the sacrificial layer, plant fibres are usually added to improve the erosion resistance of the sacrificial layer. Hemp fibres exhibit a very high toughness. Owing to their toughness, the addition of hemp fibres improves not only the overall mechanical strength (Tang et al. 2010) but also the soil ductility in general. The process of environmental factors impacting sites is not uniform, especially the effect of temperature, and water often penetrates from the outside to the inside. Therefore, the soil surface layer can produce uneven stresses. The soil structure can be destroyed under the action of nonuniform stresses. When the local stress is too high, this may even cause structural damage. Stress concentration can be effectively dispersed, and the weathering due to stress concentration can be reduced after the addition of hemp fibres to soil. The addition of hemp fibres in the sacrificial layer not only reduces soil water loss but also reduces uneven stress concentration within the soil structure. In the process of water loss, hemp fibres in soil can effectively disperse the stress attributed to shrinkage. Under the action of the suitable ductility of hemp fibres, the development of fractures is inhibited. Similarly, soil expansion can be inhibited when encountering water, and bidirectional inhibition can reduce the development of cracks. At the same time, the random distribution of hemp fibres in the sacrificial layer can reduce erosion under the influence of precipitation.
Although the physical characteristics of the sacrificial layer improve its resistance against exogenic forces, the sacrificial layer can only delay the weathering process at sites. Once the sacrificial layer is eroded by exogenic forces, the sacrificial layer loses its weathering resistance. The durability of the sacrificial layer determines its lifetime. As the sacrificial layer increasingly thins and eventually disappears, this method becomes ineffective.
Compared to penetrating consolidation, sacrificial layer protection exhibits certain limitations, especially a change in appearance. The sacrificial layer should not be the first choice. This protection measure should be implemented when other protection methods are not applicable. Therefore, at outdoor earthen sites, the sacrificial layer, as a temporary and reversible protective measure, could be used to protect the site in the short term and inhibit the weathering process due to exogenic forces in the absence of proper protection measures. Considering the thickness, erosion resistance of the sacrificial layer and external environment, the sacrificial layer can attain a suitable durability when rainfall remains below 25 mm/h (Zhang 2021). The potential areas for the application of sacrificial layers include extremely arid, arid, and semi-arid areas (Fig. 8).
Soft capping refers to the use of moss or other vegetation to cover earthen sites, which can reduce erosion at earthen sites under the influence of rain and can play a protective role. Soft capping itself provides dual effects on the protection and destruction of earthen sites. If the presence of soft capping is beneficial to long-term site preservation, this indicates that soft capping is a useful measure. At present, the following viewpoints on the protection mechanism of soft capping are mainly held: soft capping reinforces weathered soil on the site surface. The pores between weathered soil particles are filled with moss and lichens, which can improve the soil mechanical strength (Chen et al. 2017). Dense fibrous moss roots, for example, bring soil and overburden closer together, and microscopic fissures can be repaired. The relatively dense structural plane or layer formed by soft capping and weathering layers can effectively reduce the development and occurrence of weathering decay. The expansion of soft capping can absorb soluble salt ions in the weathering layer, thus reducing soluble salt enrichment in the surface layer. Moss growth can absorb salt and promote salt transport between layers. This can lower the salt content in the weathering layer and can effectively inhibit flaking. Soft capping of the soil surface can reduce the flow velocity, change the soil permeability, and improve the soil anti-scouring ability, thus reducing soil erosion due to rainfall.
Soft capping should consider the adaptability of lichens and moss to the site environment. Factors influencing the growth and distribution of soft capping include precipitation, vegetation coverage, soil nutrient content, moisture and alkalinity. Among these factors, the decisive factor is rainfall. The greatest obstacles to soft capping application include field cultivation and post-maintenance issues. Whether soft capping can repair itself and maintain a balance without human intervention constitutes the premise of the soft capping function. Although the stability of the soil surface could be maintained in the short term after the death of soft capping, vitality loss of soft capping could continue to cause site weathering. In other words, soft capping is suitable upon adaptation to the environment and can maintain self-balance and provide a certain recovery ability. The Normalised Difference Vegetation Index (NDVI) can be used as a reference for soft capping. In recent years, remote sensing spectroscopy has been introduced into the study of lichens and mosses (Karniela et al. 1999). Their spectral characteristics were studied based on normalised reflectance data. The NDVI values for wet and dry moss crusts in arid and semi-arid areas in China reached 0.65 and 0.30, respectively (Fang et al. 2008). Based on this finding, the potential range of successful soft capping application can be determined (Fig. 9). The application of soft capping must also consider the influence and preservation status of the site.
Protective structures represent an important way to protect earthen sites from weathering. This reduces the weathering degree by blocking the influence of the external environment directly or indirectly. Protective structures comprise a relatively ideal way to reduce weathering. From blocking methods to external influencing factors, protective structures can be divided into fully enclosed modes and semi-enclosed modes. The architectural forms include protective sheds, exhibition halls and museums. Through statistics of the micro-environmental characteristics of protective structures in different environments and blocking methods, fully enclosed buildings can provide a highly stable environment (Table 1). In the past, protective structures passively blocked the influence of environmental factors. In monitoring, researchers found that protective structures blocked the main factors causing weathering to achieve a relatively stable micro-environment. Protective structures, while reducing weathering rates, are still prone to other decay phenomena due to the unsuitable range of the micro-environment. Therefore, current protective structures are gradually changing from passive blocking to active control measures of environmental factors by adjusting the micro-environment to control the occurrence of secondary decay. This approach not only requires a large amount of energy but also requires maintenance fees that only rely on temperature and humidity control equipment to regulate the environment within protective structures. Therefore, the design of protective structures must consider long-term maintenance needs and costs. For example, temperature can be controlled by adjusting daylight hours through louvres, and ventilation can be designed depending on the building geometry to reduce humidity. Even the micro-environment can be controlled with low-energy consumption or green energy technology.
Protective structures are suitable for earthen sites in various climates, but each protective structure differs due to differences in site micro-environmental, display and utilisation conditions. According to the Principles for the Conservation of Heritage Sites in China (ICOMOS China 2015), the construction of protective structures should consider changes in the structure. The structure should be unobtrusive and, as much as possible, should retain the original physical characteristics of the site. The primary consideration in the design and function of such a building or shelter is its protective function. In addition, the design of protective structures should follow certain principles. First, a detailed survey of the site should be performed. The investigation of weathering characteristics to determine the factors causing site weathering and blocking of weathering factors are the primary goals. Second, preliminary research must be conducted before the construction of protective structures. Through this work, possible problems can be corrected in time. The obtained research results could guide the design of protective structures. Finally, permanent monitoring is also needed after construction. Effective maintenance and management can prevent the occurrence of secondary decay.
Backfilling protection, which is often used in the conservation of archaeological excavation sites, is also a commonly employed weather-resistant measure at earthen sites. Soil is a very good buffer material with a low thermal conductivity and low water permeability. Backfilling protection can effectively block or reduce the impact of external environmental factors. Before archaeological excavation, the site occurred in a relatively stable environment. After excavation, the site became directly exposed to the external environment, which is extremely vulnerable to exogenic forces. Under the combined action of the ambient temperature and solar radiation, the extreme surface temperature could exceed 50 °C. The surface daily temperature difference could reach much larger than 20 °C. Precipitation could also directly erode the site surface. Under the action of drying–wetting cycles, the site surface could increasingly become loose. Surface weathering could be further intensified by freezing and thawing. Under the action of temperature and water, the salinity on the surface could increase, which could accelerate weathering. Once the site is exposed to the external environment, it could be impacted by various environmental factors. Since the soil thermal conductivity is very low, the site could be protected via backfilling, and the daily temperature difference could remain very stable. The temperature below the surface could vary only annually with the ambient temperature. Even considering the influence of groundwater or capillary water, due to the low permeability coefficient of soil, the water-bearing state of the site would not change drastically within a short period, thus not causing great damage. The No. 6 Western Xia Mausoleum in Yinchuan was protected via backfilling after archaeological excavation. This method eliminated various weathering elements, such as rainfall and standing water. The site remained well preserved after backfilling (Fig. 10).
Backfilling protection is a relatively reversible process. This method can be used as a temporary or permanent protection measure. Backfilling protection is mainly used for non-exhibition archaeological sites. Backfilling protection should be implemented in conjunction with hydrogeological investigations. In principle, a stable environment should be ensured at the treated backfilling sites.
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