New Insights into Earthen Site Conservation: Methods, Techniques, Management, and Key Case Studies

1. Introduction

Earthen sites are ancient remains constructed using soil as the core building material. They carry profound historical, cultural, and scientific value and are important carriers of human civilization [1,2,3]. They not only occupy an important proportion of global cultural heritage resources but also contain rich historical information, providing key material evidence for exploring the origin and development of civilization. They possess irreplaceable historical, scientific, and artistic value. From a global perspective, earthen sites have formed a wide and regionally distinctive pattern due to the advantages of readily available raw materials and adaptability to diverse climates [4,5]. Their spatial distribution is highly consistent with the development trajectory of ancient civilizations and is also profoundly influenced by geographical environment, climate conditions, and cultural traditions. For example, in Africa, the Timbuktu and Djene-Djeno sites in West Africa and the Timgad (Algeria) and archeological site of Sabratha (Libya) in North Africa, etc. [6,7], adapted to arid and semi-arid climates with rammed earth and adobe techniques and witnessed the cultural exchanges during the trans-Saharan trade and the ancient Roman colonial period. Earthen sites in the Americas vividly demonstrate the wisdom of pre-Columbian indigenous peoples. Sites such as Mesa Verde National Park in North America [8], Paquimé in northern Mexico [9], and Sechin Bajo (Peru) in South America [10] showcase the innovative applications of earthen construction techniques by different civilizations. Eurasia, as the core cradle of ancient civilizations, boasts a greater diversity of earthen sites. East Asia, particularly China, has approximately 400 national-level earthen sites, covering 30 provinces (municipalities and autonomous regions), encompassing various types including ancient cities, tombs, and defensive structures [2]. The Mozu Kofun in Japan is a significant example [11]. Other sites include the Ashur (Qal’at Sherqat) in Iraq [12], the Persian Caravanserai in Iran in West Asia [13], the Boyne Valley site in Ireland [14], and the Çatalhöyük site in Turkey [15], recording the earthen construction traditions of different civilizations and the imprints of intercontinental exchange. While Oceania and Southeast Asia have relatively fewer sites, Aboriginal sites in Australia [16] and the ancillary buildings of Angkor Wat in Cambodia still retain traces of the application of earthen construction techniques [17]. Earthen heritage sites around the world are mostly concentrated in arid, semi-arid, and seasonally arid climate zones, while those in humid regions have been preserved through technological improvements [18]. Their functional diversity and wide distribution are not only important historical imprints but also provide important references for the protection and sustainable development of contemporary earthen architectural heritage.

Driven by this development, this Special Issue of Coatings brings together 10 original papers that collectively illustrate how earthen site conservation techniques and case studies are conducted. These articles cover fluidized catalytic cracking (FCC) technology, surface microchemical analysis, microbial diversity analysis, erosion resistance testing, lightweight crack detection and independent physical simulation, durability analysis, biocrust analysis, erosion resistance mechanisms, and composite biofilm ecological protection technology.

2. Overview of Contributions

This Special Issue primarily covers case studies from Spain, Malaysia, and China, as well as specific urban case studies from Zhuhai City, Lishui City, Quanzhou City, and Weiyuan City. This edition provides a reference for empirical analysis of conservation techniques for earthen sites across cities of different scales and types.

Frankly speaking, earthen site conservation techniques possess a rich and diverse technical framework, encompassing various experimental analytical methods. However, this Special Issue focuses on specific practical earthen site needs, emphasizing the following key models and technologies:

• X-ray diffraction (XRD);
• Scanning electron microscopy-energy dispersive spectrometer (SEM-EDS);
• High-throughput sequencing (based on the 16S rRNA gene V3–V4 region);
• QIIME2 (version 2019.4) for quality control, denoising, sequence merging, and chimera removal;
• Ion chromatography detection;
• Drone aerial photography;
• Microbial community analysis;
• Soil leaching simulations;
• Finite element mechanics analysis (FEM);
• X-ray fluorescence (XRF);
• XGRIDS Lixel L 3D laser scanner (Xgrids Limited, Shop 185 G/F, Hang Wai Ind Ctr, No 6 Kin Tai St Tuen Mun Nt, Hongkong).

By utilizing these technologies, they have provided practical methods and models for earthen site conservation and management.

2.1. New Technological Insights

María et al. [19] proposed a research scheme for characterizing earth mortars with experimentally added pozzolanic materials using fluidized bed catalytic cracking (FCC). Their ash exhibits excellent pozzolanic properties and can be used in protective processes. This paper provides a possibility for studying methods to enhance the material and structural stability of earth mortars.

Hu et al. [20] proposed a dual-path framework that combines lightweight crack detection with independent physical simulation. For detection, they developed an improved YOLOv12 model capable of lightweight and accurate identification of multiple crack types beneath complex wall textures. This framework provides an effective and interpretable solution for standardized recording and mechanistic analysis of cracks, offering a practical approach for the preventative protection and sustainable management of Fujian Tulou (earthen buildings).

Xiao et al. [21] also mentioned that future research on earthen site conservation is expected to integrate big data and cloud computing to achieve multidimensional prediction of surface degradation, while virtual reality (VR) will overcome the limitations of time and space and promote the development of conservation paradigms towards automation, intelligence, and sustainability.

2.2. Material Characterization Analysis

Zheng et al. [22] used microchemical analysis to determine that the rammed earth walls in Xiaochikan Village consist of three main minerals: Quartz (SiO₂, including alpha-type SiO₂), Calcite (CaCO₃, including synthetic calcite), and Gibbsite (Al(OH)₃). The core elemental composition of the four wall samples is consistent, mainly consisting of C, Si, Al, Ca, and Fe, with contents of over 93%. This reflects the matrix characteristics of the local alluvial soil, which consists of “silicate skeleton-carbonate cementation-organic matter residue”, and also demonstrates the “local material” attribute of the rammed earth.

Wei et al. [23] combined high-throughput sequencing (based on the V3–V4 region of the 16S rRNA gene) and microbial community analysis in an experiment on Paishan Village, the largest existing rammed earth building complex in the Pearl River Delta. They discovered that different wall orientations resulted in different microbial community structures: the north wall was rich in hygrophilic microorganisms, while the south wall was dominated by thermophilic bacteria. The coexistence rate of core microorganisms across the four walls was only 0.29%. This study elucidates the distribution pattern of microorganisms in rammed earth walls in humid and hot regions, providing a scientific basis for the ecological restoration of rammed earth walls.

Wang et al. [24] continued the above research and added ion chromatography, finding that the accumulation of acid rain and sulfates caused cracks and spalling in the south wall of the rammed earth wall; salt spray and high humidity exacerbated the surface weathering of the west wall; active biological activity and high humidity made the minerals in the north wall easy to dissolve, resulting in a loose structure; and the east wall was more susceptible to weathering due to the dynamic changes in carbonate rocks.

2.3. Performance Analysis of Rammed Earth Walls

Zheng et al. [25] determined the local erosion resistance of rammed earth wall materials in Yangjiatang Village, Zhejiang Province, through sampling and testing. They concluded that the overall corrosion resistance order was W > S > N > E. The surface deterioration of the rammed earth walls in Yangjiatang Village was mainly caused by four factors: structural cracks, surface erosion, biological erosion, and roof damage. These factors collectively led to surface cracking and peeling (3–5 cm deep).

Yang et al. [26] used drone aerial photography, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and microbial diversity detection techniques to elucidate the erosion resistance mechanism of the gray green biocrust on the surface of traditional rammed earth houses in Chenjiapeng Village, Songyang County. They found that the algal crust forms a three-dimensional network through macroscopic coverage (reducing raindrop kinetic energy and moisture retention) and microscopic extracellular polysaccharide-cemented soil particles, thereby enhancing the compressive and shear strength of the rammed earth surface. Simultaneously, they proposed a “transparent membrane-biofiber-acid-suppressing layer” composite biofilm design. This design integrates a biodegradable polylactic acid (PLA) network structure, an algal attachment matrix, and calcium carbonate microparticles, enabling dynamic neutralization of acidic substances and achieving a synergistic effect between ecological protection and cultural heritage preservation. This provides an innovative solution for the erosion protection of traditional rammed earth structures, emphasizing environmental friendliness and sustainability.

Yang et al. [27] utilized optical microscopy, scanning electron microscopy, X-ray computed tomography, and image analysis of the Warring States Qin Great Wall site in Weiyuan City, Gansu Province, China. They quantified the three-dimensional pore structure characteristics of bare soil, cyanobacteria-lichen crusts, and moss crusts within a depth range of 0–50 mm, combining these with the precipitation infiltration process. They explored in depth how biocrusts affect the microstructure of the rammed earth wall soil, alter precipitation infiltration, promote vascular plant colonization, and thus change the erosion resistance of the rammed earth wall.

Ding et al. [28] explored the durability of brick-faced clay-core walls (BCWs) in traditional Quanzhou residential architecture. They found that the key to the durability of BCWs lies in the synergistic effect of brick and clay materials and the stability of silicate mineral materials, providing theoretical and methodological support for the sustainable development research of brick-and-clay buildings.

3. Outlook

As earthen sites remain irreplaceable carriers of human civilization, this Special Issue aims to lay a solid foundation for long-term, science-based protection practices, ultimately ensuring the enduring transmission of these precious cultural legacies for generations to come. This Special Issue highlights 10 successes in conservation methods for earthen sites, which are likely to encourage more progress in preserving built heritage and motivate future research. Because of the relatively short submission period, there were fewer results, but we have been greatly encouraged by these studies. We hope these rigorously validated case studies and innovative technical frameworks will serve as actionable references for heritage practitioners, researchers, and policymakers globally, fostering cross-regional knowledge exchange and collaborative efforts in addressing shared challenges such as climate-induced deterioration and sustainable conservation. We also look forward to more new insights into the protection of earthen sites in the future.