Introduction

The Ming Dynasty silt cultural layer (SCL) of Zhouqiao Archaeological Site (AS), as the scope of this research, is considered as an important difficult heritage of China, recording a historic great flood in city of Kaifeng in mid-17th century.

Difficult heritage is a unique type of heritage, and it refers to historic sites, objects or narratives associated with traumatic, controversial or morally challenging events in a society’s past. These aspects of heritage often evoke complex emotions such as shame, guilt, anger or denial, and they challenge individuals and communities to confront uncomfortable truths about their history. This concept was introduced by Macdonald1 and has since been applied to various contexts where heritage is contested or carries an ethical and emotional burden. Some difficult heritages are designated as World Heritage, e.g. Auschwitz Concentration Camp in Poland, Hiroshima Peace Memorial, etc. Apart from above examples of difficult heritage caused by man-made incidents, there are also difficult heritages are the results of natural disasters. For example, the archaeological site of Pompeii was formed by the eruption of Mount Vesuvius2, and Lajia Archaeological Site recorded series of natural disasters of earthquake, mountain torrents, mudslides and floods in Bronze Age3. Difficult heritage has not been systematically studied yet from the perspective of World Heritage4, and there is a gap about its conservation and protection. The Ming Dynasty SCL of Zhouqiao AS is caused by a flood of the Yellow River in 17th century. Though the Yellow River had flooded times in the history5, the flood formed this SCL was a man-made incident. As a record of this incident in the region, it can be considered as a difficult heritage. Due to the particularity of the deposit layer materials, cracking and shedding on the site are extremely serious and developing. However, due to the differences in materials, conventional methods cannot work well with the site.

Zhouqiao, also named as Bianqiao (literally the Bridge of Bianhe) or Yuqiao (literally the Bridge of Imperial Boulevard), was a bridge built in late 8th century (Tang Dynasty) and was buried in silt after the flood of the Yellow River in 16426. It was a landmark of the capital city of Chinese Empire, Kaifeng, in 10th to 12th century, locating at the junction of Yujie (literally the Imperial Boulevard) and the Great Canal (Fig. 1). The site was excavated by He’nan Provincial Institute of Cultural Heritage and Archaeology in 20187, discovering various relics including river channels, hydraulic facilities, bridges, etc. Its excavation reveals that the central axis of city has not been changed over a thousand of years, which is evidence of Kaifeng’s characteristic as a palimpsest city8 and is critical to understand the urban structure of the imperial capital in 10th to 12th century. The most important discoveries of the excavation include the relics of Zhouqiao in Ming Dynasty (mid-14th century to mid-17th century), a brick-arched small bridge in late-Ming, the channel of the River of Bianhe from 8th century to early-20th century and the late-11th century stone relief sculptures on the both riverbanks. In the excavation site, the east square excavation unit (SEU) revealed the remains of the River of Bianhe, whose channel was 15 to 18 metres wide and 6 to 12 metres deep from existing ground level. On the west balk of the east SEU, the SCL was at the fourth layer which was yellow-brown clay, extremely dense, 2.8 to 4.2 metres deep from the ground and 0.15 to 1.3 metres thick. Porcelain pieces, ironware and copper coins were unearthed from this SCL, and remains of Ming Dynasty buildings were superimposed at the bottom. Among the SCLs at the site, the deepest layer was 5 metres deep, and most of them were at the depth of 1 to 2 metres.

Fig. 1: Location of Zhouqiao AS.
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a Location of the site in China; b Spatial relationship between the site and the Yellow River; c Location of the site in Kaifeng, with the trace of the city wall in Ming Dynasty mark in red line.

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The Ming Dynasty SCL of Zhouqiao AS was formed as a result of a flood in 1642 which destroyed the entire city of Kaifeng. Throughout the Ming Dynasty, Kaifeng was the prefectural capital in the region and the fief of Prince Zhou. According to historical records, the city was once ‘as bustling as the two imperial capitals’ with prosperous local economy. In the civil war in late-Ming Dynasty, the rebel army sieged in the winter of 1641 the city and broken the levees of the Yellow River to cause a flood helping to conquering the city in autumn of 16429. This flood completely submerged the city and killed 80% to 90% of the people in the city. Apart from this, there are also researchers infer the levees was broken by official army of Ming, both of the war parties, or collapsed on itself because of lacking maintenance10. This SCL has also been discovered at the archaeological sites of Shuntianmen (Shuntian Gate), Zhou Wangfu (the Residence of Prince of Zhou), and Yulongwan in Kaifeng11,12. Although the exact reason of this flood remains to be revealed, the Ming Dynasty SCL of the site evidences the existence of this flood in the history.

Silt deposits are usually the objects to be cleared during archaeological excavation. In this site, silt deposits are considered as important record of the history. The authenticity and integrity of the historical information make Zhouqiao a unique and rare archaeological site. Similar with the formation of Pompei, Zhouqiao AS was formed by unexpected incidence, and the site basically captured the moment when the flood destroyed the city. The large amount of silt buried the Ming Dynasty city, so that the thick layer of silt avoided the disturbance and damage to the site from later human activities. Zhouqiao AS was listed as one of the Top Ten Archaeological Discoveries in China in 2022 regarding to its massive significance in historical research, science, arts and society. The site is being conserved and protected at its original site, and it is already open to the public.

The Yellow River has flooded many times in history, and there are studies on the silt of the Yellow Reiver’s flood. Song et al. studied the mechanical properties of unsaturated silt in the Yellow River flooding area (YRFA)13. Gu et al. explored the effect of water content on the creep characteristics and long-term strength of the soil of silt from YRFA14. Li et al. study the effects of stress changes on soil-water characteristics and microscopic pores of silt by conducting macroscopic soil-water characteristic tests and microscopic nuclear magnetic resonance tests on compacted silt from YRFA under four stress states. Their study clarified the microscopic mechanism of the influence of stress state on soil-water characteristics of silt15. Kong et al. used lignin to provide water stability for silt16. Uday et al. studied the cracking characteristics and models of fine-grained soil17. Although these researches share the same topic about the silt of the Yellow River, their methodology is not applicable as the objectives of conservation are different from that for the Ming Dynasty SCL of the site. Li, Ma18 and Pan, Yue19 studied the modification of the soil of the Zhouqiao AS. Nevertheless, their approaches are not meet for the in-situ preservation for the Ming Dynasty SCL of Zhouqiao AS. Research on such kind of in-situ preservation approach for the SCL of archaeological site has not been found in the field of cultural heritage conservation yet.

The SCL of the site is prone to cracking and falling off because of stress release, properties of the layer and changes in the microenvironment of the site after excavation. This has seriously threatened the long-term stability of the archaeological site like Zhouqiao. At the beginning of excavation, the water content of the layers at Zhouqiao AS was 37.20%. It then decreased to 2.60% after the SCL stabilised as the water evaporated since the surface layer was gradually exposed to atmospheric environment as a result of excavation. Being led by releasing internal moisture to the environment, the Ming Dynasty SCL shrank and seriously cracked, and further caused flacking, failure and collapse (Fig. 2).

Fig. 2: Cracking of the Ming Dynasty SCL of Zhouqiao AS.
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a Panorama of the site and the layout of Ming Dynasty SCL; b Details of Ming Dynasty SCL on the site.

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The Ming Dynasty SCL is consisted of pure yellow-brown clay inside the city of Kaifeng, but the SCL of the same historical period outside the city is almost a sandy soil layer instead of pure yellow-brown clay. The cracks of Ming Dynasty SCL were crisscrossed and extremely irregular (Fig. 3). The primary cracks were distributed vertically and horizontally, and were penetrating cracks with certain connectivity. After developing to a certain extent, the surface of the site directly fell off in blocks, causing serious impact on the stability of the site. The secondary cracks were mostly in irregular polygons, with relatively small width, length and depth. Some areas were connected to the primary cracks. These were basically tortoise cracks caused by water evaporation. This type of crack morphology affected the appearance of the site and had a slight impact on the stability of the site itself.

Fig. 3
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Massive cracking and shedding disease of the Ming Dynasty SCL of Zhouqiao AS.

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This research analyses the performance of the materials of the SCL in terms of physical properties, hydraulic properties, mechanical properties and chemical properties, and explores the mechanism and influencing factors of cracking and shedding of the SCL, providing a reliable approach for future conservation and protection. This research on the properties of the SCL, especially the impact of cracking, will contribute to the conservation of similar heritage sites.

Methods

Sampling

The sample is collected from the peeling of the Ming Dynasty SCL at Zhouqiao AS(Fig. 4a), which is homogeneous and isotropic. SCL-01 and SCL-02 were sampled during the excavation process with extremely high water content(Fig. 4b), in which SCL-02 was sampled at 1 m lower than SCL-01(Fig. 4c). ZQ was sampled after the excavation while the Ming Dynasty SCL was settled and stable. All these samples are mainly in block-shaped (Fig. 4d). Another series of sample was sampled at every 1 m from the altitude of 60 m to 70 m, numbering with H-60, H-61, …, H-69 and H-70 to provide the geoarchaeological ground truth of Zhouqiao AS(Fig. 4a).

Fig. 4: Position of sampling and samples.
figure 4

a Layers at Zhouqiao AS and area of sampling. b The status of SCL immediately after excavation (when SCL-01 and SCL-02 were sampled), with extremely high water content. c Cracking and flacking after the layers were settled and stable. d Sample ZQ.

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Analysis Methodology

Physical and mechanical properties

The basic physical properties of soil were analysed according to the Chinese National Standard the Standard for Soil Test Methods (GB50123-2019), including water content, dry density, liquid limit, plastic limit, plasticity index, direct shear strength, unconfined compressive strength, and disintegration tests, permeability tests and shrinkage tests implemented.

Particle size analysis

The particle size of the sample was analysed using a Mastersizer MS2000 laser particle size analyser, water as dispersant, and size range: 0.02 to 2000 nm.

Pore size analysis

MicroActive Autopore V 9600 mercury intrusion instrument, with maximum and minimum pressures of 0.5 to 60000 psia and pore size range of 5 nm to 800μm. Among them, SCL-01 was first dried by LABCONCO FreeZone 12 L Console Freeze Dryer, with the cold hydrazine temperature at -54 °C and the vacuum degree at 0.016 mbar, following with Mercury Intrusion Porosimeter (MIP) test. The surface area and pore size (3 to 100 nm) of the sample was determined using a BET analyser Autosorb-iQ (Quantachrome instruments, USA) and using ASiQwin software. Nitrogen gas adsorbed on the samples was used to determine the surface area. Degassing was performed at 200 °C.

Optical microscope

The microscopic morphology of the sample was observed using a super-depth microscope LEICA DVM6 from Leica, Germany.

X-Ray Fluorescence Spectrometer (XRF)

Bruker S2 X-ray fluorescence spectrometer from Germany; after grinding, the sample was mixed with boric acid and pressed into a tablet, and its oxide content was determined.

X-Ray Diffractometer (XRD)

Bruker D8 X-ray diffractometer from Germany. Measurement conditions: K-ray with Cu as target, wavelength 1.5418 Å, working tube voltage 40 kV, working current 40 mA, scanning start angle 10°, end angle 80°, step length 0.02 mm.

Scanning Electron Microscope – Energy Spectrometer (SEM-EDS)

FEI QUANTA-650 environmental scanning electron microscope (SEM) and EDAX APOLLO-X energy spectrometer (EDS). Experimental conditions: high vacuum mode, working voltage 25 kV, secondary electron image and backscattered image, energy spectrum scanning time 30 s, working distance 10 mm.

Ion Chromatograph (IC)

Dionex ICS1100 ion chromatograph from U.S., sample preparation process is as follows: First, weigh 10 g of the dried sample to be tested, 0.500 g (accurate to 0.001 g) screened with a 0.2 mm (80 mesh) sieve, place it in a 250 mL beaker, add 50 mL of deionised water, ultrasonically extract for 15 min, and shake once every 5 min to ensure complete solid phase separation. Then place the solution in a 50 mL centrifuge tube, let it stand for 10 min, centrifuge it at 4000 r/min for 10 min in the centrifuge tube, filter the solution after centrifugation through a 0.45μm microporous filter membrane, and then filter it again with a C18 solid phase extraction column to obtain the sample extract, and finally perform ion chromatography detection and analysis.

Simultaneous Thermal Analysis (TG-DSC)

NETZSCH STA 449F3 from Germany, with a starting temperature of 35 °C, an end temperature of 1000 °C, and a heating rate of 10.0 (K/min).

Internal Structure

Gulmay portable DR X-ray detector from U.K., HR2530 portable digital imaging system; experimental conditions: ray focus size 3 mm, maximum ray energy 270 kV, X-ray projection mode is directional, radiation angle is 40/85 degrees. Detector imaging area 350 mm×430 mm, resolution 3072×3840. Working environment: -20 to 40 °C, 5% to 80%.

Results

Internal structure

Under an optical microscope, it can be seen that the surface of the sample is flaky (Fig. 5). In addition to obvious cracks, there are also many microcracks. In addition, the SCL at the site shows severe cracking, so it is necessary to first clarify the stability of each block formed by the cracks and its internal structure. To this end, an X-ray test was attempted on different sides of the test block (Fig. 6). There are many microcracks inside the test block, which are crisscrossed and in different directions; they are inconsistent with the apparent morphology of the test block. These microcracks are potential cracking points. Under the influence of the long-term microenvironment, they will gradually expand and gradually evolve into cracks or directly fall off in blocks. This requires higher requirements on the protection and repair of cracks.

Fig. 5
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Optical micrograph of Sample ZQ (100×).

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Fig. 6: Result of x-ray test of sample ZQ.
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a Front view true-colour image; b, c Front view x-ray image; d side view true-colour image; e, f Side view x-ray image; Dash lines in the same colour indicate the same crack in different photography conditions.

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Physical and mechanical properties

Basic physical properties

The basic physical properties of the soil samples of the SCL were analysed, and the results are shown in Tables 1 and 2. Sample ZQ were collected two years after the excavation of the site. With the volatilisation of water, the water content of the soil samples after the layer stabilised was 2.60%, the shrinkage limit is 10.88%, the plastic limit is 18.73%, the liquid limit is 34.95%, the plasticity index is 16.22, the shrinkage factor is 0.402, the compressive strength is 1.03 MPa, the internal friction angle is 18.63°, and the cohesion is 9.28 kPa. According to the engineering classification method of soil in the standard for soil test methods GB50123-2019 (GB 2019), the powder particles in the soil sample of the SCL account for 81.30%, which belongs to fine-grained soil; then, because the plasticity index is ≥10 and the liquid limit is <40, it should be low liquid limit silty clay. According to the particle size distribution curve, D30 is 17.825μm, D60 is 37.735μm, the uniformity coefficient Cu is 7.745 ( ≥5), and the curvature coefficient Cc is 1.728, that is, the soil is well-graded soil. Yue, Gao20 analysed the basic properties of soils in other SCL of Zhouqiao AS. Their liquid limit, plastic limit and plasticity index were significantly different from those in the deposit layer, indicating that the soil in the SCL was different from that in other layers. Zhu, Li21 tested the silt in the riverbed of the Yellow River in Zhengzhou. It had a water content of 71.20%, a density of 2.59 g/cm3, mass fraction of organic matter of 5.10%, a liquid limit of 63.60%, a plastic limit of 26.40%, a plasticity index of 37.20 and a particle diameter of about 1000 μm. The XRD results indicated that there is quartz, albite, montmorillonite and illite. Li, Li22 analysed the silt from the Yellow River’s bed near Ji’nan. The D10, D50 and D90 of the silt were 75.9, 126, and 206 μm, respectively. The particle size distribution was concentrated around 120 μm; the plastic limit was 7.84%, the liquid limit was 23.56%, the plasticity index was 15.72, and the loss on ignition was 5.62%. From a study about the characteristics of soil particles and their relationship with soil organic carbon components in the alluvial/sedimentary region of Kaifeng-Zhoukou, the particles in the soil particles were mainly 10-50 μm and 50–250 μm in diameter, with an average content of about 65%. The high content of particles with diameter <1000 μm was the critical factor affecting the soil properties23. By comparing the data of the silt from different section of the Yellow River (Table 3), it was clear that the liquid limit, plastic limit and plasticity index varied, which may be related to the sedimentation process of the silt.

Table 1 Analysis data of sample’s particle diameter
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Table 2 Basic physical properties of the sample
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Table 3 Critical water content of other soil samples
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Pore characteristics

MIP (Fig. 7) shows that the porosity of SCL-01 at water content of 27.2% is 37.1%, total pore area is 9.084 m2/g, median pore diameter (volume) is 363.16 nm, and the most probable pore diameter is 433.41 nm. To better understand the distribution of pores, Mehta and Monteiro24 divided the pore size into four ranges: gel micropores ( <4.5 nm), mesopores (4.5 to 50 nm), middle capillary pores (50 to 100 nm) and large capillary pores ( >100 nm). According to the MIP test results, mesopores (4.5 to 50 nm) accounted for 11.47%, middle capillary pores (50 to 100 nm) accounted for 7.87%, and large capillary pores ( >100 nm) accounted for 80.72% of SCL-01 of the SCL. And, ZQ is compact, as its porosity is 30.8%, total pore area is 11.379 m2/g, and median pore diameter (volume) is 159.12 nm, and the most probable pore diameter is 182.99 nm. Mesopores accounted for 20.82%, middle capillary pores (50 to 100 nm) accounted for 13.50%, and large capillary pores ( >100 nm) accounted for 65.68% of ZQ of the SCL. Large capillary pores were obviously decreased during the dehydration process of the samples. The numbers of mesopores and middle capillary pores in the sample increased, whereas the porosity of the sample decreased. The high content of macropores in the sample indicates that although the alluvial soil is relatively dense with good air permeability with the external environment25, so that changes in the microenvironment of the site will also affect the interior of the SCL through the macropores. The proportion of capillary pores is relatively smaller, indicating that there is a certain capillary effect in the SCL. Capillary water can travel freely in the pores inside the SCL, providing a channel for water and salt migration and freeze-thaw. By analysing the gel micropores and mesopores of ZQ by nitrogen adsorption desorption (NAD), it showed that the adsorption isotherm of the sample was Type IV with an adsorption hysteresis loop (Fig. 8). The adsorption was caused by two factors: multi-molecular layer adsorption on the pore wall and coagulation in the pore26. It can be inferred that ZQ contains slit pores with layered structure, according to the shape of the hysteresis loop27. The BET specific surface area (SSA) of ZQ was 40.958 m2/g, and the BET adsorption constant C was 172.919. Both values were high, indicating that the potential surface hydration degree of the soil sample was high and the adsorption force was strong. The total pore volume of ZQ is 0.0835 mL/g for pores smaller than 94.0 nm in diameter at P/P0 = 0.97912. The average pore diameter of ZQ is 8.153 nm. According to the BJH model and Kelvin equation, the pore size distribution of ZQ shows that mesopores contain a large amount, especially pores size of 3 to 5 nm. The pore size is distributed in a unimodal form, and the pore distribution in the soil has a certain continuity28.

Fig. 7: MIP diagram of the sample.
figure 7

Curve a: water content at 2.60% (Sample ZQ); Curve b: water content at 27.2% (Sample SCL-01).

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Fig. 8: NAD diagram of the sample.
figure 8

a Nitrogen adsorption-desorption isotherm; b distribution of pore size.

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Disintegration

The soil sample of the SCL becomes wetted while dropping a small amount of water on the surface, and then tiny layered cracks appear in the sample. When place the sample in water, it initially disintegrates in flakes. This consists with the thin layered structural characteristics of silt. This disintegration test is implemented in accordance with GB50123–2019. The calculation of disintegration follows the following equation:

$${A}_{t}=\frac{{R}_{t}-{R}_{0}}{100-{R}_{0}}\times 100$$
(1)

while \({A}_{t}\) is the disintegration amount in percent of the sample at moment \(t\), \({R}_{t}\) is the scale reading at the level of the float at moment \(t\), and \({R}_{0}\) is the instantaneous stable reading of the scale at the level of the float at the beginning of the test. The disintegration amount and time are plotted, and the slope represents the disintegration speed. It is clear from the Fig. 9 that the disintegration process is rapid. The sample immediately disintegrates after being placed in water, and the disintegration amount reaches 50% at 5 min. The disintegration speed is fastest between 5 and 6 min, and the disintegration amount is 100% at 7 min.

Fig. 9
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Plot of sample’s disintegration amount to time.

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Water permeability

The permeability coefficient k is a quantitative indicator of the permeability of the soil. The k value of sample was obtained through variable water level method. According to the test result (Table 4, Fig. 10), the k value distributes ranging from 10–7 to 10–8, which means the permeability of the sample is low. With the increase of the porosity ratio, the changes of the k value become significant and the curve trend becomes steeper. This indicates that the surface of the soil particles has a thin water film and the effective porosity is high. It leads to a strong relevancy between the k value and the porosity ratio. Fitting the scatter plot shows that the k value and the porosity ratio are in a power function relationship, and the fitting degree reaches 99.4%. By solidifying to the silt from the Zhengzhou and Ji’nan sections of the Yellow River with MICP, the k value of the silt was significantly reduced in Zhao, Wang29’s research. The k value of their sample is lower than that of conventional silty clay ranging from 10–5 to 10–6 30. Its reason can be either their sample contains a large amount of calcium carbonate, or the sample contains a very high content of silt particles (81.30%).

Table 4 Porosity Ratio and Permeability Coefficient under various Consolidation Pressure
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Fig. 10
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Plot of sample’s permeability coefficient to porosity ratio.

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Chemical properties

XRF and XRD

The component the samples’ oxides, which is mainly composed of SiO2, Al2O3 and CaO and Fe2O3, are shown in Table 5. By comparing among the SCLs and other layers at the AS, the content of CaO and Fe2O3 at SCLs are high. XRD result shows that ZQ has a mineral composition of quartz, calcite, kaolinite and illite, with a high content of calcite (Fig. 11). According to the previous researches (Table 6), the silt of the Yellow River near Zhengzhou, the content of SiO2 is greater than 60%, which means the high content of sand, and the contents of CaO, Fe2O3 and MgO are lower than those of the sample from the SCL of Zhouqiao AS. The content of SiO2 in the sample is 48.8%, indicating there are less sand in the SCL of Zhouqiao AS than that in ordinary Yellow River silt in the region. CaO, Fe2O3 and MgO were mixed into the silt when it was forming and drying. In the process of forming calcite, part of the Mg replaced the Ca in it, showing a phase of calcium magnesium carbonate ((Ca0.97Mg0.03)(CO3)2) in XRD results.

Table 5 XRF results of the samples
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Fig. 11
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XRD spectrum diagram of Sample ZQ.

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Table 6 Content of oxides of the Yellow River’s silt from various region
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SEM-EDS

The SEM result (Fig. 12) shows that the soil particles in the sample are fine, the edges are round, and the particle size difference is small. The soil is uniform with unobvious boundary between particles and pores is not obvious. Its structure appears a flocculent aggregate. The pore is in various size and different shape. There are two types of pores, which are among aggregates or inside aggregates. Microcracks inside the sample are observed, but no obvious needle-shaped or flake-shaped microstructures of calcium carbonate are found. The EDS result (Table 7) indicates that the main elements of the sample are Al, Si, Ca and O. The Ca content is relatively high.

Fig. 12: SEM images of Sample ZQ.
figure 12

a 2000× of SEM. Area 1, 2 and 3 are the locations to implement EDS analysis. b 3000× of SEM, showing the microcracks in the sample. c 5000× of SEM.

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Table 7 EDS Result of ZQ
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TG-DSC

According to the result (Fig. 13), the mass loss of the SCL can be divided into three stages. In the first stage, the temperature is below 400 °C with mass loss rate of 3.58%, where the water and organic in the sample lost. The second stage is 400 to 750 °C where some minerals undergo phase change and decomposition with mass loss rate of 8.86%. A notable endothermic peak is shown at 720.4 °C with a rapid mass decrease of 7.36%. This is primarily attributed to the decomposition of carbonates and sulfates, releasing carbon dioxide and sulfur dioxide gases. The last stage is 750 to 1000 °C, where the mass loss remains almost constant. The mass remains at 85.57% over 1000 °C. This again proves that the calcium carbonate content in the SCL of Zhouqiao AS is much higher than that of ordinary Yellow River silt22.

Fig. 13
figure 13

TG-DSC diagram of Sample ZQ.

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IC

The result (Table 8) indicates that the sample contains following three anions, with relatively low ion concentrations. This explains why no disruption and salt efflorescence by the soluble salts’ crystallisation-dissolution cycle has been found on the surface of the SCL of Zhouqiao AS.

Table 8 Content of anions of ZQ
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Source of calcium carbonate

According to the XRF results in Table 5, the samples of most layers at Zhouqiao AS contain over 5% of CaCO3, among which H-66 and H-67 are with the highest content. These two samples represent the SCL of Ming Dynasty, indicating that indicating that the Ming Dynasty SCL contains more CaO than other layers at the site. In the other words, CaO at the Ming Dynasty SCL has other sources besides the geological environment of Kaifeng. Considering that this layer was formed by the siltation of the Yellow River flood and there is no extremely high CaO in other Yellow River silts, the high content of CaO at the Ming Dynasty SCL is believed caused by dissolving lime from the buildings in Kaifeng City during the flood.

According to the record in the Complete Classics Collection of Ancient China, the city of Kaifeng was rebuilt in early Ming Dynasty of 1368. City wall had a circumference of 20 li and 195 steps (about 11.8 km) with a height of 3 zhang 5 chi (about 9.76 m), enclosing an area of about 13.1 km2. According to the record about the Kaifeng’s great flood in 1642 in the same book, the flood submerged the entire city till the level of battlement on the city wall. It can be estimated that the depth of flood was the heigh of the city wall which was approximately 9.76 m, so that the amount of water of the flood was 1.28×108 m3. The solubility of calcium carbonate in water was 0.0014 g/mL. Therefore, the amount of calcium carbonate dissolved in flood in Kaifeng was 1.79×108 kg. The excavation area of Zhouqiao AS is 4400 m2, and the average thickness of the Ming Dynasty SCL is about 2 m. Assuming that the deposition process of CaCO3 is uniform, the content of CaCO3 in each cubic meter of Ming Dynasty SCL can be estimated by applying Eq. 2, whose result is 6.83 kg.

$${m}_{s}=\frac{{m}_{z}}{{S}_{z}\times {h}_{s}}=\frac{{m}_{c}\times \frac{{S}_{z}}{{S}_{c}}}{{S}_{z}\times {h}_{s}}$$
(2)

where, \({m}_{z}\) is the total amount of CaCO3 at Zhouqiao AS in kilogram; \({S}_{z}\) is the excavated area of the site, which is 4400 m2; \({m}_{c}\) is the total amount of CaCO3 dissolved in the flood, which is 1.79×108 kg; \({S}_{c}\) is the walled area of Kaifeng City, which is 13.1 km2; \({m}_{s}\) is the amount of CaCO3 per cubic metre in the Ming Dynasty SCL, and the \({h}_{s}\) is the average thickness of the Ming Dynasty SCL, which is 2 m.

The density of this layer is 1680 kg/m3, based on which it can be estimate that the proportion of CaCO3 in the soil is around 4.06%. According to the above estimation, the high content of CaCO3 at the Ming Dynasty SCL was formed as a result of comprehensive effect of flood of the Yellow River and the local geological environment of Kaifeng. It is expected to be further clarified by refining the content and distribution of calcium carbonate from the SCLs from other archaeological sites in Kaifeng in the coming research.

Causes of cracking

The Ming Dynasty SCL was formed by the flood and sedimentation of the Yellow River, being consisted of the unique soil in China. Its cracking status is different from other layers at the site. Soil is a deformable multiphase porous material, whose dehydration cracking is a comprehensively affected by a series of factors, such as dehydration, volume shrinkage, cracking, etc. The factors affecting expansive soil’s cracking are believed as soil properties, boundary constraints, environmental conditions and admixtures31. For SCL, its cracking phenomenon is similar to that of expansive soil, but the mechanism differs. It is significantly depended on its initial water content and the content of clay particles, CaCO3 and organic matters. The reason for this phenomenon is reasoned as the following:

Initial water content

The higher the initial water content, the greater the amount of water filling the intergranular voids, so that the intergranular shrinkage is more obvious when water content decreases. It further leads to greater the soil shrinkage, and causes greater the shrinkage deformation. The initial water content and shrinkage deformation are in an exponential function relationship31. The stable ground level in the area of Zhouqiao AS is at a depth of 6.4–7.4 m (64.16–65.25 in altitude), and the historical record of highest is about 1.0 m deep (70.41 m in altitude). This high groundwater level in the area led a high initial water content of the SCL as 37.20%, which produced extremely high shrinkage deformation during the dehydration process. This is considered as the most critical reason for the obvious cracking of the SCL.

Content of clay particles and CaCO3

On the one hand, clay has a great influence on soil shrinkage and cracking mechanism. The loss of water in the macropores result the shrinking of soil and cracking caused by dehydration32. The illite and kaolinite contained in the silt are both layered hydrous aluminum silicate minerals, which will shrink significantly when the water evaporates. On the other hand, the excessive CaCO3 in the SCL does not cement the soil particles as the clay minerals do, reducing the cementation among soil particles. This is the internal factor leading to crack.

Content of organic matters

The SCL has a high content of organic matter. The mass loss rate at 500–950 °C in TG-DSC is about 7%, and that of the SCL at Xinzhengmen AS is about 5–10%12. The high organic matter content and large SSA make SCLs have strong water absorption and water retention capacity and high porosity33. The high content of macropores intensifies the exchange between the soil body and the external environment, and the high content of mesopores contributes to significant capillary effect, which result in serious cracking.

Discussion

The excavation of Zhouqiao AS confirms the constancy of Kaifeng’s central axis in more than a thousand of years, acting as a symbol of the local history of this palimpsest city. The site is conserved and protected in situ for its outstanding cultural significance. As a rare difficult heritage, the Ming Dynasty SCL at the site is an archaeological evidence of the historic Yellow River flood in 1642, which submerged the entire city. Its serious cracking and shedding, which is completely different from other layers at the site, leads necessarily and urgency to implement research and property analysis about it. The physical, mechanical and chemical properties of the Ming Dynasty SCL’s soil are comprehensively understood through this research. Microscopic observation and X-ray detection revealed that there are a large number of microcracks inside the silt, which are potential cracking points.

The properties of the layer’s soil are analysed, which is low liquid limit silty clay, with 81.30% powder particles, low permeability and fast disintegration rate. The soil is mainly composed of macropores, followed by mesopores, and a minor micropores. It contains high content of calcium carbonate and organic matter content, and very little soluble salt content. The calcium carbonate was formed during the formation process of the SCL, and part of the calcium was replaced by magnesium.

As the comprehensive effect of these factors above, the silt cracks extremely seriously and the high initial water content is believed as the most critical factor. This causes great challenges to the in-situ conservation and preservation of the archaeology site.

Moreover, this research estimates the deposited calcium carbonate content based on the volume of flood in Kaifeng in 1642 and discusses its possible source. Samples of SCLs from different archaeological sites in Kaifeng will be studied to further explore the source and formation process of calcium carbonate.