3.1 Structure of Fascine Body
In water conservancy projects, "fascine" is used as a kind of hydraulic construction, it is used for firewood, bamboo and wood and other soft materials, mixed with earth and stone, and rolled as shown in Figure 2A, several fascine bodies are connected with ropes and piles in revetment, blockage, closure and other projects. The pictures found in the Shahe bridge ruins are consistent with the traditional craftsmanship. The silt block is wrapped in a mat-like weave and is related to a man-made river bank into the embankment. The silt layer is called "fascine bank", that is, a bank built with fascine body. We mark the edges of each small block in Figure 2B and C in Figure 2D and E respectively, and we preliminarily determine that the “fascine bank” range is the river bank at that time, and the northern boundary of the bridge body is the newly discovered northernmost row of bridge piles. We can basically determine the length and scope of the No. 1 Shahe Ancient Bridge. According to the newly discovered silt layer accumulation and the range of bridge piles, it can be preliminarily determined that the plane where the silt layer is located is the river bank, and the northern boundary of the bridge body is bounded by the newly discovered northernmost row of bridge piles. Excavation was carried out on the south side of the protection greenhouses and continued to explore southward, but no new bridge piles were found. The above results confirm that the width of Shahe ancient bridge is 16 meters, and the northern end is 22 meters away from the protection hall. In 1989, when the Shahe Ancient Bridge was excavated for the first time, the C14 dating method was used to determine the age of the No. 1 Bridge about 2350-1910 years ago. In this research, we have dated two sets of braided fabric samples wrapped in a silt layer, the age is about 2210-2058 and 2230-2072, which are slightly later than the bridge piles, but the difference is not very large. The C14 dating results represent the growth age of the sample, not the construction age (Table 1). In the construction of bridge piles, it takes a long time for the trees to be planted to mature and then felled for the construction of the bridge piles, and there is often a big gap between the growth age and use age, it takes a long time for trees to grow until they are grown and then felled for use in constructing bridge piles. There is often a big gap between the age of growth and the age of use. and the soft materials used in the braid are mostly plants with a shorter growth cycle, and their growth age is close to the age of use, and the dating results of the bridge pile and the braid generally differ between 100-200 years, which is consistent with the tree’s time required for growth. According to the dating results of fascine material, it can be determined that the construction age of the Shahe Ancient Bridge is the transitional period of Qin and Han Dynasties.
Table 1. C14 dating.
3.2 Microscope Slice Observation
The surface of the Shahe ancient bridge sample is dark yellow-brown, the section is charcoal black, and there is a small amount of soil particles attached. The shape is a long strip with a thickness of about 0.5 mm, the middle part is slightly recessed inward, and both sides are slightly higher. The texture is weak and fragile (Figure 3A). The control sample is modern fresh bamboo branches (Figure 3B). As shown in Figure 3C, in the longitudinal section microstructure of the Shahe ancient bridge sample, no wood anatomical structures such as wood rays, ducts, etc. are observed. It is a typical vascular bundle structure with parallel solid beams on the periphery, irregular in shape and uneven thickness, and it may come from thick-walled fibers. As shown in Figure 3D, there are some well-preserved parenchyma cells in the vascular bundle in the cross-section of the Shahe ancient bridge sample, but the fiber bundles are connected into one piece and there are breaks, and the middle layer cannot be distinguished. The above characteristics are consistent with the anatomical structure characteristics of bamboo, and it can be determined that the plant samples unearthed at the Shahe Ancient Bridge ruins are bamboo.
3.3 Infrared Spectroscopy and X-ray Diffraction Analysis
As shown in Figure 4A, the absorption peak at 1732 cm-1 is derived from the C=O stretching vibration on the acetyl group, which is the characteristic absorption peak of hemicellulose which is different from other components. The peak was not detected in the infrared spectrum of the Shahe ancient bridge sample. Moreover, the cellulose C-H bending vibration peak at 897 cm-1 was not detected in the infrared spectrum of the Shahe ancient bridge sample. The above results show that compared with fresh bamboo, the cellulose and hemicellulose of the Shahe ancient bridge sample are both significantly degraded.
XRD was used to determine the crystallinity of fresh bamboo and Shahe ancient bridge samples (Figure 4B), using segal's calculation method, namely:
I2002θ = 22° Iam2θ = 18°
In the formula: CrI is the relative crystallinity; I200 is the maximum intensity of the diffraction surface; Iam represents the intensity of the non-crystalline background diffraction when the 2θ angle is close to 18°.
In the diffraction intensity curve of the Shahe ancient bridge sample, the strong diffraction peak at 16.1° disappeared, which indicates that the unit cell structure of the cellulose crystal area in the Shahe ancient bridge sample has changed during the long-term underground burial process. The calculation shows that the relative crystallinity of fresh bamboo is 65%, while the relative crystallinity of the Shahe ancient bridge sample is 29%, indicating that the degradation of cellulose mainly occurs in the crystallization area during the long-term underground burial of the Shahe ancient bridge sample.
3.4 Thermogravimetric Analysis
It can be seen from Figure 5A that the pyrolysis process of the Shahe ancient bridge sample can be divided into 4 stages. The first stage is from room temperature to about 200 ℃, at this stage, only water is separated out, and the weight loss is small, there is a small weight loss peak at 109 ℃, which is the drying stage; the second stage is 200-220 ℃ without obvious weight loss, mainly a small amount of depolymerization, restructuring and glass transition occurred inside the sample, which is the pre-pyrolysis stage; the third stage is about 220-410 ℃, which is the main reaction stage, this stage is mainly due to the mass loss caused by the pyrolysis of cellulose, hemicellulose and lignin, the mass change in this stage accounts for about 84% of the total process. The fourth stage is the residue pyrolysis stage, the temperature range is 450-900 ℃, in this stage, the TG curve declines slowly, mainly due to lignin cracking, entering the slow weight loss stage.
As shown in Figure 5B, the DTG curve of fresh bamboo has a shoulder peak at 316°C. Since hemicellulose starts to decompose in a large amount below 350 ℃, cellulose mainly decomposes at 315-400 ℃, and the pyrolysis peak of lignin only appears after 400 ℃, so this shoulder peak mainly corresponds to the decomposition of hemicellulose. The shoulder peak does not appear in the Shahe ancient bridge sample, indicating that the hemicellulose in the Shahe ancient bridge sample is poorly preserved.
3.5 SEM analysis
Bamboo wood is used as a flood control material and is deposited along with the sand and gravel in the river bed. After long-term deposition, the surface takes the form of bamboo, but the inside has been carbonized. It can be seen from Figure 6 (A) and (B) that the surface and cross-sectional morphology of the bamboo carbonized layer have been completely carbonized into a hard and dense layered structure. The thickness of the carbonized layer with a certain regularity is 2-4 µm, and it is speculated that the layered structure is caused by the growth rings of bamboo. Fiber carbonization has a very good effect on preventing microbial degradation. Compared with other woods, bamboo contains a higher content of starch, sugar, protein and relatively less extractives such as resin, wax, and tannin, so it is more susceptible to microbial erosion, the degradation occurs, and the preservation state is worse. The collected samples found traces of fine fibers in the gaps between the carbonized layers, as shown in Figure 6 (C) and (D). Presumably, due to the partial carbonization of the surrounding fibers, the preservation of the fine fibers is greatly promoted.
3.6 EDX spectrum analysis
Figures 7A and 7B show the energy spectrum analysis results of the carbonized layer of the bamboo sample and the bamboo fiber, respectively. It can be seen that the main components of the carbonized layer are C and O, and their component contents are 65.72% and 33.24% respectively, and there are very small amounts of Mg, Al, Si, Ca, Mn, Fe and other trace elements. Compared with the fiber layer, the element types are basically similar. Its main components are C and O, and the content is 64.63% and 28.18% respectively. The increase in the content of Ca and Fe elements is speculated to be caused by the pollution of the sand on the surface.
3.7 Microbiological analysis
The Internal Transcribed Spacer Identification results show that the dominant microbial populations are different between the fascine material and bridge pier fungi isolated, as shown in Figure 8. The microbial population of bridge pier mainly includes Trichoderma longibrachiatum, Trichoderma harzianum and Aspergillus flavus. SEM and colony morphology are consistent with ITS analysis results. Fascine materials microorganism are mainly Aspergillus versicolor, Talaromyces amestolkiae, Lecanicillium aphanocladii. and the results were consistent with colony observation and SEM. The results above showed that the microorganism populations are different for both the fascine material and bridge pier, reminding us of the independence of the environment in fascine material and bridge pier.