Through the investigation and research on the topography, stratum lithology, geological structure, groundwater and surface water, and unfavorable geological phenomena of the site, it is found that: the fluctuations in groundwater level affected the movement of water and salt, thereby influenced and accelerate the deterioration, and also caused microbes, creatures, and plants to erode the ruins. The stratigraphy revealed good correlation with the status of the ruins, the scientific analysis of the samples, the damage mechanism.
2.1 Terrain and topography
The Shahe Ancient Bridge is located on the south bank of the Weihe River in Xianyang City. The ground elevation is between 389.76m and 390.23m, and the site is relatively flat. The geomorphic unit belongs to the first-level terrace of the right bank of the Weihe River. The Weihe Plain area is formed by the alluvial deposits of the Weihe River and is divided into four small geomorphic units, namely the Weihe floodplain, first-level terrace, second-level terrace, and third-level terrace. The first-level terrace is 0.5m higher than the river bed, and it is an alluvial deposit formed in the Holocene. It is mainly composed of gravel and pebble. The first-level terrace is 380-400m above sea level and is the stacked terrace. The upper part is sandy clay and the lower part is gravel and pebble layer.
2.2 Meteorology and Hydrology
The area where the Shahe Ancient Bridge site is located belongs to a warm temperate semi-humid continental monsoon climate area, with distinct cold, warm, dry and wet seasons, rich in light, heat and water resources. The area where it is located belongs to the Weihe River system of the Yellow River Basin and is located in the hydrogeological area of the river terrace. The Weihe River is the largest surface water system in the area. It flows from west to east along the southern edge of the municipal area. The water volume changes seasonally, with an average flow of 173m3/s. The river bed is wide and shallow, with the water depth of 3.0m in the flat water period, and the river bed ratio drops by about 1 ‰; while the Feng River is a first-order tributary, and flows into the Weihe River from south to north, the river bed width is 80-250m, the river bed ratio drops by about 8.2 ‰, and the average flow rate is 9.41m3/s.
The groundwater in the area where the Shahe Ancient Bridge site is located is mainly quaternary loose layer pore diving water and confined water: the diving water is mainly found in the Quaternary Upper Pleistocene and Middle Pleistocene alluvial sand soil, it is mainly supplied by atmospheric precipitation infiltration, irrigation and lateral runoff; the runoff direction is consistent with the terrain slope, flowing from southeast to northwest, and vertical quaternary loose layer pore water and shallow confined water are connected and supplied through overflow. The drainage methods are mainly evaporation, lateral runoff drainage, artificial mining, and overflow drainage of diving. The unit water inflow is 5-10m3/h*m, and the aquifer thickness is 34-46m. It is recharged by runoff from upstream rivers and runoff from the confined aquifer of the Loess Plateau, the confined aquifer is lacustrine sediments, and the lithology is mainly medium coarse sand and gravel; the thickness of the aquifer is 27-80m, and the unit water inflow is 1-10m3/h*m; the salinity is less than 230mg/L, which is fresh water; the terrace where the site survey area is located belongs to the strong water-rich area-strong water-rich area, and the water inflow is<2500-5000m³/d.
In the survey: it is found that the natural precipitation in the site area varies greatly from year to year, and the seasonal distribution is uneven; the groundwater level is closely related to the season, climate, groundwater storage, recharge and discharge. In summer, there is abundant precipitation and the water level rises significantly; in winter, precipitation decreases and the groundwater level drops accordingly. The underground diving level has an elevation of 343.00-344.40m and a water level of 5.20-5.80m. Although the upper layer of fine sand may have water stagnant in the upper layer, generally no engineering precipitation is required.
2.3 Geotechnical characteristics
The strata within the depth of exploration is described in detail as follows:
Following the geological stratigraphy observed in borehole, the figure 4 shows that the stratum where the site is located is mainly composed of Quaternary alluvial fine sand, coarse sand, gravel sand, boulder and silty clay.
(1) Pleistocene Q3 on the fourth system
①Fine sand Q3al: layer thickness 1.0-4.0m, buried depth 1.0-7.0m, bottom elevation 342.20-348.60m. ②Medium sand Q3al: layer thickness 4.00-9.60m, buried depth 7.20-18.1m, layer bottom elevation 331.20-341.50m. ③Gravel Q3al: layer thickness 2.00-3.10m, buried depth 10.30-12.20m, layer bottom elevation 336.60-338.40m.④Boulder Q3al: layer thickness 2.00-7.80m, buried depth 14.00-20.00m, layer bottom elevation 328.80-335.60m. The overall distribution of the stratum is stable, the sedimentation rhythm is clear, the uniformity of the foundation soil of each layer is good, the bearing capacity is gradually increased from top to bottom, and the geotechnical engineering properties are gradually improved from shallow to deep. The compressibility of the formation is relatively good, and the strength is relatively high. The whole field is basically stable-stable foundation. The foundation soil of shallow foundation is mainly Quaternary Upper Pleistocene alluvial layer① fine sand and ② middle layer sand. The deep foundation adopts pile foundation, and the stratum beside the pile is composed of fine sand, coarse sand, gravel sand, round gravel and silty clay. The pile-end stratum is dominated by silty clay and sand, and the silty clay layer is discontinuous. It is recommended to use ⑥ layer of fine sand as the supporting layer.
2.4 Permeability
Tab. 1. List of indoor penetration test results
Statistics items
Test items
|
Unit
|
Layer number
|
Maximum value
|
Minimum value
|
Average value
|
Evaluation
|
Vertical permeability coefficient(Kv)
|
cm/s
|
①
|
5.44×10-3
|
2.36×10-3
|
3.67×10-3
|
Medium permeable
|
②
|
5.77×10-2
|
5.31×10-3
|
1.31×10-2
|
Strongly permeable
|
③
|
1.73×10-1
|
7.82×10-3
|
8.98×10-2
|
Strongly permeable
|
④
|
6.76×10-1
|
9.84×10-2
|
3.66×10-1
|
Strongly permeable
|
⑤
|
3.48×10-1
|
1.34×10-5
|
1.16×10-1
|
Strongly permeable
|
In this investigation, the conventional physical and mechanical properties tests are conducted on the loess-like soil, ancient soil layer, silty clay and other soil samples. The layered statistics of the physical and mechanical properties of each soil layer; A standard penetration test was conducted to evaluate the permeability of each layer of sandy soil and cohesive soil distributed in the site. The statistical results of laboratory penetration test and particle analysis are shown in the following table. According to the permeability test results, it is found that the submersible water level of the site is 343.00-344.40m, and the buried water level is 5.20-5.80m. The groundwater is quaternary pore water, and the aquifer is the sand layer of the upper Pleistocene of the quaternary system with strong permeability.
2.5 Environmental water and soil corrosion et evaluation
As shown in the above Table 2, under long-term immersion conditions or alternating wet and dry conditions, the pH of the site's environmental water is 7.16-7.19, and the Cl- content is 20.9mg/L, and the total mineralization is 557.82-575.6mg/L. The environmental water in the site area is slightly corrosive. The pH of the soil samples at the site is 8.67, and the Cl- content is 40.2-43.6mg/kg, the total soluble salt is 499.1-1128.8mg/kg, and the soil samples in the site are slightly corrosive.
Tab. 2. List of groundwater corrosion evaluation
Aquifer
|
Corrosion evaluation
|
Environment type
|
Corrosive medium
|
Content
|
Limit value
|
Corrosion level
|
Pore diving
Groundwater depth 5.20~5.80m
|
Corrosion of water to concrete structure according to environmental type
|
Ⅱ
|
Sulfate contentSO42-(mg/L)
|
153.6~165.1
|
<300
|
Micro
|
Magnesium contentMg2+(mg/L)
|
23.3~27.7
|
<2000
|
Micro
|
Ammonium contentNH4+(mg/L)
|
0.2
|
<500
|
Micro
|
Caustic content OH-(mg/L)
|
0.0
|
<43000
|
Micro
|
Total salinity (mg/L)
|
557.82~575.6
|
<20000
|
Micro
|
Corrosion of concrete structure by permeable water
|
A
|
PH value
|
7.16~7.19
|
>6.5
|
Micro
|
AggressiveCO2(mg/L)
|
0.0
|
<15
|
Micro
|
HCO3-(mmol/L)
|
3.75~3.80
|
>1.0
|
Micro
|
Corrosion to reinforcement in reinforced concrete structures
|
Long-term immersion
|
Cl-content (mg/L)
|
20.9~20.9
|
<10000
|
Micro
|
Wet and dry alternation
|
100~500
|
Micro
|
It is comprehensively inferred that the water level rises significantly during summer precipitation, but due to the protection of the greenhouse from the surface water in the area of the site, the evaporation is greater than the recharge. With the process of groundwater evaporation produced a significant phenomenon of soluble salt migration, especially for the geological environment of sandy soil layer in the site, on the one hand, the good permeability of the site soil enhanced the evaporation rate of groundwater and accelerated the speed of salt migration. More importantly, compared with the site soil, the capillary tubes of wooden bridge piles are better water transport channels, the truth is, choosing a more convenient channel is anyone would make, that was for the above reasons that the current preservation status of wooden bridge piles just verifies this inference.