Study on Deformation Characteristics and Mechanism of Q3 Loess Microstructrue Before and After Wetting

: Microstructure characteristics of loess is an effective way to study the physical and mechanical characteristics of soil under different conditions. Water content in sample of Q 3 loess is about 25% ,which was first subjected to X-Ray test to determine the main chemical components in loess and calculated the proportion in the sample. Test analysis shows the particle structure of the loess belonged to the granular structure type. CT scanning tests were performed on the sample before and after wetting. The changes of total number of pores, the maximum pore volume and the position of the centroid of pore of the sample were analyzed before and after wetting. The results shows that the soluble salts of loess have been dissolved after wetting.The volume of the pores of loess have been changed. Some increased, some decreased and others closed in volume. The change of the number of pores indicates that the sample formed obvious saturated zone, transition zone, conduction zone and humid zones after wetting. These results reveals the deformation characteristics and mechanism of Q 3 loess microstructure.


Induction
The microstructure is one of the basic characteristics of loess and reveals the formation conditions of loess, which is an effective way to study the physical and mechanical characteristics of soil and its variety under different conditions.Natural particles in loess are mainly composed of powder particles, with obvious large pores, and most of the soil particles are cemented by soluble salt crystals.The water sensitivity and dynamic vulnerability of loess are remarkable.The soluble salts in loess are caused to dissolve by rainfall infiltration, and the micro-cracks in loess changed in a certain.The water content have a great influence on cohesion and angle of internal friction, and the strength of loess can sharply decrease when it encounters water.
There are mainly two methods to study the collapsibility of loess: firstly, the qualitatively or quantitatively [1][2][3][4][5][6] study the collapsibility of loess from the opinions of the soil composition, microstructure characteristics and mechanical mechanism of the loess by laboratory tests.Polarized light microscopes [7] was used to conduct preliminary research on the structural of loess, and obtained some simple qualitative conclusions.Scanning electron microscope and mercury intrusion test [8][9][10] were used to conducted in-depth research on the microstructure characteristics and pore structure of loess, the classification of loess microstructure were proposed from different views, and were examined the relationship between microstructure and collapsibility of loess.Experimental study [11][12][13] has been conducted to reveal quantitative on the microstructure of loess.
Secondly, carrying out in-situ large-scale test pit immersion test or immersion load test,which can directly obtain the self-weight collapse deformation and the lower limit depth of the loess on the site [14][15][16][17][18][19][20][21][22][23] .However, the method is so relatively little development due to its high cost and time-consuming, the workload is large.
However, how does the structure of the Q3 loess change after the rain is humidified, how does the collapsibility process occur, what is the microstructure deformation and its mechanism.In response to these problems, the microstructure deformation and mechanism of the Q3 loess after wetting was carried out to study .Firstly, the chemical composition of the loess was determined by X-Ray.Secondly, the microstructure type of the loess was analyzed by the scanning electron microscope test.Thirdly, the immersion was scanned by CT, change in micro-cracks of loess samples were examined before and after wetting.

Microstructure test of loess
The sample of loess came from a natural loess slope in Shan-zidun Village, Lanzhou New District, and the soil belongs to Q3 Malan loess.

X-Ray test of loess
X-Ray is as shown in Figure 1. Figure 2 shows the XRD pattern of untreated loess.By comparing with the standard PDF card, except for the peak of quartz, the components of calcite (Calcite, CaCO3, PDA#47-1144) accounts for a significant proportion, while magnesium is usually generated with calcium magnesium calcite.Table 1 shows the proportion of the main chemical components of Q3 loess.Q3 loess contains the highest silica content, which accounts for about 64.18% of the total chemical substances.The content of calcium and magnesium is relatively high.These are found in Q3 loess mostly in the form of calcite and dolomite.The soluble salt of Q3 loess appears mainly in the form of calcareous nodules.

Image with magnification 500
Fig. 4 500 scanning image of SEM Scanning images of SEM in the Figures 4-5 shows that the structure of Q3 loess are an obvious granular structure, and its particles are mostly plate-shaped and long columnar, and most of the particles are single-grain structures.The particles are mostly point-to-point contact with clear pores.Under the action of earthquake load, the overhead structure of Q3 loess support is prone to damage, small loess particles fall into the pores, and the large pore structure is squeezed into a microporous structure.Loess particles constitute the skeleton of the soil, which is the main factor determining the physical and mechanical properties of loess.

Image with magnification 1000
The framework particles play a supporting role in the loess, and their contact mode reflects the structural strength of the loess and is one of the important factors that determine the physical and mechanical properties of the loess.The water sensitivity of loess refers to its special sensitivity to water.In the natural hydrated state, loess has high strength and low compressibility, but when it is immersed in water or humidified, its strength drops sharply and its deformation suddenly increases.This characteristic is collapsibility and undrain shear of loess.

CT scanning test of loess
One of the main characteristics of Q3 loess is that the soil contains a large number of pores.During the formation of Q3 loess and during the soil formation process after formation, a microstructure is formed.This is the production environment of loess and the geophysical and chemical effects in the diagenesis process.The concentrated reflection of biochemical effects.Particle morphology, pore characteristics and degree of cementation are concentrated manifestations of the microstructure characteristics of loess.

Test introduction
The dual-energy coal and rock scanning analyzer system is mainly composed of dual-energy X-ray sources, high-resolution amorphous silicon area array detectors, and computer 3D scanning to build an image processing system as shown in Figure 6.

The loess sample
The height of loess sample is about 4.31cm, the diameter is about 1.78cm, the soil weight is 30.9g, and the moisture content of the sample before wetting is about 5%.According to the results of the field rainfall test, the sample was permeated with water content of about 25%.The samples before and after wetting are shown in Figure 7.The Figure 8 are the CT scanning tests of the samples before and after wetting respectively.A total of 45 scanned images are taken, and the distance between each layer is about 0.86mm.The pores of collapsible Q3 loess are characterized by large numbers and large spaces.According to the pore size, they can be divided into three types: ultrafine pores, fine pores and large pores.Ultrafine pores and fine pores are also called intergranular pores.Intergranular pores constitute the main body of loess porosity, and the pores are slightly larger than the diameter of loess particles or aggregates.The occurrence of collapsibility causes damage to the loess structure, causing these intergranular pores to increase, decrease, or tend to close, and loess particles slip and fall into the large pores.

Analysis of the the maximum pore volume in CT image before wetting
Fig. 13 The maximum pore volume before wetting

Analysis of the maximum pore volume in CT image after wetting
Where V1 is change of maximum pore volume before and after wetting, a is the scanning layers of loess sample.

The affected area of loess collapsibility
The influence range of collapsibility (wetting radius) can be calculated by the following formula [24] : Where r is wetting radius after collapsing, H is thickness of collapsible soil layer,mβ is the coefficient of increase of water spreading to the side because the water permeability of each layer and interlayer is different, the variation range is 1~2, β is the diffusion angle from the flooding site to the side flooding; for loess and loess-like silt,β=35°;For loess-like silty clay,β=50°.
The penetration depth of Q3 loess is directly related to its micro-cracks, and it is also the fundamental factor determining the penetration depth.When Q3 loess meets water, water would first infiltrate along the cracks in loess, and the larger penetration of the cracks is , the deeper infiltration is.The soluble salt content of loess also have an important effect on the depth of soaking.After wetting, some of the soluble salts of loess would dissolved, new cracks would formed and the depth of soaking would increased .

Discussion
Figure 2 shows the high content of calcite and albite.The chemical formula of calcite is CaCO3, the chemical composition of CaO accounts for 56.03%, and CO2 accounts for 43.97%.It often contains Mn and Fe, easy to break into small square pieces along its cleavage.The chemical formula of albite is Na2O•Al2O3•6SiO2, and its theoretical chemical composition is Na2O accounts for 11.8%, Al2O3 accounts for 19.4%, SiO2 accounts for 68.8%.The appearance is generally white or off-white, the hardness is 6~6.5, and the density is 2.61~2.64g/cm 3 .The calculation of the chemical composition of the loess in Table 1 is consistent with the analysis in Figure 2.
Figures 13 and 14 show the change of the maximum pore volume of Q3 loess before and after wetting respectively.Some of pore volumes of loess increase significantly after immersion, which indicates that the soluble salt in loess dissolves and new micro-cracks are formed.
Figure 15 shows that in the upper half of the sample, the number of pores increased significantly after wetting.The increase in the number of pores decreases rapidly with the increase of the height of the sample.Near the middle of the sample, the pores after soaking are smaller than the number of pores before wetting.After that, the number of pores after soaking remains basically constant.This indicates that the soluble salt in loess has a close relationship with the height of the sample.The closer to the end of the sample, the higher the degree of dissolution.After wetting, the distribution of the largest pores along the height of the sample is relatively uniform, and new pores are formed after wetting, and from the top to the bottom of the sample, it indicates that the original pores increase, decrease or close after the soluble salt is dissolved in water.Part of the pores are connected to form new seepage channels, and water can infiltrate from these pores more easily.
Figure 17 shows the change of the maximum pore volume.After wetting, some increase and some decrease.The change of the maximum pore volume fully shows that the micro-cracks of loess has changed after wetting.It is an effective way and means to analyze loess collapsibility.

Conclusions
By carrying out X-Ray, scanning electron microscope, CT scanning analysis of loess samples before and after wetting, and using Matlab software, the law of changes in micro-cracks of the samples before and after wetting was obtained, revealing the nature of loess collapsibility after wetting.
Through the X-Ray test, the content of the main chemical components of loess are obtained.A large amount of calcium and magnesium of loess is mainly stored in the formation of calcite and dolomite, and the chemical substances constitute the soluble salt and the insoluble salt.
Scanning electron microscopy tests showed that the structure of Q3 loess belongs to the granular structure type.
The loess samples were CT scanned before and after wetting.The test showed that the gray scale of the CT image becomes deeper after wetting, and the position of the largest pore is changed after wetting.The centroid of the largest pore of the sample before wetting is located in the upper part, and it is more evenly distributed in the whole sample after wetting.The total volume of the pores is significantly reduced, and the total volume of the upper water-immersed layer tends to stabilize.The changes of these parameters reveal that the grain shape, pore characteristics and degree of cementation of loess have undergone significant changes after wetting.The saturated zones, transition zones, conduction zones and humid zones have been formed after wetting of loess samples.The loess sample before and after wetting The loess sample before and after wetting Figure 7 The loess sample before and after wetting Figure 7 The loess sample before and after wetting     The loess sample before wetting between pore and image binaryzation Figure 11 The loess sample before wetting between pore and image binaryzation Figure 11 The loess sample before wetting between pore and image binaryzation Figure 11 The loess sample before wetting between pore and image binaryzation Figure 12 The loess sample after wetting between pore and image binaryzation Figure 12 The loess sample after wetting between pore and image binaryzation Figure 12 The loess sample after wetting between pore and image binaryzation Figure 12 The loess sample after wetting between pore and image binaryzation Figure 13 The maximum pore volume before wetting Figure 13 The maximum pore volume before wetting Figure 13 The maximum pore volume before wetting Figure 13 The maximum pore volume before wetting    The number of pore change before and after wetting Figure 15 The number of pore change before and after wetting Figure 15 The number of pore change before and after wetting Figure 15 The number of pore change before and after wetting Figure 16 The maximum pore volume before and after wetting Figure 16 The maximum pore volume before and after wetting Figure 16 The maximum pore volume before and after wetting Figure 16 The maximum pore volume before and after wetting Figure 17 The number of maximum pore volume change before and after wetting Figure 17 The number of maximum pore volume change before and after wetting Figure 17 The number of maximum pore volume change before and after wetting Figure 17 The number of maximum pore volume change before and after wetting

Fig. 6
Fig.6 Dual energy scanning analyse for coal and rock

Fig. 7
Fig.7 The loess sample before and after wetting Taking into account the flatness of the two ends of the sample, in order to obtain a more reasonable image of the internal structure of the sample, the scanned image was extracted from the 150th floor and ended at the 1050th floor.One scanned image is extracted for every 20 layers, and a total of 45 scanned images are obtained.The images from top to bottom are as follows.

Figure 1 X
Figure 1

Figure 2 XRD pattern of loess Figure 2 XRD
Figure 2

Figure 2 XRD pattern of loess Figure 2 XRD
Figure 2

Figure 6 Dual
Figure 6

Figure 9 Image
Figure 9

Figure 9 Image
Figure 9

Figure 10 Image
Figure 10

Figure 14 Maximum
Figure 14

Figure 14 Maximum
Figure 14

Figure 14 Maximum
Figure 14

Figure 14 Maximum
Figure 14

Table 1
The content of main chemical components of loess