Study on Microstructure and Silting Characteristics of Soft Soil: a Case Study of Nansha District, Guangzhou

. With the development of society and economy, land subsidence in soft soil areas has become a prominent geological disaster in our country's coastal area. Land subsidence has the characteristics of irreversibility, wide influence range, long aging, etc., and these characteristics are closely related to the characteristics of soft soil, such as physical and mechanical parameters, microstructure characteristics, etc. The Nansha District of Guangzhou was taken as the study area in this paper. Through the collection of preliminary data, two-dimensional scanning electron microscopy, and three-dimensional Computed Tomography (CT), the micro-structure characteristics of soft soil are expounded and its plugging characteristics are further analyzed. The results show that the study area's soft soil has a honeycomb, granular, and flocculent structure. From the perspective of pore scale distribution, the number of macropores and ultramicro pores is small, and the continuity is poor. In most small and medium pores, namely the aggregate and granular pores. The siltation of soft soil is closely related to the microstructure of soil and can be treated according to the effect of siltation characteristics on the consolidation settlement of soft soil. The research results of this paper can provide targeted prevention and control suggestions for the soft land subsidence disaster in the Nansha area, and also provide a reference for other similar research.


Introduction
Soft soil generally refers to a soft plastic to fluid plastic clay with large natural water content, high compressibility, and low carrying capacities, such as silt, silty soil, and other saturated viscous soil and silty soil with high compressibility [1][2][3]. Soft soil is widely distributed in all parts of our country, such as delta facies deposits of soft soil in the Shanghai area of the Yangtze River and the Guangzhou area of the Pearl River [4].
The thickness and engineering characteristics of soft soil vary in different areas of different settling environments. Deep soft soil (thickness greater than 10m) is widely distributed in the southern part of Guangzhou due to its unique geographical location and complex stratigraphic conditions [5][6][7]. Land subsidence in soft soil has gradually become one of the main geological disasters in Guangzhou, which not only causes cracking and tilting of building structures, distortion of roads, deformation, and cracking of municipal pipes but also poses a threat to the safety of residents in soft soil construction areas.
With the development of the economy, urban land is gradually tense, and construction activities in soft soil areas are increasing day by day. Therefore, the strengthening and research of soft soil foundations have gradually become a hot topic. In the past, there have been various researches on the mechanism of land subsidence and corresponding achievements have been made [8][9][10]. In this paper, aiming at the silting problem which often occurs in the process of foundation drainage and consolidation, the micro-structure characteristics of soft soil are analyzed by scanning electron microscopy and three-dimensional CT, and the siltation characteristics are further elaborated. Finally, the prevention and control suggestions are put forward.

2.1SEM
Scanning electron microscopy was applied in this study to analyze the microstructural characteristics of soft soils in different areas.
The experiment was carried out under the ∑IGMATM scanning electron microscope with a magnification of 2000 times. The SEM images were statistically analyzed by using PCAS software (particle and crack image recognition and analysis system). Some results are shown as follows: (1)Sample BNS13 Fig.1 and Fig.2 showed that there are generally large minerals with a particle size greater than 50um. Large minerals are mainly quartz crystals, while fine minerals are mostly silica aluminate. At the same time, Mg, Na, Cl, Fe, and other elements are filled or replaced in the mineral lattice.
According to the scanning electron microscope analysis diagram of BNS133 ( Fig. 3-4), it can be seen that some large-grained crystals were distributed in the soil sample, and there were 217 particles, of which 10% with a particle size of 0-1um and 93% are with a particle size of 0-4um. The pores of the soil sample occupy a large area and there were many macropores. The preliminary evaluation of permeability is good.
As Fig.5 shows that the distribution of particles in the direction of the BNS13 is basically uniform. Relatively few particles are arranged in the direction of 80°~110°, which is about half of the number of particles in other directions. Pore arrangement directions are mostly concentrated in the regions of -45° and -25°. In the same way, the scanning electron microscope analysis of the BNS167 sample showed large crystalline minerals were widely disturbing, with a maximum particle size of up to 200um (Fig.6). The colloids in the BNS167 sample are closer to the spheroid and rounder, compared to BNS133. Honeycomb structure can be observed in certain colloidal granules. It can be seen from the energy spectrum that the soil sample is rich in metal oxides and silicates. In the honeycomb structure, the bright zone is mostly silicate and carbonate or organic matter, while the dark zone is mostly filled with organic matter or cemented with carbon (Fig.7). The average particle size of soil is 2.9um and the average particle area is 6.3um 2 , in which the number of 1-5um particles accounts for 87.3% of the total (Fig.8). The average shape coefficient of particles is 0.559, and the roundness is high, especially for large particles.

2.2Three dimensions CT
In order to further analyze the structural relationship of the samples, three-dimensional X-ray scanning was performed by using the beamline station of X-ray imaging and biomedical application at the Shanghai Institute of Applied Physics. Phase recovery and slice reconstruction of the scanned two-dimensional images were carried out, and Avizo 3D visualization software was used for 3D structure reconstruction.
Two-dimensional CT scanning image information of the sample at different angles will be obtained since the sample is constantly rotating. The scanning process is collected at equal Angle intervals.
Pitre software (provided by Shanghai Light Source) is used for image processing and image segmentation, and a group of continuous two-dimensional images is arranged and reconstructed to form a three-dimensional data field. In this data field, the corresponding voxel vertices of all pore Spaces are labeled 1(or 0), while the corresponding voxel vertices of all soil particles are labeled 0(or 1). Then the shape, size, and distribution of the sample region, namely the three-dimensional microstructure of the entire sample volume, can be reconstructed. Some of the results are shown below: (1)JK11-1 Sample Fig.9 shows the three-dimensional microstructure of JK11-1 and granules are represented in cyan, pores in red, and minerals and organic matter in blue. It can be seen from the figure that pores and particle aggregates occur interactively, and organic matter dispersed in the soil is small in volume, mostly overlapping with pores or located at the junction of pores and mineral aggregates. The particles are more connected together due to the high content of clay minerals. The volume of pores in the sample is large, and the pore ratio is 1.98. The arrangement of pores has certain connectivity in the horizontal direction, while the vertical connectivity has great spatial differences. Organic matter and minerals are distributed discretely in the whole sample. (2)JK22-1Sample As Fig10 shows that pores and particle aggregates occur interactively, but the content of organic matter and mineral is small. Most of the particles are connected together, the contact mode is point contact and surface contact, and the connection of the particles in the horizontal and vertical is not much different; The volume of pores is large, the pore ratio is 2.32, and the connectivity of pores is good both vertically and horizontally. This indicates that the mineral and organic matter content in this area is small, and most of them are concentrated in the shape of lines in one corner of the sample.

Silting behavior of soft soil
With the rapid economic development in the southeast coastal area, the construction of the soft soil layer is increasing day by day, and the strengthening of the soft soil foundation is the primary work of the construction. Among various methods of soft soil foundation reinforcement, drainage consolidation is the most economical and reasonable method to treat soft clay foundation, so it is widely used. However, there are some problems. There will be silting problems in the process of drainage consolidation. Blockage is the phenomenon that particulate matter accumulates or deposits on the surface or internal pores of porous media under the action of seepage, which hinders the channel of water flow and leads to the decrease of the water conductivity of porous media, namely the decrease of permeability. Thus, the drainage and reinforcement effect of the soft foundation is not good, the consolidation period becomes longer, and the economic cost is increased. Besides, If the soft gene blockage causes incomplete reinforcement, it will also lead to slow drainage and consolidation of the building foundation for a long time after the completion of construction, resulting in uneven settlement and excessive settlement of the building foundation and other problems. Therefore, it is of guiding significance to understand the properties of silt and evaluate the degree of difficulty in plugging for evaluating the consolidation and drainage characteristics of soft soil foundations and predicting the settlement during the use of buildings.

3.1Long-term penetration test
A long-term infiltration experiment was set up to study the difficulty of silting in soft soil. The test soil sample has been in the seepage state, and the change curve of the soil permeability coefficient with respect to time was observed and recorded. Besides, the proportion of 0~1um particles in the upper and lower parts of the soil sample was tested after the infiltration experiment. The variation curve of the permeability coefficient over time is shown in Fig. 11-12.
As can be seen from the figure, in the process of penetration, the permeability coefficient will decrease with time, and the decreasing speed will also decrease. Finally, the curve of permeability coefficient with time presents a downward concave curve. The initial permeability coefficient of shallow silty soft soil (BNS133) is higher than that of deep silty soft soil (BNS167). The penetration coefficient of silty soft soil in the deep position drops slightly faster than that in the shallow layer during 1-30 days. During the 30-50 days period, the decreasing rate of the permeability coefficient in deep and shallow parts decreased somewhat, but the decreasing rate of the permeability coefficient in the deep part was still greater than that in the shallow part. It can be seen that the deep silty soft soil is easier to be silted in the process of drainage and consolidation. In order to facilitate engineering application and evaluation of siltation characteristics, an evaluation standard is proposed for the siltation characteristics of soft soil according to the decreased ratio of permeability coefficient in the course of the permeability test: (1) Very severe silting: permeability decreased to less than 10% of the original value; (2) Severe silting: permeability decreased to 10%~30% of the original value； (3)Moderate silting: permeability decreased to 30%~60% of the original value； (4)Slight silting:： permeability decreased to 60%~80% of the original value； (5)Very slight： Permeability decreased to more than 80% of the original value.
The three-dimensional model of the soil sample after the experiment was reconstructed. As shown in Fig.13, it can be obviously observed that the density of particles in the sewage head is higher than those in the upper head, indicating that the particles in the soil sample have shifted along with the seepage gradient in the long-term dialysis process. Figure.13. Three-dimensional structure of upper and lower head particles According to the particle size test, before the experiment, the particles with a diameter of 0~1um accounted for 7.30% of the total particles of the BNS167 sample. After the experiment, the particles with a diameter of 0~1um at the end of the upper head accounted for 7.00%, and the particles with a diameter of 0~1um at the end of the lower head accounted for 7.42%. In the BNS133 sample, before the experiment, the particles with a diameter of 0~1um accounted for about 10.00% of the total particles. After the experiment, the particles with a diameter of 0~1um at the end of the upper head accounted for about 9.72% and the particles with a diameter of 0~1um at the end of the sewer head accounted for about 10.14%.
Based on the results of the long-term permeability experiment of the two samples, it can be seen that the decrease rate of the permeability coefficient of the two soil samples is similar, which is between the slight and very slight blockage (BNS133 belongs to slight and BNS167 belongs to very slight). BNS133 sample has a high content of fine particles and a large number of percolation migrations. However, it can be seen from its microstructure that the internal pore ratio of the BNS133 soil sample is high and there are many large pores, making the soil sample's overall permeability good. More pores and extensive macropores also make it difficult for small particles to fill pores, which makes the BNS133 soil sample less prone to blockage than the BNS167 soil sample.

3.2Suggestions
According to the grading criteria of silty soft soil (very severe siltation, severe siltation, moderate siltation, slight siltation grade, very slight siltation grade). Thus, the study area can be reclassified into different siltation grad based on the According to the influence of siltation characteristics on the consolidation settlement of soft soil, it is relatively easy to consolidate and drain the soft soil in the slight siltation blocking areas Ⅰ and Ⅱ. The construction is carried out according to the engineering specifications, and the possibility of a large settlement is low in the later stage. The difficulty of drainage consolidation in the mild silting area of area Ⅲ is not great. The construction is carried out according to the engineering specifications, and the possibility of a large settlement is not high in the later stage of construction.

Conclusion
Taking Nansha District of Guangzhou as the study area, this paper conducted a systematic study on the particle characteristics and microstructure of soft soil by means of field preliminary data collection, two-dimensional scanning electron microscopy, and three-dimensional CT analysis, and discussed the relationship between its silting characteristics and its microscopic characteristics. The conclusions are as follows: 1.
The soft soil in the working area mainly has a honeycomb structure, granular structure, and flocculent structure. From the perspective of pore scale distribution, the number of macropores and ultramicro pores is small, and the continuity is poor. In most small and medium pores, namely the aggregate and granular pores. These particles are distributed in the void, and the surrounding particles fail to form a firm contact relationship, easy to flow with water seepage and accumulation in a place to form a blockage. 2.
The silting of soft soil is closely related to its microstructure. The number of small particles without forming colloidal particles, the number of macropores, the continuity of macropores, and the microstructure of organic matter and particles in the soil are all very important factors affecting silting.