Research on the Pore Characteristics of Dredger Fill By Preloading

The granularity composition of soil refers to the percentage of various sizes particles in the soil [4]. It can be seen from Fig. 1 that after adding dispersant agent that the content of clay group in the soil samples increased, while the content of silt group decreased. It shows that the dredger fill contains aggregates formed by the combination of clay and clay, clay and silt [5]. The experimental results show that the non-uniformity coefficient of soil samples is greater than 5 which show the gradation is good. The composition of different particle sizes shows that the soil sample can be pore reduction after compaction.

The four soil samples are mainly silt and clay which reached more than 90% from Table 1. The sand and clay content of the 01 soil sample are the highest. The silt content of soil sample has increases as the depth increasing, and the clay content showed a decreasing trend. The main reason is the different material sources.

In this paper, semi-quantitative analysis and identification of some soil samples were carried out by X-ray powder diffraction [6]. From Table 2: the primary minerals of soil samples are mainly quartz, and the secondary mineral content is higher proportion, about 50%.The clay minerals is mainly illite and illite–smectite mixed layer. And contain a small amount of kaolinite and chlorite. This is also consistent with the results of particle test. The clay content of the analyzed soil sample is large. Because the clay has good hydrophilicity, it will increase the difficulty of soil drainage consolidation.

During the compression and consolidation of dredger fill, the distribution and variation of pores is one of the important factors to measure the compressive deformation. The distribution and variation of pores in soil will show significant differences with the different degree of consolidation [8].

This is the No. 9500 experimental principle: suppose the pores of the soil are cylindrical holes and the radius is r, press liquid mercury into the pores by instrument. The pressure of mercury and repulsion force reached equilibrium.where p is pressure, r is pore radius, σ is surface tension coefficient of mercury, 0.485 N/m, α is mercury wetting angle on Materials, 130°.

Porosity Distribution. It can be seen from Table 3 that the dredger fill after preloading, with the depth increasing, the overall trend of porosity is decreasing. This is because the water in the dredger fill gradually was drained out of the pores under the action of the upper preloading load. Soil particles moved and some small particles was filled in the pores of the lower soil layer, which made the soil structure more and more dense. It can be seen that the porosity of soil samples decreases rapidly from surface to 11 m and it remains virtually unchanged from 11 to 17 m by comparing the rate of porosity reduction of soil samples at different depths. It can be confirmed that the preloading effect was better within a certain depth range as the depth increased. And beyond that, the effect is worse gradually.

Interval Distribution of Aperture. It can be seen from Fig. 2: the cumulative volume fraction of pores size less than 0.01 μm is small. In the range of 0.01–0.1 μm, distribution curve of 01 soil sample shows an obviously increasing trend. The curves of 02, 03 and 04 soil samples did not change significantly. In the range of 0.1–1 μm, the curves of 02, 03 and 04 soil samples showed a significant upward trend. But the growth rate of 01 soil sample curve is slightly slow; When the aperture is larger than 1 μm, the four curves all showed an upward trend. But growth rate slowed significantly and slowly approached 100%. It can be inferred that there are mainly medium and small pores in preloading dredger fill while less macropores. It is mainly due to the high clay content of the soil.

The microstructure of dredger fill is irregularity and complexity. But its pore distribution is self-similarity. Therefore, the pores of dredger fill can be analyzed by fractal theory [9]. Assume that the pore is a sphere with radius r. V_P is the pore volume with radius less than r.where N represents the number of pores with radius less than r; D denotes the dimension of pore distribution; A is a constant.

In double logarithmic coordinates, draw the cumulative volume fraction content classification curve of soil sample pore size-pore. The fractal dimension D of pore distribution can be determined. If the slope of the straight line in the curve is K, then D = 3 − K [9].

It can be seen from Fig. 3, the curve changes obviously when the aperture is 0.06, 0.4, 4, and 40 μm. According to this, the curve can be roughly divided into 5 broken line segments. It shows that the distribution of pores has multifractal properties. Each line segment shows that the pore diameter has self-similar properties in the interval. Therefore, according to the self-similarity of pores and the experience of predecessors, the pore size can be divided into 5 levels: (a) Micropores: d ≤ 0.06 μm. (b) Small pores: 0.06 μm < d ≤ 0.4 μm. (c) Medium pore: 0.4 μm < d ≤ 4 μm. (d) Large pores: 4 μm < d ≤ 40 μm. (e) Extra-large pores: d > 40 μm.

Through the cumulative distribution curve and the above pore size division, the volume fraction of the corresponding aperture interval can be obtained. The results are shown in Fig. 4. From the figure, there is a huge different in the volume fraction concentration range of the aperture interval with depth. No. 01 soil sample is taken from the surface. The concentration range of pore volume fraction is wide, roughly 0.06–10 μm. With sampling depth increasing, the concentration range of 02 and 03 is smaller than 01. Mainly concentrated in the 0.06–4 μm interval. But the concentration range of the deepest 04 soil sample has increased compared with the front two samples. The results show that: in a certain depth range, the pores larger than 4 μm in the soil sample are crushed into smaller pores with the increase of soil depth after dredger fill preloading. The pores less than 0.06 μm decreased. It may be because small pores connect into slightly larger pores with the discharge of water from the soil. As a result, the volume fraction of pores in this interval decreases. When the depth is greater than a certain value, the influence of preloading gradually decreases. The pores larger than 4 μm show an increasing trend. It has a certain relationship with the influence depth of preloading treatment.