Effects of Moisture Content and Plasticity Index on Duncan-Chang Model Parameters of Hydraulic Fill Soft Soil

In order to explore the effects of moisture content and plasticity index on Duncan-Chang model parameters K, n, C and Rf, we selected 8 groups of soft soil with water content of 69.1% 94.3% and plasticity index of 32.2 54.1 for triaxial unconsolidated undrained shear test. The results show that Cuu, K and n values all showed a downward trend, and Rf variation was not obvious with the increase of moisture content. The variation rule of each parameter is not obvious with the increase of plasticity index. When moisture content is constant, Cuu and n values do not change much, K increases with the increase of plasticity index within the range of 70% 80% moisture content, and does not change much with the increase of plasticity index when moisture content is greater than 80%, Rf has no obvious rule. When the plasticity index is constant, Cuu, K and n decrease with the increase of moisture content, Rf has no obvious rule. The maximum value of Cuu is 20.18 kPa, the minimum is 3.72 kPa, and the maximum to minimum ratio is 5.42. The maximum value of K is 0.517, the minimum is 0.022, and the maximum to minimum ratio is 23.5. The maximum value of n is 1.198, the minimum is 0.150, and the maximum to minimum ratio is 7.99. The maximum value of Rf is 0.872, the minimum is 0.679, and the maximum to minimum ratio is 1.28.


Introduction
The constitutive relation of soil is the rule to reveal the internal mechanical properties of laterite with different compactness, and obtained the Duncan-Chang model parameters. Chen Wei et al. [2] studied the stress-strain curves and other mechanical properties of compacted loess under different basic indexes through triaxial test, and obtained the Duncan-Chang model parameters. Liao Hongjian et al. [3] obtained the stress-strain curves of remolded saturated loess through triaxial test and analyzed the influencing factors of Duncan-Chang model parameters. Wu Nengsen et al. [4] studied the influence of moisture content on Duncan-Chang model parameters of granite residual soil. Yang Xuehui [5] studied the Duncan-Chang model of unsaturated remolded loess under different conditions, and gave the intensity index of unsaturated remolded loess and the change rule of Duncan-Chang model parameters with the initial conditions. Zdravkovic and Jardin [6] discussed the anisotropy of the rotation of the middle principal stress direction caused by consolidation, and pointed out that it had obvious influence on the strength and deformation of soil, and then affected the parameters of Duncan-Chang model. Cokca et al. [7] studied the effects of moisture content on the cohesion and internal friction angle of Duncan-Chang model of unsaturated clay. Cheng Ying et al. [8] analyzed the triaxial test results of marine and fluvial soft soils by using Duncan-Chang model, and gave the change rules of model parameters and dry density. Based on the triaxial test results, Wang Shuai [9] established Duncan-Chang model of coastal soft soil with different initial degree of consolidation.
It can be seen from the above results that there are few studies on the influence of moisture content and plasticity index on Duncan-Chang model parameters of soft soil. Therefore, this experiment mainly studies the influence of moisture content and plasticity index on the parameters of Duncan-Chang model.
Firstly, the basic physical indexes of selected soil samples were determined.
Then, Duncan-Chang model parameters were obtained by triaxial unconsolidated and undrained test. Finally, the relationship between water content and plastic limit on Duncan-Chang model parameters was analyzed. E. Z. Chen et al.

Project Profile
The test soil samples were taken from a blow fill site located in tidal flats and shallow sea areas. The landform is mainly alluvial plain, and the landform unit is delta, which belongs to the tidal landform type of estuary, sand mouth and sand island. The main strata from top to bottom include: backfill sand, silt (blowing), coarse sand (blowing), fine sand (blowing), silt-silt soil (blowing), clay-silty clay, silt soil-clay. The sampling area is mainly located in the silt distribution area, and the sampling depth is 5 -15 m below the surface. Related physical indicators are shown in Table 1.
Plasticity is the characteristic of clay, which reflects the degree of interaction between clay and water. Figure 1 shows the plasticity diagram of the test soil sample. As can be seen from Figure 1, the soil samples are near line A, and all

Introduction to Duncan-Chang Model
Kondner pointed out in 1963 that the stress-strain curve of soil triaxial test can be fitted by hyperbola: ε is the axial strain. , a b are the test constant, and a is the inverse of the initial tangent deformation modulus i E . b is the inverse of the limiting deviant stress ( ) where: a P is atmospheric pressure, with a P value of 101.4 kPa, and the dimension is the same as 3 σ ; f R is the damage ratio; , c ϕ is the cohesion and internal friction Angle of the soil sample; ,

Test Results and Analysis
The effects of moisture content and plasticity index on the parameters of Duncan-Chang model was studied by triaxial unconsolidated undrained shear test.
The model parameters of the whole group are obtained by taking the experimental data of the third group of soil samples as an example. The relationship between deviator stress difference and axial strain of soil samples in group 3 is shown in Figure 2 (3) and (4), i E and f R are further obtained. f R is averaged, as shown in Table 2.  It can be seen from Table 2 that parameters C uu, K and n of Duncan-Chang model of soft soil fill have a large variation range, while R f has a small variation range. The average value of R f is 0.773, and the coefficient of variation is 0.08. From Figures 5-7, it can be seen that C uu, K and n basically show a downward trend with the increase of moisture content, and there is no obvious rule with the change of plasticity index. Figures 8-10 are the cloud diagram of Duncan-Chang model parameters C uu, K and n changing with water content and plasticity index. Figure 8 shows that when the moisture content is constant, the C uu does not change much. When the plasticity index is constant, C uu decreases with the increase of moisture content.        The maximum value of C uu is 20.18 kPa when the moisture content is 69.1% and the plasticity index is 38.0. The minimum value of C uu is 3.72 kPa when the moisture content is 84.9% and the plasticity index is 53.6. The maximum to minimum ratio is 5.42. Figure 9 shows that when moisture content is constant, K increases with the increase of plasticity index within the range of 70% -80% moisture content, and changes little with the increase of plasticity index when moisture content is greater than 80%. When the plasticity index is constant, K decreases with the increase of moisture content. The maximum value of K is 0.517 when the moisture content is 76.8% and the plasticity index is 45.1. The minimum value of K was 0.022 when the moisture content was 84.9% and the plasticity index was 53.6.
The maximum to minimum ratio is 23.5. Figure 10 shows that when the moisture content is constant, n changes little.
When the plasticity index is constant, n decreases with the increase of moisture content. The maximum value of n is 1.198 when the moisture content is 69.1% and the plasticity index is 38.0. The minimum value of n was 0.150 when the moisture content was 94.3% and the plasticity index was 51.7. The maximum to minimum ratio is 7.99.

Conclusions
Through 1) With the increase of moisture content, C uu , K and n values all showed a downward trend, and R f variation was not obvious. With the increase of plasticity index, the variation rule of each parameter is not obvious.
2) When water content is constant, C uu and n values do not change much. K increases with the increase of plasticity index within the range of 70% -80% moisture content, and changes little with the increase of plasticity index when the moisture content is greater than 80%, R f has no obvious rule. When the plasticity index is constant, C uu , K and n decrease with the increase of moisture content, R f has no obvious rule.
3) The test constant K, whose maximum value is 23.5 times of the minimum value, is most significantly affected by moisture content and plasticity index. The influence is relatively small for C uu and test constant n, the difference between the maximum and minimum values is 5.42 and 7.99 times respectively. The smallest influence is R f , whose maximum value is only 1.28 times different from the minimum value.