In this paper, extensive resonant column tests were conducted to investigate dynamic responses of subsea sandsilt mixtures. The effects of confining pressure, mixture ratio, curing age, and cement content were evaluated. For the test condition considered in this study, the measured damping ratio is the smallest when the ratio of subsea sand to silt is in a range of 1.5 to 2.0. Moreover, unsolidified subsea sandsilt mixed at a ratio of 1.5 has almost the same maximum shear stiffness as the pure sand. For solidified subsea sandsilt mixture, cement can significantly increase the dynamic shear stiffness when the curing age is less than 14 days. However, the increase of the maximum dynamic shear stiffness is negligible when the curing age is longer than 14 days. When the cement content is 2%, the damping ratio of the solidified mixtures is very close to that of the unsolidified mixture. When the cement content is higher than 4%, the damping ratio of the solidified mixtures reduces significantly. This is mainly due to hydration reactions occurring in the solidified mixtures.
Coastal levees are normally constructed to protect coastal areas, which are vulnerable to suffer the natural disasters such as typhoons and tsunamis. Construction of coastal levees requires a lot of finegrain soils, but coastal areas lack these types of soils. The coastal levee studied in this paper is located in the coastal area of Fujian Province in China. A survey shows that there are tremendous amounts of subsea sands and silts. The subsea sand is coarsegrain soil which has high strength, low compressibility, and high permeability, while the silt is finegrain soil which has low strength, high compressibility, and low permeability. Subsea sand is used mainly as a filler in highway engineering and in concrete materials for building engineering [
Because coastal levees are vulnerable to suffer typhoons and tsunamis, to evaluate the safety and serviceability of coastal levees, dynamic responses of filler materials should be investigated. Many studies have been conducted to investigate the dynamic responses of sand and clay. Hardin and Black [
A mixture containing subsea sand and silt is a new type of artificial soil. As far as the authors are aware, limited studies have been conducted to investigate the dynamic responses of subsea sandsilt mixtures. In this study, the dynamic shear stiffness and damping ratio of subsea sandsilt mixtures were explored by extensive resonant column tests. The effects of confining pressure, mixture ratio, curing age, and cement content on the dynamic shear modulus and damping ratio of the mixtures were analyzed.
Figure
GZZ50 resonance column testing apparatus.
Because the experimental investigation in this paper is based on the project in Fuzhou, the subsea sand was taken from the sand yard in Jiangyin, Fuzhou, while the silt was taken from the sea entrance in Minjiang of the eastern Lang Qi Island, Fuzhou. Once the subsea sand and silt were collected, they were sealed with two layers of plastic bags to minimize the water evaporation. By conducting indoor geotechnical tests, the engineering properties of subsea sand and silt are summarized in Tables
Properties of subsea sand.
Natural density, 
Specific gravity, 
Water content 
Maximum void ratio, 
Minimum void ratio, 
Relative density, 

2.57  2.66  62  0.76  0.52  0.35 
Properties of silt.
Specific gravity, 
Rate of water content 
Liquid limit 
Plastic limit 
Plastic index 

2.74  63.0  40.6  23.6  17 
Figure
Particle size distribution curves of subsea sand and silt.
Nonuniformity coefficients and curvature coefficients of subsea sand and silt.
Soilsamples 






Subsea sand  0.49  0.26  0.096  5.1  1.4 
Silt  0.0048  0.0019  0.0008  6.0  0.94 
In this paper, the dynamic shear stiffness (
The subsea sand was first sieved (i.e., controlled diameter was 2 mm). Then, the subsea sand is mixed with silt at ratios (i.e., the ratio of the mass of subsea sand to the mass of silt) of 2 : 1, 1.5 : 1, 1 : 1, 1 : 1.5, and 1 : 2. A summary of resonant column tests of unsolidified mixtures is shown in Table
Dry density and water content of the mixtures.
Ratio of sand/Silt  Subsea sand  2 : 1  1.5 : 1  1 : 1  1 : 1.5  1 : 2  Silt 

Controlling dry density (g/cm^{3})  1.59  1.38  1.34  1.28  1.22  1.18  0.98 
Water content (%)  6.2  12.3  16.0  20.2  26.9  32.1  63.0 
To minimize friction between soil and the chamber, a layer of Vaseline was applied on the inner wall. The inner diameter and height of the chamber were 50 mm and 100 mm, respectively. Based on the designed soil density, each soil sample was prepared by five layers to ensure the uniformity. After compaction, the surface of the last layer of soil was ensured to be smooth. Each sample was placed in a sealed box filled with wet sand for 24 hours. For the solidified mixtures, they were kept in a clear water tank.
The vacuum machine was used to saturate each soil sample. Each soil sample was placed in the vacuum chamber for at least 2 hours. Then, deaired water was added in the vacuum chamber and each soil sample was submerged underneath the water for at least 10 hours. For solidified mixtures, soil samples were kept in the deaired water for 7 days, 14 days, and 28 days (i.e., curing age), respectively.
Because dynamic responses of mixtures at each controlled confining pressure are similar, only dynamic shear stiffness and damping ratio under the confining pressure of 100 kPa are presented in this section.
Figure
Variation of measured
Figure
Variation of
From the discussion about Figures
In this study, the adopted testing apparatus can only capture the dynamic soil responses at a strain level of 10^{−4}∼2 × 10^{−2}%. The dynamic responses of soil at large strains need to be predicted by empirical equations. Based on numerous tests, Hardin and Drnevich [
By using the modified Hardin–Drnevich model to fit the measured dynamic shear stiffness at small and medium strains, the dynamic shear stiffness at large strains can be deduced. Since stiffness degradation curves (
Variation of fitted
Figure
Variation of
Figure
Variation of fitted
When silt is mixed with subsea sand, small particles of silt fill the pores among the particles of subsea sand. For cases that the ratio of subsea sand to silt is less than 1.5, with an increase in the proportion of silt, the damping ratio is improved and the dynamic shear stiffness and maximum dynamic shear modulus have little change under the confining pressure of 100 kPa. However, when the content of silt continues to increase, superfluous particles of silt cannot fill the porosity of subsea sand. The dynamic shear stiffness, maximum dynamic shear modulus, and the damping ratio begin to decrease due to silt’s poor geotechnical properties. For all the tests conducted in this study, the shear stiffness is almost the same as the pure sand when the subsea sandsilt mixture ratio is equal to 1.5. At this mixture ratio, the damping ratio is also small. It is demonstrated that again subsea sandsilt at a mixture ratio of 1.5 is a reasonable choice. Because coastal levees are vulnerable to suffer typhoons and tsunamis, dynamic loads, such as earthquake or wave actions, should be considered. At this time, higher quality subsea sandsilt mixtures are needed. In order to improve the strength and stiffness of filling materials, cement is added to solidify the mixture.
Based on the dynamic response of unsolidified mixture, the subsea sandsilt mixed at a ratio of 1.5 gives good engineering properties. Thus, the ratio of subseasand to silt of solidified mixtures is selected as 1.5. Because variations of dynamic shear stiffness of solidified mixtures at different curing ages and confining pressures are similar, only the measured dynamic shear stiffness at curing age of 7 days and confining pressure of 100 kPa is presented in Figure
Variation of measured
In order to evaluate the solidification effects, the maximum dynamic shear stiffness of solidified mixtures is compared with the unsolidified mixtures. Herein, growth rate of the maximum dynamic stiffness
Figure
Variation of
Growth rate of
Curing age (d)  Confining pressure (kPa) 



2%  4%  6%  8%  10%  
7  100  18.75  56.25  106.25  157.75  193.75 
200  11.86  35.59  65.25  100.85  124.58  
300  5.66  20.75  37.11  57.23  71.70  
400  3.03  11.61  23.73  36.36  43.43  


14  100  32.50  82.50  132.48  190.23  230.17 
200  19.49  53.39  85.59  116.10  136.44  
300  11.32  26.42  48.43  70.44  84.28  
400  6.57  16.67  31.31  45.45  52.52  


28  100  37.50  96.25  151.25  202.75  241.25 
200  24.58  61.02  94.49  128.81  145.76  
300  13.21  37.74  62.26  81.76  91.12  
400  8.58  21.71  42.93  54.55  62.63 
Based on the dynamic shear stiffness of solidified mixtures at small and medium shear strains, the shear stiffness at large strains can be deduced from the Hardin–Drnevich model. Figure
Variation of fitted
Figure
Variation of measured
Based on the measured damping ratio at small and medium strains, the damping ratio of solidified mixtures is deduced from the Zhang model. As shown in Figure
Variation of fitted
Microstructural studies of solidified mixtures show that their mechanical behavior is governed primarily by hydrates. Hydrates are the reaction products among a binding agent, minerals in the soil, and water. After hydration reactions, the hydration products (e.g., calcium silicate hydrate, hydrated calcium aluminate, and calcium hydroxide) are formed. On the one hand, these products can fill the pores among the mixture particles and produce a network structure that is stronger than the original soil skeleton. On the other hand, through ion exchange and pelletization, aggregates possessing good compactness are formed. Those gels and aggregates can bind particles together and generate a strong chain structure. Therefore, the dynamic shear stiffness, maximum dynamic shear modulus, and the damping ratio of the mixture can be improved by mixing it with cement. Note that a low cement content has limited effects on dynamic responses of solidified mixtures, and a large cement content corresponds to a stiff soil.
In this paper, extensive resonant column tests were conducted to investigate dynamic response of subsea sandsilt mixture. The effects of confining pressure, mixture ratio, curing age, and cement content were evaluated. Based on the measured results, the following conclusions may be drawn:
As the ratio of subsea sand to silt is increased, the dynamic shear stiffness of unsolidified soils increases significantly when the ratio is less than 1.5. By further increasing the subsea sandsilt ratio, the increase of dynamic shear stiffness is negligible. For the test condition considered in this study, unsolidified subsea sandsilt mixed at a ratio of 1.5 has almost the same maximum shear stiffness as the pure sand.
Since there are many pores in pure sand, the damping ratio of sand is much larger than unsolidified subsea sandsilt mixture. By adding the silt to fill the pores among sand particles, the damping ratio of subsea sandsilt mixtures is decreased. When the ratio of subsea sand to silt is in a range of 1.5 to 2.0, the measured damping ratio is the smallest. In terms of shear stiffness and damping ratio, the optimal ratio of subsea sand to silt is suggested to be 1.5.
For solidified subsea sandsilt mixture, cement can significantly increase the maximum dynamic shear stiffness when the curing age is less than 14 days. However, the increase of the maximum dynamic shear stiffness is negligible when the curing age is longer than 14 days. It implies that the appropriate curing age is 14 days.
When the cement content is 2%, the damping ratio of the solidified mixtures is very close to that of the nonsolidified mixture. If the cement content is higher than 4%, the damping ratio of the solidified mixtures reduces significantly. This is mainly due to a series of hydration reactions occurring in the solidified mixture.
The data used to support the findings of this study are available from the corresponding author upon request.
There are no conflicts of interest regarding the publication of this paper.
This work was sponsored by the Fundamental Research Funds for the Central Universities (nos. 2019B44114 and 2018B57014) and the National Natural Science Foundation of China (no. 51808193). The authors are grateful for this support.