Academic Journal of Science and Technology 
ISSN: 2771-3032 | Vol. 3, No. 2, 2022 

 

220 

Study	on	Mechanical	Properties	of	the	Compound	Soil	
of	Feldspathic	Sandstone	and	Sand	Based	on	Micro	–	
level	
Guangyu Lei 

Shaanxi Provincial Land Engineering Construction Group Co., Ltd, Xi'an 710075, China 

 

Abstract: It is of great practical significance to study the mechanical properties of composite materials of arsenic and sandstone 
to control the ecological environment in soft sandstone area. In order to understand the material and structural characteristics of 
composite materials under different compound proportions, the microstructure and the composition of the material elements 
were analyzed by the microscopic test and the test of mercury injection from the microscopic level. The dynamic three axis 
instrument was used to carry out the mechanical test under the different mixing ratio. The results showed that the main mineral 
components of feldspathic sandstone are quartz, albite, montmorillonite and calcite. The structure has the strength in the 
anhydrous state. When the water is saturated, the structure collapses, the cohesive force decreases rapidly, and it has the 
collapsibility; Due to the complexity of the structure and the optimization of the gradation, the permeability is greatly reduced; 
the macroscopic mechanics exhibits strain hardening and nonlinear characteristics. 

Keywords: Feldspathic sandstone, Mixed soil, Micro structure, Mechanical properties. 
 

1. Introduction 
Arsenic sandstone and sand are the main causes of soil 

erosion and desertification in Maowusu sandy land. Aeolian 
sand has loose structure, no cohesion, high porosity between 
grains, difficult formation and poor shear resistance; The 
arsenic sandstone is dense in texture, rich in silt particles, poor 
in cementation between particles, low in diagenesis, low in 
structural strength, poor in engineering performance, and 
loose as mud when encountering water, vulnerable to 
weathering and erosion. Therefore, the control of arsenic 
sandstone is an important step to prevent water and soil loss. 

In recent years, different scholars have made relatively 
mature research on the lithology, erosion, mechanical 
properties and physical and chemical properties of arsenic 
sandstone, such as its mixed soil with sand[1-4]. 

In this paper, starting from the micro level, through the 
composition analysis of the basic constituent minerals of 
arsenic sandstone, observation of the microstructure of the 
composite soil, understanding of the changes in the internal 
structure of the material after the combination of arsenic 
sandstone and sand, and according to the triaxial test to 
measure its macro mechanical properties, the internal causes 
of the macro characteristics of the composite soil are finally 
obtained. 

2. Basic Physical Properties of Arsenic 
Sandstone 

The physical and chemical properties of arsenic sandstone 
are mainly affected by its mineral composition and structure. 
Quartz and feldspar are relatively stable in chemical 
properties and are not prone to hydration reaction. However, 
montmorillonite and calcite are easy to be dissolved under the 
action of water due to their own microstructure characteristics, 
resulting in two phenomena: latent erosion and mechanical 
collapse. At the same time, the content of hydrophilic 
dolomite, K-feldspar and kaolinite is relatively small. The 

special mineral composition of arsenic sandstone determines 
its macro performance under the action of water, that is, it is 
not easy to be compressed, and the structure has a certain 
strength in the anhydrous state. When it is saturated with 
water, the structure collapses, the cohesion decreases rapidly, 
and it has a certain collapsibility. 

In order to understand the physical characteristics of 
arsenic sandstone, mercury intrusion test was used to measure 
the specific surface area and total pore volume of arsenic 
sandstone and composite soil structure. 

By comparing various physical indexes of arsenic 
sandstone and composite soil, it can be found that the total 
porosity of arsenic sandstone is greater than that of composite 
soil. Through further analysis, it can be concluded that due to 
its mineral composition and structural characteristics, the 
arsenic sandstone material has large inter particle pores, and 
its macro physical properties are characterized by its high 
permeability. Under the anhydrous state, its particles and 
particles are obviously cemented, and the load is mainly borne 
by its soil particle skeleton. The phenomenon of water and 
fertilizer leakage is obvious. After the arsenic sandstone is 
mixed with a certain amount of sand to become composite 
soil, due to the complementarity of its structure and the 
optimization of its grading, the total pores of the composite 
soil are reduced. At the same time, small particles are filled 
into the gaps of large particles, which increases the viscosity 
of water in the composite soil, greatly reduces the 
permeability, and plays a role in water and fertilizer 
conservation. 

From the comparison between the bulk density and 
apparent density of arsenic sandstone and composite soil, it is 
found that the bulk density and apparent density of arsenic 
sandstone are both greater than the surface density of 
composite soil, indicating that although the total porosity of 
arsenic sandstone is greater than that of composite soil, the 
actual unit weight of arsenic sandstone is greater than that of 
composite soil, that is, arsenic sandstone is relatively dense, 
the mass per unit volume is greater than that of composite soil, 



 

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and the clay content is greater than that of composite soil, 
which explains the internal reasons for the macro 
performance of arsenic sandstone. 

The total porosity of arsenic sandstone is larger than that of 
composite soil, but its porosity is far less than that of 
composite soil, which further verifies that arsenic sandstone 
has many large pores, compact texture and strong water 
permeability; However, the composite soil has many micro 
pores, loose texture and weak water permeability. 

 

 
5000x 

 
2000x 

 
1000x 

 
500x 

Figure 1. Microstructure of composite soil under different 
magnification times 

 
By comparing the microstructure maps of the composite 

soil under different magnification (Fig. 1), it is found that the 
microscopic particles and pores in the 500x and 1000x images 
are too small for data extraction and analysis, while the 5000x 
display range is small, which is difficult to reflect the overall 
layout of the microstructure. Therefore, the scanning electron 
microscope image of the composite soil microstructure with 
a magnification of 2000x is selected for analysis. 

It can be seen from the figure that the particles in the 

composite soil are composed of single silt particles and 
aggregates composed of fine particles of clay. The particles 
are mainly in face to face contact. The aggregates in the soil 
are significantly increased, and the particle sizes are uneven. 
It can be clearly seen that there are pores of different sizes, 
and these pores are not interconnected. The surface of arsenic 
sandstone particles is rough, with strong friction and bite 
force between them. However, the particle size is relatively 
single, the content of fine particles is low, and the pores 
between particles cannot be filled, resulting in water easily 
passing through the soil. 

Therefore, after the arsenic sandstone and sand are mixed, 
the particles are reconstructed. The clay particles with smaller 
particle size in the arsenic sandstone are filled into the pores 
between the sand particles with larger particle size, increasing 
the overall unit weight of the composite soil, increasing the 
specific surface area per unit volume, enhancing the water 
holding capacity of the soil mass, lengthening and thinning 
the path of free water flowing in the soil mass pores, and 
increasing the viscosity of the fluid, thus reducing the 
permeability of the soil mass, After the arsenic sandstone and 
sand are mixed, the mechanical properties have changed to 
some extent. 

3. Study on Mechanical Properties of 
Composite Soil 

In order to understand the mechanical properties of the 
composite soil and explore its internal relationship with the 
microstructure, the mechanical analysis of the composite soil 
with different mixing ratios was carried out under the water 
content. The samples were taken from Xiaojihan Township, 
Yulin, and the sand was taken from the local aeolian sand in 
Yulin. According to the preparation method of remolded soil, 
arsenic sandstone and sand are mixed to prepare a cylinder 
soil sample with a height of 80mm and a diameter of 40mm. 
GDS dynamic triaxial loading equipment was used for the test. 

The test condition is consolidated undrained test, and the 
mixture ratio of composite soil (sand: arsenic sandstone) is 0: 
1, 1: 5, 1: 4, 1: 3, 1: 2 respectively; The loading confining 
pressure is set to 50kPa, 100kPa, 200kPa; The water content 
of the mixed soil is 14%; The dry density is 1.6g/cm3. It can 
be seen from the stress-strain curve of arsenic sandstone in 
Fig. 2 that the mechanical properties of arsenic sandstone 
show strain hardening, so the control condition for failure of 
the specimen is that the strain reaches 15%. 

Because soil is composed of fragmented solid particles, the 
macroscopic deformation of soil is mainly caused by the 
change of position between particles rather than the 
deformation of particles themselves. In Figure 2, the strain 
increment caused by the same stress increment is different 
under different stress levels, which shows the nonlinearity of 
arsenic sandstone and sand composite soil. It can also be seen 
in the figure that the stress of arsenic sandstone increases with 
the increase of strain, but the increase rate is slower and 
slower, and finally approaches an asymptote. According to the 
analysis of the stress strain curve of composite soil in Figure 
3, the trend is basically the same as that of arsenic sandstone. 

 



 

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Figure 2. The stress-strain curve of feldspathic sandstone 

 
 

Figure 3. The stress-strain curve of compound soil 
 

Table 1. Soil mechanics parameters under different mixing ratios of feldspathic sandstone and sand 
mix proportion 0:1 1:5 1:4 1:3 1:2 
Cohesion (kPa) 17.5 18 18 15 13 

internal friction angle (°) 33 29 26 24 22 
 

It can be seen from Table 1 that the mechanical parameters 
of the composite soil vary greatly under different mixing 
ratios. With the decrease of arsenic sandstone content and the 
increase of sand content in the composite soil, the change of 
cohesive force is not obvious when the mixing ratio is small, 
but with the increase of sand content, the cohesive force of 
the composite soil decreases gradually, and then basically 
presents a linear relationship; The internal friction angle of 
the composite soil decreases with the increase of the 
composite proportion, which is basically a linear relationship. 

4. Conclusion 
a. The total porosity of arsenic sandstone is larger than that 

of composite soil, and its porosity is smaller than that of 
composite soil; The bulk density and apparent density of 
arsenic sandstone are greater than the surface density of 
composite soil. 

b. The particles in the composite soil are composed of 
single powder particles and aggregates composed of fine 
particles of clay. The particles are mainly in surface to surface 
contact. The pore sizes in the composite soil are different and 
not connected with each other; The surface of arsenic 
sandstone particles is rough, with strong friction and bite 
force between them, but the particle size is relatively single. 

c. The mechanical properties of arsenic sandstone show 
strain hardening phenomenon, and the strain increment 
caused by the same stress increment under different stress 
levels shows the nonlinearity of arsenic sandstone and sand 
composite soil. 

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