Keywords

1 Introduction

Loess-mudstone interface landslide refers to the loess landslide sliding along the interface between the overlying loess and the underlying mudstone, which is one of the common types of loess landslide in the Loess Plateau [1,2,3]. At present, the research results of loess landslide mainly focus on the movement characteristics of loess landslide [4, 5], sliding failure mechanism [6, 7], landslide susceptibility and risk assessment [8,9,10], landslide types and spatial–temporal distribution [11, 12]. There is still a lack of in-depth research on the mechanical properties and influencing factors of loess-mudstone and other double heterogeneous soils.

The shear strength of the contact surface of double heterogeneous soil is one of the main aspects to characterize the mechanical properties of the contact surface. In engineering practice, the contact surface of double-layer heterogeneous rock and soil structures is often regarded as the weak surface, and has attracted great attention from some scholars. Potyondy [13] was the first to study the mechanical properties of the contact surface between a variety of construction materials and rock and soil mass with the aid of strain-controlled direct shear instrument, and concluded through experimental research that the moisture content of rock and soil mass, the roughness of the structure and the vertical stress would all affect the shear strength of the contact surface. Yoshimi and Kishida [14] introduced X-ray photography technology into shear test of sand-steel contact surface and found that the shear strength was significantly affected by steel surface roughness. Hu et al. [15] applied CCD digital camera technology to the direct shear test of sand-steel contact surface, and pointed out that there is a critical contact surface roughness, which can be divided into shear failure modes of low roughness surface and high roughness surface, and the ideal elastic–plastic failure and strain localized failure with strong strain softening and dilatation respectively. Zhang Ga et al. [16, 17], based on the direct shear and single shear tests on the contact surface of coarse-grained soil and steel plate, pointed out that the shear strength is greatly affected by the properties of rock and soil, vertical stress and the roughness of the contact surface, and is independent of the shear direction. Cheng Hao et al. [18], based on large-scale direct shear tests of red clay-concrete contact surface, found that with the increase of contact surface roughness, the shear failure mode of contact surface gradually evolved from contact surface slip to internal soil failure. Although a series of in-depth studies have been carried out on double-layer heterogeneous rock and soil structures, there are still few experimental studies on the influence of moisture content and dry density of loess on the shear strength of loess-mudstone composite layer.

Based on this, taking a road cut slope in a test base in Shaanxi Province as the engineering background, loess-mudstone composite samples with different loess moisture content and dry density were prepared according to relevant test standards, and direct shear tests were carried out. The influence of loess moisture content and dry density on the shear strength of loess-mudstone composite layer was analyzed. Finally, microscopic failure mode of loess-mudstone interface were observed by SEM to further reveal the mechanism of the influence of moisture content and dry density of loess on the shear strength of loess-mudstone composite layer.

2 Experimental Design

2.1 Soil Sample Preparation

The soil sample was taken from a road cut slope containing loess-mudstone contact surface in a test base in Shaanxi Province, as shown in Fig. 1. below. As can be seen from the Fig., the site loess-mudstone assemblage can be divided into three layers according to color. The uppermost layer is loess, the middle is mudstone with high moisture content, and the bottom is mudstone with low moisture content. The main reason for this phenomenon is that mudstone is more dense than loess, and it is difficult for rainwater to penetrate mudstone layer completely, resulting in the formation of aquifer between loess and mudstone layer.

Fig. 1
A photograph of the distribution of soil into 3 layers of different textures. The layer on top with fine layers is labeled loess, and the layer at the bottom is labeled mudstone.

Distribution of soil layers in the cutting slope

Full size image

According to the Standard of Geotechnical Test Method (GB/T 50,123–2019), the basic physical property parameters of the test soil samples were determined respectively. In view of the high heterogeneity of the soil, four sites were selected to collect soil, and three groups of samples were prepared at each location for repeated measurement and the average value was obtained, as shown in Table 1.

Table 1 Basic physical parameters of samples
Full size table

The remolded loess and fully weathered mudstone samples retrieved from the site after air drying were put into the grinding dish for moderate grinding. In order to obtain well-graded test soil samples, the screening method was used to pass the loess and mudstone through sieves with pore sizes of 2, 1.18, 0.6, 0.425, 0.3, 0.212 and 0.15 mm successively. The gradation composition of loess with fully weathered mudstone with particle size <0.15 mm was analyzed by laser particle size analyzer. Finally, the gradation parameters of the test loess and mudstone are shown in Fig. 2. In order to keep the basic physical property parameters of the control test soil samples unchanged, each group of soil samples was prepared according to this grading method.

Fig. 2
A line graph plots mass fraction percentage versus particle diameter for loess and mudstone. The values are as follows. Loess, (10 to negative 1, 0), (10 to 2, 90), and (10 to 3, 100). Mudstone, (10 to negative 1, 0), (10 to 2, 30), and (10 to 3, 100). Values are estimated.

Particle size gradation of loess and mudstone for testing

Full size image

Considering that the influence of rainfall and engineering disturbance on slope soil mostly stays in the loess layer. This experiment kept the physical parameters of the underlying mudstone unchanged, and mainly explored the influence of the loess moisture content and dry density on the shear strength of the loess-mudstone interface. The field loess moisture content ranges from 10.31% to 19.80% and the dry density ranges from 1.38 to 1.56 g/cm3. Therefore, the design of the loess soil moisture content of 10, 13, 16 and 19% (at this time the dry density of loess is 1.5 g/cm3), and the dry density of 1.4, 1.45, 1.5 and 1.55 g/cm3 (at this time the moisture content of loess is 13%). Then, mudstone with moisture content of 15% and dry density of 1.7 g/cm3 is configured as loess-mudstone composite layer. The vertical stress of 100, 200, 300 and 400 kPa is applied respectively, and the fast shear without consolidation and drainage is carried out. A total of 64 kinds of composite layer conditions are obtained. Meanwhile, parallel control tests of pure loess sample and mudstone group under vertical stress are designed. The shear rate of 0.8 mm/min and shear displacement of 6 mm were used in all tests. The test was repeated 3 times for each condition, and the results were averaged.

Mudstone compresses when vertical stress is applied. In order to avoid the actual shear position deviating from the preset contact surface, the loading mass of mudstone should be calculated according to the predetermined moisture content and dry density (accurate to 0.01 g when weighing). The pure mudstone ring cutter samples were prepared, and the consolidation and compression tests of the soil samples were conducted under normal pressures of 100, 200, 300 and 400 kPa respectively. The compression amounts of the pure mudstone samples under different normal pressures were recorded, and the density of the compressed samples was calculated according to the compression amounts. That is, the actual mudstone sample density when the sample with predetermined moisture content and dry density is prepared under the corresponding vertical stress. The test results are shown in Table 2.

Table 2 Compression of mudstone samples under different normal pressures
Full size table

2.2 Direct Shear Test Scheme

The direct shear test of loess-mudstone contact surface adopts strain direct shear instrument, whose upper and lower shear boxes have the same size and can accommodate samples with a cross-sectional area of 30 cm2 and a height of 2 cm (as shown in Fig. 3.). The loess is located in the upper shear box, and the mudstone is located in the lower shear box. By applying horizontal shear stress to the lower shear box, the sample is shear at the contact point of the upper shear box, that is, the loess-mudstone contact surface.

Fig. 3
A photograph of a strain-controlled direct shearing instrument with shear boxes, soil containers, loading units, and other pieces of equipment.

Strain-controlled direct shearing instrument

Full size image

The direct shear instrument can apply shear forces under different normal pressure loads (100, 200, 300 and 400 kPa) and shear four samples simultaneously. However, in order to reduce instrument error, only one sample is placed at a fixed position for each shear. The shear displacement value of the display screen (accuracy 0.001 mm) and the deformation of the measuring ring measured by the electric dial indicator (accuracy 0.01 mm) were recorded by the software dynamometer system 3.9A, and the horizontal shear stress was obtained according to the coefficient of the measuring ring.

As the shear area gradually decreases during the test, the test result is smaller than the vertical stress applied by the actual instrument. In addition, if the set shear displacement and the actual shear displacement are not distinguished, the measured shear strength will be less than the actual shear strength of the soil. In this paper, the results of direct shear tests are modified from the aspects of effective shear area and effective vertical stress.

Due to the symmetry of the shear plane, 1/2 of the shear area is taken for area correction. The initial shear area A0 is regarded as invalid parts A1 and A2, as shown in Fig. 4.

Fig. 4
A diagram has a circle labeled as the upper part of the shear box that passes through all coordinates of the Y versus X plane with O as the origin and an overlapped circle labeled as the lower part of the shear box that passes through all coordinates of the Y prime versus X prime plane with O prime as the origin.

Schematic diagram of the effective shear area after the upper and lower shear boxes are displaced (the gray part is the actual shear area after displacement)

Full size image

According to the geometric relation in Fig. 4., the expression of A2 can be obtained:

(1)

where, D is the diameter of the sample, which is 6.18 cm here. x is the shear displacement (cm). When x = 0, the initial shear area A0 is 30 cm2, and when x = 6 mm, the initial shear area A2 is 26.29 cm2. Let the area correction factor α satisfy:

$$ {\upalpha } = \frac{{{\text{A}}_0 }}{{{\text{A}}_2 }} $$
(2)

there is:

$$ \tau_1 = {\upalpha }\tau_0 $$
(3)

where,τ1 is the shear strength of the modified sample (kPa); τ0 is the shear strength of sample without area correction (kPa).

The vertical stress on the effective shear area was corrected, and the force analysis of the upper shear soil sample was shown in Fig. 5. In Fig. 5, σ1 is the uniform vertical stress (kPa) applied to the non-effective shear area A1, σ0 is the uniform vertical stress (kPa) applied to the top of the shear soil sample, σ2 is the uniform vertical stress applied to the effective shear area A2, and P is the applied shear force.

Fig. 5
A schematic of the force analysis has two rectangular boxes with the lower box slightly dislocated. The vertical stresses labeled in the diagram are as follows. Sigma 0 on the top, and sigma 1 and sigma 2 on the bottom of the upper box. P is labeled as the horizontal force in the upper box.

The force diagram of the soil sample in the upper shear box after shear dislocation

Full size image

According to the moment equilibrium MyQ = 0 of each stress component in the y direction of the central point Q of the effective shear area in Fig. 5, it satisfies [19]:

$$ {\text{M}}_{{\upsigma }1} = {\text{M}}_{{\upsigma }0} + {\text{M}}_{{\uptau }1} $$
(4)

According to the geometric relationship in Fig. 4. and Fig. 5, it can be obtained:

$$ {\upsigma }_2 = {\upsigma }_0 - \frac{{{\uptau }_1 {\text{l}}}}{{{\pi x}}}\left[ {{\uppi } - 2{\uptheta } + \sin \left( {2{\uptheta }} \right)} \right] $$
(5)

where, I is the height of the soil sample in the upper shear box, which is 10 mm. Let the normal correction coefficient β satisfy:

$$ {\upbeta } = \frac{{{\upsigma }_2 }}{{{\upsigma }_0 }} $$
(6)

The shear strength of the comprehensively modified sample τ2 can be obtained:

$$ \tau_2 = {\upbeta }\tau_1 $$
(7)

3 Shear Test Results and Analysis of Sample

3.1 Effect of Moisture Content on Shear Strength of Sample

The relationship between shear strength and vertical stress of loess, loess-mudstone composite layer and mudstone under different moisture content is shown in Fig. 6. As can be seen from the Fig. 6, under the same vertical stress, the loess-mudstone composite sample with double heterostructure has the lowest shear strength, followed by the loess sample on the upper layer, and the mudstone sample has the highest shear strength. This indicates that the contact surface is the weak strength of loess-mudstone composite layer. When the vertical stress was the same and the vertical stress was 400 MPa, for example, when the moisture content increased from 10 to 19%, the shear strength of loess continued to decrease by 3.71, 6.93 and 12.08% to 10.02, 18.72 and 32.62 kPa, respectively. The shear strength of loess-mudstone composite layer decreases by 11.67, 22.26 and 37.51 kPa, respectively, by 4.87, 9.29 and 15.65%. This indicates that moisture content has a more significant effect on the deterioration of shear strength at the loess-mudstone interface.

Fig. 6
4 line graphs of shear strength versus vertical stress are plotted for omegas equal to 10%, 13%, 16%, and 19% in a, b, c, and d respectively. The graphs plot increasing curves for loess, mudstone, and loess mudstone in all four graphs.

Variation of shear strength of loess and composite layers with moisture content of loess

Full size image

In to the pressure, the upper loess pore structure are pressure, saturation increases, the soil particles between weakly bound water and free moisture content increases, and the lower mudstone is more dense loess, poor permeability and a relative impermeable layer, makes the bottom of the north part of the water accumulation in the loess-mudstone contact area, contact area of soil In the overwatering soft plastic-saturated state, Thus, the shear strength of the contact surface is significantly reduced. This conclusion is consistent with the conclusion proposed by Xing Lin et al. [20] that the loess shear strength decreases with the increase of saturation, that is, the higher the moisture content of the upper loess, the more water accumulated at the loess-mudstone interface, and the higher the deterioration degree of the shear strength of the interface.

3.2 Influence of Dry Density on Shear Strength of Sample

The relationship between the shear strength of loess and loess-mudstone composite layer and the dry density of loess is shown in Fig. 7. It can be seen from the Fig. 7 that, under the same vertical stress, the loess-mudstone composite layer has the lowest shear strength in various working conditions that only change the dry density of loess, that is, the contact surface is still the weak strength surface of the whole loess-mudstone double-layer heterogeneous structure. However, when the vertical stress was the same and the vertical stress was 400 kPa for example, with the dry density of loess increasing from 1.4 g/cm3 to 1.55 g/cm3, the shear strength of loess increased by 14.92, 22.81 and 39.35 kPa, respectively, by 6.29, 9.61 and 16.58%. The shear strength of loess-mudstone composite layer increases by 11.70, 15.16, 24.99 kPa, 5.50, 7.12, 11.74%, respectively. It can be seen that the dry density of loess has greater influence on the shear strength of homogeneous loess than that of composite loess.

Fig. 7
4 line graphs a, b, c, and d of shear strength versus vertical stress are plotted for rho subscript d of 1.4, 1.45, 1.5, and 1.55 grams per cubic meter respectively. The graphs plot increasing curves for loess, mudstone, and loess mudstone in all four graphs.

Variation of shear strength of loess and composite layers on dry density of loess

Full size image

The relationship between the shear strength of loess and loess-mudstone composite layer and the dry density of loess is shown in Fig. 7. It can be seen from the Fig. 7 that, under the same vertical stress, the loess-mudstone composite layer has the lowest shear strength in various working conditions that only change the dry density of loess, that is, the contact surface is still the weak strength surface of the whole loess-mudstone double-layer heterogeneous structure. However, when the vertical stress was the same and the vertical stress was 400 kPa for example, with the dry density of loess increasing from 1.4  to 1.55 g/cm3, the shear strength of loess increased by 14.92, 22.81 and 39.35 kPa, respectively, by 6.29, 9.61 and 16.58%. The shear strength of loess-mudstone composite layer increases by 11.70, 15.16, 24.99 kPa, 5.50, 7.12, 11.74%, respectively. It can be seen that the dry density of loess has greater influence on the shear strength of homogeneous loess than that of composite loess.

4 Microstructure of Loess-Mudstone Interface after Shear Failure

In order to further reveal the influence of different loess moisture content and dry density on the shear strength of loess-mudstone composite layer, it is necessary to observe the structural changes of soil samples at the interface of the composite layer under different working conditions by microscopic means. The damaged loessie-mudstone interface block was observed by FEI Quanta 400 FEG environmental scanning electron microscope (SEM), State Key Laboratory of Continental Dynamics, Northwest University.

The microstructure of contact surface after shear failure of composite layers with different loess moisture content is shown in Fig. 8. As can be seen from the Fig. 8, when the moisture content is low, the soil particles are larger on the whole, with looser arrangement and more pores. In addition, it can be seen that small clay groups fill between the larger structural units and play an internal cementation role. Under this condition, the contact particles mainly have mechanical friction under shear. With the increase of moisture content, the amount of water accumulated on the contact surface increases, the amount of clastic material and clay material on the contact surface increases, and some large particles are aggregated into a whole state under the connection of clay particles, and the pore area decreases continuously. When the moisture content increased to 19%, the cement inside the clay mass was softened, dismembered into small clay particles and filled in the pores, and there were no obvious large pores in the visual field. The shear strength of the contact surface decreases due to the thicker bonded water film adsorbed by the disintegrated slime.

Fig. 8
4 S E M scans a, b, c, and d of microstructures in composite layers at omegas of 10%, 13%, 16%, and 19% respectively. The scans have clusters of irregular structures with areas highlighted for macropores, blocky clay particles, disintegrating clay particles, and clay particles.

Scanning images of the contact surface microstructure in the composite layers with different loess moisture contents

Full size image

Figure 9. shows the microstructure scanning of the interface of composite layer with different dry density of loess after shear failure. Furthermore, PCAS software was used to binarize the image, and the results showed that the porosity of 1.4, 1.45, 1.5 and 1.55 (g/cm3) loess under dry density conditions were 8.52, 7.33, 6.17 and 5.31%, respectively. The average pore area is 228.88, 231.88, 185.94, and 183.65 (pixel). Therefore, it can be seen that with the increase of the dry density of the upper loess, the microscopic characteristics of the pores on the contact surface have a great change, and the pore number and average pore area on the whole decrease, while the porosity decreases. This is mainly because when the dry density of the upper loess increases, more soil particles will be embedded and adhered to the contact surface, filling the original pore structure on the contact surface. The large pores in the overhead are transformed into small and micro pores, and the particles are arranged from loose to tight, and the degree of occlusion is enhanced. Macroscopically, the shear strength and parameters of the contact surface increase to a certain extent with the increase of the dry density of the upper loess.

Fig. 9
4 S E M scans a, b, c, and d of microstructures in composite layers at rho subscript d equal 1.4, 1.45, 1.5, and 1.55 grams per cubic centimeter respectively. The scans have irregular structures with areas highlighted for macropore, point-to-point contact, point-surface contact, and surface-to-surface contact.

Scanning images of the contact surface microstructure in the composite layers with different loess dry density

Full size image

5 Conclusion

This paper conducted an experimental study on the shear strength of loess-mudstone composite layer with different loess moisture content and dry density, and observed the microstructure of the composite layer after shear failure with scanning electron microscopy. It can be concluded that:

  1. (1)

    The shear strength of loess-mudstone composite layer is lower than that of pure loess and mudstone samples, indicating that loess-mudstone double-layer heterostructure is the weak surface of shear strength of the whole sample, which needs to be paid more attention to.

  2. (2)

    The moisture content of loess will deteriorate the shear strength of composite layer, and its effect is greater than that of homogeneous loess. However, the dry density of loess will enhance the shear strength of composite layer, which is less than that of homogeneous loess.

  3. (3)

    The increase of loess dry density, the pore area of the contact surface of loess-mudstone composite layer will be reduced, and the degree of interlock between the two layers of heterogeneous soil particles will be increased, leading to the increase of the shear strength of the composite layer. However, the increase of loess moisture content will reduce the pore area of the contact surface of the composite layer, resulting in the accumulation of water at the contact surface, and reduce the shear strength of the loess-mudstone composite layer.