Keywords

1 Introduction

Many soil holes in karst regions threaten people’s safety and property [1], particularly during road construction. This situation has drawn the attention of domestic and foreign scholars to the stability of earth holes in karst regions.

In terms of the research factors impacting soil caverns stability, Qin et al. [2] developed an equation to determine the stability coefficient when approaching a critical state of collapse in a karst region. Santo et al. [3] discovered that the stability coefficient of soil holes is tied to the height of the upper layer of overburden soil. Chen et al. [4] and Wei and Sun [5] discovered that the stability coefficient of the soil hole is higher when the overburden layer is thicker and with the higher water level. Xu and Song [6] conducted scaled-down model tests and discovered that the arch height should be between 1.0 times and 1.5 times the width of the collapse. These studies show that the stability of earth cavities is determined by the stability coefficient, which impacted by the collapse width and upper cover layer thickness.

On the research of deformation mechanisms in the development of soil cavities, Parise and Lollino [7] conducted a study using numerical simulation to analyse the stress of soil cavities to reveal the damage mechanism of the soil cavity. Shen et al. [8] found soil cavity development involves generating and eliminating the soil arch effect of the cover soil body. On the other hand, Juan et al. [9] suggested a suspension chain line soil arch model to explain how the earth arch effect works during the collapse of earth holes. It is clear that the stability of soil holes are connected to the earth arch effect, and this effect is significant for keeping the holes stable in the evolution.

In summary, in the stability study of the soil caverns, there mainly used the methods of numerical simulations and formula derivation, and less often used test methods presently. It hinder the further disclosure of the stability mechanism of soil caves. Additionally, during the soil cave evolution, the height of the soil arch is the stable support condition of the cave, but also the key to play the soil arch effect, which should be paid attention to in the research process. Thus, this paper conducted a scaled-down model test on the collapse of soil caves, which research how the height of the arch affects cave stability when subjected to arching effect, aiming to reveal the stability mechanism of soil holes.

2 Model Test

2.1 Experiment Device and Implementation Schemes

The experimental design references the case of collapse of a long pothole (about 1.5 m long and 1 m wide) at Laiwu City, Shandong Province. Depending on the site’s structure and experiment conditions, the design similarity ratio is defined as 10. According to the similarity ratio, and consider the convenience of the test operation, the experiment box used in the test was 1.5 × 0.6 × 1.5 m (length × width × height), which has a glass perspective surface and movable floor settlement system. Additionally, in order to carry out the load-sharing test between collapse area and stable area under different collapse widths, setting 0.3 m wide collapse area and 0.6 m stabilization zone on both sides as the standard group. The specific test scheme is shown in Table 1. In terms of packing, test materials using Lijiang sand, filtered through a 2 mm sieve prior to testing. Based on geotechnical tests, the test sand found to be poorly graded silt. The material properties can be found in Table 2.

Table 1 Experiment scheme
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Table 2 Basic performance index of Li-jiang sand
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2.2 Arrange of Test Monitoring Points

To study the effects of settlement width and fill height on the load distribution and soil displacement in the collapse zone, displacement measuring points were placed every 10 cm along the center line and monitored with displacement meters. Additionally, according to the symmetry principle of the soil arching, the soil pressure in the stable area on both sides of the collapse area is assumed to be equal, hence, only the soil pressure box is placed in the right stability area to monitor the soil pressure. The data of tests are collected by the static strain meter. The arrangements of test’s measuring point are depicted in the Fig. 1. In addition, using the particle image speed measuring equipment (PIV) to monitor the vertical displacement of the soil particles. The test site picture is shown in Fig. 2.

Fig. 1
An illustration depicts a soil pressure box provided with movable and fixed bottom plates, with the location of the soil pressure cells at the base of the box and settlement markers at different heights indicated.

Measurement point layout diagram

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Fig. 2
A photograph of the experimental setup with the P I V equipment mounted on a tripod to measure the vertical displacement of the soil pressure box.

Field drawing of the retaining wall model

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3 Results and Discussion

3.1 Effect of Different Collapse Widths on Soil Cave Stability

Analysis of the soil vertical displacement results. Figure 3 displays the map of soil displacement cloud of groups Z1, Z2 and Z3 under PIV observation. Apparently, vertical displacement increasing as the soil approaches the collapsing zone, and the distribution of soil particles during collapse is in an arch. The allowable settlement value resulting from urban underground works is 0.03 m [10]. Thus, it is considered as instability, if the range of uneven ground settlement exceeds 0.03 m in the test. It revealed the range of unstable soil depths in groups Z1, Z2, and Z3. This confirms that the soil above the arch height is stable, while the soil below the arch is unstable.

Fig. 3
Three spectral graphs a to c plot fill height versus horizontal position for groups Z 1 to Z 3. All the graphs display the boundary of the collapse zone and maximum arch height, with the displacement indicated by various shades.

Displacement cloud map at different collapse widths

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Simultaneously, Fig. 4 shows cloud map of vertical displacement variations at soil locations Z1, Z2, and Z3. Overall, the filling’s displacement gradually reduced as the filling height increased. Groups Z1, Z2 and Z3 had a displacement value of zero at 0.92 m, 0.32 m and 0.19 m respectively, indicating that the soil arch had reached its maximum height. This indicates that with the collapse width decreases, the maximum arch height decreases. Thus, from the displacement perspective, the height of the arch is the crucial factor for evaluating the stability of a soil caves, which should be emphasis in engineering.

Fig. 4
A line graph plots vertical displacement versus center measurement point height. The lines are plotted for Z 1, Z 2, and Z 3. All the lines depict a downward trend.

Vertical displacement change diagram of the collapse center line measuring points under different collapse widths

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Effects on load-sharing. Figure 5 depicts the soil pressure curve in the collapse area at T1, T2, T3, and T4, alongside the relative settlement of the activity floor under various collapse widths. First, based on the analysis of the measuring points in the collapse area, it is evident that the change trend of T2 is similar to that of T1. Dividing this curve into two stages: a steep drop followed by a slow change. Additionally, the difference in soil pressure between the beginning and end of each soil group is not substantial. Filling of the boundary of the collapse zone is not only generate a displacement due to contact with the boundary, but also subject to principal stress deflection due to boundary constraints. Consequently, soil pressure decreased more near the boundary of the collapse zone than in its center. Secondly, analyzing the T3 sites located near the edge of collapse in test. Clearly, the soil pressure distribution curves at the T3 point in three groups, all with 0.02 m as the boundary are sequentially divided into two phases of sharp increase and slow decrease. In the stage of subsidence, the maximum soil pressure of Z2 and Z3 is greater than 50%. It is due to the earth arch is formed between the stable area and the subsidence area, which means the transfer of loads. Moreover, analyzing the T4 measurement points shows that the soil pressure at the T4 points of Z1, Z2 and Z3 test groups gradually stabilized from 0.005 m after different increases, but the overall rise rate decline in order. Implying that the collapse width is large while the soil pressure value is high. In addition, T3 and T4 points could raise the soil pressure in the stable area. This indicates that the existence of soil arching effects could transfer the overburden load to the stable region, causing an increase in soil pressure. The wider the collapse width, the greater the soil pressure of the stable area. Besides, due to the T5 and T6 measuring points are further from the collapse area than the T4 measuring points, the variation in soil pressure with respect to bottom plate settlement is less than T4, which is not being discussed.

Fig. 5
A line graph plots earth pressure versus depth of collapse settlement. The lines are plotted for Z 1, Z 2, and Z 3 with different measuring points. A few lines depict an initial spike, while others depict an initial dip, and all the lines remain stable thereafter.

Change curve of soil pressure at each measuring point under different collapse widths

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3.2 Influence of Different Filling Height on the Stability of Soil Cave

Effects on load-sharing. Figure 6 illustrates the trend in soil pressure for T4 points in Z4 and Z5 groups. As shown in the figure, the change trend of soil pressure in T4 measuring points in Z4 and Z5 groups is similar to that of the Z1 group, indicating that filling height has minimal impact on the load-sharing component. Meanwhile due to T4 is a representative load sharing point near the foot of the arch, the rest of the points would not be analyzed further. Because comparing to T4, T3 measuring point is susceptible by near the collapse area, while T5 points and T6 points exhibit less load sharing at the arch foot owing to their distance from the collapse zone.

Fig. 6
A line graph plots earth pressure versus depth of collapse settlement. The lines are plotted for Z 1, Z 4, and Z 5. All the lines depict a gradual, increasing trend.

Variation curve of soil pressure at T4 measurement point at different filling heights

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Analysis of the soil vertical displacement results. Figure 7 displays the vertical displacement curve of the centerline collapse area for Z4 and Z5 groups once the movable plate settles at 0.3 m. The analysis focuses solely on measuring points under 0.2 m in height. As seen in the figure, the curves of the three setting conditions nearly overlap. Upon settlement completion, the vertical displacement of Z4 and Z5 was 0.025 m, however, the Z4 and Z5 test groups had a vertical displacement of 0.021 m and 0.02 m, respectively, at the h = 0.2 m marker points. Demonstrating at a specific collapse width, a critical stability value exists in the soil arch height. Additionally, when the fill thickness reaches an adequate level, the increase in height of the overlying soil layer has minimal impact on the soil arch height.

Fig. 7
A line graph plots vertical displacement versus marker point height. The lines are plotted for Z 1, Z 4, and Z 5. All the lines depict a downward trend.

Vertical displacement curves for settlement up to 0.3 m at different fill heights

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4 Conclusion

The study analyzed the effect of changes in slump width and filling height on the maximum soil arch height and load distribution in the slump region. The main research conclusions are the following.

  • During collapse, load in the stable area increasing. This indicates that the soil arching effect transfer the load from the overlying soil to the stable area to maintain stability of the earthen cave.

  • In the process of collapse, the soil particles settle unevenly. However, soil particles with the same displacement create a maximum arch interface finally which stabilizes the settlement. This effect is referred to as the soil arching effect.

  • The narrower the collapse width, the stronger the stability of the soil cave and the lower the maximum arch height. However, the filling height does not significantly affect the stability and arch height of the soil cave.