Triaxial Test of Reinforced Sand Properties of High Strength Geocells

Abstract: The strength and deformation and failure characteristics of high-strength geogrid reinforced soil are studied by a triaxial test method. The influence factors and variation law of geogrid reinforced soil strength under different reinforcement conditions are analyzed. How to choose a more reasonable and economical reinforcement form when the amount of reinforcement is the same. The test results show that the strength of the soil and the ability to resist deformation after reinforced are obviously enhanced. Under the condition of a certain confining pressure, the improvement of the height of the reinforced concrete is far greater than the influence of the decrease of the joint spacing; When the amount of the reinforcement is the same, it is more reasonable to select the reinforcement method with high height of the chamber but less relative to the number of layers. The analysis of the reinforcement effect coefficient and strength parameter shows that the coefficient of reinforcement is reduced with the increase of confining pressure, and the geogrid Room reinforcement helps to improve the cohesion of the soil and increase the internal friction angle, in which the increase of cohesion is more significant.

Keywords: triaxial test; high strength wholesale HDPE geocell; reinforced soil;

The concept of reinforced earth was first proposed by French engineer Henri Vidal in the early 1960s. He studied the fiber-incorporated material in the soil through triaxial tests and found that it can improve the strength of the soil. From then on, the composite material was named as reinforced soil, and a new concept of reinforced soil was proposed. At present, the most widely used method for studying the reinforced soil reinforcement mechanism is the triaxial compression test.

Schlosser et al. first studied metal strip reinforced sand using a triaxial compression test. Chen Changfu et al. conducted a triaxial test on the strength characteristics of grass-rooted reinforced soil and proposed that grass-rooted reinforced soil mainly improved the cohesion of soil, but had little effect on internal friction angle. Shi Liguo et al. studied the triaxial strength characteristics of polypropylene fiber reinforced lime soil. Chen Qun et al. studied the strength properties of glass-reinforced plastic window screens and woven geotextile-reinforced soils. The cohesive force and internal friction angle of the two different materials reinforced soil samples were significantly improved, but the degree of improvement was different. Liu Fang et al. conducted a triaxial test to study the reinforcement effect of glass fiber soil. It is believed that the reinforcement effect of glass fiber soil and the amount of glass fiber blended, the compactness of glass fiber, the compactness of the sample and the confining pressure. related. Wei Hongwei et al. carried out a triaxial test on the geosynthetics reinforced cohesive soil. It is believed that there is a hysteresis in the reinforcement effect of the reinforcement on the soil strength, and the reinforcement can significantly inhibit the dilatancy deformation of the soil. Lei Shengyou studied the strength characteristics of reinforced loess with polyester fabric as reinforcement material. Zhao Chuan et al. conducted a large-scale triaxial test to investigate the strength and mechanical properties of geogrid-reinforced gravel soil. In order to compare the reinforcement effects of five kinds of domestic geosynthetics, Wu Jinghai conducted a triaxial test. Zhang Mengxi et al. conducted a comprehensive study and discussion on the three-dimensional reinforcement method through the indoor triaxial test. The results show that the three-dimensional reinforcement is greatly improved compared with the traditional horizontal reinforcement. Latha et al. studied the strength characteristics of three kinds of reinforced sand with horizontal fiber-reinforced layer, dispersed fiber strip, and fiber high quality HDPE geocell. Chen et al. studied the effects of paper wholesale HDPE geocell on the mechanical properties and strength of reinforced sand by triaxial tests. Rajagopal et al. studied the strength properties of single-layer reinforced and multi-layer reinforced sand. Nair et al. studied the strength and stiffness characteristics of geogrid-reinforced soil under static and cyclic loading through large-diameter triaxial tests. It was found that after the layer reinforcement was reached, the number of reinforcement layers would not increase. Get extra reinforcement. This provides experimental support for optimal engineering design.

For the three-dimensional new geosynthetics of HDPE geocell for sale, the reinforcement mechanism is quite different from the traditional planar reinforcement materials. At present, the research on HDPE geocell manufacturers reinforcement is not perfect. The research on this high-strength wholesale HDPE geocell is also lacking. Therefore, it is of great theoretical and practical significance to study the geogrid-reinforced soil. In this work, high-strength high quality HDPE geocell is used as the main research object. The strength characteristics of different types of HDPE geocell for sale reinforced sand are studied by the triaxial test. The stress-strain relationship of reinforced soil, the coefficient of reinforcement and the same are discussed. How to choose a more economical and reasonable reinforcement form under the amount of reinforcement material has laid a solid foundation for theoretical research and construction design of reinforced soil.

1. Test plan

1.1 Test equipment and process
The three-axis shearing instrument adopts the SJ-1A strain-controlled three-axis shearing instrument produced by Nanjing Electric Power Automation Equipment Factory. The equipment consists of four parts: the experimental machine, the pressure chamber, the measurement, and control system, and the sample preparation tool.

The prepared sample is wrapped with a rubber film and placed on the sample holder of the pressure chamber to be connected to the measuring system, and the surrounding pressure is adjusted by the adjustment knob. During the test, the experimental machine is driven by the gear transmission system to raise the pressure chamber at a certain rate, so that the sample generates axial compressive strain under the action of the piston rod. According to the amount of deformation of the force ring, the magnitude of the axial stress applied to the sample can be determined. At the same time, the volume change of the volume change tube can be measured, and the change of the internal void pressure of the sample is measured by the gap pressure measuring unit until the sample shear damage is broken.

1.2 Test materials
(1) Sand sample. In the test, the soil sample was selected as Fujian sand. In order to reduce the influence of filler moisture on the test results, the moisture content of the sand was controlled to be w=5%. The physical properties of the sand were as shown in Table 1. The particle gradation distribution curve is shown in Fig. 1. Shown.

Table 1 Physical parameters of the sand sample

Fig.1 Particle size distribution curve of the sand sample

(2) Reinforcement. The high-strength HDPE geocell manufacturers used in the test is a specially processed wholesale HDPE geocell for Yizheng City and Geotechnical Materials Co., Ltd. The strip material of the cell is modified polyethylene, and the tensile strength of the cell is higher than that of the general HDPE geocell for sale, but its elongation is relatively decreased. Due to the small size of the cells, the strips are connected by nails similar to staples (see Figure 2), and the thickness of the mesh belt is 0. 45 ± 0. 10 mm.

Fig.2 High-strength geocell and nodes

1.3 Test methods and conditions
The prepared HDPE geocell manufacturers are placed in the sand during sample preparation. Since the wholesale HDPE geocell is a flexible structure, each grid unit needs to be fully opened with a small wooden rod during the installation process and fixed at a predetermined position to ensure that the three layers of the material are aligned up and down in the sample. The location of the high quality HDPE geocell in the sample is shown in Figure 3. In order to control the sample to have the same compactness, 4 parts of the same quality sand were weighed into the casing with a balance, and each layer was compacted by the same compaction method for the same number of times. After reaching the height standard, the sand layer is leveled and shaved into the HDPE geocell for sale. After the sample is completed, the inside of the sample is evacuated by a vacuum pump (connecting port pressure valve) so that the sample can be erected after the casing is taken down. After the sample preparation is completed, water is injected and the confining pressure is applied. When the confining pressure is increased to 30-40 kPa, the sample hole pressure valve is opened and vacuum, so that the sample is in the natural state of the soil layer. When the confining pressure reaches the specified value, close the pore pressure and drain valve so that the sample is in an unconsolidated, undrained state.

Fig.3 Rein enforcement installation and layout diagram

A total of five test conditions were designed for this work (see Table 2). The working conditions are based on the height of the cell, the spacing of the cell, and the number of layers of the ribs. The amount of wholesale HDPE geocell material used in working condition 2 is M, and the working condition 4 is the same as the amount of reinforcing the material in working condition 2. The amount of HDPE geocell for sale material used in working condition 3 is N, and the working condition 5 is the same as the amount of reinforcing the material in working condition 3. The specimens of working condition 2 and working condition 3 are evenly arranged with 3 layers of ribs, and the 3 layers of ribs are aligned up and down and the position is uniform, and the spacing of the ribs is 55 mm. The samples of working condition 4 and working condition 5 are evenly arranged with two layers of ribs. To ensure the same depth of embedding, choose to embed two layers of ribs closest to the top surface of the specimen.

Table 2 Test conditions

Working conditions 2, 3 and working conditions 4, 5 are to compare the influence of the grid welding distance on the reinforcement effect. Working conditions 2, 4 and working conditions 3, 5 are to study the case where the reinforcement ratio is basically the same. The effect of height and number of layers on the reinforcement effect. The triaxial test was carried out under three confining pressures of 50, 100 and 200 kPa for each working condition.

The specimen failure criteria are as follows: 1 When there is a peak, the peak point of σ1-σ3 is the breaking point; 2 When there is no peak, the main stress difference at 15% axial strain is used to determine the failure point.

2. Test results and analysis

2.1 Stress-strain curve and analysis
2.1.1 Influence of node spacing
In order to study the influence of different node spacing on the stress-strain of reinforced soil, the stress-strain of the wholesale HDPE geocell is studied under working conditions 2 to 5 at the same confining pressure and different joint spacing. The results are shown in Figures 4 and 5.

Fig.4 Stress-strain curves of the reinforced sand(h=2cm)

Fig.5 Stress-strain curves of the reinforced sand(h=3cm)

It can be seen from Figures 4 and 5 that the peak stress of the HDPE geocell manufacturers-reinforced soil increases with the increase of the cell height. When the height of the cell is the same, the basic trend of the stress-strain curve of the geogrid-reinforced soil with pure sand and different node spacing is the same under different confining pressures, and the smaller the node spacing, the larger the peak stress, but the difference is not obvious. . Especially when the confining pressure is low, this gap is almost non-existent. When the height of the cell is 2cm and the confining pressure is 50kPa, the stress-strain curve of the high quality HDPE geocell-reinforced soil with a node spacing of 5.0 cm is particularly soft after peaking, and the bearing capacity is finally reduced to be similar to pure sand. This is because the cell has been destroyed when it reaches the ultimate bearing capacity. However, this phenomenon did not occur when the confining pressure increased, which indicates that the HDPE geocell for sale can play a role in restraining the soil under high confining pressure.

2.1.2 Influence of cell height h and number of layers
In the case of ensuring the same amount of ribs, the effect of the same spacing of the cell nodes and the stress-strain of the cell height and the number of reinforced layers are compared. The results are shown in Fig. 6. It can be seen that the height of the cell plays a key role in increasing the peak stress of the geogrid reinforced soil when the amount of the rib is the same. Even if a layer of HDPE geocell manufacturers is added, the peak load of the reinforced soil with a high compartment is increased by about 200 kPa compared with the ultimate bearing capacity of the reinforced soil with a lower compartment. At the same time, the role of node spacing is not particularly obvious. In particular, when the confining pressure is increased, the effect of reducing the node pitch on the peak stress is less noticeable. This result shows that the reinforcement effect can be improved by increasing the height of the cell, reducing the number of reinforcement layers, and simplifying the reinforcement step when the amount of the reinforcement is constant.

Fig.6 Stress-strain curves of the reinforced sand

2.2 Analysis of the coefficient of reinforcement
The peak principal stress of different types of high quality HDPE geocell reinforced sand is higher than the peak principal stress of pure sand under different confining pressures, but the degree of improvement is different. In order to better compare the strength changes of sand samples after different types of HDPE geocell for sale reinforcement, the reinforcement effect coefficient Rσ is introduced:

In the formula, Rσ is the strength reinforcement effect coefficient, which is the main stress difference when the reinforced sand is broken, and (σ1-σ3)f is the main stress difference when the reinforced sand is broken. The strength reinforcement effect coefficient of each group of samples can be calculated by the formula (1), as shown in FIG.

Fig.7 Reinforced eff etc coefficients of different conditions

It can be seen from Fig. 7 that the reinforcement effect coefficient of different forms of low price HDPE geocell reinforced sand is greater than 1 (between 1.218 and 1.995) and the reinforcement effect coefficient under low confining pressure is higher than that of the thigh circumference. Pressure. On the whole, the reinforcement effect coefficient of the HDPE geocell manufacturers reinforced sand with a height of 2cm is less than that of the high quality HDPE geocell with a height of 3cm, but the spacing of the nodes is 2. 5 and 5.0cm. The coefficient of the effect of the tendons is not much different. In practical engineering, combined with economic considerations, it is more economical to use a reinforced method with a higher compartment height and fewer reinforcement layers.

2.3 Analysis of strength characteristics
The specific values ​​of p-q for various operating conditions are shown in Table 3, and the curve is shown in Fig. 8, where p = (σ1 + σ3)/2, q = (σ1 – σ3)/2. From the p-q curve, the linear fitting regression equation and the sum of the squares of the total dispersion are obtained. The cohesive force c and the internal friction angle φ of the reinforced soil are calculated by calculation (see Table 4).

Table 3 Peak deviatoric stress of different conditions

Fig.8 p-q curves of different conditions

Table 4 Strength parameters of different conditions

From the strength parameters in Figure 8 and Table 4, it can be concluded that no matter what kind of reinforcement, the c, φ value of pure sand is increased, and the increase of c value is more obvious, from 86. 25kPa to 109.03kPa. , the value of φ is increased from 0. 80◦ to 4.23◦, and the corresponding tanφ is also increased from 0. 014 to 0. 074. This is mainly because the lateral restraint force provided by the compartment to the soil is greater than the frictional force, so the c-value of the soil is higher than the value of φ. However, the change in cell height has a much greater effect on the c and φ values ​​than on the node spacing. When the node spacing is the same, the cell height increases and the corresponding c value increases by about 17 kPa. When the cell height is the same, the node spacing is reduced, and the corresponding c value is only increased by about 5 kPa. The increase in cell height increases the c value by more than 300% of the node spacing. This again proves that the height of the cell plays a key role in increasing the strength of the reinforced soil.

2.4 Destruction form
The shape of the pure sand sample and the reinforced sand sample after shear failure is shown in Fig. 9. The failure mode of the pure sand sample is as shown in Fig. 9(a), showing plastic failure, and the middle bulging and the deformation at both ends are small. The failure mode of the high quality HDPE geocell reinforced sand sample is shown in Fig. 9(b). It can be seen that the upper and lower layers are much smaller than the pure sand sample due to the restraint effect of the low price HDPE geocell, and the deformation of the middle layer also appears. Several protruding points. This may be caused by a reduction in the binding force outside the sand body where the cell is destroyed.

The cell failure morphology (see Figure 10) is a node failure and is reflected in the stress-strain curve at the point of the mutation. When the rib joint is broken, the axial force will suddenly drop. As the axial strain increases, the axial force will rebound to a certain extent, because the damaged low price HDPE geocell is basically located in the middle layer, the upper and lower geotechnical cells are not destroyed, and the damaged tendons are generally The nodes are destroyed one by one, and the entire reinforced body can continue to withstand axial forces when the other nodes are not completely destroyed.

Fig.9 Failure modes of the soil

Fig.10 Failure modes of the reinforcement materials

3. Conclusion

(1) The peak stress of the high-strength low price HDPE geocell-reinforced soil is related to the height of the cell and the node spacing of the cell and increases with the increase of the height and the decrease of the node spacing. The influence of cell height is more significant.
(2) In the case of the same amount of low price HDPE geocell, it is better to increase the height of the HDPE geocell manufacturers and reduce the number of layers.
(3) Different types of low price HDPE geocell increase the strength of the sand after reinforcement, which not only enhances the cohesion c but also increases the internal friction angle φ, but the degree of reinforcement is different, in which the low price HDPE geocell reinforcement Mainly to enhance the cohesive force of sand, the internal friction angle of sand has also been improved.