# A Test Method for Creep Characteristics of Loess Based on the One-Dimensional Consolidator

Authors: Rongbin Huang; Shuangming Wang; Shuancheng Gu; Zhengjun Mao
Journal: Advances in Civil Engineering
 (2021)

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## Abstract

When the traditional one-dimensional consolidator is used to study the creep characteristics of loess, due to the hoop effect of the ring cutter, only the attenuation creep stage and stable creep stage of loess can be studied, but the accelerated creep stage cannot be presented. In order to avoid the influence of drilling on the creep characteristics of loess, the paper improves the consolidation instrument by drilling holes along the diameter direction in the center of the sample to provide artificial space for soil failure. At the same time, the sample size is increased to ensure that the diameter of the sample is greater than five times of the diameter of the borehole, so as to avoid the influence of drilling on the creep characteristics of loess. The creep characteristics of loess are studied by step loading (vertical pressure at all levels is 125 kPa, 175 Pa, 225 kPa, and 275 kPa), and the whole creep process characteristic curves of loess under different stress conditions are obtained. An endoscope was placed in the hole to observe the deformation and failure characteristics of loess in different stages of creep. This method makes up for the defect that the traditional one-dimensional consolidator cannot obtain the whole process characteristics of loess creep. At the same time, it has the advantages of simple operation, less external influence factors, stronger data reliability, and can directly observe the changes of loess creep soil. It has a beneficial role in promoting the experimental research of loess creep characteristics.


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## Body

## 1. Introduction

The loess covers a vast area, with a total area of 13 million km2, covering more than 9.3% of the global continental area. The total area of loess in China is about 640000 km2, accounting for 6.3% of the total land area, of which loess is the most widely distributed in Western China, accounting for 50%–60% of the total area in the western region. The creep behavior of loess is one of the most important problems to be studied [1–7].For a long time, many scholars have made a lot of progress in the experimental study of soil long-term deformation. Bjerrum carried out creep test on Norwegian marine clay and analyzed the micromechanism of cohesive soil during long-term deformation [8]. Through a series of one-dimensional consolidation and isotropic consolidation tests, Mesri and Godlewski pointed out that the soil with high compressibility in the main consolidation stage has relatively high compressibility in the secondary consolidation stage [9]. Chang et al. pointed out that when the effective stress increases to the preconsolidation pressure, both the secondary consolidation coefficient and the compression index increase. When the effective stress exceeds the preconsolidation pressure, the coefficient of secondary consolidation and compressibility do not change [10]. Wang et al. carried out a series of creep tests and obtained the influence of moisture content, dry density, and deviatoric stress on the creep characteristics of undisturbed, remolded, and saturated loess [11]. Zhu and Li carried out in situ monitoring and laboratory test by using the soil resistivity consolidation tester to reveal the creep mechanism of the loess high-fill foundation from the microperspective [12]. He studied the correlation characteristics of different confining pressures, water contents, and dry densities on the change of loess deformation with time through the triaxial compression test of remolded loess [13]. Xiao et al. took unsaturated silty clay as an example, carried out triaxial compression consolidation drainage creep test under matrix suction control, constructed the elastomer and fractional viscous body considering matrix suction, and established the creep constitutive model of unsaturated silty clay considering matrix suction [14]. Ge et al. put forward a creep model conforming to the deformation law of compacted loess based on the long-term creep test of loess with a one-dimensional consolidation instrument [15, 16]. Zhang et al. studied the creep effect of loess through the one-dimensional consolidation creep test and analyzed the effects of water content and compactness on loess creep [17]. Yang et al. and Zhang studied the compactness and moisture content characteristics of remolded loess by using the one-dimensional confined compression experiment and analyzed the creep law of soil samples [18, 19]. Chen studied the creep properties of remolded loess by using the one-dimensional consolidation instrument and three-dimensional consolidation instrument. The above tests showed two-stage creep characteristics [20].Many scholars have carried out in-depth research on the creep characteristics of loess and achieved a series of excellent results [21–27]. In practical engineering, loess is basically multidimensional stress. However, through the study of loess creep under one-dimensional stress, we can better understand the creep characteristics of loess under multi-dimensional stress, so the relevant research is still of great significance. In addition, it is easier to control the load and eliminate the external factors, so it is easier to draw valuable conclusions. Based on the long-term creep test of loess with the one-dimensional high-pressure consolidator, some scholars have proposed an empirical model that conforms to the deformation law of compacted loess [28–31]. According to the results of the unidirectional consolidation test of loess, some researchers compared undisturbed loess with disturbed loess and obtained creep curves and characteristic parameters of loess creep characteristics [32–34]. Some researchers have carried out unidirectional consolidation tests on loess which has experienced freeze-thaw cycles and studied the effects of freeze-thaw cycles and normal pressure on the consolidation creep characteristics of freeze-thaw loess [35–37].However, during the loess creep test with the one-dimensional consolidation tester, in addition to providing lateral confining pressure for the specimen, the lateral deformation of the specimen will also be limited under the hoop action of the ring cutter. Under the above joint action, only vertical compression settlement can occur in the test process, and the subsidence rate gradually decreases to zero, which means that the above tests can only show the characteristics of deceleration creep, but not the accelerated creep stage. Therefore, the paper drilled holes along the diameter direction in the center of the sample to provide artificial soil failure space. The specimen size was increased to ensure that the drilling does not affect the overall stress distribution of the specimen. At the same time, an endoscope was placed in the borehole to observe the deformation and failure characteristics of loess in various stages of creep.


## 2. Test Principle

The method of drilling holes in the middle of the sample is used to provide deformation space for loess creep. According to Saint Venant's principle and the elastic stress solution of underground cavern, the excavation of circular cavern has a significant impact on the surrounding rock within 5 times the tunnel diameter, and the surrounding rock stress outside 5 times the tunnel diameter can be approximately the original rock stress. Therefore, the sample drilling diameter shall meet the following requirements:(1)r0<15r1,where r0 is the drilling radius and r1 is the ring cutter radius.The hole can maintain self-stability under the external load. In the creep test of loess under specific confining pressure, it is necessary to ensure that the borehole remains stable at the moment of loading.On the premise of meeting the above conditions, the larger value of borehole diameter should be adopted to make the test results more obvious. According to the above principle, the experimental design is carried out, and the test schematic diagram is shown in Figure1. The size of the ring cutter increased from 2.0 cm × 6.18 cm (height × diameter) to 6.0 cm × 6.18 cm. Holes were drilled along the diameter in the center of the sample to provide the deformation space for loess creep, as shown in Figure 2. During the test, in order to obtain the deformation and failure mode of soil in real time, an endoscope was used to collect photos, as shown in Figure 3.Figure 1
Schematic diagram of the test: 1, test platform; 2, loading weight; 3, transfer lever; 4, water tank; 5, retaining ring; 6, sample; 7, ring cutter; 8, sample drilling; 9, permeable stone; 10, pressure upper cover; 11, loading guide rod; 12, displacement meter; 13, displacement meter frame.Figure 2
Schematic diagram of the ring knife and soil sample.Figure 3
5.5 mm snake tube endoscope.


## 3. Test Procedure

(1)
In order to separate the sample from the ring cutter after the test, attention should be paid to the lubrication treatment of the wall of the ring cutter.(2)
The drill bit with the specified diameter is selected to drill the sample along the diameter. Drilling should be carried out slowly and uniformly to reduce disturbance to the sample. After the completion of drilling, the hole should be cleaned to ensure that the surface around the hole is smooth, and there are no soil particles in the hole.(3)
The retaining ring, permeable plate, and thin filter paper are placed in the consolidation container. Install the ring cutter with the sample into the retaining ring and put on the guide ring. Thin filter paper, permeable plate, and pressurized upper cover are placed on the sample in turn. Place the consolidation vessel in the center of the pressurized frame so that the pressurized upper cover is aligned with the center of the pressurized frame. Finally, install the dial indicator.(4)
Apply 1 kPa prepressure to make the sample contact with the upper and lower parts of the instrument. Adjust the dial indicator or sensor to zero or measure the initial reading.(5)
Group loading is implemented. The vertical load values are 125 kPa, 175 kPa, and 225 kPa, respectively, and one group of parallel tests is set to verify the accuracy of the test results.(6)
According to the following time sequence, the height change time of the sample is 6 s, 15 s, 1 min, 2 min, 4 min, 6 min 15 s, 9 min, 12 min 15 s, 16 min, 20 min 25 s, 25 min, 30 min 15 s, 36 min, 42 min 15 s, 49 min, 64 min, 100 min, 2 h, 3 h, 4 h, etc., until stable. The stability criterion of the paper is that the soil deformation within 24 hours is less than 0.01 mm.(7)
After the test, the sample is separated from the ring cutter. The samples were sliced at the quarter diameter, half diameter, and diameter. The distribution law of plastic failure of soil around each section of the borehole was recorded.


## 4. Analysis of Test Examples

Taking the loess in Xi’an area of China as an example, the creep characteristics of loess are studied. In the experiment, the inner diameter of the ring knife is 61.8 mm, and the height is 60 mm. According to the drilling diameter requirements, the drilling diameter is determined to be 8 mm, and the drilling position is at the center line of the ring cutter height. The main physical and mechanical parameters of the sample are shown in Table1.Table 1
Main physical and mechanical parameters of loess samples.
Natural density (g/cm3)Lateral pressure coefficientLiquid limit (%)Plastic limit (%)Moisture content (%)Internal friction angle (°)Cohesion (kPa)1.670.40827.2217.2523.824.445.6In order to explore the deformation and failure law of specimens under different vertical loads, the vertical loads of 100 kPa, 200 kPa, 300 kPa, 400 kPa, 500 kPa, 600 kPa, 700 kPa, and 800 kPa were applied to the samples. After the deformation is basically stable, the deformation and failure modes of the borehole under the current stress state are photographed by using a peep. After the above load is applied, the soil sample is taken out from the ring cutter. Soil samples were sliced in different directions to analyze the plastic failure law of soil.Table2 shows the failure modes of the samples under different vertical loads. Through the analysis, it is preliminarily determined that the maximum vertical load to keep the borehole stable is between 200 kPa and 300 kPa. It can be seen from Figure 4 that, after loading 800 kPa, the specimen is obviously compressed, and the drilling hole moves downward as a whole.Table 2
Failure modes of drilling holes under different vertical loads.
Load (kPa)Pictures and introductionLoad (kPa)Pictures and introduction0100Ring cutter size: height 60 mm and diameter 61.8 mm. Drilling size: 8 mm in diameter. Drill along the ring cutter diameter at 30 mm height to ensure smooth drilling.After loading, the borehole did not change significantly.200300The surface of the borehole began to be uneven.The surface of the borehole is obviously uneven. In the upper right part of the borehole, skin exfoliation first appeared. According to the image feedback, when the iron bar coated with vaseline is gently in contact with the surface of the borehole, soil particles can be found attached, which indicates that soil is damaged and dropped into the hole.400500The surface of the borehole is further roughened.There was a large area of exfoliation on the drilling surface. The hole is flattened; that is, the hole circumference changes from circular to oval.600700There was a large area of exfoliation on the drilling surface.There was a large area of exfoliation on the drilling surface.800The surface of the borehole peeled off in large area and gradually filled the whole borehole.Figure 4
Deformation pattern of the sample after unloading. (a) The form of hole sinking. (b) The overall sinking pattern of the sample.
(a)(b)Figure5 shows the failure pattern of loess along the drilling direction. The main failure mode is the surface soil spalling, and the failure range is basically symmetrical along the borehole.Figure 5
Failure pattern of loess along the drilling direction.Figure6 shows the failure pattern of the sample at different positions perpendicular to the drilling direction. The range of the plastic zone at 1/4 radius is relatively small, the borehole becomes smaller, and there is spalling soil in the borehole. The plastic zone at 1/2 radius is obviously expanded, and the drilling hole changes from circular to oval. The drill holes in the section at the diameter are oval. The plastic zone is further expanded and distributed symmetrically in saddle shape.Figure 6
Failure mode of the slice perpendicular to the drilling direction: (a) failure mode of the drilling hole at the 1/4 radius section, (b) failure mode of the drilling hole at the 1/2 radius section, and (c) failure mode of the drilling hole at the diameter section.
(a)(b)(c)Figure7 shows the distribution of the plastic zone near the diameter. The plastic zone is symmetrically distributed in the upper part of the borehole in the form of bimodal. The maximum radius of the plastic zone is about 45° with the horizontal direction. The maximum radius of the plastic zone is about 2.9 times of the drilling radius. The radius of the plastic zone directly above the borehole is about 1.7 times of the borehole radius. The radius of the plastic zone on both sides of the horizontal direction of the borehole is about 2.0 of the borehole radius. Through the parallel contrast test, it is found that the above laws are universal. The analysis shows that the hole near the ring cutter is obviously affected by the boundary of the ring cutter, which leads to the difference between the deformation and failure of the drilling hole at the central position. The maximum radius of the plastic zone is about 45° with the positive angle of the Y-axis. The failure mode is consistent with the boundary line of the plastic zone of the soil sample calculated by relevant theories.Figure 7
Plastic failure of soil around the drilling hole.Figure8 shows the creep characteristics of loess under different loading conditions. When the vertical load is 125 kPa and 175 kPa, the samples show typical attenuation creep characteristics: the soil strain rate gradually decreases from the maximum value to 0, and the soil strain tends to a fixed value. When the load is 225 kPa, the specimen first experiences the decay creep and the stable creep. When the specimen is deformed to point a, the strain rate increases and enters the accelerated creep stage. When the load is 275 kPa, the creep law of the specimen is basically consistent with that at 225 kPa. However, due to the increase of vertical load, the total strain increases, and the time from the stable creep stage to the accelerated creep stage is advanced, and the strain increase rate in the accelerated creep stage increases.Figure 8
Creep curves of loess samples. Stage I: decay creep stage and stable creep stage. Stage II: accelerated creep stage.When establishing the creep constitutive model of loess based on the test results, it is necessary to consider not only accurately reflecting the actual creep characteristics of soil but also minimizing the undetermined parameters in the model.Burgers model can be used to describe the soil with instantaneous elastic deformation, attenuation creep, constant velocity creep, and residual deformation after unloading. At the same time, the model parameters are less, which is convenient for theoretical analysis. Therefore, Burgers model is used to analyze the creep characteristics of loess. Since Burgers model is a viscoelastic model, the relevant data of the accelerated creep stage are excluded for analysis when calculating the model parameters (Figure9).Figure 9
Burgers model.The constitutive equation of the Burgers model is as follows:(2)ε=σE1+ση1t+σE21−e−E2/η2t.Figure10 and Table 3 show the fitting results and model parameters of the Burgers model. Burgers model can accurately characterize the creep characteristics of loess. It can be seen that the application of the test method proposed in this paper, combined with the Burgers model, can scientifically analyze the deformation law of loess in elastic deformation, attenuation creep, constant velocity creep, and other stages.Figure 10
Fitting analysis with the Burgers model.Table 3
Parameters of the Burgers model.
Load (kPa)E1 (kPa)η1 (kPa.h)E2 (kPa)η2 (kPa.h)Variance27538.7768728.52372.0231.350.9332622530.7648661.80140.63114.950.9711817531.0450761.42188.3769.530.9819512528.9357486.59251.5548.680.97323


## 5. Discussion

Compared with the traditional one-dimensional consolidation apparatus, the creep test method proposed in this paper can obtain the whole creep process characteristics of loess. At the same time, the method has the advantages of simple operation, less external factors, and real-time observation of the internal failure mode of soil. It provides a new method for the further study of loess creep characteristics and has a beneficial role in promoting the related research.The test method described in this paper is generally applicable to the study of creep characteristics of general loess. The experimental results provide a basis for the study of the long-term strength change of loess, the long-term deformation law of loess, and the long-term mechanical characteristics of the supporting structure under loess geological conditions.However, this experiment only considers the vertical load conditions, so it cannot study the creep characteristics of loess under complex stress conditions. At the same time, because the collapsibility of loess has a direct impact on the deformation and failure of the borehole, it is impossible to accurately distinguish collapsible deformation from creep deformation. Therefore, the creep characteristics of collapsible loess cannot be studied by this method.In the next step, it is necessary to systematically study the creep characteristics of loess under the influence of key parameters such as moisture content and porosity.


## 6. Conclusion

(1)
Based on the one-dimensional consolidator, a creep test method of loess is proposed in this paper. Three complete stages of the loess creep process can be obtained by this method. At the same time, the change of soil mass can be observed in the whole process of creep, so as to understand the creep characteristics of loess.(2)
At the moment of loading, the sample deforms instantaneously. With the increase of time, the deformation also increases, and the higher the stress level, the greater the instantaneous deformation and creep deformation.(3)
With the increase of vertical load, soil debris appears first on the surface of the borehole, and then there are complex cracks around the hole. At about 45° above the hole inclination, the soil mass first exfoliated, and then the spalling area diffused in a large range until the whole hole was destroyed. In this process, the shape of the borehole becomes flat and finally changes from round to oval.



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*Source: 6656819-2021-10-11.xml*