Experimental Study on Electric Resistivity Characteristics of Compacted Loess under Different Loads and Drying-Wetting Cycles

Da Xing’anling Observation and Research Station of Frozen-ground Engineering and Environment, State Key Laboratory of Frozen Soil Engineering, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Jagdaqi 165000, Heilongjiang, China College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China Faculty of Civil Engineering and Mechanics, Jiangsu University, Zhenjiang 212013, China College of Civil Engineering, Northwest Minzu University, Lanzhou 730000, China College of Architecture and Civil Engineering, Xi’an University of Science and Technology, Xi’an 710054, China


Introduction
Loess is a loose aeolian deposit of yellowish silt-sized dust and is generally found in arid and semiarid regions [1,2]. In China, approximately 6.8% of the land surface is occupied by loess with a total area of approximately 640,000 km 2 [3].
in various man-made infrastructures such as buildings, highways, railways, airfields, and hydroengineering projects.
However, the engineering stability of loess is directly affected by many factors, such as the drying-wetting (DW) cycles, traffic load, and seismic activity. In the context of global climate change, extreme weather has become more frequent in many regions, and soils experience significant amounts of water loss and increase and considerable volumetric contraction and expansion under such conditions [6]. Before and after DW cycles, the size, shape, and arrangement of loess particles vary in different degrees. e proportion and average diameter of macropores in the loess particles increase with the increase in the number of DW cycles and eventually result in loess cracking [7,8], accompanied by a dramatic decrease in the mechanical strength, such as unconfined compressive strength (UCS) and shear strength [9,10]. In addition, repeated traffic loads strongly affect the long-term performance of subgrades. When a traffic load is applied to the soil subgrade, the stress fields and, thus, physical parameters such as moisture content, pore water pressure, and compactness change accordingly, which results in large and differential settlements, significantly affecting the pavement quality and highway capacity [11,12]. erefore, reliable and cost-effective systems are required to monitor the conditions of these engineering parameters and directly maintain the most vulnerable parts of the road network.
Traditionally, field-scale inspections of soil engineering properties commonly utilize field observations and measurements on soil surfaces that require surveyors to walk along the entire structure, such as slope and embankment [13][14][15], in-situ monitoring techniques such as piezometers and clinometers [16,17], and destructive techniques such as excavation and drill-holes [18][19][20]. However, direct observation and monitoring with these methods remain expensive from both human and equipment resource perspectives, and the results are based on several single-point values, which may be unreliable and incomplete and, more importantly, may disturb the original soil pattern, resulting in less accurate evaluations. In addition, remote-sensing-based methods, such as Interferometric Synthetic Aperture Radar (InSAR) [21], photogrammetry using unmanned aerial vehicles [22], and Light Detection and Ranging (LiDAR) [23], can be used to regularly monitor ground movement, but these surveys can only provide topographical information; therefore, they cannot provide real-time monitoring and early warning for structural failure. us, a reliable real-time monitoring method is required to identify changes in the internal conditions that precede failure. e electrical resistivity of soil is known to be a sensitive reflection of many soil properties that are associated with the nature of soil composition (mineralogy, fabric, and particle size distribution), structure (porosity, tortuosity, pore size distribution, and connectivity), water content, temperature, etc. [24][25][26][27][28][29][30][31]. Furthermore, these parameters determine the physical and mechanical properties of soil, which makes it possible to evaluate the engineering properties of soil using electrical resistivity. Various studies investigated the relationship between electrical resistivity and the physical and mechanical properties of soil. For example, Munoz-Castelblanco et al. [32], Seladji et al. [33], and Kibria and Hossain [34] observed that electrical resistivity is related to a variety of hydraulic characteristics of soil, such as water content, saturation, pore water salinity, bulk density, and pore structure. Rinaldi and Cuesta [35] and Liu et al. [36] studied the effect of different compaction degrees on electrical resistivity and attempted to evaluate the degree of soil compaction using electrical resistivity. Long et al. [37] compared and analyzed the relationship between soil resistivity and basic engineering properties at fifteen marine clay sites and indicated that electrical resistivity is negatively correlated with pore water salinity, clay content, plasticity index, and shear strength. Although an increasing number of studies use electrical resistivity to characterize the physical and mechanical properties of soil, the direct application of electrical resistivity methods to monitor engineering stability is still rare. is is primarily because of the uncertainty of soil electrical resistivity affected by loads, DW cycles, etc., after the construction of engineering.
Owing to the load and environmental changes after the engineering works, the relationship between soil strength and electrical resistivity when subjected to load and WD cycles must be understood to evaluate the possibility of longterm monitoring engineering stability based on electrical resistivity method. erefore, a suite of laboratory tests simulating loads (consolidation tests, unconfined compression tests, and uniaxial cyclic unloading-reloading tests) and seasonal field conditions (DW cycle tests) were conducted to resolve the effects of loads and DW cycles on geotechnical and geophysical properties, which are essential for the success of a long-term monitoring system of engineering stability based on electrical resistivity tomography.

e Experimental Soil and Sample Preparation.
e loess used in this investigation, collected from Yongdeng county (36°35′49.64″N, 103°22′45.22″E), Gansu Province, Northwestern China, was extensively used as a primary road embankment fill when the National Lianyungang-Horgos Highway (G30) in Gansu Province was built. According to the Chinese engineering geological zoning map of collapsible loess [38], the sampling site is located in a strongly collapsible loess area with the collapsible grade III or IV. e fundamental properties such as the grain size distribution, specific gravity, plastic and liquid limits, optimum water content, maximum dry density, and soluble salt content of this testing soil are summarized in Figure 1 and Table 1.
After collection from the field, the soil was air-dried naturally, crushed, and sieved using a 2 mm mesh sieve in the laboratory. Sieved loess particles were evenly mixed with an appropriate amount of distilled water to attain the optimum water content of 13%. Subsequently, the set of wet aggregates was stored in a sealed plastic bag for 24 h to homogenize the water content in the soil volume. en, the soil was placed in a cylindrical steel mould and densely compacted to two types of cylindrical soil samples of different sizes with a density of 1.81 g/cm 3 close to the Proctor maximum. e first type of loess samples of 61.8 mm diameter and 20 mm height was subjected to consolidation tests, while the second type of loess samples of 61.8 mm diameter and 125 mm height was used to conduct uniaxial compression and uniaxial cyclic unloading-reloading tests. After sample preparation, the individual soil specimen was wrapped with plastic films and placed in a foam box exposed to room temperature (20°C) in a closed system without moisture evaporation.

Apparatus.
In this study, unconfined compression and uniaxial unloading-reloading tests were performed in a developed triaxial apparatus, which could accommodate a specimen of 61.8 mm diameter and 125 mm height ( Figure 2). A pair of circular electrodes of 40 mm in diameter and 0.05 mm thick, fabricated from copper foil, were attached to the bottom surface of the upper porous stone and the top surface of the lower porous stone using insulating glue. e circular electrodes were connected to an inductance-capacitance-resistance (LCR) digital electric bridge using fine electric wires packaged with insulated skin to measure the electrical impedance of the loess samples. e samples and two pieces of ring-shaped filter paper (40 mm inner diameter and 61.8 mm outer diameter) were sandwiched between the upper and lower porous stones attached to the electrodes. e consolidation tests were performed using a developed consolidation apparatus [39,40] (Figure 3). e development of the electrodes, porous stone, and ringshaped filter paper was similar to that of the developed triaxial apparatus. To eliminate the effect of the metal chamber on resistivity measurement, the soil sample was sheathed using an insulating cutting ring fabricated from high-strength nylon material. e outside of the insulating cutting ring was a special rigid metal ring to prevent lateral deformation of the soil sample. e vertical compression strain ε si in this study is given by the following formula: where h 0 is the initial height of specimen, h i is the stabilized height at a given load, and the s means the one-dimensional compression test. e LCR digital bridge TH2810D was adopted to measure the electrical resistivity of the samples. Because the soil resistivity is significantly affected by external environmental factors [41], a series of measures were performed to obtain the soil resistivity as accurately as possible: (i) graphite was evenly applied to both surface ends of the loess sample; (ii) the digital bridge was calibrated before each test, and the measured electricity resistivity was corrected for temperature as suggested by Zha et al. [42]; (iii) to ensure sufficient contact between the sample and the electrode, a 1 kPa prepressure was applied before all tests, and then the stress-strain returned to zero. An AC frequency of 50 Hz was used in the study, and the resistivity values were calculated using where ρ is the soil electrical resistivity (Ω·m), |Z| is the impedance mode (Ω) measured using the LCR digital bridge, S is the circular electrode area (m 2 ), and L is the distance between the two circular electrodes (m).

Consolidation Tests.
Using the developed consolidation apparatus, consolidation tests were performed on loess specimens to assess their deformation and electrical resistivity behavior at the optimum water content at loads of 12.5, 25, 50, 100, 200, 400, 800, and 1600 kPa [43]. e compression deformation was assumed to be stabilized at this load when the measured vertical settlement for a particular load was not greater than 0.01 mm/h, and a new load was applied. To prevent excessive evaporation, which could cause erroneous results, the soil specimens were covered with cotton gauze during these tests. e LCR digital bridge synchronously monitored the electrical resistivity of loess samples during the consolidation tests and collected data every 2 s.

Unconfined Compression and Uniaxial Cyclic
Unloading-Reloading Tests. Unconfined compression tests were performed using the developed triaxial apparatus at a vertical strain rate of 1.25 mm/min according to the testing  Advances in Civil Engineering methods of soils for highway engineering [43]. Uniaxial cyclic unloading-reloading tests were conducted to investigate the electrical resistivity and damage properties of compacted loess specimens. First, the loess specimen was loaded at an axial strain rate of 0.5 mm/min until the axial strain reached 0.5%, and it was then unloaded at the same strain rate until the stress decreased to zero. Second, the soil specimen was reloaded up to the second peak strain of 1% at the same strain rate. A 90 s period without any loading between unloading and loading was observed to fully eliminate the effect of loading on the next loading. Such unloading and reloading form a full unloading-reloading cycle. is cycle was repeated, and the other designed peak stains were 1.5%, 2.0%, and 2.5%, respectively, in the subsequent three cycles. During the loading and unloading processes, the stress, strain, and resistivity values were recorded simultaneously.

DW Cycle Test.
After sample preparation, the individual loess specimen was air-dried at room temperature from an initial water content of 13% to a water content of 1%. Subsequently, a burette was used to gradually add distilled water up to a saturated water content of 18%. Finally, the loess specimens were air-dried at room temperature to an optimum water content of 13%. Subsequently, the specimens were wrapped with plastic films for a period of at least 24 h to achieve a uniform moisture distribution. is drying and wetting procedure is a full DW cycle test. e design DW cycle times in this study were 1, 3, 5, 7, and 10. When the designed DW cycles were achieved, an unconfined compression test or consolidation test was performed on the soil specimens. Note that three to four loess specimens were used for each test (i.e., consolidation tests, unconfined compression test, uniaxial cyclic unloading-reloading tests, and DW cycle test) for replication purposes.

Relationship between Electrical Resistivity and Compression Characteristics of Compacted Loess.
e relationship between the electrical resistivity of compacted loess specimens and loading time during the consolidation test is plotted in Figure 4. e electrical resistivity of soil samples exhibited a trend of decreasing abruptly and then gradually becoming constant with each new first load applied. Both the abrupt change in the electrical resistivity and the decreasing range of the consolidation stage were relatively large for loads from 0 to 200 kPa. When the applied load was higher 200 kPa, the electrical resistivity of the loess specimens decreased gradually. In contrast, the vertical compression strain increased with increasing load, and it changed noticeably at the beginning stage (loads less than 200 kPa) and then tended to be constant ( Figure 5). Please note that the electrical resistivity changed slightly with a further increase in load and deformation when the load was greater than 200 kPa. is indicated that compression causes the redistribution of water, air, and soil particles in the soil. When the load is small, the compression has a significant effect on the soil structure, which results in a rapid decrease in the electrical resistivity of the soil specimens. As the applied load increases, air is squeezed out of the soil and saturation increases, and the electrical resistivity gradually tends to be constant and becomes less sensitive to changes in stress and strain.
To further explain the relationship between the electrical resistivity and compression characteristics of loess, the relationship curves of void ratio, saturation, and resistivity are plotted in Figure 6. Under the condition of optimal water content, the void ratio of the soil decreases gradually, and the saturation increases with the increase in load. Since the electrical resistivities of soil particles and air in pores are significantly higher than those of pore water, the increase in saturation has a significant effect on the electrical resistivity of soil. Meanwhile, compression of internal pores will result in the connection of some unconnected water pores and form a new conductive path.
us, at the beginning of the consolidation test, with the decrease in the void ratio and the increase in saturation, the electrical resistivity of the loess decreases significantly. When the applied load is greater than 200 kPa, the change rate of both void ratio and saturation decreases rapidly with the increase in load. Hence, the conductivity channel changes slightly, and the electrical resistivity decreases gradually and tends to be stable. In this study, when the void ratio was greater than 0.56, and the saturation was less than 63%, the electrical resistivity of compacted loess was more sensitive to changes in the void ratio and saturation.

Relationship between Stress, Strain, and Resistivity of Compacted Loess.
e stress-strain-resistivity curves of loess specimens under the unconfined compression test indicated that the axial stress attains a peak and then abruptly decreases, exhibiting a typical strain-softening behavior, while the electrical resistivity tends to initially decrease and then increase rapidly (Figure 7). According to the trend of the strain-resistivity curves, it can be divided into four stages. In stage I, the electrical resistivity of loess specimens decreases exponentially with the increase in strain, which corresponds to the compression stage in the stress-strain curve. Under a uniaxial load, air in the pores is discharged rapidly, saturation increases, and pore water connects to form a large number of conductive channels; thus, the resistivity decreases rapidly. In stage II, which corresponds to the elastic stage and strengthening stage in the stress-strain curve, the electrical resistivity of loess specimens decreases with the increase in strain, but the rate of decline rapidly decreases. e soil sample becomes denser after the compaction stage, which makes it difficult for the water and air in the pores to be discharged. As a result, forming new conductive channels is difficult, and the resistivity decreases gradually in this stage. In stage III, the loess specimens are destroyed after attaining the peak strength, and the pores gradually expand and connect, but the soil specimens remain intact, and the electrical resistivity increases gradually. e soil specimens attain the residual strength in stage IV, the fractures overlap by the interconnection of the developing microcracks, and the soil structure is completely destroyed. e conductive channels are constantly broken under the uniaxial load, and the electrical resistivity of the loess specimens increases rapidly. A good real-time correspondence between the electrical resistivity and stress-strain of the loess specimens was observed in the uniaxial compression test. However, compared with stages I and IV, the variation amplitude of electrical resistivity in stages II and III is relatively small, which cannot reflect the variation of stress and strain directly.
is makes determining the peak strength of loess samples using electrical resistivity difficult. e logarithm was obtained for the strain in stages II, III, and IV, and a resistivity-strain semilogarithmic curve was plotted to compare with the uniaxial stress-strain curve more directly (Figure 8). Under a uniaxial load, the stressstrain curve had an approximately straight line when the specimen entered the elastic deformation stage from the compaction stage, and the stress increased proportionally with the strain. After the elastic stage, the stress-strain curve gradually deviated from the straight line, and the soil compressed continuously. When the stress attained the yield limit, the specimen was compressed to the densest state. Correspondingly, after entering the elastic stage, the resistivity-strain semilogarithmic curve also presented an approximately straight line, and the resistivity decreased rapidly with the increase in strain. Subsequently, the rate of resistivity decrease was slightly retarded, but the curve still presented an approximate straight line. When the yield limit was attained, the decrease rate of resistivity was retarded, and the electrical resistivity attained the lowest point when the specimen was destroyed. We observed that the resistivity-strain semilogarithmic curve can better reflect the various stages of the stress-strain curve under a uniaxial load. Before the loess specimen was destroyed, the abrupt Advances in Civil Engineering transition point of the slope of the resistivity-strain semilogarithmic curve was similar to the yield limit in the stressstrain curve, which may be used as a threshold to determine if the soil sample is about to be destroyed.

Relationship between Stress, Strain, and Resistivity of Compacted Loess under the Uniaxial Cyclic Unloading-
Reloading Tests. Figure 9 shows the variation curves of the electrical resistivity and stress of loess specimens over time during the uniaxial cyclic unloading-reloading tests. In the process of axial cyclic loading and unloading, the stress of loess specimens increased and decreased repeatedly, and correspondingly, the electrical resistivity exhibited decreasing and increasing trend. e maximum (minimum) value of stress corresponded to the minimum (maximum) value of electrical resistivity during the test, exhibiting a good corresponding relationship between them. However, the variation amplitudes of resistivity and stress were noticeably different. After each unloading, the stress decreased to 0 kPa, but the electrical resistivity increased slightly and did not increase to the initial value. is indicated that the loading process changed the structure of the loess in that the density increased and the void ratio decreased, and it could not be restored to the original state with unloading.
To explore the relationship between fatigue damage and electrical resistivity of specimens in cyclic loading and unloading tests, the variation in electrical resistivity and stress as a function of strain is shown in Figure 10. For the cyclic behavior, the envelopes of the stress-strain curves were similar to those of the monotonic curves and exhibited a deformation memory of the loess specimen. e shape of the unloading curves was nonlinear and had a concave curvature curve and did not coincide with the loading curve. e loading and unloading curves formed a closed hysteresis loop whose area increased with the increase in loading and unloading times before attaining the failure stress. is     Advances in Civil Engineering indicated that the energy dissipation and fatigue damage of loess specimens increased continuously during loading and unloading [44]. Accordingly, the electrical resistivity curve in Figure 10(b) shows a memory similar to that of the stressstrain curve. e electrical resistivity curves under loading and unloading did not coincide, and closed ring areas were formed before the residual strength was attained. However, as shown in Figure 10(c), the relationship between the area of these rings and the number of unloading-reloading was not apparent. We observed that the electrical resistivity and stress-strain of the specimen had a good correspondence, which can provide a possibility to monitor the engineering properties of loess, which is often subjected to cyclic unloading and reloading. However, there is a certain deficiency in representing the energy dissipation and fatigue damage of the loess under cyclic unloading and reloading.

Relationship between Electrical and UCS and Compression
Characteristics of Compacted Loess under DW Cycles. e variations in the vertical compression strain of loess specimens as a function of the load for different DW cycles are shown in Figure 11(a). e vertical compressive strain increased with an increase in the number of DW cycles. Under a load of 1600 kPa, as shown in Figure 11(c), a large increase in the vertical compression strain occurred in the first three DW cycles, followed by a gradual decreased growth rate in additional DW cycles as the DW cycles increased. e electrical resistivity under various loads did not exhibit a uniform monotonous trend with the number of DW cycles. e initial resistivity (without load) of the soil samples increased almost linearly from 374.72 to 508.12 Ω·m as the number of DW cycles increased ( Figure 11(d)). After the load was applied, the soil electrical resistivity decreased rapidly as the load increased ( Figure 11(b)). Under a load of 1600 kPa, the electrical resistivity decreased rapidly with an increasing number of DW cycles during the first three DW cycles and then remained constant. e UCS of the loess specimens was also significantly affected by the DW cycles ( Figure 12). During the first seven DW cycles, the UCS of loess specimens decreased rapidly with the increase in the number of DW cycles, which indicated a stronger deterioration effect on the compacted loess specimens. e UCS difference of specimens subjected to seven and ten DW cycles was not significant. In contrast, the resistivity increased almost linearly with the number of cycles.
During DW cycles, swelling upon wetting and uneven shrinkage and fissures evolution upon drying would change the soil fabric, rearrange the loess particles, and destroy the integrity of lateritic soil. us, a further decrease in strength and increase in deformation of compacted loess are caused by the repeated DW cycles. e changes in the loess inner structure will affect the electrical conductivity of the specimens. e elevated values of electrical resistivity resulting from macrocracking owing to the insulating nature of air impeded current flow. is observation implies that the presence of desiccation cracks in the loess can be identified, which provides the possibility for long-term monitoring of geotechnical properties in the field. However, for consolidation load, the electrical resistivity of the specimens decreased with an increase in the number of DW cycles. e DW cycles cause a more porous and loose structure of the soil, which provides more conductive channels. Before the load is applied, these channels are blocked by air, resulting in high resistivity. As the load increases, the air is squeezed out, and the pore water is connected. erefore, after the dry and wet cycling and soil compaction, the soil will have a denser structure and lower electrical resistivity. is proves that prewetting can improve the engineering properties of loess and provides a new method for testing the effect of prewetting by using resistivity. However, the strength, deformation, and electrical resistivity of loess specimens subjected to different DW cycles were compared based on the same water content in this study. Considering the significant effect of moisture content on soil electrical resistance [45,46], the field monitoring based on the electrical resistivity method needs the cooperation of moisture probe, and further study is required.

Conclusions
In this study, consolidation tests, unconfined compression tests, uniaxial cyclic unloading-reloading tests, and DW cycle tests on densely compacted loess specimens were conducted to assess their deterioration effects on the electrical resistivity and geotechnical properties of compacted loess. e following conclusions can be drawn based on these laboratory results: (1) Under each load level in consolidation tests, the resistivity of the loess exhibits a tendency of first changing abruptly and then decreasing. e vertical compression strain increases as the vertical load increases, and the change is initially rapid and subsequently slow, which is in contrast to the trend of electrical resistivity. When the load is lower than 200 kPa, the loess resistivity is more sensitive to the load action, while when the load level is higher than 200 kPa, with a further increase in load and deformation, the rate of change of the void ratio and saturation of soil retards, and the resistivity gradually tends to be stable. (2) During uniaxial compression of loess, the variation in resistivity is closely related to that of stress and strain, which can be divided into four stages: Stage I is the compaction stage, and the resistivity decreases rapidly with the increase in strain. e second stage corresponds to the elastic stage and the plastic failure stage. e resistivity continues to decrease with increasing strain, but the decreasing speed of the resistivity gradually is slower. In stage III, the soil specimen is damaged and attains peak strength, and  the resistivity increases gradually. In stage IV, the soil attains residual strength, the structure is completely destroyed, and the resistivity increases rapidly.
(3) For each unloading-reloading cycle, the electrical resistivity exhibits a trend of initially increasing and then decreasing, and its minimum (maximum) value corresponds to the maximum (minimum) value of the stress. Before soil is destroyed, the maximum value of resistivity after each unloading is smaller than the maximum value of the last unloading, which reflects the irreversible plastic damage caused by soil during loading and unloading. However, a certain deficiency occurs in representing the energy dissipation and fatigue damage of the loess under cyclic unloading and reloading. (4) e electrical resistivity of loess samples increases as the number of DW cycles increases but decreases with increasing cycle numbers after stabilization under consolidation load. In the unconfined compression test, the UCS of loess specimens decreased rapidly as the number of DW cycles increased during the first seven DW cycles and then remained constant, while the resistivity increased almost linearly with the number of cycles. (5) e electrical resistivity can effectively characterize the mechanical and deformation characteristics of loess samples under the loads and DW cycles, exhibiting a certain potential for long-term monitoring of soil engineering properties. However, the variation in electrical resistivity is affected by many factors such as water content, temperature, and stress state. erefore, the effect of the inner structure on electrical resistivity may be much more complex, and further study is required.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.