Prediction model for cumulative plastic strain of FAM lightweight soil under freeze-thaw cycles

Cumulative strain characteristics of compacted soil under effect of freeze-thaw cycles with water supply

Effect of Freeze-Thaw Cycles on Dynamic Characteristics of Undisturbed Silty Clay

In seasonally frozen regions, the bearing capacity of soil decreases and gradually deteriorates after undergoing freeze-thaw cycles. To resolve this problem, based on the idea of frost-resistant filling materials, a filling scheme of expanded polystyrene (EPS) particles lightweight soil in cold regions was proposed. Unconfined compressive strength, direct shear, and micro-SEM tests were carried out to study the physical and mechanical properties of EPS particles lightweight soil under freeze-thaw cycles. The results indicate that the EPS particle lightweight soil has good frost resistance and can be used as frost-resistant filling material in cold regions. Under freeze-thaw cycles, EPS particle lightweight soil maintains good integrity; EPS particles can effectively reduce the frost heave rate, mass loss rate, and compressive strength loss rate of lightweight soil. The compressive strength depends on the EPS and cement contents: it decreases with an increase in the EPS content and increases with an increase in the cement content. The strength loss rate decreases with an increase in both. When the content of EPS is larger (more than 2%), the soil cement bound with EPS particles is limited, and the performance of lightweight soil decreases. The shear strength and cohesion decrease with an increase in freeze-thaw cycles and EPS content, and the internal friction angle follows no obvious rule with regard to the increase in freeze-thaw cycles but decreases with an increase in the EPS content. Based on the experimental results, an empirical formula for the compressive strength of EPS particle lightweight soil under freeze-thaw cycles was proposed. This study can provide a reference for the engineering design and application and provide new ideas for resolving freeze-thaw problems in construction engineering in cold regions.

To investigate the dynamic stability of natural subgrade filler (NSF) and fiber-binder reinforced subgrade filler (RSF) under cyclic load after freeze-thaw (FT) cycles, a triaxial test was conducted to determine the correlation between cumulative plastic strain (CPS) and the quantity of loading cycles, as well as the evolution law of dynamic strength and critical dynamic stress (CDS) with different FT cycles. The CPS change in the NSF and RSF shows three states (stable, critical, and destructive) with increasing vibration times. However, both fillers have different failure forms, and the curve shapes of the CPS with loading cycle quantities before and after failure are also different. With the number of FT cycles increasing, the requisite dynamic stress threshold for NSF specimen failure decreases continuously. After three FT cycles, the anti-cumulative deformation ability of the NSF decreases by approximately 32%. The anti-cumulative deformation abilities of the NSF after seven and nine FT cycles, respectively, are similar. The amelioration measures could significantly enhance the FT resistance of the NSF. After zero, one, three, five, seven, and nine FT cycles, the requisite dynamic stress threshold for the RSF to reach destruction is increased 1.52, 1.89, 1.98, 2.32, 2.2, and 2.45 times, respectively, compared to that of the NSF. A mechanical model of critical dynamic stress of the NSF and RSF that considers the FT cycle was obtained using a multivariate nonlinear regression method.

Investigating Mechanical Properties of Cemented Soil under Static and Dynamic Loading after a Freeze-Thaw Cycle

A series of saturated undrained dynamic triaxial tests were carried out by using DYNTTS large-scale triaxial cyclic test system to investigate the long-term dynamic characteristics and cumulative strain evolution of round gravel foundation under cyclic subway loading. The influence mechanisms of relative density, confining pressure and dynamic stress amplitude on the long-term dynamic characteristics and cumulative deformation of round gravel were systematically investigated. A prediction model of cumulative plastic strain considering the coupling effect of 3 parameters was proposed. The results of the study show that for the subway tunnels with shallow burial depth and low relative density of the round gravel foundation, the train operation has potential safety hazards, while for the subway tunnels with deeper burial depths, the operation of the trains is relatively safe. Increasing relative density and confining pressure can effectively improve the resilient modulus of round gravel, reduce its cumulative plastic strain, elastic strain and dynamic pore-pressure ratio, and significantly enhance the foundation's liquefaction resistance. Therefore, for unfavorable working conditions such as Dr=0.3 and σ3 = 100kPa, improvement measures such as increasing tunnel depth and implementing high-pressure grouting can be adopted to reduce the settlement of the subway foundation. Based on the time-hardening theory, a cumulative plastic strain prediction model was established with comprehensive consideration of relative density, confining pressure and dynamic stress amplitude. The research results can provide theoretical support and a design basis for the dynamic stability assessment of subway round gravel foundation and its long-term settlement prediction after construction.

In order to research the dynamic stress–strain relationship of EPS (expanded polystyrene) particles light weight soil, under the condition of fixed water content and fixed curing age, the dynamic deformation properties of light weight soil with different mixing ratios are researched by dynamic triaxial tests. The results show that at the same cycle number and dynamic stress, the dynamic strain of light weight soil decreases with the increasing of cement content and the decreasing of EPS particles content. The dynamic bearing capacity of EPS particles light weight soil with cement content in the range of 10–20% and EPS particles content in the range of 0.14–0.86% is 1.5–3 times that of remolded soil. It indicates that light weight soil possesses obvious dynamic bearing capacity. In addition, when the vibration frequency increases from 0.5 to 6 Hz, the bigger the loading rate is, the more uneven the distribution of the pore water pressure in the soil is, and the less deformation can occur, which means that there is a greater constraint on the development of pore water pressure and deformation. Thus at the same dynamic strain, the dynamic stress of light weight soil under bigger vibration frequency is greater. The shape of dynamic backbone curves of light weight soil conforms to hyperbola, and its nonlinear dynamic response process can be described by Hardin–Drnevich model. Different stress state tests are used to verify the applicability of Hardin–Drnevich model. It is found that the relative errors between the measured values and calculated values of backbone curves are less than 15% by analysing the test data. It shows that Hardin–Drnevich model can be used to describe the dynamic deformation characteristics of EPS particles light weight soil.

Experimental investigation on dynamic characteristics of fiber-binder modified subgrade filler after freezing-thawing under cyclic loading

This paper aims to study the density, strength and deformation characteristics of soil and EPS particles mixtures that is a type of light weight soil. It carried out the density tests and unconfined compressive strength tests of the mixed soil with different cement content, EPS content, age and water content. The results show the following findings. When volume ratio of EPS particals is 20% to 60%, the density range of the light weight soil is about 0.64g·cm-3~1.46g·cm-3. The amount of EPS particles has the greatest impact on the light weight soil density. When the volume rate of EPS particles is increased by 10%, the density of EPS particles light weight soil is decreased by about 0.15g·cm-3~0.23g·cm-3, which shows that it is feasible to realize the light weight of mixed soil by adding EPS particles. Cement content and water content have less influence on density of light weight soil. But age has little effect on density of light weight soil. In addition, when the volume ratio of EPS is 50%, the density of light weight soil is similar to that of water. The range of unconfined compressive strength of EPS particles light weight soil in this test is 103.2 kPa~1359.0 kPa, while the unconfined compressive strength of plain soil is 79.98 kPa. So it can be stated that the light soil in the proportioning of this test has better strength properties relative to the plain soil. The unconfined compressive strength of light weight soil increases exponentially with the increase of cement content. However, the larger the volume ratio of EPS particles, the smaller the unconfined compressive strength and the linear relationship between them. In the case of more than the optimal water content, with the increase of water content, the relationship between the unconfined compressive strength is reduced exponentially. The increase of age makes the unconfined compressive strength increase in hyperbolic form. The stress-strain relationship characteristics of EPS particles light weight soil are mainly characterized by strain softening type. However, the increase of water content and the increase of the content of EPS particles can make the stress-strain relationship curves of the light weight soil change gradually to the hardening type. Damage strain and average deformation model are used to describe the deformation characteristics of soil. The damage strain of light weight soil decreases with the increase of cement content, but the volume ratio of EPS particle is opposite. The greater the water content, the greater the damage strain of light weight soil. However, the increase of the age makes the damage strain of the sample decrease. When the volume ratio of EPS particles is more than 50%, the damage strain of light weight soil is greatly increased, and its brittleness is obviously weakened. The average deformation modulus is a parameter to characterize the stiffness of EPS particles light weight soil. The larger the cement content, the larger the average deformation modulus of light weight soil. However, the average deformation modulus of light weight soil decreases linearly with the increase of the volume ratio of EPS particles. The increase of water content can reduce the stiffness of light weight soil, and the longer the age, the greater the stiffness.

Investigation of dynamic characteristics and cumulative plastic strain prediction model of clay-fouled round gravel under cyclic subway loading

Plastic strain characteristics on frozen silty clay subjected to intermittent vehicle loads

ABSTRACTSalt cavern gas storage is an important technical means to balance the demand for staggered energy supply. Due to the repeated injection and extraction of natural gas in gas storage facilities, sealing integrity failure in the wellbore of gas storage facilities frequently occurs. In response to this, considering the cyclic loading and unloading of the pressure load inside the casing, mechanical tests of set cement were carried out under alternating loads, quantifying the cumulative plastic strain change law of set cement and revealing the deterioration characteristics of its mechanical properties. A numerical model of cumulative plastic strain of casing cement sheath formation under alternating load was established based on the obtained experimental parameters. Comparative verification was conducted using experimental data, and the variation law of cumulative plastic strain of cement sheath was analyzed. The distribution of cumulative plastic strain on the cement sheath bonding surface of the entire wellbore was quantified. The research results indicate that the higher the internal pressure value of the casing, the earlier the plastic strain appears, and with the increase in the number of alternating loads, the cumulative plastic strain increases approximately linearly. After the internal pressure increased by 30 MPa, the cumulative plastic strain increased by a maximum of 46.75%. When the number of loading and unloading cycles under alternating loads is small, reducing the elastic modulus (6 GPa) of the cement sheath can effectively reduce its cumulative plastic strain. However, as the number of loading and unloading cycles under alternating loads exceeds a specific value, the cumulative plastic strain produced by high elastic modulus (15 GPa) cement sheaths decreases. Finally, the distribution pattern of cumulative plastic strain along the wellbore under different gas injection times and complex formation conditions was analyzed. Suggestions for establishing well barriers in salt cavern gas storage during cementing were proposed. The research results can provide theoretical and engineering references for evaluating the sealing integrity of gas storage wells.

Deformation Research of Silty Clay under Freeze-Thaw Cycles

Effect of freeze-thaw cycles on mechanical and porosity properties of recycled construction waste mixtures

Chlorine saline soil has adverse engineering geology such as dissolution, collapse, and pulping, the recycling of saline soil in subgrade treatment is of great research significance. First, the unconfined compressive strength (UCS) test was conducted using an orthogonal test design, and the optimal ratios of salt content, cement content, lime content, fiber content, and fiber length were determined. Second, the cyclic loading (CL) test was conducted using an orthogonal test design, and the influence of five factors, including freeze–thaw (FT) times, CL times, stress level (SL), amplitude, and frequency, on peak intensity, peak strain, and cumulative strain under FT cycles and natural air-drying (A-D) conditions were determined. Finally, the improvement mechanism of the above inorganic materials was microscopically analyzed by scanning electron microscope-energy dispersive spectrometer and X-ray diffraction tests. The UCS test results show that the improving effect of the optimal ratio for chlorine saline soil was best when the optimal ratio values were 3% (salt content), 8% (cement content), 12% (lime content), 0.2% (fiber content), and 18 mm (fiber length). CL test results show that with the increase in natural A-D time, it was found that the FT number and SL have significant effects on the strength, the number of FT times and cycle times have significant effects on the deformation, and the frequency and cycle times have significant effects on the cumulative deformation. From the microstructure analysis, the improvement mechanism is mainly the filling of pores and linking particles by water-hard gels and gas rigid gels to make the microstructure dense, effectively reduce pores, and effectively improve the engineering characteristics of chlorine saline soil. The results of this study provide a scientific basis for the engineering design of subgrade in arid areas and the in situ recycling of saline soil.