Analysis of the Deformation Characteristics of Loess Based on Thermal–Mechanical Coupling under an Energy Internet

In response to the major challenges faced by China’s transition to green low-carbon energy under the dual-carbon goal, the use of energy Internet cross-boundary thinking will help to develop research on the integration of renewable clean energy and buildings. Energy piles are a new building-energy-saving technology that uses geothermal energy in the shallow soil of the Earth’s surface as a source of cold (heat) to achieve heating in winter and cooling in summer. It is a complex thermomechanical working process that changes the temperature of the rock and soil around the pile, and the temperature change significantly influences the mechanical properties of natural loess. Although the soil temperature can be easily and quickly obtained by using sensors connected to the Internet of Things, the mechanical properties of natural loess will change greatly under the influence of temperature. To explore the influence of temperature on the stress–strain relationship of structural loess, the undrained triaxial consolidation tests were carried out under different temperatures (5, 20, 50 and 70∘C) and different confining pressures (50, 100, 200 and 400[Formula: see text]kPa), and a binary-medium model was introduced to simulate the stress–strain relationship. By introducing the damage rate under temperature change conditions, a binary-medium model of structural loess under variable temperature conditions was established, and the calculation method of the model parameters was proposed. Finally, the calculated results were compared with the test results. The calculation results showed that the established model has good applicability.

A Comparative Study On The Physical Properties of Natural Sedimentary Loess And Manual Filling Compacted Loess

With the development of human society, mega engineering projects of removing the tops of hills to infill valleys began to appear in the loess region. The thickness of the manual filling compacted loess can reach tens of meters. For such large-scale construction projects, studying the properties of compacted loess is essential to ensure the safety and reliability of land creation and artificial infrastructure. In this paper, the specimens from two exploration well profiles were carried out to study the physical properties of natural loess and compacted loess from the Loess Plateau. Here the natural loess selected was deposited in old ages (Q2 and Q1) and had strong stability. The natural water content, dry density, specific gravity, liquid limit, plastic limit, plasticity index, clay fraction, silt fraction, sand fraction, compression modulus, and permeability coefficient have been determined. Statistical theories such as t-test and correlation coefficient checks were used to describe the difference between the two kinds of loess, and the degree of correlation among various indicators. Besides, 14 groups of exploration well data in 8 studies were collected. The variation of natural water content and dry density with well depth was analyzed to supplement the existing data. Results have shown that the manual filling compacted loess is significantly different from the natural loess. On the whole, the liquid limit, plastic limit, plasticity index, clay fraction, silt fraction, sand fraction and compression modulus of the compacted loess are smaller. And compared with the natural sedimentary loess with strong stability, it deforms more easily. The difference of compression modulus between the compacted loess and natural loess is mainly controlled by the dry density and the particle composition. Moreover, the heterogeneous level of the manual filling compacted loess is greater than that of the natural loess in the horizontal direction and smaller than that of the natural loess in the vertical direction. Under a combination of external hydrologic conditions and dead weight, the compacted loess will become more stable.

Destructuration of saturated natural loess: From experiments to constitutive modeling

It has been well recognized that unsaturated natural loess shows significant volume contraction upon wetting due to its metastable internal structure. But the structural effect on stress–strain relationship of saturated natural (undisturbed) loess is much less explored. Few attempts have been made in proposing a constitutive model for saturated natural loess. This study presents both laboratory tests and constitutive modeling of a saturated natural loess, with special focus on the structural effect and evolution of structure damage during loading. Oedometer and drained triaxial compression tests have been carried out on undisturbed and remolded saturated loess samples. It is found that the natural soil structure has dramatic influence on mechanical behavior of loess, including the compressibility, dilatancy, and shear strength. Destructuration, which is the damage of soil structure with deformation, is observed in both oedometer and triaxial tests. A constitutive model is proposed for saturated loess based on the experimental observations. The model is established within the theoretical framework of subloading and superloading surface concepts. Destructuration of loess is assumed to be affected by both plastic volumetric and shear strain. A new method for determining the initial degree of structure is proposed. The model can reasonably predict the compression and shear behavior of both undisturbed and remolded saturated loess.

Farming Influence on Physical-Mechanical Properties and Microstructural Characteristics of Backfilled Loess Farmland in Yan’an, China

A gigantic project named Gully Land Consolidation (GLC) was launched in the hill-gully region of the Chinese Loess Plateau in 2011 to cope with land degradation and create new farmlands for cultivation. However, as a particular kind of remolded loess, the newly created and backfilled farmland may bring new engineering and environmental problems because the soil structure was disturbed and destroyed. In this study, current situations and characteristics of GLC are introduced. Test results show that physical-mechanical properties and microstructural characteristics of backfilled loess of one-year and five-year farmland are significantly affected by the Gully Land Consolidation project. Compared to natural loess, the moisture content, density, and internal friction angle of backfilled loess increase. On the contrary, the porosity, plasticity index, particle size index, and cohesion index decrease. Through SEM tests, it is observed that the particles of backfilled loess are rounded, with large pores filled with crushed fine particles, which results in skeleton strength weakness among particles and pores. The pore size distribution (PSD) of the four types of loess (Q3 loess, Q2 loess, one-year farmland, and five-year farmland) was measured using mercury intrusion porosimetry (MIP) tests, showing that the pore size of Q3 loess is mainly mesopores 4000–20,000 nm in size, accounting for 67.5%. The Q2, five-year, and one-year farmland loess have mainly small pores 100–4000 nm in size, accounting for 52.5%, 51.7%, and 71.7%, respectively. The microscopic analysis shows that backfill action degrades the macropores and mesopores into small pores and micropores, leading to weak connection strength among soil particles, which further affects the physical-mechanical properties of loess. The disturbance of backfilled loess leads to an obvious decrease in cohesion and a slight increase in internal friction compared to natural loess. The farming effect becomes prominent with increased backfill time, while the loess soil moisture content increases gradually. Both the cohesion and internal friction of the backfilled loess soil decrease to different degrees. This study is helpful to investigate sustainable land use in the Chinese Loess Plateau and similar areas.