Evaluation of Collapse Sensitivity of Loess Soils and Its Effect on the Distribution Pattern of Pseudokarst Sinkholes at the Regional Scale (Case Study: Golestan Province, Northeastern Iran)

The spread of loess soils in Golestan Province in northeastern Iran has caused considerable environmental issues. To evaluate the effect of loess collapse on the spatial distribution of pseudokarst sinkholes in this province, we have prepared undisturbed samples from 31 locations and characterized their physical and mechanical properties. Moreover, the collapse sensitivity in different parts of the study area has been determined by odometer tests under different pressures and calculation of three parameters, namely, the collapse coefficient (Ic), the time required for 90% settlement due to soil collapse (T90%), and initial collapse stress. Additionally, by conducting field surveying, using aerial photographs, satellite images, and drone flight, the coordinates of sinkholes were identified, and a map of their geographical distribution was prepared. The overlap of the sinkholes distribution map and the coefficient of collapse changes map showed that although the amount of Ic of sandy loess soils (Zone III) is higher than that of silty loess soils (Zone II), the extent of destructive phenomena such as sinkholes is much greater in Zone II than in Zone III. The overlap of the map of sinkholes distribution with the map of changes in collapse sensitivity (Is) of loess soils showed that the density of sinkholes in Zone II (which has higher collapse sensitivity) is higher than Zone III. In Zone II, due to severe collapse sensitivity, the highest concentration of sinkholes is observed in a unit area, more than 0.18 per square kilometer. With a relatively severe to severe collapse sensitivity in Zone III, this concentration is 0.021 per square kilometer. In Zone I (clayey loess), with slight-moderate collapse sensitivity, this destructive phenomenon is observed in the form of side instability since the study area is located in highlands. Furthermore, in this area, the sinkholes (0.004 per square kilometer) have the least concentration.

Characteristics of the Interface between Bamboo Grids and Reinforced Soil of High-Filled Embankments in Loess Areas

There are a large number of high-filled and deep-dug highways in loess areas. The differential settlement between the filled and undisturbed soils is the main cause of damage. Bamboo grids are good reinforcement and flexural tensile materials for highway subgrades, and the properties of the interface between the bamboo grid and loess soil affect the safety and stability of embankments. First, the feasibility of bamboo grid application in high-filled embankments in loess areas was verified based on a durability analysis and test of the mechanical properties of bamboo. Then, a series of large-scale direct shear tests were carried out to determine the shear properties of the interface between bamboo grids and loess soils. The influential factors of vertical stress, shear rate, grid spacing, and compactness on the shear properties were studied, and the related mechanism was discussed. The results show that bamboo grids enhance the shear strength of loess soils more than geogrids under different vertical stresses because of the passive friction resistance between the vertical and horizontal ribs and soil particles, the bite force of particle skeletons, and the surface friction of grids. Bamboo grids enhance the stability and shear resistance of soils because of their good deformation performance, and thus, the shear rate effect within 7 mm/min can be negligible. The greater the relative compaction of the subgrade soil, the better the reinforcement effect owing to the greater cohesive force, greater internal friction angle, and better bite force. The variation in grid spacing changes the embedded effect of soil, side friction resistance, and size of the contact area. The shear resistance has an optimal value, which first increases and then decreases. Therefore, in practical applications, it is necessary to test the optimal bamboo grid spacing for a project.

CPT Parameters of Loess Subsoil in Lublin Area

Loess soils were created by the wind transporting particles with later or in parallel occurred protogenetic, syngenetic and epigenetic processes. As a result, various genetic processes affected loesses strength and deformability characteristics. The aim of the study is to estimate the main CPT parameters of loess subsoil in Lublin area according to divided facies. The subsoil in the area of the Nałęczowski Plateau, where Lublin is located, consists mainly of loess from aeolian and aeolian–diluvial facies, and in the deeper parts—from aeolian–alluvial facies. Most of the results obtained for the aeolian facies at the level of qc in the range from 4.5 to 8.0 MPa indicate that these soils are a good load-bearing substrate for building structures. Cone resistances mostly at the level of 1.5–4.0 MPa for the diluvial and alluvial facies confirm that these facies constitute less favorable foundation conditions. The reduced resistance results mainly from the increased water content in ground pores. It is especially the soils of the diluvial facies that provide unfavorable foundation conditions, as they occur near the surface. Genetic processes are a very important element that should be taken into account in engineering research.

METHODS OF THERMAL STRENGTHENING OF SOILS

In the practice of construction on loess and loess soils, there are numerous cases of significant and uneven settlement of the foundations of buildings and structures due to the ability of subsidence soils to significantly decrease in volume (compaction) when moistened, which causes dangerous and often emergency deformations, and sometimes complete destruction of these structures. Thermal methods of strengthening loess soils make it possible to completely eliminate their subsidence properties and at the same time in-crease the bearing capacity of soils to a depth of 10–15 m.

RESEARCH OF POSSIBLE CAUSES OF DEFORMATION OF BUILDINGS AND STRUCTURES BUILT ON SUBSIDENCE LOESS SOILS

In this article, the authors analyzed the possible causes of deformation of buildings and structures that are built on subsiding loess soils. The research is based on a systematic analysis of the expert practice of identifying the reasons for the deterioration of the technical condition of objects built and operated on such soils. The mechanisms of the development of subsidence deformations of loess soils are considered and the reasons for the change in their hydrogeological regime are analyzed. The main factors and sources are systematized, which with a high probability can lead to changes in the hydrogeological regimes of soils during the construction and operation of buildings and structures operated on subsiding loess soils. Particular attention is paid to the study of the reasons for changes in the hydrogeological regime of soils, which lead to a discrepancy between the calculated characteristics of the soil foundations that were used in the construction of the facility, their actual value. It is shown that this discrepancy is one of the main hidden causes of flooding of buildings and structures, which can lead to uneven subsidence. The relationship between the appearance or development of damage to the building structure and the probable reasons for the deterioration of its technical condition is systematized.