# Viscosity of Clayey Soils: Experimental Studies
## 1 Introduction  
Over the last hundred years, the study of the phenomenon of the creep of clay soils has become a subject of scientific research in connection with the active development of housing, transportation and hydraulic engineering construction. The increased attention of scientists to the creep process occurring in clay soils was caused by the fact that in a number of cases, unacceptably large deformations of various buildings, structures and roads were observed, disturbing their normal operation, and in some cases, leading to destruction. The process of the creep of clay soils in nature manifests itself in the form of long-term settlements, displacements, rolls, horizontal displacements of structures, as well as in the form of the slow sliding of clay masses along natural slopes and slopes of structures made of soil materials. The historical development of soil rheology began with the study of the shear creep deformations of soils. [PERSON]. [1] mentioned in his book that the phenomenon of shear creep is caused by two mutually opposite processes: the process of hardening (the process of damped creep causes a decrease in shear strains) and the process of soil unstrengthening (the development of the process of non-damped creep causes an increase in shear strains). [PERSON] believed that in the process of shear both destruction and restoration of contacts between mineral particles in the skeleton of clayey soil occur [2]. [PERSON] proposed to take into account the change in the viscosity of the soil skeleton in time in order to take into account the hardening of clayey soil under shear [3, 4]. Based on the results of experimental studies of clay in the ring shear device, [PERSON] suggested that shear strains change discontinuously during testing under constant load [5]. To describe the process of soil creep, [PERSON]. [6; 7; 8], [PERSON]. [9] and [PERSON]. [10] applied the Boltzmann-Volterra theory of hereditary creep. This theory as applied to soils was first used by [PERSON]. [11; 12] and was subsequently developed by [PERSON] and [PERSON]. [13] to describe the creep process of hardening media (in particular, concrete), which made it possible to take into account the hardening of soils in the process of creep. A great contribution to the development of soil rheology was made by [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and many other researchers [1; 1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. Currently, a large number of scientific works are devoted to the study of the rheological properties of clay, sandy, melted and frozen soils, in which the authors derive various equations of state describing the rheological properties of soil on the basis of theoretical, experimental and numerical studies. Based on the performed experimental studies of clayey soil in the direct shear apparatus, [PERSON] and [PERSON]. [19] showed in their work that the time of action of compacting load before shear tests has a significant effect on the structure and properties of clays, which undoubtedly affects the strength and creep indices of clayey soil. A hypoplastic model (a model in which there is no clear division into elastic and plastic deformation zone) for clayey soil, which includes eight parameters that can be determined in laboratory conditions on the basis of performed experiments, was proposed by [PERSON] and [PERSON] [20]. A nonlinear creep model using a fractional order derivative and a small number of parameters capable of describing the creep process of clay was proposed by [PERSON] et al. [21]. The authors compared the proposed creep model with the Burgers model describing the behavior of viscoelastic materials. In this work, experimental soil investigations were also conducted to study the nonlinear creep properties of clay. A nonlinear creep model for frozen sand was proposed in [22].