Laboratory investigation of creep behavior of soils under constant shear stress

Dissertation for obtaining a master's degree

in the field of civil engineering, soil and foundation mechanics

Abstract:

Since many structures are built in dense urban areas, there is an increasing need to build buildings and geotechnical structures on soils. Soft clay has been created. Clay soils usually show significant creep deformation. Although a lot of research has been done on the creep behavior of natural clay, there are still many questions about this phenomenon. Even today, predicting the time-dependent deformation of clay in general has many problems. The effects of creep in soils in deviatoric stress (shear stress) have more priority than uniform stresses and hydrometric stress state. If the applied shear stress is greater than a certain limit, creep effects may lead to soil rupture; Therefore, one of the most important issues in shear creep behavior is to find the values ??of shear mobilization that lead to the occurrence of primary, secondary and tertiary creep. Detailed laboratory, field and numerical studies are needed for a better understanding and consequently a better prediction of this behavior. In this research, several consolidation and cutting tests on clay soils have been conducted and evaluated. Then, the soil creep behavior under constant shear stress, in different vertical stresses, has been studied. Finally, the rupture threshold caused by soil shear creep has been evaluated in different initial conditions and expressed as a percentage of the final shear strength of soils.

Keywords: clay soil, shear creep, strain-time behavior, rupture threshold

-1- Introduction

Noting Due to the significant development of construction and construction measures in the creation of airports, underground tunnels, the construction of huge wharves, highways, the construction of dams and related structures, huge irrigation and drainage networks, etc., the need for construction materials has increased to a great extent. Among the most important materials that are used in various construction projects, such as the construction of earthen channels, earthen dams, etc., is soil. Clay soils [1] are extremely complex natural materials containing a large amount of dispersed clay grains with a diameter smaller than 0.002 mm, which have a great effect on the physical, mechanical, and chemical-physical processes inside these materials. Clay soils have all rheological properties, and the creep [2] of clay soils is one of the most common, obvious and tangible of these properties. Due to the complex nature of clay soils, many aspects influence their creep behavior, such as composition (e.g., clay grain size), stress history, temperature variations, biochemical environment, and soil metamorphisms. Compared to sandy materials, clayey soils usually exhibit high shear deformations. For example, we can refer to long-term settlements, horizontal and sloping relocations of buildings and geotechnical structures, or slow sliding of natural slopes and soil masses.

Among the factors that cause non-linear behavior of materials, their behavior is dependent on time, the theorized type of which is known as the phenomenon of creep or secondary consolidation[3].

Creep is a gradual increase in the deformation of a material over time, which is under the effect of a constant load in a simple state. Many materials such as steel, concrete, soil, stone, etc. They show creep in their behavior.

Based on actual observations of old structures and natural slopes, the existence of creep in clay soils has been known since ancient times. However, serious investigation of this phenomenon has started in the middle of the 19th century due to the intense construction activities. We can say that during the last century and mainly during the recent years, creep deformations and shear creeps in clay soils have become one of the most important problems of soil mechanics.

Many documented examples of creep behavior can be stated, some of which have finally led to the rupture and destruction of the structure; such as the Leaning Tower of Pisa and the Vayount reservoir disaster in Italy..

It can be seen that studying the creep behavior of soils in general and the creep behavior of clay soils in particular is very important to better predict the field behavior of structures and slopes. This work is also necessary to avoid significant long-term deformation problems that can lead to the failure of structures. The use of a one-dimensional consolidation test device has been used for a long time in determining the secondary consolidation of soils. The fact is that the effects of creep in soils in deviatoric stress (shear creep [4]) have a high priority compared to uniform stresses and the oedometric stress state. If the applied deviatoric stress is large, the effects of creep may lead to soil rupture, and on the other hand, the limitations of this test in terms of boundary conditions in stress and deformation have caused us to use other equipment such as a direct shear device [5] with constant shear force to estimate creep. By using consolidation and direct shear tests with constant shear force, strain-time diagrams [6] have been extracted and investigations have been carried out on the changes of parameters and consolidation and creep components of clay soil during secondary consolidation and creep; And finally, the rupture threshold of the soil under constant shear stress is expressed in terms of the constant shear stress threshold imposed on the sample and as a percentage of the maximum soil shear strength. The creep components of clay soils are investigated during the process of secondary consolidation and shear creep.

The third chapter first examines the basic concepts related to volume changes and factors affecting it, and then examines soil stress-strain relationships. In the following, the process of consolidation and cutting will be explained. At the end, the concepts of creep and creep patterns and the creep behavior of soils under shear are examined.

In the fourth chapter, the results of experimental tests conducted in the fields of consolidation, shear and creep are analyzed and reviewed. The graphs expressing the stress-strain-time behavior of the desired soil have also been drawn and their characteristics have been described.

In the fifth chapter, the results of this thesis have been summarized.

Abstract:

As more and more constructions are concentrated in densely populated urban areas, there is an increasing need to construct buildings and geotechnical structures on soft clay materials, which usually produce significant creep deformation. Although a lot of research work has been related to the creep behavior of a natural clay material, there are still many questions about this phenomenon. Even today, prediction of the time dependent deformation is generally very problematic. Creep effects in soils, caused by constant shear stresses, are most important, in comparison with normal stresses. If constant shear stress, acting on the soil, is more than certain values, shear creep effects can lead to failure of soil during the time. So one of the main problems in the case of the shear creep behavior, is to find the values ??of the shear mobilization, which are limiting the occurrence of the primary, secondary and tertiary creep phases. Detailed laboratory, in-situ and numerical studies of soft soil creep are necessary for the better understanding and, consequently, better prognosis of this behavior. In this study, first several consolidation and direct shear tests were carried out on soil specimens and the results of these tests were evaluated. Then, creep deformation behavior of soil has been investigated, under constant shear stress condition, in different normal stresses. Finally, creep failure threshold of soil, as the ratio of acting shear stress to the maximum shear strength of the soil, has been represented.