Investigating the creep behavior of soil reinforced with geosynthetic fibers in laboratory conditions

Dissertation for receiving a master degree

in the field of civil engineering with soil and foundation orientation

Abstract:

The compressibility behavior of soils has become an important concern in geotechnical engineering. As buildings, embankments and roads are implemented, significant settlements often occur. Since long-term settlements occur due to creep, therefore, the calculation and prediction of creep settlements is very important. When the soil is subjected to constant loading, with the complete loss of pore water pressure, deformations occur over time, known as secondary compaction or creep. The mechanisms and factors influencing the creep behavior of sandy soils are not yet fully understood. In this study, single-stage, multi-stage, and loading-unloading one-dimensional consolidation tests were performed on unreinforced and reinforced clayey sandy soil samples with geotextile fibers, and the effect of soil reinforcement with geotextile fibers, the effect of stress levels, stress history, and pore water on creep deformations were studied, and the creep mechanism was stated by considering slippage, impact, and particle deformation. The obtained results are described based on the relationship between porosity and secondary density coefficient. The results of the experiments show that at low stresses, creep deformations in saturated samples are larger than dry samples, but with increasing stress, creep deformations decrease in saturated samples and increase in dry samples. In addition, in the loading-unloading test, the creep speed is higher than the single-step test, and this test is effective in accelerating the creep. In reinforced samples, with an increase in the percentage of geotextile under the same stresses, creep deformations decrease in saturated samples and creep deformations increase in dry samples.

Key words: creep, clayey sand soil, one-dimensional consolidation, geotextile, secondary compaction coefficient.

Chapter one:

Research overview

-1- Introduction

The compressibility behavior of soils has become an important concern in geotechnical engineering. By implementing buildings, embankments, roads, etc. Often significant meetings are created. These meetings can be considered very important especially when the foundation of the structure consists of deep layers of clay. Predicting soil behavior after months or years using today's knowledge is an important challenge in geotechnical engineering. When soil is subjected to constant loading, it experiences deformations over time called creep. Time-dependent deformations, especially when long-term deformations are considered, are of great importance in geotechnical engineering. These deformations include the settlement of the structure on compressible ground, movements of natural or excavated slopes, compression of soft ground around the tunnel and so on.

The use of geosynthetic materials has peaked in the world since the mid-seventies. Among these materials, geotextiles and then geomembranes have found the most use among various uses. Geosynthetics are composed of polymer materials that are used together with natural materials such as soil and stone in engineering structures. Geotextiles have a very good performance in terms of distributing and evening out stresses or forces on a larger level and effectively prevent point and local ruptures. Building materials and embankments can withstand compressive stresses in a good way, while they are weak in tensile strength. In fact, the performance of fibers in a certain depth of the soil can be considered similar to the behavior of steel bars in concrete. Another characteristic of geotextiles is their elasticity, and this causes the soil that has been reinforced with fibers and has settled under the effect of an external load to return to its original state after removing the load. Theoretical studies show that the use of geotextile fibers in soils increases the resistance and elasticity coefficient of the soil (Mohammed Balochi, Zeinab Qanad. 2019). Considering the use of geosynthetics, it seems necessary to study their effects on creep behavior.

Research background

Soil is used as a building material in civil engineering in important projects. Man lives on land and different types of structures such as houses, roads, bridges, etc. builds Therefore, civil engineers should study the properties of the soil such as origin, granularity, water drainage, settlement, shear resistance, bearing capacity, etc., and the behavior of the soil.Therefore, civil engineers should study the properties of soil such as origin, granularity, water drainage, settlement, shear strength, bearing capacity, etc. and predict soil behavior as a result of human activity. The history of understanding the creep behavior of clay soils dates back to the 19th century. Among the classic examples, we can mention the settlement of the Tower of Pisa in Italy, so that due to creep, the tower has settled about 1.5 meters and tilted to one side, and even now the tower is prone to settlement. The deviation of the tower is about 5.5 degrees. The geological conditions of the structure and the time-varying settlement are given in Figure 1-1. 

(images are available in the main file)

When the saturated soil is subjected to constant loading, effective stresses increase with time as the pore water pressure decreases, in other words, primary consolidation occurs. A significant amount of settlement occurs during initial consolidation and the ratio of initial consolidation can be expressed using the consolidation coefficient Cv. After the pore water pressure is completely dissipated, if the loading on the soil is maintained, a series of deformations will occur with time, which is called secondary compaction or creep. Secondary compression ratio can be expressed by C? coefficient. Therefore, the calculation and prediction of these long-term settlements is very important.

A series of researches in the field of secondary compaction coefficient was carried out about a decade after Terzaghi's theory[1] (1925), which expressed the compaction of clay soils due to the loss of pore water pressure. Laboratory studies conducted by Taylor [2] (1942) and Beusman [3] (1936) clearly stated the effect of time on the compressibility of clay. Beausman (1936) expressed the relationship of settlement-logarithm of time under constant stress for clay soils in a linear form. Taylor (1942) for the first time presented the time-dependent model to describe the creep behavior of clay soils in which primary consolidation and secondary compaction are considered as two separate processes. In order to describe creep behavior and overconsolidation pressure caused by creep effects, Bejerum [4] (1967) presented a model in which primary consolidation and secondary compaction act as a couple. Although primary consolidation and secondary consolidation act as a couple from the beginning of loading, secondary consolidation occurs at the end of primary consolidation. Because the creep speed is lower than the initial consolidation, therefore the creep deformations during the initial consolidation cannot be considered. When initial consolidation takes a long time (for example, for thick layers of clay), the simultaneous effect of creep and initial consolidation can be considered. Laboratory studies conducted on thin layers of clay show that primary consolidation occurs in a short period of time, so creep in the initial consolidation stage can be ignored. Zhang et al.[5] (2006), Mejia et al.[6] (1988) conducted one-dimensional consolidation creep tests on sand samples at low stresses and concluded that the amounts of creep deformations in sand increase with increasing stress levels. Studies conducted in the field of creep deformations show that mineral compounds (minerals in soil), stress levels, stress history, liquid Porosity, drainage conditions and soil structure are important parameters affecting creep behavior (Savajan Varadarjan[7]. 2011). Although the effect of these parameters has not been fully determined, in this study, using laboratory results, the effect of soil reinforcement with geotextile fibers on creep deformations, the effect of stress levels, stress history, and pore water pressure has been studied, and the creep mechanism has been described by considering slippage, impact, and deformation of particles. Long-term deformations of sandy clay soil in unreinforced and reinforced with geotextile fibers. The specific objectives of the study are:

1) Investigating the effect of stress levels and pore fluid on the creep behavior of clayey sandy soil.

2) Investigating the effect of reinforcing soil with geotextile fibers on the creep behavior of clayey sandy soil.

3) Investigating the effect of stress history in order to create suitable conditions to speed up the creep test.

Soil granularity used in this research includes sand. It is Ottawa and clay, which is Ottawa sand, passed through a 30 sieve and remaining on a 50 sieve. The clay used is Hamadan potter's clay.