Investigation of the effect of high performance polypropylene fiber additive on the undrained shear strength of clay soil

Today, with the rapidly growing urbanization, it has become inevitable to create new residential areas in problematic areas in terms of soil characteristics. Bearing capacity and settlement problems have an important role in buildings and transportation structures built on soft clay soils. In such cases, methods such as leaving the parcel or changing the route, soil improvement or piled foundations in buildings are used as alternatives. With the developing technology, geosynthetic materials are used for processes such as increasing the bearing capacity, separating the layers, and drainage. Polypropylene fibers, one of the geosynthetics, have been used in concrete and soil reinforcement for many years. In addition, it is known that concrete with alkali properties decays non-alkali-resistant fibers over time and reduces its long-term strength. To eliminate this disadvantage, alkali resistant fibers have been produced. In this thesis study, the effect of polypropylene fiber additive with high alkali and corrosion resistance and increased friction properties on the undrained shear strength of the clay soil was investigated. First of all, physical properties of clay soil taken from Kocaeli province were determined. Then, 40 mm and 54 mm long manufactured fibers were cut into 10 mm, 20 mm, 30 mm, 40 mm and 50 mm lengths. Fiber - soil mixtures were prepared by adding 0,5%, 1%, 1,5% and 2% of the dry weight to the natural soil with optimum water content. The prepared mixtures were compacted into cylindrical sample molds of two different sizes with a constant energy corresponding to 2 times the standard Proctor energy. One of the molds has a diameter of 3.50 cm and a height of 7.00 cm, while the other has a diameter of 5.00 cm and a height of 10.00 cm. In the study, unconfined compression (UC) tests were carried out on a total of 300 cylindrical samples, 20 of which were natural clay and 280 of which were polypropylene fiber reinforced. Analysis was performed by Scanning Electron Microscopy (SEM) to examine the interaction between the macroscopic images and microstructure of clay soil and polypropylene fiber reinforced samples. As a result of 300 unconfined compression tests, it was observed that polypropylene fiber additive increased the undrained shear strength of the clay soil. The maximum strength increase was obtained in mixtures with a fiber additive of 30 mm length at 2% by dry weight. In addition, due to the dimensions of the polypropylene fiber, it became difficult to obtain a homogeneous mixture as the fiber ratio and length increased.