Dynamic properties of low plasticity fine-grained soil stabilized with lime and fly ash

نوع: Type: thesis

مقطع: Segment: masters

عنوان: Title: Dynamic properties of low plasticity fine-grained soil stabilized with lime and fly ash

ارائه دهنده: Provider: Hamed Rafiei

اساتید راهنما: Supervisors: Mohammad Maleki

اساتید مشاور: Advisory Professors:

اساتید ممتحن یا داور: Examining professors or referees: Vahid Reza Ohadi - Masoud Makarchian

زمان و تاریخ ارائه: Time and date of presentation: 2022/09/21

مکان ارائه: Place of presentation: Faculty of Engineering

چکیده: Abstract: With the increase in population and urbanization, engineers in some areas are faced with a lack of proper land to implement construction projects. One of the suitable ways to improve the geotechnical properties of soils with unfavorable engineering properties on the site of construction projects is chemical stabilization of the soil with additives such as lime and fly ash. Lime is available and affordable material. Also, fly ash is a waste material that is produced from burning coal in thermal power plants, and its disposal in nature causes environmental problems. Using lime and fly ash, in addition to improving soil engineering properties, reduces environmental and economic issues in construction projects. Review studies show that adding fly ash to lime-stabilized soil plays a positive role in increasing unconfined strength. However, few studies have been done on its effect on dynamic properties such as shear modulus. In the transportation industry, where the weak soil bed is stabilized, the study of the propagation of waves caused by the vibration of heavy vehicles, trains, etc., as well as the analysis of stabilized embankments under the effect of earthquake vibration with knowledge of the dynamic properties of stabilized materials is possible. In the present study, the effect of different percentages of lime (3, 5 and 7%) and different percentages of class F fly ash (5, 10 and 15%) on the properties of soil with low plasticity index was investigated. First, the preliminary and the main tests were done to determine the soil parameters. Preliminary tests include gradation, Atterberg limits, laboratory compaction test and specific gravity, and main tests include static triaxial test, bender element and cyclic triaxial test. The samples related to the main tests were made using the results of the standard compaction test and according to the maximum dry density and the optimum moisture percentage and were tested after 28 days of curing. To further evaluate the stabilization process, a microstructural study was also conducted by performing pH determination tests, X-ray diffraction (XRD) and scanning electron microscope (SEM). The results of the static triaxial test on samples under confining stresses of 0, 100, 200 and 400 kPa showed that increasing the amount of lime from 3 to 5% increases the unconfined strength by 55.06%. still, with increasing the amount of lime from 5 to 7%, the unconfined strength decreases by 10.41%. Adding fly ash to the soil-lime mixture increases the intensity of pozzolanic reactions and significantly increases the compressive strength of the samples. Adding 15% fly ash to soil stabilized with 5% lime increases the unconfined compressive strength by 78.16%. In addition, soil stabilization reduces failure strain. According to the results, the optimal composition for stabilization of fine-grained soil with a low plasticity index (CL-ML) is 5% lime and 15% fly ash. With the increase of confining stress, the compressive strength and failure strain increase. To investigate the effect of different percentages of lime and fly ash on the maximum shear modulus of stabilized soil, the bender element test was performed under confining stresses of 0, 100, 200 and 400 kPa. The results showed that the composition containing 5% lime and 15% fly ash has the highest maximum shear modulus and the optimal composition for soil stabilization used in this research. Maximum shear modulus of soil stabilized with 5% lime and 15% fly ash compared to unstabilized soil under confining stresses of 0, 100, 200 and 400 kPa, has increased by 207.5, 180.8, 164.2 and 151.4 percent respectively. To further evaluate the stabilization process, pH, XRD and SEM tests were performed. The results of the pH test showed that adding lime and fly ash increased the pH of the reaction medium, which dissolved alumina and silica in the soil and prepared the conditions for pozzolanic reactions. X-ray diffraction curves showed a decrease in the intensity of the peaks related to kaolinite and the formation of new peaks related to cement compounds. The formation of new peaks indicates the formation of calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH) due to pozzolanic reactions. According to the SEM images, the unstabilized soil has many pores and cracks. But with the addition of lime and fly ash and the formation of CSH and CAH, the pores are filled by these cementitious compounds and the soil structure becomes dense. After determining the optimal composition, the samples made with 5% lime and 15% fly ash were subjected to loading cycles by applying confining stress of 100 kPa. The results of the cyclic triaxial test showed that the samples experienced cyclic degradation and strength reduction during cyclic loading. According to the results, with the increase in the number of loading cycles, the degradation index decreases, which indicates the increase in cyclic degradation with the increase in the number of loading cycles. Also, the degradation parameter, which indicates the rate of cyclic degradation, increases up to the first 100 cycles and then decreases. This indicates that degradation has a higher rate and occurs faster in the initial loading cycles. After loading cycles, the samples were subjected to static loading to determine the post-cycle strength. The results showed that the strength of the samples decreased by 2.15, 7.43, 14.17, 22.84 and 32.34 percent after applying 100, 200, 1000, 2000 and 3000 loading cycles, respectively, compared to the resistance of the sample without experiencing cyclic loading. The decrease in strength and cyclic degradation in samples after loading cycles is due to the destruction of cement bonds formed by pozzolanic reactions and the creation of microcracks during cyclic loading

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