Investigation of the effect of olivine and potassium hydroxide (KOH) additive on the strength of a fine-grained soil

Increasing the shear resistance of clay soils, especially under undrained conditions, is important for solving urban development problems. In this context, the soil improvement method is of great importance. The aim of this study is to investigate the effect of olivine additive on the shear resistance of low plasticity clay soils in the absence and presence of potassium hydroxide (KOH). Experimental data are presented to question and discuss the main reasons why olivine, with or without KOH activated clay strength increase. It is of great importance to identify potential additives to increase the strength values in low plasticity clay soils. Such studies play an important role in the development of soil improvement techniques and the selection of materials used in engineering applications. Within the scope of this experimental thesis study, a low plasticity clay soil brought from Kocaeli province was studied. Liquid limit, plastic limit and pycnometer tests were carried out on the part of the sample that passed through the No. 40 sieve. 4% of the sample consists of sand and 96% consists of fine-grained material. The sample with these properties was classified as low plasticity clay (CL) according to the unified soil classification system (ASTM D-4318). In this study, a three-stage experimental program was conducted to investigate the effects of various additives to increase the strength of low plasticity clay. Three different additives were used in the mixtures: olivine, sand and potassium hydroxide (KOH). By performing compaction experiments, the optimum water content and maximum dry unit weight of each natural clay and mixed blends were determined. Compaction was primarily done using Proctor and Harvard miniature compaction methods. Since it was aimed to prepare the samples with equal properties, the results of samples prepared by both methods on natural and mixed soil samples were compared. As a result of the comparison, it was seen that Proctor compaction and Harvard miniature compaction tests gave almost the same results. From a practical point of view, the process of preparing all the samples was carried out using only the Harvard miniature compaction method. In the Harvard miniature compaction, to determine the optimum water content and maximum dry unit volume weight for each sample, a 600-gr sample was taken and mixed with different water ratios (5%, 10%, 15% and 20%) until it became homogeneous. By applying the same procedure for natural samples and additive mixtures, optimum water contents and maximum dry unit volume weights were determined. Each mixture sample was compressed in a mold with a diameter of 3.5 cm and a height of 7 cm, taking into account the optimum water content and maximum dry unit volume weight values of each mixture. Since a smaller mold than the Proctor standard was used, the number of strokes was reduced from 25 to 6 in order to keep the standard Proctor energy constant. KOH alkaline activator was dissolved in distilled water at a concentration of 10 molarity and allowed to cool before adding it to natural soil, olivine and sand mixtures. The mixtures were compressed by adding enough KOH solution to the activated samples to reach the optimum water content. Non-activated samples were compressed according to their natural water content. In order to avoid any change in the water content of the samples, the samples were kept in a plastic storage box during the curing period and were previously placed in two plastic bags. All samples were cured at ambient temperature throughout the curing period. In the first stage, olivine grains were added to the natural soil at different rates (10%, 15%, 20% and 25%). To see the development of the shear resistance of the soil over time with the addition of olivine, an unconsolidated undrained triaxial cell shear test (UU) was carried out at the end of 1 day, 14 days, 28 days, 56 days and 90 days of curing periods. UU experiments were carried out under different cell pressures (100 kPa, 200 kPa, 300 kPa and 400 kPa). The results of these first stage experiments showed that olivine, when used alone, does not have a significant chemical effect that develops over time to strengthen the properties of clay soil. However, it has also been proven that olivine grains have a significant effect on increasing the mechanical properties of the soil. In this context, it has been shown that the olivine mineral affects the physical properties of the soil and the addition of olivine has the potential to increase mechanical properties. It was concluded that although chemical effects were not clearly observed, there was an improvement in mechanical properties. These findings support the evaluation of olivine mineral as a potential granular material in soil improvement applications. The second stage was carried out based on the results of the first phase. Since it was observed that the mere addition of olivine did not have a clear chemical effect over time, it was decided to use potassium hydroxide (KOH) as an activator. At this stage, when olivine was added to the clayey soil together with potassium hydroxide, a significant strengthening effect was observed over time. In the third stage, it was investigated whether the healing feature belonged only to olivine by using a sand with a grain distribution similar to olivine. In other words, it has been experimentally demonstrated whether the chemical reactions occurring with the use of KOH are specific to olivine. For this purpose, a comparison was made between olivine grains and sand grains, and the effects of olivine grains in this combination were examined. At the end of this stage, it was revealed that sand with granulometry and chemical composition similar to olivine could also provide successful improvement. In this context, KOH is considered to be the main factor of the improvement achieved in the second stage. In the fourth stage, factors such as the effect of grains, the effect of the activator and the effect of time were taken into consideration. At this final stage, a study was conducted to observe the effect of potassium hydroxide (KOH) alone. At the same time, this stage revealed which of the three different additives (olivine, sand and KOH) was the key factor in this reinforcement. As a result of the study, it was found that olivine alone was not sufficient over time, and when activated with KOH, it provided higher shear resistance. In order to see the effects of olivine grains, a comparative study was also carried out between olivine grains and sand grains, both in the absence and presence of potassium hydroxide (KOH). Here, as in the previous experiments, the experiments were carried out keeping the additive ratios and cell pressures constants. When the samples were treated with sand or olivine only, an increase in strength was observed in both cases, but the strength of sand was higher than that of olivine due to the presence of a high volume of sand. On the other hand, in the presence of KOH, the shear strength increased in both cases, but that of the sand was also higher. Considering the same proportion of olivine and sand (25% by dry weight), 100 kPa cell pressure and 90 days curing time, the increase in strength compared to natural soil is 399% for olivine and 613% for sand. Mineralogical analysis (XRD) was performed to analyze the formation of crystalline phases following the chemical reaction. Over time, the shear strength increased due to the dissolution of silicate (SiO2) and aluminate (AlO2) and the formation of crystalline phases, which allowed to improve the treated soils. The fact that the same and sometimes higher values were obtained when sand was used instead of olivine allowed questions to be answered. Therefore, this means that the improvement provided by an alternative additive can be achieved more economically. From this point of view, in order to answer whether the strength gained in the presence of KOH is specific to olivine or not, the experiments were repeated under the same conditions of the previous experiments with only KOH added to the natural soil. In the results of the experiments, in the sole presence of KOH, results close to the strength of olivine in the presence of KOH were observed over time. Based on these results, KOH is not defined as an activator, but as a stabilizer.