Browsing by Author "Indian Institute of Technology Gandhinagar"

Coal ash from the thermal power plant is conveyed to the disposal site, where it can exist in loose to medium-dense conditions. Coal ash is re-used for several engineering applications as fill material for highway and railway embankments. This material is extremely prone to liquefaction when subjected to earthquake loading conditions. Therefore, a detailed investigation is required on the cyclic response of coal ash under different magnitudes (cyclic stress ratios, CSR) of earthquake loading conditions. The three different initial states were chosen to be 86%, 90% and 95% of MDD. These specimens were consolidated under the normal effective overburden stress of 100 kPa. To analyse the effect of overburden stresses, the specimen was consolidated at three different effective vertical overburden stresses (50, 100 and 150 kPa) on a specimen prepared at 95% MDD. The frequency was employed to be 1 Hz for all the cyclic simple shear (CSS) tests. The abrupt increase in the double amplitude shear strain and pore pressure was obtained for the coal ash specimens at higher CSR as compared to lower CSR values. The liquefaction was initiated when the double amplitude shear strain reached 7.5% or the pore water pressure ratio (ru) reached 0.9, whichever occurred first. With the increase in CSR value, loading cycles at liquefaction were observed to be reducing. A rapid decay in the shear modulus was also observed for the coal specimens subjected to the higher CSR value. The evaluation of damping ratio indicated a slight increase with the increase in CSR value.

Compacted expansive soil undergoes swelling and shrinkage during seasonal moisture fluctuations and causes distress to highway and railway structures built over them. Shrinkage of expansive soil is accompanied by the development of large desiccation cracks. The present research work is focused on studying the effect of different degrees of relative compaction on the desiccation cracking of soil. Different specimens were made using static compaction having different compaction states simulating the compaction process adopted in the construction of highway and railway embankments. The digital image analysis technique was used to quantify the crack propagation using ImageJ software. Desiccation cracks were quantified based on various crack parameters such as crack area, crack length, crack width, and crack intensity factor. Specimens with higher relative compaction exhibited reduced evaporation rates causing the delayed cracking response. A series of wetting-drying tests were also conducted to study the effect of wetting-drying cycles on the desiccation cracking response of specimens compacted at different relative compaction. Only a few cracks were observed during the compaction drying stage, followed by a large number of cracks on the application of the first wettingdrying cycle. However, the crack pattern remained almost similar in further wetting-drying cycles.

The present experimental study evaluates the overburden correction factor (Kσ) of different pond ash samples under earthquake loading for liquefaction analysis. A series of 54 stress-controlled cyclic simple shear tests was conducted on pond ash specimens at different overburden pressures and cyclic stress ratios. Cyclic resistance ratio (CRR) was evaluated for each pond ash sample at different overburden pressures using two criteria based on maximum excess pore water pressure and double amplitude shear strain to evaluate the Kσ. The Kσ values obtained for the pond ash were compared with the Kσ values for natural soils (clean sand and sand-silt mixtures). The cyclic resistance ratio (CRR) and Kσ values were observed to decrease with an increase in overburden pressure from 50 kPa to 100 kPa, and a further increase in overburden pressure to 150 kPa led to an increase in CRR and Kσ values for pond ash specimens with fine particles dominated matrix. However, an opposite trend was observed for pond ash specimens with coarse particles-dominated matrix. The unique response of Kσ values for pond ash was found to be significantly different from the already available Kσ response for natural cohesionless soil (clean sand and sand-silt mixtures) as it unavoidably included the effect of OCR and void ratio along with the vertical overburden pressure.

The deposition process of ash slurry in the disposal site results in segregation of bottom ash and fly ash particles. Pond ash collected from different locations of the same disposal site can have varying proportions of bottom ash and fly ash particles. The present study evaluates the effect of varying bottom ash content on the liquefaction response of pond ash. A series of stress-controlled cyclic simple shear (CSS) tests were performed on pond ash with varying bottom ash content (0%, 20%, 40%, and 50%). The loading cycles were applied in the sinusoidal form at 1 Hz frequency and 0.12 cyclic stress ratio (CSR) under 100 kPa vertical overburden stress. All the specimens were prepared at their respective 95% maximum dry density (MDD) and optimum moisture content (OMC). The number of loading cycles required to liquefy the specimen was evaluated based on two criteria: i) Excess pore water pressure ratio, ru≥0.9, and ii) Double amplitude shear strain, γDA≥7.5%. Dynamic properties such as shear modulus (G) and damping ratio (D) were also evaluated for pond ash with varying bottom ash content. It was observed that the liquefaction resistance increased with an increase in bottom ash content. The generation of shear strain with the number of loading cycles was found to be more gradual with an increase in bottom ash content. The specimen with 0% bottom ash content showed a sharp decrease in shear modulus with an increasing number of loading cycles as compared to the specimen with 50% bottom ash content.

Combustion of coal in thermal power plants results in the generation of large quantities of ash. The finer ash (< 45 microns) is utilized by cement industries. The remaining portion along with the bottom ash is transported to the ash pond (disposal site) in the form of slurry. This disposed pond ash being a waste material needs to be utilized in raising dykes around the ash pond. However, pond ash being highly prone to liquefaction may cause catastrophic failures if used without treatment. The present study investigates the efficiency of commercially available bentonite in the mitigation of the liquefaction issues of compacted pond ash for ash dyke construction. Bentonite was used in small dosages (0%, 2.5%, 5%, 7.5%, and 10%) to treat the pond ash. Strain-controlled cyclic simple shear (CSS) tests were performed to study the effect of bentonite treatment on liquefaction behavior and dynamic properties of compacted pond ash. The hysteresis response of bentonite-treated pond ash showed higher cyclic strength than the untreated pond ash. The untreated pond ash specimens showed cyclic liquefaction in only 12 loading cycles. However, the bentonite-treated specimens showed a delayed pore pressure evolution and higher liquefaction resistance. The average shear modulus increased and the cyclic degradation parameter decreased linearly with an increase in the percentage of bentonite. Considering the liquefaction, cyclic instability, and dynamic characteristics; it was concluded that the addition of bentonite in small percentages (between 5% to 10%) could provide significant liquefaction resistance and high stiffness to the compacted pond ash under cyclic loading conditions.