Strength characteristics of fibre reinforced compacted pond ash

Abstract

At present about 130 million tonnes of ash is being produced annually from the coal based thermal power plants in India. The power requirements of the country are rapidly increasing in pace with in industrial developments. Nearly, 73% of India’s total installed power generation capacity is thermal of which coal based generation are nearly 90% (by diesel, wind, gas and steam adding about 10%). Indian coal gives 35 to 45% ash which is responsible for large volumes of pond ash. Construction of large ash disposal areas results in resettlement issues and loss of agricultural production, grazing land and habitat as well as other hand use impacts from diversion of large areas of land to waste disposal. The current practice in most of the power plants is to use large ash ponds, and nearly 75,000 acres of land is presently occupied by ash ponds sometimes in excess of 80,000 acres, which usually involves resettlement issues. Since, land holdings are typically small in size; a large ash pond development can cause hardships through loss of land-based subsistence and livelihood for literally thousands of people.
Considering these factors, effective utilization of pond ash in geo-technical constructions as a replacement to conventional earth materials needs special attention. The inherent strength of the compacted pond ash mass reduces considerably due to saturation. In this context to improve and retain the strength of compacted pond ash, cementing agents like cement or lime may be very much beneficial. The stress-strain behavior of compacted pond ash mass can be modified by inclusion of fibre reinforcements. Fibre reinforcements also improve the strength characteristics of the mass. Although, the use of reinforced earth materials has been widely accepted in many areas like embankments, foundations medium, railroads, retaining walls but the utilization of pond ash in place of earth material has not drawn much attention of researchers.
The present work aims at evaluating the geo-engineering properties of compacted pond ash and also the effectiveness of fibre inclusions in the strength characteristics of compacted pond ash specimens through a series of shear test, unconfined compression test and CBR test. For this purpose, a polyester fibre (Recron-3s) of 6mm and 12mm in length size is used with the pond ash, collected from Rourkela Steel Plant (RSP). The fibre content was varied as 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.75, and 1.0% of the dry weight of pond ash. The effect of fibre reinforcement on compacted density has been studies using the light and heavy compaction test. Compressive strength and shear strength behaviour of compacted samples were studied using unconfined compressive strength test and direct shear test respectively. The suitability of compacted pond ash fibre mixes as a road base and sub-base material have been studied by conducting laboratory CBR tests. The results have been interpreted in terms of stress-strain behavior, variation of failure stress, variation of failure strain, effect of degree of saturation, effect of fibre content, strength ratio, and secant modulus and strength parameters and are presented in this thesis.
Based on the experimental findings the following conclusions are drawn:
 The pond ash consists of grains mostly of fine sand to silt size with uniform gradation of particles. The specific gravity of particles is lower than that of the conventional earth materials.
 An increase in compaction energy results in closer packing of particles resulting in an increase in dry density where as the optimum moisture content decreases.
 Dry unit weight of compacted specimens is found to change from 10.90 to 12.70kN/m3 with change in compaction energy from 357 to 3488kJ/m3, whereas the OMC is found to decrease from 38.82 to 28.09%.
 Both the unit cohesion and angle of internal friction increase with increase in compaction energy. A nonlinear relation between these parameters is found to exist with compaction energy.
 For unreinforced compacted pond ash specimens, the value of unit cohesion increases with degree of saturation up to the OMC and thereafter the same decreases. The highest value of unit cohesion occurs at OMC for samples compacted both at standard and modified densities. However, there is a continuous decrease of angle of internal friction value with degree of saturation. Initially there is a sharp decrease which gets stabilized at moisture contents higher than OMC.
 The unit undrained cohesion of reinforced specimens is found to increase with the fibre content. However, the rate of increase of unit undrained cohesion with fibre content is not linear. Initially the rate of increase is high thereafter the increase in unit cohesion is not that prominent.
 For a given compacted density and fibre content, the 12mm size fibre gives higher strength than 6mm size fibres.
 The highest value of unconfined compressive strength is found to be 12kPa and 29kPa at a degree of saturation of 13% and 14 % for samples compacted at standard and modified proctor density. Moisture content either higher or lower than the said value results in decrease in the compressive strength.
 The failure stresses as well as initial stiffness of unreinforced samples, compacted with greater compaction energies, are higher than the samples compacted with lower compaction energy. However the failure strains are found to be lower for samples compacted with higher energies. The failure strains vary from a value of 0.75 to 1.75%, indicating brittle failures in the specimens.
 An almost linear relationship is found to exist between the compaction energy and unconfined compressive strength.
 The UCS value of unreinforced specimens is found to change from 1.2 to 17.0kPa with change in compaction energy from 357 to 3488kJ/m3 indicating that the strength can be modified suitably by changing the compactive effort. It revealed from the test results that a linear relationship exists between the initial tangent modulus with unconfined compressive strength and deformation modulus.
 The trend observed in the CBR value with moisture content is very much similar to that observe with unconfined compressive strength value of specimens. This shows that for a given compacted dry density higher unconfined compressive strength as well as CBR value can be obtained with moulding water content much lower than the OMC value.
 At low strain levels the bearing resistance is found to remain almost constant with fibre content. However at higher strain level the bearing resistance is found to increases substantially with increase in fibre content. It is observed that for a given compacted density an increase in fibre content results in decrease of initial stiffness whereas the failure strain increases.
 The inclusion of fibre gives ductility to the specimens. The reduction in post peak stress of a reinforced sample is comparatively lower than the unreinforced sample.
The strength parameters achieved in the present study is comparable to the good quality, similar graded conventional earth materials. Hence, it can be safely concluded that reinforced pond ash can replace the natural earth materials in geo-technical constructions.