Strength and Resistivity Properties of Fouled Ballast
University of Kansas
Civil, Environmental & Architectural Engineering
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Maintaining rail track in good condition is essential for ensuring the overall performance and safety of railway conditions. Track support, structural integrity, and effectiveness of the foundation structure depend on the characteristics and performance of the ballast and sub-ballast layers. The ballast of the rail track may be fouled due to intrusion and extrusion of fine particles from outside as well as particles produced within the layer due to breakage over time. This fouling can cause track support degradation and permanent settlement. Studies show that about one third of the total freight operation cost is invested for the track maintenance. Therefore, methods for locating and characterizing fouling that are faster, more effective, and less expensive would be valuable to the industry. Since there are limited methods for fouling detection and these methods are time consuming, tedious and require significant manpower; a simple approach of identification of ballast fouling within a few minutes at low cost is discussed in this thesis. Stone dust from ballast degradation caused by wear and tear of the ballast; intrusion of coal dust due to spillage from train cars; and extrusion of fine particles from the subgrade are the major contributors to ballast fouling. These particles have the capability to retain moisture and hence reduce the friction between ballast particles. Previous studies show that the fouled ballast electrical resistivity and hydraulic conductivity have certain relationships that can used to define the amount of fouling of the ballast. The fouling agents retain moisture which acts as the medium of electrical conductivity, since there is almost no flow of electricity through the air voids or solid ballast particles of the ballast layer. So, it is proposed that ballast fouling be estimated by measuring the resistivity of the ballast. Static modulus, dynamic modulus and California bearing ratio (CBR) were also investigated to determine the impact of the ballast fouling on strength properties. A vertical probe was designed at the University of Kansas (KU); Civil, Environmental and Architectural Engineering department to measure the resistivity of the fouled ballast. The probe was tested using both horizontal and vertical configurations and worked well for estimating resistivity using the fall of potential method. Forty-eight test samples of fouled ballast were prepared in a box of almost 11 cu.ft size with different degrees of fouling and with various moisture contents. Resistivity tests using a Wenner 4 probe array in horizontal alignment and fall of potential method with a vertical probe and vertical alignment were carried out. Also, the light weight deflectometer (LWD) test for the measurement of dynamic modulus, static plate loading test for determination of static modulus, and dynamic cone penetration (DCP) test for California bearing ratio (CBR) estimation were carried out. The result of the vertical probe was consistent on most of the test samples with the Wenner 4 point array method. However this method estimates the apparent resistance - which was higher than the actual resistance due to insufficient contact area with the fouled ballast. Boundary moisture content - termed as optimum moisture content for resistivity (OMCR) was found - that separates the results of each type of test conducted on the samples. The OMCR values were 6% for subgrade soil fouled ballast, 5% for Gardner track ballast dust fouled ballast, and 5.5% of coal dust fouled ballast. The resistivity of the fouled ballast can be estimated for moisture contents greater than OMCR. However, the dynamic modulus, static modulus and the CBR of the ballast decreased significantly for moisture contents greater than OMCR. Static and dynamic moduli peaked near the OMCR for all types of fouling while the CBR was constant to slightly increasing with moisture content up to the OMCR.
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