GRAIN SIZE DISTRIBUTION AND POROSITY’S EFFECT ON URBAN SEDIMENT UNDER LAVA FLOWS

The purpose of this study is to better understand how sediment characteristics, such as grain size distribution and porosity, affect heat transfer. A better understanding of the heat transfer properties of sediments and soils can help municipalities, urban planners, and private companies plan out projects which are sensitive to high temperatures. The study used an angular limestone sandy gravel, from which a 1 mm and 2 mm size fraction was separated. The sediment was sieved in order to determine grain size distribution and textural group. The size of the sieves used ranged from 4 mm to 0.063 mm. Water filled porosity was calculated by determining each sediment’s Visual Pore Volume (VPV) and Different Weight Pore Volume (DVPV). Porosity (VPV) ranged from 23% to 28.75% for the limestone gravel. Porosity for 1 mm limestone gravel was between 43 and 44%, whereas porosity for the 2mm limestone gravel was at 45.75%.

Heat transfer tests were conducted in small laboratory experiments. Heat was applied in the form of remelted basalt (50-60 ml) which was poured on the surface of a dry sediment column above a vertical array of thermocouples at 0 (melt contact), 1, 3, 5, and 7 cm. Thermal measurements were collected for 2 hours after the pour. The time for a temperature of 100 C to be recorded at 3 cm depth was 17 minutes and 37 seconds for the poorly sorted limestone gravel, 19 minutes and 26 seconds for the 2 mm gravel fraction, and temperatures never reached 100 C at a depth of 3 cm for the 1 mm gravel. Max temperatures for 1 mm gravel at a depth of 3 cm were 92.7 C. That temperature was reached 24 minutes and 42 seconds after the start of the pour. Sediments with less pore space were better insulators with slower times for transmitted heat.

This focused study compliments experiments at the University at Buffalo that use larger volumes of melt to explore the heat transfer through sediments with a wider range of characteristics and moisture contents. Future small-scale studies could focus on the relationship between particle shape and heat transfer as particle shape is an important factor of the thermal conductivity of sediments. Quantification of the sensitivity of heat transfer to sediment characteristics is a fundamental first step to providing inputs to urban design and better reading of the geologic record.