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The Case of the Exploding Bungalow Case taken from Lee, C.W. (2004) The nature of, and approaches to, teaching forensic geoscience in forensic science and earth science courses. Pye, K. & Croft, D.J. 9eds.) Forensic Geoscience: Principles, Techniques and Applications. Geological Society of London, Special Publications, 232, 301-312.
At 06:30 on 24 March 1986 a bungalow at 51 Clarke Avenue, Loscoe, some 16 km north of Derby, UK, was completely destroyed by an explosion when the central heating switched on automatically. The three occupants were lucky to escape with their lives. The day itself was overcast and a very deep atmospheric depression passed over the area with an associated barometric pressure drop of 0.04 bar (4 kPa) Immediately after the explosion, gas samples were taken from the collapsed basement and were found to contain methane and carbon dioxide in ratios between 20 – 65% CH4 to 15 – 57% CO2.
Pressure systems: Air always flows from areas of high pressure to areas of low pressure, aiming to reach the equilibrium –creating wind in the process. But, because of the deviating force caused by the rotation of the Earth (namely Coriolis effect) it does not flow in a straight line. Instead of it, the winds form a spiral: inwards and upwards in low pressure systems, downwards and outwards in high pressure systems.
The air in the lower atmosphere is composed of roughly 80% nitrogen and 20% oxygen (this is not exact, as you know there are several other components to air, but for our purposes today we are going to concentrate on just N & O). The weight of a mole of N2 and O2 are 28 g and 32 g respectively. The weight of a mole of water vapor is 18 g. A mole of any type of gas occupies the same volume. This means that the air, on a humid or stormy day is significantly lighter.
The bungalow is underlain by a sequence of coals, mudstones, siltstones, and sandstones of Carboniferous age. Most of the strata are impermeable, but the sandstones have an average porosity of 18 – 25% and a natural permeability of 600 mD (millidarcy). The coal deposits have been worked commercially since 1885 by opencast, shallow and deep mining methods.
The working of the Roof Soil Seam, beneath the area in question, produced a zone of permanently stretched strata as a result of subsidence from below. This stretching increased the permeability of the rocks above by widening and extending any of the pre-existing joints or fissures. Although the overlying soil tends to behave impermeably, house foundations and trenches for service ducts and pipes are cut to bedrock level.
In 1966 the Loscoe brick pit, located southeast of the bungalow, began accepting waste. By 1977 it was a full-fledged landfill. This was in disregard to Government guidelines that stated that no houses should lie within 200 m of a landfill site. Dumping continued until 1982 In late 1985, the site was capped by an impermeable clay to prevent water ingress and leachate production. This created a positive pressure of 0.03 bar (3kPa).
In 1983 a pear tree in the garden of 51 Clarke Ave. began to die. Later the soil became warm, dried out and crumbled. Other areas of the lawn died. At 42 Loscoe Grange, the occupier dug a hole 0.5m deep and encountered a sewer smell. British Coal installed a flame trap to allow the gas to vent (35% CH4 and 65% CO2). Additional reports of odors and distressed vegetation were reported at other locations in the neighborhood.
A bore hole at Ivy Cottage showed a decrease in soil temperature with depth from 21°C at depths of 0 - 0.5m below ground level (bgl) to 18°C at 2.27m bgl. • This was accompanied by an increase in methane from 2% at the surface to 33.4% at 2.27m bgl. • Gas samples taken at 1.65m bgl in sandstone contained 29.6% nitrogen. • At 42 Loscoe Grange, gas at 3.0m bgl in a sandstone horizon contained 58% CH4 and 39% CO2.
It is possible to identify the source of methane by examining isotopic ratio data. This can be expensive. An alternative is to examine the gases that accompany the CH4 and in what percentage. These other gases (such as CO and CO2) can determine the most likely source. So long as there has been no mixing.
Gas from un-mined coal seams (coal bed methane (CBM)) has a stable composition – normally more than 90% methane – while abandoned mine methane (AMM), which continues to be released for some time after a mine's closure, contains between 60% and 80%. However, the methane content in the CMM from active mines – such as Sasyadko – ranges from 25% to 60% and has an oxygen content of about 5% to 12%.
