The impacts of pyroclastic surges on buildings at the eruption of the Soufri�re Hills volcano, Montserrat

Abstract

We investigated the impacts on buildings of three pyroclastic surges that struck three separate villages on 25 June, 21 September and 26 December, 1997, during the course of the andesitic dome building eruption of the Soufrire Hills Volcano, Montserrat, which began on 18 July, 1995. A detailed analysis of the building damage of the 26 December event was used to compare the findings on the flow and behaviour of dilute pyroclastic density currents (PDCs) with the classical reports of PDCs from historical eruptions of similar size. The main characteristics of the PDC, as inferred from the building damage, were the lateral loading and directionality of the current; the impacts corresponded to the dynamic pressure of the PDC, with a relatively slow rate of rise and without the peak overpressure or a shock front associated with explosive blast; and the entrainment of missiles and ground materials which greatly added to the destructiveness of the PDC. The high temperature of the ash, causing the rapid ignition of furniture and other combustibles, was a major cause of damage even where the dynamic pressure was low at the periphery of the current. The vulnerability of buildings lay in the openings, mainly windows, which allowed the current to enter the building envelope, and in the flammable contents, as well as the lack of resistance to the intense heat and dynamic pressure of some types of vernacular building construction, such as wooden chattel houses, rubble masonry walls and galvanised steel-sheet roofs. Marked variability in the level of damage due to dynamic pressure (in a range 1-5 kPa, or more) was evident throughout most of the impact area, except for the zone of total loss, and this was attributable to the effects of topography and sheltering, and projectiles, and probably localised variations in current velocity and density. A marked velocity gradient existed from the outer part to the central axis of the PDC, where buildings and vegetation were razed to the ground. The gradient correlated with the impacts due to lateral loading and heat transfer, as well as the size of the projectiles, whilst the temperature of the ash in the undiluted PDC was probably uniform across the impact area. The main hazard characteristics of the PDCs were very consistent with those described by other authors in the classic eruptions of PelØe (1902), Lamington (1951) and St Helens (1980, despite differences in the eruptive styles and scales. We devised for the first time a building damage scale for dynamic pressure which can be used in research and in future volcanic emergencies for modelling PDCs and making informed judgements on their potential impacts.

Residential building and occupant vulnerability to pyroclastic density currents in explosive eruptions

“…However, Baxter et al (2005) have summarised the sparse evidence for human survival inside buildings and studied the impacts of PDCs during the eruption of the Soufriere Hills volcano on Montserrat to provide a detailed account of building vulnerability to PDCs, including the implications for the survival of occupants in future eruptions if warning and advanced evacuation fails. This work formed the conceptual basis for understanding the likely behaviour of the more dilute PDC and the causes of building vulnerability in a densely urbanised area, such as around Vesuvius.…”

“…Therefore, these PDCs are important for emergency planners to consider and they are the type studied for this paper. More dilute PDCs can be compared with dense PDCs, often termed pyroclastic flows, which are highly destructive with effects which can be difficult to mitigate (Baxter et al, 2005). Baxter et al (2005) devised for the first time a damage scale based on lateral dynamic pressure applied to the building shell by the directional flow of the PDC.…”

“…More dilute PDCs can be compared with dense PDCs, often termed pyroclastic flows, which are highly destructive with effects which can be difficult to mitigate (Baxter et al, 2005). Baxter et al (2005) devised for the first time a damage scale based on lateral dynamic pressure applied to the building shell by the directional flow of the PDC. The ingress of ash to the interior causing combustion of flammable contents such as furniture is identified as the main cause of damage at low dynamic pressure ranges.…”

“…For people inside buildings which do not collapse, the main factor which governs vulnerability is the resistance of openings. Glazing is assumed to be a building's weak point (Baxter et al, 2005), so probability density functions for window glass failure under the combination of the three loads are calculated (Sect. 2.2).…”

Potential impacts from tephra fall to electric power systems: a review and mitigation strategies

“…Blong 2003;Spence et al 2005) and predicting building damage from pyroclastic flows (e.g. Baxter et al 2005). While fragility functions have been used sparingly in probabilistic volcanic risk analysis, their usefulness has been demonstrated in other disciplines, notably in earthquake engineering to determine the probability of building failure at different ground shaking intensities (e.g.…”