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Cartografia de susceptibilidade à ocorrência de movimentos de vertente em contexto urbano: o concelho de Lisboa
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Neste trabalho ensaiam-se abordagens à determinação da susceptibilidade à ocorrência de instabilidades em vertentes em contexto urbano, aplicadas ao caso específico do concelho de Lisboa. Deste modo, procurou-se aplicar um conjunto de metodologias que permitisse não só obter resultados por diversos métodos de natureza quantitativa, como documentar as alterações que o substrato geológico foi sofrendo ao longo de um período de tempo o mais alargado possível, recorrendo à análise de fotografias aéreas de forma sistemática. O período analisado compreendeu 57 anos (1947-2004) e incluiu a análise de coberturas de fotografias de 1947, 1958, 1967, 1977/78, 1987 e 2004. Para delimitar zonas de terrenos com características geotécnicas deficientes foi realizado um inventário de aterros e escavações em cada série de fotografias, que foi completado com dados de mais de 6000 sondagens, o que permitiu melhorar a cartografia geológica de base, que apesar da sua elevada qualidade, não incluía informação sistemática sobre os terrenos de cobertura. Para a produção de cartografia de susceptibilidade à ocorrência de movimentos de vertente no concelho de Lisboa (84 km2) recorreu-se essencialmente a modelos de base física e a modelos de base estatística, que foram objecto de validação e comparação de resultados. O modelo de base física utilizado permite a determinação quantitativa da estabilidade das vertentes pela aplicação de métodos de equilíbrio limite (talude infinito) que simulam os mecanismos físicos que controlam a ocorrência de movimentos de vertente superficiais. Para a aplicação deste modelo a informação necessária inclui: as características geotécnicas (peso volúmico, ângulo de atrito interno e coesão) obtidas por consulta bibliográfica e debate com especialistas; o declive, produzido a partir da base topográfica 1:1.000 que foi objecto de processamento sistemático manual para remover os efeitos do edificado no MDT; características geológicas e geotécnicas locais como a espessura de solo potencialmente instável, derivada da cartografia de terrenos superficiais efectuada e dados de sondagens, e a espessura de solo saturado pela aplicação de ferramentas hidrogeológicas do modelo SHALSTAB. Por outro lado, o modelo de base estatística recorre a uma análise estatística bi-variada (valor informativo) na qual é verificada a relação espacial entre uma variável dependente, um inventário de movimentos de vertente e um conjunto de seis factores de predisposição (declive, curvatura, exposição das vertentes, unidades litológicas, uso e ocupação do solo e espessura de solo superficial descomprimido). Os resultados finais da modelação foram então validados com recurso à construção de curvas de sucesso, utilizando o inventário de movimentos de vertente compilado no decurso deste trabalho.
In the present work, several approaches to the evaluation of landslide susceptibility were simulated in the urban context of the Lisbon County. A wide set of methodologies were applied so that results would be of a quantitative nature, as well as document the several changes that the geological substratum suffered along a time period as wide as possible, by the means of systematic aerial photo analysis. The considered period was comprised of 57 years (1947-2004) and included the analysis of aerial photos from the years 1947, 1958, 1967, 1977/78, 1987 and 2004. In order to define areas with poor geotechnical characteristics an inventory of landfills and excavations was produced, which was later completed by a borehole database consisting of over 6000 boreholes throughout the county, this improved the basic geological mapping, which despite being of high quality, didn’t take into account the nature of the terrains that composed the land cover. In order to produce the landslide susceptibility mapping of the Lisbon County (84 km2) was mainly used physically based models were used as well as, to a lesser degree, a statistically based model, which were subject of validation and comparison of results. The physically based model allows the quantitative determination of the stability of the slopes by applying methods of limit equilibrium (infinite slope) that simulate the natural mechanisms that control the occurrence of shallow landslides. The information necessary to apply this model included: the geotechnical characteristics (material unit weight, effective friction angle and effective cohesion) obtained by consulting the literature and expert opinion; the slope, produced from the 1:1.000 topographic cartography which was the object of manual systematic processing to remove the effects of the infrastructures in the DTM; local geological and geotechnical characteristics such as the slope-normal thickness of the failure slab, derived from the land surface mapping and borehole data, and the proportion of the slab thickness that is saturated by the use of hydrogeological tools from the SHALSTAB model. On the other hand, the statistically based model is focused on bi-variate algorithms (information value) by which the spatial relationship between a dependent variable, a landslide inventory, and a set of six predisposing factors (slope, curvature, aspect of slopes, lithological units, land use and occupation and uncompressed surface soil thickness). The final results of the modeling were validated by the use of success curves using the landslide inventories compiled in the course of this work.
In the present work, several approaches to the evaluation of landslide susceptibility were simulated in the urban context of the Lisbon County. A wide set of methodologies were applied so that results would be of a quantitative nature, as well as document the several changes that the geological substratum suffered along a time period as wide as possible, by the means of systematic aerial photo analysis. The considered period was comprised of 57 years (1947-2004) and included the analysis of aerial photos from the years 1947, 1958, 1967, 1977/78, 1987 and 2004. In order to define areas with poor geotechnical characteristics an inventory of landfills and excavations was produced, which was later completed by a borehole database consisting of over 6000 boreholes throughout the county, this improved the basic geological mapping, which despite being of high quality, didn’t take into account the nature of the terrains that composed the land cover. In order to produce the landslide susceptibility mapping of the Lisbon County (84 km2) was mainly used physically based models were used as well as, to a lesser degree, a statistically based model, which were subject of validation and comparison of results. The physically based model allows the quantitative determination of the stability of the slopes by applying methods of limit equilibrium (infinite slope) that simulate the natural mechanisms that control the occurrence of shallow landslides. The information necessary to apply this model included: the geotechnical characteristics (material unit weight, effective friction angle and effective cohesion) obtained by consulting the literature and expert opinion; the slope, produced from the 1:1.000 topographic cartography which was the object of manual systematic processing to remove the effects of the infrastructures in the DTM; local geological and geotechnical characteristics such as the slope-normal thickness of the failure slab, derived from the land surface mapping and borehole data, and the proportion of the slab thickness that is saturated by the use of hydrogeological tools from the SHALSTAB model. On the other hand, the statistically based model is focused on bi-variate algorithms (information value) by which the spatial relationship between a dependent variable, a landslide inventory, and a set of six predisposing factors (slope, curvature, aspect of slopes, lithological units, land use and occupation and uncompressed surface soil thickness). The final results of the modeling were validated by the use of success curves using the landslide inventories compiled in the course of this work.
Descrição
Tese de mestrado em Geologia do Ambiente (Riscos Geológicos e Ordenamento do Território), apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2011
Palavras-chave
Aterros e escavações Deslizamentos Susceptibilidade Validação Concelho de Lisboa Teses de mestrado - 2011