Technical text

Near-Surface Geophysics applications in the context of Geotechnical Engineering

Gabriel Galdino Magalhães

, Maria Silvia Carvalho Barbosa Society of Applied Geophysics - Federal University of Ouro Preto, Brazil

Geotechnical engineering in Brazil is currently undergoing a historic moment of change, both in terms of technical knowledge and the legal aspects associated with asset management. This attention condition stems essentially from the two recent and major accidents involving a failure of mining dams, in the municipalities of Mariana (2015) and Brumadinho (2018). Both events resulted in losses of human lives in addition to significant environmental and economic impacts.

It is in this context that a wide range of technologies has been incorporated into the monitoring of tailings dams and other geotechnical structures such as dikes, pits and piles. Among the technologies that have been applied, it is possible to highlight both conventional resources, such as piezometers, water level indicators, inclinometers, in addition to new unconventional resources, such as the radars applied for monitoring displacements.

Despite the great advances brought by the various monitoring methods currently applied, it is essential to state that each one has its advantages and limitations. In this sense, even though the piezometers and water level indicators return fundamental information for the understanding of the internal phenomena of the studied structure, they still have a very punctual investigation capacity. Others, such as radars, have a spatial sampling capacity, however, limited to superficial perception.

In this sense, there is an increase in the applicability and interest in shallow geophysics, which gains space to complement other technologies. It is worth highlighting the possibility of three-dimensional and noninvasive research as the strengths of geophysics.

Among the various methods available, those with the greatest recurrence in the literature and in market offers for the Brazilian context will be briefly discussed here. These are reference works for the following considerations: Tavares (2018), Oliveira (2018) and Rocha (2019) in the context of dams; Bandeira (2018), CPRM (2019) and Gonçalves (2019) for stability studies in urban areas; Moreira (2016) and Santos (2019) for landfill investigation; Gama (2018) and Prosdocimi (2019) for cavity investigation.

One of the main demands in the geotechnical area is to understand the behavior of water in the environment. Often internal flows in masses of soil or rock are conditioned by geological structures or construction details, such as drains and galleries. Thus, the electro resistance method has as its main application the identification of potentially wet areas or the identification of voids. Generally, it is expected for these contexts, where there is presence of air or water, very characteristic resistivity responses, however, the literature indicates several contexts that can be ambiguous (Gama 2018), being some form of direct calibration, integration with others essential. geophysical methods or even other instruments for monitoring / investigating structures.

Also noteworthy is the spontaneous potential method, which tends to be applied to identify flows, internal or more superficial. The integration between piezometric data, water level meters and the geophysical methods of electro resistance and spontaneous potential, allow a robust and three-dimensional analysis when well correlated and interpreted.

The understanding of mechanical parameters such as stiffness distributions throughout the investigated environment or even in more general terms soil compaction is fundamental in geotechnical studies Refractive seismic and MASW consist of tools with great potential, with the second being highlighted by the possibility of using passive sources, which can be fundamental in the case of more fragile environments, in addition to low frequencies in general reaching greater depths.

The GPR method, although commonly applied for more shallow investigations, and in some contexts does not meet the desired penetration in the study, has great application for the recognition of pipes and galleries, eventually arranged in constructed structures and which are not always well mapped.

The seismic methods are correlated with SPT survey data. Gonçalves (2019) demonstrates the same feasibility of integration with electroresistiveness. The integration of P and S wave speed data, with other data such as soil density, allows an approximation of the dynamic deformability coefficients, which are applied in different geotechnical contexts, especially strain studies.

Naturally, all integration resources work with the propagation of existing errors at each stage, ranging

from survey to data processing. Therefore, geophysics should not be taken as a substitute for direct tests and investigations, however, a joint application can either assist in understanding how point properties are spatially distributed, or even assist in a more accurate and precise investigation, bringing the minimum damage to the structure.

The Sociedade de Geofísica Aplicada, UFOP SEG Student Chapter, has developed a keen interest in the application of Near-Surface Geophysics, a fact that has materialized in several works aimed at the investigation of natural slopes in the city of Ouro Preto. These projects allow to explore a rising field in the geophysical scenario and bring return to the community, integrating scientific knowledge to the real demands of society.

References

CPRM (2019) – http://www.cprm.gov.br/publique/ media/gestao_territorial/riscos_geologicos/ relatoriosintese.pdf Gama 2018 - http://cpga.geologia.ufrj.br/index.php/ artigos/item/173-geofisica-rasa-na-investigacao-de -cavidades-naturais-em-minerio-de-ferro Gonçalves (2019) - Gonçalves, J.T.D. 2019.

Integração do método geofísico de eletrorresistividade e ensaios SPT na investigação de perfis de subsolos da região metropolitana de

Salvador (RMS). Dissertação de Mestrado,

Universidade Federal da Bahia, Escola Politécnica,

Salvador, 67 p. Moreira (2016) - Moreira, C. A., Braga, A. C. de O.,

Hansen, M. A. F. 2016. Estimativa do tempo de produção de chorume em aterro controlado por meio de medidas de resistividade elétrica. Revista

Brasileira de Geociências, 41: 549–557. Oliveira (2018) - http://cpga.geologia.ufrj.br/ index.php/artigos/item/171-caracterizacao-debarragens-de-rejeito-atraves-de-metodosgeofisicos-eletricos Prosdocimi, G. A. S. 2019. A valiação das relações lito-estruturais em cavidades ferríferas, com base em métodos geofísicos – estudo de caso: cavidade ferrífera da província Carajás/PA. Dissertação de

Mestrado, Núcleo de Geotecnia, Universidade

Federal de Ouro Preto, Ouro Preto, p. 188. Rocha (2019) - http://cpga.geologia.ufrj.br/index.php/ artigos/item/175-geophysical-methods-for-brtailings-dam Santos (2019) - https://www.bdm.unb.br/ handle/10483/24551 Tavares (2018) - https://pantheon.ufrj.br/ handle/11422/4240