Geotechnical characterization of the Miocene formations at the location of Ivens shaft, Lisbon

Abstract

9The design of complex underground structures in an urban environment requires in the first instance 10 an appropriate characterisation and interpretation of the ground conditions and of the mechanical 11 behaviour of soil formations in the ground profile. With such information it is then possible to select 12 and calibrate appropriate soil constitutive models for application in advanced numerical analysis, with 13 the objective of predicting the induced ground movements and the potential damage to existing 14 structures and services. This paper provides an interpretation of the site investigation data collected 15 for the numerical analysis and design of the Ivens shaft excavation in Lisbon, Portugal. For the first 16 time a comprehensive set of interpreted data is obtained for two of the main formations in the Lisbon 17 area, Argilas e Calcários dos Prazeres (AP) and Areolas da Estefânia (AE), improving the understanding 18 of their mechanical behaviour and making the data available for application in most soil constitutive 19 frameworks. It is evident from the results that even with careful testing procedures the data may 20 appear to be inconsistent, requiring further assumptions when deriving soil parameters. Such 21 assumptions are discussed and an emphasis is placed on the need to combine data from laboratory 22 and field investigations. 23 24The geology of Lisbon has been influenced by several extreme geological processes (Antunes, 1979; 52 Alves et al., 1980). The oldest superficial soils date from the Cretaceous period, 95 million years ago 53 (Ma), although the majority of the city centre is founded upon Miocene formations, formed around 54 24Ma ago (Moitinho de Almeida, 1991). During this epoch several transgressive-regressive cycles, 55 each corresponding to a depositional sequence, occurred due to tectonic events and due to variations 56 in sea level (Dias & Pais, 2009). This epoch was followed by the last glacial period, when a substantial 57 climate change in the region caused intense erosion and a deflection in the course of the river Tagus 58 (Dias et al., 1997). Only at the beginning of the current Holocene epoch did the water level start to 59 rise again and new sediments began to deposit in the basin. 60The lithological profile at the Ivens shaft site is shown in Figure 2. The top 6 m is a loose sandy fill. 61Underneath are the two main formations, AE down to 35 m, followed by the AP formation. The AP 62 formation was deposited in a marine environment which changed progressively to a sub-tidal zone in 63 shallower waters. The material between 35 and 37 m depth can be considered a different unit (Top 64 AP), as it is lighter in colour (more oxygen) and is more compatible with the latter type of environment. 65

Numerical modelling of the Ivens shaft construction in Lisbon, Portugal

“…However, as this is a small gradient and there are uncertainties in the transition zones between layers due to material heterogeneity, the geological model for the shaft analysis considered these as horizontal. The water table was found at 23m depth, 8m above the TAP formation, and the soil above it was found to be saturated, apart from the coarser zones below the Limestone layer as described by Pedro et al (2018).…”

“…However, to enable the derivation of soil parameters for advanced numerical analysis, an additional ground investigation was conducted in 2010 (Pedro, 2013), involving both in-situ and laboratory experiments on the soils in the Ivens ground profile, utilising two new boreholes drilled to a depth of about 40m, measured from street level, and marked as B1 and B2 in Figure 2. A full characterisation of the ground profile, with a synergy of the historic and the new data, is presented in Pedro et al (2018) and Pedro (2013).…”

“…The ground profile is shown in Figure 5, consisting of a 6m thick loose sandy fill, underlain by the sandy "Areolas da Estefânia" (AE) formation which extends to 31m depth (measured from street level), followed by the clayey "Argilas e Calcários dos Prazeres" (AP) formation. The material between 31m and 33m depth is a transition between the AE and AP layers, having a different degree of cementation, and was hence characterised as a different unit (TAP) (Pedro et al, 2018). At depths between 8m and 13m the AE formation is bisected by a thick layer of fossiliferous and highly jointed limestone (L).…”

“…Based on the interpretation of ground conditions by Pedro et al (2018), the choice of appropriate constitutive models and their calibration were presented and discussed in Pedro et al (2017). The same reference also shows a validation of the adopted constitutive modelling, employing a 2D finite element analysis in the case study of the Baixa-Chiado station construction which benefited from the field monitoring of ground movements during construction (both West and East galleries).…”

“…before the construction in the field takes place. The numerical study presented here builds on carefully performed site investigation and characterisation of the ground conditions, reported by the authors in Pedro et al (2017Pedro et al ( , 2018.…”