seismic microzonation based on large database of ...

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OF BOREHOLE DATA: APPLICATION TO LISBON. Liliana Oliveira1, Paula Teves-Costa1,2, Cláudia Pinto3, Rui Carrilho Gomes4, Isabel Almeida2,.
SEISMIC MICROZONATION BASED ON LARGE DATABASE OF BOREHOLE DATA: APPLICATION TO LISBON Liliana Oliveira1, Paula Teves-Costa1,2, Cláudia Pinto3, Rui Carrilho Gomes4, Isabel Almeida2, Teresa Pereira5, Marta Sotto-Mayor5, Carlos Ferreira6 – European Centre on Urban Risks, Av. Elias Garcia, n.º7, 2.º Andar 1000-146 Lisboa – Portugal, [email protected] 2IDL – Instituto Dom Luiz, Faculdade De Ciências, Universidade De Lisboa, Campo Grande 1749-016 Lisboa – Portugal 3Lisbon Municipality - CML (DMGP/DC), Campo Grande, 23/27 B Lisboa – Portugal 4CERIS/IST - Universidade de Lisboa, Av. Rovisco Pais 1049-001 Lisboa – Portugal 5Lisbon Municipality - CML (DMHDL), Campo Grande, 23/27 B Lisboa – Portugal 6Lisbon Municipality - CML (DSI/DGIG), Campo Grande, 23/27 B Lisboa – Portugal 1CERU

Introduction The 1st November 1755 earthquake (M8.5) is considered by many authors has the larger earthquake occurred in Europe in historical times. Its source is located at S-SW offshore Portugal, it strongly affected the whole country, large regions of Morocco and Spain, and its effects were observed in many European countries. In particular, the town of Lisbon was severely struck, and many monuments collapsed and a large part of the population (estimated around 10%) lost their lives. Due to its historical seismicity and to its economic and social importance, the town of Lisbon is considered to have moderate to high seismic risk. It is well known that the influence of the local site conditions can strongly affect the ground motion produced by an earthquake, modifying its amplitude, duration and frequency content, which may induce damage larger than expected to buildings and infrastructures. Aware of this situation the Municipality of Lisbon supported the development of a project to characterize the soil in more detail aiming to elaborate soil classification according to EC8. Taking profit of the NSPT database of the Municipality of Lisbon, composed by 8792 boreholes, an expedited methodology was proposed to perform the site classification. The results will be compared and validated with non-invasive field experiments (surface waves seismic profiles and HVSR performed with ambient vibrations), as well as with the geology and data collected from independent geophysical reports included also in the database. The objective of this work is to contribute for long-term urban planning through the evaluation of the seismic vulnerability of the building stock as well as of the built heritage.

Data & Methodology

Results The applied methodology, based mainly on the NSPT30 estimation, gave a first soil classification for each borehole (Figure 4A). In order to make a soil zoning we made a statistical analysis, for each geological formation, of the obtained classifications. However, we noted that several geological formations presented almost equal numbers for two soil classes (Figure 4B).

The first element considered in this study is the Geological Map of Lisbon, identifying the surface geological formations present in this region of Portugal mainland (Figure 1).

A

B 140

123 115

120

Number

100

Soil Type

80 60 40 20 0

Unknow n

A

Soil Type (1 st zonation) Soil Soil Soil Soil

Figure 1. Geological Map of Lisbon (adapted from Moitinho de Almeida, 1986, 1:10 000).

The database of the Municipality of Lisbon contains a varied set of information, including NSPT values from 8792 boreholes and the thickness of cover formations, which are used in this study (Figure 2). The greater thickness of the cover formations is found in coastal areas and the thicker deposits (> 30 m) are already in the river.

0

1

0

0 B

C

D

E

Desconhecido Unknown

Soil Type

Type A Type B Type C Type D

Figure 4. A – Soil classification for each borehole; B – Statistical analysis of results for the geological formation Areolas de Cabo Ruivo.

To take into account this fact an intermediate soil class (BC) was introduced (Figure 5). B

A

140 122 120 100

Number

Soil Type 80

67

60

49

40 20

Unknow n

Soil Type (1 st zonation) Soil Soil Soil Soil

Boreholes Municipality boundary

0

0

0

1

0 A

B

Type A Type B Type C Type D

BC

C

D

E

Desconhecido Unknown

Soil Type

Figure 5. A – Soil classification for each borehole, including the intermediate BC class; B – Statistical analysis of results for the geological formation Areolas de Cabo Ruivo.

The statistical analysis was again performed and the predominant soil classification was assigned to each geological formation as a first approach. The obtained classification was analyzed, taking into account the geological constraints (mainly age and lithology), and the need of a second intermediate class AB was revealed. The final soil zoning is presented in Figure 6.

Figure 2. Boreholes location (left) and distribution of cover formation thickness (right).

From the analysis of the Geological Map of Lisbon (Figure 1), it is possible to group the various formations in 4 classes, according to Eurocode 8 (EC8) classification (Table 1), taking mainly into account the lithostratigraphy. The corresponding classification map of the Lisbon’s soils is presented in Figure 3.

Soil Type

Soil Type

Cover Formations Thickness (m)

Water plane

Figure 3. Preliminary classification of the soils of Lisbon. Figure 6. Soil Classification Map of the city of Lisbon.

Taking profit of the NSPT database of the Municipality of Lisbon an expedited methodology was proposed to perform the site classification. The value of VS30 is the primary parameter defined in the EC8 for soil classification, but considering the available data, the classification was based on estimated values of NSPT30, an alternative parameter defined for the soil classification in EC8 (Table 1). Table 1. Soil classification parameters defined in EC8 (IPQ, 2010).

Soil Classification A B C D E

VS30 (m/s) > 800 360 – 800 180 – 360 < 180 -

NSPT30 > 50 15 – 50 < 15 -

Others H < 20 m

In addition, non-invasive field experiments were performed at sites of interest: refraction microtremors (ReMi) and HVSR computed from ambient vibrations records, which complement the characterization of the site (shear wave velocities (VS) and thickness (H) of each layer; VS30 value and the fundamental period (T0) of the site). Besides, numerical modeling was also performed to confirm the obtained classification.

The final map also presents the thickness (≥ 10 m) of the cover formations not included in the alluvial valleys, as well as the boreholes identified, from the applied methodology, as soil type E.

Conclusions • • •

The applied methodology, based on the analysis of a large number of geotechnical boreholes, provides a soils classification that seems to be quite satisfactory (as first approach); The introduction of intermediate soil classes looks necessary for an adequate classification of soils (as, for instance, the one proposed by Pitilakis et al., 2013); In order to obtain the final zoning, it is suitable to confront the first results with the geological information and field data. The results can also be checked using numerical modelling.

References IPQ (2010). Eurocódigo 8: Projectos de estruturas para resistência aos sismos - Parte 1: Regras gerais, acções sísmicas e regras para edifícios, NP EN 1998-1. Instituto Português da Qualidade. Moitinho de Almeida, F (1986). Carta Geológica do Concelho de Lisboa 1:10 000. Serviços Geológicos de Portugal. Pitilakis K, Riga E, Anastasiadis A (2013). New code site classification, amplification factors and normalized response spectra based on a worldwide ground-motion database. Bull. Earthq. Eng. 11, 925–966.

Acknowledgments The work was performed with the support of CML and this communication is supported by FCT- project UID/GEO/50019/2013 – IDL.