Sandy Soil Characterisation and Site Response Analysis in the Catania Harbour (Italy)

Transcription

1 Sandy Soil Characterisation and Site Response Analysis in the Catania Harbour (Italy) Salvatore Grasso, Vercinzia La Spina, Michele Maugeri Department of Civil and Environmental Engineering, University of Catania, Viale A. Doria 6, 9525 Catania. Keywords: Earthquake, Shear modulus, Damping ratio, Soil response analysis, Bedrock. ABSTRACT The harbour of the city of Catania, located on the eastern zone of Sicily, is an area subjected to high seismic hazard. In situ investigations of sandy harbour soil were carried out in order to determine the soil profile and the geotechnical characteristics for the site under consideration, with special attention for the variation of shear modulus and damping with depth. Seismic Dilatometer Marchetti Tests (SDMT) have been carried out, with the aim to evaluate the soil profile of shear wave velocity (Vs). Moreover the following investigations in the laboratory were carried out on undisturbed samples: Resonant Column tests; Direct shear tests; Triaxial tests. The available data obtained from the Seismic Dilatometer Marchetti Tests results enabled to evaluate the shear modulus profile. In addition, using some synthetic seismograms of historical scenario earthquakes at the bedrock, the ground response analysis at the surface, in terms of time history and response spectra, has been performed by the -D nonlinear code EERA. The results of the site response analysis will be also used for the evaluation of liquefaction hazard of the investigated area. INTRODUCTION The city of Catania, located on the eastern part of Sicily, is one of the most seismically active areas of Italy. The earthquake of January,, 693 is considered one of the biggest earthquakes which occurred in Italy. It is supposed that more than 5 aftershocks occurred along a period of more than two years after the main shock. This earthquake, with an intensity of XI degree of MKS scale in many centres, struck a vast territory of south-eastern Sicily and caused the partial, and in many cases total, destruction of 57 cities and 6 casualties. The assessment of realistic earthquake damage scenarios was one of the main objective of the project Detailed Scenarios and Actions for Seismic Prevention of Damage in the Urban Area of Catania and required a reasonably detailed model of the surface geology and geotechnical characterisation. In order to study the dynamic characteristics of soils in the Catania harbour area, laboratory and in situ investigations have been carried out to obtain soil profiles with special attention being paid to the variation of the shear modulus (G) and damping ratio (D) with depth. This paper tries to summarise this information in a comprehensive way in order to provide a representative model of ground condition of an important zone in the city of Catania for realistic seismic scenarios response analysis. 2 GEOTECHNICAL CHARACTERISATION BY SDMT TESTS To evaluate the geotechnical characteristics of the soil, the following in situ and laboratory tests were performed in the Catania harbour area:

2 N. 5 Seismic Dilatometer Tests (SDMT); N. 3 Direct Shear Tests; N. 3 Triaxial CD Tests; N. 6 Resonant Column Tests (RCT); The investigation programme was performed in the zone of Acquicella Porto in the Catania harbour. The 5 Seismic Dilatometer Tests (SDMT-5) have an effective depth of 3.5 m, 32. m, 3. m, 3. m, 32. m. Figure shows the location of the SDMTs in the Catania harbour. Figure. Location of the 5 SDMTs in the Catania harbour. The SDMT (Marchetti, 98; Marchetti et al., 28; Monaco et al., 29) provides a simple means for determining the initial elastic stiffness at very small strains and in situ shear strength parameters at high strains in natural soil deposits. Source waves are generated by striking a horizontal plank at the surface that is oriented parallel to the axis of a geophone connects by a co-axial cable with an oscilloscope (Martin and Mayne, 997; 998). The measured arrival times at successive depths provide pseudo interval V s profiles for horizontally polarized vertically propagating shear waves. The small strain shear modulus G is determined by the theory of elasticity by the well known relationships: G = ρvs 2 where: ρ = mass density. SDMT obtained parameters are: I d : Material Index; gives information on soil type (sand, silt, clay), figure 2; M: Vertical Drained Constrained Modulus, figure 3; Phi: Angle of Shear Resistance, figure 4; K d : Horizontal Stress Index, figure 5 (the profile of K d is similar in shape to the profile of the overconsolidation ratio OCR. K d = 2 indicates in clays OCR =, KD > 2 indicates overconsolidation. A first glance at the K d profile is helpful to "understand" the deposit); V s : Shear Waves Velocity, figure 6; G = ρvs 2 Small Strain Shear Modulus, figure 7. The "Acquicella" site along the southern coast line of Catania is characterized by fine sands with thin limestones. Figure 2. I d : Material Index of the 5 SDMTs in the Catania Acquicella harbour. Figure 3. M: Vertical Drained Constrained Modulus of the 5 SDMTs in the Catania Acquicella harbour.

3 Figure 4. Phi: Angle of Shear Resistance of the 5 SDMTs in the Catania Acquicella harbour. Figure 6. Vs: Shear Wave Velocity of the 5 SDMTs in the Catania Acquicella harbour. Figure 5. K d : Horizontal Stress Index of the 5 SDMTs in the Catania Acquicella harbour. Figure 7. G : Small Strain Shear Modulus of the 5 SDMTs in the Catania Acquicella harbour.

4 3 SHEAR MODULUS AND DAMPING RATIO BY LABORATORY TESTS Shear modulus G and damping ratio D of Catania deposits were obtained in the laboratory from resonant column tests (RCT) and cyclic loading torsional shear tests (CLTST). A resonant column/torsional shear apparatus (Lo Presti et al. 993, Capilleri et al. 29) was used for this purpose. The experimental results of specimens (Figure 8) from uncohesive soil, such as "Plaja beach" site, obtained by Cavallaro et al. (2) were used to determine the empirical parameters of the equation proposed by Yokota et al. (98) to describe the shear modulus decay with shear strain level: G(γ) = () β G o + αγ(%) The values of α = 9 and β =.85 were obtained for uncohesive soil. The same expression was used by Carrubba and Maugeri, 988; Cavallaro et al., 999; Cavallaro et al., 2; Capilleri et al., 29 for the soil of "Piana di Catania", "Via Stellata", "Piazza Palestro", San Nicola alla Rena Church" and "Cabina Enel" sites. G/Go Test N. Test N. 2 Test N. 3 CATANIA "Uncohesive Soil" RCT.2 Test N. 4 Yokota et al..... γ [%] Figure 8. G/Go- γ curves from RCT for "Plaja beach" site. As suggested by Yokota et al. (98), the inverse variation of damping ratio with respect to the normalized shear modulus has an exponential form, as reported in Figure 9 for uncohesive soil: G( γ) D( γ)(%) = η exp λ (2) G o in which: D(γ) = strain dependent damping ratio; γ = shear strain; η, λ = soil constants. The values of η = 8 and λ = 4 were obtained for the "Plaja beach" area. D [%] Test N. Test N. 2 Test N. 3 Test N. 4 CATANIA "Uncohesive Soil" RCT Yokota et al. (98) G/Go Figure 9. D-G/Go curves from RCT for "Plaja beach" site. 4 -D LOCAL SITE RESPONSE ANALYSES La Playa beach in the city of Catania, located in the South-Eastern Sicily (Italy), has been affected by several destroying earthquakes of about magnitude 7.+ in past times. It is so reasonable to assume in Catania a maximum expected earthquake as a repetition of the January, 693 event, with intensity XI MCS and estimated magnitude M = 7.3. Synthetic seismograms (Priolo 999; 2) have been drawn for the sites long a set of receivers placed at different depths, starting from the surface up to almost 3 m, both for the 693 scenario earthquake (Grasso et al., 25) and for the 88 scenario earthquake (Laurenzano et al., 24). Using the synthetic accelerograms at the bedrock, the ground response analysis at the surface, in terms of time history and response spectra, has been obtained by a -D non-linear code in correspondence of five SDMTs. Local site response analyses have been brought for the la Plaja beach by a -D linear equivalent computer code EERA (Bardet et al. 2). The code implements a one-dimensional simplified, hysteretic model for the non-linear soil response. The Seismic Dilatometer Marchetti Tests (SDMTs) were performed up to a maximum depth of 32 meters (Figure 2). The results show a very detailed and stable shear waves profile. The S-wave propagation obtained by SDMT occur on a -D column having shear behaviour. The column is subdivided in several, horizontal, homogeneous and isotropic layers characterized by a non-linear spring stiffness G(γ), a dashpot damping D(γ) and a soil mass density ρ. Moreover, to take into account the soil non-linearity, laws of shear modulus and damping ratio against strain have been inserted in

5 the code (Cavallaro and Maugeri 25). The five -D columns have a height of 3-32 m and are excited at the base by accelerograms obtained from the synthetic seismograms of 693, with a PGA of.225g (Figure ) corresponding to a return period of 475 years in the current Italian regulatory text (NTC, 28) seismic hazard and seismic classification criteria for the national territory obtained by a probabilistic approach in the interactive seismic hazard maps by Meletti and Montaldo (27) and by Meletti et al. (28). Further analyses have been performed using scaled seismograms, to the maximum PGA of.275g (corresponding to the return period of 975 years, Figure ) and to the maximum PGA of.4g (corresponding to the return period of 2475 years, Figure 2). The analysis provides the time-history response in terms of displacements, velocity and acceleration at the surface. Figure 2. Interactive seismic hazard map of the city of Catania, with 2% probability of exceedance in 5 years (return period of 2475 years). Using this time history, response spectra concerning the investigated site have been deduced. The soil response at the surface was modeled using the Equivalent linear Earthquake site Response Analyses of Layered Soil Deposits computer code EERA (Bardet et al. 2) for calculus of amplitude ratios and spectral acceleration. Figures 3-5 show the results in terms of maximum accelerations with depth for SDMTs No. -5, respectively for the 475, 975 and 2475 return periods. Figure. Interactive seismic hazard map of the city of Catania, with % probability of exceedance in 5 years (return period of 475 years). 5 Maximum Acceleration (g),5 Depth (m) 5 2 Figure. Interactive seismic hazard map of the city of Catania, with 5% probability of exceedance in 5 years (return period of 975 years) Figure 3. Maximum accelerations with depth for SDMTs No. -5 profiles (475 years earthquake scenario return period).

6 Depth (m), Maximum Acceleration (g) Figure 4. Maximum accelerations with depth for SDMTs No. -5 profiles (975 years earthquake scenario return period). Depth (m) Maximum Acceleration (g),5,5 Figure 5. Maximum accelerations with depth for SDMTs No. -5 profiles (2475 years earthquake scenario return period). Results of the site response analysis show high values of soil amplification factors especially for the 475 and for the 975 return periods of the scenario earthquake. Probably this fact is due to a non linear behaviour of soil, especially in presence of the strong accelerations of the 975 and 2475 earthquake scenarios. Table reports stratigraphic soil amplification factors Ss obtained through -D analysis. Table. Stratigraphic soil amplification factors Ss obtained through -D EERA code. SDMT Ss (475) Ss (975) Ss (2475) Figures 6-8 show the results in terms of time history of maximum acceleration at the surface i.e. for SDMTs -5 profiles, respectively for 475, 975 and 2475 years earthquake scenario return period. Maximum acceleration at the surface reaches the higher value of.65g in correspondence of the at the time of 9.68 s for the 475 return period, the higher value of.782g in correspondence of the at the time of 9.68 s for the 975 return period and the higher value of.993g in correspondence of the SDMT 4 at the time of 9.72 s for the 2475 return period. Figures 9-2 show the results in terms of Fourier amplitude i.e. for SDMTs -5 profiles, respectively for 475, 975 and 2475 years earthquake scenario return period. Figures show the results in terms of response spectra i.e. for SDMTs -5 profiles, respectively for 475, 975 and 2475 years earthquake scenario return period. Acceleration (g),5,3, -, -,3 -,5 -, Time (sec) Figure 6. Maximum accelerations at the surface for SDMTs No. -5 profiles (475 years earthquake scenario return period).

7 Acceleration (g),7,5,3, -, -,3 -,5 -,7 -, Time (sec) Figure 7. Maximum accelerations at the surface for SDMTs No. -5 profiles (975 years earthquake scenario return period). Acceleration (g),8,6,4,2 -,2 -,4 -,6 -,8 - -, Time (sec) Figure 8. Maximum accelerations at the surface for SDMTs No. -5 profiles (2475 years earthquake scenario return period). Fourier Amplitude,35,3,25,2,5,, Frequency (Hz) Figure 9. Fourier amplitude i.e. for SDMTs -5 profiles (475 years earthquake scenario return period). been also used for the evaluation of liquefaction hazard of the investigated area. Fourier Amplitude,8,7,6,5,4,3,2, Frequency (Hz) Figure 2. Fourier amplitude i.e. for SDMTs -5 profiles (2475 years earthquake scenario return period). Spectral Acceleration (g) 2,5 2,5,5,, Period (sec) Figure 22. Response spectra i.e. for SDMTs -5 profiles (475 years earthquake scenario return period). Spectral Acceleration (g) 3,5 3 2,5 2,5,5,, Period (sec) Figure 23. Response spectra i.e. for SDMTs -5 profiles (975 years earthquake scenario return period). Fourier Amplitude,6,5,4,3,2, Frequency (Hz) Figure 2. Fourier amplitude i.e. for SDMTs -5 profiles (975 years earthquake scenario return period). The results of the site response analysis have Spectral Acceleration (g) 5 4,5 4 3,5 3 2,5 2,5,5,, Period (sec) Figure 24. Response spectra i.e. for SDMTs -5 profiles (2475 years earthquake scenario return period).

8 5 CONCLUSIONS In this paper some information concerning the geotechnical characterisation of the Acquicella Porto zone in the Catania harbour have been presented. Available data enabled one to define the small strain shear modulus profile for uncohesive soil and empirical equations to describe the G and D variation with strain level. In addition, vertical drained constrained modulus, angle of shear resistance, horizontal stress index K d and shear wave velocity V s profiles have been evaluated by SDMT tests. Finally, local site response analyses have been brought for the Acquicella Porto area by a -D linear equivalent computer code EERA. The results of the site response analysis have been also used for the evaluation of liquefaction hazard of the investigated area. Results of the site response analysis show high values of soil amplification factors especially for the 475 and for the 975 return periods of the scenario earthquake, higher than those obtained by the current Italian regulatory text. REFERENCES Bardet, J. P., Ichii, K., Lin, C. H., 2. EERA: a computer program for equivalent-linear earthquake site response analyses of layered soil deposits, user manual, University of Southern California, 2, 4 pp. Capilleri P., Cavallaro A., Grasso S. e Maugeri M., 29. Caratterizzazione Geotecnica e Amplificazione Sismica nella Zona Industriale di Catania. ANIDIS, 3 Convegno Nazionale, L'Ingegneria Sismica in Italia, Bologna, 28 Giugno - 2 Luglio 29, paper S7.5. Carrubba, P. and Maugeri, M., 988. Determinazione delle Proprietà Dinamiche di un Argilla Mediante Prove di Colonna Risonante. Rivista Italiana di Geotecnica, Vol. 22, No. 2, -3. Cavallaro A., Maugeri, M., 25. Non linear behaviour of sandy soil for the city of Catania, Chapter VII in: Seismic Prevention of Damage: A Case Study in a Mediterranean City. M. Maugeri Editor. WIT Press, Southampton (UK), pp Cavallaro, A., Maugeri, M., Lo Presti, D.C.F. and Pallara, O., 999. Characterising Shear Modulus and Damping from in Situ and Laboratory Tests for the Seismic Area of Catania. Proceedings of the 2 nd International Symposium on Pre-failure Deformation Characteristics of Geomaterials, Torino, 28-3 September 999, pp Cavallaro, A., Grasso, S. and Maugeri, M., 2. A Dynamic Geotechnical Characterization of Soil at Saint Nicola alla Rena Church Damaged by the South Eastern Sicily Earthquake of 3 December 99. Proceedings of the 5 th International Conference on Soil Mechanics and Geotechnical Engineering, Satellite Conference Lessons Learned from Recent Strong Earthquakes, Istanbul, 25 August 2, pp Cavallaro A., Grasso S., Maugeri M., 2. Sandy Soil Characterisation and Site Response Analysis at La Playa Catania Beach (Italy). ANIDIS, 4 Convegno Nazionale, L'Ingegneria Sismica in Italia, Bari, 8-22 settembre 2. Grasso, S., Laurenzano, G., Maugeri, M., Priolo, E., 25. Seismic response in Catania by different methodologies, Chapter IV in: Seismic Prevention of Damage: A Case Study in a Mediterranean City. M. Maugeri Editor. WIT Press, Southpt. (UK), pp Laurenzano, G., Priolo, E., Klinc, P., Vuan, A., 24. Near fault earthquake scenarios for the February 2, 88 M=6.2 Catanese event, Proc. of the Fourth International Conference on Computer Simulation in Risk Analysis and Hazard Mitigation: Risk Analysis 24, Rhodes, September 24, 8-9. Lo Presti, D.C.F., Pallara, O, Lancellotta, R., Armandi, M. and Maniscalco, R., 993. Monotonic and Cyclic Loading Behaviour of Two Sands at Small Strains. Geotech. Testing Journal, Vol.6, No. 4, pp Marchetti, S. 98. In Situ Tests by Flat Dilatometer. Journal of the Geotechnical Engineering Division, ASCE, Vol. 6, N. GT3, March, 98, pp Marchetti S., Monaco P., Totani G., Marchetti D. 28. In Situ Tests by Seismic Dilatometer (SDMT). In: From Research to Practice in Geotechnical Engineering", ASCE Geotech. Spec. Publ. No. 8 Honouring John H. Schmertmann, Martin, G.K. and Mayne, P.W Seismic Flat Dilatometer Tests in Connecticut Valley Varved Clay. ASTM Geotech. Testing J., 2(3), pp Martin, G.K. and Mayne, P.W Seismic Flat Dilatometer in Piedmont Residual Soils. In P.K. Robertson and P.W. Mayne (eds), Proc. st Int. Conf. on Site Characterization, Atlanta, 2, pp Rotterdam: Balkema. Meletti C., Montaldo V.,27. Stime di pericolosità sismica per diverse probabilità di superamento in 5 anni: valori di ag. Progetto DPC-INGV S, Deliverable D2, Meletti C., Galadini F., Valensise G., Stucchi M., Basili R., Barba S., Vannucci G., Boschi E., 28. A seismic source model for the seismic hazard assessment of the italian territori. Tectonophysics 45, Monaco P., Marchetti S., Totani G., Marchetti D., 29. Interrelationship Between Small Strain Modulus G o and Operative Modulus. In: Kokusho, Tsukamoto and Yoshimine (eds), Proc. International Conference on Performance-Based Design in Earthquake Geotechnical Engineering (IS-Tokyo 29), Tsukuba, Japan, June 5-7, Taylor & Francis Group, London. NTC, 28. Nuove Norme Tecniche per le Costruzioni. D.M. 4//28. G.U. n. 29 del 4/2/28 (Suppl. Ordinario n. 3) Priolo, E., D spectral element simulations of destructive ground shaking in Catania (Italy), Journal of Seismology, 999, 3(3), pp Priolo, E., 2. 2-D Spectral Element Simulation of the Ground Motion for a Catastrophic Earthquake, In Faccioli and Pessina (eds), The Catania Project: Earthquake Damage Scenarios for a high risk area in the Mediterranean, Roma 2: CNR-GNDT. Yokota, K., Imai, T. and Konno, M., 98. Dynamic Deformation Characteristics of Soils Determined by Laboratory Tests. OYO Tec. Rep. 3, pp