V S and N SPT Measures for Seismic Characterization of Soils

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1 V S and Measures for Seismic Characterization of Soils 218 Giovanna Vessia, Mario Luigi Rainone, and Patrizio Signanini Abstract Seismic classification of soils for designing purposes can be addressed by means of the measurements of shear wave velocity along thirty meters (V S30 ) according to the new Italian building code and Eurocode 8. Alternatively, especially for granular soils, the number of blow counts from standard penetration tests ( ) can be used at this aim. Although the nature of the two testing methods suggests not to relate small to large deformation measurements, corresponding V S and ranges of values are proposed by technical codes. A wide survey based on V S and measures (among others) has been performed in the most hazardous areas of Tuscan Region. There, within the VEL project (Local Seismic Effect Assessment), financed by the Tuscan Office for Seismic Prevention, a large dataset of 892 pairs of the V S and was collected. These pairs are here compared with the seismic soil categories introduced by Italian and European codes. The evident inconsistencies from the measured pairs and the code categories suggest not to rely on values for soil characterization by means of V S30. Keywords Standard penetration test Shear wave velocity Seismic soil category Tuscan region Introduction In recent years, the dynamic characterization of superficial deposits by means of the shear wave velocity V S became a relevant aspect in both seismic microzoning studies and in structural design activities. It was not considered in the 1996 Italian building code, thus a new one was issued by the G. Vessia (&) M.L. Rainone P. Signanini University G.d Annunzio of Chieti-Pescara, Via dei Vestini, 31, 66100, Chieti Scalo, Italy g.vessia@unich.it M.L. Rainone rainone@unich.it P. Signanini signanin@unich.it G. Vessia CNR- IRPI, Via Amendola 122/I, 70124, Bari, Italy Ministry Decree 14 gennaio 2008, known as DM 2008 (G.U. n. 29 del 4 febbraio 2008 Suppl. Ordinario n. 30). The new code states (according to Eurocode 8) shear wave velocity V S is the reference parameter for seismic soil classification. It can be measured by means of seismic tests (i.e. down hole) or especially for granular soils, by empirical correlation between V S and. According to this statement, four seismic soil categories are listed based on V S averaged value along 30 m depth, i.e. V S30 : Ground type A Rock or other rock-like geological formation, including at most 3 m of weaker material at the surface: V S30 (m/s) > 800 m/s; Ground type B Deposits of very dense sand, gravel, or very stiff clay, at least 30 m in thickness, characterized by a gradual increase of mechanical properties with depth: V S30 (m/s) ( m/s) and (blows/30 cm) (>50); Ground type C Deep deposits of dense or medium dense sand, gravel or stiff clay with thickness at least 30 m, G. Lollino et al. (eds.), Engineering Geology for Society and Territory Volume 5, DOI: / _218, Springer International Publishing Switzerland

2 1144 G. Vessia et al. characterized by a gradual increase of mechanical properties with depth: V S30 (m/s) ( m/s) and (blows/30 cm) (15 50); Ground type D Deposits of loose-to-medium cohesionless soil or of predominantly soft-to-firm cohesive soil, with thickness at least 30 m, characterized by a gradual increase of mechanical properties with depth: V S30 (m/s) (<180 m/s) and (blows/30 cm) (<15); The above listed soil categories do not detailed the grain size distribution of the named cohesionless soils and suggest a correspondence between V S and ranges of values. Is it reliable enough to be used for different lithologies and soil types considered over all the Italian territories? In this paper, a large dataset of 892 pairs of and V S measures, collected independently in numerous sites of Tuscany Region, are used to appreciate the feasibility and reliability of the suggested correspondence applied to different soils. Here, V S measures performed only by down hole tests have been considered and standard penetration tests are used for measuring values. A comparison with the technical code categories has been accomplished and discussed hereafter. The Italian seismic hazard map issued by DM 2008, shows a diffuse high seismic hazard throughout the Tuscan Region, especially in Garfagnana, Lunigiana, Valtiberina (Senese), Casentino, Mugello and Amiata areas (Fig ). For this reason, the regional Office for Seismic Prevention since 1998 has paid attention to the seismic response of soil deposits by means of the VEL project (Local Seismic Effect Assessment) and later by DOCUP project (Local Seismic Effect Evaluation in Industrial Areas). Especially the VEL project was aimed at investigating physical and mechanical properties of the ground by means of several different in situ and laboratory tests among which standard penetration and down hole tests. The investigated soils are characterized by variable lithotechnical properties due to the heterogeneity of geological environments: (1) Garfagnana and Lunigiana are characterized by fluviallacustrine soils belonging to the basin of Barga-Castelnuovo and Aulla-Olivola dated back to Villafranchian age. These soils are clayey-sandy units including coarser levels at the bottom and upon it, gravel cemented soils interbedded with conglomerates, sands and silts (D Amato et al. 1986). (2) Valtiberina is a large inter-mountain basin from lower Pleistocene. It is filled by more than 500 m depth of fluvial-lacustrine and fluvial deposits. The fluviallacustrine deposits can be divided into five units belonging to middle Pleistocene. These are cemented pebbles of sandstones and limestones (Principi et al. 2003). (3) Casentino is characterized by fluvial-lacustrine deposits belonging to lower Pleistocene and made up of two stratigraphic units: mainly clayey at the bottom and prevalently pebbles, sands and silts at the top (Principi et al. 2003). (4) In Mugello there are basins filled by deposits, settled between Superior Pliocene and Lower Pleistocene. These deposits are divided in three units: coarse soils are mainly present along the boundaries of the basin whereas, in the centre, there are fine deposits with subhorizontal slopes. The maximum depth of this fluviallacustrine succession is m in Borgo S. Lorenzo basin and 100 m in Barberino basin (Benvenuti et al. 2003). (5) Amiata is part of a Volcanic complex and the outcropping deposits are mainly originated by the erosion of the volcanic apparatus. They are made up of loose coarse sands (Lazzarotto et al. 2003) Main Characters of Tuscan Region Near Surface Geology Comparison of -V S Pairs with Seismic Categories Geological, geophysical and geotechnical measures performed during the VEL project are available on the website indagini/vel/centri_urbani/index.htm. Among others, 892 couples of and V S values were collected. These couples of measures are here summarized in a table where the technical code categories are also considered. A double entrance table, Table 218.1, containing all types of soils, is presented below. In this table, 351 values higher than 100 have been considered. Over the principal diagonal (italicised cells) of Table there are numbers of pairs falling within the categories introduced by the building code. On the bottom, in the last raw, there is the total amount of values falling in four considered ranges. In the last column on the right, in bold italics, there is the total amount of V S values falling into the code soil categories. As values are concerned, it can be noticed that for the first range of values (0 14) corresponding to soil category D, only 15 of 103 measures fall in this category (meaning the 15 %). In this category, there are 15 of 24 measures of V S (varying between 0 and 179 m/s). On the contrary, it seems that V S soil categories B and C better correspond to the first range of values, provided that 65 and 53 % of values fall in these ranges, respectively. The A soil category is considered as the case when V S is higher than 800 m/s and is higher than 100 (this means

218 V S and Measures for Seismic Characterization of Soils 1145 Fig. 218.1 Tuscan territories where VEL and DOCUP projects were developed Table 218.

3 218 V S and Measures for Seismic Characterization of Soils 1145 Fig Tuscan territories where VEL and DOCUP projects were developed Table V S pairs of measures relating to all soil types from Tuscany Soil category D C B A D V S [m/ (15 %) 5 (2 %) 2 (1 %) 2 (1 %) 24 C s] (65 %) 152 (53 %) 42 (28 %) 41 (13 %) 302 B (19 %) 120 (42 %) 100 (65 %) 232 (66 %) A >800 1 (1 %) 8 (3 %) 9 (6 %) 76 (22 %) that the measure cannot be performed because the test failed disregarding to the soil type). Such assumption has not been stated by the Italian technical code or the Eurocode 8 even though it can be deduced by the indicated ranges of.in this case, only the 21.7 % of V S measures (76 over 351) fall in V S range higher than 800 m/s. This last result shows that the SPT failure cannot be necessarily associated with stiffer soils although it cannot be associated to local pebbles or rocky boulders. Considering the other two ranges of values, it is shown that for both intervals, and 50 99, the majority of the V S values fall along the main diagonal: 152 values over 285 (53 %), and 100 values over 153 (65 %), respectively. This means that, for the investigated soils, these soil categories prevail although, in the interval (soil category C), 120 V S values fall in the soil category B (42 %). Finally, from Table 218.1, it can be drawn that only the 38 % of all -V S pairs of values (that is 343 over 892 cases) fall within the code categories. This result suggests that the code categories, applied to both cohesive and cohesionless soils, don t fit the dataset under study. It shows a very large scatter on each interval of values. Starting from this consideration, the authors tried to understand whether the soil grain size can interpret this scatter, indeed. Three tables, now, are considered whose structures are similar to the Table but each one relates to a different grain size fraction. 319 out of 892 pairs are those related to measured grain size curves. Such -V S pairs are divided as below: Samples with gravel and sand fraction higher than 50 % of weight (122 couples) are reported in Table Samples with silt and clay fraction higher than 50 % of weight (197 couples) are reported in Table

4 1146 G. Vessia et al. Table Mainly sandy-gravelly soil samples V S [m/s] > Table Mainly silty-clayey soil samples V S [m/s] > Table Mainly silty-sandy soil samples V S [m/s] > Samples with silt and sand fraction higher than 50 % (257 couples) are reported in Table From Table it can be seen that the amount of values falling into the code categories is only the 32 % (the couples in the grey cells). According to the ranges, the values falling outside the diagonal are: 85 % for the range 0 14, 55 % for the range 15 49, 45 % for the range and 94 % for the case of > 100. Hence, for soils with prevalent gravel and sand fractions the code categories do not fit well. A better correspondence can be found for the cases of silty-clayey (Table 218.3) and siltysandy soils (Table 218.4). From Table 218.3, the 55 % of the couples (109/197) falls into the code categories as well as from Table 218.4, the 46 % (118/257). In these two tables, the best fitting categories are and of values. These correspond to C and B categories. These outcomes show that further studies are needed to investigate the nature of the evidenced scatter by the Tuscan soil measures. This study suggests not to rely on the values for classifying soils according to V S categories Conclusions The paper deals with the seismic soil categories for designing purpose, introduced by the DM 2008 and Eurocode 8. Accordingly, in this study, a large dataset of Vs and pairs measured in Tuscan territory, has been compared with the ranges of values corresponding to the seismic soil categories. The outcomes of this study show a large scatter in this correspondence with Tuscan data. Grain size distribution does not interpret this high scatter. Thus, it seems that the is not able to be used for replacing V S values in seismic soil characterization. References Benvenuti M, Sagri M, Moretti et al (2003) Carta geologica e geomorfologica in scala 1: Programma VEL e DOCUP Toscana in Mugello, Convenzione Regione Toscana. DST Università di Firenze

5 218 V S and Measures for Seismic Characterization of Soils 1147 D Amato G, De Lucia PL, Nardi et al (1986) Carta geologica e Carta della Franosità della Garfagnana e della Valle del Serchio (Lucca) in scala 1: tipografia SELCA Lazzarotto A, Costantini A, Fontani F et al (2003) Carta geologica e geomorfologica in scala 1: Programma VEL e DOCUP Toscana in Amiata e nel Senese, Convenzione Regione Toscana - DST Università di Siena Principi G, Moretti S, Vannocci P et al (2003) Carta geologica e geomorfologica in scala 1: Programma VEL e DOCUP, Toscana in Alto Aretino (Casentino e Valtiberina), Convenzione Regione Toscana - DST Università di Firenze

In situ dynamic characterization of soils by means of measurement uncertainties and random variability

Computational Methods and Experiments in Materials Characterisation III 339 In situ dynamic characterization of soils by means of measurement uncertainties and random variability G. Vessia & C. Cherubini