Dynamic shear stiffness and damping measurements for seismic response analyses at Senigallia, Italy
Détermination expérimentale des paramètres dynamiques pour l’analyse de la réponse du sol aux tremblements de terre a Senigallia, Italie.
T. Crespellani & G. Simoni
University of Florence, Italy
ABSTRACT
This paper summarises a detailed study on the dynamic characterisation of the Quaternary alluvial deposits of Senigallia, Italy. The research was addressed to the seismic microzonation of the urban area by employing an 1-D linear equivalent model. In the subsoil of the town, Holocene deposits, overlying the marls of the substra- tum, with depths varying from 15-25m, constitute the most widespread soil of the urban area. The field inves- tigations consisted of down-hole and cross-hole testing in 8 verticals. The dynamic laboratory testing included multistage resonant column tests on undisturbed 15 samples, of which 13 are from alluvial deposits. Compari- sons with the experimental data obtained by other Authors for analogous coeval soils revealed a good agree- ment. With respect to simplified reduction factors for stiffness decay evaluation proposed by Eurocode 8 the alluvial deposits of Senigallia exhibit higher values, thus showing that Eurocode 8 in this case underestimates soil capabilities. Instead, Eurocode 8 overestimate the dissipative properties.
RÉSUMÉ
L’article présente les résultats des mesures in situ et d’essais en laboratoire qui ont été faits sur les terraines du Quaternarie et du Plio_Pleistocène de la ville de Senigallia, Italie, en vue de la détérmination des paramètres dynamiques. L’interprétation des mesures a été faite en vue de l’évaluation de la réponse aux tremblements de terre avec un logiciel basé sur un modèle linéaire equivalent, et conduit à un accord très satisfaisant avec les résultats d’autres chercheurs sur des terrains de la même epoque.
Keywords: dynamic soil characterisation, shear stiffness, damping ratio, linear and volumetric cyclic thresh- old.
1 INTRODUCTION
According to Eurocode 8 (EC8)-Part V, site re- sponse evaluation is a preliminary key step in a seismic building design. Due to their simplicity, the majority of current response analysis professional codes use equivalent linear models, which are based on the assumption that during seismic motion the maximum earthquake-induced shear strains, γmax, do not exceed the volumetric cyclic threshold shear strain, γv. In this condition, cyclic degradation of soils is negligible, in saturated soils in undrained conditions cyclic permanent pore water pressures do not develop during the earthquake, stiffness decay and damping ratios are exclusively strain amplitude dependent and do not depend on the number of load- ing cycles. So, for the evaluation of the relevant soil parameters for site response analysis in the strain range γ < 5×10-2 %, laboratory devices, such as resonant column or torsional shear cyclic apparatus,
can be employed for determining the normalised shear modulus decay G(γ)/Gmax, with G being the secant modulus and Gmax the initial stiffness, and the damping versus strain curves, D(γ).
Therefore, as the reliability of equivalent models is limited to the condition γmax < γv, even if not di- rectly required by current codes, the measurement of the volumetric cyclic threshold shear strain ap- pears to be a crucial parameter for assessing the va- lidity of response analysis results. For a given soil its value is unique (Vucetic 1994) and can be deter- mined with various measurement techniques (reso- nant column, cyclic triaxial test, simple cyclic shear, etc.).
Thus, in general, for equivalent linear response analyses the key geotechnical parameters that re- quire characterisation, are:
- stratigraphy, and for every layer:
- the initial stiffness, Gmax;
- the normalised shear modulus decay G(γ)/Gmax
and damping versus strain curves, D(γ);
- the volumetric cyclic threshold shear strain, γv. According to EC8, such stiffness and damping parameters must be measured by appropriate labora- tory or field tests and it is well known that experi- mental field and laboratory measurement accuracy affects in determinant way the equivalent linear model based predictions.
Only in some cases (for soils of classes C or D with a shallow water table and no materials with plasticity index PI > 40%), EC8 allows that, in the absence of specific data, average reduction factors be used. Such factors are given in the Table 4.1 of EC8 and are reported in this paper in Table 1. A question then rises about how much this factors may be considered valid for Italian soils.
The main purpose of the research described in this paper was the dynamic characterisation of the soils of a few deposits situated in the town of Seni- gallia. The field and laboratory investigations have been finalised to the seismic response analyses by employing a 1-D equivalent model. The use of one- dimensional analysis is justified by the very smooth geometry of the site. The results of these response analyses are described in a companion paper of Fac- ciorusso and Madiai (2007).
A minor aim of the research performed was to check the validity of EC8 simplified reduction fac- tors for the Italian site considered.
Table 1. Average soil damping ratios and average reduction factors (± one standard deviation) for shear wave velocity VS
and shear modulus G within 20m depth (Table 4.1 EC8).
Ground ac- celeraions ra-
tio, α·S
Damping ra-
tio S,max S
V V
Gmax
G
0.10 0.03 0.90 (±0.07) 0.80 (±0.10) 0.20 0.06 0.70 (±0.15) 0.50 (±0.20) 0.30 0.10 0.60 (±0.15) 0.36 (±0.20)
2 RESEARCH FRAMEWORK
The town of Senigallia is located in The Marches, in a region seismically active of Central Italy (Figure 1), which was seriously hit by the seismic sequence of September-October 1997. Starting from this event, the Regional Government promoted micro- zonation studies in most seismic prone areas, with the collaboration of specialised institutions (Mucci- arelli and Tiberi 2003; Crespellani et al. 2004).
Senigallia is one of them. A pilot microzonation study was performed with the support of the INGV (Istituto Nazionale di Geofisica e Vulcanologia) and the cooperation of a multidisciplinary team of re- searchers afferent to various Italian Universities and expert institutions. The complex of the researches performed are described in detail in a recent volume (Mucciarelli and Tiberi eds. 2007). In this study the
event of 5.9 Magnitude, occurred in 1930 producing many injuries and widespread collapses and dam- ages in the town and in many surrounding sites, has been assumed as scenario earthquake. The present study made its first move by an attempt of modelling the seismic source and effects induced by this event.
An extensive geophysical field investigation was performed, which showed that most intensive ground shaking sources are all located in the near field. In this situation, even if the magnitude of the expected earthquakes with return period of 475 years is M> 5.5, the design spectra Type 2 of the EC8 seemed to be more proper than spectra Type 1, as suggested by the Italian Code. Then, further analyses were carried out in order to evaluate local site responses in various sites of the town. The sites for detailed field and laboratory geotechnical inves- tigations and analyses were chosen after extensive geophysical surveys and instrumental registrations of site responses to micro tremors.
3 SITE DESCRIPTION
The historic centre of Senigallia is situated preva- lently in an alluvial plain formed of sediments from the Holocene to recent epoch, composed of silty and clayey layers overlying the marls of the substratum, with depths ranging from 15 to 25m. Its morphology has been altered in the course of centuries and many of these transformations are due to Romans. Other parts of the town lie on the recent sandy Adriatic coastal deposits and on Pleistocene terraces. A small part of the town penetrates in the interior of the hilly territory, where the substratum outcrops. The town is crossed by two rivers (Misa and Cesano) running towards to the sea.
The test sites selected belong to the following classes of EC8:
1) hilly soils constituted by Plio-Pleistocene marls (class B);
2) alluvial Quaternary deposits constituted of sands and clays with gravel layers with equivalent shear velocity within the first 30m from 180 to 360 m/sec and up (class C or B).
According to the actual estimates of the INGV (www.ingv.mi.it) at Senigallia an acceleration on rock or hard soil ag = 0.181g is expected with a re- turn period of 475 years. According to the actual Italian classification Senigallia is included in Zone 2 for which the design conventional acceleration on firm soil is ag = 0.20g. This last value was assumed as reference for site response evaluations.
4 TESTING PROGRAM AND PROCEDURES Before establishing the new geotechnical investi- gation program, a preliminary extensive instrumen- tal geophysical measurement of microtremors and a collection of N. 102 existing stratigraphic profiles
and geotechnical standard data were carried out. The interpretation of these data allowed to organize and optimize a new geotechnical campaign of field in- vestigations specifically oriented to the soil dynamic characterization in the sites were seismic responses could be highest.
The new campaign, performed in 2005, included:
- N. 11 boreholes with SPT tests and the extraction of 39 undisturbed samples,
- N.10 CPTU tests, - N. 7 down-hole tests, - N.1 cross-hole tests.
In addition, in order to control the influence of in- strumental equipment and testing procedures, in three verticals (S2, S6, S10) the down-hole tests were repeated with a different equipment and opera- tors of the Geotechnical Laboratory of the Depart- ment of Civil and Environmental Engineering (DICEA). As from the perspective of site response
analysis, the soils of major interest in the urban area subsoil were the Holocene alluvial deposits overly- ing the substratum, the main attention was given to these soils. In all sites the substratum resulted not clearly defined as hard soil having VS-values supe- rior to 800 m/s. Thus, in order to better define the bedrock, a new borehole drilled to 61m was per- formed and equipped for down-hole testing.
In Figure 1 the location of previous and new borehole profiles and CPT performed for seismic re- sponse analyses are shown.
A summary of the field and laboratory investiga- tions performed is given in Table 2. From the 39 un- disturbed samples only 15 resulted of high quality and appropriated for laboratory dynamic testing.
They cover the full range of in situ stress conditions relevant to assessing the dynamic response in the main part of the town.
Table 2. – Borehole, sample, depth of sampling, field and laboratory tests.
Borehole Sample Depth [m] Geological origin Laboratory tests Site tests S1 C3 13.0 – 13.5 Quaternary Class., OEDO, RC CPT,
DH1 C1 5.5 – 6.0 Quaternary Class., OEDO, RC
C2 9.0 – 9.5 Quaternary Class., OEDO, RC C3 14.5 – 15.0 Quaternary Class., RC S2
C4 21.0 – 21.5 Plio - Pleisto-
cene Class, RC
SPT, CPT, CH,
DH2 C1 3.0 – 3.5 Quaternary Class., RC
C2 9.0 – 9.5 Quaternary Class., OEDO, RC S6
C3 25.5 – 26.0 Plio - Pleisto-
cene Class., RC
CPT, DH1, DH2 C1 9.0 – 9.5 Quaternary Class.
C2 15.0 – 15.5 Quaternary Class., OEDO, RC S7
C3 24.0 – 24.5 Quaternary Class., RC
SPT, DH1 S9 C1 8.5 – 9.0 Quaternary Class., RC, OEDO SPT, DH1
C1 2.0 – 2.5 Quaternary Class., OEDO, RC S10 C2 5.5 – 6.0 Quaternary Class., OEDO, RC
CPT, SPT, DH1, DH2 S12 C1 3.0 – 3.5 Quaternary Class., OEDO, RC
SPT, CPT, DH1
Class. = Soil classification tests SPT = Standard penetration test
OEDO = Oedometric test CPT = Cone penetration test
RC = Resonant column test DH1 = Down-hole test
DH2 = Down-hole test performed with DICEA equipments
5 FIELD DYNAMIC INVESTIGATIONS
It is well known that the initial stiffness is a fun- damental soil property relevant to the prediction of amplification effects of earthquakes and, therefore, required inputs for seismic response analysis include small-strain shear modulus, Gmax, for each layer.
This parameter is directly related to small-strain shear wave velocity, VS, by
Gmax= ρ VS2
where ρ = mass density of soil. The current state of practice for determining Gmax involves estimating or measuring VS in the field. Among the various meas- urement technique, it is common opinion that the most accurate testing techniques is the cross-hole test but it is often a current practice to consider the down-hole test as a reliable solution for standard en- gineering problems.
Figure 1. Map of the territory of Senigallia whit the location of the field tests.
As said before, the field investigations carried out a Senigallia included 1 cross-hole and 7 down-hole.
In addition, Vs measurements with other equipments were performed in three boreholes (S2, S6,S10) and in a deep borehole of 61m.
The cross-hole and down-hole testing were per- formed according to conventional approaches, with respectively borehole or surface sources and 1-D geophones embedded in the receiver borehole. In the boreholes (S2, S6, S10), for the purpose of checking the influence of testing arrangements, a different ap- proach was used for down-hole measurements. In particular, two couples of solider receivers put at a distance of two meters were employed. Every re- ceiver is composed by a tern of 1-D geophones placed in particular orientations.
The results obtained are shown in Figure 2.
Measures of Gmax were also obtained in laboratory by means of the resonant column device. In Figure 2 the Gmax laboratory values (transformed into VS – values) obtained for a few specimens drawn from
the boreholes S2, S6, S10, are compared with field measurements.
By examining the results obtained it is possible to observe that:
1. The alluvial soil initial stiffness range from low to medium with average VS -values of about 200 m/s; at the top they can have softer layers of two or three meters, with VS -values ranging from 100 to 200 m/s;
2. The substratum, consisting of marls, has typi- cally average VS -values of about 300-500 m/s at the top of the substratum; this fact reveals that it is can- not be considered as hard firm soil (according to EC8 hard soils have VS -values >800 m/s); so the real seismic bedrock is situated at a depth superior to the depth explored and might coincide with the in- tact marls, but the average VS -values measured up to 61 m, are also in this case inferior to 600 m/s;
therefore the real bedrock position is unknown and this fact represents an important source of uncer- tainty in seismic response predictions;
S11 S12
S9 S10
S7
S1 S2 S6
14.5
3.0
6.0
3.5
10.5
1.5
9.5
Fill Clay Sand Silty clay Clayey silt Silty sandy clay Sandy silt Sandy gravel Clayey sandy silt Sandy silty gravel Clay-marl
700 0 100 200 300 400 500 600
2 4 6 0
8 10 12 14 16 18 20 22 24 26 28 30 32 34
700
700 700
700 0 100 200 300 400 500 600
2 4 6 0
8 10 12 14 16 18 20 22 24 26 28 30 32 34
700 0 100 200 300 400 500 600
2 4 6 0
8 10 12 14 16 18 20 22 24 26 28 30 32 34
0 100 200 300 400 500 600
2 4 6 0
8 10 12 14 16 18 20 22 24 26 28 30 32 34
700 0 100 200 300 400 500 600
2 4 6 0
8 10 12 14 16 18 20 22 24 26 28 30 32 34
0 100 200 300 400 500 600
2 4 6 0
8 10 12 14 16 18 20 22 24 26 28 30 32 34
700 0 100 200 300 400 500 600
2 4 6 0
8 10 12 14 16 18 20 22 24 26 28
0 100 200 300 400 500 600
2 4 6 0
8 10 12 14 16 18 20 22 24 26 28
DH (DIC) CH RC
DH (DIC) CH RC
DH (DIC) CH RC
Depth [m]
V [m/s]
Depth [m]
V [m/s]
Depth [m]
V [m/s]
Depth [m]
V [m/s]
Depth [m]
V [m/s]
Depth [m]
V [m/s]
V [m/s] V [m/s]
Depth [m]
Depth [m]
S S S
S S S
S S
Figure 2 Stratigraphy and shear wave velocity profiles in the boreholes examined.
3. The values of Gmax obtained from field meas- urement of VS are very similar to those obtained in laboratory with the resonant column device;
4. The different measurement techniques do not affect markedly the VS -values and variations are limited to a few thin layers.
6 DYNAMIC LABORATORY TESTS
When opting for an equivalent linear analysis, the characterization of the soil involves two main phases that include:
- The measurement of all soil index and geotech- nical parameters that are relevant for soil characteri- zation, which, even if not directly required by the seismic codes, allow a deeper understanding and interpretation of soil behaviour during dynamic and cyclic loading, such as: saturation degree, density, consistency, void ratio, plasticity index, overcon- solidation ratio, etc.;
- The measurement of the reduction curve G/Gmax
versus maximum cyclic strain γ, of the material damping D versus maximum cyclic strain γ and of the linear and volumetric cyclic threshold shear strain, γl and γv.
From the 39 undisturbed samples obtained during the borehole campaign at Senigallia using thin wall Shelby tubes, 21 samples, collected at a depth vary- ing from 2m to 29m, were selected for dynamic characterization and sent to the Geotechnical Labo- ratory of the DICEA. The remainders were analyzed at the Department of Earth Sciences of the Univer- sity La Sapienza, Rome.
As shown in Table 2, standard tests (classifica- tion, density and oedometric testing) were performed on all 21 undisturbed samples at disposal, whereas dynamic tests was carried out only on 15 samples of high quality. In relation with their geological origin the samples were grouped in the following classes:
1. Quaternary alluvium (N. 13 samples) 2. Plio-Pleistocene marls (N.2 samples)
The dynamic properties were determined by means of resonant column device of the Department of Civil Engineering Geotechnical Laboratory. The testing apparatus is the” fixed-free” Stokoe’s Reso- nant Column device (Anderson and Stokoe II 1978), slightly modified to allow for a more precise meas- urement of residual excess pore pressures.
The multistage procedure adopted consisted of the following steps. First, the apparatus pore pres- sure system is saturated, the specimen prepared and the entire system assembled. At this point, initial readings are taken. Two different stages of consoli- dation are considered before reaching consolidation pressures equal to in situ effective pressure-values s’v. Prior to each stage of consolidation, the pore water pressure coefficient, B, was measured to check that saturation is maintained. At each stage of the consolidation, low amplitude vibrations (about
0.001% strain) are applied to determine the varia- tions of shear modulus and damping ratio during the process of consolidation. Consolidation is allowed until the end of the primary consolidation (generally requiring less than 1000 minutes). Then, the drain- age valves are closed and dynamic testing is carried out. The dynamic testing covers a range of strain amplitude going from low strains (0.001%) to high strains (0.5-1%). Damping ratio was determined by the amplitude decay method.
The results of conventional laboratory testing for the measurement of index properties are reported in Figures 3 and 4. With the exception of specimens S1C3 and S2C1, the alluvial soils are of class CL, are normally consolidated or slightly overconsoli- dated, generally fully saturated, of medium plasticity (PI=11-33%), of consistency varying from 0.38 to 0.75, with K0-values ranging from 0.440 to 0.625.
Figure 3 Main soil characteristics: a) plastic limit, natural water content and liquid limit; b) void ratio; c) OCR values evaluated by means of several oedometric tests.
Figure 4. Plasticity chart with experimental data.
In Figure 5 and in Table 3 the main dynamic soil parameters are shown. In Table 3 the initial stiffness and damping ratios as well as the linear and volu- metric thresholds are related to plasticity index PI and to confining effective stress. In Figure 5, the G/Gmax and D measurements in function of the shear
0.75 1.25 1.75 2.25 2.75 OCR [-]
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 e0 [-]
0
5
10
15
20
25
30
35
0 10 20 30 40 50 60 70 80 wp, wn, wl [%]
Depth [m]
Quaternary soils Plio Pleistocene soils
0 10 20 30 40 50 60
0 10 20 30 40 50 60 70 80 90 100
WL [%]
PI [%]
Quaternary soils Plio Pleistocene soils CL
CH
CL ML
OL ML e
OH MH e
strain amplitude γ are shown. To the experimental values it has been adapted the model of Yokota et al.
(1981) and in Table 4 the fitting parameters and the coefficients of determination obtained for each specimen can be seen.
Table 3. Plasticity index PI, isotropic effective consolidation pressure used in RC test σ’0, initial shear modulus Gmax, initial damping D0, elastic cyclic threshold shear strain γl and volu- metric cyclic threshold shear strain γv from soil samples (Quaternary and Plio-Pleistocene).
Sample PI [%]
σ’0 [kPa]
Gmax
[MPa]
D0
[%]
γl [%]
γv [%]
S2C1 44 100 25.5 2.1 0.0219 0.080 S2C2 28 150 87.5 1.4 0.0094 0.040 S2C3 14 290 195.0 1.0 0.0043 0.020 S6C1 11 100 96.1 1.0 0.0023 0.021 S6C2 11 120 104.1 1.2 0.0036 0.026 S7C2 18 210 103.4 1.8 0.0056 0.020 S7C3 17 300 147.9 1.3 0.0072 0.030 S9C1 24 120 55.1 2.4 0.0096 0.038 S10C1 17 50 46.0 2.9 0.0054 0.025 S10C2 23 120 65.6 2.2 0.0058 0.030 S2C4 22 350 291.9 1.7 0.0086 -
S6C3 33 350 98.4 2.1 0.0115 -
S1C3 33 100 84.78 1.8 - -
S11C1 24 120 61.0 2.0 0.0086 0.040 S12C1 23 90 67.1 2.3 0.0055 0.030 Table 4. Coefficient of the regression and coefficient of deter- mination for the Yokota et Al. models.
Sample α β R2 λ Dmax R2
S2C1 10.656 1.475 0.984 2.154 20.901 0.996 S2C2 53.272 1.546 0.983 2.963 38.966 0.995 S2C3 62.058 1.383 0.977 2.925 30.487 0.988 S6C1 40.86 1.14 0.974 2.567 27.38 0.976 S6C2 66.896 1.334 0.969 2.872 31.059 0.954 S7C2 65.948 1.45 0.98 2.399 25.969 0.979 S7C3 46.227 1.462 0.969 2.967 32.931 0.98 S9C1 34.706 1.415 0.981 2.32 28.084 0.962 S10C1 19.382 1.129 0.987 2.054 25.371 0.992 S10C2 26.928 1.276 0.966 2.281 24.199 0.995 S11C1 22.688 1.321 0.97 2.233 29.353 0.994 S12C1 29.696 1.237 0.979 2.156 27.476 0.987 S2C4 40.346 1.242 0.995 2.265 27.002 0.986 S6C3 23.179 1.411 0.948 2.512 30.803 0.993
As, with respect to the other specimens, the specimen S2C1 can be considered an outsider, in Figure 5 two different laws were determined, the first by including all measurements with exception of specimen S2C1 and the second one for this last specimen. Figure 6 shows the laboratory measure- ments and the fitting of the law of Yokota et al. for the marls. The dispersion of the measures can be at- tributed to the different PI – values of the specimens.
In Table 5 the fitting parameters for all the examined
specimens (Quaternary and Plio-Pleistocene materi- als) are shown.
Table 5. Yokota et Al. (1981) model parameter for the Quater- nary and Plio Pleistocene materials.
Quaternary
(11 ≤ PI ≤ 33) Plio-Pleistocene
α 30.697 17.442
β 1.272 1.165
R2 0.917 0.906
λ -2.452 -2.544
Dmax 27.633 30.781
R2 0.913 0.985
7 DISCUSSION ON TESTING RESULTS
From field and laboratory measurements it ap- pears possible to state that the intrinsic and spatial variability of Quaternary alluvial deposits of Seni- gallia is generally limited.
As it can be seen in Table 6, the average values of the main parameters and their coefficients of varia- tion fall in the typical ranges generally observed in Quaternary deposits. Moreover, Figures 6 and 7 show that the linear and volumetric cyclic thresholds in function of plasticity index are included in the band of the values generally encountered. This fact allows that some analytical relationships between soil parameters be determined.
Table 6 – Statistical distribution of soil parameters of Quater- nary deposits
γ [kN/m3]
e0
[%]
w [%]
PI [%]
γl [%]
γv [%]
Gmax
[MPa]
D0
[%]
M 19.50 0.709 26.32 19.38 0.0061 0.028 114.65 1.73 SD 0.38 0.078 3.74 5.77 0.0024 0.0074 68.83 0.59 CV 1.97 10.99 14.20 29.75 38.58 26.75 60.4 34.03
N 26 19 37 13 11 9 12 10
In particular, the following relationships were ob- tained:
a)γl =0.0001⋅PI1.326 (R2 =0.865) b)γv =0.0024⋅PI0.859 (R2 =0.762) c)Gmax =510.58⋅σ'0+15.10 (R2 =0.583) where Gmax and σ’0 are expressed in MPa.
d) 8.691ln 2.122 ( 2 0.943)
max
= +
−
= R
G D G
8 STIFFNESS REDUCTION COEFFICIENTS The validity of the coefficients proposed by EC8 for stiffness decay evaluation for the soils of classes C and D was checked with the following approxi- mate iterative procedure.
For this analysis, the boreholes S2, S6 and S10 were considered and their subsoil was classified ac- cording to EC8 by considering the equivalent VS- values within the first 30m. As a result, the soils of the three boreholes belong all to class C. For these materials the EC8 gives a soil factor S equal to 1.15.
As in the site the peak ground acceleration on firm soil is 0.20g, the quantity α⋅S is 0.23. By recurring to the well known expression of Seed and Idriss (1971) for the determination of maximum shear stress at the depth z:
D vr g a σ τmax =0.65 max
(where amax is the maximum acceleration at the top of the deposit, g the gravity acceleration, σv the ver- tical total stress and rD a reduction coefficient), with a few assumptions regarding the reduction coeffi- cient rD, the secant shear modulus G and the initial value of the ratio G/Gmax, the maximum shear strain amplitudes induced by the expected earthquake at the depth z of each specimen can be determined in function of the coefficient α⋅S with the following expression:
−
=
=
=
max max max
max
) 1 015 . 0 1 ( 65
. 0
/ 65
. 0
G G G z z
S
G g r
a
D v
γ α
σ γ
By assuming initially a value of G/Gmax equal to 0.5 (that is the value indicated by the EC8) for each specimen falling in the three boreholes examined, the maximum shear strain amplitude has been evalu- ated with an iterative procedure, by evaluating from the experimental curves G/Gmax - γ, the correspond- ing values of stiffness reduction. The final results are shown in Table 7. In this table for clarity also the γv- measurement are reported. It can be seen that in oly a case (S6C1) the expected γmax-value is exceeded by γv. This fact confirms that a linear equivalent model can be used for seismic response evaluation.
With reference to Table 1, it can be seen that for α⋅S
= 0.23 the EC8 gives a D-value of 7.1% and a G/Gmax-value equal to 0.441 ± 0.20. The comparison whit values of Table 7 shows that the average values of D of the EC8 overestimate the dissipative proper- ties and underestimate the stiffness of the alluvial deposits of Senigallia. But it must outlined that EC8 values are for both classes C and D whereas the Senigallia alluvial deposit belong to class C and the water table is not so shallow as EC8 probably means.
Figure 5 Fitting of Yokota et Al. (1981) model to Quaternary experimental data (a and b) and to Plio-Pleistocene results (c and d).
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00
γ [%]
G/Gmax
Quaternary soils
Yokota et Al. model (PI = 11-33%) Yokota et Al. model (PI = 44%)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00
γ [%]
G/Gmax
Plio Pleistocene soils Yokota et Al. Model
0 5 10 15 20 25 30
1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00
γ [%]
D [%]
Quaternary soils
Yokota et Al. model (PI = 44%) Yokota et Al. model (PI = 11-33%)
0 5 10 15 20 25 30
1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00
γ [%]
D [%]
Plio Pleistocene soils Yokota et Al. model
γβ
α⋅
= + 1
1 Gmax
G
max Gmax
G
e D D= ⋅ −λ⋅
a) b)
c) d)
Moreover, for a more accurate analysis the EC8- values should be compared with the shear strain am- plitudes determined by a seismic response analysis.
1.E-04 1.E-03 1.E-02 1.E-01
0 10 20 30 40 50 60 70
plasticity index, PI [%]
linear cyclic threshold shear strain, γl [%]
Senigallia Quaternary soils Senigallia Plio-Pleistocene soils Gori (1998)
Several Authors (from Lo Presti, 1989) Silvestri (1991)
Simoni (2003)
Figure 6. Linear cyclic threshold strain γl versus plasticity index PI for quaternary and Plio Pleistocene examined soils.
1.E-03 1.E-02 1.E-01 1.E+00
0 10 20 30 40 50 60 70
plasticity index, PI [%]
volumetric cyclic threshold shear strain, γV [%]
Senigallia Quaternary soils Gori (1998)
Simoni (2003) Tika et Al. (1999) Lo Presti (1989)
Lo Presti (1989)
Lo Presti (1989)
Vucetic (1994)
Figure 7. Volumetric cyclic threshold strain γv versus plasticity index PI for quaternary examined soils.
Table 7. Estimated maximum earthquake-induced shear strain, stiffness decay values and cyclic volumetric threshold shear strain.
Sample γmax [%] D [%] G/Gmax γv [%]
S2C1 0.04802 3.2 0.88 0.080 S2C2 0.02705 3.9 0.79 0.040 S2C3 0.01917 3.8 0.74 0.020 S6C1 0.01026 4.1 0.73 0.021 S6C2 0.03476 6.6 0.59 0.026 S10C1 0.01412 4.7 0.85 0.025 S10C2 0.02602 4.5 0.75 0.030
9 CONCLUSIONS
The paper synthesizes the results of field and laboratory testing for the dynamic characterisation of Quaternary deposits and Plio-Pleistocene marls of the urban centre of Senigallia. The testing was ad-
dressed to the evaluation of seismic response analy- sis for microzoning purposes, by adopting linear equivalent 1-D model, the use of which is justified by the very smooth geometry of the site. Therefore, the main objective of the research was to determine all soil key parameters for this analysis. As second aim, the research has checked the validity for Seni- gallia alluvial deposits of simplified coefficients for stiffness decay evaluation proposed by Eurocode 8.
ACKNOLEWDGEMENTS
The research was financed by the INGV. The Au- thors are grateful to the Marche Region in the person of P. Tiberi and to Eng. R. Bardotti, director of the Geotechnical Laboratory of DICEA.
REFERENCES
ANDERSON, D.G., STOKOE, K.H., II 1978. Shear Modulus:
A Time-Dependent Soil Property. Dynamic Geotechnical Testing, ASTM STP 654, American Society for Testing and Materials, pp. 66-90.
CRESPELLANI, T. FACCIORUSSO, J. MADIAI, C. 2004.
Dynamic soil properties and site coefficients for aseismic building design in some typical soil deposits of the Marche region, Italy. 3rd Int. Conf. on Earthquake Geotechnical Engineering, vol 2, S. Francisco, 7-9 January 2004, pp 126- 133.
EC8 - 2003. Eurocode 8: Design of Structures for Earthquake Resistance - Part 5: Foundations, retaining structures and geotechnical aspects.
FACCIORUSSO, J. MADIAI, C. 2007. Local seismic response prediction and design building code provisions: the case study of Senigallia, Italy, present Conference.
GORI, S. 1998. Risposta sismica locale in un sito dell’area fiorentina. Tesi di Dottorato, Università di Firenze.
INGV (ISTITUTO NAZIONALE DI GEOFISICA E VULCANOLOGIA): www.ingv.mi.it.
LO PRESTI, D.C.F. 1989. Proprietà dinamiche dei terreni.
Proc. XIV Conferenza di Geotecnica di Torino, Politecnico di Torino.
MUCCIARELLI, M. TIBERI P., eds 2003. “Microzonazione sismica di dettaglio di 4 Comuni della Regione Marche”.
Regione Marche ed.
MUCCIARELLI, M. TIBERI, P. eds 2007. Scenari di pericolosità sismica nella fascia costiera marchigiana. La microzonazione sismica di Senigallia. Regione Marche ed.
SILVESTRI, F. 1991. Analisi del comportamento dei terreni naturali in prove cicliche e dinamiche di taglio torsionale.
Tesi di Dottorato, Università di Napoli, Federico II.
SIMONI, G. 2003. Caratterizzazione dinamica di alcuni depositi olocenici e pleistocenici dell’alta Valtiberina finalizzata alla valutazione degli effetti sismici locali”, Tesi di Dottorato, Università di Firenze.
VUCETIC, M. 1994. Cyclic threshold shear strains in soils.
Journal of Geotechnical Engineering, ASCE, 120, GT12, 2208-27.
YOKOTA, K., IMAI, T., KANEMORI, T. 1981. Dynamic de- formation characteristics of soils determined by laboratory tests. OYO Technical Report n. 3.
