A. Cavallaro,(D D.C.F. Lo Presti,(2) M. Maugeri,0> O.
( ) University of Catania, Viale Andrea Doria 6, 95125 Catania, Italy
EMail: mmaugeri@isfa. ing. unict. it
(2) Politecnico of Torino, Corso Duca degli Abruzzi 24, 10129 Torino,ItalyEMail: [email protected]
Abstract
The Saint Nicolo Cathedral of Noto was damaged by a post-seismic structuralcollapse on March 13th 1996 and is now under repair and restoration. The city ofNoto is located near Siracusa on the east coast of Sicily and actually is object ofa seismic rnicrozonation (level II) study. Assessment of the seismic risk of SaintNicolo Cathedral by means of analytical computation is also under way. To thisend, in situ and laboratory investigations have been carried out in order todetermine the soil profile with special attention being paid to the variation ofshear modulus and damping ratio with depth. This paper is aimed at providinginformation about the Saint Nicolo Cathedral site characterisation for seismicanalysis.
1 Introduction
The city of Noto is located on the east coast of Sicily, which is one of the mostseismically active areas of Italy. The 13th March 1996 the St. Nicolo Cathedralwas subject to the dome fall caused by a post-seismic structural collapse (Fig. 1).The seismic event occurred on 13th December 1990 and caused structuraldamages to the Cathedral which have not been considered as important until the
collapse of the dome that occurred six years after. In order to study the soil-structure interaction a comprehensive laboratory and in situ investigation hasbeen carried out to obtain a soil profile with special attention being paid to thevariation of shear modulus (G) and damping ratio (D) with depth. The influenceof strain rate on the G-y and D- y curves, as well as on the pore pressure buildup, was evaluated by means of laboratory tests.This paper tries to present in a comprehensive way the most importantparameters for a dynamic study of soils and buildings behaviour. This enables tothe future evaluation of site effects and the earthquake design loads for the repairand strengthening of the Saint Nicolo Cathedral.
Figure 1: The failure of the Cathedral of Noto, (Sicily)
2 Seismicity of the area
During the past centuries Sicily was struck by strong earthquakes whosecharacteristics derive by the geodynamic feature of the Western MediterraneoSee.A detailed list of the earthquake which struck this area has been given by C.N.R.[2]. A study of the most intense earthquakes which damaged the city of Noto hasbeen made by C.N.R. [3]. A brief description of the most significant earthquakescan be summarized as following reported.The Catania earthquake of February, 4, 1169 is one of the oldest shocks of greatmagnitude whose detailed studies are available. The earthquake took place in the
Southern part of Sicily whose seismiciry is characterized by very strong energyreleases which usually occurs after long quiet periods. The shock caused heavydamage. The epicentral area was located near the city of Catania in which theintensity seems to have reached the XI degree in the MCS scale. In Noto area theintensity was about the X degree so that the earthquake here caused many deathsand many buildings were destroyed.On the 10th December 1542 another earthquake struck the city of Noto,however, among the strongest, it seems one of the weakest that historicallyoccurred in Noto area. The epicentre was located near the city of Sortino whichis about 30 km far from Noto. In the epicentral area the intensity was about IXdegree while in Noto it reached about the VII degree. The "Val di Noto"earthquake of January, 11, 1693 is considered one of the biggest earthquakeswhich occurred in Italy. It is thought that more than 1500 after shocks occurredfor about two years more. The earthquake of strong intensity among the mostdestructive of all time (in many centres with intensity of 11° degree of MCSscale), struck a vast territory of the south-eastern of the Sicily and caused thepartial, and in many cases the total, destruction of 57 cities between which thegreater in the area: Catania with 19000 inhabitants, Modica (18000), Siracusa(15000), Acireale (13000), Caltagirone (12000), Noto (12000), Vizzini (11000),Lentini (10000), Ragusa (10000). There were around 60000 deads.Even in the South of Italy and in the African coasts, the effects were quietstrong.On December, 13, 1990 another earthquake struck Noto [4], [11]. Anyway theintensity was not too strong and reached, in the epicentral area, the VII degree.The magnitude was about M = 5.4. Due the low intensity, damage, this time,interested only several old buildings and ancient monuments and was mostlylimited to cornice falls and masonry fractures.
3 Investigation program and geotechnical parameters
The site investigation was performed within an area of 3200 mq (40 m x 80 m)and reached a maximum depth of 81 m. The area pertaining to the investigationprogram and the location of the boreholes and field tests are shown in Fig. 2.Undisturbed samples were retrieved by means of a 86 mm Shelby tube sampler.The Pliocene Noto deposits mainly consist of a medium stiff,normalconsolidated clayey-sand.The general characteristics and index properties of the Noto soil are shown, as afunction of depth, in Fig. 3. The value of the natural moisture content w^prevalently range from between 1 5 - 3 7 %. Characteristic values for theAtterberg limits are: Wj = 37 - 58 % and w = 17 - 22 %, with a plasticity indexof PI = 15 - 40 %. The data shown in Fig. 3 indicate a low degree ofhomogeneity with depth of the deposits. This dishomogeneity with depth is alsoconfirmed by analysing the number of blows Ngpy from mechanical standardpenetration test (SPT) performed over the investigated area (borehole S4) (Fig.
4). The soil deposits can be classified as inorganic soil of low to mediumplasticity.The Menard pressuremeter tests, piezometer tests, down-hole and cross-holetests, seismic tomography tests, ground penetrating radar tests and surfaceseismic tests have also been performed.
CF (%)0 25 50 75 100
Wp-Wn-WI(%)10 20 30 40 50 60
J1%4t
**•) «— t — i
•A
1 ^A
1. Wpp+ WnMA Wl 1— J
0o ,4gP16 -">0~>4")%32
80.9 1
1
.
i
1C0 1.1 1
J_J_
2 1.Y (KN/mc)
18.5 19 19.5 20
Figure 3: Index properties of No to soil.
10 20 30 40 50
Figure 4: Standard penetration test results.
4 Shear modulus and damping ratio
12 -1620 -24 -28 .32 _36 -
60 70
5 -^ 10 -
& MX 20 _Q*0 25-
30 -35 -
NOISPIi
i i^ i ij -Jpr- " 1
i "
TOr-S4
ii
- — i^7 i
y-! !
The small strain shear modulus G^ was determined from in situ Down Hole(DH) test. The equivalent shear modulus (G ) and damping ratio D weredetermined in the laboratory by means of a Resonant Column test (RCT) andcyclic loading torsional shear tests (CLTST) performed on Shelby tube specimenby means of a Resonant Column/Torsional shear apparatus [6]. Moreover it wasattempted to assess G^ by means of empirical correlations based either ondynamic penetration test results [10], [16] or on laboratory test results [5],
4.1 Shear modulus and damping ratio from laboratory testsThe laboratory test conditions and the obtained small strain shear modulus G^are listed in Table 1. The undisturbed specimens were isotropicallyreconsolidated to the best estimate of the in situ mean effective stress. The samespecimen was first subject to CLTST, then to RCT after a rest period of 24 hrswith opened drainage. The size of solid cylindrical specimens are Radius = 25mm and Height =100 mm.
Table 1. Test condition for No to soil specimens.
TestNumber
123456
H[m]9.0013.0015.5022.2022.5051.00
<4c[kPa]166196237294297522
e
0.64100.61130.71780.62980.74900.5840
PI
152027292236
CLTSTRCUUUUUU
Go(U[MPa]
926468100178221
Go (2)[MPa]1167784116190237
where:U = Undrained. G<> (1) from MLTST, G (2) from RCT.
100
80
^ 60— 40o
20
0
0.0001 0.001 0.01 0.1
Figure 5: G-y curves from CLTST and RCT tests.
E4 *
T
1m
0
^NOTOI Depth = 15. 50m.
Her. = 237 kPa
mI Ia
«>Q E@
. CLTST
® RCT1 "LU
a* c
a
~~
Bmi
"T - —
DIN•U
1
The GO values, reported in Table 1, indicate a moderate but measurable influenceof strain rate and type of loading even at very small strain where the soilbehaviour is supposed to be elastic [1], [7], [8], [9]. In order to appreciate therate effect on Gg, it is worthwhile to remember that the equivalent shear strain
rate (y = 240 • f • y [%/s]) experienced by the specimens during RCT can be
three orders of magnitude greater than those adopted during CLTST. The effectsof the rate and loading conditions on the shear modulus are the same over theentire strain interval investigated where G (RC)/G (CLTST) = 1.24. This
experimental finding is different than that observed by Cavallaro [1], Lo Presti etal. [7], Lo Presti et al. [9], Tatsuoka et al. [15] who have showed an increasingrate effect with a increase of the strain level. This different behaviour can betentatively explained by considering that in this study solid cilindrical specimenswith a shear strain variable from zero (at the centre of the section) to a maximumvalue at the edge have been used, while in previous study mainly hollow cilinderspecimens were used. In the case of hollow specimens, the shear strain is quiteconstant along the radius.A comparison between the damping ratio values obtained from RCT and thoseobtained from CLTST is shown in Fig. 6. It is possible to see that the dampingratio from CLTST, at very small strains, is equal to about 1 %. Greater values ofD are obtained from RCT for the whole investigated strain interval.
30
25
20
£ 15-
ft 10 -
5
0 -
i lNOTODepth = 15.50mc'c = 237 kPa
E3< \
\d Q!
-_..
_
9
m
•n
U
|CI
R(
,TST
:T 1
!
i
i
1
0 O O@ •
id=
;•
j i 1 M i iI I i : ' I 'i l | 0 , :1 1 ii ' b ' , , ' ! !
!•„••! : ! .* T ~ ~ ! ' ~ i " l
i i i i ! ' ! !
0.0001 0.001 0.01 0.1
Figure 6: Damping ratio curves from CLTST and RCT tests.
Considering that the influence of number of cycles N on D has been found to benegligible, in the case of clayey soils for strain levels of less than 0.1 %, it issupposed that RCT provide larger values of D than CLTST because of the rate(frequency) effect, in agreement with data shown by Shibuya et al. [12] andTatsuoka et al. [14]. According to these researchers the nature of soil damping insoils can be linked to the following phenomena:- Non-linearity which governs the so called hysteretic damping controlled by thecurrent shear strain level. This kind of material damping is absent or negligibleat very small strains.- Viscosity of the soil skeleton (creep) which is relevant at very small strainrates.- Viscosity of the pore fluid which is relevant at very high frequencies.Soil damping, at very small strains, is mainly due to the viscosity of the soilskeleton or of the pore fluid, depending on the strain rates or frequencies.Moreover, according to Tatsuoka et al. [14] a partial drainage condition canprovide very high values of the damping ratio.
* Ohta& Goto (1978)... Yoshida & Motonori (1988), fine sando Yoshida & Motonori (1988), any soil. Jamiolkowski et al. (1995). CLTSTo RCT
I
«
A O
*
•
W1V-/
Figure 7: G from laboratory tests and different empirical correlations.
4.2 Shear modulus from in situ tests and evaluation from SPT testsFigure 7 shows against depth the values of G^ obtained in laboratory tests andthose evaluated by means of the following empirical correlations based onstandard penetration tests results or laboratory results.a)Ohta&Goto[10]
rr \ 0.193
= 54.33-0.303 )
(1)
where: Vg = shear wave velocity (m/s), = number of blow from SPT,Z=depth (m), a=age factor (Holocene= 1.000, Pleistocene 1.303), P=geologicalfactor (clays=1.000, sands=1.086),b) Yoshida & Motonori [16]
where: Vg = shear wave velocity (m/s),
.0.14 ,-,•O vo (2)= number of blows from SPT,
<y'vo=vertical pressure, P=geological factor (any soil=55, fine sand=49)
where p=mass densityc) Jamiolkowski et. al. [5]
G = 600-
(3)
(4)
where: a'm = (a V + 2 • a 'h)/3; pa = 1 bar is a reference pressure; GO, c'm andPa are expressed in the same unit. The values for parameters which appear in eq.
(4) are equal to the average values that result from laboratory tests performed onquaternary Italian clays and reconstituted sands. A similar equation wasproposed by Shibuya & Tanaka [13] for Holocene clay deposits.The GQ values obtained with the methods above indicated are plotted againstdepth in Fig. 7. The method by Jamiolkowski et al. [5] was applied consideringa given profile of void ratio and KQ.In Fig. 7 are also shows the values of G^ measured in the laboratory from RCTand CLTST performed on undisturbed specimens. In the case of laboratorytests, the G@ values are determined at shear strain levels of less than 0.001 %. Areasonable agreement between the laboratory results and the initial shearmodulus values evaluated by mean of the proposed empirical correlations isobserved. On the whole, eq. (4) seems to provide the most accurate trend of G^with depth, as can be seen in Fig. 7.
5 Conclusions
A site characterisation for seismic response analysis has been presented in thispaper. On the basis of the data shown it is possible to draw the followingconclusions:- the small strain shear modulus obtained from CLTST and RCT is influencedby the strain rate.- the damping ratio, measured in the laboratory, resulted to be mainly influencedby rate effects.- empirical correlations between the small strain shear modulus and standardpenetration test results were used to infer GQ from SPT. The values of G^ werecompared to those measured in laboratory tests. This comparison clearlyindicates that a certain relationship exists between G^ and the standardpenetration test results, which would encourage one to establish empiricalcorrelations for a specific site. Relationships like those proposed byJamiolkowski et al. [5] or Yoshida & Motonori [16] seems to be capable ofpredicting G^ profile with depth in Pliocene deposits.
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