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Lesson Learned from Liquefaction Potential Assessment · PDF fileLesson Learned from Liquefaction Potential Assessment of Silty Sand Deposits in a Case Study in Italy Diego C.F. Lo

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  • RIVISTA ITALIANA DI GEOTECNICA 2/2011

    Lesson Learned from Liquefaction Potential Assessment of Silty Sand Deposits in a Case Study in Italy

    Diego C.F. Lo Presti, Nunzio Squeglia

    SummaryThe paper shows a case study concerning the liquefaction potential assessment of deposits which mainly consist of non

    plastic silts and sands. The site under study has been characterized by means of in situ tests (CPTU, SPT and DPSH),boreholes and laboratory tests on undisturbed samples. Liquefaction susceptibility has been evaluated by means of severalstandard procedures prescribed by codes or available in technical literature. The evaluation of liquefaction potential has beencarried out by means of different standard procedures based on in situ and laboratory tests. A qualitative estimation of possibledamages to shallow structures has been obtained on the basis of indication contained in TC4-ISSMGE [1993]. In addition,quantitative effects of minor liquefaction phoenomena on shallow structures have been studied by means of simplified soil-foundation interaction. The main scope of the paper is to show that application of standard procedures for liquefaction suscep-tibility or liquefaction potential in areas of medium low seismicity can lead to overemphasize the liquefaction problem. Inaddition, any liquefaction study should consider, at least in a simplified way, the effects of possible liquefaction on the shallowstructures. In addition useful information about the liquefaction strength of silty sands is provided.

    Introduction

    Since 2003, the Tuscany Seismic Survey (Italy)has started an investigation plan for retrofitting andrepair of existing Public Buildings (Schools, Hospi-tals etc.) and for the design of the new ones, in themost seismic areas of Tuscany (Fig. 1). Investiga-tions for the existing buildings concerned the struc-ture, the structural materials, the geology of the siteand the geotechnical characterization of the soil de-posits.

    Different levels of investigation have been un-dertaken. After a preliminary stage, a second levelof investigation has been undertaken. This usuallyconsisted of (at least) a borehole with SPT anddown-hole measurements for each existing publicbuilding. The borehole extended down to the seis-mic bedrock or at least down to 30 m depth. If pos-sible, undisturbed samples have been retrieved forlaboratory testing such as Resonant Column tests -RCT, Cyclic Triaxial tests CTX, Torsional Sheartests TST and conventional classification, odometerand direct shear tests. Special tests (RCT, CTX,TST) were aimed at obtaining shear modulus (G)and damping ratio (D). In the framework of these activities, the

    geotechnical investigations undertaken for theconstruction of a new Primary School located inFornaci di Barga, in the northern part of Tuscany,indicated that the subsoil was susceptible toliquefaction. Investigations consisted of a boreholeextending down to 52 m with down-hole seismic* Dipartimento di Ingegneria Civile, Universit di Pisa.

    Fig. 1 Map of the most seismic areas of Tuscany.Fig. 1 Carta delle aree a maggiore sismicit della Toscana.

  • 69LESSON LEARNED FROM LIQUEFACTION POTENTIAL ASSESSMENT OF SILTY SAND DEPOSITS IN A CASE STUDY IN ITALY

    APRILE - GIUGNO 2011

    measurements and a limited number of SPTmeasurements. Therefore, additional investigationshave been carried out in order to have a betterevaluation of liquefaction susceptibility andliquefaction hazards. More specifically, twoboreholes with SPT measurements, a continuousdynamic probing (DPSH), three CPTU tests and anumber of classification tests and undrainedcompression loading cyclic triaxial tests (UCTX) inthe laboratory were performed.

    On the other hand, seismic risk analysis based ona probabilistic approach indicated, for the site underconsideration, that the earthquake which mainlycontributes to the seismic risk in terms of horizontalpeak ground acceleration (HPGA) can be character-ized by a Magnitude of 5.8 with a distance of 20 km[LAI et al., 2005]. Historically, the site has experi-enced a number of earthquakes with Magnitude anddistance similar to those above indicated even in re-cent years (Tab.I), [FIALDINI, 2008]. Nonetheless, rel-evant liquefaction phenomena have never been ob-served in the study area. Therefore, it is possible toconclude that a true liquefaction can be excluded.On the other hand it is not possible to exclude theoccurrence in the past or in the future of minor liq-uefaction phenomena (e.g. sand boils, water spouts).In fact, in the past the area was not urbanised, there-

    fore in remote times minor liquefaction phenomenacould have happened without any record of them.

    The designed building is a one-storey construc-tion with a reinforced concrete cast-in-situ structure.

    The paper shows the results of the investigat-ions and analyses. The analysis results are discussedwith special attention to the effect of non-plastic silton liquefaction resistance of cohesionless soils.Moreover the prescriptions of Eurocode 8 andIta l ian code, concerning the l iquefact ionsusceptibility, are critically discussed, in the light ofthe case study.

    Seismicity of the study area

    In recent years big efforts have been done in It-aly to improve the seismic macrozonation of the ter-ritory. Actually the HPGA for different return peri-ods are available at the apexes of a square net of0.05 of side [INGV, 2005], which really representsan extremely advanced tool in the a-seismic design.Moreover a MS-Excel based software [SpettriNTCver.1.0.3, 2008] is available to interpolate the HPGAwith respect to period and referring to the effectivesite co-ordinates.

    Table I List of earthquakes with an epicentral distance minor than 20 km.Tabella I Terremoti con una distanza epicentrale minore di 20 km.

    Date Location I0 (MCS) Mw Distance [m]

    6 March 1740 Garfagnana VII 5.18 9975

    23 July 1746 Garfagnana VI 4.83 3758

    5 March 1902 Garfagnana VII 5.17 2432

    27 July 1916 Fosciandora VI 4.83 3131

    25 September 1919 Fosciandora V VI 4.63 7230

    7 September 1920 Garfagnana IX X 6.48 19904

    15 October 1939 Garfagnana VI VII 5.20 18475

    12 August 1951 Barga V VI 4.74 10245

    30 June 1934 Abetone IV V 4.38 17641

    7 June 1980 Bagni 4.70 9841

    23 January 1985 Garfagnana VI 4.69 10192

    Table II - Parametres of the seismic action.Tabella II Parametri dellazione sismica.

    Limit States TR (years) HPGA (g) rockHPGA(g) soil

    Fo (-) T*c (s)

    SLO 30 0.060 0.090 2.431 0.246

    SLD 50 0.078 0.117 2.442 0.253

    SLV 475 0.198 0.280 2.420 0.279

    SLC 975 0.254 0.340 2.422 0.283

  • 70 LO PRESTI - SQUEGLIA

    RIVISTA ITALIANA DI GEOTECNICA

    For the case study the values reported in TableII have been obtained.

    In Table II, TR is return period, F0 and T*c areparameters used in NTC2008 to define elastic spec-trum. The parameter F0 is also used in NTC2008 tocompute the stratigraphic amplification factor S =k1 F0k2HPGA(rock), in which k1 and k2 are con-stants depending on type of soil.

    Due to amplification effects, the HPGA at thetop of the soil deposit has been computed as:

    HPGA(soil) = S*HPGA(rock) (1)

    where the parameter S takes into account both top-ographic and stratigraphic amplification.

    For the case under consideration the topo-graphic amplification factor suggested by NTC2008is equal to 1.0 and S is equal to 1.5 for the first twoserviceability limit states (SLO and SLD) and to1.412 and 1.328 for the two ultimate limit states re-spectively (SLV and SLC)

    Even though there is no specification in theNTC [2008] about the limit state to be considered,it seems reasonable (for liquefaction analysis) to re-fer to the SLV limit state with a return period of 475years, which corresponds to a probability of exceed-ing equal to 10% in 50 years.

    The seismicity of the study area has been com-pletely defined by LAI et al. [2005] that performed ade-aggregation of the seismic hazard. The study hasbeen carried out using standard de-aggregationprocedures [KRAMER, 1996] obtaining the followingcouple of Magnitudes and epicentral distanceswhich mainly contribute to the hazard in terms ofHPGA with a return period of 475 years:

    M = 5.8, d = 20 km.

    After that, they selected a group of seven free-field natural accelerograms compatible with the ob-tained Magnitude-epicentral distance couples, es-tablishing a window for both M and d. They also ver-ified the capability of the selected accelerograms ofreproducing on average the prescribed spectrum onrock [NTC 2008; Eurocode 8, 2003]. The character-

    istics of the selected accelerograms are listed in Ta-ble III.

    Such accelerograms have been used to computethe seismic response at the top of the soil depositperforming 1D total stress analysis. This approachwas used as an alternative to the application of NTC[2008] in assessing the HPGA.

    More recently, SPALLAROSSA and BARANI [2007]published the results of de-aggregation of the seis-mic hazard with respect to the HPGA for a returnperiod of 475 years for the whole Italian territory.Their study indicate for the site under considerationa Magnitude of 5.0 5.5 with a distance range be-tween 5 and 25 km.

    Ground investigation

    Figure 2 shows the location in plan of prelimi-nary and integrative investigations. The ground in-vestigations consist in: 3 boreholes up to 52 m (S4)or 15 m (S15, S16); 9 Standard Penetration Tests; adown-hole test in borehole S4; a seismic refractiontest (ST4); a super heavy dynamic probing (DPSH4)up to 19 m; 3 cone penetration tests, CPTU1,

    Table III Characteristics of the selected accelerograms.Tabella III Caratteristiche degli accelerogrammi selezionati.

    Fig. 2 Location in plan of ground investigation.Fig. 2 Pianta delle indagini.

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