R. Bellotti, M. Jamiolkowski, D. C. F. Lo Presti and D. A. O'Neill anisotropy of small strain stiffness in ticino sand

T. Crespellani, D.C.F. Lo Presti, C. Madiai e G. Vannucchi

INFLUENCE OF SOIL DYNAMIC PARAMETERS ON SEISMIC RESPONSE OF A SITE AT FABRIANO, ITALY

12WCEE, Auckland 30 Jan. to 4 Feb. 2000, Paper No. 2499/10/A/ST3

Dynamic parameters play an important role in evaluating the seismic response of soil deposits. However, they are affected by many uncertainties associated with the natural variability of soils, sampling techniques, laboratory testing procedures, sensing devices, instrumental biases, operators, etc. Therefore, choosing the ‘best values’ is generally a complex problem of decision-making. In this paper, the problem of the influence on seismic response of the uncertainties connected with laboratory testing equipment and procedures used in measurement of damping ratio is considered. Reference is made to the evaluation of site effects in a cohesive deposit in the town of Fabriano, Italy, which was damaged by the Umbro-Marchigian seismic sequence that initiated on September 26, 1997

C.G. Lai, D.C.F. Lo Presti, O. Pallara e G.J. Rix

MISURA SIMULTANEA DEL MODULO DI TAGLIO E DELLO SMORZAMENTO INTRINSECO DEI TERRENI A PICCOLE DEFORMAZIONI

Atti 9° Convegno Nazionale "L'Ingegneria Sismica in Italia", Torino 20-23 Settembre 1999. ANIDIS (formato CD)

Evidenze sperimentali mostrano che un campione di terreno sottoposto a sollecitazioni cicliche esibisce sia capacità di immagazzinare energia di deformazione sia di dissiparla. Il fenomeno di dissipazione avviene anche a livelli molto bassi di deformazione, al di sotto della soglia di deformazione lineare ciclica. Da un punto di vista fenomenologico tale tipo di comportamento meccanico può venire convenientemente descritto dalla teoria della viscoelasticità. Un risultato importante previsto da questa teoria è la dipendenza funzionale tra la velocità di propagazione delle onde di volume e i coefficienti di smorzamento. Tale dipendenza ha importanti implicazioni sulla determinazione sperimentale di questi parametri, tra cui il fatto che essi dovrebbero essere determinati simultaneamente, sebbene nella prassi, essi siano misurati in modo completamente indipendente. In questo articolo gli autori propongono una nuova procedura sperimentale dove la velocità di propagazione delle onde di taglio e lo smorzamento intrinseco dei terreni sono misurati simultaneamente attraverso prove in laboratorio di colonna risonante

D.C.F. Lo Presti, I. Puci, O. Pallara, R. Maniscalco e S. Pedroni

EXPERIMENTAL LABORATORY DETERMINATION OF THE STEADY STATE OF SANDS

Soils and Foundations, Vol. 40, No. 1, February 2000, pp: 113-122.

Some experimental problems encountered when determining Steady-State (SS) or Phase Transformation (PT) envelopes and their projections in the state diagram are studied; 1) accurate assessment of the void ratio (e) at the end of the consolidation process; 2) the effects due to the membrane compliance; 3) effectiveness of lubrication ends and 4) influence of specimen slenderness on the lateral strain uniformity.

The specimen's dimensions after its formation were continuously monitored by measuring axial and radial strains locally. Both conventional undrained triaxial tests and constant volume tests with continuous correction for the membrane penetration effects were performed at constant cell pressure using loose saturated Toyoura sand.

The adopted lubrication system worked effectively up to an axial strain of 4 % for specimens with a height to diameter ratio of H/D=2, while for H/D=1 lateral strains were uniform until the end of the tests (ε25%).

The SS and PT envelope and their projection in the state diagram (SS or PT line) were established based on results from 29 tests. The condition which triggers the softening behaviour, i.e. the so called Critical Stress Ratio (CSR) envelope was also determined. The test results showed the importance of careful control of the test factors mentioned above determining the undrained behaviour of saturated very loose sand under truly constant conditions.

L. Gabrielaitis, M. Jamiolkowski, D. Lo Presti e I. Puci

OPERATIONAL SAND STIFFNESS FROM LARGE SCALE LOAD TESTS ON RECONSTITUTED SAND BEDS.

Baltic Geotechnics IX 2000, Pärnu, Estonia 9-12 May 2000. Estonian Geotechnical Society. Published by Akadeemia Trükk, Niine, 10414 Tallinn, Estonia. pp: 84-91, paper 1.14

This paper is aimed at obtaining the operational sand stiffness from two large-scale load tests performed on a square rigid foundation, 1 m side, resting on a sand bed 4.6 by 4.6 m in plan and 3 m deep. Two samples of Ticino sand were reconstituted at two different relative densities (D and 85 %). In the case of the low-density sample a maximum vertical pressure of about 100 kPa was applied. For the high-density sample the maximum vertical pressure was equal to about 300 kPa. The observed settlements have been compared to those inferred from a conventional computation methods. Particular attention was devoted to the analysis of creep deformations. The load displacement curves were used to determine the operational soil stiffness. The obtained values were compared to those inferred from plate load tests performed in Calibration Chamber on a circular, rigid, deep foundation resting on reconstituted Ticino sand samples (Ghionna et al. 1994).

S. Foti, D.C.F. Lo Presti e O. Pallara

INDAGINI GEOTECNICHE PER LA VALUTAZIONR DEGLI EFFETTI LOCALI DEI TERREMOTI A CASTELNUOVO GARFAGNANA

Atti Convegno GeoBen 2000, Geological and Geotechnical Influences in the Preservation of Historical and Cultural Heritage. Torino 7-9 Giugno. pp: 501-510

Nella salvaguardia del patrimonio storico-monumentale nazionale, l’adeguamento sismico ha un ruolo fondamentale e pertanto una accurata valutazione degli effetti locali dei terremoti è il necessario presupposto per gli interventi in tutte le regioni a rischio. In tale contesto, la caratterizzazione geotecnica dei depositi costituisce un elemento fondamentale, per la corretta modellazione numerica degli eventi sismici. L’effettuazione di prove in sito e prove di laboratorio è un requisito necessario per una modellazione che riesca a tenere debitamente in conto gli effetti della non-linearità dei terreni.

Il progetto Valutazione degli Effetti Locali, avviato dalla Regione Toscana per la definizione del rischio sismico nei comuni della Garfagnana e della Lunigiana, prevede una fase di caratterizzazione dei geo-materiali relativamente accurata in relazione all’estensione dei territori da indagare, al fine di ottenere una base dati sufficiente per una corretta modellazione.

Nel presente lavoro si riportano i risultati relativi a prove geotecniche in sito e di laboratorio eseguite nel comune di Castelnuovo in Garfagnana.

In the safeguard of the Italian National Heritage, the aspects related to the seismic retrofit are of great importance, considering the high degree of seismic risk. In this view an accurate evaluation of the possible local seismic effects is necessary. For this purpose, geotechnical characterisation of soil deposits plays a major role in the numerical modelling. In situ and laboratory tests are both necessary for a correct modelling of the non-linear behaviour of soils.

The Tuscany local government has started a comprehensive project for the evaluation of site effects in the territories of Garfagnana and Lunigiana, which are under its authority. Such a project requires an accurate characterisation of geo-materials in the whole zone interested by the study, in the view of having sufficient data for a correct modelling of seismic events.

This paper reports the results of a series of geotechnical tests, both in situ and in the laboratory, that have been conducted for the characterisation of the soil deposits in the town of Castelnuovo Garfagnana.

D.C.F. Lo Presti, C. Lai, A. Camelliti e T. Crespellani

ANALISI NON LINEARE DELLA RISPOSTA SISMICA DEI TERRENI

Atti Convegno GeoBen 2000, Geological and Geotechnical Influences in the Preservation of Historical and Cultural Heritage. Torino 7-9 Giugno. pp 601-612.

Questo articolo ha per oggetto lo sviluppo di un codice di calcolo per la determinazione della risposta sismica locale di un deposito di terreno in condizioni di sollecitazione monodimensionale e nell’ipotesi di un comportamento costitutivo dei terreni di tipo non-lineare (ONDA). La non linearità è stata modellata assumendo 1) una curva “scheletro” finalizzata a descrivere il comportamento sforzi-deformazioni di primo carico, e 2) una “legge” che consenta di modellare il comportamento di scarico-ricarico al persistere dell’eccitazione sismica (KRAMER 1996). La curva scheletro è ottenibile da prove sperimentali quasi statiche e monotone. Per quel che concerne la legge per modellare i cicli di scarico-ricarico, una scelta comunemente usata è quella della regola di Masing (MASING 1926) per cui i rami dei cicli di scarico e ricarico della curva sforzi-deformazioni sono assunti omotetici alla curva di primo carico con un fattore di scala (n) uguale a 2. In questo lavoro, i cicli di scarico-ricarico sono stati modellati con il 2° Criterio di Masing modificato (TATSUOKA et al. 1993) dall’assunzione di un fattore di scala (n) non necessariamente uguale a 2. In particolare valori di (n) maggiori di 2 sono utilizzati per modellare un comportamento di tipo incrudente (incrudimento ciclico) mentre i valori di n possono risultare persino minori di 2 per nel caso di comportamento di tipo rammollente (degradazione ciclica del materiale). ONDA pertanto, offre la possibilità di determinare le deformazioni permanenti che si sviluppano a seguito dell’eccitazione sismica e quindi di modellare la degradazione ciclica del materiale.

ONDA ed il codice di calcolo di tipo lineare equivalente SHAKE (SCHNABEL et al. 1972) sono stati utilizzati per valutare la risposta sismica locale nell’abitato di Fabriano. Le differenze ottenute con i due metodi di analisi vengono attentamente discusse.

This paper describes a newly developed computer code for performing one-dimensional, non-linear dynamic analysis (ONDA) of soil deposits. Non-linearity is modelled by assuming 1) a “backbone” curve to describe the initial loading stress-strain behaviour, and 2) a “rule” for modelling the unloading-reloading behaviour as the seismic excitation progresses (KRAMER 1996). The backbone curve can be obtained from quasi-static monotonic loading tests. Concerning the rule to model the unloading-reloading soil behaviour a common choice is the so-called “Masing rule” (MASING 1926) by which it is assumed that the unload-reload branches of the stress-strain curve have the same shape of first loading curve with a scale factor (n) equal to 2. In this work the 2° Masing rule has been modified by assuming a factor (n) not necessarily equal to 2 (TATSUOKA et al. 1993). In particular, in case of cyclic hardening, n will be greater than 2, while in the case of cyclic softening n can become even smaller than 2. ONDA gives then the possibility of computing permanent strains that are developed during seismic excitation and hence of taking into account the phenomenon of material degradation.

ONDA and the equivalent linear code SHAKE (SCHNABEL et al. 1972) have been used to evaluate the seismic response at Fabriano. Differences between the results obtained by ONDA and SHAKE are discussed in detail.

M. Ferrini, V. Petrini, D. Lo Presti, I. Puci, L. Luzi, F. Pergalani, e P. Signanini

NUMERICAL MODELLING FOR THE EVALUATION OF SEISMIC RESPONSE AT CASTELNUOVO GARFAGNANA IN CENTRAL ITALY

XV ICSMGE, Istanbul 27-31 August 2001, Special Volume of TC4, Lessons Learned from Recent Strong Earthquakes. pp 149-154.

ABSTRACT: This paper mainly focuses on the evaluation of the seismic response in the town of Castelnuovo Garfagnana. It represents the conclusive part of a multidisciplinary work that includes geological, geophysical and geotechnical investigations and the evaluation of the expected strong ground motion at the bedrock. Several computer codes have been used to compare 1D and 2D approaches and to evaluate, for the 1D case only, the effect of non-linear vs. linear-equivalent stress-strain soil models.

The main objectives of this paper are i) to point out capabilities and limitations of different models and ii) to provide the local authorities with a tool for land use planning, building construction and retrofitting of pre-existing structures.

The results are expressed as response spectra and amplification coefficients for two representative geologic cross-sections in the territory of Castelnuovo Garfagnana.

E. Calosi, M. Ferrini, A. Cancelli, S. Foti, D. Lo Presti, O. Pallara, G. D’Amato Avanzi, A. Pochini, A. Puccinelli, L. Luzi, M. Rainone e P. Signanini

GEOLOGICAL AND GEOTECHNICAL INVESTIGATIONS FOR THE SEISMIC RESPONSE ANALYSIS AT CASTELNUOVO GARFAGNANA IN CENTRAL ITALY

XV ICSMGE, Istanbul 27-31 August 2001, Special Volume of TC4, Lessons Learned from Recent Strong Earthquakes. pp 141-148.

ABSTRACT: The Tuscany regional government (Regione Toscana) has started a comprehensive project for the evaluation of site effects in about 60 municipalities, located in the territories of Garfagnana and Lunigiana (Ferrini et al. 2000). For this purpose a multidisciplinary task force is working in order to assess the elastic response spectra taking into account both topographic and stratigraphic amplifications. The main objective is to provide a tool for land use planning, design of new structures and retrofitting of the existing ones. Such activity has been completed for Castelnuovo Garfagnana, that is a town of about 6500 inhabitants with a territory of about 28 square Km.

This paper mainly focuses on the geological, geophysical and geotechnical investigations concerning the town of Castelnuovo Garfagnana. The results are summarised with special attention to the following aspects:

- interplay between geological, geophysical and geotechnical investigations;

- comparison among different geophysical testing methods (Down-Hole, SASW-fk, Seismic refraction);

- comparison among different laboratory tests (Monotonic and cyclic triaxial tests, resonant column tests, cyclic torsional shear tests);

- assessment of the input parameters for seismic analysis, from the experimental results;

- creation of an integrated database.

The main objectives of this paper are i) to point out capabilities and limitations of different in situ and laboratory testing methods and ii) to provide enough information on a well-documented case history. The results of seismic analysis at Castelnuovo Garfagnana are reported in a companion paper.

A.Cavallaro, V. Fioravante, G. Lanzo, D. Lo Presti, Oronzo Pallara S. Rampello, A. d'Onofrio, F. Santucci de Magistris e F. Silvestri

REPORT ON THE CURRENT SITUATION OF LABORATORY STRESS-STRAIN TESTING OF GEOMATERIALS IN ITALY AND ITS USE IN PRACTICE

XV ICSMGE, Istanbul 27-31 August 2001, Special Volume of TC29, Advanced Laboratory stress-strain testing of geomaterials. pp 15-44.

ABSTRACT: Laboratory testing for routine geotechnical design in Italy is mainly based on the results of oedometer, triaxial and direct shear tests. These tests provide compressibility, undrained shear strength and shear strength characteristics generally used in simplified design procedures. The undrained shear strength and/or shear strength parameters are inferred from UU or CU triaxial compression tests performed on specimens 38 mm in diameter and H/D≈2. The shear strength parameters are also inferred from direct shear tests performed under drained conditions.

A booklet prepared by the Italian Geotechnical Society (AGI, 1994) suggests the procedures recommended for laboratory tests such as: grain size analysis, odometer, direct shear and triaxial tests. Anyway, none of these suggested methods deal with the assessment of the stress-strain relationship of geomaterials. In a separate booklet (AGI, 1977) recommendations are provided in order to:

- program geotechnical investigations

- retrieve soil samples

- draw stratigraphic and geotechnical logging

- install piezometers

- determine in situ soil permeability

- perform in situ tests (CPT, SPT and FVT)

The above mentioned documents are written in Italian and mainly follow the recommendations of the International Society (ISSMGE). ). Associations of commercial laboratories (ALGI) and contractors (ANISIG) also guarantee that laboratory and in situ tests (including sampling and borehole logging) are carried out according to the AGI recommendations.

A comprehensive review of AGI recommendations on both in situ and laboratory tests is currently in progress. The reviewed version will closely refer to the principles addressed by Parts 2 and 3 of Eurocode 7 (Geotechnical design).

The Italian Ministry of Public Work (MLLPP) has recently established that Geotechnical Laboratories in Italy should guarantee some requirements and should follow given standards in performing laboratory and in situ tests on soils and rocks (MLLPP, 1999). According to the MLLPP the AGI recommendations will become mandatory. For those tests not considered by AGI it is suggested to follow the international standards such as ASTM, ISRM recommendations and BS 1377 - 5930.

Advanced stress-strain testing is in practice required only for special projects such as microzonation studies and/or design of very important infrastructures especially in seismic areas.

Lai C.G., Pallara O., Lo Presti D. C. F. & Turco E.

LOW-STRAIN STIFFNESS AND MATERIAL DAMPING RATIO COUPLING IN SOILS.

XV ICSMGE, Istanbul 27-31 August 2001, Special Volume of TC29, Advanced Laboratory stress-strain testing of geomaterials. pp 265-274.

ABSTRACT: Experimental evidence shows that soils subjected to dynamic excitations have both the ability to store and to dissipate strain energy. The phenomenon of energy dissipation takes place even at very small strain levels, below the so-called linear cyclic threshold shear strain. During low-strain harmonic oscillations, mechanical energy is exchanged between the loading apparatus and a soil specimen. Because this energy exchange takes place in a finite amount of time (which is called the relaxation time), stress and strain are out of phase and energy is dissipated during the process of cyclic loading. This type of material behaviour, where the phenomenon of energy dissipation takes place over a finite amount of time can be conveniently described from a phenomenological point of view by the theory of viscoelasticity. It is an experimental fact in soils that at strain levels below the linear cyclic threshold shear strain, phenomena of instantaneous energy dissipation do not take place or if they do they are negligible. An important result predicted by the linear theory of viscoelasticity, is the functional dependence between the velocity of propagation of body waves and material-damping ratio. The coupling existing between these two important parameters is a consequence of material dispersion, a phenomenon by which the velocity of propagation of mechanical disturbances in dissipative media is frequency dependent. Viscoelastic materials, and hence soils subjected to low-strain harmonic excitations, are inherently dispersive. A corollary of the functional dependence of the velocity of propagation of body waves upon material damping ratio is that a correct procedure for the experimental measurement of these parameters should determine them simultaneously and at the same frequency of excitation. However, it is a common practice in laboratory measurements to determine V_S and D_S separately. In this article the authors present a new experimental procedure to be conducted with the resonant column apparatus where the shear wave velocity and shear-damping ratio are determined simultaneously. Since these parameters are determined at specific frequencies of excitation, the proposed method is also well suited to investigate the frequency dependence laws of these important soil parameters. The novel procedure, which offers a re-interpretation of the resonant column test, has been applied in a series of experiments using both remoulded and undisturbed clay specimens. The paper shows the results obtained and compare them with those determined with conventional techniques.

Cavallaro A., Lo Presti D.C.F., Maugeri M.

THE DEGRADATION BEHAVIOUR OF FABRIANO SOIL DURING CYCLIC LOADING.

Italian Geotechnical Review, Vol. XXXV, No. 2, pp. 107-117.

When the shear strain is higher than the so called volumetric trheshold, degradation phenomena occur. In this paper the possibility of modelling the degradation behaviour of Fabriano soil with a simple law is examined. The study shows that the modified 2^nd Masing Rule (Masing 1926, Tatsuoka et al. 1993) is capable of well reproducing the experimental stress strain curve of natural clays during cyclic loading. The modified 2^nd Masing rule assumes that the scale amplification factor for unload-reload branches (n) is not necessarily equal to 2 as postulated by the original 2^nd Masing rule. In particular it was found that n=6 in the case of stable cycles. When degradation phenomena occur, n decreases. The limited experimental data did not allow the authors to determine the dependence of n on shear strain and numer of loading cycles. On the other hand, the degradation parameter (t) proposed by Idriss et al. (1978) was computed according to the procedure suggested by Lin and Chen (1991). The values of t resulted to be much higher than those available in literature.

The results obtained for fabriano soil have been compared with others data available in literature and in particular with the results obtained for undisturbed Augusta, Catania, Pisa clays and reconstituted Toyoura sand (Cavallaro 1997, Lo Presti et al. 1998, Ionescu 1999, Lo Presti et al. 2000).

Lo Presti D.C.F. e Puci I.

IMPIEGO DELLE PROVE PENETROMETRICHE DINAMICHE PER LA CARATTERIZZAZIONE MECCANICA DEI TERRENI

XVIII Ciclo di Conferenze di Geotecnica di Torino

La presente nota descrive le prove penetrometriche dinamiche a punta aperta come la Standard Penetration Test (SPT) ed a punta chiusa come la Standard Cone Penetration Test (SCPT). Negli anni più recenti la sigla SCPT è stata utilizzata per designare una particolare prova penetrometrica statica (Seismic Cone Penetration Test), cosicché nel proseguimento la prova penetrometrica dinamica a punta chiusa sarà designata con la sigla DP (Dynamic Probing) in accordo con la terminologia adottata dal Technical Committee on Penetration Testing dell’International Society of Soil Mechanics and Geotechnical Engineering – ISSMGE (Stefanoff et al. 1988). In particolare, per entrambe le metodologie di prova, saranno descritte le attrezzature (molto brevemente), le modalità di esecuzione (confrontando i diversi standards) ed i metodi di interpretazione al fine di pervenire alla caratterizzazione dei depositi ed alla definizione dei parametri di progetto. Quest’ultimo aspetto sarà trattato ponendo particolare attenzione al caso dei depositi ghiaiosi.

In una precedente edizione delle Conferenze di Geotecnica di Torino lo stesso argomento era stato trattato nei lavori di Bruzzi e Cestari (1983) e Pasqualini (1983) che rappresentano punti di riferimento fondamentali per quel che concerne la letteratura tecnica in lingua italiana su tale argomento. Un altro lavoro in lingua italiana riguardante le attrezzature e le modalità di prova è quello di Cestari (1990).

Lo scopo delle prove penetrometriche dinamiche è quello di determinare il numero di colpi necessari ad infiggere, per una data profondità (tipicamente 300 mm), un tubo campionatore o una punta chiusa di dimensioni standard mediante battitura. La prova SPT può essere effettuata in fori di sondaggio, oppure in fori appositamente eseguiti. Tale requisito non risulta invece necessario per la prova DP che in tal senso risulta economicamente vantaggiosa, tuttavia, al fine di evitare l’attrito laterale sulla batteria delle aste, è necessario eseguire la prova utilizzando dei tubi di rivestimento.

Le prove vengono preferibilmente eseguite in terreni granulari (sabbie e ghiaie). Tuttavia è possibile realizzarle in qualsiasi terreno sciolto ed anche nel caso di alcune rocce tenere. Unicamente in presenza di grossi elementi lapidei, questa tipologia di prova perde di significato.

I valori misurati nel corso della prova consentono una stima qualitativa della consistenza del terreno. Inoltre è possibile stimare mediante correlazioni empiriche i seguenti parametri:

- la densità relativa , l'angolo di resistenza al taglio e la resistenza alla liquefazione dei terreni granulari;

- il modulo di taglio a piccole deformazioni ;

- la resistenza al taglio non drenata di terreni a grana fine e rocce tenere.

E’ anche diffuso l’impiego diretto dei risultati delle prove per il calcolo dei cedimenti delle fondazioni superficiali in sabbia (Burland e Burbridge,1984, Berardi e Lancellotta, 1991) e per il calcolo della capacità portante dei pali di fondazione. Tale impiego è basato sull’esperienza locale oppure su un’estensione di metodi che utilizzano i risultati delle prove penetrometriche statiche (CPT). In questo secondo caso, è necessario utilizzare delle correlazioni empiriche tra ed . Nella presente nota vengono illustrate e commentate le correlazioni empiriche utilizzate per la determinazione dei parametri sopra indicati e vengono brevemente riassunti e commentati i metodi che utilizzano direttamente i risultati delle prove penetrometriche dinamiche per la verifica delle fondazioni superficiali e profonde.

Lo Presti D.C.F., Shibuya S. e Rix G.

INNOVATION IN SOIL TESTING

IS Torino 99, Theme Lecture, Balkema, Vol. II, pp:1027-1076.

This paper describes the experimental determination of the stress-strain properties and stiffness of geomaterials in the pre-failure range. In particular, the following aspects are described in detail: i) importance of accurate measurements of stress and strain in laboratory tests, ii) assessment in the laboratory of sample disturbance, iii) development of appropriate laboratory reconsolidation techniques to minimise the effects of sampling disturbance and iv) use of seismic tests to obtain reference stiffness (, and damping ratio values.