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

R. Bellotti, M. Jamiolkowski, D.C.F. Lo Presti and D.A. O'Neill

ANISOTROPY OF SMALL STRAIN STIFFNESS IN TICINO SAND

Geotechnique, 1996, Vol. 46, No. 1, pp. 115-131.

A series of comprehensive tests performed in a large calibration chamber and consisting of the propagation of seismic body waves in dry Ticino silica sand are presented. By virtue of the fact that both shear and constrained compression waves have been propagated in vertical, horizontal and oblique directions, five independent constants of the cross-anisotropic elasticity model of the test sand have been established. The tests have also made it possible to evaluate the influence of the structural (inherent) and stress induced anisotropy on the velocity of the propagated seismic body waves. Information concerning the influence of the principal effective stresses on the velocity of these waves is also given. The influence of the principal effective stresses on the velocity of these waves is also described.


D.C.F. Lo Presti

VERIFICA DI STABILITA' DI UNA RAMPA IN TERRA ARMATA: APPLICAZIONE ALLA PIRAMIDE DI CHEOPE (Stability Analysis of a Reinforced Earth Ramp: the Cheope Pyramid Case; in Italian)

Atti dell'Accadamedìa delle Scienze, Torino, Atti Scienze Fisiche, No. 130, pp. 95-109, Anno 1996.

La presente nota considera la stabilità di una rampa in mattoni crudi con elementi di rinforzo in legno di palma e stuoie di canna. Si presume che una simile struttura sia stata utilizzata per il sollevamento dei blocchi di pietra necessari alla costruzione delle Piramidi. L'esempio analizzato in particolare fa riferimento alla geometria delle grandi Piramidi. Si sono considerati tre differenti fenomeni di collasso: scivolamento dell'intera rampa lungo la superficie di appoggio, rottura locale della rampa all'estradosso, danno strutturale agli elementi di rinforzo in legno di palma. Le verifiche effettuate garantiscono la stabilità della struttura.

This note concerns the stability of a ramp obtained using clay bricks and reinforcements made up of palm wood and reed mat. It is supposed that such a structure was used during the construction of the Pyramids in order to transport the construction material. Cheope Pyramid is studied in detail. Three different kinds of failures were considered: global instability of the slope, local failure, structural damage of the reinforcing elements of the palm wood. The examined structure resulted to be safe.

O. Pallara, D.C.F. Lo Presti, M. Jamiolkowski e S. Pedroni

CARATTERISTICHE DI DEFORMABILITA' DI DUE SABBIE DA PROVE MONOTONE E CICLICHE

Rivista Italiana di Geotecnica, 1998, Vol. XXXII, n. 1, pp. 61 - 80

Questo lavoro, frutto di una collaborazione tra l'ENEL CRIS. di Milano e il Politecnico di Torino, riporta i risultati di uno studio sperimentale riguardante le caratteristiche di deformabilità di due sabbie ricostituite in laboratorio.

I materiali utilizzati sono: la sabbia di Toyoura, che è prevalentemente quarzosa, e la sabbia carbonatica di Quiou con grani altamente frantumabili.

Per realizzare le indagini sperimentali si sono utilizzate due apparecchiature di laboratorio: una cella triassiale, con caratteristiche tali da poter determinare accuratamente la relazione sforzi deformazioni anche per piccoli livelli deformativi, ed un apparecchio di colonna risonante in grado di eseguire prove di taglio torsionale in condizioni di carico statico monotono e ciclico. Sia nel caso delle prove triassiali, che per le prove di colonna risonante e taglio torsionale è stato possibile determinare in modo affidabile la rigidezza dei provini anche per livelli deformativi molto piccoli pari a 103 %.

Le indagini sperimentali effettuate erano soprattutto tese a investigare l'influenza dei fattori sottoriportati sulla rigidezza di due sabbie aventi diversa mineralogia: stato del materiale (indice dei vuoti, tensioni di consolidazione); livello deformativo; storia tensionale; tipo di prova (triassiale, torsionale).

Inoltre la ricerca era tesa a valutare l'influenza degli errori di misura della deformazione assiale sulla rigidezza determinata in prove triassiali.

This paper shows the experimental results of a joint research between E.N.E.L C.R.I.S of Milan and the Politecnico di Torino, which was aimed at investigating the deformation characteristics of two reconstituted sands.

The tested soils were: quartz Toyoura sand and carbonatic Quiou sand with high crushable grains.

Tests were performed by means of a triaxial apparatus and a Resonant Column/Torsional Shear apparatus. Both the apparatusa allow the assessment of very small strains of about %.

Tests were performed in order to investigate the influence of the following factors on the stiffness of two sands having different mineralogical composition: soil state (void ratio, pressures); strain level; stress-history; type of test (triaxial vs. torsional).

Moreover, the influence on the stiffness of possible experimental errors in the axial strain measurements during triaxial tests was also examined.

D.C.F. Lo Presti, M. Jamiolkowski, O. Pallara e A. Cavallaro

RATE AND CREEP EFFECT ON THE STIFFNESS OF SOILS

ASCE Convention, 10 - 14 Nov. 1996, Washington, Geotechnical Special Publication No. 61, pp 166- 180.

The shear modulus of two undisturbed Italian clays was measured in the laboratory by means of a Resonant Column/Torsional shear apparatus for a strain range of 0.0001 % up to 1 %. Three different kinds of tests were performed on hollow cylindrical specimens reconsolidated to the in situ geostatic stress: a) static monotonic loading tests at constant stress rate, b) cyclic loading tests at constant strain rate, c) Resonant Column tests. Moreover the small strain shear modulus , determined at strain level less than 0.001% was measured during the drained creep following reconsolidation. The increase of G, with time during the drained creep was therefore assessed and compared with available correlations from literature. The data obtained from the above mentioned research activity enabled one to examine the influence of the strain rate, loading conditions (cyclic vs. monotonic) and strain level on the deformation moduli of the tested clays. It was possible to assess that the normalised curves (G/G vs. γ) are dependent on loading conditions and strain rate.

C. Fretti, F. Froio, M. Jamiolkowski, D.C.F. Lo Presti, A. Olteanu and S. Pedroni

DILATOMETER TESTS IN CALIBRATION CHAMBER: STIFFNESS AND Ko ASSESSMENT

A VIII-a Conferinta Nationala De Geotehnica Si Fundatii, Iasi, Romania 25-28 Septembrie 1996, Vol. I, pp. 85-94.

This paper analyses the Marchetti's dilatometer tests (DMT) performed in Calibration Chamber (CC) on reconstrituted specimens of Ticino (TS) and Toyoura (TOS) sands. These tests were carried out to assess the capability of DMTs to determine the stiffness and the coefficient of earth pressure at rest Ko of cohesionless deposits.

D.C.F. Lo Presti, A. Lazzari, V. Peisino e L. Vicentini

DAMAGE TO THE ARTISTICAL PATRIMONY IN PIEDMONT DUE TO THE NOVEMBER 1994 FLOOD

Arrigo Croce Memorial Symposium, Geotechnical Engineering for the Preservation of Monuments and Historic Sites, Napoli, Italy 3-4 October 1996, Balkema, Rotterdam, pp. 713-722.

From the 4th to the 6th November 1994, Piedmont (Italy) was affected by heavy floods which caused several deaths and more than 6.5 billion US dollars of damage to firms and civil buildings. The transport infrastructure was also heavily damaged. The flood was caused by peculiar meteorological conditions which have already occurred four times over the last century and another twice during this century. The majority of the observed casualties occurred because of river inundation and slope instability. Three different kinds of slope instability were observed: a) debris and mud flows, which mainly occurred in concurrence with the peak of the rainfall intensity. These instabilities evolved very quickly and caused the majority of deaths. The volume of soil that collapsed ranged from 10 to more than 500 cubic meters; b) translational sliding along planar surfaces. This kind of instability, which mainly affects schistose marls, concerned a very large volume of soil and was usually activated a day after the end of the rainfalls. The evolution of the phenomenon was quite quick; c) rotational failure, evolving in mud flow during heavy rainfalls.

Both old and new slope instabilities were activated by the heavy rainfalls.

The artistical patrimony was also damaged. Slope instabilities of first type damaged the Barolo fortress, the Castellinaldo Castle, the Corneliano and Barbaresco medieval towers and the medieval church of Brosso, among others.

This paper, after a brief description of the meteorological conditions which caused the flood and a brief analysis of the pluviometric data, describes the slope instabilities which damaged the above listed monuments. The countermeasures and repairs to the monuments are also considered.

D.C.F. Lo Presti, O. Pallara e A. Cavallaro

DAMPING RATIO OF SOILS FROM LABORATORY AND IN-SITU TESTS

Proc. of discussion special technical session on Earthquake Geotechnical Engineering, XIV ICSMFE, Hamburg, 6-12 Settembre 1997, Seismic Behaviour of Ground and Geotechnical Structures, Balkema, Rotterdam, pp. 391-400.

This paper considers the influence of N and on D of two undisturbed Italian clays and of three reconstituted granular soils. The experimental results were obtained in the laboratory by means of a Resonant Column/Torsional Shear apparatus.

Moreover, the damping ratio of reconstituted granular soils was determined, at small strains, from seismic tests performed in a large Calibration Chamber. The spectral ratio and the spectral slope methods were used for this purpose. Capability and limits of these methods are discussed in the light of the strong influence of test conditions on damping ratio even at very small strains.

Eventually the experimentally determined values of D are compared against those predicted by means of simple non-linear models incorporating the Masing rules.

D.C.F. Lo Presti, M. Maugeri, A. Cavallaro e O. Pallara

SHEAR MODULUS AND DAMPING OF A STIFF MARINE CLAY FROM IN SITU AND LABORATORY TESTS

International Conference on Geotechnical Site Characterization, ISC 1998, Atlanta, Georgia, Balkema, Vol. 2, pp. 1293, 1300

This paper describes and compares the results of in situ and laboratory investigations which were carried out in order to determine the soil profile of the Saline di Augusta site (also Saline site), with special attention being paid to the variation of shear modulus and damping ratio with depth. The soil deposits at this site mainly consist of a medium stiff, overconsolidated (OCR = 2.0 to 6.0), Pleistocene marine clay with low to medium PI, overlaid by a 15 m thick Holocene silty clay stratum. The Saline site is located near the city of Augusta on the east coast of Sicily which is one of the most seismically active areas of Italy.

D.C.F. Lo Presti, M. Jamiolkowski, O. Pallara, A. Cavallaro e S. Pedroni

SHEAR MODULUS AND DAMPING OF SOILS

Géotechnique Symposium in Print, 1997, Vol. 47, No. 3, pp. 603-617.

The shear modulus (G) and damping ratio (D) of two natural clays and both siliceous and carbonatic pluvially deposited dry sands have been measured at the authors' laboratory by means of a Resonant-Column/Torsional-Shear apparatus for a strain range of 0.0001 % up to 1 %. Different kinds of tests were performed above all on hollow cylindrical specimens: a) monotonic loading torsional shear tests (MLTST) at constant stress rate, b) cyclic loading torsional shear tests (CLTST) at constant strain rate or at constant frequency and c) Resonant Column tests (RCT).

The results of the above tests enabled the authors to analyse the influence of strain level (γ), loading rate (, number of loading cycles (N) and type of loading (monotonic or cyclic) on the measured values of G and D.

D.C.F. Lo Presti, A. Cavallaro e O. Pallara

MODELLAZIONE DELLA NON LINEARITA' DEL LEGAME SFORZI-DEFORMAZIONI DEI TERRENI IN PROBLEMI DI SOLLECITAZIONE CICLICA

8° Convegno Nazionale di Ingegneria Sismica, Taormina, Settembre 1997, Vol. 1, pp. 91-98.

La presente nota analizza criticamente la capacità di semplici modelli non lineari, basati sui criteri di Masing, di riprodurre i risultati di prove di taglio torsionale ciclico e monotono eseguite su argille naturali e sabbie ricostituite. Viene anche considerato un modello che introduce alcune modifiche significative al secondo criterio di Masing consentendo una rappresentazione puù realistica della relazione sforzi-deformazioni a taglio dei terreni.

This paper critically analyses the capability of simple non-linear models based on the Masing rules, to fit the experimental stress-strain curves obtained from monotonic and cyclic loading torsional shear tests that were performed on two undisturbed clays and reconstituted sand. A model which modifies the second Masing rule profidin more realistic fit of the experimental data is also considered.

D.C.F. Lo Presti, M. Maugeri, A. Cavallaro e O. Pallara

RAPPORTO DI SMORZAMENTO DEI TERRENI DA PROVE DI LABORATORIO

8° Convegno Nazionale di Ingegneria Sismica, Taormina, Settembre 1997, Vol. 1, pp. 99-108.

Questo lavoro esamina l'influenza del numero di cicli e della velocità di deformazione sul rapporto di smorzamento di due argille naturali e tre sabbie ricostituite, per mezzo di un apparecchio di taglio torsionale-colonna risonante.

Dall'analisi dei risultati emerge chiaramente che il rapporto di smorzamento è in realtà una grandezza complessa che riflette fenomeni di diversa natura (non linearità, viscosità dello scheletro solido e viscosità del fluido interstiziale). Ciò comporta per il progettista, una particolare attenzione nella scelta dei valori di smorzamento da utilizzare in relazione al problema in esame.

This paper considers the influence of number of loading cycles and strain rate on the damping ratio of two undisturbed Italian clays and three reconstituted granular soils. The experimental results were obtained in the laboratory by means of a Resonant Column-Torsional Shear Apparatus. Due to the high variability of D, especially with frequency, it is extremely important when using linear equivalent methods to compute the seismic response of soil deposits to chose the most appropriate values of damping in relation to the examined problem.

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

DETERMINAZIONE DELLO SMORZAMENTO MEDIANTE MISURE DI TIPO SISMICO IN CAMERA DI CALIBRAZIONE SU TERRENI GRANULARI RICOSTITUITI

8° Convegno Nazionale di Ingegneria Sismica, Taormina, Settembre 1997, Vol. 2, pp. 943-952.

La nota esamina criticamente la possibilità di determinare il rapporto di smorzamento dei terreni D da misure di propagazione di onde di volume eseguite in Camera di Calibrazione su diversi terreni granulari. In particolare sono stati esaminati i metodi cosidetti del rapporto spettrale e della pendenza spettrale.

I valori di smorzamento ottenuti sono stati confrontati con quelli ricavati per i medesimi materiali da prove di laboratorio. I metodi considerati forniscono valori di D più elevati in confronto con quelli ricavati da prove di laboratorio.

Questa discrepanza può essere imputata alla dipendenza di D dalla frequenza e/o al fatto che la legge di attenuazione geometrica assunta non è in grado di rappresentare in modo realistico l'attenuazione con la distanza in un mezzo anisotropo

The paper critically analysed the capability of determining damping ratio D of granular soils from seismic tests performed in Calibration Chamber, by means of the so called spectral ratio and spectral slope methods.

The determined values of D are greatre in comparison to those obtained from laboratory tests. As a possible explanation of such a dicrepancy the following factors should be considered:

-frequency dependence of D, due to the viscosity of the pore fluid;

- need of a more complex law for geometric damping in anisotropic materials.


D.C.F. Lo Presti, O. Pallara, V. Fioravante e M. Jamiolkowski

ASSESSMENT OF QUASI-LINEAR MODELS FOR SANDS

Géotechnique, Pre-failure deformation of geomaterials, 1998, 363-372.

Appropriate modelling of the strain level dependency of soil stiffness is of great importance in order to correctly estimate the displacements of foundations under working load conditions. It is worthwhile remembering that soil deformations which mainly occur in the case of well designed foundations are within the range of 0.001% to 1%, therefore the assumed model should be able to reproduce stress-strain curves, above all in this interval.

In the light of this observation, a review of quasi-linear stress-strain relationships has been made. This review includes the models recently proposed by Tatsuoka and Shibuya (1992), Fahey and Carter (1993) and Puzrin and Burland (1996), that overcome some intrinsic limitations of the conventional hyperbolic model (Cox 1850, Kondner 1963, Kondner and Zelasko 1963).

Model parameters were determined for a selected database of drained compression loading triaxial tests that refer to four sands with different mineralogical and physical characteristics. The sands were tested in a triaxial apparatus equipped with gauges for the local axial and radial strain measurements, so that the stress strain curve is accurately defined, especially in the strain interval of interest.

A parameter was introduced to evaluate the capacity of each model to fit the experimental results and especially those concerning the variation of Young's modulus with axial strain. An attempt was also made to find a correlation between the model parameters and test conditions (void ratio, consolidation stresses, consolidation stress-ratio, overconsolidation ratio). It was seen that the last two of these factors significantly affect the model parameters. It was also attempted to reproduce stress-strain curves obtained for different stress-paths using these correlations.

D.C.F. Lo Presti e M. Jamiolkowski

DISCUSSION ON ESTIMATE OF ELASTIC SHEAR MODULUS IN HOLOCENE SOIL DEPOSITS

Soils and Foundations, 1998, Vol. 38, No. 1, 263-265

The authors are to be complimented on presenting and summarising well documented in situ and laboratory data concerning the "pseudo-elastic" shear modulus G. The writers, because of their own research, are greatly interested in the study of the factors which influence G and would like to contribute to the following points:

a) influence of OCR on G

b) void ratio function

c) general equation

M. Jamiolkowski, D.C.F. Lo Presti e F. Froio

DESIGN PARAMETERS OF GRANULAR SOILS FROM IN SITU TESTS

XI Europe-Danube Conference, Porec, May 1998, Proc. Balkema, pp. 65-94

The capability of in situ testing techniques to assess the mechanical properties of granular soils is here reviewed after a brief qualitative picture of the mechanical soil behaviour. Particular attention is paid to recent innovations and the capabilities of in situ testing methods to assess the relative density, stiffness and secant angle of shear resistance of coarse grained soils. In particular, the advantages and limitations of penetration, dilatometer, pressuremeter and seismic tests are critically reviewed.

V. Fioravante, M. Jamiolkowski, D.C.F. Lo Presti, G. Manfredini e S. Pedroni

ASSESSMENT OF COEFFICIENT OF EARTH PRESSURE AT REST FROM SHEAR WAVE VELOCITY MEASUREMENTS

Géotechnique, Vol. 48, No. 4, pp. 1-10.

The paper presents an attempt to evaluate the feasibility to predict the value of the coefficient of earth pressure at rest (Ko) of natural soil deposits on the basis of the measured velocities of the seismic shear waves. The experimental data presented have been obtained from seismic tests performed in the calibration chamber (CC) on laboratory prepared specimens of seven granular materials with grade ranging from fine sand to gravelly sand. The relationship between Ko and shear wave velocity established by means of the CC results was used successfully to predict K at two well documented sites where both Cross-Hole Seismic (CH) and Seismic Piezocone Penetration (SCPTU) tests are available.


D.C.F. Lo Presti, O. Pallara, A. Cavallaro e M. Maugeri

NON.LINEAR STRESS-STRAIN RELATIONS OF SOILS FOR CYCLIC LOADING

Proceedings of the 11th European Conference on Earthquake Engineering, 6-11 September 1998, Paris, Balkema, p 187.

This paper critically analyses the capability of Masing rules to represent the non-linear stress-strain relations of soils under cyclic loading. The analysis is performed in the light of the experimental results obtained from monotonic and cyclic loading torsional shear and resonant column tests performed on two undisturbed Italian clays and various reconstituted sands.

The second Masing rule implicitly assumes that cyclic and monotonic loading tests provide the same skeleton curve. This assumption is not confirmed by the experimental results shown in this paper.

For a given strain level the secant shear modulus from monotonic loading tests is smaller than the unload-reload shear modulus from cyclic loading tests. Moreover the loops originated by the second Masing rule are too large in comparison to those experimentally observed.

The model proposed by Tatsuoka and Kohata (1995) modifies the second Masing rule in order to overcome these intrinsic limits. This model is analysed and it seems to give more realistic prediction of the non-linear stress-strain relations of soils.

The original formulation of the Masing rules assumes stable cycles and disregards the possible occurrence of cyclic hardening or cyclic degradation.

The cyclic hardening is experimentally observed in dry sand for shear strain level greater than about 0.01 %. Cyclic degradation is typical of clays. The strain level at which this phenomena is triggered seems to depend on soil plasticity because of the rate dependent behaviour of clays.

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

DAMPING MEASUREMENT OF RECONSTITUTED GRANULAR SOILS IN CALIBRATION CHAMBER BY MEANS OF SEISMIC TESTS

Proceedings of the 11th European Conference on Earthquake Engineering, 6-11 September 1998, Paris, Balkema, p 206.

This paper analyses the results of seismic tests performed in a large Calibration Chamber (CC) in order to determine the damping ratio of seven different granular soils by means of the spectral ratio and spectral slope methods. Seismic measurements were performed with arrays of three miniature geophones, using the geophone located at one end of the arrays as a source and the other two as receivers. The damping ratio values were determined by analysing the traces of shear waves propagating along the horizontal direction with particle motion in the vertical direction and those propagating in the vertical direction with particle motion in the horizontal direction . These shear waves practically corresponds to those generated during Cross hole (CH) and Down Hole (DH) tests, respectively. It is reasonable to assume that the seismic tests in CC simulate CH and DH tests performed in ideal conditions: i) ideal coupling between soil and geophone, ii) homogeneous soil, iii) well known boundary conditions, soil density and imposed stresses. Seismic measurements were performed in the CC of Ismes at Bergamo which houses 1.5 m height and 1.2 m diameter specimens. Soil sample were reconstituted by means of pluvial deposition in air. During formation of the specimens, the pluviation process was interrupted at a pre-established elevation in order to place on the specimen surface several miniature cylindrical geophones. The specimen was saturated with deaired water after formation. The source geophone was excited by means of a 50 V peak-to-peak sine wave with frequencies of 2000 and 3000 Hz for P and S waves respectively. Typical lengths of the travel path between two receivers ranged from 45 to 60 cm. The above conditions minimise the so called "near field" effects. The CC specimens were subjected to stepwise loading with stress increments for σ of 25-50 kPa with σ corresponding to either σ or σ depending on the chosen path. The seismic tests were performed, at the end of each consolidation step, by generating the groups of body waves and measuring their velocities. These waveforms have been pre-processed by means of a cosine bell filter before their use for damping determination. The damping ratio obtained from seismic tests in the CC were compared with those obtained in the laboratory on soil element from cyclic loading torsional shear tests or Resonant Column tests. The paper tries to explain the observed differences and to improve the capability of the seismic tests in predicting the damping ratio of soils.