1,721,059 research outputs found
Centrifuge tests to evaluate the Po river embankment seismic response
No full textOn the behalf of the Italian Department for the Civil Protection, the Po River Basin Authority carried out a research project to evaluate the seismic hazard of about 90 km of the right embankment of the Po river, from Boretto (Reggio Emilia Province) to Ro (Ferrara Province). The project aim is the assessment of 1) the regional seismic risk of the areas under study, 2) the soil prop-erties of the embankment and the foundation ground via site investigation and laboratory tests, 3) the local seismic response of the foundation soil, 4) the liquefaction potential of the foundation soil and 5) the stability of the bank under static and dynamic conditions.
In order to investigate the seismic response of the embankment, a series of dynamic centrifuge tests on model banks were per-formed using a single degree of freedom shaking table. The models were reconstructed using silty and sandy soils retrieved in situ. Two soil profiles and two geometries were tested. Four time histories were applied to each model; the accelerograms were deduced from a local seismic response analysis carried out referring to one of the investigated site.
The tests showed a significant tendency of the embankments to amplify the input motions. The amplification is significant in a wide range of frequencies and increases from the base towards the top. No signs of local instability were observed during tests. The experimental results are compared with the preliminary results of a numerical 2D response analyses of the embankments
A correlation to evaluate cyclic resistance from CPT applied to a case history
No full textA methodology to evaluate the undrained cyclic resistance of sandy deposits from cone penetration tests through the state parameter is applied to the case history of the village of San Carlo (Italy), where widespread liquefaction phenomena occurred during the 2012 Emilia earthquake. The mechanical behaviour of the sand retrieved in the area of San Carlo was characterised within the framework of critical state soil mechanics via a series of monotonic and cyclic triaxial tests carried out on both undisturbed and reconstituted samples. Centrifuge cone penetration tests were also performed on reconstituted models of the same material. The results of centrifuge and undrained cyclic triaxial tests were interpreted through the state parameter to calibrate a direct correlation between the cone resistance, qc and the undrained cyclic resistance ratio (CRR). CRR profiles were deduced from the CPTs performed at sites in San Carlo where liquefaction took place using the correlation calibrated herein and a liquefaction assessment was carried out. The results of the proposed method was compared to a well-known simplified approach
Contact versus non-contact piled raft foundations
No full textIn the last few decades there has been a rapid increase in the number of piled foundations where the piles have been employed as settlement reducers; in some recent projects, the piles have been separated from the raft by a granular layer, which creates a more uniform pressure distribution on the raft bottom and reduces constraint reactions in the soil, foundation, and superstructure. A series of centrifuge model tests has been performed to investigate the load transfer mechanisms between a square rigid raft and a group of instrumented piles jacked in dry dense sand, in direct contact with the raft or separated from the raft by an interposed granular layer. The test results have shown that contact piles act as settlement reducers by diffusing the load applied to their heads to greater and deeper volumes of soil. The insertion of a deformable layer between a raft and pile heads does not ensure displacement compatibility, and the pressure diffused by the granular fill acts partly on the pile heads and partly produces shallow soil settlements, which mobilize negative skin friction on the upper part of the pile shaft. Noncontact piles act mainly as soil reinforcement
Density specifications for hydraulic fills
No full textUn problema comune che presentano i riempimenti idraulici e che insorge durante la costruzione è la definizione dei criteri di accettazione. Le controversie contrattuali sono principalmente legate al grado di compattazione e sorgono in gran parte a causa di specifiche scadenti. Una buona specifica deve poter essere misurabile senza ambiguità sul campo, utilizzando apparecchiature affidabili. Il contributo descrive il processo ingegneristico che dovrebbe essere seguito durante la progettazione e la costruzione di riempimenti idraulici, a partire dalla progettazione concettuale fino alla costruzione e al successivo utilizzo del riempimento, concentrandosi sui requisiti di densità, le specifiche e i criteri di accettazione per la densità del riempimento. Fornisce una metodologia per specificare la densità del riempimento, che eviterà la maggior parte delle tipiche insidie riscontrabili nei riempimenti idraulici. Il presente contributo prende come punto di partenza la pubblicazione CIRIA sui Riempimenti Idraulici (Hydraulic Fill Manual for Dredging and Reclamation Works, VAN’T HOFF e VAN DER KOLFF, 2012) sviluppando ulteriori considerazioni a integrazione di quanto riportato in tale documento e così facendo cerca di stabilire qual è la migliore pratica attuale.A common problem with hydraulic fills arises during construction when acceptance criteria are considered. Contractual disputes are mainly related to degree of compaction and they arise largely because of poor specifications. A good specification is one that can be measured unambiguously in the field using reliable equipment.
This paper describes the engineering process which should be followed during design and construction of hydraulic fills, starting from conceptual design all the way through to construction and subsequent use of the fill, focusing on the density requirements, specifications and acceptance criteria for density of the fill.
It provides a methodology to specify the density of the fill, which will avoid most of the typical pitfalls related to hydraulic fills.
This paper takes the CIRIA publication on Hydraulic Fills (Hydraulic Fill Manual for Dredging and Reclamation Works”, van’t Hoff and van der Kolff, 2012) as its starting point developing further considerations to supplement what is given in that document and in doing so tries to establish what is current best-practice
Centrifuge Modelling of Vertical and Horizontal Drains to Mitigate Earthquake-Induced Liquefaction
Full text linkThis paper reports the results of dynamic centrifuge tests carried out on sandy models alternatively equipped with vertical or horizontal drains. The main aim of the experimentation was to investigate the use of horizontal drains to mitigate the liquefaction susceptibility of sandy deposits and to validate their applicability as a remediation technique applicable in urban and industrial areas to protect existing buildings from liquefaction. The assessment and validation were carried out by comparing the seismic behavior of models treated with horizontal drains with that of the untreated model and models equipped with vertical drains
Centrifuge modelling of liquefaction triggering in sandy soils
No full textIn this paper are presented some of the results of a dynamic centrifuge experimentation carried out to investigate several aspects of the phenomenon of liquefaction, including application of remediation techniques. The focus of the contribution is the effect of soil permeability and state parameter on excess pore pressure generation and dissipation
Unidirectional cyclic resistance of Ticino and Toyoura sands from centrifuge cone penetration tests
No full textThe evaluation of the undrained cyclic resistance of sandy deposits is required to forecast the soil behaviour during an earthquake (liquefaction, cyclic mobility); due to the difficulties in obtaining undisturbed samples of most liquefiable soils, it is usually deduced from field test results such as cone penetration tests. This paper proposes a methodology to evaluate the undrained cyclic resistance from normalised cone resistance of two wellstudied silica sands (Ticino and Toyoura), with different mineralogy, one mainly composed of feldspar, the other of quartz. The etermination of the cyclic resistance of Ticino and Toyoura sands was achieved through undrained cyclic triaxial tests on reconstituted specimens. The tip resistance was deduced from CPTs performed in centrifuge with a miniaturised piezocone on homogeneous reconstituted models. Both the undrained cyclic and tip resistances were correlated with the state parameter w. Results of centrifuge and triaxial tests were combined through w to deduce the cyclic resistance ratio CRR directly from the normalised cone resistance. The shape of the curve relating CRR to the normalised cone resistance resulted unusual respect to all the recognised curves widespread in the geotechnical literature. The aim of the proposed correlations is to provide a useful instrument to improve the actual knowledge on liquefaction and to give a contribution based on the critical state soil mechanics framework to the development of refined correlations between the cyclic resistance of a sand and the results of cone penetration tests
Load transfer mechanisms of piled raft foundations
No full textWithin an extensive research programme on piles as settlement reducers, multi-g centrifuge tests have been performed on models of piled rafts with the piles arranged in two configurations: in direct contact with the raft and separated from the raft by a granular fill layer, introduced to create a uniform pressure distribution on the raft bottom and to reduce constraint reactions between the piles and the slab. The aim of this paper is to present some of the observations concerning the load transfer mechanisms which take place within a mixed foundation and to evidence the influence of a granular cushion, inserted between the raft and the pile heads, on the foundation load – settlement behaviour
Physical Modeling of Liquefaction in Various Granular Materials
No full textThis paper compiles numerous experiences gained from physical models, to highlight the phenomena of triggering, propagation, and mitigation of liquefaction in granular soils. Results of tests at different scales, from the element volume (cyclic triaxial tests) to small-scale models in centrifuge, performed using several granular soils, will be presented to provide behavioral tools for predicting the phenomenon. Furthermore, the efficacy of vertical and horizontal drains as liquefaction mitigation techniques will be discussed
MODELLING OF PILED RAFT FOUNDATIONS IN SAND
No full textA piled raft is a composite foundation in which the piles are used as
settlement reducers and they share, with the raft, the load from the
superstructure. The applied load is transferred from the raft to the
shallow soil and to the pile heads, and from the piles it is diffused
through the shaft and the base to deeper soil. The pile–raft and pile–pile
interactions represent the distinctive aspect of the piled raft foundations
since they modify the load–bearing behaviour of each foundation
component, compared to an analogous isolated element, thus determining
the overall foundation behaviour.
The main aim of this thesis is to highlight the effects of the raft–soil–pile
interactions on the resistance and stiffness of axially loaded piled raft
foundations in sand.
A series of centrifuge tests on models of rigid circular piled rafts in loose
saturated sand has been performed to this end, employing both non
displacement and displacement piles. The raft settlement and the load
transmitted to the pile heads and bases were monitored during the tests,
which also included unpiled raft and isolated pile tests.
The test results have been analysed in terms of bearing capacity and
stiffness; the former according to a load efficiency method, the latter by
comparing the values obtained from centrifuge tests with those evaluated
through a simplified analytical method.
ii Modelling of Piled Raft Foundations in Sand D. Giretti
In order to clarify the effect of the pressure transmitted by the raft to the
soil on the behaviour of a single capped pile, some of the centrifuge tests
were simulated via finite element numerical analyses, using an elasto–plastic strain hardening constitutive model for the sand. The geometry
and the dimensions of the numerical models corresponded to those of the
physical ones and the simulations were carried out applying an
accelerated gravitational field to the mesh.
The influence of a granular layer, interposed between the raft and the pile
heads, on the load transfer mechanism has also been analysed, through an
additional series of centrifuge tests which was performed on square rigid
raft models on displacement piles in dry dense sand
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