Luc Thorel

Centrifuge Modeling of a Pile-Supported Granular Earth-Platform

AbstractSoil reinforcement by vertical rigid piles is a suitable technique to solve soft-soil foundation problems. This technique consists of transferring the load of a pile-supported structure to a resistant, less compressible soil layer. The loads are transferred on pile heads by arching mechanisms in an earth-platform located between piles and structure. The role of the earth-platform is to distribute the loads among the piles and to minimize the fraction of the load applied on the soft compressible soil. Centrifuge tests are performed at 12 and 20×g levels of acceleration with a specific apparatus containing 63 rigid piles. The behavior of a granular earth-platform with rigid pile reinforcement is studied. The granular earth-platform is simulated with Hostun sand. The physical model is widely instrumented to compare the load-transfer efficiency and the surface settlements for several geometrical configurations. The influence of the height of the earth-platform and the spacing between the piles on load...

Study of scale effect in an internal erosion mechanism: centrifuge model and energy analysis

Suffusion is an internal erosion mechanism, which means detachment and transport of fine particles within the soil skeleton due to hydraulic seepage flows. Different researchers have observed that the value of the critical hydraulic gradient required to initiate suffusion decreases with the length of the specimen tested. A specific centrifuge bench was designed to study the suffusion process and to study this scale effect under a controlled effective stress. Clayey sand specimens were subjected to centrifuge acceleration and to a downward flow under a constant hydraulic head. The study underlines the influence of specimen length on critical hydraulic gradient and also on the rate of erosion. A new energy analysis of tests is developed, linking the erosion rate to the power expended by fluid flow, and the eroded clay mass to the energy dissipation. This method permits the effect of specimen length to be avoided. La suffusion est un mécanisme d’érosion interne qui correspond au détachement et au transport de particules fines au travers du squelette granulaire sous l’action d’un écoulement. Différents chercheurs ont observé que la valeur du gradient hydraulique critique nécessaire à l’initiation de la suffusion décroît avec la longueur de l’échantillon testé. Un dispositif d’essais en centrifugeuse est spécialement développé afin de caractériser la suffusion et d’étudier cet effet d’échelle sous contrainte effective contrôlée. Des échantillons sablo-argileux sont centrifugés et sont soumis à un écoulement vertical descendant sous charge hydraulique constante. L’étude met en évidence l’influence de la longueur de l’échantillon sur le gradient hydraulique critique et sur le taux d’érosion. Une nouvelle approche énergétique est proposée et permet de relier le taux d’érosion avec la puissance dissipée par le fluide ainsi que la masse érodée avec l’énergie dissipée. Cette approche permet de s’affranchir de l’effet d’échelle.

Centrifuge modelling of capillary rise

This paper reports results from centrifuge tests designed to investigate capillary rise in soils subjected to different gravitational fields. The experimental programme is part of the EU-funded NECER project (Network of European Centrifuges for Environmental Geotechnic Research), whose objective is to investigate the appropriateness of geotechnical centrifuge modelling for the investigation of geoenvironmental problems, particularly with reference to partially saturated soils. The tests were performed at the geotechnical centrifuge laboratories of Cardiff. Bochum. Manchester, and LCPC in Nantes. The aim was to determine the scaling laws of capillary rise under both equilibrium and transient conditions. In all laboratories, column wetting tests in fine poorly graded sands (Congleton Sand, Bochum Normsand, HPF5 Sand, and Fontaineblau Sand) were performed. Capillary rise above the phreatic surface of the sand model was distinguished in a continuous capillary zone (completely saturated) and a discontinuous capillary zone (partially saturated). The Cardiff Geotechnical Centrifuge Laboratory used matrix potential probes to follow the capillary rise of the continuous zone and, therefore, determine the suction above the phreatic zone during centrifuge testing. Ar Bochum, two cameras were used for optical and volumetric measurements, in order to follow the rise of the visible wetting front (upper limit of discontinuous zone) in the sand within the sample column. At Manchester. the movement of the wetting front was observed by video cameras over periods up to 8 h, whereas in LCPC pore pressure transducers recorded the changes in pressure caused by capillarity. A simple centrifuge similitude law for capillary rise in these sands has been established and the kinetic phenomena have been measured as a function of the gravitational field. The results from these experiments verify that both the continuous and discontinuous capillary zones are scaled at a factor 1/N whereas the time for rise seems to be scaled at a factor 1/N-2. This research suggests that capillary phenomena can be modelled using a geotechnical centrifuge. Therefore, centrifuge testing can be a useful tool for future modelling of boundary value problems involving complex transport phenomena

Climatic Chamber With Centrifuge to Simulate Different Weather Conditions

Increasing interest in thermo-hydro-mechanical (THM) studies of soil responses to hydrological variations has heightened the need for improvements in the basic understanding of the heat and mass transport taking place at the soil-atmosphere interface. Numerous hydrological parameters affect this thermo-hydro-mechanical process including solar radiation, air temperature, atmospheric pressure, wind velocity, rain intensity and hygrometry. Since field tests of soil-atmosphere interaction require measurements over long periods of time, only a small number of these results are available for calibration of the numerical models that are based on atmospheric data as boundary condition. The number is even more limited for results which focus on cyclic wetting and drying. Centrifuge modeling is a powerful tool for studying these problems since it can accelerate the time needed for diffusion processes taking place at the soil-atmosphere interface. Nevertheless, modeling this interaction adequately with a centrifuge requires development of new types of equipment such as a climatic chamber that allows control of weather variables while respecting the centrifuge’s scaling laws. This paper describes the design of an apparatus for simulating tropical weather conditions which combines a climatic chamber with a centrifuge. The scaling laws are studied, and the feasibility of reproducing tropical weather conditions around a centrifuge is discussed. Finally, the paper presents some validation results that highlight the working principles of this new apparatus.

Arching and Deformation in a Piled Embankment: Centrifuge Tests Compared to Analytical Calculations

AbstractMeasured loads in centrifuge tests on piled embankments were compared with values calculated by three analytical arching models. The Zaeske and Van Eekelen models use sets of arches that give the direction of the load transfer, and result in an increasing load on the soft soil with increasing embankment thickness or surcharge load, which matches the measurements. The Svano model, however, assumes that the load on the soft soil remains constant with increasing embankment thickness or surcharge load, thereby being transferred directly to the piles. Therefore, the Zaeske and Van Eekelen models match the measurements better than the Svano model. The Van Eekelen model slightly overestimates the pile efficiency for relatively thin embankments, but matches the measurements better than the Zaeske model for increasing embankment thickness or surcharge load.

A Miniature Falling Weight Device for Non-Intrusive Characterization of Soils in the Centrifuge

Non-intrusive and non-destructive techniques are used for characterizing multilayer soil models in the centrifuge. Based on the spectral analysis surface waves analysis, a miniature falling weight-type (FW) device is developed for centrifuge testing. A description of the mini-FW with application to characterize homogeneous and bilayer models is presented. The mini-FW principle consists of the fall in a steel ball generating vibrations within the centrifuged models. These vibrations are recorded by accelerometers placed on the model’s surface. On condition that the vibrations remain within the first five upper centimeters in order to avoid near-field effects, the results obtained for both wave velocity values and location of the layer interfaces agree satisfactorily with already available data.

The occurrence of residual stresses in helical piles

Residual stresses generated during the pile installation are usually observed and considered in the interpretation of conventional pile load tests. However, in the case of helical piles, residual stresses can be generated by previous loadings. At the end of the unloading stage in tension tests, the shaft resistance is reversed, and the helices are maintained forced against the soil. The occurrence of residual stresses measured at the lower portion of the shaft, close to the helices, was observed in four tension tests performed on instrumented helical piles in Brazil. This paper discusses the outcomes of these tests on helical piles installed in two different sites of tropical residual soil. The results illustrate that the amount of residual stresses is greater for piles with less contribution of shaft resistance in the pile uplift capacity.

Centrifuge modelling of unsaturated soils

The use of centrifuge modeling where unsaturated soils are involved is more limited than its use in problems involving dry sand or saturated clays. This limitation is certainly due to the well-known experimental complexities related with unsaturated soils that increase in centrifuge modeling; on the other hand, few data is available about the scaling laws for unsaturated soils. In this paper, physical modeling of unsaturated soil problems in centrifuge is evaluated considering some phenomena involved in the behavior of those materials such as water migration, expansion and collapse. An overview of the scaling laws that has to be used is presented, with a selection of geotechnical problems studied on small-scale models in centrifuge.

Dynamic Actuator for Centrifuge Modeling of Soil-Structure Interaction

This paper presents a new dynamic actuator useful to study soil-structure interactions in a centrifuge. This new dynamic apparatus is based on an amplified piezoelectric actuator. Using this device it is possible to create vibrations in the soil sample of different frequencies and amplitudes. The dynamic actuator consists of a set of weights in a single degree of freedom system plus a piezoelectric actuator and a piezoelectric load cell, which measures the dynamic load. A description of the dynamic actuator and its application to dynamic soil-structure interaction under wind turbines is presented. The calibration of the dynamic actuator and some results of centrifuge tests for shallow and embedded foundations are presented.

Effect of Seabed Trenching on the Holding Capacity of Suction Anchors in Soft Deepwater Gulf of Guinea Clays

Seabed trenching, related to large motions of relatively taut deepwater mooring systems in the long Gulf of Guinea swell, can reach the suction anchor and the chain padeye depth, thus raising serious concerns about the holding capacity of the suction anchors and the safety of the mooring systems. The potential reduction of the anchor capacity, estimated by means of 3D finite element (FE) calculations, needs to be verified by means of testing, and this is the objective of the study presented here. The paper describes the IFSTTAR geotechnical centrifuge test facility and the model soil preparation procedure for obtaining large open trenches with natural deepwater Angola clay in the swinging centrifuge. The scope of the study includes a number of holding capacity tests on suction anchors without trenches (reference cases) and with large trenches reaching the suction anchor and padeye depth. Tests were performed under vertical and inclined loading (actual case of taut deepwater moorings with significant vertical loading component), with the measurement of pore pressures at different levels below the anchor tip for verifying the effect of the trench on the reverse end bearing (passive suction) capacity. The centrifuge test results are compared with Plaxis 3D FE calculations for the purpose of comparison with previously published results. The preliminary results presented show that the REB capacity is mobilized, even with large trenches reaching the anchor and padeye depth. It is also confirmed that Plaxis 3D FE modelling offers a reliable tool for the assessment of the post-trench holding capacity of the suction anchors.