A new apparatus for modelling excavations

Underground space is commonly exploited both to maximise the utility of costly land in urban development and to reduce the vertical load acting on the ground. Deep excavations are carried out to construct various types of underground infrastructure such as deep basements, subways and service tunnels. Although the soil response to excavation is known in principle, designers lack practical calculation methods for predicting both short- and long-term ground movements. As the understanding of how soil behaves around an excavation in both the short and long term is insufficient and usually empirical, the judgements used in design are also empirical and serious accidents are common. To gain a better understanding of the mechanisms involved in soil excavation, a new apparatus for the centrifuge model testing of deep excavations in soft clay has been developed. This apparatus simulates the field construction sequence of a multi-propped retaining wall during centrifuge flight. A comparison is given between the new technique and the previously used method of draining heavy fluid to simulate excavation in a centrifuge model. The new system has the benefit of giving the correct initial ground conditions before excavation and the proper earth pressure distribution on the retaining structures during excavation, whereas heavy fluid only gives an earth pressure coefficient of unity and is unable to capture any changes in the earth pressure coefficient of soil inside the zone of excavation, for example owing to wall movements. Settlements of the ground surface, changes in pore water pressure, variations in earth pressure, prop forces and bending moments in the retaining wall are all monitored during excavation. Furthermore, digital images taken of a cross-section during the test are analysed using particle image velocimetry to illustrate ground deformation and soil-structure interaction mechanisms. The significance of these observations is discussed.

Development of a new apparatus for modeling deep excavation related problems in geotechnical centrifuge

Underground space is commonly exploited both to maximise the utility of costly land in urban development and to reduce the vertical load acting on the ground. Deep excavations are carried out to construct various types of underground infrastructure such as deep basements, subways and service tunnels. Although the soil response to excavation is known in principle, designers lack practical calculation methods for predicting both short- and long-term ground movements. As the understanding of how soil behaves around an excavation in both the short and long term is insufficient and usually empirical, the judgements used in design are also empirical and serious accidents are common. To gain a better understanding of the mechanisms involved in soil excavation, a new apparatus for the centrifuge model testing of deep excavations in soft clay has been developed. This apparatus simulates the field construction sequence of a multi-propped retaining wall during centrifuge flight. A comparison is given between the new technique and the previously used method of draining heavy fluid to simulate excavation in a centrifuge model. The new system has the benefit of giving the correct initial ground conditions before excavation and the proper earth pressure distribution on the retaining structures during excavation, whereas heavy fluid only gives an earth pressure coefficient of unity and is unable to capture any changes in the earth pressure coefficient of soil inside the zone of excavation, for example owing to wall movements. Settlements of the ground surface, changes in pore water pressure, variations in earth pressure, prop forces and bending moments in the retaining wall are all monitored during excavation. Furthermore, digital images taken of a cross-section during the test are analysed using particle image velocimetry to illustrate ground deformation and soil–structure interaction mechanisms. The significance of these observations is discussed.

Spray drying: An alternative synthesis method for polycationic oxide compounds

Synthesis of polycationic compounds by the spray-drying technique is an interesting alternative in the domain of aqueous precursor synthesis methods. Spray drying yields high quality samples with good reproducibility. The possibility of scaling up for production of large quantities with fast processing time is well established by the commercial availability of powders of various compositions. In this paper, we have discussed the advantages and limitations of this method and demonstrated its interest by synthesizing a few polycationic compounds selected for their attractive properties of thermoelectricity [Bi1.68Ca2Co1.69O 8, La0.95A0.05CoO3 (A=Ca, Sr, Ba)] or magnetoresistance [La0.70A0.30MnO3 (A=Sr, Ba)]. We have confirmed the quality of these samples by reporting their structure, magnetic and transport properties. © 2010 Elsevier Ltd All rights reserved.

Electrical transport and magnetic properties of Mn3O4-La0.7Ca0.3MnO3 ceramic composites prepared by a one-step spray-drying technique

La0.7Ca0.3MnO3/Mn3O4 composites can be synthesized in one step by thermal treatment of a spray-dried precursor, instead of mixing pre-synthesized powders. Another advantage of this composite system is that a long sintering step can be used without leading to significant modification of the manganite composition. The percolation threshold is reached at ∼20 vol% of manganite phase. The 77 K low field magnetoresistance is enhanced to ∼11% at 0.15 T when the composition is close to the percolation threshold. © 2007 Elsevier Ltd. All rights reserved.

Low-field magnetoresistance in La0.7Ca0.3MnO 3 manganite compounds prepared by the spray drying technique

Calcium-substituted lanthanum manganite compounds were synthesized by the spray drying technique. This method - whose main advantages are versatility, high reproducibility and scalability - yields small grain materials of high homogeneity and displaying low-field magnetoresistance effects. We report about the physical and chemical characterizations of these samples in order to investigate the potential interest of spray drying for the production of materials for low-field magnetoresistance applications. We have studied the dependence of the low-field magnetoresistance on the temperature and duration of the thermal treatment applied to the pelletized powders. The issue of the shape anisotropy (demagnetisation effects) influence on the magnetoresistance properties has also been dealt with. © 2005 Springer Science + Business Media, Inc.

Strength and drying shrinkage of reactive MgO modified alkali-activated slag paste

Conventional alkali-activated slag (AAS) cements suffer from significant drying shrinkage which hinders their widespread application. This paper investigates the potential of using commercial reactive MgO to reduce the drying shrinkage of AAS. Two different reactive MgOs were added at a content of 2.5-7.5 wt% of the slag, which was activated by sodium hydroxide and water-glass. The strength and the drying shrinkage of those reactive MgO modified AAS (MAAS) pastes were measured up to 90 days. It is found that MgO with high reactivity accelerated the early hydration of AAS, while MgO with medium reactivity had little effect. The drying shrinkage was significantly reduced by highly reactive MgO but it also generated severe cracking under the dry condition. On the other hand, medium-reactive MgO only showed observable shrinkage-reducing effect after one month, but the cement soundness was improved. The hydration products, analysed by X-ray diffraction, thermogravimetric analysis and scanning electron microscopy techniques, showed that Mg was mainly incorporated in the hydrotalcite-like phases. It is concluded that the curing conditions and the time of hydrotalcite-like phases formation and their quantity are crucial to the developed strength and shrinkage reduction properties of MAAS, which are highly dependent on the reactivity and content of reactive MgO. Crown Copyright © 2013 Published by Elsevier Ltd. All rights reserved.

Substrate treatment and drying conditions effect on the properties of roll-to-roll gravure printed PEDOT:PSS thin films

The optical, structural and electrical properties of poly(3,4- ethylenedioxythiophene):poly(4-styrenesulfonic acid) (PEDOT:PSS) thin films printed by roll-to-roll gravure have been investigated. Corona treatment has been applied to enhance the adhesion of PEDOT:PSS on PolyEthylene Terephthalate (PET) web. It has been found that there was a stronger in-depth surface modification of PET with the increase of corona efficiency; however, the adhesion of PEDOT:PSS was not actually affected. Also, Spectroscopic Ellipsometry and Atomic Force Microscopy have been used to extract information on the mechanisms that define PEDOT:PSS properties. The increase of the drying temperature of the PEDOT:PSS films has been found to reduce the remaining water inside the films and lead to the decrease of the PEDOT:PSS particles size. © 2011 Elsevier B.V. All rights reserved.

Fracture of colloidal single-crystal films fabricated by controlled vertical drying deposition

Controlled vertical drying deposition method was used to make high-quality single crystal close-packed colloidal films formed of different radii polystyrene latex spheres on glass substrates coming from a low concentration water suspension (0.1% volume fraction). Regardless of the spheres radii the film thickness was about 6.3 microns. However, cracks destroyed the crystalline film structure during the colloidal film growth. The effect of particle radius (85-215 nm range) on film cracking was systematically studied using in situ optical fracture monitoring. Primary parallel cracks run along the vertical growth direction, later followed by secondary branched cracks in-between the primary cracks due to residual water evaporation. Quantitative theoretical relationship between the cracks spacing and particles radius was derived and shows good agreement with experimental observations. Normalized cracks spacing is related to a reciprocal ratio of the dimensionless particle radius.

Plio-Quaternary stepwise drying of Asia: Evidence from a 3-Ma pollen record from the Chinese Loess Plateau

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