Improving Indigenous Technologies for Sustainable Land Use in Northern Mountainous Areas of Vietnam

More than 30 ethnic groups are now living in northern mountainous regions, Vietnam, mainly relying on shifting cultivation with the fallow period being shortened from time to time. Naturally, soil fertility reduces from cycle to cycle, entailing the reduction of productivity. Large areas of moderately sloping lands suitable for upland agriculture have become bare after many cultivation-fallow cycles. The soils there have been severely degraded with more toxicity, low porosity, low water retention capacity and poor floral diversity. Normally, these lands cannot be used for food crop cultivation. So farmers in uplands have to rely on slash-and-burn practices for their livelihood. As there is no more forest with good soil in medium slopes, farmers go to cut forests in watershed, high slope lands and old forests up to the mountains‘ top. There are ecologically and environmentally very sensitive areas, so their destruction will inevitably cause hazardous consequences in the whole basin. Meanwhile,cultivation in these areas has low economic efficiency and sustainability because the crop yield may decrease very fast due to severe erosion as the higher the slope, the more serious erosion. Consequently living standards of highland farmers remain low and unstable. Sustainable farming on these lands in the perspective of a seriously deteriorated ecology and environmental is not an easy task. There have been many projects trying to help mountainous farmers get out of their vicious circle. However, due to different reasons, the results gained are low, and in some cases,things ceased to move after the projects phased out. During past few years, based on the farmer experiences, the Vietnam Agricultural Science Institute has cooperated with local and international partners to implement different projects in order to solve the problems by developing simple, easy and cheap cultivation technologies, which can be accepted and applied by local poor farmers for sustainable agricultural production. The first results of our activities offered good opportunities for sustain food production, improve soil health, recharge of aquifers,and enhanced household income for better rural lively hoods in the upland eco-regions of northern Vietnam.     

Compressibility and permeability of sand–kaolin mixtures. Experiments versus non‐linear homogenization schemes

This study deals with the behaviour of mixtures of sand and saturated kaolin paste considered as composite materials made of permeable and deformable (with non‐linear behaviour) matrix (the kaolin paste) with rigid and impervious inclusions (the sand grains). Oedometric and permeability tests highlight the key role of the state of the clay paste, and show the existence of a threshold of sand grain concentration above which a structuring effect influences both compressibility and permeability. At the light of these experiments two homogenization schemes (with simplifying assumptions to make the problem manageable) are considered to model these two parameters. Qualitative and quantitative comparisons with experimental data point out their respective domain of interest and limitations: a tangent homogenization scheme is shown to be sufficient to describe the macroscopic properties for dilute sand concentration; above the concentration threshold, the structuring effect is captured by the new homogenization scheme developed in this paper. Copyright © 2010 John Wiley & Sons, Ltd.     

Time-Dependent Behaviour and Static Liquefaction Phenomenon of Sand

Recent laboratory and field experiments have confirmed that sand does indeed exhibit time-dependent behaviour. Based on these findings, it was considered necessary to revisit some of the published experimental results on the static liquefaction phenomenon of loose anisotropic Hostun RF sand. Time-dependency might have had a significant influence on the observed undrained response of anisotropic consolidated sand specimens. Specific triaxial tests have been performed and a qualitative analysis is presented in this paper. It is shown that, despite the differences on the anisotropic consolidation path employed, different specimens show qualitatively identical undrained responses if creep periods are performed at identical test stages or if the anisotropic consolidation takes place very slowly. With time, the undrained stiffness and strength are considerably improved and this may explain why the static liquefaction phenomenon is not as common in practice as could be predicted based on an instability line concept. Whereas the original instability line concept was developed independently of time-dependency, in field situations, the liquefaction resistance of the sand can increase with time.     

Measurements of Rare Earth Element and Other Element Mass Fractions in Environmental Reference Materials (NIST SRM 1646a,NIST SRM 1400, IAEA‐395 and IAEA‐450) by INAA,ICP‐AES and ICP‐MS

INAA, ICP‐AES and ICP‐MS were used to elementally characterise four environmental reference materials – NIST SRM 1646a (Estuarine Sediment), NIST SRM 1400 (Bone Ash), IAEA‐395 (Urban Dust) and IAEA‐450 (Algae). An analytical scheme consisting of the three methods was first applied to NIST SRM 1646a to validate the methodology because it has been extensively analysed and has certified values for many elements. With repeated analyses of NIST SRM 1646a, the accuracy and measurement repeatability of the data obtained were evaluated based on two statistical calculations (zeta‐score and Horwitz ratio) and were observed to be good enough for the analytical scheme to be applied to similar sorts of environmental/geochemical samples. Applying the same approach to NIST SRM 1400, IAEA‐395 and IAEA‐450, enabled mass fractions of 29, 38 and 28 elements to be determined, respectively. Among these results, the data for rare earth elements are of particular interest, not only for IAEA‐450 but also for the other three reference samples. The data for Pr, Gd, Dy, Ho, Er and Tm in NIST SRM 1646a are newly reported in this study. By using small test portions (< 100 mg) for NIST SRM 1646a and IAEA‐395, and recommended minimum amounts for NIST SRM 1400 and IAEA‐450, sample homogeneity was evaluated.     

Exploring the undrained induced anisotropy of Hostun RF loose sand

The effects of recent history on the undrained behaviour of very loose and saturated Hostun RF clean sand are investigated in this paper. From an initial isotropic stress state in the triaxial plane, recent histories are generated by isotropic consolidation followed by standard drained triaxial preshear in compression, up to a desired value of axial strain or mobilized stress ratio, and unloading to an initial stress ratio. Subsequent undrained behaviour in triaxial compression is analysed in detail. This paper contributes to the traits explaining the progressive transformation of a compressive and unstable behaviour of loose sand into a dilative and stable behaviour of dense-like sand by previous history, while remaining in the same state of loose density. Experiments show a large pseudo-elastic domain induced by recent history in terms of effective stress paths, function of the initially mobilized stress ratio level, a unique initial gradient of the effective stress paths depending on the stress ratio at the beginning of the undrained shearing, a progressive appearance of dilatancy and a surprising evolution the undrained behaviour of loose sand. Experimental results evidence the important role of the recent deviatoric strain history, from any initial isotropic or anisotropic stress state. This paper also offers a comprehensive understanding of the history mechanisms created by simple linear stress paths with fixed direction in the classical triaxial plane.     

On the undrained strain-induced anisotropy of loose sand

An experimental study was carried out to investigate the effects of previous deviatoric strain histories on the undrained behaviour of loose and saturated Toyoura sand and compared with known results of Hostun RF sand. From an initial isotropic stress state, recent deviatoric strain histories in the compression side of the triaxial plane were generated by a standard drained presheared cycle up to a specified mobilized stress ratio. Mainly, the fully liquefied, contractive, unstable and softening behaviour of loose sand was progressively transformed into the non-liquefied, dilative, fully stable and hardening behaviour of dense-like sand, while remaining within a narrow range of loose density. The paper validates and extends the current understanding of strain-induced anisotropy of loose sand. New experimental data support the directional dependency of the instability cone on the stress increment direction, suggest the bifurcation characteristics of loose sand and evidence the important role of past deviatoric strain histories.     

Simulating the effects of induced anisotropy on liquefaction potential using a new constitutive model

The effects of induced anisotropy on the undrained behaviour of very loose and saturated sands have been a subject of intensive investigation, both experimentally and theoretically, by several authors in the past few years. This paper proposes an original constitutive model well‐adapted to simulate the behaviour of sands subject to complex stress histories, in particular, the preloading cycle along the classical drained stress path in compression. The developed model belongs to the family of critical state models. Its construction is based on a few theoretical concepts taken from the theory of ‘Bounding Surface Plasticity’ developed among others by Y. Dafalias and Popov (1975), the ‘Clay And Sand Model’ (CASM) of H. Yu (2006), the CJS model (B. Cambou and K. Jafari (1988)) and the hyperelastic isotropic model of P. Lade (1987). To accurately simulate volume changes, which represent a key element in soil behaviour, a state‐dependent dilatancy rule is proposed, which can account for the influences of stress and void ratio. The current void ratio depends implicitly on the irreversible strains already accumulated hence the strain history. A kinematic hardening is combined with an isotropic hardening, involving rotation and distortion of the bounding surface, in order to capture correctly the experimental observations. Comparisons of experimental results to numerical simulations show that the model is able to simulate with a good precision the major trends of undrained responses of loose and presheared sands. It predicts correctly rapid static liquefaction at small or null drained preloading, as well as the progressive transition to a completely stable behaviour typical of dense sands, while the sample is loose in reality. At intermediate to large amplitudes of preloadings, the model also predicts correctly the temporary stage of instability when the deviatoric stress decreases slightly before rising up again. Copyright © 2013 John Wiley & Sons, Ltd.     

Dynamic instabilities under isotropic drained compression of idealized granular materials

The compressibility behaviour of loose and contracting granular assemblies, normally consolidated and overconsolidated, under isotropic drained compression is investigated in this paper. Short cylindrical samples of water-saturated monodisperse glass beads, initially assembled in loose state by moist-tamping technique, are isotropically compressed in a classical axisymmetric triaxial machine. Very loose glass bead samples experience numerous unexpected events, sometimes cascading, under undetermined triggered effective isotropic stress in loading and in unloading, while the classical compressibility behaviour of granular material is recovered once these events ignored. Each event, resembling the stick–slip instability during shear in triaxial compression, is characterized by a transient dynamic phase I with very fast drop of effective isotropic stress \(\sigma ^{'}\) due to an excess pore pressure development at nearly constant volume and constant axial strain, followed by a quasi-static phase II with gradual increase in axial \(\varepsilon _\mathrm{a}\) (contraction) and volumetric \(\varepsilon _\mathrm{v}\) (compaction) strain, and a full progressive recovery of \(\sigma ^{'}\) to the previous level before event. A short-lived liquefaction with null \(\sigma ^{'}\) measured in the first phase I results in a local collapse state. Collapse events also happen on unsaturated moist and dry states. Rare events even occur during the unloading of subsequent isotropic compression cycles. The effects of triggered isotropic stress are discussed, the instability characteristics measured, the comparison with stick–slip instability made and the hypothesis of micro-structural instability with local collapse of contact networks and rapid micro-structural rearrangement argued.     

Effects of Previous Deviatoric Strain Histories on the Undrained Behaviour of Hostun RF Loose Sand

The effects of previous deviatoric strain histories on the undrained behaviour of very loose and saturated Hostun RF clean sand are investigated in this paper. From an initial isotropic stress state in the triaxial plane, strain histories are generated by isotropic consolidation followed by standard drained triaxial preshear or presheared cycle, either in compression or in extension, up to a desired value of axial strain or mobilized stress ratio. Deviatoric strain histories are achieved by having nearly the same void ratio at the beginning of the undrained shearing for all tested samples. Subsequent undrained behaviour in triaxial compression and extension is analyzed in detail. Previous deviatoric strain histories can progressively transform the compressive and unstable behaviour of loose sand into a dilative and stable behaviour of dense-like sand, while being loose. Experiments show a common response induced by recent strain histories in terms of effective stress paths, independently of the axial strain attained during the drained presheared cycle, a unique initial gradient of the effective stress paths, a progressive appearance of dilatancy, an evolution the undrained behaviour and a systematic partial static liquefaction associated with softer behaviour when sheared in the opposite direction of the initial presheared direction. This paper offers a comprehensive understanding of the mechanisms of a specialized induced anisotropy created by simple linear stress paths in the classical triaxial plane.