Evolution of surface roughness of single sand grains with normal loading

The surfaces of soil grains are not perfectly smooth, especially examined at small scale. In geotechnical engineering, surface roughness has been found to be able to influence the inter-particle friction angle at micro scale and small-strain stiffness at macro scale. However, the quantity and quality of the studies on surface roughness of natural soils are still limited. In this study, the evolution of surface roughness of natural sand grains with increasing normal load was investigated by a single-particle compression apparatus. Thirty Leighton Buzzard sand (LBS) grains coarser than 2·36 mm were tested, and the surface roughness was measured before and after compression by an optical interferometer. The deformations of the asperities and of the bulk of the sand grains in the vicinity of the contact were mapped. Three stages were identified as the normal load increased: (a) plastic deformation of the asperities; (b) asperities and bulk plastic deformation; and (c) bulk only plastic deformation. At very small normal load, only the asperities were found to deform plastically, and the surface roughness of the sand grains decreases due to the flattening of the asperities. Within this regime, the load–displacement relationship of LBS grains under compression could be simulated by the modified Hertz model, which takes surface roughness into consideration. With increasing normal load, the bulk of the sand grains began to yield near the contact. The geometry of the surfaces of LBS grains in contact with the loading platen is the main factor that influences the plastic deformation of the bulk. Differently from the plastic deformation of the asperities, the plastic deformation of the bulk could both smoothen and roughen the surfaces. When plastic deformation of the bulk occurred, both Hertz and modified Hertz theory could not predict the load and displacement relationship of sand grains. Through analysing the cumulative distributions of surface roughness of 30 LBS grains at different normal loads by the Weibull function, the surface roughness was found to decrease dramatically with increasing normal load at first and then tended to be constant

Optimising the hydrophobicity of sands by silanisation and powder coating

Sands are naturally hydrophilic granular materials, yet, rendering them hydrophobic could lend them to a wide range of geotechnical applications. This study describes a powder-coating procedure performed after chemically modifying the surfaces of coarse, medium and fine sands and examines its effect on their hydrophobicity. The purpose is to render these granular materials more hydrophobic than what is conventionally achieved by chemical methods using a simple technique. The procedure consists of first silanising both the sands and silica powder at a similar concentration by means of an organosilane to modify their surface chemistry, then the silica powder is adhered to the sands at a mass mixing ratio to alter their hydrophobicity. Irrespective of the concentrations and mixing ratios, the powder-coating procedure enhances the hydrophobicity of sands in comparison to the sole use of the chemical method. Changes in the morphology of the sand grains, such as their particle size, particle shape and surface roughness, resulting from the powder-coating procedure are examined by means of dynamic image analysis, profilometry and scanning electron microscopy. The effects of surface chemistry, surface roughness and air on the hydrophobicity of the sands are discussed based on theoretical wetting models to analyse the experimental results

Wettability decay in an oil-contaminated waste-mineral mixture with dry-wet cycles

The dependency of soil particle wettability on soil water content implies that soils subjected to drying-wetting cycles become wettable with wetting and water repellent with drying. While this has been demonstrated widely, the results are contradictory when water repellent soils are subjected to a sequence of cycles. Added to this, past wettability measurements were seldom done in batches of samples collected from the field at natural or dry water contents, with little appreciation that slight particle size variations, different drying-wetting histories and fabric (as required by different wettability measurement methods) may alter the results. This note presents soil particle wettability—soil water content relations by means of an index test following staged drying and wetting paths over a period of 8 months for an untreated, oil-contaminated anthropogenic soil (a mixture of slag, coal particles, fly ash and mineral particles) from Barry Docks (UK), a site formally used for oil storage, which is to be remediated and redeveloped for housing. The results revealed a decrease in the water repellency and increasing mineralization and bacterial activity with the wetting and drying cycles.postprin

Cavitation in high-capacity tensiometers:effect of water reservoir surface roughness

High-capacity tensiometers (HCTs) are sensors made to measure negative pore water pressure (suction) directly. In this paper, a new approach is proposed to expand the range and duration of suction measurements for a newly designed HCT. A new technique is employed to reduce significantly the roughness of the diaphragm’s surface on the water reservoir side in order to minimise the possibility of gas nuclei development and the subsequent early cavitation at the water–diaphragm interface. The procedures employed for the design, fabrication, saturation and calibration of the new tensiometers are explained in detail. Furthermore, the performance of the developed HCTs is examined based on a series of experiments carried out on a number of unsaturated clay specimens. An improvement in maximum sustainable suction in the range of 120–150% of their nominal capacity was obtained from different surface treatment methods. Moreover, the results show an improvement of up to 177% for the long-term stability of measurements, compared to the developed ordinary HCTs with untreated diaphragms

Geomorphologic features related to gravitational collapse: Submarine landsliding to lateral spreading on a Late Miocene-Quaternary slope (SE Crete, eastern Mediterranean)

Detailed geological mapping, complemented by sedimentary facies analysis, photomosaics and topographic measurements (height, width) allowed the investigation of the geomorphological features related to the Late Miocene-Quaternary gravitational collapse of a palaeoslope located in SE Crete (eastern Mediterranean). In the study area, carbonate megablocks indicative of submarine landsliding during the Late Miocene alternate with collapse features more typical of subaerial settings; the latter generated after a major event of tectonic uplift initiated in Crete during the Early-mid Pliocene. Submarine features typically show basal shear zones, rather than planes, generated in near-seafloor strata deformed in ductile form during the gravitational collapse of the megablocks. The lithology of failed carbonate strata differs from that of their basal shear surfaces, a characteristic providing a reliable estimate for the degree and styles of basal deformation during submarine slope instability. Styles of submarine collapse include, by order of magnitude; (i) lateral spreading of fractured segments of fan cones and carbonate sheet flows, eventually transported 100s of metres downslope; (ii) aperture of ravines and chasms in gravitationally unstable fan cones and boulder conglomerates; (iii) gliding of megablocks over a ductile basal layer through a distance of up to several kilometres; and (iv) rolling of subcircular blocks, often within a debris-flow matrix in fan cones and deltas, or embedded in slope siliciclastic strata. This work highlights the existence of prominent 2-10. m basal shear zones in strata underneath the larger megablocks deposited on marine slope strata. Basal shear zones comprise a melange of reworked conglomerates and breccia clasts from overlying megablocks, large ripped blocks of rock and faulted near-seafloor strata, at places showing remnant beds and sand injection features. Consequently, the outcrop data show an average 5:1 ratio between the maximum observed thickness of megablocks and the thickness of basal shear zones (R), a value of similar magnitude to published examples from offshore landslides. © 2010 Elsevier B.V.link_to_subscribed_fulltex

Aspects of sand behaviour by modified constant shear drained tests

Constant shear drained tests (CSD) are probably the most suitable to simulate the strength and deformation behaviour of soils in slopes under water infiltration conditions or lateral stress relief. This is significant because soil behaviour following a CSD stress path could differ from that of traditional compression triaxial tests. In this paper, CSD tests on sand following an alternative procedure are presented and discussed. The modified CSD tests were conducted by increasing the pore water pressure at a constant rate from one end of the specimen with water free to drain from the opposite end. Among the results from specimens consolidated at variable initial void ratios and principal stress ratios it was revealed that specimens showed a tendency to dilate even for loose sands; failure was reached at low axial strains; and a pre-failure type of instability could be identified. The modified procedure has the potential to provide new insights into the failure mechanisms of slopes under a water infiltration condition. © 2010 Springer-Verlag.link_to_subscribed_fulltex

Volumetric behavior of saturated sands under poor drainage conditions

Permeability variations have been identified as a key factor in controlling slope failure locations in rainfall-induced landslides. In this research, failure behavior in limited drainage conditions was investigated. Tests were performed on saturated sands by means of a modified triaxial system that could mimic the effect of low-permeability barriers present in the field. The tests were conducted by increasing the pore water pressure at different rates to study the effects of the speed of pore water pressure rise on soil failure. The results revealed a dependence of soil volume changes on the rate of pore water pressure increase. In general, the results showed that volume change of granular soils, which are under shear and confined laterally by low-permeability materials, depends on the initial porosity and the pore water pressure rate. These results are particularly valid during the early stages of soil deformation that precede wholesale slope failure. Copyright 2006 by the American Geophysical Union.link_to_subscribed_fulltex

Cavitation in high suction tensiometers: Effect of temperature, time of use, and stone drying

In high suction tensiometers, cavitation occurs when the tension in the water inside the device exceeds a certain threshold. The threshold corresponds to the formation of air bubbles either by air entry into or formation within the porous stone or reservoir. The standard procedure to inhibit air bubble formation in the device is to flood under vacuum and apply water at high pressure to force any remaining air bubbles to dissolve. A series of factors are, however, likely to influence the cavitation behaviour of high suction tensiometers. This paper demonstrates that: (1) there are no precursory phenomena leading to cavitation, (2) higher pressures are not essential for saturation, (3) cooler temperatures and tensiometer usage decrease the pore water pressure at cavitation, and (4) high suction tensiometers record increasing pressures when plunged dry in free water. The findings provide useful guidance to researchers using these devices for suction measurement. © 2012 IEEE.link_to_subscribed_fulltex

Discussion "The development of a suction control system for a triaxial apparatus," by Jotisankasa, A., Coop, M., and Ridley, A

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