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Precise observation of soil surface curling
Marcin Zielinski a,b,⁎, Marcelo Sánchez b, Enrique Romero c, Alvis Atique a
a Department of Civil and Environmental Engineering, University of Strathclyde, Glasgow, UKb Zachry Department of Civil Engineering, Texas A&M University, College Station, USc Department of Geotechnical Engineering and Geosciences, Universitat Politècnica de Catalunya, Barcelona, Spain
⁎ Corresponding author at: University of StrathclyEnvironmental Engineering, James Weir Building, LevelGlasgow, UK.
The process of drying in soil is often associated with complex deformations. Due to the complexity of both thesoil structure and the coupled hydro mechanical processes occurring during drying, different forms and shapescan be created during drying. Soil curling is recognized as one of the typical phenomena taking place duringshrinkage, when the surface layer is curled up or down by different mechanisms triggered during drying. A pre-cise non-contact electro-optical technique based on a 2D laser profile scanner with motion controller to system-atically track the evolution of the exposed surface of a natural soil during controlleddrying conditionswas used inthis research. Different stages of curling were identified at different elapsed times. These stages are discussed indetail. To gain a better understanding of the different curling stages and their associatedmechanisms, a set of dry-ing experimentswere performed on artificially preparedmixtures of kaolin and silica sand. Particularly, two basicmechanisms were studied: differential drying and differential shrinkage effects. A simple conceptual model isalso proposed and discussed to help in the interpretation of the test results involving the curling.
The desiccation process in soils generally leads to shrinkage with amajor rearrangement of the soil particles and significant changes inthe stress state. The shrinkage deformations can be divided into twomain groups: i) intra-deformations, those locatedwithin the particle ar-rangement (visible under the microscope); and ii) global deformations,observed visually volumetric changes of a given sample or specimen(i.e. shrinkage, cracking or curling). The first group includes the deforma-tions associated with grain redistribution (i.e. Kirkpatrick and Rennie,1973) and/or aggregate formation (Jim, 1990; Tang et al., 2011). Bothkinds of deformations (i.e. global and intra) are intimately related,because most of the global deformations during drying depend onintra-deformations. At that level the whole process initiates, driven bythe changes in the capillary pressures (pc); defined as the excess of theair pressure (pa) over the (negative) pore liquid pressure (pl). It iswell known that the increase of capillary pressure (or matric suction‘s’, i.e. s = pc = pa − pl) leads to the strengthening of the soil mass(i.e. Fredlund et al., 1978; Ho and Fredlund, 1982; Nearing and West,1988; Nearing et al., 1988).
de, Department of Civil and5, 75 Montrose Street, G1 1XJ
nski).
The changes in the soil state during drying described above maycause the soil to curve. This phenomenon, called soil curling, can takeplace in two different ways: i) with edges curling upwards also knownas concave-up and ii) edges curling downwards known as convex-up.Curling is a natural soil deformation which occurs during the desicca-tion process. Soil curling can develop in different environments, suchas mud (Allen, 1986), remoulded samples (Nahlawi and Kodikara,2002, 2006) and compacted construction material (Berney et al.,2008). Curling in soils is generally closely associated with desiccationcracking. Amongst other effects, it has been observed that curling en-hances the formation/enlargement of the sub-horizontal cracks in thesoil mass (Berney et al., 2008).
Curling has been a topic of a long research tradition in soil physicswith the emphasis on physical modelling (Dow, 1964; Kindle, 1917;Kindle and Cole, 1938) and numerical simulations (Allen, 1986;Kodikara et al., 1999). Past research clearly highlight three main factorscontrolling the curling in soils:
a) Material properties (e.g. soil grain size distribution, mineralogy andsoil microstructure);
b) hydraulic boundary conditions (e.g. moisture gradient, differentialdrying); and
c) other coupled processes involving chemical interactions.
The following phenomena are the dominant ones when lookingat the effect of material properties on curling (i.e. factor a) above): differ-ential contraction due to non-homogeneous distribution of soil particles,
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re-distribution of grain particles, and particle aggregation during drying(e.g. Allen, 1986; Kindle, 1917, 1926; Kodikara et al., 2004; Nahlawi andKodikara, 2002; Valentin and Bresson, 1992). As for factor b), previousstudies have beenmainly focused on the effect of differential drying con-ditions; moisture gradients; access of relative humidity to sub-horizontalcracks; and evaporation/infiltration boundary conditions (e.g. Berneyet al., 2008; Bradley, 1933; Longwell, 1928; Minter, 1970; Style et al.,2010; Ward, 1923). Other processes that affect the curling (i.e. factor c)are as follows: gradients of salt concentration; solvent evaporation duringdrying, presence of salt crystals; and changes in matric and osmoticsuction and their spatial distribution (e.g. Bradley, 1933; Dow, 1964;Kindle, 1926; Minter, 1970).
The differential drying and differential contraction due to particle dis-tribution are among those mechanisms most researched in the past(Allen, 1986; Bradley, 1933; Kindle, 1917, 1923; Kindle and Cole, 1938).In addition to the aforementioned curling mechanisms, some other fac-tors were recognized to be associated with the curling, for example, theelevation of deposits, clay presence and its type, underlying layer andsalt content (e.g. Bradley, 1933; Dow, 1964; Kindle, 1926; Minter, 1970;Ward, 1923). Kindle (1917) appears to be one of the first scientists pub-lishing about soil curling. His drying-pan experiments revealed that theclay content can influence the direction of polygon curling. He was thefirst to observe that high salinity mud polygons curl downward andthose formed in the fresh water curl upwards. A similar observationwas later performed by Plummer and Gostin (1981). Some exceptionsare known, for example: Minter (1970) reported concave downwardpolygons forming in the sediments left after flooding on the elevatedriver embankments. This was due to the contraction at the bottom ofthe layer, in which drainage was greater than evaporation causing differ-ential drying. Ward (1923) highlighted that the difference in the surfaceunderlying mud flats can also have some influence on the direction ofcurling. According to Bradley (1933) and Valentin and Bresson (1992),the soil layer will curl towards the finest grained material and the rateof curling will depend on the grain size distribution. Allen (1986) pro-posed a model to calculate the curvature of curled polygons which wasvalidated against field and laboratory observations.
Curling in soils is strictly associatedwithwater evaporation and gen-erally occurs at the edge of drying polygons formed by the desiccationcracks. Laboratory desiccation experiments looking at soil curlingunder non-constrained conditions are usually carried out using a slurrypaste placed either in circular or rectangular plates (e.g. Kindle, 1923;Kodikara et al., 2004; Nahlawi and Kodikara, 2002). Peron et al.(2009) studied the curling in the laboratory under constrained condi-tions. In that research, a metallic base with 2 mm spaced parallelgrooves across its length was used. Rectangular sample of Bioley siltprepared at around 1.5 the limit liquid (i.e. 1.5 × LL) were used in thisstudy. Although Peron et al. (2009) do not report curling, it is evidentthat even the shrinkage was restricted in the longitudinal direction,some moderate concave up took place in the uniformly structured soilbar. This curling was moderate compared with the one observed onthe Werribee clay (i.e. 1 × LL) and reported by Nahlawi and Kodikara(2002). The presence of smectite minerals in the last one may explainthis difference in soil curling.
Curling in soils has been studied using traditional measuringmethods (e.g. callipers) and the interest has been mainly focused onthe description of the direction of curling (Allen, 1986; Bradley, 1933;Kodikara et al., 2004; Minter, 1970; Ward, 1923). A problem associatedwith the study of curling in soils is that measurements have to be per-formed in an extended area; so conventional point-wise measuring-devices are not convenient because a set of synchronized devices willbe required. Moreover, the measuring device should not be in contactwith the desiccating soil. This is recommended to prevent any perturba-tion in the stress field that may spuriously affect the phenomenonof curling.
Previous researches have beenmainly focused on field and laborato-ry observations of mud polygons and the slurry paste rectangular
samples. However, no attempt has been made to measure the rate of acurvature during drying. Moreover, the spatial nature of curling andthe soft nature of the soil at high water contents make it almost impos-sible to measure the soil curvature using conventional (contact) tools(such as linear vertical displacement transducer (LVDT), calliper or sim-ilar devices). In this context, recent measuring techniques based on theprocessing of digital images appear very convenient for this kind ofstudies. Digital image analyses are widely used in a number of soilscience-related investigations. For example, they are used to study thevolume change in soils (e.g. Alshibli and Al-Hamdan, 2001; Alshibliand Sture, 2000; Gachet et al., 2006; Macari et al., 1997; Örenet al., 2006; Puppala et al., 2004), the displacement field in soil samples(e.g. Guler et al., 1999; Messerklinger and Springman, 2007; Obidat andAttom, 1998; Romero et al., 1997), and the morphology of desiccationcracks (e.g. Lakshmikantha et al., 2009, 2012; Li and Zhang, 2010;Velde, 2001). However, a shortcoming of this technique is that a lim-ited number of parameters can be gathered using one camera only.This limitation is dictated because digital images flatten the objectto a 2-dimensional set of pixels; whichmake the third (vertical) dimen-sion not visible. This problem can be overcomeby taking pictures at twodifferent angles with two digital cameras (e.g. Kikkawa et al., 2006;Kitzhofer et al., 2010), but this technique is not very common becauseit makes the measurements much more complicated.
Linear displacement laser sensors (i.e. 1D laser) have been recentlyused with success in soil-related research (e.g. Hong et al., 2006;Romero et al., 1997). Various 3D laser scanners have also been used inthe agricultural science to measure the soil surface roughness (Aguilaret al., 2009; Bertuzzi et al., 1990; Huang and Bradford, 1990, 1992;Römkens et al., 1986) and to study erosion under different rainfalls(Römkens et al., 2001; Wells et al., 2003). However, they have notbeen used yet to study features of drying soil behaviour, as for examplecurling.
Some recently developed non-contact technology brings up newpossibilities on how to overcome the problem of tracking spatiallydistributed deformation such as the evolution of curling. In the presentwork, a high precision profile laser is proposed as a new tool to study theformation of curling. Two main experimental campaigns were carriedout to study the process of curling and to understand the mechanismscontrolling it. Two different kinds of samples were studied: a naturalsoil and reconstituted mixtures of kaolin and silica sand. The soil andkaolin/silica mixtures studied in this research and other experimentalaspects related to the drying tests are presented in detail. Backgroundinformation associated with the problem of curling in soils is also intro-duced. In this paper, the main focus is on the phenomenon of curling,with particular emphases on a better understanding of the differentstages of curling.
2. Material and methods
The soil curling phenomenon is governed by physical and mechani-cal processes which are usually not easy to capture in the natural envi-ronment. The proposed laser setup, provides very precise non-contactmeasurement of the distance between the group of the points organizedas a linear beam generated by the laser and the surface. It gives very ac-curate and real time profile which is obtained at the current position ofthe laser above the scanned object. A number of parallel 2D profiles andfurther data processing allow the construction (with high resolution) ofa 3D image of the scanned surface. The results gathered with the laserdevice can be used to explore different features associated with thebehaviour of drying soils. The two different samples studied in thiswork are briefly described below.
2.1. Samples description
Two different materials were investigated in this work: natural soilsamples and reconstituted mixtures made up of kaolin and sand. The
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natural soil used in the first experimental campaign is called‘Bengawan-Solo’ and was sourced from a depth of 0.5–1.0 m at a sitelocated along the flood defence embankment of the Bengawan SoloRiver, in the village of Kedungharjo, East Java, Indonesia. More informa-tion about this soil can be found elsewhere (El Mountassir et al., 2011).Particle size distribution tests carried out on the soil sample reveal thefollowing fractions: 30% sand, 57% silt, 13% clay, with 8% of organicmat-ter. The Atterberg's limits were found to be: liquid limit (LL) — 54%;plastic limit — 36%; and shrinkage limit — 14%. According to the BS1377-2 (BSI, 1996), the investigated soil can be described as an organicsilt of high plasticity. The soil particle density is around 2.73 (Mg/m3).The X-ray diffraction analysis, carried out on the powdered materialshows the presence of quartz, calcite and feldspar (plagioclases),montmorillonite and kaolinite clay minerals. More precise measure-ments carried out on the b2 μm fraction (El Mountassir et al., 2011)highlighted the presence of montmorillonite and kaolinite, with themain clay mineral present in this soil to be montmorillonite (88% ofthe clay fraction). A high percentage of montmorillonite explains thehigh activity of the soil, which was found to be 1.4 (A N 1). The salinitywas found to be EC1:5 = 0.36 dS/m and can be described as slightlysaline. The total sodium content was found to be 1249 mg/kg.
The reconstituted samples were made up either from pure kaolin orfrom a mixture of kaolin and Ottawa sand. The X-ray diffraction test in-dicates that kaolin clay contains 97% of kaolinite and the averageparticle size is 1.36 μm. The salinity of kaolin was found to be EC1:5 =0.12 dS/m and the clay can be described as non-saline. The total sodiumcontent was found to be 84.8 mg/kg. The liquid limit was found to be26%, linear shrinkage 5.8% and particle density 2.65Mg/m3. The Ottawasand, used in this experiment is a specially graded natural silica sandgraded to retain 98% on a No. 100 (150 μm) sieve, 75% on a No. 50(300 μm), 30% on a No. 40 (425 μm) and 2% on a No. 30 (600 μm). Thespecific particle density is 2.65 Mg/m3.
Fig. 1. Experimental setup.
2.2. Experimental setup
The experimental setup presented in Fig. 1 is composed of: i) a 2Dprofile ScanControl 2700-100 laser; ii) axial motion controller; iii) rela-tive humidity and temperature sensor; and iv) a personal computer,with data logging system.
Thehigh precision 2Dprofile compact laser scanner, so-called profilescanner, was supplied by Micro Epsilon. This high performance andaccuracy laser is mainly used in industrial applications. The workingprinciple is based on the laser triangulation allowing the creation of atwo-dimensional profile of a given surface. The laser used in this exper-iment can quickly and accurately generate the 2D profile by using ahigh-speed sensor matrix and a tuned line generator.
The laser provides a two-dimensional profile and the two valuesmeasured are the distance from the sensor to the target (z-axis) andthe position along the laser beam (y-axis).When the sensor is subjectedto traversing, the third coordinate can be collected along the x-axis.Individual profiles may be collected to create a 3D point cloud of thescanned object. The system described above is presented in Fig. 2.
A special motion system was designed to achieve an accurate mea-surement along the traverse axis. Themotor is controlled by a controllerconnected to the PC using a USB port. The laser was attached to theplate, which is actuated axially by the transmission system connectedto the motor. The entire motion system was attached to the wall usinga custom-made mounting system.
A commercially available relative humidity (RH) and temperature(T) chart recorder were used to monitor the RH and T in the closevicinity of the soil sample. The sensor has a detachable probe whichsenses the ambient conditions. Live measurements (graphs) aredisplayed on the LCD and the related data is transferred to the PC.
Fig. 2. Schematic operation of the 2D profile scanner with the corresponding 3D pointcloud of the soil sample.
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2.3. Experimental procedure
The experiment was carried out in a temperature-controlled roomspecially conditioned to operate the laser described in Section 2.2.More details can be found elsewhere (Sanchez et al., 2013). The soilsample was carefully crashed, sieved and oven dried prior to the exper-iment. The dried soil was mixed with de-ionized and de-aired water atgravimetric water content near the liquid limit. The resulting slurry wasthen vigorously mixed and left in a tightly sealed container for 24 h toensure perfect homogenization and even saturation of the soil sample.After the 24-hour period, the sample was transferred to a circularglass plate, measuring 96.9 mm in diameter and 12.9 mm in height.The air bubbles were removed from the soil by carefully tapping thewalls of the plate and the excess of soil was removed by moving asharp edge of a penknife on the top of the plate. The moisture contentwas determined before and after the test and was found to be 57.7%and 6.2% respectively.
The underlying aim of the second study was to explore the effectthat two different fabrics may have on the curling during soil desicca-tion. In order to prevent other factors (e.g. swell/shrink of clayminerals)that could affect the volumetric deformation of the investigated sam-ples, non-active commercially available pulverized kaolin was used inthe second study. Two types of samples were studied: a) samples pre-pared from pure kaolin; b) and mixtures of kaolin and silica sand,mixed at 90:10 (i.e. percentage in term of sample mass). The intentionwas to test two different kinds of particle arrangements: a) sampleswith (almost) evenly distributed particles (i.e. pure kaolin), and b) sam-ples with gradually textured skeleton (i.e. containing particles of differ-ent fractions). Six samples in total were prepared as follows. Threesamples of pure kaolin were mixed with distilled water at three differ-ent liquid limits: 2 × LL, 2.5 × LL and 3 × LL. Other three samples wereprepared using the mixture of kaolin and silica sand at the same liquidlimits (e.g. 2 × LL, 2.5 × LL, 3 × LL). In this case distilled water wasalso used.
During both experiments, the relative humidity and temperaturechart recorderwere placed in the close vicinity of the samples to contin-uously monitor these two variables. The recorded T and RH were oscil-lating around 19.5 °C and 37% respectively and no significant variationwas noticed during the experiment.
The laser and themotion control were programmed taking into con-sideration the length of the sample in the traversal direction (x-axis).The motion controller was set to move at 3.88 mm/s with the laser tak-ing measurements of 100 lines/s. This allowed projecting a linear scan-ning beam at 38 μm increments. At the standard laser configuration,a 100mm long beamwas projected on the surface. The 2D contour pro-file was then calculated by the scanner's microprocessor from the pixeldata of the diffusely-reflected laser line consisting of 320 points. Thissetup gives a densely distributed grid of over 1 million points. Thescans were performed at almost constant time intervals. In order toavoid any disturbance of the measurements, the sample remained un-touched for the duration of the test.
Fig. 4 presents the profile obtained with the laser for different timescorresponding to a cross-section at a distance of 60 mm (i.e. a cross-section in the middle of the sample). The dashed and dotted lineswere used to distinguish three possible different stages: i) the initialstage at which curling started to develop, ii) the maximum stage atwhich curling reached the maximum height, and iii) the final stagewhere no further curlingwas observed. This can be seen on the zoomedcrack edge presented in Fig. 4.
Fig. 4 shows that theupward curling started to develop at the edge ofthe crack at 9 h from the start of the drying process (identified as‘Initial’) andwas then progressively developing until it reached its max-imum elevation (identified as ‘Max’) at 16 h. At this point, the deforma-tion reversed and the curled edge started to retreat downwards to thefinal position (identified as ‘Min’) at 24 h. A similar reversible phenom-enon was also observed by Berney et al. (2008) using time laps photog-raphy. It is worth noticing that the period of time between Initial andMax; and between Max and Min positions is almost the same. More-over, it also appears from Fig. 5 that the settlement (i.e. rather uniformdescend of the soil surface) develops during the first stages of theexperiment and it continues until the cracks open and the lateralshrinkage takes place. This is in line with the conceptual model pro-posed by Shin and Santamarina (2011). The mechanisms behind thedevelopment of curling and its reverse behaviour are explained inthe next section.
The availability of evenly distributed sets of data points, measuredby the laser, allows the creation of a contour map for each scanningtime. Fig. 6 presents a set of contour maps created for the duration ofan experiment. The difference in colour intensity represents the z-axisposition of the points with respect to the scanning sensor. In otherwords, it shows the evolution of surface deformation by graduallydarkening the areas according to the elevation above the base surfaceand creating a very precise topography map.
Volumetric deformations of soils subjected to drying, depend on thesoil type, soil density, initial water content, andmost importantly on thepresence of clay minerals, their type and amount. From the resultspresented above, it can be clearly seen that the settlement is the firstprocess associated with soil shrinkage. Furthermore, the progressingdrying process leads to more complex deformations such as crackingand curling. The specific nature of the sample used in this experiment(i.e. uniform texture) suggests that differential drying and differentialshrinkage could explain the curving behaviour observed during thetest (Allen, 1986; Bradley, 1933; Kindle, 1917, 1923).
3.2. Test on artificial samples
The stresses developed during drying in a soil mass that lead tocurling are explained in detail in Kodikara et al. (2004). However,there are different conditions and factors that may affect the kinematicsof curling, as for example: material type; particle size; material fabric;boundary conditions (both mechanical and hydraulic); initial stresses;rate of drying; sample heterogeneities. In order to gain a better
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understanding on the effect of particle size and material fabric on themagnitude of curling, a set of simple drying experiments was carriedout. Six samples were placed in rectangular Perspex moulds measuring15.9 × 2.9 × 1.4 cm (Fig. 7). All samples were air dried at the constantroom temperature and relative humidity, 23 °C and 45% respectively.A thin film of silicone grease was applied inside each mould in orderto reduce the adhesion and allow the free shrinkage of the sample.
It is anticipated that after sedimentation, samples prepared at highwater content (e.g. 3 × LL) will present a distribution of particles(in vertical) with the bigger particles settling at the bottom of thesample and the smaller ones at the top (Fig. 9a)). This graduation willbe more notorious in samples made up of mixture of kaolin/sand,where the coarser sand grains will be overlaid by the clay particles(i.e. Bradley, 1933). In samples prepared at lower water contents(e.g. 2 × LL), the graduation of particles in vertical direction willbe less pronounced (i.e. the sedimentation process will be lessimportant). It is anticipated that pure kaolin samples at low water con-tent will present a rather uniform destitution of particles (Fig. 9b)).
It can be noticed from Fig. 8 that all the samples experienced somecurling on either one or both ends. However, the amount of curlingdiffers between the samples made up of pure kaolin (Fig. 8a)–c)) andthose with made up from the mixture (Fig. 8d)–f)). The difference be-tween the two types of soils is especially pronounced in samples atthe higher water contents (Fig. 8c and f), where, as mentioned above,
Fig. 4. Evolution of the cross-sectional profiles at
the effect of the sedimentation is more marked and a more gradualsoil size distribution is expected through the sample (i.e. coarser sandgrains are overlaid by the clay particles, i.e. Fig. 9a)). The difference ob-served in curling can then be explained by considering that near the topof the sample, the particles are significantly smaller and so the contrac-tion at the beginning of the drying is greater than those of the silica sandsettled at the bottom. This phenomenon leads to a differential shrinkageduring desiccation.
It is worth mentioning that the curling observed in this experimentis also controlled by the phenomenon of differential drying explainedas follows. The evaporation is initially limited to the surface of the sam-ple, and restricted (by the mould) at the sides and the bottom of thesample (Fig. 10). Evaporation of water from the surface of the initiallysaturated soil sample results in the development of the meniscus atthe interface among water, air and the neighbouring soil particles. Theassociated surface tension (Ts) and capillary pressure (pc) also start atthis stage.
During early desiccation the vertical resultant of the surface tensionand the grain self-weight (w) trigger vertical/downward movement ofthe sample. This causes the settlement of the particles in the surfacelayer, as indicated by grey arrows in Fig. 10b). Note that particles atthe top edges of the sample do not have neighbouring particles on oneside, so the resultant of surface tension forces is smaller leading to thesettlement of those grains at a different rate. This phenomenon induces
Fig. 5. Evolution of the sliced point clouds during desiccation.
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an initial curling. As mentioned before, the sample adhesion to themould surface was reduced by using silicone grease allowing a freeshrinkage to take place.
As drying continues, the shrinkage leads to disconnect the samplefrom the mould (black arrow Fig. 10c)). As a consequence of the differ-ential drying, the increasing stresses result in the densification of theparticles in the surface layer. The water can then evaporate sideways,exacerbating the mechanisms explained above and leading to morecurling involving now a bigger mass (Fig. 10d)). Evaporation may startthen at the bottom of the sample. The drying front is being reversed asthe evaporation rate from the bottom exceeds the one from the top(e.g. Adams and Hanks, 1963). Hence, the expected increasing stressesat the bottom (indicated by black arrows) may cause the soil skeletonto move downwards (Fig. 10e)). The ability of this later mechanism to
Fig. 6. Surface contour maps. Colour s
reverse the initial curling will depend, amongst other factors, on thesoil fabric, as explained below.
Fig. 11 shows the comparison of curling that can take place in thetwo different textures studied: Fig. 11a), which corresponds to a skele-ton with a quite uniform texture (e.g. pure kaolin sample for the studypresented above); and Fig. 11b), associated with a non-uniform texture(e.g. mixture of graded sand and kaolin). In the first case (i.e. quite uni-form grain size distribution) the curling initially induced by the differ-ential drying is almost reversible at later stages when the bottomlayers are subjected to shrinkage. In other words, the fabric at the topand bottomare quite similar, so under similar stress-fields the deforma-tions are expected to be similar and the initial curling observed at thetop layerswill almost reverse atfinal drying stages. However, in the sec-ond case (i.e. course particles at the bottom) it is evident that the drying
Fig. 7. Soil sample prepared for desiccation test.
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at latest stages will not be able to reverse the curling as the retraction ishindered by the larger particles settled at the bottom of the sample.Thus, the downward movement is restricted.
Fig. 12 presents the laser results obtained for the sample made upfrom the soil mixture at 3 × LL (i.e. soil gradually textured skeleton).These results confirmed the observations of the digital images. Perma-nent curling is observed after drying due to a non-reversible curling.
Fig. 8.Curled reconstituted soil samples: (a) pure kaolin prepared at 2 × LL, (b) pure kaolinprepared at 2.5 × LL, (c) pure kaolin prepared at 3 × LL, (d) kaolin silica sandmixture pre-pared at 2 × LL, (e) kaolin silica sand mixture prepared at 2.5 × LL, (f) kaolin silica sandmixture prepared at 3 × LL.
Fig. 9. Schematic representation of the two studied textures: (a) gradually textured skel-eton, (b) uniformly textured skeleton.
Fig. 10. Mechanism of curling due to differential drying.
Fig. 11. Schematic representation showing the differences between the rates of curling for samples with: (a) uniformly textured skeleton, (b) gradually textured skeleton.
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4. Discussion
The effect of the material fabric on drying and curling discussedabove could assist to understand the differences observed betweenthe pure kaolin samples and the ones made up of a mixture, and alsoto clarify the effect of the mixing water on the final shape of the driedsample. Comparing the samples mixed at the same high water content(e.g. 3 × LL), the curling observed in the kaolin sample (i.e. Fig. 8c)) ismuch less marked than the one observed in the mixture (i.e. Fig. 8f)).This can be related to the fact that a uniformly graded sample (i.e. kaolinsample; which looks like Fig. 9b) will be more susceptible to reversiblecurling than a sample with a gradually textured skeleton (i.e. kaolin/silica mixture, which resemble Fig. 9a)).
It is also evident that for the same type of material, the samplesprepared with higher water contents present a more marked curling.This can be checked by comparing the kaolin/silica mixture samplesprepared at 2 × LL (Fig. 8d) against the one prepared at 3 × LL(Fig. 8). The fabric of the samples prepared at high water contents(e.g. 3 × LL) is mainly controlled by the sedimentation process thattakes place just after moulding the material. The end result of the sedi-mentation process under these conditions is a sample with a well grad-ed distribution of particles (i.e. bigger particles at the bottom and thesmallest one at the top), resembling the scheme presented in Fig. 9a).As the amount of mixing water reduces, the effect of sedimentation isless important, leading to samples with more uniform distribution ofparticle sizes. Comparing these two different samples, the second one(i.e. the one moulded with a lower water content) will have a fabricmore susceptible to reversible drying and, therefore, the associatedcurling at the end of the desiccation process will be less marked.
Fig. 12. Cross-sectional evolution during drying of t
The aim of the experiments described above was to provide an in-sight into the possible effects of different fabrics (and particle sizes)may have on soil curling. They represent very simple conditions (withthe associated basic soil fabrics). Obviously more detailed studiesshould be carried out to develop more general models to describe soilcurling. However, these observations may help to understand the re-versible/irreversible behaviour observed sometime in soil curling dur-ing drying. For example, the mechanisms discussed above can assist toexplain the three stages of soil curling observed in Begawan-Solo soilduring drying (Section 3, Figs. 4 and 5). Initially, the drying induced arather uniform settlement of the soil sample. This uniform settlementtook place until around the time of 7 h where the first cracks were evi-dent. At that point, the initial curling was triggered by the mechanismsexplained above (i.e. Fig. 10b and c). The drying crack was acting in thiscase as the sideway evaporating surface. Curling tended to reverse asthe stages of drying advance. Note that the sample used in thisexperiment was sieved under a 3 mm sieve to ensure a quite uniformdistribution of soil grains.
5. Conclusions
The experimental study on soil curling presented in this paper wasperformed using recent measuring techniques based on a 2D profilescanner. The proposed devicewas used to study soil curling during des-iccation. Recent advances in laser technology allow very precise real-time cross profile scan of the drying sample, providing (at a very highresolution) the coordinates on the surface crossing line. The informationprovided by this device is very valuable; for example the point cloud
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created from the laser profiles can be used to study in detail the changesin the evolving surface of soils during drying.
The proposed device was used in this paper to study the behaviourof a natural soil from Indonesia subjected to drying in a circular sampleunder controlled conditions. The analysis of the geometrical model ofthe tested sample showed that an (almost) reversible behaviour interms of soil curling took place during drying. These changes were pre-cisely tracked with the 2D laser and discussed in detail in this work. Inaddition, a series of desiccation experiments on rectangular samplesusing a mixture of kaolin and silica sand was performed to study the ef-fect on grain size and fabric on curling. This study showed the differencein curling between uniformly and non-uniformly textured skeletons.Although, the uniformly textured skeleton also undergoes some curling,the amount of the curvature is significantly smaller than in the non-uniformly textured skeleton due to a reverse curling phenomenon.
This study has shown for the first time the capabilities of theproposed device to precisely observe relevant features associated withsoil curling. It has also contributed to the better understanding of themechanisms behind curling in drying soils and how the soil fabricmay affect this phenomenon. Further works are in progress to gain abetter understanding on the influence of other factors affecting soildrying and curling.
Acknowledgements
The first author would like to acknowledge the financial supportprovided by the EC funded project RISMAC (Grant agreement: PIOF-GA-2009-254794).
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