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Grain shape effects on the mechanical behavior of compacted earth
Journal Pre-proof Grain shape effects on the mechanical behavior of compacted earth A. Koutous, E. Hilali
PII:
S2214-5095(19)30405-X
DOI:
https://doi.org/10.1016/j.cscm.2019.e00303
Reference:
CSCM 303
To appear in:
Case Studies in Construction Materials
Received Date:
22 August 2019
Revised Date:
26 October 2019
Accepted Date:
29 October 2019
Please cite this article as: Koutous A, Hilali E, Grain shape effects on the mechanical behavior of compacted earth, Case Studies in Construction Materials (2019), doi: https://doi.org/10.1016/j.cscm.2019.e00303
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Grain shape effects on the mechanical behavior of compacted earth KOUTOUS (1) * E. HILALI (1)
(1)
MMGC Lab., National School of Applied Sciences, Ibn Zohr University, Agadir 80000, Morocco.
(*)
Corresponding author. Tel.: +212668 82 17 97, E-mail address: [email protected].
HIGHLIGHTS Sand-earth, rounded gravel and angular gravel are the three materials used in this study. Sand-earth, rounded gravel-earth and angular gravel-earth mixtures were prepared and compacted. The three earthen materials produced were tested in uniaxial compression tests. Axial strain has been measured using displacement transducers attached to cylindrical specimens. The mechanical behavior of compacted earthen materials appears to be influenced by grain shape and size.
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ARTICLE INFO Article history:
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Submitted 22 August 2019
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Revised 26 October 2019
ABSTRACT
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The main purpose of this paper is to experimentally explore the effects of grain shape and size on the mechanical behavior of compacted earthen materials. Sand-earth, natural rounded gravel and crushed angular gravel are the three materials used in this study. Both gravels are from the same site and characterized by the same grain size curve. The uniaxial compression tests, the results of which are presented and discussed in this paper, were performed on cylindrical specimens of three materials: sand-earth mixture, rounded gravel-earth mixture and angular gravel-earth mixture. The tested specimens were prepared under optimum compaction references, using the Proctor test procedures. For each compression test, four parameters are determined: the compressive strength, the initial tangent modulus, the secant modulus at maximum stress and the peak axial strain corresponding to the maximum compressive stress. The results obtained show that the mechanical behavior of gravel-earth mixture may be influenced by grain shape of the gravel used, thus introducing a new parameter to be taken into account when preparing unstabilized rammed earth material. Further experimental studies are recommended to better assess these results
Keywords: Rammed earth; Mechanical behavior; Grain shape; Compressive strength; Initial tangent modulus; Secant modulus; Peak strain
1. Introduction Compacted earth is an ancestral building material. The most common method employed to use this material is called rammed earth, which may or may not be stabilized using additives such as cement, lime, natural fibers, etc. Over the past few decades, studies have shown that physical and mechanical characteristics of compacted earth depends on many parameters including water content [1][2][3], compaction method and energy [4], clay content [5], grain size distribution, etc. As for grain size distribution, optimal granular spindles have been recommended for the rammed earth method in order to obtain the most suitable material without adding stabilizers (i.e. unstabilized rammed earth). The most well-known spindle seems to be the one established by
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Houben and Guillaud (1995, [6]). These grain size spindles are included in Standards used in some countries such the Australian code HB 195 [7]. Others spindles can be found in a review on rammed earth by Maniatidis and Walker (2003, [8]). The curves, describing these grain size spindles and recommended by these engineering references, are based, as explained in a recent article by Koutous and Hilali (2019, [9]), on the following Fuller-Thompson formula [10]: 𝑑 𝑛 𝑃(%) = 100 ( ) 𝐷
(𝐸1)
Where "𝐷" represents the maximum grain diameter and "𝑃" the percentage of grains with diameters less than a given diameter "𝑑". And where "𝑛" is a parameter called gradation index, whose value depends on the grain shape (𝑛 = 0.5 for spherical grains and 𝑛 = 0.20 𝑡𝑜 0.25 for earths used as building materials). Grain shape was therefore taken into account in some way when establishing the optimal grain size curve. Indeed, the Fuller-Thompson formula, used to define this curve, was established in order to enhance the material density, and consequently improve its strength, because it is known that the strength increases as the density increases [11]. However, these optimal curves are approximate because the criteria for choosing the value of the gradation index " 𝑛 " are not clear.
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On the other hand, and as mentioned at the beginning of this introduction, the grain size is not the only factor that influences the strength of the compacted earth. There is another factor as important as grain size, which the plasticity of the earth fine-grained fraction. Indeed, a study by S. Naeini et al (2012, [12]) reported that plasticity index has a significant effect on uniaxial compressive strength, as the latter decreases when plasticity index increases. Also, they found that « as the plasticity index increases, the [earth] yields at a higher strain. Therefore, the higher plasticity index could change the stress-strain behavior of [earth] samples from a brittle to ductile manner ».
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For these reasons, it is of scientific interest to carry out studies comparing the mechanical behavior of earth compacted under the same conditions, and having the same characteristics in terms of grain size, plasticity and mineral composition, but whose grain shape is different. This is the purpose of the present study.
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While it is known that the mechanical behavior of conglomerates such as concrete is influenced by the shape of the aggregates used [13], there are currently no well-known studies that can be found in the literature about the impact of grain shape of compacted earth on its mechanical behavior under uniaxial compression. But there are some articles that deal with grain shape effects on the mechanical behavior of granular materials (generally noncohesive materials).
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Among these studies, an article by D. Sarkar et al (2019, [14]) in which the authors compare the mechanical behavior of three different granular materials under triaxial compression or direct shear conditions. The results of this comparative study show that « crushed angular materials (having low values of regularity) exhibit comparatively higher strength than the rounded materials ». The same observation was made regarding the friction angle. As for strain, rounded material seems to have a « very small contraction even for loose materials after which dilative tendencies start creeping in ».
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A study by G-C. Cho et al. (2006, [15]) about the influence of grain shape on physical and mechanical characteristics of natural sands compared to crushed sands shows that the decrease in sphericity and/or roughness of sand grains leads to « increase in extreme void ratios and void ratio interval, decrease in smallstrain stiffness, yet increased sensitivity to the state of stress, increase in the compressibility under zero-lateral strain loading, increase in the constant volume critical state friction angle; and increase in the critical state line intercept, and a weak effect on the slope of the critical state line (void-stress space) ». Similar results have been reported in an article by K.-A. Alshibli et al (2018, [16]) on the influence of grain morphology on the mechanical behavior of sands under triaxial compression. A brief note by S. Yagiz (2001, [17]), on the effects of the shape and percentage of gravel on the shear strength of sand-gravel mixtures, showed that the shear strength and internal friction angle increase linearly with the percentage of gravel in the mixture and that when crushed angular aggregates are used, this increase becomes much more noticeable. Almost the same results were presented in an article by N. S. Salimi et al. (2008, [18]). In a conference paper, J-C. Santamarina and G-C. Cho (2004) reported that « ellipticity and platiness […] promote inherent anisotropy and affect the evolution of stress-induced anisotropy » [19]. In an article by I. Cavarretta et al (2010, [20]), it was reported that at the micro-scale, grain compression tests show an initial behavior dominated by plasticity that depends on the intrinsic characteristics of the grains (size, roundness, roughness, elastic properties and surface hardness). On the other hand, at the macroscopic level, the mechanical behavior of granular materials seems to be less sensitive to the characteristics of grain roughness (and hence inter-grain friction). The explanation given for this lack of sensitivity to friction is that « the stress levels were too high for the increase in inter-particle friction to be retained during initial, isotropic compression of
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the sample […] The contact forces between particles along the strong force chains are likely to be high enough to remove the roughening effect ». These research studies, as interesting as they are, deal only with cohesionless granular materials. However, earth used as building material must be cohesive, especially when it comes to unstabilized rammed earth. This is particularly the case of the earth covered by the present paper. To describe grain shape in detail, J-K. Mitchell and K. Soga, (2005, [21]) distinguish three terms for three scales: sphericity (antonym: elongation) on a large-scale scale, roundness (antonym: angularity) on intermediate scale and roughness (antonym: smoothness) on small-scale. The present paper discusses the effects of grain morphology (large to intermediate scale) on the mechanical behavior of compacted earth. The parameter used to quantify this shape is flakiness index [acc. to NF EN 933-3 [22]]. The determination of flakiness index consists of double sieving. First, by using standard sieves, the sample is fractionated into different " 𝑑𝑖 /𝐷𝑖 " granular fractions, as shown in table 3 (Tab. 3). Then, each of the granular fraction " 𝑑𝑖 /𝐷𝑖 " is sieved using grid sieves with a spacing width " 𝐷𝑖 /2 ". The flakiness index is calculated as a percentage of the total mass of particles passing through the grid sieves to the total dry mass of the particles under test.
2. Materials and methods
2.1.
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Three materials are used in this case study: a sand-earth, a rounded gravel and an angular gravel. The two types of gravel (rounded and angular) actually form a single semi-crushed gravel from a local river (the Souss River, near Agadir, Morocco). They were first sorted and then used separately. Standards used in the study
The following table (Tab. 1) shows the Standards used in the study covered by this paper. 2.2.
Sand-earth
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The sieving and sedimentation tests [acc. to NF P 94-056 [24] and NF P 94-057 [25] respectively], show that the grain size curve of the earth used in this experimental study (Fig. 3) fits well into the grain size spindle recommended by H. Houben and H. Guillaud (1995, [6]) for rammed earth method.
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The sand-earth material (Fig. 1-a) used in this case study is a mixture of two local materials: a clayey soil and a crushed sand. The mass proportions of the two materials in the mixtures, [2⁄3 𝑒𝑎𝑟𝑡ℎ + 1⁄3 𝑠𝑎𝑛𝑑], are determined in such a way that the mixture has a grain size curve that fits well into the above-mentioned grain size spindle. More details can be found in a previous study [9].
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The characteristics of the fine-grained fraction of the Sand-earth are summarized in the following table (Tab. 2).
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Based on these characteristics, the sand-earth studied is a clayey sand (type "SC") according to USCS – the Unified Soil Classification System [acc. to ASTM D3287 [30]], or type "A-6" according to AASHTO – the American Association of State Highway and Transportation Officials Classification System [acc. to ASTM D3282 [31]] (Tab. 5). Note that these results are not affected when gravel is added, since Atterberg limit tests are performed on 0/0.4 𝑚𝑚 fraction [acc. to NF P 94-051 [26]]. Gravels
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2.3.
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The two gravels are originally mixed and come from the same crushing quarry in a local river (the Souss River, near Agadir City, Morocco). They were therefore manually separated into rounded grains characterized by their smoothness and angular grains characterized by their roughness (Fig. 1-b and 1-c). Sieving tests were then carried out on the tow gravels. Comparing the two gravels grain size curves, there is a slight difference. To make them the same, adjustments were made keeping the lowest proportion of each granular fraction as shown in the table above (Tab. 3). Then, their absolute densities, flakiness indexes and water absorption percentages were determined (Tab. 4). The procedures followed to determine these parameters are those of the Standards given in the table (Tab. 1). Once the adjustments have been made, the two gravels have the same grain size curve as shown in figure below (Fig. 2). After these adjustments, two gravel-earth mixtures were prepared with the following mass proportions: ¾ 𝑒𝑎𝑟𝑡ℎ + ¼ 𝑔𝑟𝑎𝑣𝑒𝑙. These mass proportions are determined so that the grain size curve of the mixture is as close as possible to the optimal curve -ICD20 [RE]- (Fig. 3). This optimal curve -ICD20 [RE]- is the curve representing the equation « E1 » with the coefficient " 𝑛 " taken equal to 0.25, according to Houben and Guillaud recommendations [6].
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The two mixtures, thus prepared, which are rounded gravel-earth and angular gravel-earth are clayey gravel, type "GC" according to USCS system, or type "A-2-6" according to AASHTO system (Tab. 5). 2.4.
Preparation of specimens
Specimens dimensions and compaction references
Taking into account grain size distributions of the materials used, the minimum specimen size is taken equal to 100 𝑚𝑚 (i.e. five times the maximum grain size) [acc. to NF P 94-074 [32]]. Thus, for cylindrical specimens, the diameter is taken equal to 100 𝑚𝑚 while the height is taken equal to 200 𝑚𝑚 (i.e. twice the diameter). As a result, the dimensions of the mold for specimen manufacturing are 100 𝑚𝑚 × 200 𝑚𝑚. Compared to those of the molds used in Proctor tests, the standard Proctor test is chosen to determine compaction references using five samples with different water content per each material [acc. to NF P 94-093 [29]].
Manufacturing process
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Once the compaction references were determined, three specimens were prepared per each of the three materials studied. To manufacture these specimens, a maximum dry density equal to that obtained in Proctor test ±5% is targeted. To achieve this purpose, and in order to keep the compaction energy the same, the rammer and compaction procedures of standard Proctor test were used with 5 layers for a height of 200 𝑚𝑚 (height of the mold to be used for specimen manufacturing) instead of 3 layers for a height of 116.5 𝑚𝑚 (height of the Proctor mold). This means that the compaction of the specimens consists of 5 layers compacted with 25 blows each (3 sequences of 8 blows distributed in a circular manner, plus a 25th blow in the center) as illustrated in the figure below (Fig. 4). The water content used in each mix is the corresponding optimum moisture content given in the part 3.1 (Tab. 6). To improve the quality of specimen bearing surfaces, a thin layer containing only grains with sizes of less than 5 𝑚𝑚 is provided on each of the two bearing surfaces. Consequently, each layer is approximately 3.6 𝑐𝑚 thick, except the thin layers (bearing surfaces) whose thickness is about 1 𝑐𝑚.
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As no additional material is used for the preparation of the specimen bearing surfaces, the 0/5 angular fraction is considered as a sort of plaster for this preparation, because it is easier to level without damaging the specimens. The preparation method applied are detailed in section 2.5 (§ 2.5).
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The demolding is done immediately. Then, the specimen wet mass is measured and thus the wet density determined (Fig. 5). Concerning drying conditions, specimens were stored in a room with temperatures varying from 20 to 25 °C and relative humidity of about 60%.
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After four weeks, the weights and dimensions of each specimen are measured, then the densities are determined, according current European standard on concrete material [acc. to NF EN 12390-7 [33]], and compared to the aimed dry densities.
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These comparisons are used to “check” the drying of the specimens. The drying of a specimen is achieved if its density is equal to the one targeted by compaction ±5% and if the weight of the specimen does not decrease by more than 2/1000 after 48 hours under the drying conditions above.
2.5.
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Note that the “dry state” of the compacted material means that the water content has reached its equilibrium value close to and not equal to zero. Uniaxial compression test
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The preparation of the bearing surfaces is performed in the wet state by levelling using a metal ruler. In the dry state, an abrasive paper and a spirit level are used, if necessary, for finishing. The aim of this preparation is to provide parallel and flat bearing surfaces. After that, the dry specimens are compressed using a machine that complies with the European Standard [acc. to NF EN 12390-4 [34]] and allow a constant loading speed of 0.05 𝑀𝑃𝑎 ⁄𝑠 (Fig. 6). A value of 0.05 𝑀𝑃𝑎 has been chosen as peak sensitivity that corresponds to the decrease in stress during the test at which the machine will consider the sample to have failed, and will stop the test. The test apparatus used in this study is equipped with two displacement transducers characterized by a sensitivity of 0.02 𝜇𝑚. The two transducers are arranged symmetrically with respect to the axis of the tested specimen (Fig. 7). The specimen axial strain is the average of the strains calculated from the measurements of the tow transducers.
3. Results and discussions 3.1.
Effects on the compaction references
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The results of the Proctor test show an increase in the maximum dry density and a decrease in the optimum compaction moisture content after adding gravel to sand-earth (Fig. 8). Regarding to the decrease in the optimum compaction moisture content, it can be said that it is obvious that when the proportion of gravel in a soil increases, its optimum moisture content will decrease, because the specific surface area of the soil decreases with the diameter of the grains, especially in the case of smoothgrained soil. This is shown, for example, by a technical note by J-J. Wang et al. (2014, [35]), in which the authors conclude that « the optimum moisture content is generally decreasing with the increase of the median particle diameter or gravel content ». Also, it can be observed that the optimum compaction moisture content of rounded gravel-earth is slightly lower than that of angular gravel-earth, and that inversely, the maximum dry density of rounded gravel-earth is slightly higher than that of angular gravel-earth (Tab. 6). The difference in the optimum compaction moisture content can be explained by the difference in water absorption percentage, which is higher for crushed gravel than for round gravel (1.4% > 0.7%).
3.2.
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As for the maximum dry density, the sand-earth absolute density is smaller than the gravel absolute density (2710 𝑘𝑔⁄𝑚3 > 2570 𝑘𝑔/𝑚3 ), which increases the absolute density of gravel-earth mixture and therefore the dry density of the latter. In addition to that, it appears that the rounded shape of the gravel grains promotes compaction, unlike the angular and flattened shape, which makes compaction harder to achieve as it can be noticed when comparing the coefficients of variation of the dry densities of the specimens compacted (Tab. 9). Effects on the mechanical behavior
The compaction of the specimens is carried out under the optimum requirements previously determined.
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The first observation that can be made is that the average dry densities of the specimens are slightly higher than maximum dry densities obtained during compaction tests (1.04 to 1.05 times higher), and this is the case for the three materials studied (Tab. 7). A possible explanation for this is that the layers thickness applied when making the specimens is lower than that of Proctor test (3.6 𝑐𝑚 < 4 𝑐𝑚), while the compaction energy remains the same.
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This increase in dry density may be due to the fact that the specimens hold onto some moisture content, because, as compared to Proctor tests, the specimens were not dried in an oven (Fig. 9). However, tests carried out on samples of materials recovered after the compression tests show that their water content does not exceed 1% in the worst cases (about 0.9% in the case of angular gravel-earth material). Therefore, this explanation can be dismissed.
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Will this residual moisture content due to the environment relative humidity, no matter how low it is, have an impact on the mechanical behavior of the materials studied? According to a study carried out on three different compacted earths by F. Champiré et al (2016), it appears that an increase in relative humidity in the environment leads to an increase in the moisture content of compacted earth, which affects its mechanical characteristics by decreasing its strength at its Young's modulus [36]. Another study conducted by E. Araldi et al. (2018) leads to similar results [1].
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This means that the values of these two parameters obtained in the present study are actually lower than the results that could be obtained if compression tests were carried out in a real dry state, with a zero-moisture content. Nevertheless, the study presented here is of scientific interest as all samples were prepared, stored and tested under the same conditions. However, it should be kept in mind that the interpretations made in this paper are valid only when these conditions are met.
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Once the “drying” has been reached and the dry density determined, the compression tests are conducted according to the procedures described above (§ 2.5). For each compression test, the stress-strain curve is established (Fig. 11). From this curve, the initial tangent modulus and peak strain are determined graphically as shown in the figure above (Fig. 10). The crushed specimens are carefully examined by looking at the propagation of cracks (Fig. 12). The test is considered successful if the cracks are regular and symmetrical with respect to the axis of the specimen. This is a way of checking whether the load has been applied uniformly to the bearing surfaces, otherwise the test has to be repeated. The table below summarizes the results obtained on a test basis of the compression tests (Tab. 8). Note that the dry density is given to the nearest 10 𝑘𝑔/𝑚3 . As shown in the figure (Fig. 10), the secant modulus is determined as the slope of a line drawn from the origin of the stressstrain diagram and intersecting the curve at the peak stress point.
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The results of these compression tests show a slight but beneficial improvement in compressive strength after adding gravel (Fig. 13). It appears, also, that there is a small difference between the two gravels in terms of strength, which remain within the error range given the number of specimens (3 specimens per mixture). Indeed, there is an increase of 0.10 𝑀𝑃𝑎 in the case of rounded gravel-earth mixture compared to sand-earth, and a relatively better increase of 0.21 𝑀𝑃𝑎 in the case of angular gravel-earth mixture (Tab. 9). Taking into account the range of uncertainties commonly observed for earthen materials and the coefficient of variation of the results obtained (Tab. 9), it can be said that in terms of strength, only the improvement provided by the angular gravel may be considered significant (Fig. 14). It is true that the improvement of 0.1 MPa provided by rounded gravel is significant for an earthen material, but this value remains within the error range and cannot therefore be taken into consideration. While the possible improvement in compressive strength after adding gravel can be explained by the influence of the grain size distribution, which has been made closer to the optimal distribution (ICD20 [RE]) (Fig. 3), the small difference between the two gravel-earth mixtures in compressive strength can be explained on the basis of the studies presented in the introduction to this paper. Obviously, the angular shape and roughness of the crushed gravel grains promote the gravel-clay bond, which increase the mixture strength.
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Concerning the initial tangent modulus, the results show that it is only for rounded gravel-earth material that a significant increase can be observed (Fig. 15): increase of 97 𝑀𝑃𝑎 for rounded gravel-earth material compared to only 12 𝑀𝑃𝑎 for angular gravel-earth material. Considering the error bars, it appears that adding angular gravel had practically no influence on the initial tangent modulus (Fig. 16).
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As the two gravel-earth mixtures studied have the same mineralogical composition and grain size distribution, it can be said, at least at this stage of the study, that the initial tangent modulus depends mainly on the dry density of the compacted material (and thus its compactness), which is influenced by grain shape. Moreover, a correlation can be observed between these two parameters as shown in the figure below (Fig. 17). These results only confirm what has already been reported in previous studies [37].
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On the other hand, when analyzing each material results, it can be noted that, for both sand-earth and angular gravel-earth materials, the initial tangent modulus and compressive strength both increase with the increase of dry density. In contrast, it appears that the compressive strength and initial tangent modulus of rounded gravelearth material vary in opposite ways with dry density. In other words, when the dry density increases, the initial tangent modulus increases and the compressive strength decreases (Fig. 18).
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This finding, which may seem unexpected, cannot be established on the basis of the results obtained for only three specimens. Therefore, a study on more samples is necessary to confirm or contradict it. Nevertheless, this result, if confirmed, can be explained by the following reasons:
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The very smooth grains of rounded gravel have the effect that, when compacting, they tend to leave their imprints in the fine-grained fraction of the wet earth and thus promote the creation of more humid gaps between coarse grains and the clayey fraction. These gaps may become more significant after drying due to clay shrinkage. In this way, the fine fraction is practically the only one to support stress at the beginning of compression. This may also justify why the tangent modulus of the rounded gravel-earth material decreases faster than that of the angular gravel-earth material, as the compressive stress increases. This dissociation of the clay fraction from the gravel grains leads, obviously, to a decrease in strength. Note that the more intense the compaction is, the higher the density increases and the more simultaneously this dissociation worsens.
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Concerning the peak strain corresponding to the maximum compressive stress, the results obtained show that it increases when adding angular gravel and relatively decreases when adding rounded gravel (Fig 19). Which doesn't necessarily mean that the angular shape provides better improvement in the compacted earth elasticity, because this increase in peak strain is associated with an increase in maximum stress (i.e. compressive strength). Instead, secant modulus at maximum stress may be a better parameter to assess the stiffness of compacted earthen materials. The secant modulus " 𝐸𝑠 " can be calculated using the following equation: 𝑓 𝐸𝑠 = 𝑐⁄𝜀𝑠
(𝐸2)
Where “𝑓𝑐 ” is the compressive strength and “𝜀𝑠 ” the peak strain. As for the three mixtures studied, the results show that this parameter increases when adding rounded gravel and decreases when adding angular gravel (Fig. 19), meaning that it is impacted in the same way as the initial
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tangent modulus. Moreover, in both gravel cases, the secant modulus is slightly more than a half of the initial tangent modulus as shown in the table below (Tab. 10). Another phenomenon that may be inf luenced by the grain shape, which was not investigated in this study and which can be investigated in future studies, is the anisotropy of the compacted earth. Indeed, according to a study by Q.-B. Bui et al (2009), « results enable [Authors] to initiate the hypothesis that rammed earth is an isotropic material of the first order if the layers remain adherent to each other » [38]. Except that this study did not take into consideration grain shape. Yet, J.-C. Santamarina et al (2004) found that ellipticity and platiness can lead to anisotropy of mechanical characteristics [19]. Intuitively, it can be said that, given the layered compaction procedure, elongated grains are more likely to be compacted in parallel with layers, especially if their thickness is low, which will certainly increase strength in the perpendicular direction to layers, because in this way the compacted earth will take advantage of what can be called the « reinforced earth effect ».
4. Conclusions and prospects In this experimental study, two types of gravel with the same grain size distribution (rounded natural gravel and angular crushed gravel) were added to sand-earth for use as rammed earth materials. Results show that the optimum compaction references and mechanical behavior in uniaxial compression of compacted earth are impacted by grain shape. Indeed, as illustrated in the table 10 (Tab. 11), it can be concluded that:
The peak strain corresponding to the maximum stress appears to be greater when using angular grain shape, which makes the material relatively less sensitive to displacements.
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Whatever is the grain shape, the optimum compaction moisture content decreases with the main grain size. The optimum compaction moisture content of rounded gravel-earth mixture is relatively lower than the one of angular gravel-earth mixture. Within the limits of the grain size distribution presented, the maximum dry density of gravel-earth mixtures increases when the gravel used is rounded and remains practically unchanged when using angular gravel. Within the limits of the grain size distribution presented, compressive strength appears to be more improved when using angular gravel than when using rounded gravel. Globally, the rounded grain shape makes the stiffness of compacted earth materials increase while the angular shape makes it decrease.
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In brief, the angular grain shape provides strength for compacted earthen materials, while the rounded grain shape improves their stiffness. Therefore, grain shape should be taken into account when considering adding aggregates to an earth in order to modify its grain size distribution as a building material.
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Of course, it is not always relevant to consider selecting crushed gravel because it usually involves extra work and costs. Indeed, much better effects can be obtained by using a small amount of stabilizer such as lime or cement. However, the results obtained suggest that the grain shape is a parameter that should not be ignored when establishing the optimal grain size curve. This may explain why some earth used in ancient unstabilized rammed earth constructions perform well despite not complying with the recommendations mentioned in this paper. Anyway, this is a possibility that is worth exploring as a perspective for this work.
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5. Limitations of the study
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The results discussed in this paper were obtained for gravel-earth mixtures prepared in such a way as to have the same grain size distribution. Note that the transducers used during compression tests for measuring axial strain, are disconnected from the specimen due to cracks appearing just after the failure, which does not provide reliable measurements to analyze the post-failure behavior of the materials studied. This work can be improved by testing several mixtures in order to find the optimum gravel percentage. Also, the targeted optimal grain size curve is based on the Thomson-Fuller equation, which uses a certain gradation coefficient that depends on grain shape. It would therefore be more appropriate if, for each gravel-earth, a different value for this coefficient were used to establish its optimal grain size curve. But since gravel represents only 25% of the gravel-earth mixture and as the value taken for this coefficient (𝑛 = 0.25) in this study is an indicative value, one would be tempted to consider that a slight change in this coefficient will not have significant impact on mechanical behavior. Anyway, this remains one of the possibilities to be explored as in further research projects. It is appropriate to contextualize the problem covered by this paper. Indeed, not all earths require the addition of aggregates (sand and/or gravel) as a building material. Also, the specimen manufacturing procedures are not the same as those applied on site when building with rammed earth.
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Conflict of interest No conflict of interest to declare.
Acknowledgment
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The experimental work described in this paper was carried out in the civil engineering laboratory of National School of Applied Sciences (ENSA), Ibn Zohr University of Agadir. The contribution of the laboratory team in terms of technical assistance is duly acknowledged. Thanks also to all the staff of the Civil Engineering Department at the same Institution for their support and encouragement.
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References
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38]
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[11]
-p
[10]
re
[5] [6] [7] [8] [9]
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[4]
na
[3]
ur
[2]
E. Araldi, E. Vincens, A. Fabbri, and J. P. Plassiard, “Identification of the mechanical behaviour of rammed earth including water content influence,” Mater. Struct., vol. 51, no. 4, 2018. Q. B. Bui, J. C. Morel, S. Hans, and P. Walker, “Effect of moisture content on the mechanical characteristics of rammed earth,” Constr. Build. Mater., vol. 54, pp. 163–169, 2014. P. A. Jaquin, C. E. Augarde, D. Gallipoli, and D. G. Toll, “The strength of unstabilised rammed earth materials,” Geotechnique, vol. 59, no. 5, pp. 487–490, 2009. S. Kenai, R. Bahar, and M. Benazzoug, “Experimental analysis of the effect of some compaction methods on mechanical properties and durability of cement stabilized soil,” J. Mater. Sci., vol. 41, no. 21, pp. 6956–6964, 2006. F. S. Khan, S. Azam, M. E. Raghunandan, and R. Clark, “Compressive strength of compacted clay-sand mixes,” Adv. Mater. Sci. Eng., 2014. H. Houben and H. Guillaud, Traité de Construction en Terre, 2nd ed. Marseille, France: Editions Parenthèses, 1995. Standards Australia and P. Walker, HB 195 - The Australian earth building handbook. Australia, 2002. V. Maniatidis and P. Walker, A review of rammed earth construction. Bath, UK: University of Bath, 2003. A. Koutous and E. Hilali, “A proposed experimental method for the preparation of rammed earth material,” Int. J. Eng. Res. Technol., vol. 8, no. 07, pp. 345–354, 2019. A. S. Rajagopal, A. Veeraragavan, and C. E. G. Justo, “A simplified approach for mix design based on shape factors of coarse aggregates,” Bull. Int. Assoc. Eng. Geol., vol. 30, pp. 123–126, 1984. H. N. Abhilash and J.-C. Morel, “Stress–Strain Characteristics of Unstabilised Rammed Earth,” in Earthen Dwellings and Structures, Springer Nature Singapore Pte Ltd., 2019, pp. 203–214. S. A. Naeini, B. Naderinia, and E. Izadi, “Unconfined compressive strength of clayey soils stabilized with waterborne polymer,” KSCE J. Civ. Eng., vol. 16, no. 6, pp. 943–949, 2012. C. G. Rocco and M. Elices, “Effect of aggregate shape on the mechanical properties of a simple concrete,” Eng. Fract. Mech., vol. 76, pp. 286–298, 2009. D. Sarkar, M. Goudarzy, and D. König, “An interpretation of the influence of particle shape on the mechanical behavior of granular material,” Granul. Matter, vol. 21, no. 53, pp. 1/24-24/24 (online), 2019. G.-C. Cho, J. Dodds, and J. C. Santamarina, “Particle Shape Effects on Packing Density, Stiffness, and Strength: Natural and Crushed Sands,” J. Geotech. Geoenvironmental Eng., vol. 132, no. 5, pp. 591–602, 2006. K. A. Alshibli and M. B. Cil, “Influence of Particle Morphology on the Friction and Dilatancy of Sand,” J. Geotech. Geoenvironmental Eng., vol. 144, no. 3, p. 04017118, 2018. S. Yagiz, “Brief note on the influence of shape and percentage of gravel on the shear strength of sand and gravel mixtures,” Bull. Eng. Geol. Environ., vol. 60, pp. 321–323, 2001. S. Salimi, V. Yazdanjou, and A. Hamidi, “Shape and size effects of gravel grains on the shear behavior of sandy soils,” in Proceedings of the 10th International Conference on Landslides and Engineered Slopes., 2008, pp. 469–474. J.-C. Santamarina and G.-C. Cho, “Soil behaviour : The role of particle shape,” in Proceedings of The Skempton Conference, 2004. I. Cavarretta, M. Coop, and C. O’Sullivan, “The influence of particle characteristics on the behaviour of coarse grained soils,” Géotechnique, vol. 60, no. 6, pp. 413–423, 2010. J. K. Mitchell and K. Soga, Fundamentals of soil behavior, 3rd Editio. Hoboken, New Jersey: John Wiley & Sons, Inc., 2005. AFNOR, NF EN 933-3 Tests for geometrical properties of aggregates - Part 3 : Determination of particle shape - Flakiness index. France, 1997, p. 11. AFNOR, NF P 94-095 Soils : Investigation and testing - Determination of moisture content - Oven drying method. France, 1995, p. 7. AFNOR, NF P 94-056 Soils : Investigation and testing - Particle-size analysis - Method by dry sieving after washing. France, 1996, p. 15. AFNOR, NF P 94-057 Soils : Investigation and testing - Particle-size analysis - Sedimentation method. France, 1992, p. 17. AFNOR, NF P 94-051 Soils : Investigation and testing - Determination of Atterberg’s limits. Liquid limit test using Casagrande apparatus. Plastic limit test on rolled thread. 1993, p. 15. AFNOR, NF P 94-054 Soils : Investigation and testing - Determination of particle density - Pycnometer method. France, 1991, p. 6. AFNOR, NF EN 1097-6 Tests for mechanical and physical properties of aggregates — Part 6 : Determination of particle density and water absorption. France, 2001, p. 34. AFNOR, NF P 94-093 Soils : Investigation and testing - Determination of the compaction characteristics of a soil - Standard Proctor test Modified Proctor test. France, 1999, p. 18. ASTM, D 2487 - 06 Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System). USA, 2006, p. 12. ASTM, D 3282 - 93 Standard Practice for Classification of Soils and Soil-Aggregate Mixtures for Highway Construction Purposes. USA, 2004, p. 6. AFNOR, NF P 94-074 Soils : Investigation and testing - Shear strength tests with revolving triaxial test apparatus - § 5.2 Specimen size requirements. France, 1994, p. 36. AFNOR, NF EN 12390-7 - Testing hardened concrete - Part 7 : density of hardened concrete. 2001, p. 9. AFNOR, NF EN 12390-4 - Testing hardened concrete - Part 4 : compressive strength - Specification for testing machines. France, 2000, p. 14. J. J. Wang, Y. Yang, and H. P. Zhang, “Effects of Particle Size Distribution on Compaction Behavior and Particle Crushing of a Mudstone Particle Mixture,” Geotech. Geol. Eng., vol. 32, no. 4, pp. 1159–1164, 2014. F. Champiré, A. Fabbri, J. C. Morel, H. Wong, and F. McGregor, “Impact of relative humidity on the mechanical behavior of compacted earth as a building material,” Constr. Build. Mater., vol. 110, pp. 70–78, 2016. V. Maniatidis and P. Walker, “Structural Capacity of Rammed Earth in Compression,” J. Mater. Civ. Eng., vol. 20, no. 3, pp. 230–238, 2008. Q. B. Bui and J. C. Morel, “Assessing the anisotropy of rammed earth,” Constr. Build. Mater., vol. 23, no. 9, pp. 3005–3011, 2009.
Jo
[1]
9
b
Jo
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na
lP
re
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ro of
a
c Fig. 1 : Photos showing the three materials used (Sand-earth, Rounded gravel and Angular gravel).
10
100
Rounded Gravel
90
Angular Gravel
80
Adjusted Gravel
Passing (%)
70 60 50 40 30 20 10 0
4
5
6.3
8
10
12.5
16
20
Grains sizes (mm)
Fig. 2 : Grain size curves of the two gravels before and after adjustments made.
90
R.E. Spindle
80
Sand-Earth Gravel-earth
70
ICD20 [RE]
60 50 40
-p
Passing (%)
ro of
100
30
10
0.01
0.1 1 Grains sizes (mm)
10
100
lP
0 0.001
re
20
Fig. 3 : Grain size curves of sand-earth before and after adding gravel, compared to the recommended rammed earth spindle (R.E. Spindle by Houben and Guillaud) and to the ideal curve (ICD20 [RE]).
na
10
0/5 Granular fraction
10
Jo
5 × 36
ur
0/20 Granular fraction
6
7
5 4
25 8
3 2
1
Distribution of rammer blows on a layer of material.
100
Fig. 4 : Illustration of the manufacturing methods applied to the specimens.
11
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2.25 2.00 1.75
re
1.50 1.25 1.00 0.75 0.50 0.25 0.00 10
20
30
40
na
0
lP
Stress ( MPa )
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Fig. 5 : Measurement of specimen wet mass after demolding (RG2 specimen).
Time ( s )
Jo
ur
Fig. 6 : Typical loading speed curve during the compression tests (RG1 specimen).
12
ro of
Earth + R. Gravel
2.00
Earth + A. Gravel
1.95
lP
1.90 1.85 1.80 1.75 1.70 1.65 1.60 6.0
8.0
10.0
na
Dry Density (103 kg.m-3)
Sand + Earth
100% Saturation lines
2.05
re
2.10
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Fig. 7 : Compression device with the two displacement transducers fixed on one of the tested specimens (SE1 specimen).
12.0
14.0
16.0
ur
Moisture Content (%)
Jo
Fig. 8 : Compaction test curves of the three materials studied (Proctor test).
13
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Fig. 9 : One of the series of specimens after drying (SE1, SE2 and SE3 sand-earth).
2.50 𝑓𝑐 = 2.24 𝑀𝑃𝑎
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1.50
𝐸𝑖 = 347 𝑀𝑃𝑎 𝐸𝑠 = 189 𝑀𝑃𝑎
1.00
re
Stress (MPa)
2.00
lP
0.50 𝜀𝑝 = 0.0119
0.00 0.0000
0.0025
0.0050
0.0075
0.0100
Axial strain (mm/mm)
0.0125
0.0150
na
Fig. 10 : Typical stress–strain curve for the compacted earthen material in the compression test (AG1 specimen).
SE
ur
2.50
RG
2.00
Stress ( MPa )
1.50
1.00
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AG
0.50
0.00 0.0000
0.0025
0.0050
0.0075
0.0100
0.0125
0.0150
Axial strain ( mm/mm )
Fig. 11 : Superimposed stress-strain curves resulting from all compression tests
14
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Fig. 12 : One of the specimens after being compressed until failure (AG1 specimen).
2.40 SE 2.30
AG
2.20 2.10
lP
Stress ( MPa )
re
RG
2.00
1.80 0.0050
0.0075
na
1.90
0.0100
0.0125
0.0150
Axial strain ( mm/mm )
Jo
ur
Fig. 13 : Superposed stress-strain curves resulting from all compression tests, Enlarged view of the end of the tests
15
Compressive strength (MPa)
2.50
2.30
2.10
1.90
1.70 2.03
2.13
2.24
SE
RG
AG
1.50
Fig. 14 : Comparison of the average compressive strengths of the three materials tested (𝑬𝒓𝒓𝒐𝒓 𝒃𝒂𝒓𝒔 = 𝑺𝒕𝒂𝒏𝒅𝒂𝒓𝒅 𝒆𝒓𝒓𝒐𝒓𝒔)
0.45
SE
0.40
RG
0.35
AG
ro of
0.50
0.25 0.20
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Stress ( MPa )
0.30
0.15 0.10
0.00 0.0000
0.0005
0.0010
0.0015
Axial strain ( mm/mm )
re
0.05
lP
Fig. 15 : Superposed stress-strain curves resulting from all compression tests, Enlarged view of the beginning of the tests
na ur
450
400
350
300
Jo
Initial tangent modulus (MPa)
500
340
437
352
SE
RG
AG
Fig. 16 : Comparison of the average initial tangent modulus of the three materials tested (𝑬𝒓𝒓𝒐𝒓 𝒃𝒂𝒓𝒔 = 𝑺𝒕𝒂𝒏𝒅𝒂𝒓𝒅 𝒆𝒓𝒓𝒐𝒓𝒔)
16
500
SE
Initial tangent modulus (MPa)
480
RG
460
AG
440 420 400 380 360 340 320 300 1960
2010
2060
2110
Dry density (kg/m3)
Fig. 17 : Initial tangent module against the dry density
ro of
2.50
2.30
AG
2.20
RG
2.10
-p
Compressive strength (MPa)
2.40
SE
2.00
1.80 325
375
425
475
Initial tangent modulus (MPa)
re
1.90
lP
Fig. 18 : Compressive strength against initial tangent modulus
na
2.50
ur
1.50
1.00
0.50
SE
ES = 215 MPa
RG
ES = 232 MPa
Jo
Compressive strength (MPa)
2.00
ES = 195 MPa
0.00 0.000
AG
0.005 0.010 Peak strain (mm/mm)
0.015
Fig. 19 : Compressive strength against peak strain (𝑬𝒓𝒓𝒐𝒓 𝒃𝒂𝒓𝒔 = 𝑺𝒕𝒂𝒏𝒅𝒂𝒓𝒅 𝒆𝒓𝒓𝒐𝒓𝒔)
17
Tab. 1 : List of Standards used in the present study.
Standard applied
Water content
NF P 94-050 [23]
Sieving
NF P 94-056 [24]
Sedimentation
NF P 94-057 [25]
Atterberg limits
NF P 94-051 [26]
Absolute density
NF P 94-054 [27]
Flakiness index
NF EN 933-3 [22]
Water absorption
NF EN 1097-6 [28]
Compaction (Proctor)
NF P 94-093 [29]
Earth classification
ASTM D2487 [30] & D3282 [31]
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Method / Test
Specimen requirements NF P 94-074 [32] NF EN 12390-7 [33]
Compression machine
NF EN 12390-4 [34]
re
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Specimen density
Tab. 2 : Characteristics of the fine-grained fraction of the sand-earth used.
Liquid limit 𝝎𝒍
2570 𝑘𝑔/𝑚3
32.8%
Plasticity index 𝑰𝑷
lP
Grains density 𝝆𝒔
10.6%
Rounded Gravel 𝑹𝑮%
Angular Gravel 𝑨𝑮%
Adjustment to make 𝑨𝑮% − 𝑹𝑮%
4/5
4.6 %
6.5 %
+ 1,9 %
5/6.3
8.4 %
13.4 %
+ 5,0 %
6.3/8
17.2 %
25.1 %
+ 7,9 %
8/10
20.2 %
15.5 %
– 4,7 %
10/12.5
17.5 %
10.8 %
– 6,7 %
12.5/16
19.5 %
15.3 %
– 4,2 %
16/20
12.6 %
13.4 %
+ 0,8 %
ur
Granular Fraction 𝒅/𝑫
Jo
na
Tab. 3 : Calculation of adjustments to be made to the gravels grain size distributions.
18
Tab. 4 : Characteristics of the two gravels used.
Angular Gravel
Gravel
Rounded Gravel
The two gravels come from a crushing quarry located in a local river (Souss River), whose sediments are petrographically heterogeneous.
Origin (Fig. 1-b & 1-c)
Absolute density 𝝆𝒔 (𝒌𝒈⁄𝒎𝟑 ) [27]
2 710
2 710
Flakiness index 𝑰𝒇 (%) [22]
23 %
11 %
Water absorption 𝑾𝒂𝒃𝒔 (%) [28]
1.4 %
0.7 %
Material
Sand-Earth
Gravel-Earth
AASHTO [31]
A-6
A-2-6
USCS [30]
SC
GC
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Tab. 5 : Classification of sand-earth used before and after adding gravel.
R. GravelEarth
A. GravelEarth
𝝎𝑶𝑴𝑪 (%)
13.0
9.5
10.5
𝝆𝒎𝒂𝒙 (𝒌𝒈/𝒎𝟑 )
1 900
2 000
1 970
re
SandEarth
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Material
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Tab. 6 : Effects of adding gravel to sand-earth on its compaction references: Optimum compaction moisture content (𝝎𝑶𝑴𝑪 ) and Maximum oven dry density (𝝆𝒎𝒂𝒙 ).
19
Tab. 7 : Average compaction conditions of the specimens of the three materials studied.
Material
SandEarth
A. GravelEarth
Compaction water content
13.0 %
Wet mass 𝒎𝒉 (𝒌𝒈)
3.490
3.580
3530
Dry mass 𝒎𝒅 (𝒌𝒈)
3.110
3.280
3.220
Dry density 𝝆𝒅 (𝒌𝒈/𝒎𝟑 )
1 980
2 090
2 050
Average compactness
77.0%
80.1%
78.5%
𝝆𝒅 ⁄𝝆𝒎𝒂𝒙 Average Ratio
104%
105%
104%
9.5 %
10.5 %
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R. GravelEarth
Tab. 8 : Summary of the results of the compression tests conducted on the three materials studied.
233
0.0083
1.94
SE2
1990
347
199
0.0109
2.16
SE3
1980
340
214
0.0093
1.98
RG1
2110
463
251
0.0081
2.03
RG2
2070
411
223
0.0099
2.22
RG3
2090
436
221
0.0097
2.15
AG1
2040
347
189
0.0119
2.24
AG2
2100
367
198
0.0120
2.37
AG3
2010
341
199
0.0107
2.12
re
332
Jo
ur
Angular Gravel-Earth (AG)
1970
lP
Rounded Gravel-Earth (RG)
SE1
na
Sand-Earth (SE)
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Initial tangent Dry density Secant modulus Peak strain Compressive modulus 3 𝜌𝑑 (𝑘𝑔⁄𝑚 ) 𝐸𝑠 (𝑀𝑃𝑎) 𝜀𝑠 (𝑚𝑚⁄𝑚𝑚) strength 𝑓𝑐 (𝑀𝑃𝑎) 𝐸𝑖 (𝑀𝑃𝑎)
Specimens
20
Tab. 9 : Average characteristics of the three compacted earthen materials (coefficient of variation).
Dry density 𝜌𝑑 (𝑘𝑔⁄𝑚3 )
Initial tangent modulus 𝐸𝑖 (𝑀𝑃𝑎)
1980 (0%)
340 (2%)
215 (6%)
0.0095 (11%)
2.03 (5%)
Rounded GravelEarth (RG)
2090 (1%)
437 (5%)
232 (5%)
0.0092 (9%)
2.13 (4%)
Angular GravelEarth (AG)
2050 (2%)
352 (3%)
195 (2%)
0.0115 (5%)
2.24 (5%)
Material
Sand-Earth (SE)
Compressive Secant modulus Peak strain strength 𝐸𝑠 (𝑀𝑃𝑎) 𝜀𝑠 (𝑚𝑚⁄𝑚𝑚) 𝑓𝑐 (𝑀𝑃𝑎)
Tab. 10 : Secant modulus compared to Initial tangent modulus (coefficient of variation)
𝑬𝒔 (𝑴𝑷𝒂)
𝑬𝒔 /𝑬𝒊
SE
340 (2%)
215 (6%)
0.63 (8%)
RG
437 (5%)
232 (5%)
0.53 (3%)
AG
352 (3%)
195 (2%)
0.55 (4%)
(𝐸3)
-p
𝐼𝑛 𝑎𝑣𝑒𝑟𝑎𝑔𝑒: 𝐸𝑠 ≈ 0.57 𝐸𝑖
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𝑬𝒊 (𝑴𝑷𝒂)
Material
Gravel grain shape
Rounded
Angular
−−
−
Effect on 𝜌 𝑑𝑚𝑎𝑥
++
±
Effect on 𝑓𝑐
±
Effect on 𝐸𝑖 and 𝐸𝑠 Effect on 𝜀𝑝
+
++
+
na
±
−
(+) : increase
(±) : to be tested
Jo
ur
(−) : decrease
lP
Effect on 𝜔𝑂𝑀𝐶
re
Tab. 11 : Grain shape effects on compacted earth proprieties.
21
PII:
S2214-5095(19)30405-X
DOI:
https://doi.org/10.1016/j.cscm.2019.e00303
Reference:
CSCM 303
To appear in:
Case Studies in Construction Materials
Received Date:
22 August 2019
Revised Date:
26 October 2019
Accepted Date:
29 October 2019
Please cite this article as: Koutous A, Hilali E, Grain shape effects on the mechanical behavior of compacted earth, Case Studies in Construction Materials (2019), doi: https://doi.org/10.1016/j.cscm.2019.e00303
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Grain shape effects on the mechanical behavior of compacted earth KOUTOUS (1) * E. HILALI (1)
(1)
MMGC Lab., National School of Applied Sciences, Ibn Zohr University, Agadir 80000, Morocco.
(*)
Corresponding author. Tel.: +212668 82 17 97, E-mail address: [email protected].
HIGHLIGHTS Sand-earth, rounded gravel and angular gravel are the three materials used in this study. Sand-earth, rounded gravel-earth and angular gravel-earth mixtures were prepared and compacted. The three earthen materials produced were tested in uniaxial compression tests. Axial strain has been measured using displacement transducers attached to cylindrical specimens. The mechanical behavior of compacted earthen materials appears to be influenced by grain shape and size.
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ARTICLE INFO Article history:
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Submitted 22 August 2019
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Revised 26 October 2019
ABSTRACT
Jo
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The main purpose of this paper is to experimentally explore the effects of grain shape and size on the mechanical behavior of compacted earthen materials. Sand-earth, natural rounded gravel and crushed angular gravel are the three materials used in this study. Both gravels are from the same site and characterized by the same grain size curve. The uniaxial compression tests, the results of which are presented and discussed in this paper, were performed on cylindrical specimens of three materials: sand-earth mixture, rounded gravel-earth mixture and angular gravel-earth mixture. The tested specimens were prepared under optimum compaction references, using the Proctor test procedures. For each compression test, four parameters are determined: the compressive strength, the initial tangent modulus, the secant modulus at maximum stress and the peak axial strain corresponding to the maximum compressive stress. The results obtained show that the mechanical behavior of gravel-earth mixture may be influenced by grain shape of the gravel used, thus introducing a new parameter to be taken into account when preparing unstabilized rammed earth material. Further experimental studies are recommended to better assess these results
Keywords: Rammed earth; Mechanical behavior; Grain shape; Compressive strength; Initial tangent modulus; Secant modulus; Peak strain
1. Introduction Compacted earth is an ancestral building material. The most common method employed to use this material is called rammed earth, which may or may not be stabilized using additives such as cement, lime, natural fibers, etc. Over the past few decades, studies have shown that physical and mechanical characteristics of compacted earth depends on many parameters including water content [1][2][3], compaction method and energy [4], clay content [5], grain size distribution, etc. As for grain size distribution, optimal granular spindles have been recommended for the rammed earth method in order to obtain the most suitable material without adding stabilizers (i.e. unstabilized rammed earth). The most well-known spindle seems to be the one established by
1
Houben and Guillaud (1995, [6]). These grain size spindles are included in Standards used in some countries such the Australian code HB 195 [7]. Others spindles can be found in a review on rammed earth by Maniatidis and Walker (2003, [8]). The curves, describing these grain size spindles and recommended by these engineering references, are based, as explained in a recent article by Koutous and Hilali (2019, [9]), on the following Fuller-Thompson formula [10]: 𝑑 𝑛 𝑃(%) = 100 ( ) 𝐷
(𝐸1)
Where "𝐷" represents the maximum grain diameter and "𝑃" the percentage of grains with diameters less than a given diameter "𝑑". And where "𝑛" is a parameter called gradation index, whose value depends on the grain shape (𝑛 = 0.5 for spherical grains and 𝑛 = 0.20 𝑡𝑜 0.25 for earths used as building materials). Grain shape was therefore taken into account in some way when establishing the optimal grain size curve. Indeed, the Fuller-Thompson formula, used to define this curve, was established in order to enhance the material density, and consequently improve its strength, because it is known that the strength increases as the density increases [11]. However, these optimal curves are approximate because the criteria for choosing the value of the gradation index " 𝑛 " are not clear.
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On the other hand, and as mentioned at the beginning of this introduction, the grain size is not the only factor that influences the strength of the compacted earth. There is another factor as important as grain size, which the plasticity of the earth fine-grained fraction. Indeed, a study by S. Naeini et al (2012, [12]) reported that plasticity index has a significant effect on uniaxial compressive strength, as the latter decreases when plasticity index increases. Also, they found that « as the plasticity index increases, the [earth] yields at a higher strain. Therefore, the higher plasticity index could change the stress-strain behavior of [earth] samples from a brittle to ductile manner ».
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For these reasons, it is of scientific interest to carry out studies comparing the mechanical behavior of earth compacted under the same conditions, and having the same characteristics in terms of grain size, plasticity and mineral composition, but whose grain shape is different. This is the purpose of the present study.
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While it is known that the mechanical behavior of conglomerates such as concrete is influenced by the shape of the aggregates used [13], there are currently no well-known studies that can be found in the literature about the impact of grain shape of compacted earth on its mechanical behavior under uniaxial compression. But there are some articles that deal with grain shape effects on the mechanical behavior of granular materials (generally noncohesive materials).
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Among these studies, an article by D. Sarkar et al (2019, [14]) in which the authors compare the mechanical behavior of three different granular materials under triaxial compression or direct shear conditions. The results of this comparative study show that « crushed angular materials (having low values of regularity) exhibit comparatively higher strength than the rounded materials ». The same observation was made regarding the friction angle. As for strain, rounded material seems to have a « very small contraction even for loose materials after which dilative tendencies start creeping in ».
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A study by G-C. Cho et al. (2006, [15]) about the influence of grain shape on physical and mechanical characteristics of natural sands compared to crushed sands shows that the decrease in sphericity and/or roughness of sand grains leads to « increase in extreme void ratios and void ratio interval, decrease in smallstrain stiffness, yet increased sensitivity to the state of stress, increase in the compressibility under zero-lateral strain loading, increase in the constant volume critical state friction angle; and increase in the critical state line intercept, and a weak effect on the slope of the critical state line (void-stress space) ». Similar results have been reported in an article by K.-A. Alshibli et al (2018, [16]) on the influence of grain morphology on the mechanical behavior of sands under triaxial compression. A brief note by S. Yagiz (2001, [17]), on the effects of the shape and percentage of gravel on the shear strength of sand-gravel mixtures, showed that the shear strength and internal friction angle increase linearly with the percentage of gravel in the mixture and that when crushed angular aggregates are used, this increase becomes much more noticeable. Almost the same results were presented in an article by N. S. Salimi et al. (2008, [18]). In a conference paper, J-C. Santamarina and G-C. Cho (2004) reported that « ellipticity and platiness […] promote inherent anisotropy and affect the evolution of stress-induced anisotropy » [19]. In an article by I. Cavarretta et al (2010, [20]), it was reported that at the micro-scale, grain compression tests show an initial behavior dominated by plasticity that depends on the intrinsic characteristics of the grains (size, roundness, roughness, elastic properties and surface hardness). On the other hand, at the macroscopic level, the mechanical behavior of granular materials seems to be less sensitive to the characteristics of grain roughness (and hence inter-grain friction). The explanation given for this lack of sensitivity to friction is that « the stress levels were too high for the increase in inter-particle friction to be retained during initial, isotropic compression of
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the sample […] The contact forces between particles along the strong force chains are likely to be high enough to remove the roughening effect ». These research studies, as interesting as they are, deal only with cohesionless granular materials. However, earth used as building material must be cohesive, especially when it comes to unstabilized rammed earth. This is particularly the case of the earth covered by the present paper. To describe grain shape in detail, J-K. Mitchell and K. Soga, (2005, [21]) distinguish three terms for three scales: sphericity (antonym: elongation) on a large-scale scale, roundness (antonym: angularity) on intermediate scale and roughness (antonym: smoothness) on small-scale. The present paper discusses the effects of grain morphology (large to intermediate scale) on the mechanical behavior of compacted earth. The parameter used to quantify this shape is flakiness index [acc. to NF EN 933-3 [22]]. The determination of flakiness index consists of double sieving. First, by using standard sieves, the sample is fractionated into different " 𝑑𝑖 /𝐷𝑖 " granular fractions, as shown in table 3 (Tab. 3). Then, each of the granular fraction " 𝑑𝑖 /𝐷𝑖 " is sieved using grid sieves with a spacing width " 𝐷𝑖 /2 ". The flakiness index is calculated as a percentage of the total mass of particles passing through the grid sieves to the total dry mass of the particles under test.
2. Materials and methods
2.1.
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Three materials are used in this case study: a sand-earth, a rounded gravel and an angular gravel. The two types of gravel (rounded and angular) actually form a single semi-crushed gravel from a local river (the Souss River, near Agadir, Morocco). They were first sorted and then used separately. Standards used in the study
The following table (Tab. 1) shows the Standards used in the study covered by this paper. 2.2.
Sand-earth
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The sieving and sedimentation tests [acc. to NF P 94-056 [24] and NF P 94-057 [25] respectively], show that the grain size curve of the earth used in this experimental study (Fig. 3) fits well into the grain size spindle recommended by H. Houben and H. Guillaud (1995, [6]) for rammed earth method.
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The sand-earth material (Fig. 1-a) used in this case study is a mixture of two local materials: a clayey soil and a crushed sand. The mass proportions of the two materials in the mixtures, [2⁄3 𝑒𝑎𝑟𝑡ℎ + 1⁄3 𝑠𝑎𝑛𝑑], are determined in such a way that the mixture has a grain size curve that fits well into the above-mentioned grain size spindle. More details can be found in a previous study [9].
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The characteristics of the fine-grained fraction of the Sand-earth are summarized in the following table (Tab. 2).
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Based on these characteristics, the sand-earth studied is a clayey sand (type "SC") according to USCS – the Unified Soil Classification System [acc. to ASTM D3287 [30]], or type "A-6" according to AASHTO – the American Association of State Highway and Transportation Officials Classification System [acc. to ASTM D3282 [31]] (Tab. 5). Note that these results are not affected when gravel is added, since Atterberg limit tests are performed on 0/0.4 𝑚𝑚 fraction [acc. to NF P 94-051 [26]]. Gravels
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2.3.
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The two gravels are originally mixed and come from the same crushing quarry in a local river (the Souss River, near Agadir City, Morocco). They were therefore manually separated into rounded grains characterized by their smoothness and angular grains characterized by their roughness (Fig. 1-b and 1-c). Sieving tests were then carried out on the tow gravels. Comparing the two gravels grain size curves, there is a slight difference. To make them the same, adjustments were made keeping the lowest proportion of each granular fraction as shown in the table above (Tab. 3). Then, their absolute densities, flakiness indexes and water absorption percentages were determined (Tab. 4). The procedures followed to determine these parameters are those of the Standards given in the table (Tab. 1). Once the adjustments have been made, the two gravels have the same grain size curve as shown in figure below (Fig. 2). After these adjustments, two gravel-earth mixtures were prepared with the following mass proportions: ¾ 𝑒𝑎𝑟𝑡ℎ + ¼ 𝑔𝑟𝑎𝑣𝑒𝑙. These mass proportions are determined so that the grain size curve of the mixture is as close as possible to the optimal curve -ICD20 [RE]- (Fig. 3). This optimal curve -ICD20 [RE]- is the curve representing the equation « E1 » with the coefficient " 𝑛 " taken equal to 0.25, according to Houben and Guillaud recommendations [6].
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The two mixtures, thus prepared, which are rounded gravel-earth and angular gravel-earth are clayey gravel, type "GC" according to USCS system, or type "A-2-6" according to AASHTO system (Tab. 5). 2.4.
Preparation of specimens
Specimens dimensions and compaction references
Taking into account grain size distributions of the materials used, the minimum specimen size is taken equal to 100 𝑚𝑚 (i.e. five times the maximum grain size) [acc. to NF P 94-074 [32]]. Thus, for cylindrical specimens, the diameter is taken equal to 100 𝑚𝑚 while the height is taken equal to 200 𝑚𝑚 (i.e. twice the diameter). As a result, the dimensions of the mold for specimen manufacturing are 100 𝑚𝑚 × 200 𝑚𝑚. Compared to those of the molds used in Proctor tests, the standard Proctor test is chosen to determine compaction references using five samples with different water content per each material [acc. to NF P 94-093 [29]].
Manufacturing process
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Once the compaction references were determined, three specimens were prepared per each of the three materials studied. To manufacture these specimens, a maximum dry density equal to that obtained in Proctor test ±5% is targeted. To achieve this purpose, and in order to keep the compaction energy the same, the rammer and compaction procedures of standard Proctor test were used with 5 layers for a height of 200 𝑚𝑚 (height of the mold to be used for specimen manufacturing) instead of 3 layers for a height of 116.5 𝑚𝑚 (height of the Proctor mold). This means that the compaction of the specimens consists of 5 layers compacted with 25 blows each (3 sequences of 8 blows distributed in a circular manner, plus a 25th blow in the center) as illustrated in the figure below (Fig. 4). The water content used in each mix is the corresponding optimum moisture content given in the part 3.1 (Tab. 6). To improve the quality of specimen bearing surfaces, a thin layer containing only grains with sizes of less than 5 𝑚𝑚 is provided on each of the two bearing surfaces. Consequently, each layer is approximately 3.6 𝑐𝑚 thick, except the thin layers (bearing surfaces) whose thickness is about 1 𝑐𝑚.
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As no additional material is used for the preparation of the specimen bearing surfaces, the 0/5 angular fraction is considered as a sort of plaster for this preparation, because it is easier to level without damaging the specimens. The preparation method applied are detailed in section 2.5 (§ 2.5).
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The demolding is done immediately. Then, the specimen wet mass is measured and thus the wet density determined (Fig. 5). Concerning drying conditions, specimens were stored in a room with temperatures varying from 20 to 25 °C and relative humidity of about 60%.
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After four weeks, the weights and dimensions of each specimen are measured, then the densities are determined, according current European standard on concrete material [acc. to NF EN 12390-7 [33]], and compared to the aimed dry densities.
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These comparisons are used to “check” the drying of the specimens. The drying of a specimen is achieved if its density is equal to the one targeted by compaction ±5% and if the weight of the specimen does not decrease by more than 2/1000 after 48 hours under the drying conditions above.
2.5.
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Note that the “dry state” of the compacted material means that the water content has reached its equilibrium value close to and not equal to zero. Uniaxial compression test
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The preparation of the bearing surfaces is performed in the wet state by levelling using a metal ruler. In the dry state, an abrasive paper and a spirit level are used, if necessary, for finishing. The aim of this preparation is to provide parallel and flat bearing surfaces. After that, the dry specimens are compressed using a machine that complies with the European Standard [acc. to NF EN 12390-4 [34]] and allow a constant loading speed of 0.05 𝑀𝑃𝑎 ⁄𝑠 (Fig. 6). A value of 0.05 𝑀𝑃𝑎 has been chosen as peak sensitivity that corresponds to the decrease in stress during the test at which the machine will consider the sample to have failed, and will stop the test. The test apparatus used in this study is equipped with two displacement transducers characterized by a sensitivity of 0.02 𝜇𝑚. The two transducers are arranged symmetrically with respect to the axis of the tested specimen (Fig. 7). The specimen axial strain is the average of the strains calculated from the measurements of the tow transducers.
3. Results and discussions 3.1.
Effects on the compaction references
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The results of the Proctor test show an increase in the maximum dry density and a decrease in the optimum compaction moisture content after adding gravel to sand-earth (Fig. 8). Regarding to the decrease in the optimum compaction moisture content, it can be said that it is obvious that when the proportion of gravel in a soil increases, its optimum moisture content will decrease, because the specific surface area of the soil decreases with the diameter of the grains, especially in the case of smoothgrained soil. This is shown, for example, by a technical note by J-J. Wang et al. (2014, [35]), in which the authors conclude that « the optimum moisture content is generally decreasing with the increase of the median particle diameter or gravel content ». Also, it can be observed that the optimum compaction moisture content of rounded gravel-earth is slightly lower than that of angular gravel-earth, and that inversely, the maximum dry density of rounded gravel-earth is slightly higher than that of angular gravel-earth (Tab. 6). The difference in the optimum compaction moisture content can be explained by the difference in water absorption percentage, which is higher for crushed gravel than for round gravel (1.4% > 0.7%).
3.2.
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As for the maximum dry density, the sand-earth absolute density is smaller than the gravel absolute density (2710 𝑘𝑔⁄𝑚3 > 2570 𝑘𝑔/𝑚3 ), which increases the absolute density of gravel-earth mixture and therefore the dry density of the latter. In addition to that, it appears that the rounded shape of the gravel grains promotes compaction, unlike the angular and flattened shape, which makes compaction harder to achieve as it can be noticed when comparing the coefficients of variation of the dry densities of the specimens compacted (Tab. 9). Effects on the mechanical behavior
The compaction of the specimens is carried out under the optimum requirements previously determined.
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The first observation that can be made is that the average dry densities of the specimens are slightly higher than maximum dry densities obtained during compaction tests (1.04 to 1.05 times higher), and this is the case for the three materials studied (Tab. 7). A possible explanation for this is that the layers thickness applied when making the specimens is lower than that of Proctor test (3.6 𝑐𝑚 < 4 𝑐𝑚), while the compaction energy remains the same.
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This increase in dry density may be due to the fact that the specimens hold onto some moisture content, because, as compared to Proctor tests, the specimens were not dried in an oven (Fig. 9). However, tests carried out on samples of materials recovered after the compression tests show that their water content does not exceed 1% in the worst cases (about 0.9% in the case of angular gravel-earth material). Therefore, this explanation can be dismissed.
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Will this residual moisture content due to the environment relative humidity, no matter how low it is, have an impact on the mechanical behavior of the materials studied? According to a study carried out on three different compacted earths by F. Champiré et al (2016), it appears that an increase in relative humidity in the environment leads to an increase in the moisture content of compacted earth, which affects its mechanical characteristics by decreasing its strength at its Young's modulus [36]. Another study conducted by E. Araldi et al. (2018) leads to similar results [1].
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This means that the values of these two parameters obtained in the present study are actually lower than the results that could be obtained if compression tests were carried out in a real dry state, with a zero-moisture content. Nevertheless, the study presented here is of scientific interest as all samples were prepared, stored and tested under the same conditions. However, it should be kept in mind that the interpretations made in this paper are valid only when these conditions are met.
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Once the “drying” has been reached and the dry density determined, the compression tests are conducted according to the procedures described above (§ 2.5). For each compression test, the stress-strain curve is established (Fig. 11). From this curve, the initial tangent modulus and peak strain are determined graphically as shown in the figure above (Fig. 10). The crushed specimens are carefully examined by looking at the propagation of cracks (Fig. 12). The test is considered successful if the cracks are regular and symmetrical with respect to the axis of the specimen. This is a way of checking whether the load has been applied uniformly to the bearing surfaces, otherwise the test has to be repeated. The table below summarizes the results obtained on a test basis of the compression tests (Tab. 8). Note that the dry density is given to the nearest 10 𝑘𝑔/𝑚3 . As shown in the figure (Fig. 10), the secant modulus is determined as the slope of a line drawn from the origin of the stressstrain diagram and intersecting the curve at the peak stress point.
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The results of these compression tests show a slight but beneficial improvement in compressive strength after adding gravel (Fig. 13). It appears, also, that there is a small difference between the two gravels in terms of strength, which remain within the error range given the number of specimens (3 specimens per mixture). Indeed, there is an increase of 0.10 𝑀𝑃𝑎 in the case of rounded gravel-earth mixture compared to sand-earth, and a relatively better increase of 0.21 𝑀𝑃𝑎 in the case of angular gravel-earth mixture (Tab. 9). Taking into account the range of uncertainties commonly observed for earthen materials and the coefficient of variation of the results obtained (Tab. 9), it can be said that in terms of strength, only the improvement provided by the angular gravel may be considered significant (Fig. 14). It is true that the improvement of 0.1 MPa provided by rounded gravel is significant for an earthen material, but this value remains within the error range and cannot therefore be taken into consideration. While the possible improvement in compressive strength after adding gravel can be explained by the influence of the grain size distribution, which has been made closer to the optimal distribution (ICD20 [RE]) (Fig. 3), the small difference between the two gravel-earth mixtures in compressive strength can be explained on the basis of the studies presented in the introduction to this paper. Obviously, the angular shape and roughness of the crushed gravel grains promote the gravel-clay bond, which increase the mixture strength.
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Concerning the initial tangent modulus, the results show that it is only for rounded gravel-earth material that a significant increase can be observed (Fig. 15): increase of 97 𝑀𝑃𝑎 for rounded gravel-earth material compared to only 12 𝑀𝑃𝑎 for angular gravel-earth material. Considering the error bars, it appears that adding angular gravel had practically no influence on the initial tangent modulus (Fig. 16).
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As the two gravel-earth mixtures studied have the same mineralogical composition and grain size distribution, it can be said, at least at this stage of the study, that the initial tangent modulus depends mainly on the dry density of the compacted material (and thus its compactness), which is influenced by grain shape. Moreover, a correlation can be observed between these two parameters as shown in the figure below (Fig. 17). These results only confirm what has already been reported in previous studies [37].
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On the other hand, when analyzing each material results, it can be noted that, for both sand-earth and angular gravel-earth materials, the initial tangent modulus and compressive strength both increase with the increase of dry density. In contrast, it appears that the compressive strength and initial tangent modulus of rounded gravelearth material vary in opposite ways with dry density. In other words, when the dry density increases, the initial tangent modulus increases and the compressive strength decreases (Fig. 18).
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This finding, which may seem unexpected, cannot be established on the basis of the results obtained for only three specimens. Therefore, a study on more samples is necessary to confirm or contradict it. Nevertheless, this result, if confirmed, can be explained by the following reasons:
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The very smooth grains of rounded gravel have the effect that, when compacting, they tend to leave their imprints in the fine-grained fraction of the wet earth and thus promote the creation of more humid gaps between coarse grains and the clayey fraction. These gaps may become more significant after drying due to clay shrinkage. In this way, the fine fraction is practically the only one to support stress at the beginning of compression. This may also justify why the tangent modulus of the rounded gravel-earth material decreases faster than that of the angular gravel-earth material, as the compressive stress increases. This dissociation of the clay fraction from the gravel grains leads, obviously, to a decrease in strength. Note that the more intense the compaction is, the higher the density increases and the more simultaneously this dissociation worsens.
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Concerning the peak strain corresponding to the maximum compressive stress, the results obtained show that it increases when adding angular gravel and relatively decreases when adding rounded gravel (Fig 19). Which doesn't necessarily mean that the angular shape provides better improvement in the compacted earth elasticity, because this increase in peak strain is associated with an increase in maximum stress (i.e. compressive strength). Instead, secant modulus at maximum stress may be a better parameter to assess the stiffness of compacted earthen materials. The secant modulus " 𝐸𝑠 " can be calculated using the following equation: 𝑓 𝐸𝑠 = 𝑐⁄𝜀𝑠
(𝐸2)
Where “𝑓𝑐 ” is the compressive strength and “𝜀𝑠 ” the peak strain. As for the three mixtures studied, the results show that this parameter increases when adding rounded gravel and decreases when adding angular gravel (Fig. 19), meaning that it is impacted in the same way as the initial
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tangent modulus. Moreover, in both gravel cases, the secant modulus is slightly more than a half of the initial tangent modulus as shown in the table below (Tab. 10). Another phenomenon that may be inf luenced by the grain shape, which was not investigated in this study and which can be investigated in future studies, is the anisotropy of the compacted earth. Indeed, according to a study by Q.-B. Bui et al (2009), « results enable [Authors] to initiate the hypothesis that rammed earth is an isotropic material of the first order if the layers remain adherent to each other » [38]. Except that this study did not take into consideration grain shape. Yet, J.-C. Santamarina et al (2004) found that ellipticity and platiness can lead to anisotropy of mechanical characteristics [19]. Intuitively, it can be said that, given the layered compaction procedure, elongated grains are more likely to be compacted in parallel with layers, especially if their thickness is low, which will certainly increase strength in the perpendicular direction to layers, because in this way the compacted earth will take advantage of what can be called the « reinforced earth effect ».
4. Conclusions and prospects In this experimental study, two types of gravel with the same grain size distribution (rounded natural gravel and angular crushed gravel) were added to sand-earth for use as rammed earth materials. Results show that the optimum compaction references and mechanical behavior in uniaxial compression of compacted earth are impacted by grain shape. Indeed, as illustrated in the table 10 (Tab. 11), it can be concluded that:
The peak strain corresponding to the maximum stress appears to be greater when using angular grain shape, which makes the material relatively less sensitive to displacements.
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Whatever is the grain shape, the optimum compaction moisture content decreases with the main grain size. The optimum compaction moisture content of rounded gravel-earth mixture is relatively lower than the one of angular gravel-earth mixture. Within the limits of the grain size distribution presented, the maximum dry density of gravel-earth mixtures increases when the gravel used is rounded and remains practically unchanged when using angular gravel. Within the limits of the grain size distribution presented, compressive strength appears to be more improved when using angular gravel than when using rounded gravel. Globally, the rounded grain shape makes the stiffness of compacted earth materials increase while the angular shape makes it decrease.
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In brief, the angular grain shape provides strength for compacted earthen materials, while the rounded grain shape improves their stiffness. Therefore, grain shape should be taken into account when considering adding aggregates to an earth in order to modify its grain size distribution as a building material.
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Of course, it is not always relevant to consider selecting crushed gravel because it usually involves extra work and costs. Indeed, much better effects can be obtained by using a small amount of stabilizer such as lime or cement. However, the results obtained suggest that the grain shape is a parameter that should not be ignored when establishing the optimal grain size curve. This may explain why some earth used in ancient unstabilized rammed earth constructions perform well despite not complying with the recommendations mentioned in this paper. Anyway, this is a possibility that is worth exploring as a perspective for this work.
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5. Limitations of the study
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The results discussed in this paper were obtained for gravel-earth mixtures prepared in such a way as to have the same grain size distribution. Note that the transducers used during compression tests for measuring axial strain, are disconnected from the specimen due to cracks appearing just after the failure, which does not provide reliable measurements to analyze the post-failure behavior of the materials studied. This work can be improved by testing several mixtures in order to find the optimum gravel percentage. Also, the targeted optimal grain size curve is based on the Thomson-Fuller equation, which uses a certain gradation coefficient that depends on grain shape. It would therefore be more appropriate if, for each gravel-earth, a different value for this coefficient were used to establish its optimal grain size curve. But since gravel represents only 25% of the gravel-earth mixture and as the value taken for this coefficient (𝑛 = 0.25) in this study is an indicative value, one would be tempted to consider that a slight change in this coefficient will not have significant impact on mechanical behavior. Anyway, this remains one of the possibilities to be explored as in further research projects. It is appropriate to contextualize the problem covered by this paper. Indeed, not all earths require the addition of aggregates (sand and/or gravel) as a building material. Also, the specimen manufacturing procedures are not the same as those applied on site when building with rammed earth.
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Conflict of interest No conflict of interest to declare.
Acknowledgment
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The experimental work described in this paper was carried out in the civil engineering laboratory of National School of Applied Sciences (ENSA), Ibn Zohr University of Agadir. The contribution of the laboratory team in terms of technical assistance is duly acknowledged. Thanks also to all the staff of the Civil Engineering Department at the same Institution for their support and encouragement.
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References
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38]
ro of
[11]
-p
[10]
re
[5] [6] [7] [8] [9]
lP
[4]
na
[3]
ur
[2]
E. Araldi, E. Vincens, A. Fabbri, and J. P. Plassiard, “Identification of the mechanical behaviour of rammed earth including water content influence,” Mater. Struct., vol. 51, no. 4, 2018. Q. B. Bui, J. C. Morel, S. Hans, and P. Walker, “Effect of moisture content on the mechanical characteristics of rammed earth,” Constr. Build. Mater., vol. 54, pp. 163–169, 2014. P. A. Jaquin, C. E. Augarde, D. Gallipoli, and D. G. Toll, “The strength of unstabilised rammed earth materials,” Geotechnique, vol. 59, no. 5, pp. 487–490, 2009. S. Kenai, R. Bahar, and M. Benazzoug, “Experimental analysis of the effect of some compaction methods on mechanical properties and durability of cement stabilized soil,” J. Mater. Sci., vol. 41, no. 21, pp. 6956–6964, 2006. F. S. Khan, S. Azam, M. E. Raghunandan, and R. Clark, “Compressive strength of compacted clay-sand mixes,” Adv. Mater. Sci. Eng., 2014. H. Houben and H. Guillaud, Traité de Construction en Terre, 2nd ed. Marseille, France: Editions Parenthèses, 1995. Standards Australia and P. Walker, HB 195 - The Australian earth building handbook. Australia, 2002. V. Maniatidis and P. Walker, A review of rammed earth construction. Bath, UK: University of Bath, 2003. A. Koutous and E. Hilali, “A proposed experimental method for the preparation of rammed earth material,” Int. J. Eng. Res. Technol., vol. 8, no. 07, pp. 345–354, 2019. A. S. Rajagopal, A. Veeraragavan, and C. E. G. Justo, “A simplified approach for mix design based on shape factors of coarse aggregates,” Bull. Int. Assoc. Eng. Geol., vol. 30, pp. 123–126, 1984. H. N. Abhilash and J.-C. Morel, “Stress–Strain Characteristics of Unstabilised Rammed Earth,” in Earthen Dwellings and Structures, Springer Nature Singapore Pte Ltd., 2019, pp. 203–214. S. A. Naeini, B. Naderinia, and E. Izadi, “Unconfined compressive strength of clayey soils stabilized with waterborne polymer,” KSCE J. Civ. Eng., vol. 16, no. 6, pp. 943–949, 2012. C. G. Rocco and M. Elices, “Effect of aggregate shape on the mechanical properties of a simple concrete,” Eng. Fract. Mech., vol. 76, pp. 286–298, 2009. D. Sarkar, M. Goudarzy, and D. König, “An interpretation of the influence of particle shape on the mechanical behavior of granular material,” Granul. Matter, vol. 21, no. 53, pp. 1/24-24/24 (online), 2019. G.-C. Cho, J. Dodds, and J. C. Santamarina, “Particle Shape Effects on Packing Density, Stiffness, and Strength: Natural and Crushed Sands,” J. Geotech. Geoenvironmental Eng., vol. 132, no. 5, pp. 591–602, 2006. K. A. Alshibli and M. B. Cil, “Influence of Particle Morphology on the Friction and Dilatancy of Sand,” J. Geotech. Geoenvironmental Eng., vol. 144, no. 3, p. 04017118, 2018. S. Yagiz, “Brief note on the influence of shape and percentage of gravel on the shear strength of sand and gravel mixtures,” Bull. Eng. Geol. Environ., vol. 60, pp. 321–323, 2001. S. Salimi, V. Yazdanjou, and A. Hamidi, “Shape and size effects of gravel grains on the shear behavior of sandy soils,” in Proceedings of the 10th International Conference on Landslides and Engineered Slopes., 2008, pp. 469–474. J.-C. Santamarina and G.-C. Cho, “Soil behaviour : The role of particle shape,” in Proceedings of The Skempton Conference, 2004. I. Cavarretta, M. Coop, and C. O’Sullivan, “The influence of particle characteristics on the behaviour of coarse grained soils,” Géotechnique, vol. 60, no. 6, pp. 413–423, 2010. J. K. Mitchell and K. Soga, Fundamentals of soil behavior, 3rd Editio. Hoboken, New Jersey: John Wiley & Sons, Inc., 2005. AFNOR, NF EN 933-3 Tests for geometrical properties of aggregates - Part 3 : Determination of particle shape - Flakiness index. France, 1997, p. 11. AFNOR, NF P 94-095 Soils : Investigation and testing - Determination of moisture content - Oven drying method. France, 1995, p. 7. AFNOR, NF P 94-056 Soils : Investigation and testing - Particle-size analysis - Method by dry sieving after washing. France, 1996, p. 15. AFNOR, NF P 94-057 Soils : Investigation and testing - Particle-size analysis - Sedimentation method. France, 1992, p. 17. AFNOR, NF P 94-051 Soils : Investigation and testing - Determination of Atterberg’s limits. Liquid limit test using Casagrande apparatus. Plastic limit test on rolled thread. 1993, p. 15. AFNOR, NF P 94-054 Soils : Investigation and testing - Determination of particle density - Pycnometer method. France, 1991, p. 6. AFNOR, NF EN 1097-6 Tests for mechanical and physical properties of aggregates — Part 6 : Determination of particle density and water absorption. France, 2001, p. 34. AFNOR, NF P 94-093 Soils : Investigation and testing - Determination of the compaction characteristics of a soil - Standard Proctor test Modified Proctor test. France, 1999, p. 18. ASTM, D 2487 - 06 Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System). USA, 2006, p. 12. ASTM, D 3282 - 93 Standard Practice for Classification of Soils and Soil-Aggregate Mixtures for Highway Construction Purposes. USA, 2004, p. 6. AFNOR, NF P 94-074 Soils : Investigation and testing - Shear strength tests with revolving triaxial test apparatus - § 5.2 Specimen size requirements. France, 1994, p. 36. AFNOR, NF EN 12390-7 - Testing hardened concrete - Part 7 : density of hardened concrete. 2001, p. 9. AFNOR, NF EN 12390-4 - Testing hardened concrete - Part 4 : compressive strength - Specification for testing machines. France, 2000, p. 14. J. J. Wang, Y. Yang, and H. P. Zhang, “Effects of Particle Size Distribution on Compaction Behavior and Particle Crushing of a Mudstone Particle Mixture,” Geotech. Geol. Eng., vol. 32, no. 4, pp. 1159–1164, 2014. F. Champiré, A. Fabbri, J. C. Morel, H. Wong, and F. McGregor, “Impact of relative humidity on the mechanical behavior of compacted earth as a building material,” Constr. Build. Mater., vol. 110, pp. 70–78, 2016. V. Maniatidis and P. Walker, “Structural Capacity of Rammed Earth in Compression,” J. Mater. Civ. Eng., vol. 20, no. 3, pp. 230–238, 2008. Q. B. Bui and J. C. Morel, “Assessing the anisotropy of rammed earth,” Constr. Build. Mater., vol. 23, no. 9, pp. 3005–3011, 2009.
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re
-p
ro of
a
c Fig. 1 : Photos showing the three materials used (Sand-earth, Rounded gravel and Angular gravel).
10
100
Rounded Gravel
90
Angular Gravel
80
Adjusted Gravel
Passing (%)
70 60 50 40 30 20 10 0
4
5
6.3
8
10
12.5
16
20
Grains sizes (mm)
Fig. 2 : Grain size curves of the two gravels before and after adjustments made.
90
R.E. Spindle
80
Sand-Earth Gravel-earth
70
ICD20 [RE]
60 50 40
-p
Passing (%)
ro of
100
30
10
0.01
0.1 1 Grains sizes (mm)
10
100
lP
0 0.001
re
20
Fig. 3 : Grain size curves of sand-earth before and after adding gravel, compared to the recommended rammed earth spindle (R.E. Spindle by Houben and Guillaud) and to the ideal curve (ICD20 [RE]).
na
10
0/5 Granular fraction
10
Jo
5 × 36
ur
0/20 Granular fraction
6
7
5 4
25 8
3 2
1
Distribution of rammer blows on a layer of material.
100
Fig. 4 : Illustration of the manufacturing methods applied to the specimens.
11
-p
2.25 2.00 1.75
re
1.50 1.25 1.00 0.75 0.50 0.25 0.00 10
20
30
40
na
0
lP
Stress ( MPa )
ro of
Fig. 5 : Measurement of specimen wet mass after demolding (RG2 specimen).
Time ( s )
Jo
ur
Fig. 6 : Typical loading speed curve during the compression tests (RG1 specimen).
12
ro of
Earth + R. Gravel
2.00
Earth + A. Gravel
1.95
lP
1.90 1.85 1.80 1.75 1.70 1.65 1.60 6.0
8.0
10.0
na
Dry Density (103 kg.m-3)
Sand + Earth
100% Saturation lines
2.05
re
2.10
-p
Fig. 7 : Compression device with the two displacement transducers fixed on one of the tested specimens (SE1 specimen).
12.0
14.0
16.0
ur
Moisture Content (%)
Jo
Fig. 8 : Compaction test curves of the three materials studied (Proctor test).
13
ro of
Fig. 9 : One of the series of specimens after drying (SE1, SE2 and SE3 sand-earth).
2.50 𝑓𝑐 = 2.24 𝑀𝑃𝑎
-p
1.50
𝐸𝑖 = 347 𝑀𝑃𝑎 𝐸𝑠 = 189 𝑀𝑃𝑎
1.00
re
Stress (MPa)
2.00
lP
0.50 𝜀𝑝 = 0.0119
0.00 0.0000
0.0025
0.0050
0.0075
0.0100
Axial strain (mm/mm)
0.0125
0.0150
na
Fig. 10 : Typical stress–strain curve for the compacted earthen material in the compression test (AG1 specimen).
SE
ur
2.50
RG
2.00
Stress ( MPa )
1.50
1.00
Jo
AG
0.50
0.00 0.0000
0.0025
0.0050
0.0075
0.0100
0.0125
0.0150
Axial strain ( mm/mm )
Fig. 11 : Superimposed stress-strain curves resulting from all compression tests
14
ro of
-p
Fig. 12 : One of the specimens after being compressed until failure (AG1 specimen).
2.40 SE 2.30
AG
2.20 2.10
lP
Stress ( MPa )
re
RG
2.00
1.80 0.0050
0.0075
na
1.90
0.0100
0.0125
0.0150
Axial strain ( mm/mm )
Jo
ur
Fig. 13 : Superposed stress-strain curves resulting from all compression tests, Enlarged view of the end of the tests
15
Compressive strength (MPa)
2.50
2.30
2.10
1.90
1.70 2.03
2.13
2.24
SE
RG
AG
1.50
Fig. 14 : Comparison of the average compressive strengths of the three materials tested (𝑬𝒓𝒓𝒐𝒓 𝒃𝒂𝒓𝒔 = 𝑺𝒕𝒂𝒏𝒅𝒂𝒓𝒅 𝒆𝒓𝒓𝒐𝒓𝒔)
0.45
SE
0.40
RG
0.35
AG
ro of
0.50
0.25 0.20
-p
Stress ( MPa )
0.30
0.15 0.10
0.00 0.0000
0.0005
0.0010
0.0015
Axial strain ( mm/mm )
re
0.05
lP
Fig. 15 : Superposed stress-strain curves resulting from all compression tests, Enlarged view of the beginning of the tests
na ur
450
400
350
300
Jo
Initial tangent modulus (MPa)
500
340
437
352
SE
RG
AG
Fig. 16 : Comparison of the average initial tangent modulus of the three materials tested (𝑬𝒓𝒓𝒐𝒓 𝒃𝒂𝒓𝒔 = 𝑺𝒕𝒂𝒏𝒅𝒂𝒓𝒅 𝒆𝒓𝒓𝒐𝒓𝒔)
16
500
SE
Initial tangent modulus (MPa)
480
RG
460
AG
440 420 400 380 360 340 320 300 1960
2010
2060
2110
Dry density (kg/m3)
Fig. 17 : Initial tangent module against the dry density
ro of
2.50
2.30
AG
2.20
RG
2.10
-p
Compressive strength (MPa)
2.40
SE
2.00
1.80 325
375
425
475
Initial tangent modulus (MPa)
re
1.90
lP
Fig. 18 : Compressive strength against initial tangent modulus
na
2.50
ur
1.50
1.00
0.50
SE
ES = 215 MPa
RG
ES = 232 MPa
Jo
Compressive strength (MPa)
2.00
ES = 195 MPa
0.00 0.000
AG
0.005 0.010 Peak strain (mm/mm)
0.015
Fig. 19 : Compressive strength against peak strain (𝑬𝒓𝒓𝒐𝒓 𝒃𝒂𝒓𝒔 = 𝑺𝒕𝒂𝒏𝒅𝒂𝒓𝒅 𝒆𝒓𝒓𝒐𝒓𝒔)
17
Tab. 1 : List of Standards used in the present study.
Standard applied
Water content
NF P 94-050 [23]
Sieving
NF P 94-056 [24]
Sedimentation
NF P 94-057 [25]
Atterberg limits
NF P 94-051 [26]
Absolute density
NF P 94-054 [27]
Flakiness index
NF EN 933-3 [22]
Water absorption
NF EN 1097-6 [28]
Compaction (Proctor)
NF P 94-093 [29]
Earth classification
ASTM D2487 [30] & D3282 [31]
ro of
Method / Test
Specimen requirements NF P 94-074 [32] NF EN 12390-7 [33]
Compression machine
NF EN 12390-4 [34]
re
-p
Specimen density
Tab. 2 : Characteristics of the fine-grained fraction of the sand-earth used.
Liquid limit 𝝎𝒍
2570 𝑘𝑔/𝑚3
32.8%
Plasticity index 𝑰𝑷
lP
Grains density 𝝆𝒔
10.6%
Rounded Gravel 𝑹𝑮%
Angular Gravel 𝑨𝑮%
Adjustment to make 𝑨𝑮% − 𝑹𝑮%
4/5
4.6 %
6.5 %
+ 1,9 %
5/6.3
8.4 %
13.4 %
+ 5,0 %
6.3/8
17.2 %
25.1 %
+ 7,9 %
8/10
20.2 %
15.5 %
– 4,7 %
10/12.5
17.5 %
10.8 %
– 6,7 %
12.5/16
19.5 %
15.3 %
– 4,2 %
16/20
12.6 %
13.4 %
+ 0,8 %
ur
Granular Fraction 𝒅/𝑫
Jo
na
Tab. 3 : Calculation of adjustments to be made to the gravels grain size distributions.
18
Tab. 4 : Characteristics of the two gravels used.
Angular Gravel
Gravel
Rounded Gravel
The two gravels come from a crushing quarry located in a local river (Souss River), whose sediments are petrographically heterogeneous.
Origin (Fig. 1-b & 1-c)
Absolute density 𝝆𝒔 (𝒌𝒈⁄𝒎𝟑 ) [27]
2 710
2 710
Flakiness index 𝑰𝒇 (%) [22]
23 %
11 %
Water absorption 𝑾𝒂𝒃𝒔 (%) [28]
1.4 %
0.7 %
Material
Sand-Earth
Gravel-Earth
AASHTO [31]
A-6
A-2-6
USCS [30]
SC
GC
ro of
Tab. 5 : Classification of sand-earth used before and after adding gravel.
R. GravelEarth
A. GravelEarth
𝝎𝑶𝑴𝑪 (%)
13.0
9.5
10.5
𝝆𝒎𝒂𝒙 (𝒌𝒈/𝒎𝟑 )
1 900
2 000
1 970
re
SandEarth
Jo
ur
na
lP
Material
-p
Tab. 6 : Effects of adding gravel to sand-earth on its compaction references: Optimum compaction moisture content (𝝎𝑶𝑴𝑪 ) and Maximum oven dry density (𝝆𝒎𝒂𝒙 ).
19
Tab. 7 : Average compaction conditions of the specimens of the three materials studied.
Material
SandEarth
A. GravelEarth
Compaction water content
13.0 %
Wet mass 𝒎𝒉 (𝒌𝒈)
3.490
3.580
3530
Dry mass 𝒎𝒅 (𝒌𝒈)
3.110
3.280
3.220
Dry density 𝝆𝒅 (𝒌𝒈/𝒎𝟑 )
1 980
2 090
2 050
Average compactness
77.0%
80.1%
78.5%
𝝆𝒅 ⁄𝝆𝒎𝒂𝒙 Average Ratio
104%
105%
104%
9.5 %
10.5 %
ro of
R. GravelEarth
Tab. 8 : Summary of the results of the compression tests conducted on the three materials studied.
233
0.0083
1.94
SE2
1990
347
199
0.0109
2.16
SE3
1980
340
214
0.0093
1.98
RG1
2110
463
251
0.0081
2.03
RG2
2070
411
223
0.0099
2.22
RG3
2090
436
221
0.0097
2.15
AG1
2040
347
189
0.0119
2.24
AG2
2100
367
198
0.0120
2.37
AG3
2010
341
199
0.0107
2.12
re
332
Jo
ur
Angular Gravel-Earth (AG)
1970
lP
Rounded Gravel-Earth (RG)
SE1
na
Sand-Earth (SE)
-p
Initial tangent Dry density Secant modulus Peak strain Compressive modulus 3 𝜌𝑑 (𝑘𝑔⁄𝑚 ) 𝐸𝑠 (𝑀𝑃𝑎) 𝜀𝑠 (𝑚𝑚⁄𝑚𝑚) strength 𝑓𝑐 (𝑀𝑃𝑎) 𝐸𝑖 (𝑀𝑃𝑎)
Specimens
20
Tab. 9 : Average characteristics of the three compacted earthen materials (coefficient of variation).
Dry density 𝜌𝑑 (𝑘𝑔⁄𝑚3 )
Initial tangent modulus 𝐸𝑖 (𝑀𝑃𝑎)
1980 (0%)
340 (2%)
215 (6%)
0.0095 (11%)
2.03 (5%)
Rounded GravelEarth (RG)
2090 (1%)
437 (5%)
232 (5%)
0.0092 (9%)
2.13 (4%)
Angular GravelEarth (AG)
2050 (2%)
352 (3%)
195 (2%)
0.0115 (5%)
2.24 (5%)
Material
Sand-Earth (SE)
Compressive Secant modulus Peak strain strength 𝐸𝑠 (𝑀𝑃𝑎) 𝜀𝑠 (𝑚𝑚⁄𝑚𝑚) 𝑓𝑐 (𝑀𝑃𝑎)
Tab. 10 : Secant modulus compared to Initial tangent modulus (coefficient of variation)
𝑬𝒔 (𝑴𝑷𝒂)
𝑬𝒔 /𝑬𝒊
SE
340 (2%)
215 (6%)
0.63 (8%)
RG
437 (5%)
232 (5%)
0.53 (3%)
AG
352 (3%)
195 (2%)
0.55 (4%)
(𝐸3)
-p
𝐼𝑛 𝑎𝑣𝑒𝑟𝑎𝑔𝑒: 𝐸𝑠 ≈ 0.57 𝐸𝑖
ro of
𝑬𝒊 (𝑴𝑷𝒂)
Material
Gravel grain shape
Rounded
Angular
−−
−
Effect on 𝜌 𝑑𝑚𝑎𝑥
++
±
Effect on 𝑓𝑐
±
Effect on 𝐸𝑖 and 𝐸𝑠 Effect on 𝜀𝑝
+
++
+
na
±
−
(+) : increase
(±) : to be tested
Jo
ur
(−) : decrease
lP
Effect on 𝜔𝑂𝑀𝐶
re
Tab. 11 : Grain shape effects on compacted earth proprieties.
21