Influence of the Diatomite Specie on the Peak and Residual Shear Strength of the Fine-Grained Soil

applied sciences

Article Inﬂuence of the Diatomite Specie on the Peak and Residual Shear Strength of the Fine-Grained Soil

Carlos J. Slebi-Acevedo 1, Daniel A. Zuluaga-Astudillo 2, Juan C. Ruge 3 and Daniel Castro-Fresno 1,*

1 GITECO Research Group, Universidad de Cantabria, 39005 Santander, Spain; [email protected] 2 Escuela de Ingenieros Militares, Ejército de Colombia, Bogotá 111211, Colombia; [email protected] 3 Facultad de Ingeniería, Universidad Militar Nueva Granada, Bogotá 1101111, Colombia; [email protected] * Correspondence: [email protected]

Abstract: Diatomite is a powdering mineral mainly composed of diatom microfossils present in marine and lacustrine soils, which influences their physical and mechanical properties. Although many articles have been found in the literature concerning the influence of diatomite in the overall behavior of natural soils, few research efforts have been carried out to evaluate the influence of the diatom microfossil species on their shear resistance. Therefore, in this research, the influence of the diatomite species and the content in the peak and the residual shear strength of diatomite-fine grained soil mixtures was analyzed using the annular shear strength test. Scanning electron microscopy (SEM) and Atterberg limits were also carried out as additional tests to explain the interlocking effect between the microfossils and the soil. Overall, both diatomite species increased both peak and residual shear strength of the soil similar to dense sands. Nevertheless, the Mexican species reveal higher friction angle values compared with Colombian species. Keywords: annular shear strength test; residual; diatomite; kaolin; diatom; marine soil Citation: Slebi-Acevedo, C.J.; Zuluaga-Astudillo, D.A.; Ruge, J.C.; Castro-Fresno, D. Influence of the Diatomite Specie on the Peak and 1. Introduction Residual Shear Strength of the The soil constitutes the support of many infrastructures such as buildings, bridges, Fine-Grained Soil. Appl. Sci. 2021, 11, highways, and dams, among others; therefore, understanding the various elements that are 1352. https://doi.org/10.3390/ part of it seems to be an interest topic in the recent years. Due to the heterogeneity of the soil, app11041352 the shape of distinct particles conditions some properties, including volumetric properties, permeability, suction, compressibility, shear strength, and cyclic loading [1–3]. Diatoms Received: 18 November 2020 are unicellular photosynthetic algae which have a skeleton denoted as frustule composed Accepted: 28 January 2021 Published: 3 February 2021 mostly of silica. These microorganisms whose size can vary from 20 to 200 microns have become fossilized through the geological ages, becoming an inherent part of the soil and

Publisher’s Note: MDPI stays neutral changing its properties [4–6]. Diatomaceous earth, or diatomite as it is known in the with regard to jurisdictional claims in scientific literature [7], are the fossilized diatoms which have settled in seas and lakes published maps and institutional affil- (periods near 40 million years) forming marine and lacustrine soil deposits in many parts iations. of the world, such as Mexico City, Bogotá, the capital of Colombia, Osaka Bay in Japan, California in the US, seabed sediments in the Antarctic, Yunnan in China, and the Indian Ocean [8–11]. Given its morphology of hollow structure, the diatomite has a high porosity which helps it to retain a high amount of water. Other characteristics found in the literature are that they present a rough and abrasive surface, low density, large surface area, and Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. absorbent capacity [12,13]. These singular characteristics govern the behavior of some soil This article is an open access article deposits, leading to an increment in the void ratio and Atterberg limits but also increasing distributed under the terms and the friction angle, controverting the fundamentals established in the classic mechanical conditions of the Creative Commons of soils. Attribution (CC BY) license (https:// Previous studies have been carried out to analyze the influence of the diatomite in the creativecommons.org/licenses/by/ geotechnical properties of fine-grained soils [7]. Díaz-Rodriguez [9] studied the influence 4.0/). of diatomite on geotechnical properties of soils manufacturing artificial mixes of diatom

Appl. Sci. 2021, 11, 1352. https://doi.org/10.3390/app11041352 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 1352 2 of 13

microfossils and kaolin. Atterberg limits and shear strength in undrained conditions were performed in soil specimens normally and over-consolidated. Based on the author’s conclusion, Atterberg limits increased with the diatom content as well as the friction angle. Similar results were found by Shiwakoti et al. [14], who prepared remolded specimens of soil with diatomite. According to the authors, they conclude that the presence of these microfossils alters the index properties and increases the compressibility as well as the friction angle of the soil. An explanation of this phenomenon could be attributed to the hollow structures of the diatoms and the rough surface which helps to increase the interlocking among the particles. Exploratory studies have also been carried out in lacustrine soils with presence of diatoms. Caicedo et al. [10] developed an experimental plan in diatomaceous soils located in Bogotá, where more than 2400 tests were performed. Based on their analysis, they concluded that high values of Atterberg limits and friction angle disagree between the classification system based on Atterberg limits and the classification system based on grain size [10]. Although some studies have examined the influence of the diatomite in a standard soil at different dosages [15], a gap was found in studying the influence of a specific species in the geotechnical behavior of the soil. No research efforts have been found in analyzing the type of diatomite in the shear strength properties of the soil. Accordingly, the main aim of this research was to study the presence of two species of diatom microfossils on the shear strength properties of soils. The shear resistance of the soil was determined by the friction angle and the interlocking phenomenon which exists between the particles [16]. Most research focuses on measuring the friction angle at the peak shear strength because, in the majority of geotechnical problems (e.g., foundations), the strains are small and the stresses are below the peak. However, it is better to consider the residual shear strength as parameter design [17]. Therefore, in the current research, the mechanical response of the soil samples was analyzed through the ring shear equipment, whose main advantage is the evaluation of shear strength in both peak and residual conditions.

2. Materials and Methods 2.1. Materials Characterization and Experimental Designs In this research, an untreated commercial kaolin available in Bogota (Colombia) with a specific gravity (GS) of 2.69 was employed as the fine-grained soil matrix, and two distinct types of diatom microfossils commonly used as pesticide in the field of agriculture were selected to be studied. The first type of diatom microfossil corresponds to the Alucoseira Granulata species (GS = 2.55), extracted from the lacustrine soil of Tunja in Colombia (Colombian diatomite species), and the second is the Cosconodiscus Centralis (GS = 2.34) imported directly from Mexico City in Mexico (Mexican diatomite species). Scanning electron micrographs (SEM) were taken of the three materials, as shown in Figure1. According to the SEM images, the kaolin presents a set of many laminar layers overlapping together and without pores within the internal structure. Concerning diatom microfossils, the Colombian species has a cylindrical shape with pores along its walls. Similarly, the Mexican species has a shape of discs with pores, too, along its surface. In both cases, the pores sizes range from 200 to 900 nm, approximately. Since the particle size is less than 0.075 mm remaining in the fine range, the particle size distribution for the three materials was obtained through the hydrometer, as ian be seen in Figure2. From the graph, it can be observed that the three materials fit in the size of the silt. Similar gradation curve was also noticed between Colombian species and kaolin with a more regular size distribution, while in the Mexican species, the slope of the curve is steeper. Appl. Sci. 2021, 11, x FOR PEERAppl. REVIEW Sci. 2021 , 11, 1352 3 of 13 3 of 13

(a)

(b)

(c) Figure 1. SEM images of: (a) kaolin; (b) Colombian diatom species; (c) Mexican diatom species. Figure 1. SEM images of: (a) kaolin; (b) Colombian diatom species; (c) Mexican diatom species.

Since the particle size is less than 0.075 mm remaining in the fine range, the particle size distribution for the three materials was obtained through the hydrometer, as ian be seen in Figure 2. From the graph, it can be observed that the three materials fit in the size of the silt. Similar gradation curve was also noticed between Colombian species and kaolin with a more regular size distribution, while in the Mexican species, the slope of the curve is steeper. Appl. Sci. 2021, 11, x FOR PEERAppl. REVIEW Sci. 2021, 11, 1352 4 of 134 of 13

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Passing percentage % 20

0 0.001 0.01 0.1 Particle size (mm) Figure 2. Particle size distributionFigure. 2. Particle size distribution.

For the experimental designs, artificial mixtures of kaolin and the two diatom micro- For the experimentalfossils were designs, manufactured artificial to investigate mixtures the of influence kaolin and of the the diatomite two diatom species inmicro- the shear fossils were manufacturedstrength of to the investigate soil. The mass the ratioinfluence expressed of the in percentagediatomite for species all the in mixture the shear designs strength of the soil.is depictedThe mass in Tableratio1 .expressed For the specimen’s in percentage conditioning for beforeall the the mixture annular desi sheargns strength is depicted in Table test,1. For the diatomite-kaolinthe specimen’s mixtures conditioning were initially before blended the inannular dry conditions shear andstrength then were added to distilled water so that the mixture reached the moisture content corresponding to test, the diatomitethe-kaolin liquid mixtures limit (LL). were Later, theinitially mixes wereblended homogenized in dry conditions and kept wrapped and then in plastic were bags added to distilled forwater 24 h so thatthat all the the mixture mixtures werereached completely the moisture humidified. content corresponding to the liquid limit (LL). Later, the mixes were homogenized and kept wrapped in plastic bags for 24 h so that allTable the 1. mixturesMass ratio in were percentage completely of the mixture humidified. designs. Sample N◦ Id Mixture Composition Liquid Limit Plastic Limit 1Table 1. Mass Kaolinratio in 100% percentage of the mixture 100% designs. kaolin 44 24 2 Col. Diatomite 20% 80% Kaolin-20% Colombian diatomite 50 28 Plastic 3Sample N° Col. DiatomiteId Mixture 40% 60% Kaolin-40%Compo Colombiansition diatomite Liquid 56 Limit 32 4 Col. Diatomite 60% 40% Kaolin-60% Colombian diatomite 70Limit 38 51 Col. DiatomiteKaolin 80%100% 20% Kaolin-80%100% Colombian kaolin diatomite 8344 24 42 6 Col. Diatomite 100% 100% Colombian diatomite 99 48 72 Mex.Col. Diatomite Diatomite 20% 20% 80% 80% Kaolin Kaolin-20%-20% Mexican Colombian diatomite diatomite 4950 28 28 83 Mex.Col. Diatomite Diatomite 40% 40% 60% 60% Kaolin Kaolin-40%-40% Mexican Colombian diatomite diatomite 5656 32 34 94 Mex.Col. Diatomite Diatomite 60% 60% 40% 40% Kaolin Kaolin-60%-60% Mexican Colombian diatomite diatomite 8070 38 45 10 Mex. Diatomite 80% 20% Kaolin-80% Mexican diatomite 110 50 115 Mex.Col. Diatomite Diatomite 100% 80% 20% Kaolin 100% Mexican-80% Colombian diatomite diatomite 13283 42 60 6 Col. Diatomite 100% 100% Colombian diatomite 99 48 7 Mex. Diatomite 20% 80% Kaolin-20% Mexican diatomite 49 28 2.2. Annular Shear Strength Test 8 Mex. Diatomite 40% 60% Kaolin-40% Mexican diatomite 56 34 The annular—or the ring shear strength test, as it is also commonly known—was 9 Mex. Diatomite 60% 40% Kaolin-60% Mexican diatomite 80 45 employed in the current study to measure peak and residual strength of the diatomite- 10 Mex. Diatomitekaolin mixtures. 80% It is20% worth Kaolin pointing-80% out Mexican that estimating diatomite the peak and110 the residual50 strength 11 Mex. Diatomiteis extremely 100% important to100% find out Mexican the limit diatomite states of a material or a132 geotechnical 60 structure, due to the fact that they present different levels of strains and stress paths. For instance, 2.2. Annular Shear Strength Test The annular—or the ring shear strength test, as it is also commonly known—was employed in the current study to measure peak and residual strength of the diatomite- kaolin mixtures. It is worth pointing out that estimating the peak and the residual strength is extremely important to find out the limit states of a material or a geotechnical structure, due to the fact that they present different levels of strains and stress paths. For instance, in terms of design, an excavation supported by a retaining structure could present two situations. On the one hand, the soil subjected to a passive condition could reveal small strains reaching its peak resistance. On the other hand, the soil mass supported by the Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 13 Appl. Sci. 2021, 11, 1352 5 of 13

retaining structure could already be subjected to large strains and, therefore, be closer to in terms of design, an excavation supported by a retaining structure could present two its residual strength.situations. On the one hand, the soil subjected to a passive condition could reveal small To perform thestrains test, reachingthe reconstituted its peak resistance. humidified On the diatomite other hand,-kaolin the soil specimen mass supported was re- by the moved from the plasticretaining bag structure and placed could already in the be annular subjected shear to large cell. strains The and, annular therefore, shear be closer cell to its consisted of two concentricresidual strength. rings confined vertically and totally filled with water. Then, To perform the test, the reconstituted humidified diatomite-kaolin specimen was normal stress was removedapplied fromto the the sample plastic bag by and the placed top inplate the annularby means shear of cell. a Thelever annular-arm shear(see cell Figure 3). Similar toconsisted the consolidation of two concentric test, ringsthe samples confined verticallywere consolidated and totally filled by performing with water. Then, the water pore pressure.normal Once stress the was mixture applied towa thes consolidated, sample by the topthe plateshear by streng meansth of wa a lever-arms meas- (see ured. To shear the sample,Figure3). Similarthe upper to the lid consolidation and the bottom test, the ring samples must were rotate consolidated relative by to performing each the water pore pressure. Once the mixture was consolidated, the shear strength was other. In that sense, the bottom ring rotated at a controlled angular speed whereas the measured. To shear the sample, the upper lid and the bottom ring must rotate relative to upper part was restrainedeach other. by In a thathorizontal sense, the beam bottom, and ring a rotated torque at cell a controlled device help angulared speedto meas- whereas ure the shear resistancethe upper of the part soil. was Both restrained the normal by a horizontal stresses beam, applied and aon torque the lever cell device arm helpedand to the measured shearmeasure stress thewere shear previously resistance of calibrated the soil. Both to the avoid normal measurement stresses applied errors. on the lever To arm obtain drained conditionsand the measured of the test, shear this stress was were carried previously out calibratedat an angular to avoid speed measurement of 0.032 errors. To obtain drained conditions of the test, this was carried out at an angular speed of 0.032 radians per second.radians The normal per second. stresses The normal to which stresses the tosamples which the were samples consolidated were consolidated were 100, were 100, 200, and 400 kPa, respectively.200, and 400 kPa, The respectively. vertical displacement The vertical displacement was also recorded was also recorded to analyze to analyze the the specimen volume change.specimen More volume details change. of More the pr detailsocedure of the are procedure described are described in the United in the United States States Bureau of ReclamationBureau USBR of Reclamation 5730. USBR 5730.

Figure 3. Annular shear Figurestrength 3. Annular test. Adapted shear strength from test.[18] Adapted. from [18].

3. Results and Discussion3. Results and Discussion Initially, to have moreInitially, comprehensible to have more comprehensible knowledge knowledge with respect with respect to the to diatomite the diatomite-kaolin-ka- mixtures, the Atterberg limits for the different diatomite concentrations were recorded, as olin mixtures, the shownAtterberg in Figure limits4. Although for the it different is very typical diatomite to obtain concentrations the LL of the samples were by rec- applying orded, as shown inthe Figure Casagrande 4. Although0s method, it is in very this research,typical to the obtain fall-cone the method LL of wasthe adoptedsamples since by the applying the Casagrandecalculations′s method, for Mexican in this species research was not, possiblethe fall with-cone the method conventional was method. adopted As can since the calculationsbe seenfor Mexican in the Figure species4, the was trend not suggests possible that with the higher the conventional the diatom content method. was, the higher was the increase in liquid and plastic limit. This result ties well with previous As can be seen in thestudies Figure wherein 4, the limit trend and suggests plastic limits that increase the higher with the the increase diatom in diatomite content contentwas, [9]. the higher was the Anincrease explanation in liquid of the and above plastic could belimit. that theThis hollow result structure ties well and w theith pores previous along their studies wherein limitsurfaces and plastic help to retain limits more increase water, increasingwith the increase their Atterberg in diatomite limit values. content Concerning [9]. the An explanation of thetype above of diatom could species, be that the Mexicanthe hollow displayed structure higher and values the ofpores both liquidalong andtheir plastic limits in relation to Colombian species. In agreement with the above, the Mexican species surfaces help to retain more water, increasing their Atterberg limit values. Concerning the displayed a lower speciﬁc gravity than the Colombian species, indicating that this species type of diatom species,could presentthe Mexican higher porosity displayed and, ashigher a result, values retain moreof both water liquid inside and its structure. plastic limits in relation to Colombian species. In agreement with the above, the Mexican species displayed a lower specific gravity than the Colombian species, indicating that this species could present higher porosity and, as a result, retain more water inside its structure. Appl. Sci. 2021, 11, 1352 6 of 13 Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 13

140

100

Kaolin 100% 60

PI Liquid Limit (%) / Plastic

20 0 0.2 0.4 0.6 0.8 1 Diatomite content FigureFigure 4. 4.Liquid Liquid limit limit (LL) (LL) and and plastic plastic limit limit (PL). (PL).

ConcerningConcerning the the shear shear resistance, resistance, the annularthe annular shear s testhear presents test presents a particular a particular advantage ad- overvantage other over kinds other of shear kinds tests of (e.g.,shear direct tests shear)(e.g., direct considering shear) theconsidering possibility the to evaluatepossibility the to residualevaluate strength the residual parameters strength in theparameters same fail in surface the same and fail in the surface same and shear in stress the same direction shear becausestress direction of the radial because movement of the radial of the movement soil container. of the Thesoil residualcontainer. shear The residual strength shear is a propertystrength of is ﬁnea property soils which of fine evaluates soils which the gradualevaluates loss the of gradual shear strength loss of shear along strength a developed along failurea developed surface failure and, thus, surface in some and, cases, thus, it in is some better cases, to use it this is better value ratherto use thanthis value peak shearrather strengththan peak for shear design strength purposes. for design For all purpos diatomite-kaolines. For all diatomite mixtures,-kaolin the mixtures, shear-strength the shear vs.- displacementstrength vs. displacement curves at the threecurves normal at the stressesthree normal were calculated.stresses were calculated. FigureFigure5 shows5 shows the the results results of of shear shear strength strength vs. vs. displacement displacement for for the the two two diatomite diatomite speciesspecies under under study. study. The The increase increase of of diatomaceous diatomaceous material material in in the the mixtures mixtures with with kaolin kaolin revealedrevealed slight slight increases increases in in resistance. resistance. Regarding Regarding the the stress–strain stress–strain response, response, it it is is noted noted that the shear stress increased in those specimens subjected to greater consolidation steps. that the shear stress increased in those specimens subjected to greater consolidation steps. Besides, at higher normal stresses, the behavior of the curves looked more typical of a Besides, at higher normal stresses, the behavior of the curves looked more typical of a compacted sand than of a normally consolidated clay, especially with higher contents of compacted sand than of a normally consolidated clay, especially with higher contents of Mexican diatomite. Unlike clays, the fossilized diatoms did not present electrochemical Mexican diatomite. Unlike clays, the fossilized diatoms did not present electrochemical properties; thus, the improvement in the shear stress could be attributed to the friction properties; thus, the improvement in the shear stress could be attributed to the friction produced between the particles. The above is because the higher the diatomite content was, produced between the particles. The above is because the higher the diatomite content the greater the shear resistance was. With some exceptions, the residual shear stress was was, the greater the shear resistance was. With some exceptions, the residual shear stress kept constant after reaching 4 mm of horizontal displacement. was kept constant after reaching 4 mm of horizontal displacement. A point of interest worth highlighting is that, for all the samples, regardless of the stress level and the amount of diatomite added, they adequately reached the critical state of the soil during the residual condition of the material. This concept is fundamental to understand the response of the soil that has been previously remolded, as it was in the current case, where the specimens underwent a previous peak resistance [19,20]. Appl. Sci. 2021, 11, x FOR PEERAppl. REVIEW Sci. 2021 , 11, 1352 7 of 137 of 13

Figure 5. IllustrativeFigure example 5. Illustrative for shear example stress for vs shear. displacement stress vs. displacement curve. curve. To analyze the volumetric behavior of the diatomite-kaolin mixtures, the vertical A point of interestdisplacement worth washighlighting also recorded is for that all, the for specimens, all the samples, as can be observed regardless in Figure of the6. From stress level and thethe amount graphs, of two diatomit importante added, aspects they were adequately deemed for analysis:reached ﬁrstly, the critical the deformability state of the soil during theobserved residual in the condition specimens of when the theymaterial. were subjected This concept to different is fundamental consolidation stressesto prior to the torque shear stage and secondly, the behavior observed in the composite understand the responsematerial of by the inﬂuencesoil that of has diatomite. been Inpreviously the 100% kaolin remolded, mixture, theas it material was in presented the a current case, wherecontractive the specimens behavior underwent during the sheara previous stage being peak very resistance similar to a[19,20]. normally consolidated To analyze theclay volumetric or, at most, behavior a slightly over-consolidatedof the diatomite clay,-kaolin as was mixtures, expected. the The vertical same phenomenon displacement was alsowas recorded displayed for until all reachingthe specimens the combination, as can with be observed 40% diatomite in andFigure 60% 6. kaolin, From where the graphs, two importantthe mixture asp beganects were to reveal deemed a dilatancy for analysis response,: especiallyfirstly, the at deformability the highest consolidation ob- stress (i.e., 400 kPa). The above could be due to the presence of diatomaceous material served in the specimenswhich when provided they frictional were subjected properties to thedifferent specimens. consolidation The behavior stresses at 400 kPa prior was that to the torque sheartypically stage and shown secondly, by a dense the granular behavior material. observed in the composite material by the influence of diatomite. In the 100% kaolin mixture, the material presented a contractive behavior during the shear stage being very similar to a normally consolidated clay or, at most, a slightly over-consolidated clay, as was expected. The same phenomenon was displayed until reaching the combination with 40% diatomite and 60% kaolin, where the mixture began to reveal a dilatancy response, especially at the highest consolidation stress (i.e., 400 kPa). The above could be due to the presence of diatomaceous material which provided frictional properties to the specimens. The behavior at 400 kPa was that typically shown by a dense granular material. Appl. Sci. 2021, 11, x FOR PEERAppl. REVIEW Sci. 2021 , 11, 1352 8 of 138 of 13

Figure 6.Figure Vertical 6. Vertical vs. horizontal vs. horizontal displacement displacement for bothboth species species analyzed analyzed at 60% at diatomite 60% diatomite content. content.

The same phenomenonThe same wa phenomenons more noticeable was more in noticeable the diatomite in the- diatomite-kaolinkaolin mixtures mixtures manu- manu- factured with Mexican diatomite content equal to or greater than 40%, where the behavior factured with Mexicanat stresses diatomite of 400 content kPa was equal always to revealing or greater dilatancy. than 40%, This where may have the been behavior influenced at stresses of 400 kPaby thewas microstructural always revealing configuration dilatancy. of the This Mexican may diatomitehave been where influenced the discs by present the microstructuralcould configuration interlock more of the easily Mexican than the diatomite cylinders shown where by the the discs Colombian present species. could This is interlock more easilyconsistent than the with cylinders what has been shown found by in previousthe Colombian studies [21 species.]. In samples This prepared is con- with 100% diatomite, the dilatancy was also observed at a medium normal stress (e.g., 200 kPa) sistent with what hasfor been both species. found Therefore,in previous it could studies be believed [21]. In that, samples when the prepared specimen with has the 100% presence diatomite, the dilatancyof only was diatomaceous also observed material, at a the medium behavior normal is more similarstress (e.g., to a dense 200 kPa) or compacted for both species. Therefore,granular it could material. be believed that, when the specimen has the presence of only diatomaceous material,Figure7 the reveals behavior the failure is more envelopes similar obtained to a dense from the or annular compacted shear strengthgranu- test lar material. for the peak shear strength resistance for both diatom microfossil species. With respect to the modified mixtures with Colombian diatomite, at low normal stresses, the shear stress Figure 7 reveals the failure envelopes obtained from the annular shear strength test for the peak shear strength resistance for both diatom microfossil species. With respect to the modified mixtures with Colombian diatomite, at low normal stresses, the shear stress seemed to be very similar for any content of diatom. Meanwhile, with increasing normal effort, the variability tended to be slightly higher. Regarding mixtures manufactured with Mexican diatomite, more pronounced changes are noted from diatomite contents equal to or above 60%. The trend also indicates that, as the diatom content increased from 60% to 100%, the shear stress tended to increase, especially at high normal stresses. Appl. Sci. 2021, 11, 1352 9 of 13

seemed to be very similar for any content of diatom. Meanwhile, with increasing normal effort, the variability tended to be slightly higher. Regarding mixtures manufactured with Mexican diatomite, more pronounced changes are noted from diatomite contents equal to Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 13 or above 60%. The trend also indicates that, as the diatom content increased from 60% to 100%, the shear stress tended to increase, especially at high normal stresses.

300

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Col. species 80 Kaolin 100% Diat. 20% 100 Peak Shear Stress (kPa) Diat. 40% Diat. 60% 40 Diat. 80% Diat. 100%

100 200 300 400 100 200 300 400 Normal Stress (kPa) Normal Stress (kPa) Figure 7.Figure Relationship 7. Relationship between between peak peak shear shear stress stress vs vs.. normalnormal stress stress at differentat different diatom diatom contents. contents. Comparing the results from both diatomite species, the peak shear strength was much Comparing thehigher results when from modifying both the diatomite kaolin mixture species, with Mexicanthe peak diatomite shear insteadstrength of Colombian was much higher whendiatomite, modifying especially the kaolin when mixture the confined with vertical Mexican pressure diatomite was higher instead and withof Co- diatom lombian diatomite,contents especially superior when to 60%. the To confined cite an example, vertical Mexican pressure diatomite was 60% higher displayed and incrementswith of 116%, 33%, and 45% when compared to Colombian diatomite 60% on peak shear stresses diatom contents superior to 60%. To cite an example, Mexican diatomite 60% displayed at 100, 200, and 400 kPa, respectively. In the same way, Mexican diatomite 80% showed increments of 116%,values 33% of, and peak 45% shear when stress compared of 93.85, 154.36, to Col andombian 279.32while diatomite Colombian 60% diatomiteon peak 80% shear stresses at 100,presented 200, and values 400 of kPa, 34.08, respectively. 81.64, and 161.83 In the at vertical same pressuresway, Mex ofican 100, 200, diatomite and 400 kPa, 80% showed valuesrespectively. of peak shear These stress findings of 93.85, support 154.36 the notion, and that279.32 the shapewhile and Col theombian species di- of the atomite 80% presendiatomiteted values influence of 34.08, the peak 81.64 shear, and resistance 161.83 ofat soil. vertical pressures of 100, 200, Similar to peak shear strength, the residual shear strength for all diatomite-kaolin and 400 kPa, respectively.mixtures These was obtained, findings as cansupport be observed the notion in Figure that8. Itthe was shape evident and that, the as species the residual of the diatomite influenceshear strength the peak was recordedshear resistance at large displacements, of soil. the values attained would be lower Similar to peakthan shear the peakstreng shearth, strength.the residual Nonetheless, shear strength similar trends for all were diatomite observed.-kaolin Regarding mixtures was obtainedmixtures, as modifiedcan be observed with Colombian in Figure diatomite, 8. It wa at 100s evident kPa of normal that, stress,as the the residual results were very similar despite the diatom content. However, at large normal stresses, the variance shear strength was recorded at large displacements, the values attained would be lower among the mixtures was higher. On the contrary, mixtures with Mexican diatomite showed than the peak shearhigher strength. dispersion Nonetheless, at 100 kPa and similar lower variabilitytrends were at 400 observed. kPa. Adding Regarding Colombian mix- diatomite, tures modified withno Colombian clear improvement diatomite, in residual at 100 shear kPa stress of wasnormal observed. stress, Mexican the results diatomite were denoted very similar despitehigher the valuesdiatom of residualcontent. shear However, strength, at especially large normal with diatom stresses, contents the equal variance to or higher among the mixturesthan wa 60%,s ashigher. was observed On the in thecontrary, peak shear mixtures strength aswith well. However,Mexican this diatomite improvement was more noticeable at lower vertical pressures. showed higher dispersionFrom at the 100 normal kPa and shear lower stress plots,variability the peak at and400 the kPa. residual Adding friction Colombian angles, which diatomite, no clear areimprovement indicators of thein residual effect interlocking shear stress between was particles observed. and theMexican variation diatomite of the friction denoted higher valuesangle, of were residual calculated shear for allstrength kaolin-diatomite, especially specimens with diatom (see Figure contents9). The cohesion equal for to or higher than 60%both, as peak was and observed residual states in the was peak nulled shear in view strength of the contractive as well. However, volumetric response,this typical for normally consolidated materials. Concerning Colombian diatomite contents, improvement was themore variation noticeable of the frictionat lower angle vertical was almost pressures. negligible. In fact, as the diatomite content increased, the gap between peak and friction angles became smaller. This result suggests

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Mex. species 80 Col. species 80 Kaolin 100% Kaolin 100% Diat. 20% Diat. 20% Residual Shear Stress (kPa) Diat. 40% Diat. 40% Diat. 60% Diat. 60% 40 40 Diat. 80% Diat. 80% Diat. 100% Diat. 100%

100 200 300 400 100 200 300 400 Normal Stress (kPa) Normal Stress (kPa) Figure 8. Relationship between residual shear stress vs. normal stress at different diatom contents. Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 13

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Col. species 80 Kaolin 100% Diat. 20% 100 Peak Shear Stress (kPa) Diat. 40% Diat. 60% 40 Diat. 80% Diat. 100%

100 200 300 400 100 200 300 400 Normal Stress (kPa) Normal Stress (kPa) Figure 7. Relationship between peak shear stress vs. normal stress at different diatom contents.

Comparing the results from both diatomite species, the peak shear strength was much higher when modifying the kaolin mixture with Mexican diatomite instead of Co- lombian diatomite, especially when the confined vertical pressure was higher and with diatom contents superior to 60%. To cite an example, Mexican diatomite 60% displayed increments of 116%, 33%, and 45% when compared to Colombian diatomite 60% on peak shear stresses at 100, 200, and 400 kPa, respectively. In the same way, Mexican diatomite 80% showed values of peak shear stress of 93.85, 154.36, and 279.32 while Colombian diatomite 80% presented values of 34.08, 81.64, and 161.83 at vertical pressures of 100, 200, and 400 kPa, respectively. These findings support the notion that the shape and the species of the diatomite influence the peak shear resistance of soil. Similar to peak shear strength, the residual shear strength for all diatomite-kaolin mixtures was obtained, as can be observed in Figure 8. It was evident that, as the residual shear strength was recorded at large displacements, the values attained would be lower than the peak shear strength. Nonetheless, similar trends were observed. Regarding mixtures modified with Colombian diatomite, at 100 kPa of normal stress, the results were very similar despite the diatom content. However, at large normal stresses, the variance Appl. Sci. 2021, 11, 1352 10 of 13 among the mixtures was higher. On the contrary, mixtures with Mexican diatomite showed higher dispersion at 100 kPa and lower variability at 400 kPa. Adding Colombian diatomite, no clearthat improvement this diatomite leadsin residual to an increase shear in stress the residual was friction observed. angle atMexican a higher ratediatomite than denoted higher valuesthe friction of residual angle. On theshear other strength hand, the, variation especially of the with friction diatom angle was contents higher as equal the to or higher than 60%Mexican, as diatomitewas observed content increased. in the peak Although shear the strength addition of as Mexican well. diatomiteHowever, may this increase both peak and residual friction angles, the growth rate is greater for the peak improvement wasrather more than noticeable the residual at one. lower vertical pressures.

160 160

Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 13

120 120 From the normal shear stress plots, the peak and the residual friction angles, which are indicators of the effect interlocking between particles and the variation of the friction Mex. species 80 angle, wereCol. calculated species for all k80aolin-diatomite specimens (see Figure 9). The cohesion for Kaolin 100% both peak andKaolin residual 100% states was nulled in view of the contractive volumetric response, Diat. 20% Diat. 20% Residual Shear Stress (kPa) typical for normally consolidated materials. Concerning Colombian diatomite contents, Diat. 40% the variation ofDiat. the 40% friction angle was almost negligible. In fact, as the diatomite content Diat. 60% Diat. 60% 40 40 increased, the Diat.gap 80% between peak and friction angles became smaller.Diat. 80% This result suggests that this diatomiteDiat. 100% leads to an increase in the residual friction Diat.angle 100% at a higher rate than

100 200the friction300 angle. On400 the other hand,100 the variation200 of the300 friction angle400 was higher as the NormalMexican Stress (kPa) diatomite content increased. AlthoughNormal the Stress addition (kPa) of Mexican diatomite may Figure 8. Relationshipincrease between both residual peakshear and stressresidual vs. normal friction stress angle at differents, the diatomgrowth contents. rate is greater for the peak Figure 8. Relationshiprather between than theresidual residual shear one. s tress vs. normal stress at different diatom contents.

100

Diatomite content (%) 40

10 15 20 25 30 35 40 Residual/Peak friction angle (°) FigureFigure 9. 9.Variation Variation of theof the effective effective friction friction angle angle respect respect diatomite diatomite content incontent both species. in both species.

A correlation between peak and residual friction angles was also observed for both types of diatomite, as shown in Figure 10. In the case of mixtures modified with Colom- bian diatomite, the relationship between the peak and the residual friction angle was positively direct with a R2 of 0.88, whereas in the case of mixtures modified with Mexican diatomite, a good correlation with a R2 value of more than 0.80 was obtained. However, while the residual friction angle tended to increase, the peak angle was getting lower values. From the graph, it can also be inferred that the addition of higher amounts of Colom- bian diatomite led to an increase in the interlocking effect between particles in short and large deformation levels. Meanwhile, the inclusion of Mexican diatomite also interlocked well with soil particles as its content increased. Nonetheless, at large displacement levels, it tended to dissipate them. Appl. Sci. 2021, 11, 1352 11 of 13

A correlation between peak and residual friction angles was also observed for both types of diatomite, as shown in Figure 10. In the case of mixtures modiﬁed with Colombian diatomite, the relationship between the peak and the residual friction angle was positively direct with a R2 of 0.88, whereas in the case of mixtures modiﬁed with Mexican diatomite, a good correlation with a R2 value of more than 0.80 was obtained. However, while the residual friction angle tended to increase, the peak angle was getting lower values. From the graph, it can also be inferred that the addition of higher amounts of Colombian diatomite led to an increase in the interlocking effect between particles in short and large deformation levels. Meanwhile, the inclusion of Mexican diatomite also interlocked well Appl. Sci. 2021, 11, x FOR PEER REVIEW 11 of 13 with soil particles as its content increased. Nonetheless, at large displacement levels, it tended to dissipate them.

36 80% 100%

60% 32

Peak friction angle (°) 100% 40% 60% 24 20% 20% 40% 22 80%

20 100% Kaolin 18 12 14 16 18 20 22 24 26 Residual friction angle (°) FigureFigure 10. 10.Correlation Correlation between between peak peak and and residual residual friction friction angle. angle. 4. Conclusions 4. Conclusions In this research, the inﬂuence of diatomite species and content in peak and residual In this research, the influence of diatomite species and content in peak and residual shear strength of diatomite-kaolin mixtures was analyzed using the annular shear strength shear strength of diatomite-kaolin mixtures was analyzed using the annular shear test. Scanning electron microscopy and Atterberg limits were also carried out as additional strength test. Scanning electron microscopy and Atterberg limits were also carried out as tests to explain the interlocking effect between the microfossils and the kaolin. Based on theadditional experimental tests to results, explain the the following interlocking conclusions effect between are drawn: the microfossils and the kaolin. Based on the experimental results, the following conclusions are drawn: • According to SEM images, both diatomite species have larger voids content due to • theAccording pores disposed to SEM alongimages, their both surfaces. diatomite The species presence have of larger these poresvoids cancontent be related due to tothe aspects pores disposed such high along water their retention surfaces. that The inﬂuences presence the of these results pores of tests can suchbe related as the to Atterbergaspects such limits. high For water both retention species, thethat higher influences the diatomite the results content of tests was, such the as higherthe At- theterberg liquid limits. limit andFor theboth plastic species, limit the values higher were. the diatomite However, content diatomite-kaolin was, the higher mixtures the manufacturedliquid limit and with the Mexican plastic limit species values displayed were. However, greater values, diatomite suggesting-kaolin thatmixtures this diatomitemanufactured has larger with porosityMexican in species its structure. displayed greater values, suggesting that this • Fordiatomite the annular has larger shear porosity strength in test, its structure. the results show the increment in the peak and • theFor residual the annular strength shear as strength the normal test, stress the results increased. show A linearthe increment behavior in was the observedpeak and the residual strength as the normal stress increased. A linear behavior was observed in almost the totality of the mixtures. Comparing the results from both diatomite species, the peak shear strength was much higher when modifying the kaolin mixture with Mexican diatomite instead of Colombian diatomite, especially when the confined vertical pressure was higher and with diatom contents superior to 60%. Re- garding residual shear strength, Mexican diatomite also denoted higher values of residual shear strength, especially with diatom contents equal to or higher than 60% as was observed in the peak shear strength as well. • The increase of diatomite content in the samples polarized their behavior towards materials similar to compacted sands. Likewise, when the specimens had a higher proportion of clay, their response was contractive, analogous to a normally consolidated clay. Appl. Sci. 2021, 11, 1352 12 of 13

in almost the totality of the mixtures. Comparing the results from both diatomite species, the peak shear strength was much higher when modifying the kaolin mixture with Mexican diatomite instead of Colombian diatomite, especially when the conﬁned vertical pressure was higher and with diatom contents superior to 60%. Regarding residual shear strength, Mexican diatomite also denoted higher values of residual shear strength, especially with diatom contents equal to or higher than 60% as was observed in the peak shear strength as well. • The increase of diatomite content in the samples polarized their behavior towards materials similar to compacted sands. Likewise, when the specimens had a higher proportion of clay, their response was contractive, analogous to a normally consolidated clay. • With respect to the friction angle results, as the diatom content increased, the peak friction angle was also higher. That phenomenon was observed in both species. Nev- ertheless, the Mexican diatomite revealed higher values compared with Colombian diatomite species. Regarding the residual friction angle, mixtures manufactured with Mexican diatomite also displayed higher values when compared to mixes with Colom- bian diatomite. In addition, the difference between peak and residual friction angle was higher in Mexican diatomite mixes. • As has been observed, diatomite has large porosity, low density, and interlocks well with other particles, increasing the shear resistance. Due to these properties, future research is recommended to study the inﬂuence of these microfossils in other composites such as concrete or bituminous mixtures.

Author Contributions: Conceptualization, C.J.S.-A., D.A.Z.-A., J.C.R. and D.C.-F.; methodology, C.J.S.-A. and D.A.Z.-A.; formal analysis, C.J.S.-A. and D.A.Z.-A.; resources, C.J.S.-A. D.A.Z.-A. and D.C.-F.; writing—original draft preparation, C.J.S.-A., D.A.Z.-A. and J.C.R.; writing—review and editing, D.C.-F.; funding acquisition, D.A.Z.-A. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Centro de Investigaciones-Escuela de Ingenieros Militares (ESING), budget item A-02-02-02-008-001, Ejército Nacional de Colombia. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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