Article history:Received 26 December 2012Received in revised form 20 April 2013Accepted 2 May 2013Available online 19 May 2013
Keywords:NanoindentationHardnessResidual modulusSandsFine-grained soilsThermal energy
In the scenario of rapid industrialization and infrastructural development, many situations are encounteredwhere in soil like bentonite is subjected to elevated temperatures (about 200 °C). This may lead to alterationin the mechanical and engineering characteristics (such as hardness and residual modulus) of the soil. In thiscontext, these characteristics can be quantified by measuring deformation of individual soil grains exposed todifferent elevated temperatures, by employing nanoindentation. With this in view, an attempt was made toexplore the potential of this technique, normally used for material characterization by material scientiststo study heat induced alteration in mechanical and engineering characteristics of the metals, to study thebehavior of soils when they get exposed to elevated temperatures. As such, soils of entirely different charac-teristics were exposed up to 200 °C (in steps of 50 °C), nanoindentation studies were conducted on the res-idues and the results are reported in this technical note. It has been observed that there is a significant changein the hardness, residual modulus and resistance to indentation of the soils due to their exposure to elevatedtemperatures. It has also been demonstrated that the effect of elevated temperature is more pronounced onfine-grained soils as compared to coarse-grained soils.
The present day geotechnical engineering related projects dealwith laying of buried power supply cables and air condition ducts(Gangadhara and Singh, 1999), ground modification or stabilizationtechniques using chemicals and thermal treatment (Ma and Hueckel,1992; Alcocer and Chowdhury, 1993; Akinmusuru, 1994; Joshi et al.,1994; Yang and Farouk, 1995; Krishnaiah and Singh, 2006), disposalof high level radioactive (Varlakov et al., 1997) and industrial toxicwastes (Farag, 1993), designing foundations for the furnaces (Li etal., 2011), boiler units, forging units, brick kilns, rocket launchingpads, volcanic eruptions, underground explosions, etc. In such situa-tions the soil gets exposed to elevated temperatures (about 200 °C),which might result in particle breakage and alteration of surface char-acteristics such as physical (changes in the specific gravity, Yilmaz,2011; specific surface area and particle size, Utkaeva, 2007), chemical(variation in CEC, pH, EC; Parlak, 2011) and mineralogical (Ghumanand Lal, 1989; Certini, 2005; Hatten et al., 2005). Furthermore,changes in these properties would also influence their engineeringbehavior (such as hardness and residual modulus). Hence, it becomesessential to quantify and analyze, accurately, the influence of elevatedtemperatures on the soil, which is a conglomerate of various grainsor particles (Lado and Ben-Hur, 2004). However, due to lack of un-derstanding related to such exposure conditions of the soil, proper
instrumentation and analytical techniques, little efforts have beenmade in the past to understand and measure variations in the behaviorof the soil when it is exposed to elevated temperatures. Incidentally,nanoindentation technique appears to be a panacea for establishingchanges in mechanical properties of soils under these circumstancesand it has been observed that the potential of nanoindentation hasbeen exploited in the various fields of material characterization bymaterial scientists [viz., determination of residual modulus, hardness(Doerner and Nix, 1986; Oliver and Pharr, 1992; Field and Swain,1993, 1995; Swain, 1998); cracking, phase transformations, creep andenergy absorption (Fischer-Cripps, 2004); hardness and modulus ofultrathin films and coatings (Hakiri et al., 2009; Han and Joost, 2009);hardness and modulus of minerals (Dutta and Penumadu, 2007;Bathija, 2009; Wei, 2009; Zhang et al., 2009); quantifying fracturetoughness and interlayer adhesion of semiconductors (Volinsky et al.,2003); mechanical properties and creep behavior of lyocell fibers andconcrete (Lee et al., 2007; Vandamme and Ulm, 2009); mechanicalproperties of shale (Ulm and Abousleiman, 2006; Ulm et al., 2007;Bobko and Ulm, 2008; Ortega et al., 2010; Deirieh et al., 2012);mechan-ical properties of concrete (Dejong and Ulm, 2007; Ulm et al., 2007;Miller et al., 2008; Vandamme and Ulm, 2009)]. These studies also indi-cate that nanoindentation technique has been widely used in the fieldof metallurgical engineering to evaluate material performance.
Though, in this context earlier researchers (Dutta and Penumadu,2007; Bathija et al., 2009; Penumadu et al., 2009; Wei, 2009; Zhanget al., 2009) have demonstrated application of nanoindentation ingeomechanics successfully, this technique has not yet been employedto establish changes undergone by the soil when it gets exposed to
15S. Kadali et al. / Engineering Geology 162 (2013) 14–21
elevated temperatures. With this in view, attempts were made toexplore the utility of the nanoindentation for establishing changesin the mechanical properties (viz., hardness and residual modulus) ofthe fine- and coarse-grained soils when they get exposed to elevatedtemperatures. Details of the methodology adopted for this purposeare presented in this technical note and based on a critical synthesis oftest results; the utility of nanoindentation in understanding the behav-ior of soils exposed to elevated temperature has been demonstrated.
2. Experimental investigations
2.1. Materials and their characterization
Two commercially available soils; bentonite and white clay desig-nated as BT and WC, respectively, two naturally occurring soils, sam-pled from western region of India, designated as S1 and S2, and onestandard sand sample, designated as SS, were used in this study.These five soils were characterized for establishing their physical,chemical and mineralogical characteristics by conducting a series ofinvestigations. For the sake of completeness, and clarity, details ofthese investigations are presented in the following.
2.2. Physical characterization
2.2.1. Specific gravityThe specific gravity, G, of the soil sample was determined with
the help of an ULTRA-PYCNOMETER (Quanta-chrome, USA), which em-ploys helium gas as a displacing fluid as per the guidelines provided byASTM D 5550-06. For the sake of accuracy, the average specific gravityobtained from the results of three tests is reported in Table 1.
2.2.2. Gradational and consistency characteristicsThe particle-size distribution characteristics and consistency limits
(i.e., the Atterberg limits) of the soil sample were determined as perthe guidelines provided by ASTM D 422-63; ASTM D 4318-93 andASTM D 427, respectively. Consequently, soil samples have been clas-sified based on the Unified Soil Classification System, USCS, ASTM D2487-10. The test results are presented in Table 1 from which it canbe noticed that the soils considered for this study are of entirely differ-ent characteristics.
2.2.3. Specific-Surface AreaThe Specific Surface Area, SSA, of the soil samples was determined
by employing ethylene glycol monoethyl ether, EGME, method whichhas been shown to be the most efficient method for determining theSSA (Carter et al., 1986; Cerato and Lutenegger, 2002; Arnepalli et al.,2008). The amount of EGME, Wa, that gets absorbed on per gram ofthe soil, Ws, was computed by subtracting the dry weight of the sam-ple from the weight of the EGME mixed sample. Subsequently, byemploying Eq. (1), the SSA of the sample was determined and theresults are presented in Table 1.
SSA ¼ Wa⋅ 0:000286⋅Wsð Þ−1: ð1Þ
Table 1Physical characteristics of different soils.
Note: CH: clay of high plasticity; CL: clay of low plasticity; SP: poorly graded sand.
2.3. Mineralogical characterization
The mineralogical composition of the soil sample was determinedwith the help of an X-ray diffraction spectrometer (Phillips, Eindhoven,the Netherlands), which is fitted with a graphite monochromatic andemploys Cu-Kα as the source. Minerals present in the sample wereidentified with the help of the Joint Committee on Powder DiffractionStandards, (JCPDS, 1994) search files, from the diffractograms and arelisted in Table 2. It can be observed from the table that the soil samplesconsist of awide range ofminerals andhence theirmineralogy is entirelydifferent.
2.4. Chemical characterization
2.4.1. Chemical compositionChemical composition of the soil sample, in the form ofmajor oxides,
was determined using an X-ray Fluorescence setup, XRF (Phillips 1410,Holland). Details of the sample preparation are presented in the follow-ing. 4 g finely powdered sample, 1 gmicrocrystalline cellulose and a fewdrops of isopropyl alcohol were mixed thoroughly and the mixture waskept below an infrared lamp for slow drying. A small aluminum dish(with inner diameter of 33 mm and height of 12 mm) was taken andtwo thirds of this dish was filled with mixture of 70% methyl-celluloseand followed by filling up the container by the dried sample. For makinga pellet, the dish was compressed with the help of a hydraulic jack byapplying a load of 15 ton. The chemical composition of the sample hasbeen determined by mounting the pellet in the sample holder of theXRF setup and the results are presented in Table 3.
2.4.2. Cation-exchange capacityThe cation-exchange capacity, CEC, of the soil signifies its capacity
to retain cations up to its highest limit; or it can also be defined asthe power of the soil to combine with cations in such a manner thatthey cannot be easily removed by leaching with water, but can be ex-changed by an equivalent amount of other cations. Capacity of a soil tohold cationsmainly depends on pH and ionic strength of the soil–fluidsystem, and the presence of salts. The guideline presented by IS, 2720Part XXIV-1976 were followed for determination of the CEC of thesamples used in this study. CEC of the sample can be obtained byemploying Eq. (2). The chemical properties of the soils used in thisstudy are listed in Table 4, for the sake of completeness.
CEC ¼Concentration of Na
μgml
� �� 100� Vol: of extract mlð Þ
Equivalent wt: of the cation� 1000�wt: of sample gð Þ : ð2Þ
16 S. Kadali et al. / Engineering Geology 162 (2013) 14–21
Furthermore, pH, electrical conductivity, EC, and total dissolvedsolids, TDS, of the soil were measured by employing a water qualityanalyzer (Model PE 136, Elico Ltd., India) and the results are presentedin Table 4.
2.5. Nanoindentation experimentation
Response of the soils considered in this study was obtained byresorting to the methodology mentioned in the following. In case offine-grained soils, as soil grains are very fine in size (≤75 μm), itis practically impossible to carry out nanoindentation on individualparticles. In this context, some researchers have employed samplesdried up from their wet state to study the nanoindentation responseof clays (Bathija, 2009). However, in authors' opinion, such atechnique is not recommended for the soils with active minerals
a bAll dimensions in mm an
50
12
12
11.8
50
35
Fig. 1. The die used for preparation
a
MicroscopeNanoindenter
Platform
Fig. 2. Details of the test setup (a) nanoindenter with the
(viz., montmorillonite and illite), which would result in crackedsamples when they dry. Also, due to the swelling and shrinking be-havior of the minerals, predominant in clays, the surface of the soilsample will not be uniform and hence nanoindentation resultswould be erroneous. To overcome these difficulties, authors haveadopted a novel methodology for preparing the sample fornanoindentation, as mentioned in the following.
As depicted in Fig. 1, a stainless steel die of 50 mm outer diameter,50 mm height and containing a central hole of 12 mm diameter(refer Figure 1a) was specially fabricated for preparing the specimensof fine-grained soils. A stainless steel cylindrical base cap (diameter35 mm and 15 mm in height) with a small plunger (11.8 mm indiameter and 12 mm in height), refer Fig. 1b, was fabricated andfixed at the bottom portion of the die and it acts as the cushion andfacilitates ease of sample preparation. 2 g of the soil was filled inthe hole of the die and with the help of a plunger/piston (66 mm inlength and 11.8 mm in diameter), which has a circular loading pad(15 mm thickness and 35 mm in diameter) attached to it (referFigure 1c), soil was compressed by installing it on a universal testingmachine and applying a load of 3 T (≈0.265 GPa). Subsequently, thepellet (12 mm in diameter, 6 mm in height and 2.15 ± 0.1 g/cm3,refer Figure 1d) was extracted from the die by pushing it out withthe help of plunger. This pellet was exposed to elevated temperature;up to 200 °C in steps of 50 °C, by mounting it on the heating plate,refer Fig. 2(b), of the nanoindenter (Model: TI 900Tribolndenter,
c dd figure not to scale
12
6
35
66
11.8
15
(a) The die
(b) Base unit
(c) Piston/Plunger
(d) The Pellet (specimen)
of fine-grained soil specimens.
b
Heating platePellet (specimen)
microscope and (b) sample mounted on the platform.
Fig. 4. Scanning electron micrograph of the indentation on a typical specimen of thefine-grained soil (soil S1).
All dimensions in mm and figure not to scale
12
30
15
12
12
(a) The die(b) The pellet (specimen)
a b
Soil grains
Fig. 6. The die used for preparation of coarse-grained soil specimens.
17S. Kadali et al. / Engineering Geology 162 (2013) 14–21
marketed by Hysitron, USA). This machine contains a CCD cameraattached to an optical microscope; refer Fig. 2(a). A Berkovich in-denter which is basically a three sided pyramid of radius of curvature~150 nm, refer Figs. 2(a) and 3, has been used in this study. Thisindenter measures the force imposed on the pellet (read specimen)and the corresponding displacement is recorded with the help ofLVDTs. Fig. 4 depicts impression of the indenter on a typical speci-men of the fine-grained soil (soil S1).
However, for preparing specimens of the coarse-grained soils(viz. soil SS) the observations and guidelines reported by earlier re-searchers (Dutta and Penumadu, 2007) have been found to be quitehelpful. These researchers have opined that though it is possible tostudy the response of an individual particle of the coarse-grained soils(viz. sands), holding these particles in place is a challenging task. Also,the epoxy used for binding these particles melts at elevated
temperatures and it is not advisable to conduct nanoindentation oncoarse-grained soils at higher temperatures. Keeping this in view, spec-imens of the soil SS, for nanoindentation, were prepared by employing adie of outer and inner diameters of 30 mmand12 mm, respectively, andheight 15 mm, as depicted in Fig. 6. The sequence of specimen prepara-tion is depicted in Fig. 5. First, the hole of the die is lubricated by usingsilica grease and the die is placed on a smooth glass plate. Later, thecoarse-grained soil is filled in the die up to 2 to 3 mm height, withoutcompacting it, and subsequently the die is filled up with an epoxy(Acralyn ‘R’, Asian Acrylates, Mumbai, India). After about one hour,which is the time taken by the epoxy to set; the specimen is retrievedout of the die, by pushing it with a small plunger. The pellet/specimenis depicted in Fig. 6(b). To overcome the difficulties associated with theheating of the specimen with epoxy, sand grains were pre-heated in amuffle furnace at different temperatures. A typical specimen beforenanoindentation, as seen by employing the SEM, is depicted in Fig. 7.
Calibration of the nanoindenter was accomplished by employingfused Quartz, PN:5-0098, which is 3 mm thick and a square of10 mm, refer Fig. 8. The indentation load, P, was applied at a constantrate of loading of 100 μN/s to obtain load versus displacement, h,relationship (subsequently referred as P vs. h relationship). Typicalresults for the Quartz crystal are depicted in Fig. 9. From the figureit can be noted that after the peak load, Pmax, (=8000 μN) has beenattained, it was maintained for 5 s, for obtaining the hold segmentof the P vs. h relationship, and subsequently, the unloading wasdone (at the rate of 100 μN/s). This facilitates unloading responseof the sample as depicted in the figure. Incidentally, the Er and Hvalues for the Quartz crystal are found to be 69.92 GPa and 8.63 GPa,respectively, which match very well with the values reported bythe manufacturer for the standard Quartz crystal, 69.6 GPa ± 5%and 9.25 GPa ± 5%, respectively (Hysitron Inc., Fused Quartz SamplePN: 5-0098).
) coarse-grained soil, (4) epoxy (5) the pellet ecimen)
18 S. Kadali et al. / Engineering Geology 162 (2013) 14–21
Incidentally, Table 5 depicts that even for different values ofthe applied load, P, mechanical properties of the soil (i.e., Er and H)do not change, significantly, and hence 1000 μN and 10,000 μNloads were selected for indentation of fine- and coarse- grained soils,respectively.
0 50 100 150 200 250
0
2000
4000
6000
8000
1
Hold
P (
μN)
h (nm)
Standard fused Quartz crystal H = 8.63 GPa Er = 69.92 GPa
Pmax
S
hmax
hf
LoadingUnloading
Fig. 9. The load versus displacement response of the standard Quartz crystal.
2.6. Computation of residual modulus and hardness of the sample
Following the methodology presented in the previous section,the nanoindentation response of different soils (original as well asexposed to elevated temperatures) was obtained (refer Figure 11).The mechanical properties (Residual Modulus and Hardness) of thematerial, which are mainly dependent on indentation parameters(refer Figure 10, Oliver and Pharr, 1992), can be computed from theload–displacement relationships obtained from the nanoindentationexperiments. Subsequently, the residual modulus, Er, and hardness,H, of the sample were obtained by Eqs. (3) and (4), respectively, asexplained below.
H ¼ PAc
ð3Þ
Er ¼ffiffiffiπ
p
2βffiffiffiffiffiAc
p S ð4Þ
Ac ¼ 24:5 h2c þ Σ7i¼0Ci h
1=2c ð5Þ
hc ¼ hmax−hsð Þ ð6Þ
hs ¼ εPmax
Sð7Þ
where P is the applied load(in μN), Ac is the projected contact areabetween the indenter and the sample (in nm2) and is expressed byEq. (3), β is a dimensionless correction factor for the indenter tipshape (Oliver and Pharr, 1992, 2004), Ci is the coefficient defining‘tip wearing condition’ and can be obtained from calibration indenta-tion tests on standard Quartz, hc is contact depth, hs is surface dis-placement at the contact perimeter (refer Figure 10), ε is a constantdepends on indenter tip geometry and S is the contact stiffness,which is defined as the slope of the initial unloading curve at themaximum indentation depth, hmax, as depicted in Fig. 9.
P: applied load hs: surface displacement at the contact perimeterhc: contact depth hr: plastic deformation after unloading hmax:total indentation displacement LS: Line of symmetry
IndenterInitial Surface
Deformed Surface profile (loading)
hmaxhs
hr
hc
P
LS
Residual Surface profile (unloading)
Fig. 10. Details of various indentation parameters.
19S. Kadali et al. / Engineering Geology 162 (2013) 14–21
0.01
0.1
1
10
1
10
100
25 50 75 100 125 150 175 200100
1000
SoilBT WC S1 S2 SS
H (
GP
a)
a
b
Er (
GP
a)
ch m
ax (n
m)
(oC)θ
Fig. 12. Variation of the (a) hardness, (b) residual modulus and (c) maximum depth ofindentation with respect to temperature for different soils.
20 S. Kadali et al. / Engineering Geology 162 (2013) 14–21
3. Results and discussion
Following the abovementionedmethodology the nanoindentationwas performed on samples of soils BT, WC, S1, S2 and SS, correspond-ing to the ambient (25 ± 1 °C) and elevated temperatures. For thesake of completeness, P vs. h relationship for various soil samplesis depicted in Fig. 11. Furthermore, by employing Eqs. (3) and (4),residual modulus, Er and hardness, H, of these samples was computed,respectively and the same are listed in Table 6.
The results of the nanoindentation experiments revealed thatthere is appreciable change in the mechanical properties of the soilswhen exposed to elevated temperatures. From the load vs. displace-ment plots (refer Figure 11) it can be clearly observed that, as thetemperature increases the plots get shifted i.e., towards left for allthe samples (BT, WC, S1 and S2). This indicates that when fine-grained soils are subjected to elevated temperatures, their resistanceto penetration of the indenter increases. On the contrary, temperaturehas no appreciable influence on the load versus displacement plots ofthe coarse-grained soil (viz., soil SS). Further from Fig. 11, it can benoticed that at ambient temperature slopes of the loading portion ofthe P vs. h relationship for different soils are gentle as compared totheir counterparts at elevated temperature (at 200 °C). This impliesthat alteration in the material properties occurs at higher tempera-ture. It may be attributed to the variation in elasto-plastic behaviorof the material with temperature.
Furthermore, using the data presented in Table 6, the variation ofH, Er and hmax with temperature, θ, was developed as depicted inFig. 12. The data presented in the table corresponds to average often trials and the standard deviation is found to be 10 to 15%. Thisfigure indicates that with increase in temperature, surface hardnessof the soil particles as well as the residual modulus increases withtemperature for soils (BT, WC, S1 and S2). It can be observed fromthe trends depicted in the figure that contrary to the other soils, soilSS, which is a coarse-grained soil, does not exhibit any appreciablechange in the three parameters with respect to elevated temperature.This could be attributed to the presence of Quartz (refer Table 2), asthe primary mineral in this soil, and for which the properties remainunaffected at elevated temperatures (θ ≤ 200 °C). On the contrary, asdepicted in Fig. 12, for soil WC, exhibits minimum changes at elevatedtemperatures, when compared to soils BT, S1 and S2, because WC,which mainly constitutes of passive and soft mineral Kaolinite (referTable 2). It should also be noted that the H value for the Kaoliniteand Quartz have been reported to be 0.017 and 8.157 GPa, respectively.A good matching of the results from those reported in the literature(Oliver and Pharr, 1992; Pharr, 1998; Dutta and Penumadu, 2007;Mikowski et al., 2007; Bathija et al., 2009; Daphalapurkar et al., 2011)is a testimony of good experimentation methodology adopted by theauthors.
4. Concluding remarks
In this study potential of the nanoindentation technique, which hasbeen employed bymaterial scientists for characterization of metals, hasbeen exploited to demonstrate the changes undergone by the fine- andcoarse- grained soils when they are exposed to elevated temperatures.This study, being novel in itself, can also be employed for quantifyingthe response of the swelling and shrinking minerals, predominantlypresent in fine-grained soils. It has been demonstrated that the fine-grained soils, in contrast to the coarse-grained soils, are more suscepti-ble to changes in hardness, residual modulus and resistance to indenta-tion, when exposed to elevated temperatures. Hence, the methodologypresented in this paper would be helpful in understanding the changesundergone by the soil when it is subjected to adverse environmentwhich prevails in landfills, nuclear waste repositories, undergroundstorage tanks and thermally stabilized soils. It is believed that sucha study, being fundamental in nature, will also be quite useful in
developing constitutive laws for the soils when they get exposed toelevated temperatures.
Nomenclatureε constant for indenter tip geometryθ temperatureβ correction factorAc area of contactCEC cation exchange capacityEC electrical conductivityEGME ethylene glycol monoethyl etherEr residual modulusG specific gravityh displacementH hardnesshc contact depthhmax maximum displacementhr plastic deformationhs surface displacementLL liquid limitP indentation loadPI plasticity index
21S. Kadali et al. / Engineering Geology 162 (2013) 14–21
PL plastic limitpmax maximum loadS slope of the load deformation curveSL shrinkage limitSSA specific surface areaTDS total dissolved solidsUSCS unified soil classification systemWa weight of EGME absorbed on the soilWs weight of dry soilXRD X-ray diffractionXRF X-ray fluorescence
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