DISPOSICIONES GENERALES
II. PROCESO DE CONTROL DE PLAZAS 4. ASPECTOS GENERALES
10. LIMITES DE COMPETENCIA PARA AUTORIZAR MOVIMIENTOS DE PERSONAL
Principle Differential thermal analysis (DTA) con-
sists of simultaneously heating a test sample and a thermally inert substance at constant rate (usually
OTHER METHODS FOR COMPOSITIONAL ANALYSIS 75
Table 3.7 X-ray Diffraction Data for Clay Minerals and Common Nonclay Minerals
d (A˚ ) Minerala d (A˚ ) Minerala
14 Mont. (VS) Chl. Verm. (VS)b 2.93–3.00 Felds.
12 Sepiolite, heated corrensite 2.89–2.90 Carb.
10 Illite, Mica (S), Halloysite 2.86 Felds.
9.23 Heated Verm. 2.84 Carb. Chl.
7 Kaol. (S). Chl. 2.84–2.87 Chl.
6.90 Chl. 2.73 Carb.
6.44 Attapulgite 2.61 Attapulgite
6.39 Felds. 2.60 Verm., Sepiol.
4.90–5.00 Illite, Mica, Halloysite 2.56 Illite (VS), Kaol.
4.70–4.79 Chlor. (S) 2.53–2.56 Chlor., Felds., Mont.
4.60 Verm. (S) 2.49 Kaol. (VS)
4.45–4.50 Illite (VS), Sepiolite 2.46 Quartz, heated Verm.
4.46 Kaol. 2.43–2.46 Chlorite
4.36 Kaol. 2.39 Verm., Illite
4.26 Quartz (S) 2.38 Kaol.
4.18 Kaol. 2.34 Kaol. (VS)
4.02–4.04 Felds. (S) 2.29 Kaol. (VS)
3.85–3.90 Felds. 2.28 Quartz, Sepiol.
3.82 Sepiol. 2.23 Illite, Chl.
3.78 Felds. 2.13 Quartz, Mica
3.67 Felds. 2.05–2.06 Kaol. (WK)
3.58 Carbonate, Chl. 1.99–2.00 Mica, Illite (S), Kaol. Chl.
3.57 Kaol. (VS), Chl. 1.90 Kaol.
3.54–3.56 Verm. 1.83 Carb.
3.50 Felds., Chlor. 1.82 Quartz
3.40 Carb. 1.79 Kaol.
3.34 Quartz (VS) 1.68 Quartz
3.32–3.35 Illite (VS) 1.66 Kaolin
3.30 Carb. 1.62 Kaolin
3.23 Attapulgite 1.54B Verm. (S), Quartz
3.21 Felds. 1.55 Quartz
3.20 Mica 1.58 Chl.
3.19 Felds. (VS) 1.53 Verm., Illite
3.05 Mont. 1.50 Ill. (S), Kaol.
3.04 Carb. (VS) 1.48–1.50 Kaol. (VS), Mont.
3.02 Felds. 1.45B Kaol.
3.00 Heated Verm. 1.38 Quartz, Chl.
2.98 Mica (S) 1.31, 1.34, 1.36 Kaol. (B)
a(B)⫽broad; (S)⫽strong; (VS) ⫽very strong; (WK)⫽weak; Mont.⫽montmorillonite; Ch1.⫽chlorite; Verm.⫽
vermiculite; Kaol.⫽kaolinite; Carb.⫽carbonate; Felds.⫽feldspar; Sepiol.⫽sepiolite.
bItalics indicates (001) spacing.
about 10C / min) to over 1000C and continuously measuring differences in temperature between the sam- ple and the inert material. Differences in temperature between the sample and the inert substance reflect re- actions in the sample brought about by the heating.
Thermogravimetric analyses, based on changes in
weight caused by loss of water or CO2 or gain in ox-
ygen, are also used to some extent. Thermal analysis techniques are described in detail by Tan et al. (1986). The results of differential thermal analysis are pre- sented as a plot of the difference in temperature between sample and inert material ( T ) versus tem-
perature (T ) as indicated in Fig. 3.39. Endothermic re-
actions are those wherein the sample takes up heat,
Copyrighted Material
Table 3.8 X-ray Identification of the Principal Clay Minerals (⬍2m) in an Oriented Mount of a Clay Fraction Separated from Sedimentary Material
Mineral Basal d Spacings (001)
Glycolation Effect
(1 h, 60C) Heating Effect (1 h)
Kaolinite 7.15 A˚ (001); 3.75 A˚ (002) No change Becomes amorphous 550–
600C Kaolinite, disordered 7.15 A˚ (001) broad; 3.75 A˚
broad
No change Becomes amorphous at lower
temperatures than kaolinite Halloysite, 4H2O
(hydrated)
10 A˚ (001) broad No change Dehydrates to 2H2O at 110C Halloysite, 2H2O
(dehydrated)
7.2 A˚ (001) broad No change Dehydrates at 125–150C;
becomes amorphous 560– 590C
Mica 10 A˚ (002); 5 A˚ (004)
generally referred to as (001) and (002)
No change (001) becomes more intense on heating but structure is maintained to 700C
Illite 10 A˚ (002), broad, other
basal spacings present but small
No change (001) noticeably more intense on heating as water layers are removed; at higher temperatures like mica Montmorillonite group 15 A˚ (001) and integral
series of basal spacings
(001) expands to 17 A˚ with rational sequence of higher orders
At 300C (001) becomes 9 A˚
Vermiculite 14 A˚ (001) and integral series of basal spacings
No change Dehydrates in steps
Chlorite, Mg-form 14 A˚ (001) and integral series of basal spacings
No change (001) increases in intensity;
⬍800C shows weight loss but no structural change Chlorite, Fe-form 14 A˚ (001) less intense
than in Mg-form; integral series of basal spacings
No change (001) scarcely increases; structure collapses below 800C
Mixed-layer minerals Regular, one (001) and
integral series of basal spacings
No change unless an expandable component is present
Various, see descriptions of individual minerals
Random, (001) is addition
of individual minerals and depends on amount of those present
Expands if
montmorillonite is a constituent
Depends on minerals present in interlayered mineral
Attapulgite (palygorskite)
High intensity d reflections at 10.5, 4.5, 3.23, and 2.62 A˚
No change Dehydrates stepwise (see
description) Sepiolite High intensity reflections at
12.6, 4.31, and 2.61 A˚
No change Dehydrates stepwise (see
description) Amorphous clay,
allophane
No d reflections No change Dehydrates and loses weight
Compiled by Carroll (1970).
OTHER METHODS FOR COMPOSITIONAL ANALYSIS 77
Figure 3.38 Pictorial representation of response of phyllosilicates to differentiating treat-
ments. Approximate spacings in nm (1 nm ⫽ 10 A˚ ) (from Whittig and Allardice, 1986).
Reproduced with permission from The American Society of Agronomy, Inc., Madison, WI.
and in exothermic reactions, heat is liberated. Analysis of test results consists of comparing the sample curve with those for known materials so that each deflection can be accounted for.
Apparatus Apparatus for DTA consists of a sample holder, usually ceramic, nickel, or platinum; a furnace; a temperature controller to provide a constant rate of heating; thermocouples for measurement of tempera- ture and the difference in temperature between the sample and inert reference material; and a recorder for the thermocouple output. The amount of sample re-
quired is about 1 g. Although the temperatures at which thermal reactions take place are a function only of the sample, the size and shape of the reaction peaks depend also on the thermal characteristics of the ap- paratus and the heating rate.
Reactions Producing Thermal Peaks The impor- tant thermal reactions that generate peaks on the ther- mogram are:
1. Dehydration Water in a soil may be present in
three forms in addition to free pore water: (1)
Figure 3.39 Thermogram of a sandy clay soil.
adsorbed water or water of hydration, which is driven off at 100 to 300C, (2) interlayer water such as in halloysite and expanded smectite, and (3) crystal lattice water in the form of (OH) ions, the removal of which is termed dehydroxylation. Dehydroxylation destroys mineral structures. The temperature at which the major amount of crystal lattice water is lost is the most indicative property for identification of minerals. Dehydration reac- tions are endothermic and occur in the range of 500 to 1000C.
2. Crystallization New crystals form from amor-
phous materials or from old crystals destroyed at a lower temperature. Crystallization reactions usually are accompanied by an energy loss and, thus, are exothermic, occurring between 800 and 1000C.
3. Phase Changes Some crystal structures change
from one form to another at a specific tempera- ture, and the energy of transformation shows up as a peak on the thermogram. For example, quartz changes from the to form reversibly at 573C. The peak for the quartz phase change is sharp, and its amplitude is nearly in direct pro- portion to the amount of quartz present. The quartz peak is frequently masked within the peak for some other reacting material, but may be readily identified by determining the thermogram during cooling of the sample or by letting it cool first and then rerunning it. The other minerals are destroyed during the initial run while the quartz reaction is reversible.
4. Oxidation Exothermic oxidation reactions in-
clude the combustion of organic matter and the oxidation of Fe2⫹ to Fe3⫹. Organic matter oxi- dizes in the 250 to 450C temperature range.
Beside quartz, the only common nonclay minerals in soils that give thermal reactions with large peaks are carbonates and free oxides such as gibbsite, brucite, and goethite. The carbonates give very large endother- mic peaks between about 800 and 1000C, and the ox- ides have an endothermic peak between about 250 and
450C. Thermograms for many clay and nonclay min-
erals are presented by Lambe (1952).
Quantitative Analysis Theoretically, the area of the reaction peak is a measure of the amount of mineral present in the sample. For sharp, large amplitude peaks such as the quartz inversion at 573C and the kaolinite
endotherm at 650C, the amplitude can be used for
quantitative analysis. In either case, calibration of the apparatus is necessary, and the overall accuracy is of the order of plus or minus 5 percent.
Optical Microscope
Both binocular and petrographic microscopes can be used to study the identity, size, shape, texture, and con- dition of single grains and aggregates in the silt and sand size range; for study in the thin section of the fabric, that is, the spatial distribution and interrelation- ships of the constituents; and for study of the orien- tations of groups of clay particles. Because the in-focus depth of field decreases sharply as magnification in- creases, study of soil thin sections is impractical at magnifications greater than a few hundred. Thus, in- dividual clay particles cannot usually be distinguished using an optical microscope.
Useful information about the shape, texture, size, and size distribution of silt and sand grains may be obtained directly without formal previous training in petrographic techniques. Some background is needed to identify the various minerals; however, relatively simple diagnostic criteria that can be used for identi- fication of over 80 percent of the coarse grains in most soils are given by Cady et al. (1986). These criteria are based on such factors as color, refractive index, birefringence, cleavage, and particle morphology. The nature of surface textures, the presence of coatings, layers of decomposition, and so on are useful both for interpretation of the history of a soil and as a guide to the soundness and durability of the particles.
Electron Microscope
With modern electron microscopes it is possible to re- solve distances to less than 100 A˚ , thus making study of small clay particles feasible. Electron diffraction study of single particles may also be useful. Electron diffraction is similar to X-ray diffraction except an electron beam instead of an X-ray beam is used.
QUANTITATIVE ESTIMATION OF SOIL COMPONENTS 79
Magnetic lenses that refract an electron beam form the basis of the transmission electron microscope (TEM) optical system. An electron beam is focused on the specimen, which is usually a replica of the surface structure of the material under study. Some of the elec- trons are scattered from the specimen, and different parts of the specimen appear light or dark in proportion to the amount of scattering. After passing through a series of lenses, the image is displayed on a fluorescent screen for viewing. Probably the most critical aspect of successful transmission electron microscopy is spec- imen preparation.
In the scanning electron microscope (SEM), second- ary electrons emitted from a sample surface form what appear to be three-dimensional images. The SEM has
a ⫻20 to ⫻150,000 magnification range and a depth
of field some 300 times greater than that of the light microscope. These characteristics, coupled with the fact that clay particles themselves and fracture surfaces through soil masses may be viewed directly, have led to extensive use of the SEM for study of clays. Ex- amples of electron photomicrographs of clays and soils are given earlier in this chapter and in Chapter 5. Prin- ciples of electron microscopy techniques and addi- tional examples are presented in McCrone and Delly (1973) and Sudo et al. (1981).
3.24 QUANTITATIVE ESTIMATION OF SOIL
COMPONENTS
Qualitative X-ray diffraction and a few simple tests will generally indicate the minerals present in a soil. More data are needed, however, for more precise quan- titative estimates. As a rule, the number of different analyses needed is equal to the number of mineral spe- cies present. The results of glycol adsorption, cation exchange capacity, X-ray diffraction, differential ther- mal analysis, and chemical tests all give data that may be used for quantitative estimations. Some pertinent identification criteria and reference values for the clay minerals are given in Table 3.9.
After the quantities of organic matter, carbonates, free oxides, and nonclay minerals have been deter- mined, the percentages of clay minerals are estimated using the appropriate glycol adsorption, cation ex-
change capacity, K2O, and DTA data. The nonclays
can be identified, and their abundance determined, us- ing the microscope, grain size distribution analysis, X- ray diffraction, and DTA. The amount of illite is estimated from the K2O content since this is the only
clay mineral containing potassium. The amount of ka- olinite is most reliably determined from the 600C
DTA endotherm amplitude. If X-ray has indicated montmorillonite, chlorite, and / or vermiculite, then quantitative estimates are made based on the glycol adsorption and exchange capacity data. The total exchange capacity and glycol retention are ascribed to the clay minerals, and the measured values must be accounted for in terms of proportionate contributions by the different clay minerals present.
As a simple example, assume that quartz, illite, and smectite are identified in the⫺2m fraction of a soil. Additional data indicate 4.0 percent K2O, ethylene gly-
col retention of 100 mg / g, and a cation exchange ca- pacity of 35 meq / 100 g. Then, assuming 9 percent as an average value of for pure illite (Table 3.9), the con- tent of illite is estimated at 4.0 / 9.0, or 44 percent. Be- cause only the illite and smectite will contribute to the glycol adsorption, the amount of smectite may be es- timated:
0.44⫻ 60⫹ S⫻ 300⫽ 100
100⫺ 26.4
⬖S⫽ ⫽ 25%
300
The remaining 31 percent can be ascribed to quartz and other nonclay components. For this clay mineral composition, the theoretical cation exchange capacity should be, based on the reference values in Table 3.9:
0.44⫻ 25 ⫹ 0.25 ⫻ 85⫽ 11 ⫹ 21⫽ 33 meq / 100 g
This compares favorably with the measured quantity of 35 meq / 100 g. Thus, the composition of the clay size fraction is
Illite 44%
Smectite 25%
Quartz and other nonclays 31%
The main difficulty in this method for quantitative min- eralogical analysis is the uncertainty in the reference values for the different clay minerals.
A semiquantitative analysis is sufficient for most ap- plications. This may be done as follows. The silt and sand fraction can be examined by microscope and the approximate proportion of nonclay minerals deter-
mined. The amount of clay size material ⫺2 m can
be estimated by grain size distribution analysis. As a first approximation, it may be assumed that the amount of clay mineral equals at least the amount of clay size. This assumption is justified for the following reasons. Nonclay minerals, principally quartz, are found in the clay size fraction. On the other hand, for most soils,
Table 3.9 Summary of Clay Mineral Identification Criteria—Reference Data for Clay Mineral Identification (⫺2-m fraction) Clay X-ray d(001) Glycol (mg / g) CEC (meq / 100 g) K2O (%) DTAa
Kaolinite 7 16 3 0 End. 500–660 ⫹Sharpb
Exo. 900–975Sharp
Dehydrated halloysite 7 35 12 0 Same as kaolinite but 600 peak
slope ratio⬎2.5
Hydrated halloysite 10 60 12 0 Same as kaolinite but 600peak
slope ratio⬎2.5
Illite 10 60 25 8–10 End. 500–650Broad
End. 800–900Broad Exo. 950 Vermiculite 10–14 200 150 0 Smectite 10–18 300 85 End. 600–750 End. 900 Exo. 950 Chlorite 14c 30 40 0 End. 610 10or 720 20
aFor clays prepared at same relative humidity the size of the 100–300C endotherm (adsorbed water removal) increases
in the order kaolinite–illite–smectite.
bFor samples started at 50% RH the amplitude of 600peak / amplitude of adsorbed water peak ⬎⬎⬎1. cHeat treatment will accentuate 14 A˚ line and weaken 7 A˚ line.
the amount of clay mineral exceeds the amount of clay size. This most probably results from cementation of small clay particles into aggregates larger than 2 m in diameter. Approximate proportions of the different clay minerals in the clay fraction can be estimated from the relative intensities of the X-ray diffraction reflections for each mineral. The presence of organic matter and carbonates can be easily detected using the tests listed in Section 3.21.
3.25 CONCLUDING COMMENTS
The sizes, shapes, and surface characteristics of the particles in a soil are determined in large measure by their mineralogy. Mineralogy also determines interac- tions with fluid phases. Together, these factors deter- mine plasticity, swelling, compression, strength, and fluid conductivity behavior. Thus, mineralogy is fun- damental to the understanding of geotechnical prop- erties, even though mineralogical determinations are not made for many geotechnical investigations. In- stead, other characteristics that reflect both composi- tion and engineering properties, such as Atterberg limits and grain size distribution, are determined.
Interatomic bonding, crystal structure, and surface characteristics determine the size, shape, and stability of soil particles and the interactions of soil particles with liquids and gases. The structural stability of the different minerals controls their resistance to weath- ering and hence accounts in part for the relative abun- dance of different minerals in different soils.
Because interatomic bonds in soil particles are strong, primary valence bonds, whereas usual interpar- ticle bonds are of the secondary valence or hydrogen bond type, individual particles are strong compared to groups of particles. Thus, most soil masses behave as assemblages of particles in which deformation proc- esses are dominated by displacements between parti- cles and not by deformations of particles themselves, although grain crushing becomes important in coarse- grained soils such as sands and gravels when they are under very high stresses.
The type of bonding between the unit layers of the clay minerals, coupled with the adsorption properties of the particle surfaces, controls soil swelling. Ad- sorption and desorption processes are important in interactions between chemicals and soils. These inter- actions in turn determine the flow and attenuation of various substances through soil. Changes in surface
QUESTIONS AND PROBLEMS 81
forces owing to changes in chemical environment may alter the structural state of a soil.
Mineralogy is related to soil properties in much the same way as the composition and structure of cement and aggregates are to concrete, or as the composition and crystal structure of steel relate to its strength and deformability. With these engineering materials—soil, concrete, and steel—mechanical properties can be measured directly; however, they cannot be explained without consideration of the composition and structure of their components.
Since about 1980, environmental problems, espe- cially those related to the safe disposal and contain- ment of municipal, hazardous, and nuclear waste and to the clean up of contaminated sites and the protection of groundwater, have assumed a major role in geo- technical engineering practice. This has required a greatly increased focus on the compositional charac- teristics of soils and their relation to the long-term physical and chemical properties that control soil be- havior under changed and extreme environmental con- ditions.
QUESTIONS AND PROBLEMS
1. A montmorillonite has a cation exchange capacity of 130 meq / 100 g and a total external and internal surface area of 800 m2/ g.
a. How many calcium ions will there be on a par- ticle that is 0.4m ⫻ 0.2m ⫻ one unit cell in thickness?
b. What percentage of the dry weight of the clay is composed of calcium?
2. An orthorhombic crystal has axial ratios of 0.6, 0.3, and 1.0. The (500) plane is 2.0 A˚ horizontally from the origin. This crystal is irradiated with CuK X-rays (wave length of 1.54 A˚ ). At what value of does the second-order (010) reflection occur?
3. Sketch the following planes relative to crystallo- graphic axes: (001), (243), (hk0), (hkl), (111), (060), (010).
4. Consider an orthorhombic crystal of dimensions
a ⫽ 6A˚ , b ⫽ 12 A˚ , c ⫽ 8 A˚ . With the aid of sketches determine the angle of intersection be- tween the planes of each pair indicated below. If the planes do not intersect, then so indicate. a. (002) and (020)
b. (001) and (002) c. (111) and (222)
d. (111) and (111)
e. (112) and (001)
5. A clay has a surface density of charge of one charge per 150 A˚2. Its cation exchange capacity is
10 meq / 100 g. Determine the specific surface area.
6. Why are soils containing smectite often expansive, whereas soils containing illite and / or kaolinite are not?
7. As the geotechnical engineer on a project, you find an inorganic soil containing 15 percent by weight
of particles finer than 100 m, as measured by
hydrometer analysis. What soil components do you expect? Why?
How could you confirm this expectation? Be spe- cific in terms of tests and diagnostic criteria. 8. What is the smallest interplanar spacing that can
be measured by X-ray diffraction using copper K
radiation?
9. You suspect that a fine-grained soil sample con- tains kaolinite, illite, and smectite minerals. De- scribe in logical sequence the tests you would do to verify that these clay minerals are present. In- dicate the reasons why you choose these tests and the criteria for distinguishing among the minerals. 10. An inorganic clay has a liquid limit of 350 percent. a. What is the most probable predominant clay
mineral in this soil?
b. Explain the high liquid limit in terms of the crystal structure of this mineral.
c. Would you recommend founding light struc-