Measurement of Properties
2.2 Basic and Index Properties
2.2.3 Soils GeneralGeneral
The basic and index properties of soils are generally considered to include volume–weight and moisture–density relationships, relative density, gradation, plasticity, and organic content.
Rippable Marginal Non rippable
Velocity (m/s × 1000) Velocity (ft/s × 1000)
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
1 2 3 4
Rippability based on caterpillar D9 with mounted hydraulic No.9 ripper Topsoil
Rock Rippability as Related to Seismic p-Wave Velocities (Courtesy of Caterpillar Tractor Co.)
Volume–Weight Relationships
Definitions of the various volume–weight relationships for soils are given in Table 2.8.
Commonly used relationships are void ratio e, soil unit weight (also termed density or mass density and reported as total or wet density γt, dry density γd, and buoyant density γb), moisture (or water) content w, saturation degree S, and specific gravity Gsof solids.
Determinations of basic soil properties are summarized in Table 2.9. A nomograph for the determination of basic soil properties is given in Figure 2.2.
Sand Cone Density Device (Figure 2.3a)
A hole 6 in. deep and 6 in. in diameter is dug and the removed material is stored in a sealed container. The hole volume is measured with calibrated sand and the density is cal-culated from the weight of the material removed from the hole (ASTM D1556-00).
Rubber Balloon Device
A hole is dug and the material is stored as described above. The hole volume is measured by a rubber balloon inflated by water contained in a metered tube (ASTM D2167-94).
Nuclear Moisture-Density Meter (ASTM D2922)
A surface device, the nuclear moisture-density meter, measures wet density from either the direct transmission or backscatter of gamma rays; and, moisture content from the transmis-sion or backscatter of neutron rays (Figure 2.3b). The manner of measurement is similar to that of the borehole nuclear probes (see Section 1.3.6). In the direct transmission mode a rod containing a Celsium source is lowered into the ground to a desired depth. In the backscat-ter mode, the rod is withdrawn and gamma protons are scatbackscat-tered from the surface contact.
A rapid but at times approximate method, measurement with the meter yields satisfactory results with modern equipment and is most useful in large projects where soil types used as fills do not vary greatly. Frequent calibration is important to maintain accuracy.
Borehole Tests
Borehole tests measure natural density and moisture content. Tests using nuclear devices are described in Section 1.3.6.
Moisture content (w)
The moisture meter is used in the field (ASTM D4444-92). Calcium carbide mixed with a soil portion in a closed container generates gas, causing pressure that is read on a gage to indicate moisture content. Results are approximate for some clay soils.
For cohesive soils, moisture content is most reliably determined by drying in the labo-ratory oven for at least 24 h at 104°C.
Moisture–Density Relationships (Soil Compaction)
Optimum moisture content and maximum dry density relationships are commonly used to specify a standard degree of compacting to be achieved during the construction of a load-bearing fill, embankment, earth dam, or pavement. Specification is in terms of a percent of maximum dry density, and a range in permissible moisture content is often specified as well (Figure 2.4).
Description
The density of a soil can be increased by compacting with mechanical equipment. If the moisture content is increased in increments, the density will also increase in increments
TABLE 2.8
Volume–Weight Relationships for Soilsa
Property Saturated Unsaturated Illustration of Sample
Sample (Ws, Ww, Sample (Ws, Ww, Gs, are Known) Gs, V are Known) Volume Components
Volume of solids Vs
Volume of water Vw
Volume of air or gas Va Zero V(Vs Vw)
Volume of voids Vv V
Total volume of sample V Vs Vw Measured
Porosity n or
Void ratio e (Gras)-1
Weights for Specific Sample
Weight of solids Ws Measured Weight of water Ww Measured Total weight of sample Wt Ws Ww
Weights for Sample of Unit Volume
Dry-unit weight γd
Wet-unit weight γt
Saturated-unit weight γs
Submerged (buoyant) unit γsγwc
weight γb Combined relations Moisture content w
Degree of saturation S 1.00 γd γsγdγw冢 冣
Specific gravity Gs
a After NAVFAC, Design manual DM-7.1, Soil Mechanics, Foundations and Earth Structures, Naval facilities Engineering Command, Alexandria, VA, 1982.
b γwis unit weight of water, which equals 62.4 pcf for fresh water and 64 pcf for sea water (1.00 and 1.025 g/cm3).
c The actual unit weight of water surrounding the soil is used. In other cases use 62.4 pcf. Values of w and s are used as decimal numbers.
Ws
under a given compactive effort, until eventually a peak or maximum density is achieved for some particular moisture content. The density thereafter will decrease as the moisture content is increased. Plotting the values of w% vs. γt, or w% vs. γdwill result in curves sim-ilar to those given in Figure 2.5; 100% saturation is never reached because air remains trapped in the specimen.
Factors Influencing Results
The shape of the moisture–density curve varies for different materials. Uniformly graded cohesionless soils may undergo a decrease in dry density at lower moisture as capillary forces cause a resistance to compacting or arrangement of soil grains (bulking). As mois-ture is added, a relatively gentle curve with a poorly defined peak is obtained (Figure 2.5).
Some clays, silts, and clay–sand mixtures usually have well-defined peaks, whereas low-plasticity clays and well-graded sands usually have gently rounded peaks (Figure 2.6).
Optimum moisture and maximum density values will also vary with the compacted energy (Figure 2.7).
Test Methods
Standard compaction test (Proctor Test) (ASTM D698): An energy of 12,400 ft lb is used to compact 1 ft3of soil, which is accomplished by compacting three sequential layers with a 5 1/2-lb hammer dropped 25 times from a 12-in. height, in a 4-in.-diameter mold with a volume of 1/30 ft3.
Modified compaction test (ASTM D1557): An energy of 56,250 ft lb is used to compact 1 ft3of soil, which is accomplished by compacting five sequential layers with a 10 lb hammer dropped 25 times from an 18 in. height in a standard mold. Materials containing signifi-cant amounts of gravel are compacted in a 6-in.-diameter mold (0.075 ft3) by 56 blows on each of the five layers. Methods are available for correcting densities for large gravel par-ticles removed from the specimen before testing.
Relative Density DR
Relative density DRrefers to an in situ degree of compacting, relating the natural density of a cohesionless granular soil to its maximum density (the densest state to which a soil can be compacted, DR 100%) and the minimum density (the loosest state that dry soil grains can attain, DR 0%). The relationship is illustrated in Figure 2.8, which can be used to find DRwhen γN(natural density), γD(maximum density), and γL(loose density) are known.
DRmay be expressed as
DR (1/γL 1/γN )/ (1/γL 1/γD ) (2.2) TABLE 2.9
Determination of Basic Soil Properties
Determination Basic Soil Property Laboratory Test Field Test
Unit weight or density, γd, γt, γs, γb Weigh specimens Cone density device Figure 3.3a, ASTM U1556 Rubber ballon device, ASTM D2167 Nuclear moisture-density meter, ASTM D2922
Specific gravity Gs ASTM D854 None
Moisture content w ASTM D4444 Moisture meter
ASTM D2922 Nuclear moisture-density meter Void ratio e Computed from unit dry weight and specific gravity
Significance
DRis used for classification of the degree of in situ compactness as given in Figure 2.8 or, more commonly, to classify in situ density as follows: very loose (0–15%), loose (15–35%), medium dense (35–65%), dense (65–85%), and very dense (85–100%) (see Table 2.23 for correlations with N values of the Standard Penetration Test (SPT)). Void ratio and unit weight are directly related to DR and gradation characteristics.
Permeability, strength, and compressibility are also related directly to DRand gradation characteristics.
EXAMPLE
Given : wef = 123.6 lbs./ ft. Gs = 2.625 = 20.0%
WET DENSITY, wet, IN LBS. PER CU. FT.
wet / 62.4 VOID RATIO, e DRY DENSITY, d, IN LBS.CU.FT.
WATER CONTENT FOR COMPLETE SA TURA TION,
sat IN %
POROSITY, IN % APPARENT SPECIFIC GRAVITY OF SOIL, Gs WATER CONTENT, , IN PERCENT OF DRY WEIGHT
Find: d =
Nomograph to determine basic soil properties. (From USBR, Earth Manual, U. S. Burean of Reclamation, Denver, CO, 1974. With permission.)
Measurements of DR
Laboratory testing: See ASTM D4254-00 and Burmister (1948). Maximum density is deter-mined by compacting tests as described in the above section, or by vibrator methods wherein the dry material is placed in a small mold in layers and densified with a hand-held vibrating tool. Minimum density is found by pouring dry sand very lightly with a funnel into a mold. DRmeasurements are limited to material with less than about 35% nonplastic soil passing the No. 200 sieve because fine-grained soils falsely affect the loose density. A major problem is that the determination of the natural density of sands cannot be sampled undisturbed. The shear-pin piston (see Section 1.4.2) has been used to obtain values for γN, or borehole logging with the gamma probe is used to obtain values (see Section 1.3.6).
Field testing: The SPT and Cone Penetrometer Test (CPT) methods are used to obtain esti-mates of DR.
Correlations: Relations such as those given in Figure 2.10 for various gradations may be used for estimating values for γDand γL.
Gage Gage
Detectors
Detectors
Photon paths Backscatter Mode Direct Transmission
Surface
Surface
Source Photon paths Source
Min = 50mm (2 in.) (a)
(b) FIGURE 2.3
(a) Sand cone density device being used to measure in situ density of a compacted subgrade test section for an airfield pavement. (b) Nuclear moisture density meter used to measure in situ density.
Gradation (Grain Size Distribution)
Gradation refers to the distribution of the various grain sizes in a soil specimen plotted as a function of the percent by weight passing a given sieve size (Figure 2.9):
● Well-graded — a specimen with a wide range of grain sizes.
● Poorly graded — a specimen with a narrow range of grain sizes.
● Skip-graded — a specimen lacking a middle range of grain sizes.
Water content (w %) S = 100%
(zero air v
oids)
d
FIGURE 2.4
The moisture–density relationship. The soil does not become fully saturated during the compaction test.
Water content (w %) Dry
d
FIGURE 2.5
Typical compaction curve for cohesionless sands and sandy gravels. (From Foster, C. R., Foundation Engineering, G. A. Leonards, Ed., McGraw-Hill Book Co., New York, 1962, pp. 1000–1024. With permission. Reprinted with permission of the McGraw-Hill Companies.)
131
120 114
S = 100%
15 12
8
Water content (w %)
d (pcf)
Silty sand (SM)
Sandy clay (SC)
Low plasticity clay(CL)
FIGURE 2.6
Typical standard Proctor curves for various materials.
Water content (w %) (All in 6-in molds)
FIGURE 2.7
Effect of different compactive energies on a silty clay. (After paper presented at Annual ASCE Meeting, January 1950.)
90
Relative density, percent DR
Maximum Density or Dense State,γD Saturated moisture content. w ′ Percentage of dry weight GS = 267
L-105 Minimum density or loose state, L unit dry weight, lb per cu. ft.
FIGURE 2.8
Relative density diagram. (From Burmister, D. M., ASTM, Vol. 48, Philadelphia, PA, 1948. Copyright ASTM International.
Reprinted with permission.)
● Coefficient of uniformity Cu— the ratio between the grain diameter at 60% finer to the grain diameter corresponding to the 10% finer line, or
Cu D60/ D10 (2.3)
Significance
Gradation relationships are used as the basis for soil classification systems. Gradation curves from cohesionless granular soils may be used to estimate γDand γL, and, if γNor DR is known, estimates can be made of the void ratio, porosity, internal friction angle, and coefficient of permeability.
Gradation Curve Characteristics (Burmister, 1948, 1949, 1951a)
The gradation curves and characteristic shapes, when considering range in sizes, can be used for estimating engineering properties. The range of sizes CRrepresents fractions of a uniform division of the grain size wherein each of the divisions 0.02 to 0.06, 0.06 to 0.02, etc., in Figure 2.9 represents a CR 1. Curve shapes are defined as L, C, E, D, or S as given in Figure 2.9 and are characteristic of various types of soil formations as follows:
● S shapes are the most common, characteristic of well-sorted (poorly graded) sands deposited by flowing water, wind, or wave action.
CR − 2.7
Percentage finer by weight Series of type S curves
Regularly varying Fineless and range of grain sizes
31−1/2 3/4 3/4 4 8 16 30 50 100 200 Sieves
Uniform scale of fractions Grain size (mm)
Log of grain size Log of grain size
Balance plus and minus area for upper and lower branch of curve
Upper br
Percentage finer by weight
Boulders cobbles
Gravel
M Sand
M Clay-soil plasticity and clay-qualities Nonplastic
Distinguishing characteristics of grain size curves: fineness, range of grain sizes, and shape: (a) type S grain size curves and (b) type of grain size curve. (From Burmister, D. M., ASTM, Vol. 48, Philadelphia, PA, 1948.
Copyright ASTM International. Reprinted with permission.)
● C shapes have a high percentage of coarse and fine particles compared with sand particles and are characteristics of some alluvial valley deposits in an arid cli-mate where the native rocks are quartz-poor.
● E and D shapes include a wide range of particle sizes characteristics of glacial tills and residual soils.
Relationships
General relationships among gradation characteristics and maximum compacted densi-ties, minimum densidensi-ties, and grain angularity are given in Figure 2.10 (note the signifi-cance of grain angularity). Gradation characteristics for soils of various geologic origins, as deposited, are given in Figure 2.11.
Test Methods
Gradations are determined by sieve analysis (ASTM D422) and hydrometer analysis (ASTM D422), the latter test being performed on material finer than a no. 200 sieve. For sieve analysis, a specimen of known weight is passed dry through a sequence of sieves of decreasing size of openings and the portion retained is weighed, or a specimen of known weight is washed through a series of sieves and the retained material dried and weighed.
The latter procedure is preferred for materials with cohesive portions because dry sieving is not practical and will yield erroneous results as fines clog the sieves.
140
Range of grain sizes Cr, in units of soil fractions Units dry weight s (pcf) reduced to GS = 2.67 basis
(b) Approximate Minimum Densities, 0% Dr
(c)Approxomate Influence of Grain Shape on Density Decrease in Density (pcf) Range in grain sizes (Cr ) Coarser soils Finer soils
Grain shape Change in density (pcf)
Very angular
Maximum compacted densities, approximate minimum densities, and influence of grain shape on density for various gradations.
(From Burmister, D. M., ASTM, Vol. 48, Philadelphia, PA, 1948. Reprinted with permission of the American Society for Testing and Materials.)
Plasticity
Definitions and Relationships
Atterberg limits, which include the liquid limit, plastic limit, and the shrinkage limit, are used to define plasticity characteristics of clays and other cohesive materials.
Liquid limit (LL) is the moisture content at which a soil passes from the liquid to the plas-tic state as moisture is removed. At the LL, the undrained shear strength su≈0.03 tsf.
Plastic limit (PL) is the moisture content at which a soil passes from the plastic to the semisolid state as moisture is removed.
Plasticity index (PI) is defined as PI LL PL.
Shrinkage limit (SL) is the moisture content at which no more volume change occurs upon drying.
Activity is the ratio of the PI to the percent by weight finer than 2 µm (Skempton, 1953) Liquidity index (LI) is used for correlations and is defined as
LI (w PL)/(LLPL) (w PL)/PI (2.4)
Significance
A plot of PI vs. LL provides the basis for cohesive soil classification as shown on the plas-ticity chart (Figure 2.12). Correlations can be made between test samples and characteris-tic values of natural deposits. For example, predominantly silty soils plot below the A line, and predominantly clayey soils plot above. In general, the higher the value for the PI and LL, the greater is the tendency of a soil to shrink upon drying and swell upon wetting. The relationship between the natural moisture content and LL and PI is an indication of the soil’s consistency, which is related to strength and compressibility (see Table 2.37). The
0 10 20 30 40 50 60 70 80 90 100 Beach sand deposits, wave formed
Glacial outwash Medium compact Compact Loose
C L MC
_ + _ + _ +
Initial-depositional relative density DR (%)
Dune sands
Quiet water to very low velocities CR − 1.0 to 2.5 S type
Probable initial depositional relative densities produced by geologic process of granular soil formation as a tentative guide showing dependence on grain-size parameters, grading-density relations, and geological processes. (From Burmister, D. M., ASTM Special Technical Publication No. 322, 1962a, pp. 67–97.
Reprinted with permission of the American Society for Testing and Materials.)
liquidity index expresses this relationship quantitatively. The controlling factors in the val-ues of PI, PL and LL for a given soil type are the presence of clay mineral, and the per-centages of silt, fine sand, and organic materials.
Test Methods
Liquid limit (ASTM D4318-00) is performed in a special device containing a cup that is dropped from a controlled height. A pat of soil (only material passing a no. 40 sieve) is mixed thoroughly with water and placed in the cup, and the surface is smoothed and then grooved with a special tool. The LL is the moisture content at which 25 blows of the cup are required to close the groove for a length of 1 cm. There are several test variations (Lambe, 1951).
Plastic limit (ASTM D4318-00) is the moisture content at which the soil can just be rolled into a thread 1/8 in. in diameter without breaking.
Shrinkage limit (ASTM D427) is performed infrequently. See Lambe (1951) for discussion.
Organic Content General
Organic materials are found as pure organic matter or as mixtures with sand, silt, or clay.
Basic and Index Properties
Organic content is determined by the loss by ignition test that involves specimen combus-tion at 440°F until constant weight is attained (Arman, 1970). Gradacombus-tion is determined after loss by ignition testing. Plasticity testing (PI and LL) provides an indication of organic matter as shown in Figure 2.12 (see also ASTM D2914-00).
Bentonite Organic silt and clay (Flushing meadows L.I.) Kaolin and alluvial clays Micaceous silts
Plasticity chart for Unified Classification System.