Soil Mechanics ME 420
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4 Boll Clauttlcatlon W e p S 680 LL Pl etc Classification System Language Engineering Properties permeability compressibility shrinkswell shear strength etc I Engineering Purpose highways airfields foundation dams etc Classification and Index Properties Fig 31 Role of classification system in geotechnioal engineering practice civil engineering had their roots in agricultural soil science This is why the first systems used by civil engineers classified soil by grain size or soil texture Atterberg 1905 apparently was the first to suggest that something other than grain size could be used for soil classification To this end in 1911 he developed his consistency limits for the behavior of finegrained soils Sec 27 although at that time for agricultural purposes Later the US Bureau of Public Roads based the classification of finegrained soils almost entirely on the Atterberg limits and other simple tests Casagrande 1948 describes several other systems that have been used in highway engineering airfield construction agriculture geology and soil science Today only the Unified Soil Classification System USCS and the American Association of State HighWay and Transportation Officials AASHTO system are commonly used in civil engineering practice The Unified Soil Classification System is used mostly by engineering agencies of the US Government US Army Corps of Engineers and US Depart ment of the Interior Bureau of Reclamation and many geotechnical engineering consulting firms and soil testing laboratories With slight modification this system is also in fairly common use in Great Britain and elsewhere outside the United States Nearly all of the state Departments of Transportation and Highways in the United States use the AASHTO system which is based upon the observed behavior of soils Holtz 17313 and Kowzcs H313 CF18 71 InfraaUCT I39aI Z a Geafechm39Ca Engheating lorethee I d 32 The Unified Soil Classification System USCS 49 under highway pavements The Federal Aviation Administration FAA of the US Department of Transportation had its own soil classification system for the design of airport pavements but it now uses the Unified Soil Classification System Once you become familiar with the details both the USCS and AASHTO systems are easy to use in engineering practice 32 THE UNIFIED SOIL CLASSIFICATION SYSTEM USCS This system was originally developed by Professor A Casagrande 1948 for use in airfield construction during World War II It was modified in 1952 by Professor Casagrande the US Bureau of Reclama tion and the US Army Corps of Engineers to make the system also applicable to dams foundations and other construction US Army En gineer Waterways Experiment Station 1960 The basis for the USCS is that coarsegrained soils can be classified according to their grain size distributions whereas the engineering behavior of finegrained soils is primarily related to their plasticity In other words soils in which fines silts and clays do not affect the engineering performance are classified according to their grain size characteristics and soils in which fines do control the engineering behavior are classified according to their plasticity characteristics Therefore only a sieve analysis and the Atterberg limits are necessary to completely classify a soil in this system The four major divisions in the USCS are indicated in Table 3 1 They are l coarsegrained 2 finegrained 3 organic soils and 4 peat Classification is performed on the material passing the 75 mm sieve and the amount of oversize material is noted on the drill logs or data sheets Particles greater than 300 mm equivalent diameter are termed boulders while materials between 75 mm and 300 mm are called cobbles Coarse grained soils sands and gravels are those having 50 or more material retained on the No 200 sieve These fractions have been arbitrarily but conveniently subdivided as shown in Table 3 1 Finegrained soils are those having more than 50 passing the No 200 sieve The highly organic soils and peat can generally be identified visually The symbols in Table 31 are combined to form soil group symbols which correspond to the names of typical soils as shown in Table 32 The coarsegrained soils are subdivided into gravels and gravelly soils G and sands and sandy soils S The gravels are those having the greater percentage of the coarse fraction particles larger than 475 mm diameter retained on the No 4 sieve and the sands are those having the greater portion passing the No 4 sieve Both the gravel G and the sand S 50 Soil Classification TABLE 31 USCS Definitions of Particle Size Size Ranges and Symbols Soil Fraction or Component Symbol Size Range Boulder None Greater than 300 mm Cobble None 75 mm to 300 mm 1 Coarsevgrained soils Gravel G 75 mm to No 4 sieve 475 mm Coarse 75 mm to l9 mm Fine 19 mm to No4 sieve 475 mm Sand S No 4 475 mm to No 200 0075 mm Coarse No 4 475 mm to No l0 20 mm Medium No l0 20 mm to No 40 0425 mm Fine No 40 0425 mm to No 200 0075 mm 2 Finegrained soils Fines Less than No 200 sieve 0075 mm Silt M No specific grain size use Atterberg limits Clay C No specific grain size use Atterberg limits 3 Organic Soils 0 No specific grain size 4 Peat Pt No specific grain size Gradation Symbols Liquid Limit Symbols Wellgraded W High LL H Poorlygraded P Low LL L groups are divided into four secondary groups GW and SW GP and SP GM and SM GC and SC depending on the grain size distribution and nature of fines in the soils Wellgraded W soils have a good representa tion of all particle sizes whereas the poorly graded P soils are either uniform or skip or gapgraded Fig 24 Whether a gravel or sandy soil is well graded can be determined by plotting the grain size distribution curve and computing the coefficient of uniformity Cu and the coefficient of curvature CclThese coefficients are defined in Chapter 2 as D60 C 2 19 D10 and the coefficient of curvature is D30 Cc m 220 32 The Unified Soil Classification System U808 51 where D50 grain diameter at 60 passing D30 grain diameter at 30 passing and D10 grain diameter at 10 passing by weight or mass Gradation criteria for gravelly and sandy soils are sh0wn in Table 32 column 6 The CW and SW groups are well graded gravelly and sandy soils with less than 5 passing the No 200 sieve The GP and SP groups are poorly graded gravels and sands with little or no nonplastic fines The finegrained soils those having more than 50 passing the N0 200 sieve are subdivided into silts M for the Swedish terms m0 very fine sand and mjc ila silt and clays C based on their liquid limit and plasticity index Organic soils 0 and peat Pt are also included in this fraction although as shown in Table 31 no grain size range is specified Finegrained soils are silts M if their liquid limits and plasticity indices plot below the Aline on Casagrande s 1948 plasticity chart Fig 32 The fines are clays C if the LL and PI plot above the Aline The Aline generally separates the more claylike materials from those that are silty and also the organics from the inorganics The exception is organic clays 0L and OH which plot below the Aline However these soils do behave similarly to soils of lower plasticity The silt clay and organic fractions are further subdivided on the basis of relatively low L or high H liquid limits The dividing line between the low and high liquid limits has been arbitrarily set at 50 Representative soil types for finegrained soils are also shown in Fig 32 This figure columns 4 and 5 of Table 32 and Table 33 will be helpful in the visual identification and classification of fine grained soils You can see from Fig 32 that several different soil types tend to plot in approximately the same area on the LLPI chart which means that these soils tend to have about the same engineering behavior This is why the Casagrande chart is so useful in the engineering classifica tion of soils For example Casagrande 1948 observed the behavior of soils at the same liquid limit with plasticity index as compared with their behavior at the same plasticity index but with an increasing liquid limit and he obtained the following results Soils at Equal LL Soils at Equal PI Characteristic with Increasing Pl with Increasing LL Dry strength Increases Decreases Toughness near PL increases Decreases Permeability Decreases Increases Compressibility About the same Increases Rate of volume change Decreases Toughness near the PL and dry strength are very useful visual classification properties and they are defined in Table 33 The other characteristics are engineering properties and they are discussed in great TABLE 32 Unified Soil Classificathn System Soil Classification Group Field ldentilication Procedures Major Divisions Symbols Typical Names leacluding particles larger than 75 mm Y and basing lractions on estimated weightsl 1 2 3 4 5 6w Wailgraded grayels gravel sand mix Wide range in grain sizes and substantial El 5 5 turns little or no ines amounts ol ail intermediate particle sites 0 In N u y 8 5 a N a GP Poorly graded graveis graveiisand mi Predominantly one size or a range ol sizes a i 2 g a tures little or no lines with some intermediate sizes missing 2 a E 2i 3 g 3 E e C 2 U N g quot E 3 39139 GM Silty gravels gravelsandsilt mixtures Nonoiarmc 39in of quot5 mm 390 quot Ian39ch 5 Q 3 n a E a tor identification procedure see ML below t is 52 3 his u 5i 39 u S E 0 E g E GE Clayey gravels graveisandcay Plastic lines ilor identification procedures a e a m g o L mixtures ae CL bel wi E E g a Bi 3 39 i g 3 a SW Wellwaded sands gravelly sands Wide range in grain sizes and substantial El 5 g 3 g g 2 3 E g little or no ines amounts at all intermediate particle sites in g t v 8 f z g 3i 5 c T quotquot 7 E E j i d T g 3 SP Poorly graded sands graveiiy sands Predominantly one size or a range of sizes 2 2 g a f g i E 0 little or no lines with some intermediate sizes missing 2 e c e 2 u a j k 3 i3 g E E Ear 3 SM 5quot d d Nonolastic lines or lines with low plasticity 6 2 39E 5 9 g m 3 g E E V 53quot 539 n 39539 l mum39squot for identilication procedures we ML below 5 E u E a o 2 E as u E z z u g b E I r 5 5 a N O c C d d I Plastic lines lor identtlication procedures in m 2 S ayey sari 5 san c av mixtures see CL Imam re E identi cation Procedures 5 on Fraction Smaller than No 40 Steve Size 3 Dry Strength Dilatancy Toughness cru hin reaction consistency o 1 characteristics to shaking near PL 2 inorganic slits and very line sands rock 2 Z ML our silly Dr clayey 391 53M 0 None to slight Quick to slow None I E E3 clayey silts with slight plasticity x 4 8 a g inorganic clays ol low to medium N E m er 3 g N g g 5 CL Dlasllmlv gravenv clays sandy Medium to high 3 V V Medium D m a g g g claysl silty clays lean clays u Z l 2 2 m Organic silt and organic silty clays oi Slight to a Equot t OL low oiasticrty medium Slaw Shy if E E inorganic sills micaceous or n u r 39 l I a 3 MH diatomaceous line sandy or Slow to none i 393 3 silty soils elastic slits E E 5 39EE inorganic clays ol high olasticitv lat H n 1 u ig to very N H h E I g 3 a CH may high one i9 2 quot 3 E 3 a r g o 39 39 d Wquot N Si39 htt rganic c ays 0 me rum to l one to very 399 0 h 0 plasticity organic tilts M39d39um m 399quot slow medium Readily identitied by color odor spongy feel V 39 l 39 Highly Organic 39Sorls Pi Feat and other highly organic sot s and frequmuy by humus mum 39 Boundary ciassrlications soils Bossessmg characteristics at two groups are destignated by combinations ol group symbols For example GW GC wellgraded gravel sanu mutiule wrih clay Dinner All sieve sizes on lhls chari are U s Siandero After US Anny Eng nccr Waterways Experiment Station 1960 and Howard 1977 4 32 The Unified Soil Classi cation System USCS TABLE 32 Continued Laboratory Classi cation Criteria 6 Use grain size curve in identifying the tractions as given under field identification D c 22 greater than 4 u D Sea Sec 25 g 3 u s E 2 c 3 DIV 7 2 C between 1 and 3 E 3 Dm x D a a 2 E 3 x Not meeting all gradation requirements lor GW 5 e 9 Q 55 B a Atterbarg limits below A line or Above A39W E Willquot 3 Pi between 2 5 39 Pllesstan4 55 a m u 9 4 and 7 are borderline U g 5 E 390 Atterbarg limits above Aline cases requiring use at E 0 5 g u with Pl greater than 7 dual symbols a V 339 2 a i D g 5 o o m I I l greater than 6 b 5 3 See Sec 2 5l o 8 a 32 125 02quot C Dn1 betweanlandil 2 8 8 g g S Dt quot DBO 5 a A 5 quot 0 Egg Not meeting all gradation requirements lot SW g E Atterberg limits below Arline or Liwis olqlimg In hatched 5 5 7 PI less than Wllh 939 WWW E c 8 4 and 7 are borderline 5 EN Atterberg limits above A line cases requiring use ol 3 with Pi greater than 7 dual symbols Plasticity Chart For laboratory classi cation oi linegrained soils so i l l l l l I l Comparing soils at equal liquid limit toughness and dry strength increase x with increasing plasticity index Q1 19 40 CH r 90 a 30 W 4 t w CL OH 3 or a 20 MH CL Mi 10 K I 7 or n l t i 1 i L 0 10 20 30 40 50 60 70 BO 90 too LIQUID LiMlT 53 100 90 ivi rl caceous or diatomaceous fine sandy and silty soils elastic silts organic silts clays and silty clays I l l 39 Inorganic and organic silts l l 70 4 I cH ganic clays of I 60 high plasticity V 50 silty or clayey fine sands Liquid limit and silty clays of low plasticity rock flour VS inorganic r la T IOL 4 7 Medium 40 2 plastic 0r LL 7 ML 30 A 32 The Unmod Soll Claulllcatlon System U868 55 TABLE 33 Fleld Identification Procedures for FlneGralned Soils or Fractions These procedures are to be performed on the minus No 40 sieve size particles approximately 04 mm For field classification pur poses screening is not intended simply remove by hand the coarse particles that interfere with the tests Dilatancy reaction to shaking Dry Strength crushing characteristics After removing particles larger than No 40 sieve size prepare a pat of moist soil with a volume of about 5 cm Add enough water if necessary to make the soil soft but not sticky Place the pat in the open palm of one hand and shake vigorously against the other hand several times A positive reaction consists of the appearance of water on the surface of the pat which changes to a livery consistency and becomes glossy When the sample is squeezed between the fingers the water and gloss disappear from the surface the pat stiffens and finally it cracks or crum bles The rapidity of appearance of water during shaking and of its disappearance dur ing squeezing assist in identifying the char acter of the fines in a soil Very fine clean sands give the quick est and most distinct reaction whereas a plastic clay has no reaction Inorganic silts such as a typical rock flour show a mod erately quick reaction After removing particles larger than No 40 sieve size mold a pat of soil to the consistency of putty adding water if neces sary Allow the pat to dry completely by oven sun or air and then test its strength by breaking and crumbling between the fingers This strength is a measure of the character and quantity of the colloidal fraction con tained in the soil The dry strength increases with increasing plasticity High dry strength is characteristic for clays of the CH group A typical inorganic silt possesses only very slight dry strength Silty fine sands and silts have about the same slight dry strength but can be distinguished by the feel when powdering the dried speci men Fine sand feels gritty whereas a typical silt has the smooth feel of flour Toughness consistency near plastic limit After removing particles larger than the No 40 sieve size a specimen of soil about i in cube in size is molded to the consistency of putty If too dry water must be added and l Silty clays clayey silts Low plastic inorganic 10 Hand sand clays sandy and silty clays A Casagrande39s plasticity Chart showing several representative soil types developed from Casa grande 1948 and Howard 1977 60 50 20 Fig 32 if sticky the specimen should be spread out in a thin layer and allowed to lose some moisture by evaporation Then the specimen is rolled out by hand on a smooth surface or between the palms into a thread about 3 mm in diameter The thread is then folded and refolded repeatedly During this manipulation the moisture content is gradually reduced and the specimen stiffens finally loses its plasticity and crumbles when the plastic limit is reached After the thread crumbles the pieces should be lumped together and a slight kneading action continued until the lump crumbles The tougher the thread near the plastic limit and the stiffer the lump when it finally crumbles the more potent the colloidal clay fraction in the soil Weakness of the thread at the plastic limit and quick loss of coherence of the lump below the plastic limit indicate either inorganic clay of low plasticity or materials such as kaolintype clays and organic clays which occur below the Aline Highly organic clays have a very weak and spongy feel at the plastic limit After US Army Engineer Waterways Experiment Station 1960 and Howard 1977 58 Soil Claulilcailon detail later in this book For now just rely on your general knowledge and ingenuity to figure out what those words mean The upper limit line Uline shown in Fig 32 indicates the upper range of plasticity index and liquid limit coordinates found thus far for soils A Casagrande personal communication Where the limits of any soil plot to the left of the Uline they should be rechecked Some highly active clays such as bentonite may plot high above the Aline and close to the Uline Itis shown in Chapter 4 that Casagrande s plasticity chart can even be used to identify qualitatively the predominant clay minerals in a soil Coarsegrained soils with more than 12 fines are classified as GM and SM if the fines are silty limits plot below the Aline on the plasticity chart and GC and SC if the fines are clayey limits plot above the Aline Both well graded and poorly graded materials are included in these two groups Soils having between 5 and 12 passing the No 200 sieve are classed as borderline and have a dual symbol The first part of the dual symbol indicates whether the coarse fraction is well graded or poorly graded The second part describes the nature of the fines For example a soil classified as a SPSM means that it is a poorly graded sand with between 5 and 12 silty fines Similarly a GWGC is a wellgraded gravel with some clayey fines that plot above the Aline Finegrained soils can also have dual symbols Obviously if the limits plot within the shaded zone on Fig 32 PI between 4 and 7 and LL between about 12 and 25 then the soil classifies as a CL ML Howard 1977 makes the practical suggestion that if the LL and PI values fall near the Aline or near the LL 50 line then dual symbols should be used Possible dual symbols then are MLMH CL CH OL OH CLML CLOL GLMH CHOH Borderline symbols can also be used for soils with about 50 fines and coarse grained fractions In this case possible dual symbols are GMML GMMH GCCL GCCH SMML SMMH SGCL SCCH 32 The Unified Soil Classi cation System USCS 57 Figure 33 is a practical guide for borderline cases of soil classifica tion UNIFIED SOIL CLASSIFICATION SYSTEM Borderline Classifications CoarseGrained Soils Fine Grained Soils ML ow 3 GM 5 MH Gravel GP 8 Gravel 6C 8 Silt OH in Q E g 7 5 CL SW 399 5M 12 Sand SP 000 Sand SC lg CIaY j l l 0 5 12 45 50 55 100 Percent passing the No 200 sieve Note Only two group svmbols may be used to describe a soil Borderline classifications can exist within each of the above groups Fig 33 Guide for borderline cases of soil classification after Howard 1977 A stepbystep procedure for USCS classification of soils conveni ently presented in Fig 34 shows a process of elimination of all the possibilities until the only one left indicates the specific classification The following steps adapted from the Corps of Engineers may help in this process US Army Engineer Waterways Experiment Station 1960 Clas sification should be done in conjunction with Table 32 and Fig 34 1 Determine if the soil is coarse grained fine grained or highly organic This is done by visual inspection and or by determining the amount of soil passing the No 200 sieve If coarse grained a Perform a sieve analysis and plot the grain size distribution curve Determine the percentage passing the No 4 sieve and classify the soil as gravel greater percentage retained on No 4 or sand greater percentage passing No 4 b Determine the amount of material passing the No 200 sieve if less than 5 passes the No 200 sieve examine the shape of the grain size curve if well graded classify as GW or SW if poorly graded classify as GP or SP If between 5 and 12 of the material passes the No 200 sieve it is a borderline case and the classification should have dual N P 53 Soquot Clanmcnuon Mata vlmll ulmlmllnn of soil In datumlm mum II II HIGHLY ORGANIC COARSE GRAINED or FINE GRAINED In humanlyquot um dcllrmlnu nmuum DIIIII39ID No 200 mu HIGHLY ORGANIC SOILS Pt Fltku mtun olor odor my Mgh moimm conunl nIcIu u vlqcubIl mum InInku In COANSE GRAINED 60 m Im pm No 200 SIM CA IG Gmm nunnun 01 tonne lvacunn nnimd on No 4 I M Batman 8 Ind I2 pm No 200 IIquot m nun 6 pm Nu 200 Ii I39 Len hm 5 u Human 55 Ind 12 N11 200 liuva39 Mora hm IZS pm Na 200 II Boydm ll l lo hlvl doubIl Bardtriinc In hwy douhll baI Iwmnnuo Io Run LL Ind FL on E mine in in 39 x on I mmul Na 40 IIquot CUM lrquon charcmlmu 41 SW SM Below Allm Ind leIII plol In Abavt AIinl and BIIow AIlnI Ind hllchod mm on hatched IDquot on I on huc rd mm on oImIchv chm plnI IciIv chm plumin chm plmlclty chm Eel In Shn IIquot In us Sundard II My inmIru wivh Imduinirq Nocqu use double Ivm GM ug Flg 34 Auxiliary laboratory Idemi ca on procedure after USAEWES1960 32 The Unlllod Sol Claumcauon System U568 59 FINE GRAINED Mm than 50 pm No 20D Ilm Run LL Ind PL on mlnul N0L 0 IIM mlllrlll I H quuld Ilmil In un Lluuld IImIl wrnruv hm 60 60 Abovv AIIM m plmiclly chm Bulww Avllnu an ullnchw cmn BnInw AIlne Ind Abovn AIIM Ind hlmhod was an plmIcIry chm LImIU all In almich chad nlhlicnv chm Color ndotl pmlbly LL Ind PL on own dry roll m odor poulblv nun LL m1 PL on W LL Ind FL on oven dvy Io n40llm Abow AIlnc Ind LImlu plan In and mm on Mn n planle chm pImIcllv chm Flg 34 Contlnued 60 Salt Clanmeatless symbols appropriate to grading and plasticity characteristics GWGM SWSM etc d If more than 12 passes the No 200 sieve perform the Atter berg limits on the minus No 40 sieve fraction Use the plastic ity chart to determine the correct classification GM SM GC SC GMGC or SM SC 3 If fine grained 39 a Perform Atterberg limits tests on minus No 40 sieve material If the liquid limit is less than 50 classify as L and if the liquid limit is greater than 50 classify as H b For L If the limits plot below the Aline and the hatched zone on the plasticity chart determine by color odor or the change in liquid limit and plastic limit caused by ovendrying the soil whether it is organic 0L or inorganic ML If the limits plot in the hatched zone classify as CLML If the limits plot above the Aline and the hatched zone on the plasticity chart Fig 32 classify as CL c For H If the limits plot below the Aline on the plasticity chart determine whether organic OH or inorganic MH If the limits plot above the Aline classify as CH d For limits which plot in the hatched zone on the plasticity chart close to the Aline or around LL 50 use dual border line symbols as shown in Fig 33 Although the letter symbols in the USCS are convenient they do not completely describe a soil or soil deposit For this reason descriptive terms should also be used along with the letter symbols for a complete soil classification Table 34 from US Army Engineer Waterways Experiment Station 1960 provides some useful information for describing soils In the case of all soils such characteristics as color odor and homogeneity of the deposit should be observed and included in the sample description For coarsegrained soils such items as grain shape mineralogical content degree of weathering in situ density and degree of compaction and presence or absence of fines should be noted and included Adjectives such as rounded angular and subangular are commonly used to describe grain shape see Fig 25 The in situ density and degree of compaction is normally obtained indirectly by observing how difficult the material is to excavate or to penetrate with devices called penetrometers Terms such as very loose loose medium dense and very dense are used to describe in situ density A granular deposit which can for example be excavated readily 32 The Unmed Soil Claul callon System USCS 61 TABLE 34 Information Required for Describing SoHs Coarsegrained soils For undisturbed soils add informa tion on stratification degree of compact ness cementation moisture conditions and drainage characteristics Give typical name Indicate ap proximate percentages of sand and gravel maximum size angularity surface condi tion and hardness of the coarse grains local or geologic name and other pertinent descriptive information and symbol in parentheses Example Silly sand gravelly About 20 hard angular gravel particles 12 mm maxi mum size rounded and subangular sand grains coarse to fine about 15 nonplastic fines with low dry strength well com pacted and moist in place alluvial sand SM Finegrained soils Give typical name Indicate degree and character of plasticity amount and maximum size of coarse grains color in wet condition odor if any local or geo logic name and other pertinent descriptive information and symbol in parentheses For undisturbed soils add informa tion on structure stratification con sistency in undisturbed and remolded states moisture and drainage conditions Example Clayey silt brown slightly plastic small percentage of fine sand numerous vertical root holes firm and dry in place loess ML Note Be prepared for wide variations in soil description among organizations and testing laboratories They all have their own ways of doing things After US Army Engineer Waterways Experiment Station 1960 by hand would be considered very loose whereas a deposit of the same material which requires power tools for excavation would be described as very dense or perhaps cemented For the finegrained fraction natural water content consistency and remolded consistency should be noted in the sample description Con sistency in the natural state corresponds in some respects to degree of compaction in coarsegrained soils and is usually evaluated by noting the ease by which the deposit can be excavated or penetrated Such terms as very soft soft medium stiff very stiff and hard are employed to describe consistency Sometimes the word rm is used synonymously with the term stiff Finegrained soils may be additionally described by using the tests explained in Table 33 for dilatancy toughness and dry strength Other techniques for visual classification of soils should be learned and practiced in the laboratory Excellent descriptions of visual classification and identi fication procedures are found in the USBR 1974 Earth Manual Ap pendix E3 and ASTM I980 Designation D 2488 EXAMPLE 31 leen new 10 4653M Aq 1851903 luai llid 100 Sieve analysis and plasticity data for the following three soils 001 0005 Soil 1 Soil 2 Soil 3 i Sieve Size Finer Finer Finer No 4 99 97 100 No 10 92 90 100 3 No 40 86 40 100 E No 100 78 a 99 E No 200 so 5 97 LL 20 124 pl 15 47 pl 5 Ni 77 Nonplastic J Requlred Classify the three soils according to the Unified Soil Classification System iiiiiiii 5 illTl Solution US Slandard sieve numbers I Use Table 32 and Fig 34 3 4 6 8101415 20 30 4O 50 70100140200 I Soil 5 K lLLLil 1 Plot the grain size distribution curves for the three soils shown in Fig Ex 31 A 1 V 2 For soil 1 we see from the curve that more than 50 passes the inquot L No 200 sieve 60 thus the soil is a finegrained soil and the 39 1 Atterberg limits are required to further classify the soil With g39l 4 LL 20 and P1 5 the soil plots in the hatched zone on the 393 plasticity chart Therefore the soil is a CLML i 3 Soil 2 is immediately seen to be a coarsegrained soil since only 5 u passes the No 200 sieve Since 97 passes the No 4 sieve the soil 3 is asand rather than a gravel Next note the amount of material passing the No 200 sieve 5 From Table 32 and Fig 34 the i l l J L l L i l l J 0001 j Silt or clay Ex3i ProjECl Boring no Date Pl NP 15 1 LL 20 1 24 49 Sand Medium Nat w Flg Ex 31 Grain size mml Coarse Fine Classificaiian Gradahon Curves Gravel oarse I r Elev or depth Cobbles l L 8 o 3 c 8 3 g 3 O N D in 391 l 8 soil is borderline and therefore has a dual symbol such as g SPSM or SWSM depending on the values of Cu and Cc From new 10 iu iaM Aq tau iuacuad Sample No the grain size distribution curve Fig Ex 31 we find that D so 071 mm D30 034 mm and D10 018 mm The coefficient of uniformity Cu is and the coefficient of curvature Cc is 00 034 39 D0 gtlt D60 7 018 x 071 0911 For a soil to be considered well graded it must meet the criteria shown in column 6 of Table 32 it does not so the soil is considered poorly graded and its classification is SPSM The soil is SM because the fines are silty nonplastic 4 A quick glance at the characteristics for soil 3 indicates the soil is fine grained 97 passes the No 200 sieve Since the LL is greater than 100 we cannot directly use the plasticity chart Fig 32 Use instead the equation for the Aline on Fig 32 to determine if the soil is a CH or MH PI 073LL 20 073124 20 759 Since the PI is 78 for soil 3 it lies above the Aline and thus the soil is classified as a CH 33 TlIE AASHTO SOIL CLASSIFICATION SYSTEM In the late 1920 s the US Bureau of Public Roads now the Federal Highway Administration conducted extensive research on the use of soils especially in local or secondary road construction the socalled farmto market roads From that research the Public Roads Classification System was developed by Hogentogler and Terzaghi 1929 The original system was based on the stability characteristics of soils when used as a road surface or with a thin asphalt pavement There were several revisions since 1929 and the latest in 1945 is essentially the present AASHTO 1978 system The applicability of the system has been extended considerably AASHTO states that the system should be useful for determining the relative quality of soils for use in embankments subgrades subbases and bases But you might keep in mind its original purpose when using the system in your engineering practice See Casagrande 1948 for some comments on this point Soquot Clanmention 33 The AASHTO Soll Claumcallon System 65 TABLE 35 AASHTO Definitions of Gravel Sand and SiltClay Soil Fraction Size Range Boulders Above 75 mm Gravel 75 mm to No 10 sieve 20 mm Coarse sand No 10 20 mm to No 40 0425 mm Fine sand No 40 0425 mm to No 200 0075 mm Siltclay combined Material passing the 0075 mm No 200 silt and clay sieve Soil fractions recognized in the AASHTO system are listed in Table 35 Boulders should be excluded from the sample to be classified but as with the USCS the amount of boulders present should be noted Fines are silty if they have a PI less than 10 and clayey if the PI is greater than 10 The AASHTO system classifies soils into eight groups A l through A8 and it includes several subgroups Soils within each group are evaluated according to the group index which is calculated by an empirical formula The only tests required are the sieve analysis and the Atterberg limits Table 36 illustrates the current AASHTO 1978 soil classification Granular materials fall into classes Al to A3 Al materials are well graded whereas A3 soils are clean poorly graded sands A2 materials are also granular less than 35 passing the No 200 sieve but they contain a significant amount of silts and clays A 4 to A7 are finegrained soils the silt clay materials They are differentiated on the basis of their Atterberg limits Figure 35 can be used to obtain the ranges of LL and PI for groups A4 to A7 and for the subgroups in A 2 Highly organic soils including peats and mucks may be placed in group A 8 As with the USCS classification of A8 soils is made visually The group index is used to further evaluate soils within a group It is based on the service performance of many soils especially when used as pavement subgrades It may be determined from the empirical formula given at the top of Fig 36 or you may use the nomograph directly Using the AASHTO system to classify soils is not difficult Once you have the required test data proceed from left to right in the chart of Table 36 and find the correct group by the process of elimination The first group from the left to fit the test data is the correct AASHTO classifica tion A complete classification includes the group index to the nearest whole number in parentheses after the AASHTO symbol Examples are A263 A45 A6l2 A75 17 etc Figure 37 will be helpful in classifying soils according to the AASHTO system