Using Soil Classification Data

Using Soil Classification & Description Data (What you Need to Know, Especially for Risk Assessment and Modeling)VAP Spring 2015Certified Professional CoffeeSoil Classification & DescriptionPurpose of trainingBriefly review the most commonly used soil classification systems (USDA & USCS) Identify some limitations and (associated misuses) of each systemAddress some concerns about the use of soil classification and description data (especially for risk assessment and modeling)

Soil Classification & DescriptionPurpose of soil classification and description?Provides soil property data in an organized, systematic manner to support an activity (environmental remediation, agriculture, construction, etc.)Foundation of the conceptual site model for environmental remediation projects

Soil Classification & DescriptionJohnson and Ettinger model for estimating subsurface vapor intrusion into buildingsUses USDA Soil Conservation Survey soil classification data to provide site-specific data for the following model parameters:Soil bulk densityTotal porosityWater-filled porositySoil Classification & DescriptionPoor quality data often result fromA laissez-faire approach to soil (geologic) logging, which leads toFailure to learn and consistently follow standard soil classification & description methodsIndiscriminately using different classification systems (mix and match), or making it up as you log along

Soil Classification & DescriptionEven good quality data can be misused when the user doesnt understandThe classification system and what the data actually indicate about soil propertiesData limitations, e.g., Field vs. laboratory classificationProperties used as basis for a particular classification (USDA silt vs. USCS silt)

Soil Classification SystemsUnified Soil Classification SystemEngineering and environmental applicationsASTM D2486-11 (lab), ASTM D2488-09a (field)USDA Soil Classification SystemAgricultural, land development and environmental applicationsUSDA Soil Survey Manualhttp://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/ref/?cid=nrcs142p2_054262

USDA Soil Classification SystemOrigin and purposeEarly soil science work, 1930s through 1940s by American and Russian scientistsU.S. National Cooperative Soil Survey officially adopted the USDA system in 1965 and published it in 1975 (Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys)Evaluates soils for agriculture, ranching, forestry (land development, non-farm uses important after 1950)

USDA Soil Classification SystemUSDA Texture Triangle DiagramTwelve soil classes defined by %s sand, silt, claySilty Clay = 40-60% clay, 40-60% silt, 0-20% sandLab analyses or field texture by feel analysisGravel not included (used as modifier); %s sand, silt clay normalized to 100% without gravel

USDA Soil Classification SystemParticle Size Limits

SANDSILTCLAYGRAVEL2.00 mm0.050 mm0.002 mm(Hydrometer)(Sieves)75 mmUSDA Soil Classification SystemSoil class identification (e.g., sandy loam) May be based on field examination or lab analysis of soil samplesField examination, estimate % sand/silt/clay byPlasticity (ability to form a soil ribbonFeel of soil (gritty vs. smooth vs. greasy)Accuracy of field identification? At best, +/- 10% to 15% (sand/silt/clay)

USDA Soil Classification System(Field Testing)

USDA Soil Classification System(Field Testing)

Soil ribbon length estimates % clayGrittySmoothGrittiness estimates % sand versus % siltUnified Soil Classification System (USCS)Origin and purposeDeveloped in 1942 by Professor Arthur Casagrande for airfield construction during WWIIMost commonly used soil classification systemApplicable ASTM standardsASTM D2487-11, Standard Classification for Engineering Purposes (laboratory testing)ASTM D2488-09a, Standard Practice for Description and Identification of Soils (field evaluation)Unified Soil Classification System (USCS)Separates soils into two broad categories and then uses a flow-chart approach to classify based on soil properties (%s by weight)Coarse-grained soils, > 50% sand and gravelG stands for gravel or gravelly soilsS stands for sand or sandy soils Fine-grained soils, >= 50% silt and clayM stands for silt or silty soilsC stands for clay or clayey soilsUnified Soil Classification System (USCS)Coarse-Grained Soil ClassificationLess than 5% fines (silt and clay)If the % sand is >= % gravel, SAND (S)If the % gravel is > % sand, GRAVEL (G)Greater than 15% finesSILTY SAND (SM) or CLAYEY SAND (SC)SILTY GRAVEL (GM) or CLAYEY GRAVEL (GC)

Unified Soil Classification System (USCS)Coarse-Grained Soil Classification: GradingWell Graded(Poorly Sorted)

Poorly Graded(Well Sorted)

Well Graded vs Poorly Graded

Unified Soil Classification System (USCS)Coarse-Grained Soil ClassificationBetween 5 and 15% fines (approx. 10%)?Dual Symbol: two symbols separated by a hyphen, e.g., SP-SC, Poorly Graded Sand with Clay1st symbol (SP) identifies sand or gravel and grading2nd symbol (SC) identifies the type of fines (silt or clay)Hydraulic conductivity significance (fines lower K by reducing effective porosity) + other soil propertiesUnified Soil Classification System (USCS)Dual Symbols, Coarse-Grained SoilsPoorly Graded Gravel with Silt (GP-GM)Poorly Graded Gravel with Clay (GP-GC)Well-Graded Gravel with Silt (GW-GM)Well-Graded Gravel with Clay (GW-GC)Poorly Graded Sand with Silt (SP-SM)Poorly Graded Sand with Clay (SP-SC)Well-Graded Sand with Silt (SW-SM)Well-Graded Sand with Clay (SW-SC)

Unified Soil Classification System (USCS)Fine-Grained Soil ClassificationFine-grained soil > or = 50% silt and/or clayThe USCS grain size limit for the silt (M) and clay (C) fractions is the same 0.075 mm (passes #200 sieve)(USDA system: silt fraction is between 0.05 and 0.002 mm, clay fraction < or = 0.002 mm)USCS silt and clay classification is based on plastic properties (Atterberg Limits) and not particle size distribution

Unified Soil Classification System (USCS)Atterberg Limits (based on laboratory testing, ASTM D2487)Plastic Limit (PL), water content at which soil starts to exhibit plastic behaviorLiquid Limit (LL), water content at which soil starts to exhibit liquid behaviorPlasticity Index, PI, numeric difference between LL and PL, measure of plasticity; clays have a high PI, silts have a low PI

Unified Soil Classification System (USCS)Fine-Grained Soil Classification, Plasticity Chart

Unified Soil Classification System (USCS) Fine-Grained Soil Classification, Field Plasticity Tests (based on Table 12, ASTM D2488-09a)

Unified Soil Classification System (USCS) Fine-Grained Soil ClassificationPercentages of sand or gravel?If < 15% sand or gravel, no qualifier needed, e.g., lean clayIf 15-25% sand or gravel, add with sand (%S > or = %G) or with gravel (%G > %S), e.g., lean clay with sandIf > 30 % sand or gravel, add sandy (%S > or = %G) or gravelly (%G > %S), e.g., sandy lean clay

USCS vs. USDA Particle Size Comparison

Unified Soil Classification SystemGRAVELSANDSILT and CLAY75 mm4.75 mm0.074 mm (#200 sieve)GRAVELSANDCLAYSILTUSDA Soil Classification System75 mm2.00 mm0.05 mm0.002 mmATTERBERG LIMITSMisuse of Soil Classification SystemsScenario #1A model utilizes USDA soil classification data (the results will be used to evaluate risk). Is substituting USCS soil type data for equivalent USDA soil type data appropriate? For example, can we assume that that USCS lean clay and sandy clay are equivalent to USDA clay and sandy clay?

USDA Soil TypesDirect Comparison of USCS & USDA LabClassification of 62 Soil Samples FromCuyahoga, Franklin, Hamilton, Lucas andMontgomery Counties (DERR County Background Sampling)Misuse of Soil Classification SystemsScenario #1Ohio EPA geotechnical laboratory data indicatesUSCS lean clay may classify as USDA silty clay, clay loam, silty clay loam, loam or silt loam (but not clay)USCS clayey sand classifies as USDA sandy loam (not clayey sand)Misuse of Soil Classification SystemsScenario #1USCS and USDA are not mix and match systems (soil types arent equivalents)Some soil classification types may not occur in a given geographic area or site locationsUSCS elastic silt (MH) is not common in OhioBased on DERRs county soil data to date, USDA clay, sandy clay, and sandy clay loam are not common in Ohio

U.S. Army Corps of Engineers, Electromagnetic Power Attenuation in Soils,ERDC/EL TR-05-5, August 2005USCS vs. USDA Particle Size Comparison

Unified Soil Classification SystemGRAVELSANDSILT and CLAY75 mm4.75 mm0.074 mm (#200 sieve)GRAVELSANDCLAYSILTUSDA Soil Classification System75 mm2.00 mm0.05 mm0.002 mmATTERBERG LIMITSMisuse of Soil Classification SystemsScenario #1 (Continued)If you perform hydrometer analysis (ASTM D422) to determine the clay (> 0.002 mm) fraction for a soil sample classified according to the USCS, can you use the clay data with the USCS sand and silt %s to determine the USDA soil type? USCS vs. USDA Particle Size Comparison

Unified Soil Classification SystemGRAVELSANDSILT and CLAY75 mm4.75 mm0.074 mm (#200 sieve)GRAVELSANDCLAYSILTUSDA Soil Classification System75 mm2.00 mm0.05 mm0.002 mmATTERBERG LIMITSUSCS vs. USDA Lab TestingUSCS Must HavesUSDA Must HavesSieve Sizes#4 (gravel/sand)#200 (sand/[silt+clay])Atterberg LimitsSieve Sizes#10 (gravel/sand)Hydrometer0.05 mm fraction (sand/silt)0.002 mm fraction (silt /clay)USCS vs. USDA Lab TestingUSCS OptionsUSDA OptionsSieve Sizes (c. gravel/f. gravel)#10 (c. sand/m. sand)#40 (m. sand/f. sand)Hydrometer0.005 mm or 0.002 mm fractions (clay)Sieve Sizes#18 (v.c. sand/c. sand)#35 (c. sand/m. sand)#60 (m. sand/f. sand)#140 (f. sand/v.f. sand)Misuse of Soil Classification Systems Scenario #2You are reviewing existing soil boring logs for a new site. The previous consultants assessment report states that they logged soils using the USCS per ASTM D2488-09a. You note that the terms loam and loamy are used with USCS terminology, and that silty clay appears frequently as a field soil description. Do you have any concerns?Misuse of Soil Classification Systems Scenario #2Problems with consultants soil classification:Never appropriate to combine USCS and USDA soil classification terminologyUSCS silty clay or USDA silty clay? (significant difference)USCS silty clay? Should have used the term lean clay instead per ASTM D2488-09a and ASTM D2487-11 (silty clay cant be classified in the field)Unified Soil Classification System (USCS)Fine-Grained Soil Classification, Plasticity Chart

Misuse of Soil Classification Systems Scenario #3You need to determine USDA soil types for a Phase II property assessment in Hamilton County. You have soil type data from several sites in Franklin County. Both counties are covered with till, which consistently classifies as clay loam or silty clay loam at the Franklin County sites.

Would assuming that the Hamilton County soils are clay loam or silty clay loam based on the Franklin County data be appropriate?

Comparison of USDA Soil Classification for Franklin and Hamilton Counties(DERR County Background Sampling, Soil Laboratory Results)

Comparison of USDA Soil Classification for Franklin, Lucas, and Hamilton Counties(DERR County Background Sampling, Soil Laboratory Results)Recommendations for Using Soil Classification DataBecome familiar with both the USCS and USDA soil classification system basicsNever substitute USCS data for USDA soil classification data (directly or by manipulation)Request soil laboratory testing/classification for modeling and risk assessment purposesRecommendations for Using Soil Classification DataQuestion soil classification (field or lab) that doesnt appear to follow a standard system Never assume the soil type(s) at one site is/are comparable to another site based on the origin of geologic material (or proximity)Questions?Thanks for your time and attention!