IIT Bombay 20/08/2009 Lecture: 10 Soil Classification Sub-topics � AASHTO � ISSCS Surface densification to reduce compressibility and increase strength (11th Century Chinese Building Standards) CE 303 10 Instructor: AJ
IIT Bombay AASHTO soil classification system AASHTO AASHTO classifies soil into 8 groups: A-1 through A-7, and A-8 for peat Soils are evaluated according to group index: Group index, GI = 0.2a + 0.005ac + 0.01bd Group index, GI = 0.2a + 0.005ac + 0.01bd CE 303 10 Instructor: AJ
IIT Bombay Group index, GI = 0.2a + 0.005ac + 0.01bd a = that part of per cent passing 75 μ m (-75 μ m) greater than 35; and not exceeding 75 (expressed as positive whole number == range 1 to 40) b = that part of per cent passing 75 μ m (-75 μ m) greater than 15; and not exceeding 55 (expressed as positive whole number == range 1 to 40) c = that part of LL greater than 40; and not greater than 60 (expressed as positive whole number == range 1 to 20) d = that part of PI greater than 10; and not exceeding 30 (expressed as positive whole number == range 1 to 20) CE 303 10 Instructor: AJ
IIT Bombay Note: 1. If any term becomes negative, it is dropped 2. GI is rounded off to nearest whole number 3. Negative GI is reported as zero 4. Proceed from left to right in the table and select the first group that fits the data CE 303 10 Instructor: AJ
IIT Bombay PI of A-7-5 subgroup is equal to or less than LL minus 30 PI of A-7-6 subgroup is greater than LL minus 30 CE 303 10 Instructor: AJ
IIT Bombay Indian Standard Soil Classification System Division of soil fraction on basis of grain size Coarse-grained soils Fine-grained soils Gravel Sand Boulder Cobbl e Coarse Fine C o ars e Medium Fine Silt & clay size >300 300 80 20 4.75 2.0 0.425 0.075 <0.075 Classification criteria for coarse-grained soils is the same as USCS Only difference in classification of fine grained soils In ISSCS : fine-grained soils are subdivided into 3 instead of 2 subgroups – low, intermediate and high plasticity CE 303 10 Instructor: AJ
IIT Bombay CE 303 10 Instructor: AJ
IIT Bombay Step-by-step procedure for ISSCS: Determine PSD 50% or more of soil by 50% or more of soil by weight is retained on 75 μ m weight passes the 75 μ m sieve sieve � It is coarse grained � It is fine grained CE 303 10 Instructor: AJ
IIT Bombay 50% or more of the soil retained on 75 μ m sieve Coarse grained If 50% or more of this fraction is If 50% or more of this fraction retained on 4.75mm sieve passes 4.75mm sieve � Classify the soil as gravel (G) � Classify the soil as sand (S) CE 303 10 Instructor: AJ
IIT Bombay If 50% or more of this fraction is If 50% or more of this fraction retained on 4.75mm sieve passes 4.75mm sieve � Classify the soil as gravel (G) � Classify the soil as sand (S) If fraction passing 75 μ m sieve If fraction passing 75 μ m is less than 5% sieve is more than 12% � Calculate C u and C c from � Perform LL and PL tests (fraction passing 0.425mm) PSD � Use IS plasticity chart to � If well-graded : classify as determine the classification : GW or SW GM, SM, GC, SC, GM-GC or � If poorly graded : classify SM-SC as GP or SP CE 303 10 Instructor: AJ
IIT Bombay � Classify the soil as gravel (G) � Classify the soil as sand (S) If fraction passing 75 μ m sieve is less than 5% If fraction passing 75 μ m sieve is more than 12% � Calculate C u and C c from PSD � Perform LL and PL tests (fraction passing 0.425mm) � If well-graded : classify as GW or SW � Use IS plasticity chart to determine the classification : GM, SM, GC, SC, GM-GC or SM-SC � If poorly graded : classify as GP or SP 5% < If fraction passing 75 μ m sieve is < 12% ?? Assign dual symbol : appropriate to gradation and plasticity (GW-GM, GW-GC, GP-GM, GP-GC, SW-SM, SW-SC, SP-SM, SP-SC) CE 303 10 Instructor: AJ
IIT Bombay Step-by-step procedure for ISSCS: Determine PSD 50% or more of the soil by weight passes the 75 μ m sieve � It is fine grained CE 303 10 Instructor: AJ
IIT Bombay Determine LL and PL (on fraction passing 0.425mm) Use PI chart to classify the soil e.g. CL, CI or CH or ML, MI or MH PI chart : can also be used for slightly organic soils If in hatched zone : use dual symbols CE 303 10 Instructor: AJ
IIT Bombay If the soil has about 50% each of fine- and coarse-grained fraction 1. Determine coarse fraction : G or S 2. Determine LL and PL 3. Assign dual symbols; e.g. GM-ML, GM-MI, etc. CE 303 10 Instructor: AJ
IIT Bombay Field identification procedure : (1)Visual examination: (coarse-grained soils) examine for size, angularity, touch and grading (2) Dilatancy: Squeeze a pat of saturated sample If water appears and disappears � fine sands and silts (3) Toughness: (Useful for clays) Roll a pat of sample in the palm of hand until it starts to crumble at 3mm dia Soils of high toughness would require high pressure to re-roll. Soils of low toughness would be hard to re-roll without breaking them CE 303 10 Instructor: AJ
IIT Bombay (4) Dry strength: (fine grained soils) Break a pat of air dried sample between fingers If the sample is powdered easily � low dry strength If it cannot be powdered � high dry strength (e.g. clays) (5) Organic content: Fresh, wet, organic soils � pungent odour and dark colouration (6) Other identification tests: “Developed” by individual at site based on experience CE 303 10 Instructor: AJ
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