20/08/2009 Lecture: 10 Soil Classification Sub-topics AASHTO - - PowerPoint PPT Presentation

▶

Oct 14, 2023 222 likes •398 views

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

SLIDE 1

IIT Bombay

CE 303 10 Instructor: AJ

20/08/2009 Lecture: 10 Soil Classification Sub-topics

AASHTO
ISSCS

Surface densification to reduce compressibility and increase strength (11th Century Chinese Building Standards)

SLIDE 2

IIT Bombay

CE 303 10 Instructor: AJ

AASHTO soil classification system

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

AASHTO

SLIDE 3

IIT Bombay

CE 303 10 Instructor: AJ

Group index, GI = 0.2a + 0.005ac + 0.01bd a = that part of per cent passing 75μm (-75μm) greater than 35; and not exceeding 75 (expressed as positive whole number == range 1 to 40) b = that part of per cent passing 75μm (-75μm) 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)

SLIDE 4

IIT Bombay

CE 303 10 Instructor: AJ

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

SLIDE 5

IIT Bombay

CE 303 10 Instructor: AJ

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

SLIDE 6

IIT Bombay

CE 303 10 Instructor: AJ

Indian Standard Soil Classification System

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

>300 300 80 20 4.75 2.0 0.425 0.075 <0.075 Silt & clay size Fine Medium Coarse Fine Coarse Sand Gravel Fine-grained soils Coarse-grained soils Cobble Boulder

Classification criteria for coarse-grained soils is the same as USCS Division of soil fraction on basis of grain size

SLIDE 7

IIT Bombay

CE 303 10 Instructor: AJ

SLIDE 8

IIT Bombay

CE 303 10 Instructor: AJ

Step-by-step procedure for ISSCS:

Determine PSD 50% or more of soil by weight is retained on 75μm sieve It is coarse grained 50% or more of soil by weight passes the 75μm sieve It is fine grained

SLIDE 9

IIT Bombay

CE 303 10 Instructor: AJ

If 50% or more of this fraction is retained on 4.75mm sieve Classify the soil as gravel (G) If 50% or more of this fraction passes 4.75mm sieve Classify the soil as sand (S) 50% or more of the soil retained on 75μm sieve Coarse grained

SLIDE 10

IIT Bombay

CE 303 10 Instructor: AJ

If 50% or more of this fraction is retained on 4.75mm sieve Classify the soil as gravel (G) If 50% or more of this fraction passes 4.75mm sieve Classify the soil as sand (S) If fraction passing 75μm sieve is less than 5% Calculate Cu and Cc from PSD If well-graded : classify as GW or SW If poorly graded : classify as GP or SP If fraction passing 75μm sieve is more than 12% Perform LL and PL tests

(fraction passing 0.425mm)

Use IS plasticity chart to determine the classification : GM, SM, GC, SC, GM-GC or SM-SC

SLIDE 11

IIT Bombay

CE 303 10 Instructor: AJ

If fraction passing 75μm sieve is less than 5% Calculate Cu and Cc from PSD If well-graded : classify as GW or SW If poorly graded : classify as GP or SP If fraction passing 75μm sieve is more than 12% Perform LL and PL tests (fraction passing 0.425mm) Use IS plasticity chart to determine the classification : GM, SM, GC, SC, GM-GC or SM-SC

Classify the soil as sand (S) Classify the soil as gravel (G) 5% < If fraction passing 75μm 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)

SLIDE 12

IIT Bombay

CE 303 10 Instructor: AJ

Step-by-step procedure for ISSCS:

Determine PSD 50% or more of the soil by weight passes the 75μm sieve It is fine grained

SLIDE 13

IIT Bombay

CE 303 10 Instructor: AJ

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

SLIDE 14

IIT Bombay

CE 303 10 Instructor: AJ

1. Determine coarse fraction : G or S
2. Determine LL and PL
3. Assign dual symbols; e.g. GM-ML, GM-MI, etc.

If the soil has about 50% each of fine- and coarse-grained fraction

SLIDE 15

IIT Bombay

CE 303 10 Instructor: AJ

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

f low toughness would be hard to re-roll without breaking them

SLIDE 16

IIT Bombay

CE 303 10 Instructor: AJ

(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