6: Consistency of Soils

CE 6: Geotechnical Engineering 1J.L.M. Tirao

Consistency of Soils• Describe the strength with which soil

materials are held together or the resistance of soils to deformation and rupture.

• Consistence varies with moisture content, and can be described as dry, moist, and wet consistence.

Rupture Resistance• Rupture resistance is the field measure of the

ability of the soil to withstand and applied stress or pressure as applied using the thumb and forefinger.

• Consistency and rupture resistance largely depends on soil minerals and water content.

When describing rupture resistance• When describing rupture resistance, we

attempt to describe it using:– Moist Consistency– Dry Consistency– Wet Consistency

- Difficult to obtain!!!- Not considered at all times

Wet Consistency• Water is added to soil to increase water content• Rupture resistance is best described using stickiness and

plasticity.– Stickiness – the ability of soil materials to adhere to other

objects– Plasticity – the ability of soil materials to change shape (but not

volume), continuously under the influence of a constant pressure and to retain the impressed shape when the pressure is removed.

Field Tests for Stickiness• (0) Non-sticky – No soil or practically no soil

sticks to fingers

Field Tests for Stickiness• (1) Slightly sticky – soil begins to stick to

fingers but comes off one or the other cleanly; does not stretch when the fingers are opened.

Field Tests for Stickiness • (2) Sticky – soil sticks to both thumb and

forefinger and tends to stretch a little and pull apart rather than pulling free from your fingers

Field Tests for Stickiness • (3) Very sticky – soil sticks firmly to both

thumb and forefinger and stretches when the fingers are opened

Plasticity of Solids• The degree to which a reworked soil can be

permanently deformed without rupturing• In other words, a plastic soil can undergo

deformation without cracking.

Plasticity of Soils

SOLID SEMISOLID PLASTIC LIQUID

Shrinkage Limit Plastic Limit Liquid Limit

Increasing moisture content

Plasticity of Soils• Shrinkage (SL), plastic (PL) and

liquid (LL) limits are also known as the Atterberg limits.

• Atterberg limits were observed by Albert Mauritz Atterberg in early 1900s but were later refined by Arthur Casagrande.

Why plasticity?• Water content significantly affects properties

of silty and clayey soils (unlike sand and gravel)

• Plasticity describes the response of soil to physical change through (increasing) moisture content

Why plasticity• Responses:

– Strength decreases as water content increases– Soils swell-up when water content increases– Fine-grained soils at very high water content possess

properties similar to liquids– As the water content is reduced the volume of the soil

decreases and the soils become plastic– If the water content is further reduced the soil becomes semi-

solid when the volume does not change

Atterberg limits

Atterberg limits• Atterberg limits are basically the limits of

water content used to define soil behavior.

Atterberg limits • Liquid Limit (LL) – moisture content at which soil begins

to behave as a liquid material and begins to flow• Plastic Limit (PL) – moisture content at which soil

begins to behave as a plastic material• Shrinkage Limit (SL) – moisture content at which no

further volume change occurs with further reduction in moisture content.

Liquid Limit (LL) Test

Liquid Limit Test• In the lab, cup apparatus or Casagrande cup is

used.

Liquid Limit Test

Liquid Limit Test• The moisture content (%) required to close a

distance of 12.7 mm along the bottom of the groove after 25 blows is the liquid limit.

Flow CurveLiquid limit – Moisture content along the flow curve, at 25 blows.

Liquid limit flow curve is a plot of water content (%) versus number blows (N), blow number in logarithmic scale.

Flow Index• Flow index is the slope of the flow curve.

Fall cone test method• Liquid limit is the moisture content at which a

standard cone of apex angle 30 degrees and weight of 0.78 N will penetrate a distance 20 mm in 5 seconds when allowed to drop from a position of point contact with the soil surface.

Fall cone test method

Plastic Limit (PL) Test

Plastic Limit Test• Plastic limit is the moisture content (%) at

which the soil, when rolled into threads of 3.2 mm (1/8 inch), crumbles (ASTM D-4318).

• The sample is remolded and the test is repeated until the thread begins to break at large diameters.

Plastic Limit Test

Plastic Limit Test

Plastic Limit Test• Another method for determining PL is the fall

cone test method by using a cone of similar geometry but with a mass of 2.35 N.

• The PL is the moisture content corresponding to a cone penetration of 20 mm.

Shrinkage Limit (SL) Test

Shrinkage Limit• Shrinkage limit is the moisture content (%) at

which the volume change of the soil mass ceases (or the volume no longer goes smaller).

Δ𝑤

w 𝑖

𝑆𝐿=𝑤𝑖 (% )− Δ𝑤 (%)From the figure,

Where: is the initial moisture content when the soil is placed in the shrinkage limit dish

is the change in moisture content = initial moisture content – moisture content at the shrinkage limit

𝑤𝑖 (% )=𝑚1−𝑚2

𝑚2

×100Where: is the mass of the wet soil at the beginning of the test (g) is mass of the dry soil (g)

Δ𝑤 (% )=𝜌𝑤

𝑚2(𝑉 1−𝑉 2 )×100

Where: is the initial volume of the wet soil (g) is the final volume (oven-dried soil) (g) is the density of water (1 g/cm2)

Shrinkage Limit• Substituting to original equation,

Derived Limits

Plasticity Index (PI)• Plasticity index is the measure of plasticity of

the soil. Plasticity index is also the range of moisture contents by which the soil exhibits plastic properties.

Plasticity Index (PI)Plasticity Index Description

0 Non-plastic

1 – 5 Slightly plastic

5 – 10 Low plasticity

10 – 20 Medium plasticity

20 – 40 High plasticity

Greater than 40 Very high plasticity

Liquidity Index (LI)• Liquidity index provides the relative

consistency of a cohesive soil with respect to the Atterberg limits. It is also the measure of current state of the soil.

Note: is the in-situ moisture content of the soil.

Liquidity Index (LI)Liquidity Index Description

LI is less than 0 Semi-solid or semi-plastic solid state- High strength, brittle (sudden fracture is expected)

LI is between 0 and 1 Plastic state- Intermediate strength, soil deforms like a plastic material

LI is greater than 1 Liquid state- Low strength, soil deforms like a viscous fluid

Activity (A or Ac)• Activity is an index for identifying the swelling

potential of clay soils.

Note: is the percent finer than 0.002 mm (clay size particles)

Activity (A)Activity Classification

Activity is less than 0.7 Inactive clay

Activity is between 0.7 and 1.2 Normal clay

Activity is greater than 1.2 Active clay

Activity controls how much wetting is required to move a soil from one phase to another (plastic to liquid state)

Shrinkage Ratio (SR)• Shrinkage ratio indicates how much volume

change is possible as changes above water content above the shrinkage limit occur.

Where and are the masses and volumes of oven-dried soil, respectively.

Shrinkage Ratio (SR)• Shrinkage ratio is also equal to: (verify at

home)

• If the soil has higher dry density, then the shrinkage ratio is also higher, thus, volume expansion is larger if water is added to it.

Specific Gravity

Plasticity Chart

Plasticity Chart• A-line separates inorganic clays from inorganic

silts• U-line is the upper limit of the relationship of

the PI to the LL for any soil found so far• The region between LL 30-50 and below the A-

line belongs to organic silts.

Example 1The following results were obtained from a liquid limit test on a clay using a Casagrande cup device. The natural water content of this clay is 38% and the plastic limit is 21%.

Number of blows 6 12 20 28 32

Water content % 52.5 47.1 42.3 38.6 37.5

Example 1

1 10 1000

10

20

30

40

50

60

Number of blows (logarithmic scale)

Wat

er c

onte

nt (%

)

Example 11. What is the liquid limit of this clay?2. What is the plasticity index of this clay?3. What is the liquidity index of this clay?4. What is the flow index?

Answers: 40%, 19%, 0.895

Example 2 (1.45)

1 10 1000

10

20

30

40

50

60

Number of blows (logarithmic scale)

Wat

er c

onte

nt (%

)

Example 3From the Atterberg limit tests for a soil, it was found that its liquid limit is 41% and its plastic limit is 21.1%. Determine (a) plasticity index of the soil, (b) liquidity index of the soil is the in situ moisture content is 30%, (c) and its nature corresponding to its liquidity index.Answers: (a) 19.9%, (b) 0.447, (c) Plastic

Example 4Following are the results of a shrinkage limit test:

– Initial volume of soil in saturated state = 24.6 cc– Final volume of soil in dry state = 15.9 cc– Initial mass in saturated state = 44 g– Final mass in dry state = 30.1 g

Determine (a) shrinkage limit of the soil, (b) the shrinkage ratio and the (c) specific gravity of the solids.Answers: (a) 17.28%, (b) 1.893, (c) G = 2.813

NEXT WEEKAugust 9, 2014More example of soil consistency computationsLab Experiment #3 Plastic Limit Test