soils enci 5791 engineering properties of soils soil types engineering properties of soils...

Soils ENCI 579 1

Engineering Properties of SoilsSoil Types

• Engineering Properties of Soils– foundation for the project– construction material (road embankments,earth

dams)

• Soil Definition (Engineering)– “refers to all unconsolidated material in the

earth’s crust, all material above bedrock”• mineral particles (sands, silts, clays)

• organic material (topsoil, marshes) + air + water

Soils ENCI 579 2


• Mineral Soil Particles– weathering of rock from the crust of the earth– physical weathering and chemical weathering

• Physical Weathering– action of frost, water, wind, glaciers, plant/animals,

etc. breaking particles away from original bedrock– particles transported by wind, water, ice >rounding

and reducing their size– soils formed are called granular soil type– “grains are similar to the original bedrock”

Soils ENCI 579 3


• Chemical weathering– occurs when water flows through rocks and

leaches out some of the mineral components of the rock

– soils formed are called clays– “clay particles are mineral crystals that have

very different properties from those of the original bedrock”

Soils ENCI 579 4

Engineering Properties of SoilsMineral Soils

Soils ENCI 579 5


• Granular and Cohesive soil types– difference in engineering properties result from

the large variation in size and shape of the grains

• Cohesive soil type (clays)– grains are extremely small and flat

• the mass of a grain as a force is negligible when compared to the forces resulting from the surface properties of the grain

Soils ENCI 579 6


• Water Holding Capacity of Clays– Shrinkage

• evaporation of exposed clays

• loading

– Expansion• dry side may absorb moisture

• Structure of Clays– deposited by settling out in lakes

Soils ENCI 579 7


• Structure of Clays– surface charges forces grains to edge to side

pattern– flocculent structure as opposed to granular soils

which are deposited in a denser configuration because the force of gravity on the mass of these grains is more important

Soils ENCI 579 8


• Clays have surface charges due to the very large surface area per gram of material

• Chemical composition results in:– negative charges along the sides of a grain

– positive charges at the ends of a grain

• Results of these surface properties– water holding capacity of clays surface charges attract

water

– structure of clay deposits

clay grain

Soils ENCI 579 9


Soils ENCI 579 10


• Clay Soils– Small flat shape– Negative/positive surface charges– Bound water on the surface– Different clay minerals are different in size– Swelling clays absorb water into the crystal

lattice– Shrinkage due to evaporation or loading

Soils ENCI 579 11


• Granular Soils– Larger grain sizes than clays– Particles tend to be more or less spheres/cubes– Bound water is small compared to overall mass– Silt particles may not be visible to eye but tend

to be gritty, have dull appearance and lack cohesion when dry

Soils ENCI 579 12


• Organic Soils– Tend to be fibrous and/or amorphous– Brown to Black in color– High moisture holding capacity– Water may run out when squeezed– Dried organic soils may combust

Soils ENCI 579 13


• Silts are coarser than clays and not bond tightly together

• Silts are gritty, less plastic and dull when cut

• Dry Strength-silts loose “apparent cohesion” when dried

• Shaking test-saturated silt samples become denser water seeps to the surface - “dilantancy”

Soils ENCI 579 14

Engineering Properties of SoilsMass-Volume Relationships

Soils ENCI 579 15


Example 1-2 A soil sample has a

volume of 175cm3 and a total mass of 300g. Mass when dried is 230g. Relative density of the soil solids is 2.70. Find , D, w, e, S and n

Given:

Mw = M - MD = 70g

MD = 230g

M = 300g

V = 175cm3 WaterSolids

Air

Soils ENCI 579 16


Calculations:Vw = Mw/w

= 70g/(1 g/cm3)

=70 cm3

VD = MD/(RD x w)

= 230g/(2.70 x 1 g/cm3)

= 85 cm3

VA = V- (VD + Vw )

= 175 - 155 = 20 cm3

VA = 20 cm3

VW = 70 cm3

VD = 85 cm3

V = 175 cm3

Soils ENCI 579 17


Answer:

= M/V = 300g/175cm3 = 1.71 g/cm3

D = MD/V = 230g/175cm3 = 1.31g/cm3

w = Mw / MD = 70g/230 g = 30.4 %

e = Vv / VD = 90 cm3/85 cm3 = 1.06

S = VW /Vv = 70 cm3/90 cm3 = 78 %

n = Vv /V = 90 cm3/ 175 cm3 = 51 %

Soils ENCI 579 18

Engineering Properties of SoilsMass-Volume Relationships - Rules

1. Density is given assume total unit volume 1 cm3 or 1 m3

2. Water content is given along with total density or total mass.

Use MD = M or D =

1 + w 1+w

3. Void Ratio is given and RD assume a unit volume of soil solids VD = 1 m3

Soils ENCI 579 19

Engineering Properties of SoilsMass-Volume Relationship

• Density Index– Field soil condition referred to as loose or dense– Density Index is insitu soil’s density relative to

the maximum and minimum for that type of soil– Assessing the stability of granular soils– Known as relative density

– ID = DRY MAX x DDRY MIN

D DRY MAX - DRY MIN

Soils ENCI 579 20

Engineering Properties of Soils

Soils ENCI 579 21

Engineering Properties of SoilsClassification Tests

• Two types of tests used in classifying soils– Grain size, measures grain sizes– Plasticity, measures grain types

• Grain Size– grain size distribution curve

• Sieve analysis gravel and sand

• Hydrometer test for silt and clay

Soils ENCI 579 22


• Hydrometer Test– Used to find the size of smaller grains to plot a

grain size distribution curve– Stokes Law

• particles in suspension settle out at a rate which varies with their size

• hydrometer measures the density of a soil-water mix at various times as the grain settles

• The size of particle to the center of the bulb can be calculated and density of the solution indicates the percentage of the sample still in solution

Soils ENCI 579 23

Engineering Properties of SoilsSieve Analysis

Soils ENCI 579 24


Soils ENCI 579 25


• Grain Size Distribution Curve– Shape

• Uniform soil is composed of mainly one size grain

• Well graded soil contains a wide range of grain sizes

– Effective Size• Effective size is the grain size that only 10% of the

grain sizes are finer than.

• The amount and type of fine grains in a soil are important in assessing the properties of that soil

Soils ENCI 579 26


• Grain Size Distribution Curve– Uniformity Coefficient Cu

• indication of the shape of the curve and range of particle sizes that the soil contains

• Cu = D60 / D10

– Coefficient of Curvature Cc• indication of the shape of the curve.

• Cc = (D30)2 / (D60 x D10)

Soils ENCI 579 27


• Curve A - Uniform Soil• Curve B - Well Graded

Soil• Soil B

– Effective size = .09 mm– Cu = 7/.09=78– Cc = 1.12/(7x.09)=1.9

Soils ENCI 579 28


– Textural Classification• based entirely on grain size

– ASTM System• Gravel larger than 4.75 mm (No. 4)

• Sand 4.75 mm - 0.075 mm (No.4-No. 200)

• Silt 0.075 mm - 0.005 mm

• Clay smaller than 0.005 mm

– Identify % of the grains as classified

Soils ENCI 579 29


Soils ENCI 579 30


• Plasticity Test (grain type)

• Measures the amount of water that a soil absorbs– Plastic Limit

• w/c where the soil behaves like a plastic material

– Liquid Limit• w/c where the soil behaves more like a liquid

– Plasticity Index (Atterberg Limits Test)• the range of w/c’s where soil is plastic

Soils ENCI 579 31


Soils ENCI 579 32


• Atterberg Limits test measures the consistency of the soil deposit– Soil A: Wp=25 Wl=32 w/c=35– Soil B: Wp=40 Wl=80 w/c=45

• Atterberg Limits Test– soil rolled into a thread Wp

– soil flow measured in a cup Wl

Soils ENCI 579 33

Engineering Properties of SoilsSoil Classification

• Unified Soil Classification ASTM D2487– general engineering purposes

• AASHTO Soil Classification– highway engineering purposes– classifies soils to it’s suitability as a subgrade

soils enci 5791 engineering properties of soils soil types engineering properties of soils...

Documents

soils enci

size soils

rock soils

surface properties water

different properties

clays particles

clays clay particles

cohesive soil types