Course3_SoilMechanics_EngineringGeology

Geological Engineering DepartmentFaculty of Engineering

Introduction to Soil Mechanics

Rock Cycles

(Das, 1998)

The final products due to weathering are soils

Bowen’s Reaction Series– The reaction series are similar to the weathering stability

series.

•More stable

•Higher weathering resistance

(Das, 1998)

Soils• Glacial soils: formed by transportation and deposition of glaciers.• Alluvial soils: transported by running water and deposited along

streams.• Lacustrine soils: formed by deposition in quiet lakes (e.g. soils in

Taipei basin).• Marine soils: formed by deposition in the seas • Aeolian soils: transported and deposited by the wind (e.g. soils in the

loess plateau, China).• Colluvial soils: formed by movement of soil from its original place by

gravity, such as during landslide

Three Phases in SoilsS : Solid Soil particle

W: Liquid Water (electrolytes) A: Air Air

Three Volumetric Ratios• (1) Void ratio e (given in decimal, 0.65)

• (2) Porosity n (given in percent 100%, 65%)

• (3) Degree of Saturation S (given in percent 100%, 65%)

)V(solidsofVolume)V(voidsofVolumee

)V(samplesoilofvolumeTotal)V(voidsofVolumen

%100)V(voidsofvolumeTotal

)V(watercontainsvoidsofvolumeTotalSv

)e1(VeVn

• Completely dry soil S = 0 %• Completely saturated soil S = 100%• Unsaturated soil (partially saturated soil) 0% < S <

• Demonstration:• Effects of capillary forces

• Engineering implications:– Slope stability– Underground excavation

%100)V(voidsofvolumeTotal

)V(watercontainsvoidsofvolumeTotalSv

Engineering Applications• 80 % of landslides are due to

erosion and “loss in suction” in Hong Kong.

• The slope stability is significantly affected by the surface water.

(Au, 2001)

Density and Unit Weight• Mass is a measure of a

body's inertia, or its "quantity of matter". Mass is not changed at different places.

• Weight is force, the force of gravity acting on a body. The value is different at various places (Newton's second law F = ma) (Giancoli, 1998)

• The unit weight is frequently used than the density is (e.g. in calculating the overburden pressure). w

mkN8.9,Water

secm8.9g

gravitytodueonaccelerati:g

VolumegMass

VolumeWeight,weightUnit

VolumeMass,Density

Weight Relationships

• Water Content w (100%)

• For some organic soils w>100%, up

to 500 %• For quick clays, w>100%

• Density of water (slightly varied with temperatures)

Density of soila. Dry density

b. Total, Wet, or Moist density (0%<S<100%, Unsaturated)

c. Saturated density (S=100%, Va =0)

d. Submerged density (Buoyant density)

%100)(

MsolidssoilofMassMwaterofMassw )V(samplesoilofvolumeTotal

)M(solidssoilofMass

)V(samplesoilofvolumeTotal)MM(samplesoilofMass

)V(samplesoilofvolumeTotal)MM(watersolidssoilofMass

333w m/Mg1m/kg1000cm/g1

Weight Relationships (Cont.)• Submerged unit weight:

• Consider the buoyant force acting on the soil solids:

• Archimede’s principle:• The buoyant force on a body

immersed in a fluid is equal to the weight of the fluid displaced by that object.

%)100S(V

)VV(WVVW

Engineering Applications (w)• For fine-grained soils, water

plays a critical role to their engineering properties (discussed in the next topic).

• For example,• The quick clay usually has a

water content w greater than 100 % and a card house structure. It will behave like a viscous fluid after it is fully disturbed.

Clay particle

(Mitchell, 1993)

Other Relationships

(1) Specific gravity

• Proof:

GweSweS

Typical Values of Specific Gravity

(Lambe and Whitman, 1979)(Goodman, 1989)

Solution of Phase Problems

• Remember the following simple rules (Holtz and Kovacs,

1981):

• Remember the basic definitions of w, e, s, S, etc.

• Draw a phase diagram.• Assume either Vs=1 or Vt=1, if not given.

• Often use wSe=ws, Se = wGs

Problem:

1. Suatu sampel lempung jenuh mempunyai kadar air 56%, jika Gs = 2.72, hitunglah e dan n?

2. Suatu sampel pasir seragam mempunyai porositas sebesar 43% dan kadar air 12%, anggap Gs = 2.65, hitung angka pori dan tingkat kejenuhan!

SOIL TEXTURE Particle Sizes

Coarse-Grained Soil Texture

• Typical values• Engineering

applications:

– Volume change tendency– Strength

(Lambe and Whitman, 1979)

Simple cubic (SC), e = 0.91, Contract

Cubic-tetrahedral (CT), e = 0.65, Dilate

Link: the strength of rock joint

)itan(strengthShear n

Engineering Implications (e)(Cont.)

– Hydraulic conductivity• Which packing (SC or

CT) has higher hydraulic conductivity?

e = 0.91

e = 0.65

The fluid (water) can flow more easily through the soil with higher hydraulic conductivity

Engineering Applications (e)(Cont.)

e = 0.91

e = 0.65

The finer particle cannot pass through the void

•Clogging

Critical state soil mechanics

Filter

Soil Texture: Grain Shape

Important for coarse granular soils Angular soil particle higher friction Round soil particle lower friction

Rounded Subrounded

Subangular Angular

(Holtz and Kovacs, 1981)

Coarse-grained soils

Coefficient of Friction, = 0.4

Weight, N = 4T

Force, T = 1.6T

T = N.

Friction

T = N.

T = N. tan ()

= angle of internal friction

EXTERNAL

INTERNAL

Grain Shape and

Rounded 30 - 35Rounded 30 - 35oo Sub-rounded 32 - 37Sub-rounded 32 - 37oo

Sub-angular 34-39Sub-angular 34-39oo Angular 36-41Angular 36-41oo

Mineral type and

• QuartzQuartz 3030oo

• CalciteCalcite 3838oo

• KaoliniteKaolinite 1515oo

• IlliteIllite 1010oo

• SmectiteSmectite 5 5oo

Cohesion

Some soils, and all rocksdisplay some interparticlebonding, which gives them

strength even when the normal stress is zero –

we call this COHESION

Fine Grained Size: Atterberg Limits• The presence of water in fine-grained soils can significantly affect

associated engineering behavior, so we need a reference index to clarify the effects.

Atterberg Limits (Cont.)

Liquid Limit, LL

Liquid State

Plastic Limit, PL

Plastic State

Shrinkage Limit, SL

Semisolid State

Solid StateDry Soil

Fluid soil-water mixture

Typical Values of Atterberg Limits

(Mitchell, 1993)

Indices•Plasticity index PI •For describing the range of water content over which a soil was plastic•PI = LL – PL

• Liquidity index LI • For scaling the natural

water content of a soil sample to the Limits.

contentwatertheiswPLLL

LI <0 (A), brittle fracture if sheared0<LI<1 (B), plastic solid if sheared LI >1 (C), viscous liquid if sheared

Liquid Limit, LL

Liquid State

Plastic Limit, PL

Plastic State

Shrinkage Limit, SL

Semisolid State

Solid State

• The Atterberg limits are usually correlated with some engineering properties such as the permeability, compressibility, shear strength, and others. In general, clays with high plasticity have lower permeability,

and they are difficult to be compacted. The values of SL can be used as a criterion to assess and

prevent the excessive cracking of clay liners in the reservoir embankment or canal.

Engineering Applications

The Atterberg limit enable clay soils to be classified.

OTHER SOIL DESCRIPTORSMOISTURE CONTENTDry - Absence of moistureMoist - Damp, but no visible waterWet - Visible water

CEMENTATIONWeakly - Crumbles or breaks easilyModerately - Crumbles or breaks with considerable finger pressureStrongly - Will not crumble or break with finger pressure

COLOR (Soil Color Chart)ODORADDITIONAL COMMENTS

COARSE-GRAINED SOIL

FINE-GRAINED SOIL

Course3_SoilMechanics_EngineringGeology

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