1

Introduction to Soil Mechanics

2

LABORATORY TESTS AND EQUIVALLENT FIELD EXPERIMENTS

Soil Index (, , e, GS etc.)· Measurement of soil volume and mass· Sieve analysis test· Atterberg test

Compaction : proctor, field compaction

Shear Strength (c, )· Triaxial Test (UU, CU, CD)· Direct Shear· Unconfined Compression Test · CPT, SPT,Vane Shear test,….

Compresibility (Cc, Cv)Consolidation test

Permeability (k)· Constant Head· Falling Head

3

Three Phases in SoilsS : Solid Soil particle

W: Liquid Water (electrolytes) A: Air Air

4

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

s

v

)V(samplesoilofvolumeTotal

)V(voidsofVolumen

t

v

%100)V(voidsofvolumeTotal

)V(watercontainsvoidsofvolumeTotalS

v

w

e1

e

)e1(V

eVn

s

s

5

Engineering Applications (e)

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

i

6

Engineering Implications (e)(Cont.)

· Hydraulic conductivity–Which packing (SC or CT)

has higher hydraulic conductivity?

SC

e = 0.91

CT

e = 0.65

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

7

Engineering Applications (e)(Cont.)

SC

e = 0.91

CT

e = 0.65

The finer particle cannot pass through the void

• Clogging

· Critical state soil mechanics

Filter

8

Engineering Applications (S)Completely dry soil S = 0 %

Completely saturated soil S = 100%

Unsaturated soil (partially saturated soil) 0% < S < 100%

Demonstration:

Effects of capillary forces

Engineering implications:·Slope stability·Underground excavation

%100)V(voidsofvolumeTotal

)V(watercontainsvoidsofvolumeTotalS

v

w

9

Engineering Applications (S) (Cont.)

• 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)

10

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

s

w

s

w

ss

3

2

g

gG

mkN8.9,Water

secm8.9g

gravitytodueonaccelerati:g

Volume

gMass

Volume

Weight,weightUnit

Volume

Mass,Density

11

Weight Relationships

(1)Water Content w (100%)

For some organic soils w>100%, up to 500 %

For quick clays, w>100%

(2)Density of water (slightly varied

with temperatures)

(3) 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)(

)(

s

w

MsolidssoilofMass

MwaterofMassw )V(samplesoilofvolumeTotal

)M(solidssoilofMass

t

sd

)V(samplesoilofvolumeTotal

)MM(samplesoilofMass

t

ws

)V(samplesoilofvolumeTotal

)MM(watersolidssoilofMass

t

wssat

wsat'

333w m/Mg1m/kg1000cm/g1

12

Weight Relationships (Cont.)

Submerged unit weight:

Consider the buoyant force acting on the soil solids:

wsat'

wsat

t

wtws

t

wwts

t

wwts

t

wss

V

VWW

V

WVW

%)100S(V

)VV(W

V

VW

13

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 sensitive structure. It will behave like a viscous fluid after it is fully disturbed.

Clay particle

Water

(Mitchell, 1993)

14

Other Relationships

(1) Specific gravity

(2)

Proof:

w

s

w

ssG

s

sw

GweS

weS

s

w

w

w

s

s

s

w

w

s

s

ws

s

w

s

v

v

w

s

V

V

VM

VM

M

M

M

MGw

V

V

V

V

V

VeS

GweS

15

Typical Values of Specific Gravity

(Lambe and Whitman, 1979)

(Goodman, 1989)

16

Determination of Atterberg limits

17

CASAGRANDE METHOD (LL)

18

CASAGRANDE METHOD

19

CASAGRANDE METHOD

SINGLE-POINT

20

Plastic behaviour

The test is done by rolling up the soil sample to 3.2mm diameter

Defined as the water content, in percent, at which the soil crumbles, when rolled into threads of 1/8 in (3.2mm) in diameter.

PLASTIC LIMIT (PL)

21

Ü Test Standard : ASTM D 427

Ü After drying the soil sample in an oven, and determining the mass and volume of sample before(i) and after (f) drying:

SHRINKAGE LIMIT (SL)

[ ( ) ] 100mi mf vi vf

SL wmf mf

22

23

Sieve Analysis

Test Standard

ASTM D422, AASHTO T88

The testing should be only carried out once for one sample

24

Curve of Particle Size Distribution

10

60

D

DCU

6010

230

.DD

DCC

25

SOIL COMPACTION

26

INTRODUCTION

Soil compaction is defined as the method of mechanically increasing the density of soil.  In construction, this is a significant part of the building process.  If performed improperly, settlement of the soil could occur and result in unnecessary maintenance costs or structure failure

27

SOIL COMPACTION

PURPOSE· Improving the soil quality by:

– Increasing the shear strength of soil

– Improving the bearing capacity of soil

· Reduces the settling of soil· Reduces the soil permeability· To control the relative volume

change

28

TYPES OF COMPACTION

4 types of compaction effort on soil : · Vibration · Impact · Pressure

29

BASIC THEORY

Developed by R.R. Proctor at 1920-an with 4 variables :· Compaction efforts (Compaction Energy)· Soil types· Water content· Dry Unit Weight

LABORATORY COMPACTION TEST· Standard Proctor Test· Modification Proctor Test· Dietert Compaction· Harvard Miniatur Compaction

SOIL COMPACTION

30

STANDARD PROCTOR TEST

The soil is compacted at cylindrical tube

Specification of test and equipments· Hammer weight = 2,5 kg (5,5 lb)· Falling height = 1 ft· Amount of layers = 3 · No. of blows/layer = 25· Compaction effort = 595 kJ/m3

· Soil type = pass sieve no. 4The test is carried out several time

with different water contentAfter compacted, the weight, moisture

content and unit weight of samples are measured

Test Standard :· AASHTO T 99· ASTM D698

31

MODIFIED PROCTOR TEST

• The soil is compacted at cylindrical tube

• Specification of test and equipments– Hammer weight = 4.5 kg (10

lb)– Falling height = 1.5 ft– Amount of layers = 5 – No. of blows/layer =

25, 56– Compaction effort = 2693

kJ/m3

– Soil type = pass sieve no. 4• The test is carried out several

time with different water content

• After compacted, the weight, moisture content and unit weight of samples are measured

• Test Standard :– AASHTO T 180– ASTM D1557

32

TEST RESULT

.

..

GSS

SGSwd

33

FIELD COMPACTION

Type of Compaction Equipment :· Smooth Wheel Roller :

compaction equipment which supplies 100% coverage under the wheel, with ground contact pressures up to 400 kPa and may be used on all soil types except rocky soils. Mostly use for subgrades and compacting asphalt pavements.

34

FIELD COMPACTION

Type of Compaction Equipment :· Rubber Tire Roller :

A heavily loaded wagon with several rows of three to six closely spaced tires with tire pressure may be up to about 700 kPa and has about 80% coverage (80% of the total area is covered by tires).

This equipment may be used for both granular and cohesive highway fills.

35

FIELD COMPACTION

Type of Compaction Equipment :· Sheepsfoot Roller :

This roller has many round or rectangular shaped protrusions or “feet” attached to a steel drum.The area of these protusions ranges from 30 to 80 cm2.Area coverage is about 8 – 12% with very high contact pressures ranging from 1400 to 7000 kPa depending on the drum size and whether the drum is filled with water.The sheepsfoot roller is best suited for cohesive soils.

36

FIELD COMPACTION

Type of Compaction Equipment :· Grid Roller :

This roller has about 50% coverage and pressures from 1400 to 6200 kPa, ideally suited for compacting rocky soils, gravels and sand. With high towing speed, the material is vibrated, crushed, and impacted.

37

FIELD COMPACTION

Type of Compaction Equipment :· Baby Roller :

Small type of smooth wheel roller yang, which has pressure ranges from 10 to 30 kPa. The performance base on static weight and vibration effect.

38

FIELD COMPACTION

Type of Compaction Equipment:· Vibrating Plate :

Compaction equipment, which has plate shape. In Indonesia this equipment sometimes called as “stamper”. Usually used for narrow area and high risk when use large compaction equipment like smooth wheel roller etc.

39

CONDITIONER FACTORS

Characteristic of compaction equipment· Weight and size· Operation frequency and frequency range

Soil Characteristic· Initial density· Soil type· Size and shape of soil particle· Moisture Content

Compaction Procedure· No. of passes of the roller· Layer thickness· Frequency of operation of vibrator· Towing speed

40

FIELD COMPACTION CONTROL

Excavate a hole with certain diameter and depth. Determine the mass of excavated material.

Determine the moisture contentMeasure the volume of excavated

material by:· Ottawa Sand Sand cone· The balloon method· Pouring water or oil

Compute the total density, and d,field

Compare d, field with d,max and calculate the relative compaction

41

SPECIFICATION OF COMPACTION

End Product Specification

Method of Specification· Minimum soil sample 100 kg· Need special experience to find out the optimum moisture

content in order to get optimum compaction performance

%100(max)

)( xRCd

fieldd