Handout Num 1-3

8/3/2019 Handout Num 1-3

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ENCI252

Geotechnical Engineering 1

Powrie: 5-7

Origin of soils

Soil Unbonded granular material

topsoil ~ top 1m soil Variable, so often consider only as a

surcharge for deep / largefoundations

But NZ houses often founded in top300mm

Origins Mostly breakdown of rocks

Some organic (e.g. peat)

Modified by breakdown process (chemical,

biological, mechanical)

Transportation

Can be man-made (fill)

Geotechnical Engineering

Some typical geo terms: Geotechnical engineer

Geologist

Engineering geologist

Geoenvironmental engineer

Geomechanics

Soil mechanics Rock mechanics

Geotextiles; ground engineering; groundimprovement; geo-remediation; foundationengineering

dams

tunnels

landfills

pavements / roading

slopes

retaining walls

foundations Transportation

Wind:

aeolian => loess

Powrie: 7-10

Ice:

glacial (moraine) => tillfluvial glacial => drift

deposits

Water:

alluvial => alluvialdeposits

None => residual soil

Mineralogy 1 Most soils are silicates (SiO4

4-)

Clay minerals are phyllosilicates or sheet silicates(Si4O10)

4-

E.g. kaolinite, illite, montmorillonite (sometimes called smectiteand used as bentonite) & others

Different structures can lead to very different mechanicalbehaviour!

Powrie: 10-16

SEM images courtesy Mineralogical Society, London (2007)

KaoliniteMontmorilloniteIllite

Mineralogy 2 Non-clay minerals

Quartz, SiO2 (most abundant, framework crystal silicate, hard &stable)

Feldspars (some Si replaced by Al, less hard, can be easilycleaved)

Micas (Si4O10)4-, phyllosilicates (flakey and can be flexible)

Mechanical behaviour of non-clay soils(i.e. silts, sands, gravels)mostly governed by particle size and packing

Photos 2&3 courtesy Stewart Mccallum (2006) & University of New Hampshire (2007)

Quartz sand (SEM) Feldspar rocks

(note cleavage planes)

Mica structure


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2

Phase relationships

Soil is made up of Solid particles (skeleton / matrix)

with Spaces or voids filled with air

and/or water

Soil is therefore a 3 phasematerial To simplify we either assume

Voids are filled completely withwater = saturated

Voids are filled completely with air= dry

Reduces problem to 2 phases!

Powrie: 16-18 Microscope section of silica sand fixed in resin

Particle Void

MassesVolumes

Phase relationships

Total

volume

VT

Ma = 0Va

MwVw

MsVs

Volumevoids

VV

Volume

solids

VS

There are a number of volumetric and mass relationships that are useful:

Phase relationships

Voids ratio e:S

V

V

Ve =

T

V

V

Vn=

VS

VVV

V

+= e

e

+=1 Porosity n:

Typical values for sands/gravels: 0.4 - 1.0

Typical values for clays: 0.3 - 1.5

Theoretically 0 - 1.0 (if e varies from 0 to ),

Reported as percentage or decimal: e.g. 50% or 0.5

Phase relationships

e, n & Vare indicators of the efficiency ofpacking

the lower the value, the denser the soil

e used most often,

V useful mathematically,

n used more in hydrology

S

T

V

VV=

S

VS

V

VV += e+=1 Specific volume V:

Phase relationships

Saturation ratio Sr:V

Wr

V

VS =

S

W

M

Mw =

Powrie: Example 1.1

Moisture (or water) content w:

Cant measure Srand wdirectly. Need to weigh soilsample with water then dry to find dry weight of soil &water see example

Lies between 0 and 1.0

Dry soil: Sr= 0; fully saturated soil Sr= 1

If 0


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More definitions

Unit weight water: gWW =

( )e

eSG rSW

+

+=

1

Powrie: 19-22

( )e

wGWS

+

+=

1

1

Unit weight soil:

w~ 9.81kN/m3 (can use 10kN/m3)

It is more complicated

Includes solids & voids either filled with air or water(or both)

We can derive this equation (Powrie & Aysen)

More definitions

There are special cases for saturated and dry

unit weights of soil:( )

e

eGSWsat

+

+=

1

e

GSWdry

+=

1

( )e

GSW

+

=

1

1'

Dry unit weight (Sr=0):

Saturated unit weight (Sr=1):

Buoyant unit weight:

( = w)

Particle size

>300mm>200mmBoulders

75-300mm60-200mmCobbles

4.75-75mm2-60mmGravel

0.075-4.75mm0.06-2mmSand

0.005-0.075mm0.002-0.06mmSilt


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Particle size distribution, PSD

Coefficient of Uniformity,CUorU10

60

D

DU =

6010

2

30

DD

DZ

=

Powrie: 32-35, Example 1.5

For 1 Z10 well-graded

Particle size distribution, PSD

0

10

20

30

40

50

60

70

80

90

100

0.001 0.01 0.1 1 10 100 1000

Particle size (mm)

Percentagefinerbyweig

ht(%) Uniform (gravel)

Uniform (sand)

Gap-graded

Well-graded

D10 = 0.05mm, D30 = 0.1mm, D60 = 0.9mm U=18, Z=0.22

D10 = 10mm, D30 = 20mm, D60 = 35mm U=3.5, Z=1.14

D10 = 0.004mm, D30 = 0.5mm, D60 = 4mm U=1000, Z=15

Gap-graded fine gravely SAND

Uniform medium GRAVEL

Well-graded clayey sandy GRAVEL;

w.g. gravelly SAND, some clay?

Soil classification

Classification figure courtesyNZGS (2005)

If coarse, usePSD, colour, shape

If fine, use indextests

Systematic classification used in NZ is a hybridof USCS and BS systems, follows BS divisionsbut some terms from USCS

Initially determine soil type based on particlesize, then further classify:

Coarse soils: Grain shape Shape of particles is important to mechanical behaviour

of coarse soils (sands & gravels) Affects interlock of particles

Affects compressibility of whole system

Typically use a visual classification system: Compare with chart below (from Field description of soil and

rock, NZGS 2005).

Can also have platey, flakey, flat, elongated etc. as terms

Example classification: Sub-rounded coarse SAND with platey micaAngular medium GRAVEL

Granular soils: void ratio

For granular soil (silt, sand, gravel), soil

packing (how dense) depends on how soil was

deposited and loaded

Theoretical maximum and minimum void ratios,

emax and emin can be found for a particular soil

emax and emin depend on:

PSD

Particle shape

S

V

V

Ve = Recall void ratio

Granular soils: emax

At maximum void ratio, soil has lowest possible density

BS1377 Pt 4: method to measure emax (other methods inASTM, USCS etc).

Tip gently

(upsidedown)

Set down &

measure height

Repeat at least 10 times and take highest value for emax

Known mass soil

in 1L gas jar


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Granular soils: emin At minimum void ratio, soil has highest possible density

BS1377 Pt 4: method to measure emin (other methods in

ASTM, USCS etc).

3 layers soil in mould (with collar) of known volume &

mass. Vibrate on vibrating table with weight on top

between each of 3 layers. (Wet?)

Smooth soil with

straight edge, weigh

whole.

Vibrate,

remove

weight,

add soil

Remove

weight, collar

& smooth

Repeat at least once with different soil batch and take lowest value for emin

emax and emin of 300 granular soils

0

0.5

1

1.5

2

2.5

0 0.5 1 1.5

Clean sands

Sands with fines

Sands with claySilty soils

Maximumvoidratio,e

max

Minimum void ratio, emin

30 gravels

12 coarse sands(ASTM & non-standard procedures)

Clean sands, sands with fines and silty soils

(JGS procedures for emax

and emin

)

(non-standard procedures)

12 gravelly sands(P

G= 17-36 %, F

C< 6 %)

(FC

= 0 -5%)

(5 < FC

15 %)

(15 < FC

30 %, PC

= 5-20 %)

(30 < FC

70 %, PC

= 5-20 %)

Granular soils: relative density

ID varies between 0.0 (minimum possible density) and 1.0(maximum possible density), often expressed as %

minmax

min

minmax

max

=

=

ee

eeDorI RD

Note, Powrie and other UK

authors use symbol ID, but

many other authors (US, etc.)

use DR. NZGS uses RD!

Powrie: 19

85Very dense

Relative density (%)Descriptive term

Once theoretical emax and emin are determined, the

actual density of a granular soil can be defined in termsof a density index called relative density:

Fine-grained soils: index tests

Fine-grained soils (silts and clays) are difficult to classifyusing visual inspection alone

Albert Atterberg (1911) proposed tests linking changes inclay consistency with moisture content: Atterberg limits Analogous to emaxand emin for granular soils

Are empirical index tests based on mechanical response

Relate to clay mineralogy and amount in soil

Plastic limit, wPL, is water content below which fine soilbehaves brittle and crumbly

Liquid limit, wLL, is water content above which fine soilbehaves as a liquid

Plasticity index: IP = wLL - wPL

Fine grained soils: index tests

To determine wLL: BS: use a standard fall-cone

apparatus; for clay mixed at wLL,standard cone will penetratespecified distance

US: use a Casagrandeapparatus, cut V-shape in soil;link number of standard taps tocause to disappear to wLL.

Pictures courtesy Verruijt(2001)

Standard fall-cone

Casagrande apparatus

Powrie: 40-43 & 1stPro Lab.

To determine wPL: Roll out threads of soil to 3mm

diameter. wPL when threadsbecome brittle

Fine grained soils: index tests Once w, wPL and wPP are known for a particular clay,

can determine Liquidity index, IL

Powrie: 43-44

PLLL

PLL

ww

wwI

=

Determine moisture content, w, of a clayey soil by weighing,oven drying and then weighing again (e.g. Powrie Example1.1)

IL analogous to ID (relative density) for granular soils

If IL = 1.0, w=wLL, clay at liquid limit (runny like a liquid)

If IL = 0.0, w=wPL, clay at plastic limit (dry, brittle and hard)


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Fine soils: classification

Figure from BS5930:1990 Section 6Powrie: 44, Fig 1.15

wPL = 22%, wll = 65%,

IP = 43% CH: High

plasticity CLAY

wPL = 25%, wll = 29%,

IP = 4% ML: Lowplasticity SILT

Clays above A-line

Silts & organicsbelow A-line

Compaction

Soils are often used as a structural

material in the construction of

embankments, land reclamation,

backfills and earth dams.

Dense soils have superior engineering

properties to loose soils:

- Higher stiffness

- Higher strength

Smaller deformability

Improved stability

It is necessary to compact thesoils in the field and compressthem into a smaller volume(reduce the volume of thevoids) to increase strength.

Performance of loose fills Compaction methodsSpecially designed equipment is

used to compress or densify soils.

All equipment uses one or more ofthe following techniques:

Sheepsfoot roller

Steel-wheel roller

Portable

equipment

pressure (the most important factor)- impact load(dynamic component)- vibration (15-60 Hz; enhances compaction)

- complex loads involving shearing

Selection of equipment depends on the type of the soil, size

of the project and compaction requirements

Proctor Compaction Test

- Compaction of a soil sample ina cylindrical mould (1 litre

capacity and Di= 105 mm)

- Standard rammer (m = 2.5kg,drop height 300mm)

- 3 equal layers, each receiving

27 blows of the rammer

Used to investigate compaction characteristics of aparticular soil in the lab Standard Proctor Test:

Modified Proctor Test:

- m = 4.5kg, h = 450mm, 5layers (more energy)

The test is carried out on at least 5 samples, each prepared at a

different water content.


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Calculation procedure

1. Trim the sample and then weigh it:

3. Calculate the water content, w:w

s

Mw

M=

4. Calculate the dry mass density,d: d1 w

=+

5. Plot the data on a graph ofd against w

ms m

m

M M

V =

2. Calculate the mass density of the sample,(since

we know the volume Vm and mass of the mould Mm):

Mms Compaction curve

Optimum water content (wo)

Compaction curve

d dg

Sometimes is

expressedas:

Low wHigh suction

High w High SrNo / little air

Mechanics of the compaction curve

Water provides lubrication Improves compactibility

Usually the peak of the compaction curve occurs at about: Sr~ 80 %

In this case:optimum water contentwO = 11.7%

Characteristics of compaction curves

- The Proctor test results

suggest that all fills should

be compacted at wo

- However, the compactioncurve depends on the

compactive effort

d increases with theapplied energy while wo

decreases

- The Proctor test result is not directly applicable to fieldconditions

- Usually field compaction will lie between the Standardand Modified Proctor Test results

Standard Proctor Test results for 8 typical soils

Note the compaction curve shifts

up and leftwards with increasing

grain size

Higher D50 leads tohigherdand lower wo

Zero airvoids curve

Sr= 100 %

- Gravels: Make good fillsHigh strength and low compressibility; high permeability

- Sands: Usually make good fills

High strength and low compressibility; can be easilybrought to wo

- Low plasticity silts: Less desirable than gravels and sandsLose more strength and require more moisturecontrol

- High plasticity clays: Only when very low permeability is required(landfill caps; clay core of earth dams)

- Organic soils and peat: Extremely poor; weak, compressible

Suitability of different soils as fills


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Ground investigation - brief intro

Site Investigation (SI) is fundamental to geotechnicalengineering

Process by which geological, geotechnical & other informationwhich might affect civil engineering project is acquired

Soil Classification Identification of material, composition and intrinsic properties

(can used disturbed soil)

Soil Description In situproperties of material (prefer to use undisturbed soil)

Ground investigation is part of SI Aims to determine ground and groundwater characteristics

Enables soil classification & description by:

Drilling boreholes (BHs) and digging trial pits (TPs)

Testing soil in situ (in the ground) and enabling soil to be removed(sampled) for testing in the lab.

Ground investigation

Cannot eliminaterisk but canreduce it by goodplanning

Try to locate BHs and TPs as efficiently as possible. Soil strata may be of variable thickness

May be discontinuous soil lenses & cavities

Powrie: 56-60

Try to infer 3Dpicture fromrelatively fewholes!

Better to spend $earlier than $$$later


Boreholes are usually150mm diameter, drilled todepths up to 30m

Trial pits are typically 2mdeep by 2m wide holes

Figures courtesy DJ Douglas & Partners (2006)

Borehole core Borehole log


Soil samples can be taken for lab testing: Strength, compressibility, permeability

Stress-strain behaviour, classification, etc.

Disturbance is a big issue! Particularly forgranular soils

Soil behaviour depends on deposition & stresshistory, i.e. soil remembers past events

Taking samples (sampling) from the groundcan wipe this memory!

Sometimes we test in situ to avoid this

Undisturbed soil sample

tools used in boreholes -

REALLY?

Undisturbed sampling

method from trial pit

OKAY

Figures courtesy USBR (1998)

Handout Num 1-3

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Transcript of Handout Num 1-3