Lecture Exploration

7/31/2019 Lecture Exploration

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Soil Exploration

COSC 323: Soils in Construction


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Steps of Soil Exploration

Reconnaissance Map of aerial photographs

Site visit

Boring Auger boring

Wash boring

Test pit

____________ Sampling

Testing _____________


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Question

How many borings do we need?

How deep the borings should be?


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How many borings do we need?

The more the better, but what about thecost?

Rule of thumb

_______(50ft~100ft) for multi-story buildings

_______(100ft~200ft) for one-story building,earth dams

__________(500ft~1000ft) for highway


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How deep the borings should be?

Until acceptable_______________isobtained

Need to extend to_______for very heavystructure (tall building)


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More serious question

What would be the minimum depth of test boringwhen

An 8ft square footing is subjected to a contact

pressure of 4000lb/ft2

The wet unit weight of the soil supporting the footingis estimated to be 120lb/ft3

The water table is estimated to be 30ft beneath the

footing

We need more scientific ways


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Core Boring

For Infinite Strip Loading

Submerged Unit Weight ?Wet Unit Weight ?


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Core Boring

For Square Loading


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Core Boring

For Square Loading (Low Pressure)


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Solution

Water table = 30ft < 8ft (=footings width) use wet unit

weight (=120lb/ft3)

22ft


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Standard Penetration Test

Useful in determiningproperties of cohesionless soil

Simple and inexpensive

Use a split-spoon sampler

Method Use a___________falling

_____

Drive the split-spoon sampler_____into the soil

Standard penetrationresistance value (N-Value)

Number of blows required topenetrate the last_____

Number of blows per foot.


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SPT results are influenced by overburdenpressure

How to correct N-values

po is effective overburdenpressure

2

2

10

/25.0

/

20log77.0

fttonp

fttonsinp

pC

o

o

o

N


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)(kips/ftpressureoverburdeneffectivep

fieldin thedeterminedvalue-NN'

value-NcorrectedN

)/5.1(5.025.3

'4

)/5.1(21

'4

2

o

2

2

ftkipspp

NN

ftkipspp

NN

o

o

o

o

)(kN/mpressureoverburdeneffectivep

100'

2

o

2/1

opNN


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Example

An SPT was performed at a depth of 20ft in sandof unit weight 135 lb/ft3. The blow count was 40.

What is the corrected N-value?

20ft


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Example

36)901.0)(40(

901.0/35.1

20log77.0

/35.1/2000

)/135)(20(

20log77.0

210

23

0

0

10

corrected

N

N

N

fttonsC

fttonstonlb

ftlbftp

pC


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Example

35)/70.2)(5.0(25.3

)40)(4(

)/5.1/70.2(5.025.3'4

/70.2/1000

)/135)(20(

2

22

0

0

23

0

correctedNftkips

N

ftkipsftkipspp

NN

ftkipskiplb

ftlbftp


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Example

35/3.129

100)40(

/3.129/1

/76.95)/35.1(

100'

2/1

2

2

2

22

0

2/1

0

correctedNmkN

N

mkNftton

mkNfttonsp

pNN


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Relative Density of Sand andSPT

SPT N-Value Relative Density

0-4 Very loose4-10 Loose

10-30 Medium

30-50 Dense

Over 50 Very dense


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SPT for Clay

Consistency qu (kN/m

2

)VerySoft

Soft Medium Stiff VeryStiff

Hard

SPT N-value

30

qu 400

Relation of Consistency of Clay, and Unconfined Compressive (qu)


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How to use SPT

Bearing capacity factors and f

f vs. SPT N-value


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Cone Penetration Test

Popular in Europe, Gaining favor in US

Rapid subsurface exploration without taking soilsamples

Base area = 1000mm2

Methods

Static cone test when pushed

Dynamic cone test when driven

Mechanical cone penetrometer


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Cone Penetration Method

How to measure

Push rods to advance the penetrometers tip

Push the inner rods to extend the tip

Repeat these steps What to measure

________________________

Depth of soil penetrated


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Cone Penetration Method

Cone Resistance Friction Resistance Friction Ratio


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Animation
http://www.fugro.nl/geotech_envir/Cone_penetration_techniques/cone_test.shtmhttp://www.fugro.nl/geotech_envir/Cone_penetration_techniques/cone_test.shtm


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Vane Test

Determine in-place shear strength forsoft clay soil

Good for____________that lose part oftheir strength when disturbed

How to? Push the vane tester into the soil and apply

a torque to the vertical shaft

62

32dhd

Tc

c = cohesion of the clay

T = torque required to shear the soil

d = diameter of vane tester

h = height of vane tester


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Vane Test

Correction factor for vane shear test


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Report Format

Scope and purpose

Introduction

Geologic setting

Field studies

Laboratory tests

Analysis

Conclusions and recommendations

Appendix


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Record of Soil Exploration


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Boring log network

SB1

SB2SB3

SB4 SB5

SB6

Lecture Exploration

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