road soil ch t i ticharacterization - college of … design course notes-may 18-20 2010...road soil...

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Road Soil Ch t i tiCharacterization

By:yCurtis F. Berthelot Ph.D., P.Eng.Department of Civil Engineering

Road Soil Introduction

Roads are constructed of layered heterogeneous multiphase geo-materials that exhibit internal friction,

h i dil i d i l l i b h icohesion, dilation, and viscoelastoplastic behavior under load.

Behavior is often influenced by confinement because the material is non-linear stress dependent.

Several methods have evolved to quantify performance-related properties or road soils.

Road Soil Characterization2

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Granular Pavement Structure

Asphalt wearing coarse

S bbGranular Base

Subgrade: in-situ material used to construct grade. Subbase: select material (usually low quality granular)

that is borrowed from a pit and placed over the b d

Subgrade

Subbase


subgrade. Base: high quality granular layer placed directly under

asphalt concrete. Wearing coarse : asphaltic or hydraulic bound

aggregate

PCC Cross Section

Granular Base

Portland Cement Concrete

Subgrade

Granular Base

Subgrade: in-situ material used to construct grade.

Subbase: none.

Base: high quality coarse crush with high air voids for


g q y gdrainage.

Wearing coarse : Portland Cement Concrete

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Road Soil Characterization

Physical properties:

• Grain size analysis (AASHTO T27, ASTM C136)

• Sand equivalent (AASHTO T176)

• Classification (ASTM D3282):

• Unified Soil Classification System

• AASHTO



Phenomenological properties:

• Atterberg limits and plasticity index (ASTM D4318, AASHTO T89 , AASHTO T90)

• Proctor water-density (AASHTO T99, ASTM D 698)

• California bearing ratio (AASHTO T193, ASTM D1883)

• Hveem Resistance


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Grain Size Analysis

80

100T

han

0

20

40

60

0.001 0.010 0.100 1.000 10.000

Per

cen

t F

iner

T


0.001 0.010 0.100 1.000 10.000

Grain Size (mm)Clay Subgrade (Hydrometer Sieve) Clay Subgrade (Mechanical Sieve)Clay Till Subgrade (Hydrometer Sieve) Clay Till Subgrade (Mechanical Sieve)Subbase (Mechanical Sieve) Granular Base (Mechanical Sieve)

Atterberg Limits

Introduction

• Water significantly affects behavior of fine grained soils

• Different fine grained soils behave differently

• How can different fine grained soils be categorized?


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Soil Classification

Most common methods:

• USCS: Unified Soil Classification SystemUSCS: Unified Soil Classification System.

• AASHTO: American Association of State Highway and Transportation Officials.


Four main groups of soils:

1 Coarse-grained

USCS Soil Classification

1. Coarse-grained

2. Fine-grained

3. Organic soils

4. Peat


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Coarse-grained:

• less than 50% of soil passes No. 200 sieve

• includes sands and gravels

• Sands: more than 50% passes No. 4 sieve

• Gravels: more than 50% retained on No. 4 sieve

• With or without fines

• Little or no fines: clean

d d/ d d


• Well graded/poorly graded

• Well graded: distribution of all sizes

• Poorly graded: one sized particles


Fine-grained: more than 50% of soil passes No. 200 sieve

• Further subdivided by liquid limit, organic content, and silt or clay behavior

• Liquid limit: greater than or less than 50?

• Silty behavior, or clayey behavior?

Organic soils and peat: visually identifiable


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Letter system for soil classification:

• G – Gravel

• S – Sand

• M – Silt

• C – Clay

• W – Well Graded

• P – poorly graded

• U – uniformly graded

• L – low liquid limit

• H – high liquid limit


USCS Soil Classification ASTM D 2487

Group symbol Group name

GW well graded gravel, fine to coarse gravel

GP poorly graded gravel

GM silty gravel

GC clayey gravel

SW well graded sand, fine to coarse sand

SP poorly-graded sand

SM silty sand

SC clayey sand

ML silt

CL clay

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CL clay

OL organic silt, organic clay

MH silt of high plasticity, elastic silt

CH clay of high plasticity, fat clay

OH organic clay, organic silt

Pt peat


8


50

60

CV

10

20

30

40

Pla

stic

ity

Ind

ex

CI

CH

CL


0

0 10 20 30 40 50 60 70 80 90 100

Liquid Limit (%)

MLCL-ML

AASHTO Soil Classification

Group Index (GI):

• Used to further evaluate soils within a group

• Based on service performance of soils

• Range from 0 to 20• GI = (Fines-35)[0.2+0.005(LL-40)]+0.01(Fines-15) (PI-10)

• Fines = % passing 0.075mm sieve

• LL = Liquid limit

• PI = Plasticity Index


• PI = Plasticity Index

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Sand Equivalent

Introduction:

• A small amount of clay can impact soil behavior significantly

• Clay – slippery, cohesive

• Important to quantify amount of clay in aggregate


Sand Equivalent

Used as a rapid field test to determine relative amount of plastic fines in an aggregate.p gg g

A specific amount of material (< 5mm maximum size) mixed with water/calcium chloride solution and allowed to settle for a set time.

Clay level and sand level are measured and SE value is calculated.

SE < 40: poor aggregate


SE < 40: poor aggregate.

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Sand Equivalent


Ref: Hot Mix Asphalt Materials, Mixture Design and Construction, 1996

Proctor Moisture-Density

Compaction improves engineering properties of soils

• Minimizes settlements in road surface.

• Increases soil strength.

• Increases bearing capacity.

• Helps control volume change.

Amount of moisture in the soil affects compaction.


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Using specific amount of compaction effort, density and moisture content varies.


Test used to determine optimum moisture content to get maximum soil density for a level of compaction energy.

• Standard Proctor is minimum acceptable.

• Modified is preferred




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2100

2300

1300

1500

1700

1900

Dry

Den

sity

(k

gs/m

3)


1100

0 5 10 15 20 25 30 35 40

Gravimetric Moisture Content(%)

GranularBase Sand Clay Till Silt Clay

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Developed in late 1920’s.

Used by highway departments for evaluation of road

California Bearing Ratio (CBR)

soils.

Bearing ratio of soils determines amount of load that soil can carry.

Saskatchewan Highways uses bearing ratio for pavement design.

Higher soil bearing ratio = thinner pavement, base, or


Higher soil bearing ratio thinner pavement, base, or sub-base layers = less $$$.

Soil sample prepared according to Proctor compaction procedure.


Sample is soaked for 4 days.

Swelling during soaking is measured.


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Determined by pushing a cylindrical piston with an area of 1935 mm2 into a prepared soil sample at a standard

f 1 3 / irate of 1.3 mm/min.

Applied pressure required to maintain the penetration rate is recorded at 2.5mm intervals up to a penetration depth of 12.5mm.

Measures the “relative” shearing resistance of soil.

Does not directly relate to field loading conditions, but


it does provide a repeatable relative measure of soil strength.

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P

Penetration(mm)

Pressure(MPa)

10.4

15.9

Crushed Rock CBR

Surcharge49.6 mm

200

mm

15 m

m

Compacted Soaked Sample

2.5

7.6 13.15.0

10.2

6.9


15.9

152.4 mm dia.

1 12.7 17.90.

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CBR is calculated as ratio of pressure at a given penetration increment during the soil test to that of a p gstandard high-quality crushed rock.

npenetratio 12.5mm to2.5mm @rock Crushed

npenetratio 12.5mm to2.5mm @ Soil

P

PCBR


SDHT GI-CBR Correlation

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14

y = -3E-05x 5 + 0.002x 4 - 0.0546x 3 + 0.7184x 2 - 4.9707x + 20.172

R2

= 0.9994

4

6

8

10

CB

R


0

2

0 2 4 6 8 10 12 14 16 18 20

Group Index

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DiscussionDon’t hold back. You have a great deal tocontribute. Join in the spirit and camaraderief ki hi i i h Thof making something exciting happen. There

are many ways and places to make a living.We must do that, but each of us also desires toaccomplish something more exhilarating andcreate lasting value through our efforts.


road soil ch t i ticharacterization - college of … design course notes-may 18-20 2010...road soil...

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