Subgrade:
Subgrade is that portion of the earth roadbed which after
having been constructed to reasonably close conformance with the lines, grades,
and cross-sections indicated on the plans, receives the base or surface
material. In a fill section, the
subgrade is the top of the embankment or the fill. In a cut section the subgrade is the bottom
of the cut .
The subgrade
supports the subbase and/or the pavement section. To ensure a stable, long-lasting, and
maintenance free roadway, the subgrade is required to be constructed using
certain proven procedures that provide satisfactory results.
After the rough grading is completed, the fine grade stakes
are set and the final processing of the subgrade may begin.
The rough grade is the top grade of the
embankment as built using the information provided on the grade sheets. The grade is normally within 2 in. at this
point. The finish grading operation
consists of trimming the excess material down to the final grade. Filling any low spots with thin lifts of
materials tends to slide these lifts around if not properly worked into the
underlying materials.
Subgrade Preparation
1.
Types of Subgrade
preparation
2.
Typical equipment used
3.
Purpose of equipment
4.
Grading
5.
Compaction
6.
Soils
7.
Testing
Importance of Quality Subgrade
Provide good
support for placement and compaction of pavement
pavement
deflections to acceptable limits
Minimize
differential movement due to frost and Shrinking/swelling soils
Promote
uniformity of support (Key element for good long term pavement performance)
Types of subgrade preparation
·
Modification and
Stabilization
·
Removal and Replace
Modification and Stabilization
·
Subgrade treatment that is
intended to provide a stable working platform during construction.
·
–Adding chemicals like fly ash or cement
·
–Replacing existing soils with aggregates
·
–Geosynthetic reinforcement with aggregates
·
–Moisture conditioning
Chemical Stabiliztion
This work consists of treating the subgrade by combining chemicals such
as fly ash, lime, or cement and water with the pulverized soil material to the
specified depth and compaction requirements
Benefits of fly ash
Drying agent
Reduces the shrink-swell potential
Increases
the strength of subgrade
Benefits of lime
Lime dries
wet soils
Lime
modifies clay soils
Lime
permanently stabilizes clay soils
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Benefits of Cement
Reduces
swell potential
Produces a weather resistant working platform
for asphalt paving
Gains
strength with time
Significantly
reduces the freeze thaw movement
Soil Stabilization Benefits
Lower
material costs – reduces base and pavement thickness
Lower
construction costs – eliminates cost of material removal and replacement. 30% -
50% savings
Increased
Strength – a dramatic increase in the CBR can be achieved
Soil Stabilization Benefits
Longer
durability – stabilized soil is highly resistant to water and frost, which
increases the lifespan of the subgrade
Increased
environmental responsibility – stabilizing the existing soil eliminates the
need to export the poor undesirable soil and import new fill
Remove and Replace
Simple
Procedure that does not require specialized equipment
Removal and
Replace
This process
will consist of removing the unsuitable soil and replacing it with aggregates
such as base or sand
Geosynthetics
are often placed on the surface of the excavated subgrade prior to placement of
aggregates
Geosynthetics
The primary
purpose of using geosynthetics in the pavement design is to reduce reflective
cracking in the asphalt and resist moisture intrusion into the underlying
pavement structure
Geosynthetic Benefits
Improves
structural capability of soil
Allows the use of poorer quality of soils to
be used in construction
Construction
time can be reduced
Drastically
increases the durability of subgrade
Moisture Conditioning
This work
consists of blading, shaping, wetting, and compacting the subgrade with
moisture and density control
Moisture
Conditioning reduces or increases the soil moisture content to be compacted to
the required density
Controls
shrinking and swelling of soils
Moisture Conditioning Benefits
Allows
contractors to expedite work
Reduction of construction costs Improved
utilization of existing materials
Greater
environmental protection
Increased short-term and long-term savings
Full Depth Reclamation Process
The total
asphalt surface is pulverized plus a predetermined portion of the base
The
pulverized base is compacted and graded
The new
stabilized base is now ready for paving
FDR Benefits
Cheaper – At least 50% less
expensive than traditional road repair methods
Faster – Get 2-3 times more
repairs done in the same time period
Easier – No excavating, no
loading, no hauling off and dumping old asphalt
Base is stabilized with
pulverized road surface
No reflective cracking!
Permanent repair, not a
temporary fix
Environmentally friendly
Types of Equipment Needed
Reclaiming machine
Compactors
Motor Graders
Tanker Trucks (Water and Emulsion)
Water Trucks
Loaders and Excavators
Reclaiming Machine
Pulverization
of existing materials
–Asphalt
–Dirt
–Asphalt with bas
Sizing Material
Controlled
by the operator who controls the speed and rear door opening
Mixing Additives
Reactive –
Lime
Self
cementing
– Portland
cement and fly ash
Water
Compactors
Compact
material to desired density
Typical compaction sequence
–Initial or
breakdown
– Pad foot
roller
–Intermediate
– rubber tire rollers and vibratory rollers
–Finish
– smooth
drum and rubber tire rollers
Motor Graders
Placing
material at desired grade
Scarify material
Process
material
Tanker Trucks
Deliver
water
Deliver
emulsion materials to declaimer
Water Trucks
Apply water
to subgrade surface directly
Proof rolls
Loaders and Excavators
Removing
existing pavement
Excavating excess or unsuitable soils
Loading
trucks
Fine Grading
Fine grade is
required for the final trimming and checking of the cross section.
Setting grade
Stakes are
usually set at variable intervals near each edge of the subgrade and the
centerline
When the
distance is too far apart from stake to stake, intermediate stakes may be
required
How
to establish cross slope
Measure distance from edge of road to centerline
Multiply the distance to the desired cross slope –For
example: 25’ * 2% (.02) = .5 or 6”
That is the elevation difference from the edge of the road
to centerline
Checking grade
A string
line can be stretched across adjoining grade stakes
The subgrade is checked by measuring down to
the known offset distance from the string line to the dirt
Straight
edge
– easiest way to check uniformity of subgrade
Importance of uniform
subgrade
Strength – Make
sure you have full pavement section.
Yield –
Uniform subgrade will optimize yield.
Cost – Asphalt is for more expensive than base
Smoothness –
HMA compacts differentially, thicker areas compact more than thinner areas
which will affect pavement smoothness
Compaction
Compaction
occurs when a force compresses the soil and pushes air and water out of it so
that it becomes more dense. Compaction is achieved easer when the soil is wet
and less able to withstand compression
Why Compact?
5 reasons to
compact
- Increases load-bearing capacity
- Prevents
soil settlement and frost damage
- Provides
stability
- Reduces
water seepage, swelling and contraction
- Reduces
settling of soil
Types
of compaction
There are four types of compaction effort on soil or
asphalt:
Vibration
Impact
Kneading
Pressure
Static Force
Static force
is the deadweight of the machine, applying downward force on the soil surface,
compressing the soil.
Static compaction is confined to upper soil
layers.
Kneading and pressure are two examples of
static compaction.
Vibratory Force
Vibratory
force uses a mechanism, usually enginedriven, to create a downward force.
The
compactors deliver a rapid sequence of blows (impacts) to the surface,
affecting the top layers as well as deeper layers. Vibration moves through the
material, setting particles in motion and moving them closer together for the
highest density possible.
Based on the
materials being compacted, a certain amount of force must be used to overcome
the cohesive nature of the soil.
Compaction Equipment
Choosing the
right equipment for the job is vital to achieving proper compaction.
Deciding
Factors
Soil type
–Cohesive
–Granular
Thickness of Lift and Machine Performance
Compaction Specifications
Cohesive
soils
A machine
with a high impact force is required to ram the soil and force the air out to
achieve compaction.
Pad Foot or
Sheep Foot Roller
Jumping Jack
Granular soils
Require a
shaking or vibratory action to move them
Smooth Drum Roller
Plate Compacter
Wheel
Rolling
Thickness of
Lift and Machine Performance
The thicker
the lift the heavier piece of equipment needs to be.
Materials
Vibrating
Sheepsfoot Rammer
Static
Sheepsfoot Grid Roller Scraper
Vibrating
Plate Compactor Vibrating Roller Vibrating Sheepsfoot
Scraper
Rubber-tired Roller Loader Grid Roller
Lift
Thickness Impact Pressure (with kneading)
Vibration
Kneading (with pressure)
Gravel 12+
Poor No Good Very Good
Sand 10+/-
Poor No Excellent Good
Silt 6+/-
Good Good Poor Excellent
Clay 6+/-
Excellent Very Good No Good
Compaction Specifications
Method
Specification –Detailed instructions specify machine type, lift thickness,
number of passes, machine speed and moisture content.
End-result
Specification – Engineers indicate final compaction requirements, allowing the
contractor to choose what is the best method to achieve compaction.
Types of Equipment
Rollers
Smooth drum
Padded drum
Sheep Foot
Rubber-tired
Static and vibratory sub-categories
Walk-behind and ride-on
Smooth Drum
Roller
Smooth Drum
Smooth-drum
machines are ideal for both soil and asphalt
Sheep Foot
Roller
Padded Drum
/ Sheep Foot
Appropriate
for cohesive soils. The drum pads provide a kneading action on soil.
Rubber Tire Roller
7 to 11 tires that have an overlapping pattern
Typically a
static roller
Compaction effort is pressure and kneading
Wheel
Rolling
Jumping Jack
Rammers / Jumping Jack
Deliver a high impact force ( high amplitude)
making them an excellent choice for cohesive and semi-cohesive soils
Three types of compaction:
impact,
vibration and kneading.
Plate
Compacter
Vibratory
Plates
Low amplitude and high frequency, designed to
compact granular soils and asphalt. Type of compaction: vibration
Walk Behind
Appropriate
for cohesive soils
Ideal for
small areas
Soil types
Soil types
are classified by grain size, determined by passing the soil through sieves to
screen or separate the different grain sizes.
A well -
graded soil consists of a wide range of particle sizes with the smaller
particles filling voids between larger particles.
The are three basic soil groups:
Three Soil
Groups
Cohesive
Granular
Organic
(this soil is not suitable for compaction)
Cohesive
• Cohesive
soils have the smallest particles. Clays range from .00004" to .002".
• Cohesive
soils are dense and tightly bound together. They are plastic when wet and can
be molded, but become very hard when dry.
• Proper
water is essential for proper compaction.
• Cohesive
soils usually require a force such as impact or pressure.
• Silt has a
noticeably lower cohesion than clay. However, silt is still heavily reliant on
water content.
Granular
• Granular
soils range from .003" to .08" (sand) and .08" to 1.0"
(fine to medium gravel).
• Granular
soils are known for their waterdraining properties.
• Sand and
gravel obtain maximum density in either a fully dry or saturated state.
• Testing
curves are relatively flat so density can be obtained regardless of water
content.
Guide to Soil Types
What to look
for Appearance/Feel Water Movement When Moist When Dry
Granular
soils, fine sands and silts
Coarse
grains can be seen. Feels gritty when rubbed between fingers
When water
and soil are shaken in palm of hand, they mix. When shaking is stopped they
separate
Very little
or no plasticity Little or no cohesive strength when dry. Soil sample will
crumble easily.
Cohesive
soils, mixes and clays
Grains
cannot be seen by naked eye. Feels smooth and greasy when rubbed between
fingers
When water
and soil are shaken in palm of hand, they will not mix
Plastic and
sticky. Can be rolled
Has high
strength when dry.
Crumbles
with difficulty.
Slow saturation in water.
Materials
Vibrating
Sheepsfoot Rammer
Static
Sheepsfoot Grid Roller Scraper
Vibrating
Plate Compactor Vibrating Roller Vibrating Sheepsfoot
Scraper
Rubber-tired Roller Loader Grid Roller
Lift
Thickness Impact Pressure (with kneading)
Vibration
Kneading (with pressure)
Gravel 12+
Poor No Good Very Good
Sand 10+/-
Poor No Excellent Good
Silt 6+/-
Good Good Poor Excellent
Clay 6+/-
Excellent Very Good No Good
Fill Materials
Permeability
Foundation Support Pavement Sub grade Expansive Compaction Difficulty
Gravel Very
High Excellent Excellent No Very Easy
Sand Medium
Good Good No Easy
Silt Medium
Low Poor Poor Some Some
Clay None+
Moderate Poor Difficult Very Difficult
Organic Low
Very Poor Not Acceptable Some Very Difficult
Moisture vs. Soil Density
Moisture or water content is key to achieving
density in compaction.
Water allows
the particles of material to move together and decrees voids.
Not enough
water particles can not slide past each other.
Too much
water and water voids are created.
The optimum moisture content is when
compaction will be achieved the easiest.
Density
Curve
Testing
Why Should
We Test?
Measures
density of soil for comparing the degree of compaction vs. specs
Measures the effect of moisture on soil
density vs. specs
Provides a
moisture density curve identifying optimum moisture
Save Money
Types of Tests
Proctor Test
Modified Proctor Test
Hand Test
Proof Role
Sand Cone
Nuclear Density
Proctor Test
Determines
the maximum density of a specific soil.
Tests the effects of moisture on soil density
Standard Proctor
Modified Proctor
Standard
Proctor
A small soil sample is taken from the jobsite.
A standard weight is dropped several times on the soil. The material weighed
and then oven dried for 12 hours in order to evaluate water content
Modified Proctor
This is similar
to the Proctor Test except a hammer is used to compact material for greater
impact, The test is normally preferred in testing materials for higher shearing
strength.
Hand Test
Pick up a
handful of soil.
Squeeze it
in your hand.
If the soil
is powdery and will not retain the shape made by your hand, it is too dry.
If it shatters when dropped, it is too dry.
If the soil
is moldable and breaks into only a couple of pieces when dropped, it has the
right amount of moisture for proper compaction.
If the soil
is plastic in your hand, leaves traces of moisture on your fingers and stays in
one piece when dropped, it has too much moisture for compaction.
Proof Rolling
Sand Cone
Test (ASTM D1556-90)
A small hole
(6" x 6" deep) is dug in the compacted material to be tested. The
soil is removed and weighed, then dried and weighed again to determine its
moisture content.
A soil's moisture is figured as a percentage.
The specific volume of the hole is determined by filling it with calibrated dry
sand from a jar and cone device. The dry weight of the soil removed is divided
by the volume of sand needed to fill the hole. This gives us the density of the
compacted soil in lbs per cubic foot. This density is compared to the maximum
Proctor density obtained earlier, which gives us the relative density of the
soil that was just compacted.
Nuclear Density (ASTM
D2292-91)
Nuclear
Density meters are a quick and fairly accurate way of determining density and
moisture content. The meter uses a radioactive isotope source (Cesium 137) at
the soil surface (backscatter) or from a probe placed into the soil (direct
transmission).
The isotope
source gives off photons (usually Gamma rays) which radiate back to the mater's
detectors on the bottom of the unit. Dense soil absorbs more radiation than
loose soil and the readings reflect overall density. Water content (ASTM D3017)
can also be read, all within a few minutes. A relative Proctor density with the
compaction results from the test.
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