Civil Engineering: Subgrade Preparation for New Pavements |Subgrade details |

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.