INTRODUCTION
Grade preparation lays the foundation for the entire pavement system. Uniformity and stability of the subgrade affect both the long-term per- formance of the pavement and the construction process. Stability of the subgrade is needed to provide adequate support of the pavement sec- tion and to provide an acceptable construction platform. The pavement design begins with the identification of the pavement’s foundation. The construction begins with the preparation of that foundation. See ACPA’s EB204P, “Subgrades and Subbases for Concrete Pavements,”for more general and detailed discussion on sub- grade and base/subbase concepts.
Important elements of subgrade preparation in- clude:
1. Evaluation of subgrade stability,
2. Subgrade modification to improve stabil- ity, and
3. Evaluation of surface tolerances.
Areas where the assistance of a geotechnical engineer experienced in subgrade preparation may be required are:
1. Variability of soil condition.
2. Soil with low bearing strength (≤ 1 ton/square foot [tsf]).
3. Organic soil.
4. Swelling/expansive soil.
5. Frost-susceptible soil.
GRADING AND COMPACTING
Important items to consider include the follow- ing:
1. Fill material is usually obtained from cut operations. The geotechnical report may be used to evaluate the potential use of this material as engineered fill.
2. If the in-place material is not of sufficient quantity or it has material properties that are adverse to good pavement perform- ance, borrow areas need to be identified to obtain fill material.
3. The contractor needs to be aware of the local subgrade conditions as they relate to pre-grading and other construction ac- tivities.
The second phase of pre-grading is the con- struction staking process. It is a good practice for the owner/ engineer to perform an independ- ent verification of the staking accuracy. Auto- mated grading equipment using global
positioning are an increasingly common means of establishing the grade. If these systems are employed, it is a good practice to periodically verify the results through the use of conventional surveying.
Removal of Unsuitable Subgrade
When preparing the grade, unsuitable soil can be encountered. Materials such as peat, or- ganic silt, silt, and soil with high organic content are classified as unsuitable. To deal with these materials the following actions can be taken:
1. Remove and replace with soil similar to the surrounding subgrade,
2. Remove and replace with granular mate- rial, or
3. Alter the properties through compaction or stabilization.
Protection of Grade
During grading operations, protect the grade by performing two essential activities:
1. Provide temporary drainage via trenches, drains or ditches to divert or intercept sur- face water. If water ponds on the sub- grade, the material will soften and can be damaged by construction traffic, resulting in delays and the need for repairs.
2. Implement procedures to manage site traffic over the grade. Do not use chan- nelized traffic patterns over one portion of the grade. Make sure the traffic is distrib- uted over the grade.
Grading Operations
The grading operation for an airport pavement may involve construction of an embankment:
1. The embankment is constructed by plac- ing material in successive horizontal lay- ers for the full width of the cross section.
2. Most specifications include a maximum loose depth of fill to be placed. Use of thicker fill layers may require the contrac- tor to demonstrate to the engineer that the thicker fill layer can be compacted to the specified density.
PAVING ALERT
Mandatory Requirements:
Measures to control water pollution, soil ero- sion, and siltation that are shown on the plans or that are needed for the site should be pro- vided. All pertinent local, State, and federal laws should be followed.
Chapter 4 —Grade Preparation
3. During construction of the embankment, the hauling equipment needs to travel evenly over the entire width of the em- bankment. If equipment travel is chan- nelized, permanent deformation and shear failure can occur.
4. In the construction of embankments, layer placement should begin at the deepest portion of the fill. As placement progresses, layers should be con- structed approximately parallel to the finished pavement grade line.
5. In areas where subgrade transitions occur, the subgrade materials are mixed by disc at the boundary of the transition zone. Subgrade mixing must be per- formed over a distance of about 100 ft along the transition zone (50 ft on either side of the transition). This practice re- duces the potential for differential settle- ment or frost heave.
Compaction Requirements
Compaction of the subgrade is essential to building a stable work platform. Typical com- paction requirements are:
1. For pavements trafficked by aircraft greater than 60,000 lb, the Modified Proc- tor procedure (ASTM D1557) needs to be used to determine the maximum density.
The Standard Proctor procedure (ASTM D698) may be used for pavements used by light-weight aircraft.
2. Cohesive soils:
a. In fill sections – Entire fill com- pacted to 90 percent maximum den- sity.
b. In cut sections – The top 6 in. to be compacted to 90 percent maximum density.
3. Cohesionless soil:
a. In fill sections – The top 6 in. of sub- grade is compacted to 100 percent maximum density, the remainder of fill to 95 percent maximum density.
b. In cut sections – The top 6 in. com- pacted to 100 percent maximum den- sity, and next 18 in. compacted to 95 percent maximum density.
Moisture control is essential to obtaining a stable subgrade. The following items should be ad- hered to:
1. Specifications for compaction usually re- quire the moisture content in the sub- grade to be within ± 2 percent of optimum moisture content before rolling to obtain the prescribed compaction.
2. For expansive soils, moisture content needs to be 1 to 3 percent above opti- mum prior to compaction to reduce swell potential.
3. For fine-grained soils, that do not exhibit swelling properties, it is best to keep the moisture content at 1 to 2 percent below optimum.
4. Cohesive soils compacted wet of opti- mum can become unstable under con- struction traffic even when the target den- sity is achieved.
QUALITY ALERT
Subgrade Compaction:
Due to the weight of construction equipment, it is a good practice to compact all subgrade materials to 95 percent of the maximum den- sity using the Modified Proctor method. This assists in providing a stable working platform.
If the standard proctor method is used to con- trol compaction, subgrade instability can occur.
1 2 2 1 2 3 1 2 4 1 2 5 1 2 6 1 2 7 1 2 8 1 2 9 1 3 0 1 3 1
5 6 7 8 9 1 0 11 1 2 1 3
Dry Density, pcf
c la y e y s a n d s ilty c la y
Moisture density curves of typical soils can pro- vide insight into field performance. In Figure 4.1, the shape of the curve suggests that clayey sand soil is moisture sensitive. A small change in moisture content results in compaction diffi- culty. Additional compaction related items to consider include:
1. Sheepsfoot rollers need to be used for cohesive soil. The pads need to pene- trate 70 percent of the lift thickness.
2. Discing of cohesive soil is necessary if moisture is to be controlled.
3. Static steel drum rollers can be used to smooth the surface of the grade after compaction.
4. Vibratory steel drum rollers are used for cohesionless soil. If the water table is close to the surface, or if subgrade soils are saturated, vibration should be used with caution.
Nuclear density testing procedure can be used to check the density of the compacted soil.
Gauges need to be calibrated to local materials.
If problems achieving density are encountered, the following techniques can be used to trouble- shoot:
1. Perform additional moisture density test- ing to ensure that the proper maximum density value is being used to control field compaction.
2. Use a sand cone or volume measure to perform the density tests.
3. Use traditional methods to determine moisture content.
4. Probe the subgrade to determine if soft layers are present below the problem area.
Field density control must be a full time function.
This allows observation of the material as it is placed. If the material appears to change, one- point field Proctor tests can be used to check the maximum density.
QUALITY ALERT
High Volume and Heavy Load Pavements:
It is recommended that the subgrade be stabi- lized to provide stable and uniform support condition. In any case, subgrade soil with a California Bearing Ratio (CBR) of 5 or less should be stabilized or a granular layer be used to enhance constructability.
Chapter 4 —Grade Preparation