Specification for Construction of Unbound Granular Pavement Layers

1. Introduction

Unbound granular pavement layers form the structural foundation of road infrastructure, providing essential load-bearing capacity and drainage characteristics. This specification outlines the requirements for materials, construction methods, and quality control measures necessary for the successful implementation of unbound granular pavement layers in road construction projects. These layers, comprising aggregates without hydraulic binders, serve as critical components in flexible pavement systems, distributing traffic loads and protecting subgrade layers from excessive stress.

2. Scope

This specification applies to the construction of unbound granular pavement layers including sub-base and base courses in flexible pavement structures. It covers the requirements for material selection, preparation, placement, compaction, and quality control during construction. The specification is applicable to new construction, rehabilitation projects, and resurfacing works where unbound granular layers form part of the pavement structure. The guidelines provided herein constitute minimum requirements and may be supplemented by project-specific conditions or jurisdictional standards.

3. Materials Requirements

3.1 Aggregate Properties

Aggregates used in unbound granular pavement layers shall comply with the following specifications:

• Hard, durable particles from crushed rock, gravel, or recycled materials approved by the engineer
• Free from deleterious materials such as clay, organic matter, or excessive dust
• Abrasion resistance (Los Angeles Abrasion) not exceeding 35% loss for base courses and 40% for sub-base materials
• Sodium sulfate soundness loss not exceeding 12% after five cycles
• Flakiness index not exceeding 30% for base course aggregates

3.2 Grading Requirements

The grading of aggregates shall conform to the following sieve analysis:

Sieve Size	Percentage Passing (Base Course)	Percentage Passing (Sub-base)
37.5mm	100	100
26.5mm	85-100	90-100
19mm	70-90	75-95
9.5mm	45-75	50-80
4.75mm	30-60	40-70
2.36mm	20-45	30-60
0.425mm	10-25	15-35
0.075mm	2-10	5-15

3.3 Moisture Content

The moisture content of aggregates during placement shall be maintained within 2% of the optimum moisture content as determined by the modified Proctor compaction test. Materials that are too dry or wet shall be adjusted before placement through the addition of water or through aeration, respectively.

4. Construction Methods and Procedures

4.1 Preparation of Subgrade

The subgrade shall be prepared to the required elevation, grade, and cross-section profile in accordance with the design drawings. This includes:

• Removal of all organic material, topsoil, and unsuitable material
• Proper compaction of the subgrade to not less than 95% of maximum dry density
• Installation of any required drainage systems prior to placement of granular layers
• Final shaping and grading to ensure uniform thickness of unbound layers

4.2 Spreading and Placement

Unbound granular materials shall be spread in loose layers not exceeding 200mm in loose thickness after compaction. Placement shall be achieved using approved equipment such as motor graders, pavers, or spreader box attachments. The following requirements shall be observed:

• Materials shall be placed in a uniform manner without segregation of aggregate sizes
• Layer thickness shall be monitored regularly to ensure design requirements are met
• Additional water shall be added uniformly during placing if necessary to achieve the required moisture content
• Irregularities or segregation during placement shall be corrected immediately

4.3 Compaction

Compaction of unbound granular layers shall commence immediately after spreading and moisture conditioning. The following compaction requirements apply:

• Vibratory rollers with a minimum static weight of 10 tons shall be used
• Compaction shall proceed from the edges toward the center, overlapping each pass by at least one-third of the roller width
• Minimum of eight passes with the compactor, or until field density requirements are achieved
• Vibration frequency and amplitude shall be adjusted according to material characteristics and layer thickness
• Tamping rollers may be required near edges, corners, or transitions where conventional rollers cannot operate effectively

4.4 Shaping and Finishing

The final shaping and finishing of compacted unbound layers shall produce a uniform surface conforming to the specified grade, cross-section, and smoothness requirements. Excessive irregularities exceeding 10mm when tested with a 3-meter straight edge shall be corrected by loosening the area, adding or removing material as required, and recompacting.

5. Quality Control and Testing

5.1 Material Testing

The following tests shall be performed on source materials:

• Gradation analysis (ASTM D136 or equivalent) for every 1,000 cubic meters or part thereof
• Los Angeles Abrasion test (ASTM C131) for material sources or when significant changes in material properties are suspected
• Atterberg limits (ASTM D4318) for material passing the 0.425mm sieve to evaluate plasticity characteristics
• California Bearing Ratio (CBR) testing to evaluate strength characteristics

5.2 In-place Testing

Field testing during construction shall include:

• Nuclear density gauge or sand cone tests to verify field density (minimum of one test per 500 square meters or as directed by the engineer)
• Visual inspection for segregation, uniformity, and proper moisture condition
• Surface smoothness testing using a 3-meter straight edge
• Thickness verification through level surveys or core sampling

5.3 Acceptance Criteria

The constructed unbound granular layers shall meet the following requirements:

• Compaction not less than 98% of maximum dry density for base courses and 95% for sub-base layers
• Thickness within 10mm of design thickness
• Surface irregularities not exceeding 10mm when tested with a 3-meter straight edge
• Gradation within the specified limits
• Moisture content within 2% of optimum moisture content during compaction

6. Weather Limitations

Construction of unbound granular layers shall not proceed when rainfall conditions would adversely affect the work. Operations shall be suspended when rain approaches or during precipitation. If unplaced materials become wet, they shall be removed from the site or aerated until acceptable moisture content is achieved. In cold weather conditions, construction shall be suspended when the material temperature drops below 2C, unless special provisions are made for heating materials and protecting placed layers until cure is achieved.

7. Maintenance Considerations

Properly constructed unbound granular pavement layers require appropriate maintenance to ensure long-term performance. Maintenance activities may include:

• Regular inspection for surface failures, potholes, or deformation
• Prompt repair of localized failures to prevent deterioration progression
• Re-compaction of areas exhibiting strength loss due to moisture infiltration
• Periodic re-grading to maintain proper cross-slope for drainage
• Application of surface treatments as needed to prevent erosion and loss of fines

8. Advantages and Limitations

8.1 Advantages

• Lower construction costs compared to stabilized or bound pavement layers
• Excellent drainage characteristics reducing moisture-related distress
• Simplified construction techniques that can be implemented with readily available equipment
• Reduced environmental impact through potential use of recycled materials
• Easier maintenance and rehabilitation processes

8.2 Limitations

• Lower structural capacity compared to stabilized or bound layers
• Greater susceptibility to deformation under heavy or repeated loads
• Requires proper thickness design to prevent excessive stresses on subgrade
• Susceptible to erosion at the surface without appropriate treatment
• Potential for particle migration and segregation without proper gradation design