Sub Base Construction

1.0 Introduction
This would consist of placing and compacting of sub-base material in layers on an existing pavement or prepared sub-grade in accordance with lines, levels, grades, dimensions and cross sections.

2.0  Abbreviations
SE       – Site Engineer
SEO       – Safety & Environmental Officer
ME         – Material Engineer
PPE       – Personal Protective Equipment
MDD      – Maximum Dry Density
OMC      – Optimum Moisture Content

3.0 Resources
Man power
•    SE
•    ME
•    SEO
•    Lab Technicians & Helpers
•    Supervisor/Foreman
•    Skilled & Unskilled laborers

Equipment/ Tools
•    Pick-axes
•    Crow bars
•    Mammoties
•    Back-hoe loader
•    Motor grader
•    Vibrating tamper/rammers/ plate compactor
•    Walk-behind double drum roller
•    Vibrating roller
•    Field density test kit (Sand replace type)

1. The material would be obtained from approved source in borrow cut and natural soil or material obtained by blending two or more soils or by blending soil and sand.
2. The completed sub-base would contain no aggregate having a maximum dimension exceeding two thirds of the compacted layer thickness.

4.0 Methodology
4.1 Procedures
1. Sub-base construction would be carried out after checking the underlying layer conforms to the specified requirements.
2. The sub-base material would be placed, spread, broken down, watered and compacted in accordance with the requirements of the specification.
3. All materials deposited in place prior to compaction would be evenly spread over the whole of the designated area for the layer concern.
4. Thickness of any layer, when measure after compaction would comply with requirements specified.
5. Water required before material is compacted would be added to the material in successive applications by means of water bowsers fitted with sprinklers.
6. The water would be thoroughly mixed with the sub-base material to be compacted by means of a motor grader.

7. Mixing would be continued until the required amount of water is added and a uniform homogeneous mixture is obtained.
8. The field moisture content would be within 2% of the predetermined optimum moisture content at the time of compaction.
9. If the material is too wet, it would be dried by aeration and if it is too dry the material would be sufficiently watered prior to compaction.
10. Where an existing sub-base layer is to be improved by addition of a layer of material less than 100mm thick the existing surface would be scarified to a depth instructed by the Engineer, mixed with the imported material to form a homogeneous layer of minimum depth of 100mm, recompacted to the dry density specified for the layer and formed to the lines and levels.

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