SS308-002020-01.docx
DOCX document 33 KB Posted
- Attached to
- Construction, University Drive Bike Lanes
- Solicitation number
- IFB-114424
- Issued by
- Fairfax County, Virginia
About this file
This document is supplemental specifications from the Virginia Department of Transportation for Section 308 – Subbase Course, effective July 13, 2021, and applies to all road and bridge projects within the state. The specifications establish procedures for subbase course construction, requiring contractors to construct the subgrade in accordance with Sections 304 and 305, then mix subbase material at an approved central mixing plant before placement on the subgrade using an approved aggregate spreader. Material must be placed and compacted in two or more layers of approximately equal thickness when the required thickness exceeds 6 inches, with individual compacted layers not exceeding 6 inches, though the Engineer may approve up to 10 inches for a single layer when vibrating or other approved special compacting equipment is utilized.
The specifications establish density requirements based on the percentage of material retained on a No. 4 sieve, requiring 100 percent density for materials with 0-50 percent retention, 95 percent for 51-60 percent retention, and 90 percent for 61-70 percent retention, with measurements compared to theoretical maximum density per VTM-1. Compaction must occur at optimum moisture within 2 percentage points of optimum, and density may be reduced by 5 percent when using a portable nuclear density gauge in direct transmission mode per VTM-10. The Department will conduct field density determinations using a portable nuclear density gauge and density control strips specified in Section 304 or by other Engineer-approved methods. Contractors are responsible for scarifying, reshaping, and recompacting uneven or distorted surfaces, and for addressing any deficiencies in thickness or surface depressions at their own expense before adding additional material.
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Guidelines: Use on all projects.
SS308-002020-01 July 13, 2021
VIRGINIA DEPARTMENT OF TRANSPORTATION
2020 ROAD AND BRIDGE SUPPLEMENTAL SPECIFICATIONS
SECTION 308 – SUBBASE COURSE
SECTION 308 – SUBBASE COURSE of the Specifications is amended as follows:
Section 308.03 – Procedures is replaced by the following:
Prior to placement of the subbase course, the subgrade shall be constructed in accordance with Section 304 and Section 305 as applicable.
Subbase material shall be mixed in an approved central mixing plant of the pugmill or other mechanical type in accordance with Section 208.05. The Contractor shall place the mixed material on the subgrade by means of an approved aggregate spreader. The Engineer will not require the use of such spreader when the material is being applied solely for the temporary maintenance of traffic or where the width of the course shown on the plans is transitional and impracticable to place with a spreader box.
The Contractor shall spread and compact the material in two or more layers of approximately equal thickness where the required thickness is more than 6 inches. The compacted thickness of any one layer shall be not more than 6 inches, however the Engineer may approve increasing the compacted depth of a single layer of the subbase course to 10 inches when vibrating or other approved types of special compacting equipment are used.
Each layer of subbase course shall be compacted at optimum moisture, within ±2 percentage points of optimum. The density of each layer of subbase aggregate material, when compared to the theoretical maximum density as determined in accordance with VTM-1, shall conform to the following:
| % Material Retained on No. 4 Sieve |
| Min. % Density |
| 0-50 |
| 100 |
| 51-60 |
| 95 |
| 61-70 |
| 90 |
Percentages shall be reported to the nearest whole number. The above density requirements may be reduced by 5% per VTM-10 when using the portable nuclear density gauge in direct transmission mode.
The Department will perform field density determinations with a portable nuclear density gauge using the density control strip as specified in Section 304 and VTM-10, or by other approved methods as directed by the Engineer.
The Contractor shall scarify, reshape, and recompact the surface of the subbase if it becomes uneven or distorted and sets up in that condition. If the subbase when compacted and shaped shows a deficiency in thickness or if depressions occur in the surface, the Contractor shall scarify such sections at his own expense before additional material is added.
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