- 1 300.1 Design
- 2 300.2 Construction Inspection for Bases and Aggregate Surfaces
Bases are provided for several reasons, including to provide drainage, to reduce or eliminate pumping (ejection of fine-grained soil and water from beneath the pavement), to provide a working platform for overlying pavement placement, for improvement of subbase characteristics, or as a separator layer.
The Construction and Materials Division will provide a recommendation for base type with the pavement thickness determination, along with a recommendation for modified subgrade if necessary.
Pay limits for base are shown on Standard Plan 401.00
300.1.1 Stabilized Permeable Base
Stabilized permeable base, sometimes referred to as open graded treated base, provides rapid positive drainage. The components are coarse-graded crushed stone with virtually no fines stabilized with asphalt cement or Portland cement. The layer thickness is typically 4 in. (100 mm). Type 1 aggregate base is used as a separator layer under stabilized permeable base to prevent infiltration of fines.
300.1.2 Rock Base
The preferred base for all new rigid and flexible pavements is an 18 in. (0.45 m) rock base whenever available in suitable quantities on or in close proximity to the job. Rock base consists of 18 in. (0.45 m) of Class C excavation. It is placed full roadbed width and daylighted to the in-slope or fill-slope, except on the high side of superelevated curves and when not economically feasible under light traffic pavements. In this case the rock base is not daylighted to the ditch and soil is used as a fill material. Undergrading in rock cuts is performed to allow placement of the full specified lift thickness. A 4 ft. (1.2 m) deep ditch is preferred. Determination of rock base availability must be made early in the project through consultation with the District Geologist and materials personnel, as elevations, pavement structure design and other considerations may be affected when used.
300.1.3 Type 5 Aggregate Base
Type 5 base is used to provide moderate drainage, reduce or eliminate pumping and provide a working platform. Drainage capability is much lower than permeable base, but still enough to keep water from contacting the pavement bottom when properly sloped. Typical thickness is 4 in. (100 mm). Longitudinal pipe aggregate edge drains are normally used in conjunction with the Type 5 base.
300.1.4 Type 1 Aggregate Base
Type 1 aggregate base is used to provide a working platform or a separator layer between pavement or permeable base and the soil subgrade. It is a more stable material with higher fines content.
300.1.5 Modified Subgrade
Subgrade soils are modified with lime, flyash, lime kiln dust, or other material to decrease the plasticity index (PI) and increase stiffness. The dosage rates of modifying material vary depending on the soil type to provide a working platform and/or pavement structure, but generally no drainage. Typical minimum thickness is 6 in. (150 mm), but may range up to 12 in. (300 mm) or greater. District or Central Office Soils and Geology personnel may identify the need for this item during design; however, the need for modified subgrade is usually determined during construction operations.
Aggregate stabilized shoulders are appropriate on Minor Roads as explained in Shoulder Surface.
300.2 Construction Inspection for Bases and Aggregate Surfaces
Base courses are structural foundations and provide drainage to maintain stability for the various types of roadway surfaces.
Pavement bases include all types of aggregate courses or stabilized permeable bases treated with portland cement or bituminous binders. The specific type is specified in the contract.
300.2.3 Construction Requirements
The construction procedures are common to most types of bases. Uniformity is an essential part of the construction of satisfactory base courses and shall receive the strict attention of the inspection forces as well as the contractor’s forces, during all the phases of production, placing, blending and mixing of the materials.
300.2.3.1 Inspection Checklist
1. Check and record finished subgrade for proper crown and grade base for conformance with allowable tolerance of 1/2 inch. prior to starting base operations. Spot-check areas both between stations where stakes are set as well as at staked locations. Document in bound field book.
2. If blading and manipulation of the base on the subgrade is necessary to achieve proper crown, sufficient moisture must be present in the material to prevent segregation.
3. Aggregate should not be deposited in excessive thicknesses in one lane with the top half of the material then bladed to the adjacent lane. Excessive manipulation promotes segregation and the material in place must meet gradation requirements in order to provide the necessary drainage and structure.
4. In the case where the material was deposited on the subgrade or a prior lift of base, and the desired density was not obtained, the aggregate can be aerated, wet down in place and rerolled to obtain the required required density. However, it must not become an accepted part of the normal operation. This practice typically produces segregation of the material.
5. A stable subgrade is essential and it can be secured only if adequate attention is given to surface drainage. Water pockets create soft or spongy spots in the subgrade that are unfit for pavement. Inspect the rolling operation. If the base is pumping, cracking or is showing other signs of failure, determine the magnitude of the failure and take corrective action to address the problem. Rutting will produce areas with deficient thickness and will result in a poorly drained subgrade.
6. Perform the required density tests for control purposes. Inspector should at all times be sure that minimum density requirements are being met and that compactive effort is uniform.
7. Inspect finished section and grade for conformance with acceptable tolerance.
8. All checks made and test results obtained should be properly documented in diaries or in appropriate inspection books.
300.2.3.2 Frequent Problems
Frequent problems occurring during base construction are:
1. Contractor produces or purchases aggregate base material within, but close to, specification limits, leaving no tolerance. In some cases handling and manipulating material causes segregation or breakdown so that it is no longer within specification limits when tested on the roadbed. This is the contractor's responsibility. If this occurs, the inspector should immediately contact the resident engineer for his handling of this matter.
2. Failure to add sufficient water to base material before hauling to grade for laying. This problem is created by attempting to avoid hauling costs for excess moisture which will be deducted from hauling weight. It is, however, imperative that proper moisture content be uniformly distributed in the material to obtain consistent and satisfactory density. If proper moisture content is not originally put in the delivered base or maintained through laying and compacting operations, the contractor must be required to take immediate corrective action.
3. Segregation of base material frequently occurs during the laying operation. Uniform density cannot then be obtained. Corrective action should be taken immediately. This problem normally occurs when insufficient moisture is in the base material, when water has not been thoroughly and uniformly mixed throughout the base material, or when equipment is used on-site to spread and manipulate the base material.
4. Insufficient or inconsistent compactive effort sometimes occurs. This is often due to high production in the laying operation without sufficient compactive equipment. Observe if the compactive equipment sits idle too often. In either case, the inspector should immediately inform the contractor that corrective measures are needed. If uniform density is to be obtained, compactive effort must be consistent at all times.
5. Controlling crown, width, and grade in base work is essential. This work is of a finishing nature and is often treated too lightly. Tolerance for finish grade for bases on which flexible pavement is to be constructed should generally be a maximum of + 1/2 inch.