Chapter 4: Construction Details, Section 4-25 Aggregate Subbases

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Published: July 2019


Section 25 Aggregate Subbases

4-2501 General

This section provides guidelines for inspecting aggregate subbases for work specified under Section 25, “Aggregate Subbases,” of the Standard Specifications.

Aggregate subbase is designated by class. The bid item list and plans specify the class of aggregate subbase, and the Standard Specifications and special provisions provide the requirements for each class.

Aggregate subbase is usually the lowest layer in the pavement structure, as shown in the typical cross sections of the contract plans. Typical cross sections show the thickness of aggregate subbase and layout sheets show where to place it.

4-2502 Before Work Begins

Before placement begins, review contract plans and specifications to determine the aggregate subbase requirements. For sampling and testing requirements, including frequency of testing, refer to Chapter 6, “Sampling and Testing,” of this manual.

Include the following steps in the preliminary review and inspections:

  • Obtain the contractor’s quality control plan, which details the methods the contractor will use to assure quality of work and conformance with the Standard Specifications
  • If mutually agreed with the contractor, hold a preconstruction meeting with the contractor and the district Materials Unit before construction to discuss the quality control plan and contractor’s method for performing each element of work affecting material quality, including acceptance testing priorities, shipping of samples, lines of communication for test results, timeframes for reporting quality control and acceptance test results, and any contractual testing dispute resolution processes.
  • Verify the design R-value by testing the basement material at the grading plane to verify the planned thickness shown in the pavement structure. Testing should be completed early enough before the placement of aggregate subbase to allow time for redesign if necessary. Refer to Topic 614.4, “California R-Value,” in the Highway Design Manual for a discussion of R-value and pavement structure design.
  • Test potential sources of aggregate subbase when the contractor requests such testing in writing. Deduct applicable Caltrans costs for sampling and testing from contract payments as required under Section 6-1.03, “Local Materials,” of the Standard Specifications.
  • Review compaction tests of the subgrade that is to receive the aggregate subbase. Examine the subgrade to make sure that it has not deteriorated since it was tested and that it is still firm and stable. Give special attention to isolated areas where pumping occurs.
  • Measure the subgrade grading plane for compliance with Section 19-1.03C, “Grade Tolerance,” of the Standard Specifications. When measuring for compliance, spot-check areas between stations where stakes are set, as well as the staked locations. Determine the extent of this measurement based on factors such as the nature of material, the efficiency of the contractor’s operation, and the accuracy of the grading operation, as indicated during the early stages of checking. The grade will be established from markings on the final grade stakes that Caltrans Surveys set. Refer to the Staking Information Booklet for information on state-furnished construction surveys.
  • When subgrade is cohesionless soil and you decide that the subgrade is unstable for placing aggregate subbase on the roadbed in layers or windrows, give the contractor written permission to dump aggregate subbase in piles and spread ahead.
  • Determine whether the contractor has complied with all requirements related to the use of local materials. Refer to Section 5-1.20B(4), “Contractor-Property Owner Agreement,” of the Standard Specifications.

4-2503 During the Course of Work

During work operations, do the following:

  • Sample the aggregate subbase at the time it is deposited on the roadbed. Observe delivered aggregate subbase to assure that it is clean of debris and other harmful materials. For requirements related to material quality, perform the tests at the frequencies shown in Section 6-1, “Sample Types and Frequencies,” of this manual. The frequency table has a provision for reducing R-value testing, but exercise caution when doing so. Consider reducing frequency when initial test results show R-values significantly exceeding the specified R-value and ongoing gradation and sand equivalent test results indicate acceptable and consistent material quality. Do not reduce R-value frequency when failing or borderline test results occur. Include in the project records an explanation of why you reduced R-value testing.
  • Compare sand equivalent and grading test results with requirements for operating range and contract compliance. Refer to Section 3-609A, “Operating Range and Contract Compliance,” of this manual. Note that it is prudent to take frequent samples, especially with borderline test results, but test only at the frequency shown in the table in Section 6-1 of this manual. If a test result fails to meet the requirement for contract compliance, you may test additional previously taken samples to determine the quantity of material represented by the failing test result.
  • Verify that aggregate subbase is spread on the subgrade without significant segregation. Normally, you would verify this step through observation, but if problems persist, support your observations with a sieve analysis. If segregation is taking place, it can sometimes be avoided by wetting the material before it is hauled to the job or before spreading operations start. Watering and compacting go hand in hand. It is important that the proper amount of water is evenly distributed in the aggregate at the time of compaction.
  • Where geosynthetic materials are shown, assure materials are properly placed, including overlapping requirements, and securely holding materials in place during aggregate subbase placement. Make sure that the geosynthetic materials are not damaged during placement, spreading, and compaction of the aggregate subbase. Specifications provide limits on the contractor’s equipment and operations. Verify that any damaged materials are repaired or replaced. Refer to Section 4-96, “Geosynthetics,” of this manual for additional guidance on geosynthetics.
  • Observe the spreading and compacting operation to make sure that it conforms to the layer thickness requirements of the specifications.
  • Test the relative compaction of aggregate subbase layers using the area concept procedures under California Test 231, “Method of Test for Relative Compaction of Untreated and Treated Soils and Aggregates Using Nuclear Gages.”
  • Observe the compacting operation to see that the material forms a uniformly firm, stable base.
  • Measure the surface of the finished aggregate subbase for conformance with tolerances specified in Section 25-1.03E, “Compacting,” of the Standard Specifications. Use the markings on the final grade stakes Caltrans set to determine compliance with the planned elevation of the aggregate subbase surface. Require corrective action for any deficiencies.
  • Measure the thickness of the completed aggregate subbase. Use your judgment to determine the number of measurements necessary. The minimum acceptable thickness equals the planned thickness minus the sum of the specified tolerance for high subgrade and the specified tolerance for low finished aggregate subbase surface. A thin section is acceptable if an increased thickness of the base material placed above the aggregate subbase makes up the deficiency. The Standard Specifications allow the engineer to accept a deduction for deficient thickness in lieu of other corrective action. Caltrans policy is to verify that thickness complies with requirements by ordering corrective action if it is deficient. Therefore, apply the deduction in only the most extenuating circumstances. Keep adequate records for payments on progress pay and final estimates.
  • Note in the daily report any inspections performed on items that are not otherwise part of a permanent record. For instance, you do not need to note any compaction tests taken because these are recorded elsewhere. However, you do need to explain in the daily report any absence of testing. You also need to note that construction is being performed according to specified layer thicknesses, because this information is not recorded elsewhere.
  • If the contractor disputes Caltrans’ acceptance results, follow Section 23-1.01D(1)(b), “Test Result Disputes,” of the Standard Specifications. An independent third party performs referee testing as specified, must have no previous direct involvement with the contract, and be mutually selected with the contractor.

4-2504 Level of Inspection

Suggested levels of inspection for typical aggregate subbase work activities are:

  • Benchmark inspection of subgrade grading plane.
  • Intermittent sampling and testing of aggregate subbase materials.
  • Intermittent inspection of placement, spreading, and compaction operations.
  • Intermittent review of contractor’s quality control program including quality control test results.
  • Benchmark inspection of finished surface grading plane.

4-2505 Quality Control

Guidance for quality control activities included in this section is summarized as follows:

  • Check that the contractor is actively performing quality control on aggregate subbase materials throughout production operations by reviewing copies of quality control records, including quality control test results.
  • The quality control plan must include, but is not limited to:
    • Frequency of quality control sampling and testing that meets or exceeds specification requirements as listed in Section 25-1.01D(2)(d), “Quality Control Testing,” of the Standard Specifications.
    • Time and frequency of submitting test results.
    • Action and suspension limits, and details of corrective action to be taken if any process is outside of those limits. Suspension limits must not exceed specified acceptance criteria.
    • Responsibilities of subcontractors and testing laboratories.
    • Quality control manager if the quantity of aggregate subbase exceeds the requirements listed in the “QC Testing Frequencies” table of Section 25-1.01D(2)(d), “Quality Control Testing,” of the Standard Specifications.

4-2506 Payment

Review quantity calculations found in the resident engineer’s pending file to determine if they are sufficiently detailed and accurate to be used in the project records. Calculate the aggregate subbase volume based on the dimensions shown on the plans. Make quantity calculations as early in the project as possible.

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