500-1 DESCRIPTION
The work covered by this section involves the preparation, shaping, manipulation of moisture content, and compaction of either an unstabilized or stabilized roadbed to a condition suitable for placement of base course, pavement, and shoulders. By definition of
Section 101, the roadbed is the graded portion of a highway from ditch to ditch in cuts and shoulder point to shoulder point in fills. This section requires the Contractor to prepare and shape the subgrade; clean, shape, and maintain roadway ditches; strip the existing vegetation, and place and compact in accordance with
Sections 235 and
560 material resulting from the shaping operations. This section shall only be applicable on those portions of the project, which will be paved under the contract.
On any project or portion of a project where
grading is the only work to be performed under the terms of the contract (no base or pavement to be placed), the requirements of this section are
not applicable. Such work shall be covered under the provisions of
Article 225-3, Unclassified Excavation, or
Section 230, Borrow Excavation, depending upon the source of material. Therefore, this provision will only apply to turnkey projects (where the fills have been built during the same contract that requires fine grading and paving) and paving projects.
When a project graded under a previous contract is to be paved under a second contract, the Engineer should, determine the profile elevation of the existing grade along both edges and the centerline of the future pavement before the Contractor begins work. All vertical clearances for overhead structures shall be checked and recorded and the vertical tie-in alignment to structures shall be checked to eliminate any possible grade differences. If a grade revision appears necessary and feasible, the Engineer should consult with the Division Engineer before making any revision. Before making a revision, several things must be considered. The vertical clearances at overhead structures and tie-ins to structures must be checked. A minimum of
200 feet should be utilized to make the transition to the new grade at each end of any adjustment.
If proposed grade requires the Contractor to excavate more than
0.4 foot below an existing graded surface, the excavation below
0.4 foot must be paid for as unclassified excavation. Refer to
Article 500-5 of the Specifications. On those projects or portions of a project which will be paved under the contract (such as turnkey projects), the Engineer shall consider the above vertical clearance and tie-ins prior to establishing the subgrade elevations. In addition, the Engineer must consider the type of equipment to be used by the Contractor to cut this grade. Automatically controlled fine grade equipment may require approximately
1 inch of cut material in order to function effectively. All records relative to the above adjustment must be retained until the project is complete and the final estimate has been paid.
Also, before any work is begun by the Contractor, the Engineer in accordance with
Article 104-10 of the Specifications, should set up an inspection of all existing drainage facilities and structures and record their condition prior to beginning work. The description of the condition of drainage facilities should include any previous damage and the degree of filling by silt or sand in existence at the time of the inspection. A representative of the Contractor should be given an invitation in writing to be present during this inspection. The inspection should be held on the date of availability or the day the Contractor begins work, whichever occurs first.
500-2 CONSTRUCTION METHODS
(A) GENERAL
Material generated from fine grading shall be utilized to the fullest extent possible. There will be no compensation for the direct utilization, other than completion of the unclassified excavation, or stockpiling and hauling of the material generated from fine grading. It is the responsibility of the Contractor to balance the suitable excavation throughout the limits of the project.
On projects or portions of projects where the grading was completed under a previous contract, suitable material excavated below a level of
0.4 foot below the existing graded roadbed and above subgrade is considered as unclassified excavation and is to be utilized in fine grading, earth shoulder construction, or in any other work requiring the utilization of suitable material. Any surplus material upon completion of this work shall be disposed of by the Contractor, in accordance with
Section 802.
On turnkey projects the provision concerning
0.4 foot below the existing graded roadbed and above subgrade does not apply (see paragraph 3 of
Article 500-5).
The Contractor shall conduct his operations in such a manner as to avoid damage to any previously constructed structures and facilities. By conducting an inspection of drainage facilities and structures early in the project, as outlined in
Article 500-1 herein, the Engineer will be able to determine if the damage occurred during the life of the project or is a pre-existing condition.
(B) PREPARATION OF SUBGRADE
The Contractor may utilize any method or equipment he desires in preparing the subgrade when the Special Provisions do not require the use of an automatically controlled fine grade machine and when pavement is to be constructed.
When conventional methods and equipment such as self-loading scrapers and motor graders are utilized by the Contractor, it may be necessary to scarify or plow the subgrade to a depth of
3 to 4 inches before shaping and compaction are begun in order to break up slippage planes in the subgrade material and to provide a surface in which the moisture content may be more readily controlled in the event that it is necessary to add moisture or dry the material to obtain the required density.
When an automatically controlled fine grade machine is utilized by the Contractor, the anticipated benefit is a more uniform subgrade finish with less equipment and less effort. However, the use of this equipment does not eliminate the necessity for scarifying, plowing, and wetting or drying.
The Engineer and Technician do not have the authority to direct the Contractor to change his methods inasmuch as this is an end result Specification. However, if the methods employed do not produce an acceptable subgrade, density and surface they do have the right to offer suggestions as to how the required end product might be achieved. See
Article 108-5 of the Specifications and this
Manual.
Project personnel should, wherever possible, be on the lookout ahead of the finish fine grade operations for areas of unsuitable material, areas requiring underdrain, and areas requiring chemical or aggregate stabilization. These areas should be promptly investigated and decisions rendered as expeditiously as possible. Prompt action in these matters will allow the Contractor to organize his work schedule such that unnecessary delays will be avoided. Whenever possible, decisions on these matters should be made by the Engineer, or if the authority is delegated, the project Technician. If assistance is needed, the Engineer should contact the Division Engineer, Area Construction Engineer, or the
Geotechnical Engineering Unit.
Any surplus material generated in the preparation of a subgrade and not needed in the immediate adjacent areas of subgrade being prepared may be stored or stockpiled by the Contractor. This material is considered as unclassified excavation and should be utilized as such in the remaining phases of project construction. This applies only to previously graded projects and does not apply to turnkey projects. The Engineer should discuss the location of stockpiles with the Contractor and determine that proposed locations do not interfere with proper drainage or any subsequent operations of stabilization, placing base, or placing pavement. Since the cost of repair seeding and mulching made necessary as a result of stockpiling is normally borne by the Department, the Engineer should not permit the Contractor to stockpile material at such locations and in such manner as to cause any greater damage to the seeding and mulching than is absolutely necessary. Any surplus material remaining in the stockpiles is to be disposed of by the Contractor in accordance with the provisions of
Article 225-3.
On projects where the Contractor is required to grade the project, place base and pavement, and construct earth shoulders, the Contractor may leave the grade high prior to subgrade preparation by an amount sufficient to produce the majority of the earth shoulder material by trenching and windrowing the earth material from the subgrade to the shoulder area. In this type of operation, adequate drainage of the subgrade shall be maintained by the Contractor at all times and the provisions of
Article 500-4 of the Specifications will apply. In accordance with
Article 560-4, no direct payment will be made for shoulder construction utilizing this method. Additional material, if needed, shall be obtained from roadway excavation or borrow sources and shall be paid for accordingly.
When an automatically controlled fine grade machine is utilized by the Contractor and the machine is of the type which is capable of placing surplus material directly into the earth shoulder construction area, such methods are permissible provided the Contractor keeps the subgrade adequately drained. In accordance with
Article 560-4, no direct payment will be made for shoulder construction utilizing this method. Additional material, if needed, shall be obtained from roadway excavation or borrow sources and shall be paid for accordingly.
(C) COMPACTION OF SUBGRADE
If possible, before beginning compactive efforts, a decision should be made whether stabilization by external means will be required. Stabilization is normally performed with aggregate and/or chemicals. In compacting a subgrade, the work necessary to obtain the required density is directly related to the type of soil, the proper control of moisture in the soil and the equipment and methods used in the work. The Density Technician should work closely with the Contractor's supervisory personnel in determining the moisture content of the soil at which the required density can best be obtained. Optimum moisture as determined by moisture density curves is, of course, the ideal; but the range above or below optimum moisture in which the required density can be obtained with a reasonable effort is extremely variable.
Subarticle 500-2(C) of the Specifications requires the Contractor to “dry or add moisture to the subgrade when required.” This article places the burden of when and how much upon the Contractor. The Technician or Engineer should consult with the Contractor in this matter and furnish information based upon results of testing performed, but should not direct the Contractor's operations relative to the moisture content at which the Contractor attempts to compact the subgrade. Should the Contractor attempt to obtain compaction when the moisture content of the soil is not within the range determined to be reasonable for satisfactory compaction by the Technician or Engineer, the Contractor should be so informed and the conditions should be fully documented in the
Weekly Project Report.
It should be further noted that “a uniformly compacted and acceptable subgrade” also indicates that the completed subgrade is to be uniformly stable and free of soft wet areas that weave or rut under traffic.
500-3 TOLERANCES
The established grade is the finished subgrade elevation as set by the Engineer. A tolerance of plus or minus
½ inch of this elevation is allowed. Subgrade tolerance of plus or minus
¼ inch from the established grade is required for subgrade under concrete pavement mainline lanes.
Moreover, the maximum difference between the established grade and the graded subgrade within any 100 foot section longitudinally is ½ inch and ¼ inch for subgrade for concrete pavements. This holds true for each transverse point checked, i.e., centerline, edge of travelway, edge of pavement, etc.
For Example: At Station 0+00 the centerline grade checked +½ inch and at Station 1+00 the centerline checked - ½ inch. Although each point individually is within ½ inch of the established grade, this section would not be in tolerance since the
overall difference is 1” which is greater than ½ inch. The tolerance permitted by
Article 500-3 of the Specifications is intended to give a reasonable amount of tolerance in the vertical control as long as such tolerance results in a reasonably uniform riding surface. Short dips or humps and warped cross section shall be avoided especially in vertical curves, horizontal curves, and superelevation runoffs.
When the Technician is informed by the Contractor that a section of subgrade is completed to line and grade, the Technician or the field party should check and record the finished elevation of the subgrade at intervals of
50 feet or less longitudinally and transversely at the centerline (if a crown section), at the edge of each travel lane [usually every
11-12 feet], and at each edge of pavement line. The record of subgrade elevation may be made either by utilizing the blue tops and a string line or with a level and should be recorded in accordance with the
Records and Reports Section of this Manual.
Upon completion of the check of grade and cross section conformity, the Contractor is to be informed of any necessary corrections. Upon completion of any necessary corrections and verifications that density is acceptable, the Contractor should be advised that, barring damage before base or pavement is placed, the section of subgrade is approved for placement of base material.
500-4 MAINTENANCE OF SUBGRADE
Continuous maintenance of surface drainage is an absolute necessity in order to avoid soft areas that may develop due to the subgrade holding water for long periods of time. This requires the Technician to be on the alert for stopped-up drainage and depressed areas in the grade. These conditions should be immediately brought to the attention of the Contractor's Superintendent and corrected.
Neither should the subgrade be allowed to become dry to the extent that binder material is lost through dust. The Contractor shall be required to maintain the subgrade in a moist consolidated condition ahead of the base course operations.
Maintenance of the subgrade also requires the exclusion of hauling equipment, which causes rutting, weaving, or other distortion of the grade. Careful attention should be given to windrowed material on shoulders to assure that drainage outlets are provided through the windrowed material so as not to hold water on a subgrade.
Refer to
Article 104-10 of the Specifications, and this
Manual for guidelines in implementing these provisions.
500-5 MEASUREMENT AND PAYMENT
The work of fine grading covered by this section is paid for on a lump sum basis and no actual measurement is required. The project records must verify that the density and surface finish were acceptable. The
Weekly Project Report should document the Contractor's election to utilize construction methods which result in no payment for shoulder borrow as provided for in
Article 560-4 of the Specifications (Ex: trenching and windrowing earth material from the subgrade to the shoulder area).
In addition, cross sections must be entered in a pay record book to document any material removed from a level of
0.4 foot below the level of the existing graded surface for which the Contractor is to be paid as unclassified excavation. This provision does not apply to turnkey projects.
ADDITIONAL RESOURCES
