6-24-97

FLY ASH STABILIZATION OF SOILS 
207-R-336

DESCRIPTION: This work shall consist of stabilizing the subgrade soils by mixing fly ash in conjunction with lime or cement with the soils. This may be used to stabilize the natural ground under embankments and to the subgrade under pavements.

MATERIAL: The materials shall be in accordance with the following:

Fly ash ............................901.02.
Hydrated lime ......................913.04.
Portland cement ...................901.
Water ..............................913.01

        Asphalt material for curing seal shall be AE-150 Type I in accordance with 404.

        Reactive Soils are those soils which, when mixed with at least 9% fly ash, compacted in accordance with AASHTO T 99 Method C, and cured for 48 hours at 49E C (120E F), shall show a strength gain of at least 345 KPa greater than that obtained for similarly prepared untreated control specimens.

EQUIPMENT: The machinery, tools and equipment necessary for proper execution of the work shall be available on the project site and approved by the Engineer prior to the commencing of construction operations.

STORAGE AND HANDLING:

  1. Fly Ash shall be stored in weather protected conditions with adequate protection from ground dampness. The storage facility shall be approved by the Engineer prior to commencement of any fly ash work.

  2. Each shipment shall be accompanied by a bill of lading and by a certificate of compliance stating conformance to the applicable specification requirements.

  3. The Contractor's safety measures shall be in accordance with applicable OSHA requirements.

MIXTURE COMPOSITION:

  1. Sample: The contractor shall provide 140 kg (310 lb), minimum, samples of the soil to be modified and 25 kg (55 lb), minimum, samples of the fly ash to be used at least 45 days prior to the use of the fly ash.

  2. Mix Design: Fly ash shall be proportioned within, but not limited to, an approximate range of 9% to 15% of soil as determined by the oven - dry test basis. The required proportion of fly ash will be established by the Engineer prior to construction using samples of soil and fly ash and the Department's laboratory design procedure for soil stabilization. The Engineer reserves the right to make such adjustments of fly ash proportioning as are considered necessary during the progress of the work.

    If a lime, cement, and fly ash combination is used, the proper combination of lime and cement to fly ash ratio will be determined in the laboratory. An approximate range of ratios of lime and cement to fly ash will be 1:9 to 1:1 and 1:3 to 1:4 respectively.

    The source or type of fly ash shall not be changed during the progress of the work without written permission of the Engineer. The Engineer may, however, based on laboratory test results, direct that different types of fly ash be used on different portions of the project. The different types shall not be mixed in the same project portion.

 

CONSTRUCTION REQUIREMENTS:

  1. Temperature and Weather Limitations: Fly ash modification shall only be performed with a minimum soil temperature, measured 100 mm (4 in.) below the surface, of 7E C (45E F), and with the air temperature rising. Fly ash shall not be mixed with frozen soils or with soil containing frost.

  2. Preparation of Existing Soils: All deleterious material, such as stumps, roots, turf, etc. and aggregate larger than 75 mm (3 in.) shall be removed. Any soft organic soils shall be removed as directed by the Engineer.

  3. Scarification and Pulverizing: The soil surface shall be scarified to the required depth with a machine of such design that a visible indication is given at all times the cutting is to the required depth. Moisture shall be added if necessary.

  4. Fly Ash Application: The Department reserves the right to increase or decrease the quantity of the fly ash used, based on test results, as directed by the Engineer.

    The fly ash application shall include a curing period of seven days if the specified gradation is achieved. Curing periods may be extended at the discretion of the Engineer if the gradation is not being achieved.

    Fly ash shall be spread only on that area where the initial mixing operation can be completed during the same working day.

    Any fly ash that has been exposed to the open air for a period of 6 hours or more before mixing will not be accepted unless approved by the Engineer.

    Heavy traffic or equipment shall not be permitted on the spread fly ash except for that required for spreading, watering, or mixing.

    Spreading Fly Ash:
    Fly ash shall be uniformly spread at the specified quantities. Fly ash shall not be applied when wind conditions are such that blowing fly ash becomes objectionable to adjacent property owners or creates a hazard to traffic on adjacent highways.

    When fly ash is used in conjunction with lime or cement, the lime, cement, and fly ash shall be spread separately. They may, however, if directed by the Engineer, be pre-blended and stored dry. Pre-blended mixes shall be placed dry unless directed otherwise by the Engineer. If both lime and cement and fly ash are used and spread separately, conditioned fly ash, at water contents between 15% and 25%, and dry lime shall be used.

    Fly ash which has been damaged by rain prior to or during the mixing operations, which has been damaged while spreading, or which has been displaced by the Contractor's equipment or other traffic, shall be replaced.

  5. Moisture Control: At the completion of the mixing operation, the moisture content of the mixture shall be within 3 percentage points below to optimum moisture content.

  6. Test Section: The first section of each mixing operation, shall be a test section to demonstrate the acceptability of the Contractor's equipment and methods, and to provide a check of the resulting finish grade elevation and the compacted thickness of the stabilized layer. Changes in equipment shall be made as needed based on the results of the test section. If changes in methods or requirements are made, additional test sections shall be constructed as required by the Engineer. Additional test sections shall be constructed if
    recommended in the Geotechnical Report. The length of the section shall be as long as necessary to use one truck of fly ash. Test sections may be waived by the Engineer.

  7. Mixing: After the fly ash has been spread, it shall be mixed with the pulverized soil. Fly ash shall not be mixed below the specified depth. Mixing shall be accomplished with mixing devices approved by the Engineer and shall be continued until the resulting mixture is homogeneous and uniform in appearance. Water shall be added to the mixture so the moisture content is no less than optimum for fly ash-soil mixture, but no more than optimum plus 3%. The friable mixture of soil and fly ash shall be free from clods and lumps exceeding 25 mm (1 in.) in size. After initial mixing, the fly ash treated layer shall be shaped to the approximate section and lightly compacted, prior to curing, to minimize the evaporation loss and to prevent excessive wetting from rain. The surface shall be crowned to provide proper drainage.

    Prior to compaction, the fly ash stabilized soil mixture shall be pulverized such that 100% of the mixture passes a 25 mm (1 in.) sieve and 60% passes a 4.75 mm (No. 4) sieve, excluding the non-slaking fraction.

  8. Compaction: Compaction of the mixture shall begin immediately after the mixing process is complete. The mixture shall be compacted to at least 100% of the maximum dry density of the fly ash stabilized mixture within the special subgrade treatment zone and to at least 95% of the maximum dry density of the fly ash stabilized mixture below the special subgrade treatment zone. The maximum dry densities will be determined in accordance with AASHTO T 99 or one point proctors in accordance with AASHTO T 272. The density of the soil-fly ash mixture will be determined by the Engineer near the end of the finishing operations. Any portion of the soil-fly ash mixture having density less than specified shall be corrected or removed and replaced. The in-place dry density shall be determined by the Engineer in accordance with AASHTO T 191 or T 239.

    The average compacted thickness of modified soils constructed shall be within 10% of the specified thickness, except that the thickness at any one place may be within 15% of that specified.

    The Department reserves the right to determine the actual thickness of the completed and cured layer by coring or by other means. The Engineer will perform at least one test every 280 m2 (335 syd) for each compacted layer to ensure that the fly ash is spread at the specified rate. All deficient areas shall be corrected.

    During the compaction phase, the shape of the course shall be maintained by blading and the surface shall be sloped to the required lines, grades and cross-sections. Compaction shall be continued until the minimum specified density is obtained.

    Compaction of some stabilized soils may result in the formation of horizontal shear planes, resulting in a thin, platey structure in the upper part of the compacted layer. These compaction planes shall be removed by light scarification with a spike-toothed harrow, weeder, or nail drag, and the loosened material then moistened and recompacted with a pneumatic-tired roller.

  9. Curing and Protection: After the mixture has been finished as specified, it shall be protected against drying for 14 calendar days. The surface shall be maintained in a moist condition by sprinkling for the above mentioned period or until a curing seal coat is placed. Equipment used to apply the curing seal and curing water shall be of a size and weight such that the surface is not damaged.

    Heavy traffic or equipment other than curing equipment shall not be allowed on the finished surface until completion of curing, unless permitted by the Engineer. A satisfactory curing day shall be any day when the temperature of the completed subgrade does not fall below 7E C (49E F) and the air temperature rises. All damage to the curing seal prior to completion of curing, shall be immediately resealed.

    The curing seal shall consist of the asphalt material specified and shall be uniformly applied at the rate of 1 kg/m2 (2 lb/syd) with approved distributing equipment. The actual rate and application temperature of asphalt material will be determined by the Engineer.

    If the asphalt material is tacky, a sand blotter material shall be applied at a rate by approximately 3 kg/m2 (5 lb/syd) when directed by the Engineer.

    Other acceptable curing material or methods may be used upon written permission of the Engineer.

    The stabilized surface shall be protected against freezing for seven calendar days after placement.

    The stabilized subgrade shall be completely covered with the specified pavement courses before work is suspended for the winter months. When the stabilized subgrade is not completely covered by the specified pavement courses, the Contractor shall be responsible for all work necessary to protect and maintain the uncompleted work during the winter months. The Contractor shall perform any work necessary to repair or restore the uncompleted work before the beginning of spring paving operations.

  10. Maintenance: The Contractor shall maintain the entire fly ash stabilized soil mixture in a manner satisfactory to the Engineer. Maintenance shall include immediate repairs of any defective or damaged portions of the stabilized subgrade.

 

METHOD OF MEASUREMENT: Stabilized soils will be measured in square meters (square yards) of the thickness specified. The width for measurement shall be as shown on the plans. Asphalt material used as a curing seal will be measured in megagrams (tons). Water will not be measured for payment. Fly ash, lime, and cement will be measured megagrams (tons). The fly ash, lime, and cement, may be weighed in trucks or freight cars. If the fly ash, lime, or cement is shipped in trucks, it may be weighed at the place of loading, at the place of unloading, or at such other place as the Engineer may designate. The Engineer may accept original signed bills in lieu of weighing.

        Extra earth excavation or embankment due to the Contractor's method of construction requiring more than one lift to construct the fly ash stabilized soil layer as shown on the plans will not be measured.

        Contract Quantities: When the project is constructed essentially to the lines, grades or dimensions shown on the plans and the Contractor and Engineer have previously agreed in writing that the plan quantities are accurate, no further measurement shall be required. If errors are discovered after work has been started corrective measurements shall be made.

        When the plans have been altered or when disagreement exists between the Contractor and the Engineer as to the accuracy of the plan quantities before any work is started which would affect the measurement, either party shall have the right to request in writing, and thereby cause the quantities involved to be measured.

BASIS OF PAYMENT: Fly ash stabilized soil will be paid for at the contract unit price per square meter (square yard) at the thickness specified, complete in place. Fly ash, lime, cement, and asphalt curing seal will be paid for at the contract unit price per megagram (ton).

        Payment will be made under:

Pay Item              

Metric Pay Unit Symbol
(English Pay Unit Symbol)

Asphalt Curing Seal

 

......................Mg (TON)

Cement

 

......................Mg (TON)

Fly Ash

 

......................Mg (TON)

Fly Ash Stabilized Soil

 

.......................m2 (SYD)

Lime

 

......................Mg (TON)

 

        The costs of replacing fly ash damaged by rain prior to or during the mixing operations; the replacement of fly ash damaged by the Contractor's operations; the water necessary for moisture control or for the stabilization process; correction of deficient areas; repair of damage to the curing seal; furnishing and spreading the sand blotter; the extra materials, methods, and construction techniques required to protect, maintain, or repair portions of the uncompleted work left during the winter months; and all extra earth excavation or embankment due to more than one lift being required to construct the fly ash stabilized soil layer will be included in the costs of the pay items.