Bridges & Structures

Geotechnical Aspects of Pavements Reference Manual

Appendix F: Determination Of Admixture Content For Subgrade Stabilization

(Adopted from Joint Departments of the Army and Air Force, USA,
TM 5-822-14/AFMAN 32-8010, Soil Stabilization for Pavements,
25 October 1994.)

Lime Content for Lime-Stabilized Soils

To determine the design lime content for a subgrade soil, the following steps are suggested:

1. Determine whether the soil has at least 25% passing the 75-µm sieve and has a plasticity index (PI) of at least 10. The soil screening criteria also limit soluble sulfates to less than 0.3 % by weight in a 10:1 water-to-soil solution.
2. Determine the initial design lime content by mixing varying amounts of lime with the soil in water and measuring the pH levels in 1-hour intervals. Select the lowest lime mixture level for which a pH of 12.4 occurs as the initial design lime content.
3. Using the initial design lime content conduct moisture-density tests to determine the maximum dry density and optimum water content of the soil lime mixture defined by the user agency, e.g., AASHTO T-99, AASHTO T-180, ASTM D 698, or ASTM D 1557. The procedures in ASTM D 3551 will be used to prepare the soil-lime mixture.
4. Prepare specimens at optimum moisture content and specified density requirement (e.g., 90% of AASHTO T-180) using the initial design lime content and at about 2% and 4% lime above that lime content from Step 1. Cure the test specimens in sealed plastic bags for 28 days at 21°C (73°F). (Alternative - cure for 7 days at 40°C (104°F)).
5. Determine the unconfined compressive strength for all cured test specimens (e.g., ASTM 5102). Select as the construction design lime content the minimum percent required to achieve the required compressive strength (e.g., 150 psi). Either prepare a sample at the design lime content and perform resilient modulus test (e.g., AASHTO T 294-94) or estimate from Unconfined compression strength Q_u. A conservative estimate for lime-stabilized soils has been reported to be obtained from (Thompson, 1970):

   M_R = 0.124 q_u + 9.98

   where,

   MR	=	resilient modulus, ksi,
   qu	=	unconfined compressive strength, psi, as tested in accordance with ASTM D 5102, "Standard Test Method for Unconfined Compressive Strength of Compacted Soil-Lime Mixtures"

6. Add 0.5 - 1% additional lime in the lower percentage ranges to compensate for problems associated with non-uniform mixing during construction.

Laboratory testing should always be performed to check whether the stabilization has the desired effect on other engineering properties like plasticity and strength.

Cement Content for Cement-Modified Soils

1. Improve plasticity. The amount of cement required to improve the quality of the soil through modification is determined by the trial-and-error approach. If it is desired to reduce the PI of the soil, successive samples of soil-cement mixtures must be prepared at different treatment levels and the PI of each mixture determined. The Referee Test of ASTM D 423 and ASTM D 424 procedures will be used to determine the PI of the soil-cement mixture. The minimum cement content that yields the desired PI is selected, but since it was determined based upon the minus 40 fraction of the material, this value must be adjusted to find the design cement content based upon total sample weight expressed as:

   A = 100 BC

   where,

   A	=	design cement content, percent total weight of soil
   B	=	percent passing No. 40 sieve size, expressed as a decimal
   C	=	percent cement required to obtain the desired PI of minus 40 material, expressed as a decimal

2. Improve gradation. If the objective of modification is to improve the gradation of a granular soil through the addition of fines, then particle-size analysis (ASTM D 422) should be conducted on samples at various treatment levels to determine the minimum acceptable cement content.
3. Reduce swell potential. Small amounts of Portland cements may reduce swell potential of some swelling soils. However, Portland cement generally is not as effective as lime, and may be considered too expensive for this application. The determination of cement content to reduce the swell potential of fine-grained plastic soils can be accomplished by molding several samples at various cement contents and soaking the specimens along with untreated specimens for 4 days. The lowest cement content that eliminates the swell potential or reduces the swell characteristics to the minimum is the design cement content. Procedures for measuring swell characteristics of soils are found in ASTM D 4546 and MIL-STD-621A, Method 101. The cement content determined to accomplish soil modification should be checked to see whether it provides an unconfined compressive strength great enough to qualify for a reduced thickness design in accordance with criteria established for soil stabilization.
4. Condition frost areas. Cement-modified soil may also be used in frost areas, but in addition to the procedures for mixture design described in (1) and (2) above, cured specimens should be subjected to the 12 freeze-thaw cycles prescribed by ASTM D 560 (but omitting wire-brushing) or other applicable freeze-thaw procedures. This should be followed by determination of frost design soil classification by means of standard laboratory freezing tests. If cement-modified soil is used as subgrade, its frost susceptibility, determined after freeze-thaw cycling, should be used as the basis of the pavement thickness design if the reduced subgrade design method is applied.

Cement Content for Cement-Stabilized Soil

The following procedure is recommended for determining the design cement content for cement-stabilized soils.

Step 1. Determine the classification and gradation of the untreated soil following procedures in ASTM D 422 and D 2487, respectively.

Step 2. Using the soil classification, select an estimated cement content for moisture-density tests from Table F-1.

Step 3. Using the estimated cement content, conduct moisture-density tests to determine the maximum dry density and optimum water content of the soil-cement mixture. The procedure contained in ASTM D 558 will be used to prepare the soil-cement mixture and to make the necessary calculations; however, the procedures outlined in AASHTO T180 or ASTM D 1557 will be used to conduct the moisture density test.

Step 4. Prepare triplicate samples of the soil-cement mixture for unconfined compression and durability tests at the cement content selected in Step 2 and at cement contents 2% above and 2% below that determined in Step 2. The samples should be prepared at the density and water content to be expected in field construction. For example, if the design density is 95% of the laboratory maximum density, the samples should also be prepared at 95%. The samples should be prepared in accordance with ASTM D 1632, except that when more than 35% of the material is retained on the 4.75 mm (# 4) sieve, a 100-mm (4-in.) diameter by 200-mm-high (8-in.) mold should be used to prepare the specimens. Cure the specimens for 7 days in a humid room before testing. Test three specimens using the unconfined compression test in accordance with ASTM D 1633, and subject three specimens to durability tests, either wet-dry (ASTM D 559) or freeze-thaw (ASTM D 560) tests, as appropriate. The frost susceptibility of the treated material should also be determined, as indicated in appropriate pavement design manuals.

Step 5. Compare the results of the unconfined compressive strength and durability tests with the requirements. The lowest cement content that meets the required unconfined compressive strength requirement and demonstrates the required durability is the design cement content. If the mixture should meet the durability requirements, but not the strength requirements, the mixture is considered to be a modified soil. If the results of the specimens tested do not meet both the strength and durability requirements, then a higher cement content may be selected and Steps 1 through 4 above repeated.

Selection of Lime-Flyash Content for LF and the Determination of the Ratio of Lime to Fly LCF Mixtures.

1. Step 1. The first step is to determine the optimum fines content that will give the maximum density. This is done by conducting a series of moisture-density tests using different percentages of flyash and determining the mix level that yields maximum density. The initial flyash content should be about 10%, based on dry weight of the mix. It is recommended that material larger than 19 mm (¾ in.) be removed and the test conducted on the minus 19 mm (¾ in.) fraction. Tests are run at increasing increments of flyash, e.g., 2%, up to a total of about 20%. Moisture density tests should be conducted following procedures indicated in AASHTO T99, AASHT T180, and ASTM D 1557. The design flyash content is then selected at 2% above that yielding maximum density. An alternate method is to estimate optimum water content and conduct single point compaction tests at flyash contents of 10 - 20%, make a plot of dry density versus flyash content, and determine the flyash content that yields maximum density. The design flyash content is 2% above this value. A moisture density test is then conducted to determine the optimum water content and maximum dry density.
2. Step 2. Determine the ratio of lime to flyash that will yield highest strength and durability. Using the design flyash content and the optimum water content determined in Step 1, prepare triplicate specimens at three different lime-flyash ratios, following the selected density procedure. Use LF ratios of 1:3, 1:4, and 1:5. If desired, about 1% of Portland cement may be added at this time.
3. Step 3. Test three specimens using the unconfined compression test. If frost design is a consideration, subject three specimens to 12 cycles of freeze-thaw durability tests (ASTM D 560), except wire brushing is omitted. The frost susceptibility of the treated material shall also be determined as indicated in the appropriate design manual.
4. Step 4. Compare the results of the unconfined compressive strength and durability tests with the requirements. The lowest LF ratio content, i.e., ratio with the lowest lime content that meets the required unconfined compressive strength requirement and demonstrates the required durability, is the design LF content. The treated material must also meet frost susceptibility requirements, as indicated in the appropriate pavement design manuals. If the mixture should meet the durability requirements, but not the strength requirements, it is considered to be a modified soil. If the results of the specimens tested do not meet both the strength and durability requirements, a different LF content may be selected, or additional Portland cement used and Steps 2 through 4 repeated.

Selection of Cement Content for LCF Mixtures.

Portland cement may also be used in combination with LF for improved strength and durability. If it is desired to incorporate cement into the mixture, the same procedures indicated for LF design should be followed except that, beginning at Step 2, the cement shall be included. Generally, about 1 - 2% cement is used. Cement may be used in place of or in addition to lime; however, the total tines content should be maintained. Strength and durability tests must be conducted on samples at various LCF ratios to determine the combination that gives best results.

Selection of Asphalt Content for Bituminous-Stabilized Soil

Guidance for the design of bituminous-stabilized base and subbase courses is contained in U.S. Army TM 5-822-8/AFM 88-6, Chap. 9. For subgrade stabilization, the following equation may be used for estimating the preliminary quantity of cutback asphalt to be selected:

where

The preliminary quantity of emulsified asphalt to be used in stabilizing subgrades can be determined from Table F-2. The final design content of cutback or emulsified asphalt should be selected based upon the results of the Marshal Stability test procedure (AASHTO T 245, ASTM D 5581, MIL-STD 620A). The minimum Marshall Stability recommended for subgrades is 2.2 kN (500 lb). If a soil does not show increased stability when reasonable amounts of bituminous materials are added, the gradation of the soil should be modified, or another type of bituminous material should be used. Poorly graded materials may be improved by the addition of suitable tines containing considerable material passing the 75 µm (No. 200) sieve. The amount of bitumen required for a given soil increases with an increase in percentage of the liner sizes.

1 lb = 0.454 kg