206 Chemically Stabilized Subgrade

Specification Changes

1.        Item 206 Lime Stabilized Subgrade (LSS) has been combined with Supplemental Specification 804 Cement Stabilized Subgrade (CSS) and renamed Item 206 Chemically Stabilized Subgrade. Both Lime Stabilized Subgrade and Cement Stabilized Subgrade are covered by the revised specification.

2.        The subgrade is constructed to 1” above grade and allow for cure.  Once curing period is complete the subgrade is fine graded removing the extra 1” bring the surface to grade. This top inch often is dry and unsuitable material and this will allow it to be removed.

3.        The minimum water content requirement of CSS is optimum.

 

Uses and Application for Chemically Stabilized Subgrade

1.        Use chemically stabilized subgrade for long sections generally more than a mile.

a.        It is not cost effective for short spot locations.

2.        Lime is most effective in treating soils with a PI greater than 20.

a.        A-7-6 and A-6-b’s soils.

3.        Lime Stabilized Subgrade is used to modify the subgrade to bridge soft areas.

a.        See Figure 204.H Subgrade Treatment Chart and the other pertinent sections.

b.        Use LSS for subgrades with an average N values as low as 10, average U of 1.0 tons/ ft2 and to remove ruts < 6 inches (150mm).

4.        Cement is most effective in treating soils with PI less than 20.

a.        Mainly Sandy and Silty Soils.

b.        A-3-a, A-4-a, A-4-b, A-6-a, and some A-6-b’s.

5.        Cement is used to modify the subgrade to bridge soft areas.

a.        See the Subgrade Treatment Chart in Figure 204.H and the other pertinent sections.

b.        Use CSS for subgrades with an average N values as low as 5,  an average U of 0.5 tons/ ft2 and to remove ruts > 6 inches (150mm).

A brief outline of the specification requirements follows:

 

Materials (206.02)

1.        LSS. Use hydrated or quick lime according to 712.04.B. 

a.        100% of the quick lime must pass the # 4 sieve.

b.        Use between 4 to 8 % of lime by weight.

c.        Plan will include 5 % hydrated lime.

                                                               i.      If quick lime is used

1.        Use 4 %

2.        Use the conversion in 206.07 for payment

2.        CSS. Use Type 1 Cement according to 701.04

a.        Use between 4 to 10 % cement by weight.

b.        Plan will include 6 %.

3.        Soil must weigh at least 100 lb/ft3.

4.        Use prime coat or curing compound for the curing coat.

a.        Curing coats can leach off the subgrade if not cured completely prior to a rain event.

 

Limitation (206.03)

1.        Limitations

a.        Temperatures greater than > 40oF.

b.        Material not Frozen.

c.        No Severe Wet Conditions.

 

Test Rolling (206.04)

1.        If stabilization is specified in spot locations.

a.        Test roll to locate soft areas with a proof roller.

                                                               i.      For a cement application, investigate if rutting is greater than 6 inches.

                                                              ii.      For lime application, investigate if rutting is greater than 4 inches.

2.        Proof Roll after stabilization, if stabilization is specified throughout the project.

a.        See Item 204 in this manual.

Construction (206.05)  Spreading (206.05.A)

During the spreading operation, fill a pan, with a known volume, with the chemical as it is applied on the prepared surface.  Once the spreader has passed and filled the pan with the chemical weight the chemical.  Then calculate the pounds per cubic yards of chemical mixed in the soil.  The contractor is to submit a spreading plan to obtain the correct pounds of chemical per cubic yard.

To calculate the number of pounds of the chemical per square yard, use Equation 206.1 as follows:

Equation 206.1 – Pounds of Chemical per Square Yard

where:

D = depth (inches)

WD = average dry density (lb/ft3)

% = percent chemical (expressed as a decimal)

For example, if using 5 % of a chemical, 110 lb/ft3 of soil material, at 12 inch lifts:

¾ X 12 inches X 110 lb/ft3 material X 0.05 = 49 lb/yd2 of chemical

When a mechanical spreader is used, dusting is minimized by using a shroud around the spreader bar that extends to the surface.  A distribution bar with a maximum height of 3 feet (1 meter) above the subgrade can be used.  The chemical should not be spread if wind conditions are such that blowing cement exceeds the limits in 107.21.

Mixing  (206.05.B)

1.        (LSS)Apply the lime and mix into the soil.

a.        Use a power driven mixer.

b.        During initial mixing.

                                                               i.      All clods less than 2 inches (50mm).

                                                              ii.      Depth 9 to 16 inches.

2.        Add water.

a.        For hydrated lime at optimum.

b.        For quick lime 3 % above optimum.

                                                               i.      Prevents expansion.

3.        Lightly compact.

4.        Cure with water for 24 hours to 7 days.

a.        Mellows the soil.

5.        Re-Mix the soil.

a.        Soil 100 % < 1” and 60 % P- #4 Sieve.

b.        Add water to the same moisture contents.

c.        Check soil for uniform color.

 

1.        (CSS). Apply the cement and mix into the soil.

a.        Use a power driven mixer.

2.        During initial mixing.

a.        Ensure 100 % of mix is < an inch and 80 % s < #4 sieve.

b.        Depths 9 to 16 inches.

c.        Do not add water.

3.        Second pass.

a.        Add water.

                                                               i.      At least optimum.

                                                              ii.      Add water within 2 hours of first mixing.

                                                            iii.      Insures hydration of the cement.

4.        If the procedure gets the desired results may one pass maybe used.

5.        Check soil for uniform color.

Compaction (206.03.D)

1.        For CSS complete the compaction within 2 hours of adding water.

a.        Do not use vibration during final rolling.

2.        Compact to 98% of the maximum density.

3.        For compaction testing use the one point proctor method in Supplement 1015.

4.        Measure the dry density with a gauge and take the proctor soil from under the gauge.

5.        Use the Ohio Typical Density Curves to pick the maximum dry density.

6.        In some rare cases, a test section may be constructed or the curves developed in section 206.06 may be used.

7.        Check the depth of the stabilization by digging a hole.

a.        Add phenolphthalein to the sidewalls.

                                                               i.      Should turn purple.

b.        Can use diluted hydrochloric acid.

                                                               i.      Should fizz.

8.        Cure the chemically stabilized subgrade for 5 days.

a.        Make sure the surface is wet prior to adding the curing coat.

                                                               i.      This will prevent drying of surface.

9.        Fine grading.

a.        The 2005 specification requires fine grading after curing.

b.        The chemically stabilized subgrade is constructed an inch high.

                                                               i.      This material is taken off during the fine grading.

1.        This top inch is often dry and unsuitable.

Contractor Designed Chemically Stabilized Subgrade (206.06)

Use this section to verify that the soil will increase in strength when the chemical is added to the soil.

Unconfined strengths are determined to detail the strength gain.  If problems develop in the field, this information can be used to make field adjustments.

The common increase in strength ranges from 20 psi to 100 psi with lime.  With cement, the typical strength gain is from 50 to 200 psi.

If there is any question about the durability of the subgrade then durability testing can be performed.

 

Documentation Requirements - 206 Chemically Stabilized Subgrade

1.        Materials per 206.02

2.        Check contractor design lime or cement percentage.

3.        Verify subgrade line and grade.

4.        Verify subgrade stability with proof roller, if variable locations. 

5.        Document the Construction:

a.        Spreading

b.        Mixing

c.        Compaction

6.        Perform the compaction testing according to 1015.

7.        Proof Rolling.

a.        Make Corrections.

1.        If required perform test pits

1.        Use the Subgrade Test Pit Investigation

8.        Measure and Pay according to 206.07 and 206.08

9.        Document on the CA-EW-2, CA-EW-3, CA-EW-8, CA-EW-12 and CA-D

a.         Do not duplicate the information on all forms unless necessary.