205 Chemically Stabilized Embankment

Uses of Chemically Stabilized Embankment

Chemically stabilized embankment is generally used to repair landslides, as shown in the following figure. This situation involves the removal of wet soil, treating it with cement, lime, or lime kiln dust, and then using the treated soil to rebuild the embankment. Notice the addition of drainage to the slide repair.

Figure 205.A – Typical Use of Chemically Stabilized Embankment to Repair Landslide

Cement is most effective in treating soils with a Plasticity Index (PI) less than 20, such as sandy and silty soils like A-3a, A-4a, A-4b, A-6a, and some A-6b soils. Use cement stabilized subgrade to treat areas with N values (SPT blow counts) as low as 5 and unconfined strengths (hand penetrometer) as low as 0.5 tsf.

Lime is most effective in treating soils with a PI greater than 20, such as A-7-6 and A-6b soils. Use lime stabilized subgrade to treat areas with N values (SPT blow counts) as low as 10 and unconfined strengths (hand penetrometer) as low as 1.0 tsf.

Lime kiln dust (LKD) may be used to stabilize unstable subgrade soils which have a PI from 10 to 20. Consult the Office of Geotechnical Engineering when using LKD.

Materials (205.02)

If using cement, use Type 1 cement according to 701.04. If using lime, use quick lime conforming to 712.04.B. Quick lime must pass through the No. 4 (4.75 mm) sieve. Lime must come from a certified supplier.

Lime kiln dust is another material that can be used for soil stabilization. The Department has begun using lime kiln dust for soil stabilization on some projects, but it is generally not included in the plans. However, the Contractor may propose to use it in a value engineering change proposal. Lime kiln dust must conform to 712.04.C.

Construction (205.04)

Limitations

Chemical stabilization should not be performed when the temperature is below 40 ºF (5 ºC) or when the ground is frozen. In order to stabilize the soil, the chemical needs to react with the water in the soil. It cannot do that if the water is frozen. If it is raining, the free water will react with the lime or cement instead of the water in the soil.

Spreading lime, cement, and lime kiln dust creates some dust; therefore, chemical stabilization should not be performed when it is windy, as this will spread the dust outside of the project area.

Spreading (205.04.A)

The amount of chemical applied to each lift is based on a percentage of the dry weight of the soil that will be treated. The percentage is typically 4 to 10 percent for cement and 4 to 8 percent for lime. If the Mixture Design for Chemically Stabilized Soils pay item is included in the plans, the Contractor will determine the appropriate percentage of chemical based on a testing program. If the pay item for the mixture design is not included in the plans, use the percentage given in the plans or the percentage given in 205.04.A.

To calculate the spreading rate (number of pounds of chemical per square yard), use the following equation:

C = 0.75 × T × D × P

Where:

C = Spreading rate for chemical (pounds per square yard).

T = Thickness of embankment lift (inches).

D = Average dry density of soil (if not known, assume 110 lb/ft³).

P = Percentage of chemical, expressed as a decimal

(e.g., 5% = 0.05)

0.75 is a unit conversion factor (9 ft² / 1 yd² × 1 ft / 12 in).

For example, if using 4 percent of a chemical and an 8-inch embankment lift:

C = 0.75 × 8 in × 110 lb/ft³ × 0.04 = 26.4 lb/yd² of chemical

The Contractor must submit a spreading plan or report two days before the work and indicate how the Contractor will achieve the required spreading rate. After the spreading operation, check the Contractor’s spreading rate by taking the total weight of chemical spread and dividing it by the area that was treated.

For slide repair work, a loader is sometimes used to spread the chemicals. The exact amount of chemical in each lift of soil is not as critical as it is in subgrade stabilization work.

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 m) above the subgrade can be used. The chemical should not be spread if wind conditions are such that blowing dust exceeds the limits in 107.19.

Figure 205.B – Spreading Cement with a Shroud around the Spreader Bar

Mixing (205.04.B)

The Contractor should mix the chemical into the soil immediately after spreading the chemical. The Contractor may use a spring tooth harrow, a disk harrow, or a power driven rotary mixer which looks like a big rotary tiller. However, if the area is beneath a pavement or paved shoulder, the Contractor must use the power driven rotary mixer.

Figure 205.C – Rotary Mixer for Chemical Stabilization

If using a power driven rotary mixer, mix the soil and chemical until all the soil clods are reduced to a maximum size of 2 inches (50 mm). Add water, if necessary, to bring the mixture to at least the optimum moisture content if using cement or lime kiln dust and bring the mixture to at least 3 percent above optimum moisture content if using lime. Quick lime reacts more strongly with water and the additional water prevents future expansion problems. Determine the optimum moisture content from the moisture-density curves developed in the test program from the mixture design or by using the Ohio Typical Moisture Density Curves and the one-point Proctor method in Supplement 1015.

Figure 205.D – Disk Harrow

If using a spring tooth or disk harrow, break-up the soil with the harrow before spreading the chemical. Mix the chemical into the soil and add water as described above. During mixing, use at least 20 passes of the harrow: 10 in one direction and 10 in a direction which will be perpendicular to the first 10. Reduce all the soil clods to a maximum size of 1 inch (25 mm). The harrows do not mix the chemical as well as the rotary mixer; therefore, more effort is required with the harrow to mix the chemical and soil.

Note that for Chemically Stabilized Embankment, the mixing is the same for all the chemicals. This is different from Chemically Stabilized Subgrade.

Compacting (205.04.C)

Compact to 98 percent of the maximum dry density and use the one-point Proctor method in Supplement 1015 to determine the maximum dry density. In some cases, the test section method or the moisture-density curves developed by the Contractor may be used to determine the maximum dry density.

Mixture Design for Chemically Stabilized Soils (205.05)

If a pay item for Mixture Design for Chemically Stabilized Soils is included in the plans, then the Contractor uses the testing program described in Supplement 1120 to determine the spreading rate for the chemical. Different mixtures of the soil and chemical are tested for unconfined compressive strength. The results are used to determine the percentage of chemical used in the field. The common increase in strength ranges from 20 psi to 100 psi with lime and from 50 psi to 200 psi with cement.

Method of Measurement (205.06)

The mixture design pay item also includes verification testing of the stabilized subgrade. For this reason, the Department pays for only two-thirds of the lump sum item for mixture design testing after the mixture design is complete. The other one-third is paid after the chemically stabilized subgrade is completed.

Documentation Requirements - 205 Chemically Stabilized Embankment

1. Materials per 205.02.

2. Check spreading rate for lime, cement, or lime kiln dust.

3. Verify cross-sections.

4. Temperature must be 40 °F (4 °C) or above and the soil cannot be frozen.

5. Document the construction: spreading, mixing, and compaction.

6. Perform the compaction testing according to Supplement 1015.

7. Measure and pay according to 205.06 and 205.07.

8. Final cross-sections.

9. Document on CA-EW-1, CA-EW-12 and CA-D-1, and CA-D-3. Do not duplicate the information on all forms unless necessary.