Chemically stabilized embankment is generally used to repair landslides, such as shown in the following figure. This situation involves the removal of wet soil, treating it with cement or lime, and then using the treated soil to rebuild the embankment. Notice the addition of drainage to the slide repair.
Typical Use of Chemically Stabilized Embankment to Repair Landslide
There are primarily two types of chemicals used to treat soil: cement and lime.
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.
If using cement, use Type 1 cement according to 701.04. If using lime, there are two types of lime used for soil stabilization: quick lime and hydrated lime. Quick lime is used most of the time. Quick lime must all pass through the No. 4 (4.75 mm) sieve. Lime must come from a supplier listed on the QPL.
Lime kiln dust is another material that can be used for soil stabilization. The Department has limited experience with lime kiln dust for soil stabilization, so 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.
Chemical stabilization should not be performed when the temperature is below 40 ºF (5 ºC) or the ground is frozen. In order to stabilize the soil, the lime or cement needs to react with the water in the soil. It cannot do that if the water is frozen. If it is raining, then the free water will react with the lime or cement instead of the water in the soil.
Spreading lime and cement creates some dust. For this reason, the chemical stabilization should not be performed when it is windy, as this will spread the dust outside of the project area.
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 Contractor Designed Chemically Stabilized Embankment 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 contractor designed subgrade is not included in the plans, then use the percentage given in the plans or the percentage given in 205.04.A.
When lime is shown on the plans, the quantities and percentage of lime are based on the use of hydrated lime. If the contractor chooses to use quick lime and the percentage is not determined from the Contractor Designed Chemically Stabilized Embankment test program, adjust the percentage to account for the fact that hydrated lime is not being used (typically this means using 4 percent quick lime rather than 5 percent hydrated lime).
To calculate the spreading rate (number of pounds of the chemical per square yard), use the following equation.
C = 0.75 × T × D × P
C = spreading rate for chemical (pounds per square yard)
T = thickness of embankment lift (inches)
D = average dry density of soil (110 pounds per cubic foot)
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% 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 that indicates 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.
Spreading cement with a shroud around the spreader bar
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, then the contractor must use the power driven rotary mixer.
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 hydrated lime and to at least 3 percent above optimum moisture content if using quick 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 Contractor Designed Chemically Stabilized Subgrade, or by using the Ohio Typical Moisture Density Curves and the one point proctor method in Supplement 1015.
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 perpendicular to the first 10. Reduce all the soil clods to a maximum size of 1 inch (25 mm). The harrows do not do as good a job mixing in the chemical as the rotary mixer, so 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 both cement and lime. This is different from Chemically Stabilized Subgrade.
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.
If the pay item for Contractor Designed Chemically Stabilized Embankment 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.
When lime is shown on the plans, the quantities and percentage of lime are based on the use of hydrated lime. If the contractor chooses to use quick lime, the quantity of lime measured for payment is increased by the equation in 206.07. This is done because generally less quick lime is required than hydrated lime to have the same soil improvement.
1. Materials per 205.02
2. Check contractor designed lime or cement percentage.
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.
8. Final cross sections.