§ 3.2.1. Stabilization Selection and Mix Design for Subgrade and Base Materials

The stabilization selection and mix design approach shall include the selection of the type of stabilizer (see Figure 3.2.1) and the development of an appropriate stabilized mix design (see Figures 3.2.2 and 3.2.3) based upon the gradation (TxDOT Test Method Tex-110-E), plasticity index (TxDOT Test Method Tex-106-E) and pH (TxDOT Test Method Tex-128-E) of the candidate soil mixture.

Appropriate mix designs for any stabilized/treated subgrade, subbase and base layers shall be developed by a Registered Professional Engineer, Licensed in the State of Texas.

A.

Lime Stabilization

The principal goal of the mixture design process is the establishment of an appropriate lime content for construction. However it should be noted that there may be instances where acceptable soil-lime mixtures may not be obtained regardless of the lime percentages used to treat the base and subbase materials. The flow diagram presented in Figure 3.2.1 shall be used as an aid in defining those soil mixtures that are expected to be amenable to lime treatment.

In general the addition of lime to a fine-grained soils results in spoil mixtures that display decreased plasticity, improved workability, reduced volume change characteristics and strength increases. Improvement in soil strength, however, does not always develop with the addition of lime. In general soils classified by the AASHTO method as A-4, A-5, A-6, A-7 and sometimes A-2-7 and A-2-6 are more readily susceptible to stabilization with lime. It should be noted that a number of variables, including soil type, lime type, lime percentage and curing conditions can impact the properties of soil-lime mixtures.

The impact of lime on the post-conditioned properties of materials proposed for lime treatment can range from reduction in the plasticity properties (with minimal strength increases) to significant strength increases. The latter impact (i.e. significant strength increase) is identified as stabilization of lime-reactive soils/materials (i.e. normally soils/materials with pH values greater than 7.0), while the former impact (i.e. reduction in plasticity properties) is identified as conditioning of non-lime-reactive soils/materials (i.e. normally soils/materials with pH values less than 7.0). The type of lime treatment proposed for the work should be indicated in the mix design report (i.e. lime stabilization for strength increase or lime conditioning for plasticity reduction).

Most fine-grained soils can generally be conditioned/stabilized effectively with 3% to 10% of lime addition (dry weight of soil basis). The lower percent lime additions are normally identified with lime conditioning (with minimal strength increases) of the soil material, while the higher percent lime additions are normally necessary to achieve lime soil mixtures with significant strength increases.

In the case of lime conditioning of soil mixtures (with minimal strength increases) the lime conditioned soil mixture design for the City of Austin shall be developed using TxDOT Test Method Tex-112-E, "Method of Admixing Lime to Reduce Plasticity Index of Soils."

In development of a lime stabilized soil mix design for the City of Austin, the mix design approach presented in Figure 3.2.2 and the procedures specified in TxDOT Test Method Tex-121-E, "Soil-Lime Testing," shall be used to establish the lime content that would produce a 28 day unconfined compressive strength (TxDOT Test Method Tex 117-E) of 50 psi for a lime stabilized subgrade and 100 psi for a lime stabilized base layer.

The minimum rate of lime solids application shall be 5% by weight (mass) for non-lime-reactive materials (pH of 7.0 or less) or 7% by weight (mass) for lime-reactive materials (pH greater than 7.0), unless indicated otherwise in the mix design process or as directed by the Engineer or designated representative.

B.

Cement Stabilization

A wide range of soil types may be stabilized using Portland cement. The greatest effectiveness is with sands, sandy and silty soils, and clayey soils of low to medium plasticity. However, Portland cement is difficult to mix into soils with a plasticity index that exceeds 30. The flow diagram presented in Figure 3.2.1 shall be used as an aid in defining those soil mixtures that are expected to be amenable to cement treatment.

Soil mixtures that are acid, neutral or alkaline may well respond to cement treatment; however the higher pH soils react more favorably to cement addition and undergo significant strength increases. Although some organic matter such as un-decomposed vegetation may not influence stabilization adversely, other organic compounds of lower molecular weight, such as nucleic acid and dextrose, act as hydration retarders and reduce strength.

A special pH test (see Table 3.2.1) shall be used to provide an indication of the impact of organics on normal hardening of the cement stabilized soil mixture. In essence a 10:1 mixture (by weight) of soil and cement is mixed with distilled water for a minimum of 15 minutes and the pH of the combined mixture is then measured. If the pH value is at least 12.1, then it is probable that organic matter, if present, will not interfere with normal hydration/hardening of a soil-cement mixture. This pH measurement is a principal feature in identifying the soil mixtures that can likely be stabilized with cement and are candidates for development of a cement-soil mix design (see the mix design flow diagram presented in Figure 3.2.3).

Since sulfate attack is known to adversely affect some cement stabilized soil, the sulfate content of a soil should be considered in the selection of cement as a stabilizer. The impact of the sulfate factor on the mix design is also identified in Figure 3.2.3, where cement stabilization of soils with sulfate contents greater than 0.9% is discouraged. Procedures for determining sulfate content of soils are presented in Tables 3.2.4 and 3.2.5.

There are additional selection criteria based on gradation and Atterberg limits results that should be used in establishing the acceptability of a soil mixture for cement stabilization, specifically:

1.

Fine-grained soils - Plasticity Index should be less than 20 and the Liquid Limit less than 40;

2.

Sandy soils - Plasticity Index should be less than 30;

3.

Coarse-grained (gravel) soils - Minimum of 40% passing the no. 4 sieve; and

4.

All soils - Plasticity Index should not exceed the number calculated in the following equation:

The properties of cement-treated soils are principally dependent on cement content, density, moisture content and confining pressure. It should also be noted that the addition of cement to a soil mixture could produce some change in both the optimum water content and maximum dry density for a given compactive effort. The principal goal of the cement stabilization mixture design process is therefore the establishment of an appropriate cement content-optimum moisture-density relationship appropriate for construction.

Most soils can generally be stabilized effectively with 5% to 16% of cement addition (dry weight of soil basis). The lower percent cement additions are normally identified with coarser soil mixtures (AASHTO classifications A1 and A2), while the higher percent cement additions are normally necessary for the fine-grained soils (AASHTO A6 and A7). Estimates of cement requirements for various soil classifications are presented in Table 3.2.2 below.

In development of a cement stabilized soil mix design for the City of Austin, the mix design approach presented in Figure 3.2.3 and the procedures specified in TxDOT Test Method Tex-120-E, "Soil-Cement Testing," shall be used to establish the design cement content that would produce a mix that meets the allowable durability requirements presented in Table 3.2.3. The mix design report should include the molding moisture content, the dry density to the nearest 0.1 pcf, the seven-day unconfined compressive strength to the nearest psi and the recommended cement content to the nearest whole percent.

The seven-day compressive strength associated with the recommended cement content should be used as the field control measure during construction. The seven-day compressive strength for cement stabilized soils can vary between 100 psi for fine-grained soils to more than a 1,000 psi for coarse-grained soils.

If a mix design is not developed in the laboratory in accordance with in TxDOT Test Method Tex-120-E, "Soil-Cement Testing," the minimum rate of cement solids application shall be the percent by weight for the specific soil classification (i.e. AASHTO or Unified Classification) identified with the percent cement for moisture-density testing (column four of Table 3.2.2), unless indicated otherwise by the Engineer or designated representative.

C.

Lime-Cement Stabilization

Cement stabilization alone is normally not desired with high plasticity soil mixtures (i.e. soils with a plasticity Index greater than 30) because of difficulties in the mixing phase. In this instance combinations of lime and cement can often produce an acceptable combination. Lime is initially added to the soil mixture to increase the workability and mixing characteristics of the soil, as well as to reduce its plasticity. Cement is subsequently added to the lime-soil mixture to provide rapid strength gain. The lime-cement combination stabilization of high plasticity soils is especially advantageous when rapid strength gain is required for placement during cooler weather conditions.

The lime content to reduce the plasticity index below 30 should be established using TxDOT Test Method Tex-112-E, "Method of Admixing Lime to Reduce Plasticity Index of Soils," while the TxDOT Test Method Tex-120-E, "Soil-Cement Testing," shall be used to establish the design cement content that would produce a mix that meets the allowable durability requirements presented in Table 3.2.3.

The mix design report should include the molding moisture content, the dry density to the nearest 0.1 pcf, the seven-day unconfined compressive strength to the nearest psi and the recommended lime and cement contents to the nearest whole percent. Expected lime contents range from 1% to 3%, while the expected subsequent cement contents range from 3% to 10%. The amount of lime and cement additions is dependent upon the type of soil.

The seven-day compressive strength associated with the recommended lime and cement contents should be used as the field control measure during construction.

Figure 3.2.1 Selection matrix for base/subgrade stabilization (Source: Epps, Dunlap and Gallaway, "Soil Stabilization: A Mission Oriented Approach")

Figure 3.2.2 Mix Design Subsystem for lime stabilization

Figure 3.2.3 Mix Design Subsystem for cement stabilization

Table 3.2.1
pH Test on Soil-cement Mixtures

Materials	PH meter (range of 14)
	150 ml plastic bottles with screw-top lids
	50 ml plastic beakers
	Distilled water
	Balance
	Oven
	Moisture cans
Procedures	1.  Standardize pH meter with buffer solution with pH of 12.0.
	2.  Weigh 25.0 gms (to nearest .01 grams) of representative air dried soil sample that passes no. 40 sieve.
	3.  Transfer air-dried soil sample to 150 ml bottle with screw-top lids.
	4.  Add 2.5 grams of Portland cement to bottle.
	5.  Add distilled water to the bottle until a thick paste is created (Caution: too much water will effect the pH value).
	6.  Stir the soil-cement and water until thoroughly blended.
	7.  After 15 minutes, transfer part of the paste to a plastic beaker and measure the pH.
	8.  If pH is 12.1 or greater, the soil organic matter content should not interfere with cement stabilization.

 

Table 3.2.2 Estimates of Cement Requirements for Various Soils

Soil Classification		Percent cement by wgt (mass)
AASHTO	Unified	Usual range*	Moisture-density
A-1-a	GW, GP, GM, SW, SP, SM	3 to 5	5
A-1-b	GM, GP, SM, SP	5 to 8	6
A-2	GM, GC, SM, SC	5 to 9	7
A-3	SP	7 to 11	9
A-4	CL, ML	7 to 12	10
A-5	ML, MH, CH	8 to 13	10
A-6	CL, CH	9 to 15	12
A-7	OH, MH, CH	10 to 16	13

 

•  Note: For most A horizon soils, the cement content should be increased four percentage points if the soil is dark gray to gray and six percentage points if the soil is black.

Table 3.2.3
Criteria Based on Wet-Dry and Freeze-Thaw Durability Tests

Soil Classification		Maximum allowable
AASHTO	Unified	Weight Loss, %
A-1-a	GW, GP, GM, SW, SP, SM	14
A-1-b	GM, GP, SM, SP	14
A-2	GM, GC, SM, SC	14*
A-3	SP	14
A-4	CL, ML	10
A-5	ML, MH, CH	10
A-6	CL, CH	7
A-7	OH, MH, CH	7

 

* 10% is maximum allowable weight loss for A-2-6 and A-2-7

Table 3.2.4 Gravimetric Method for Determination of Sulfate in Soils

Reagents	Barium chloride:  10% solution of BaCl2 2H2O. (Add 1 ml of 2% HCl to each 100 ml of solution to prevent formation of carbonate)
	Hydrochloric acid, 2% solution (0.55N)s
	Magnesium Chloride, 10% of MgCl2.6H2O
	Demineralized water
	Silver Nitrate, 0.1 N solution
Apparatus	Beaker, 1,000 ml   Burner and ring stand   filtering flask, 500 ml
	Buchner funnel, 90 mm   Filter paper, Whatman No. 40, 90 mm
	Filter paper, Whatman No. 42, 90 mm   Saranwrap
	Crucible, ignition, or aluminum foil, heavy grade analytical balance
	Aspirator or other vacuum source
Procedures	1.  Select a representative sample of air-dried soil and weigh approximately 10 gm. to the nearest 0.01 gm. Determine the moisture content of the air-dried soil. (Note: When sulfate content is expected to be less than 0.1%, a sample weighing 20 gms. or more should be used).
	2.  Boil the soil sample for 1½ hours in a beaker with mixture of 300 ml water and 15 ml HCl.
	3.  Filter through Whatman No. 40 paper, wash with hot water, dilute combined filtrate and washings to 50 mls.
	4.  Take 100 ml of this solution and add MgCl2 solution until no more precipitate is formed.
	5.  Filter through Whatman No. 42 paper, wash with hot water, dilute combined filtrate and washings to 200 mls.
	6.  Heat 100 mls. of this solution to boiling and add BaCl2 solution very slowly until no more precipitate is formed. Continue boiling for about 5 minutes and let stand overnight in a warm place, covering the beaker with Saranwrap.
	7.  Filter through Whatman No. 42 paper, wash with hot water until free from chlorides (filtrate should show no precipitate when a drop of AgNO3 solution is added).
	8.  Dry filter paper in crucible or on sheet of aluminum foil. Ignite the paper. Weigh the residue on the analytical balance as BaSO4.
Calculations
Note: If precipitated from a cold solution, barium sulfate is so finely dispersed that it cannot be retained when filtering by the above method. Precipitation from a warm, dilute solution will increase the crystal size. Due to the adsorption (occlusion) of soluble salts during the precipitation by BaSO 4 , a small error is introduced. This error can be minimized by permitting the precipitate to digest in a warm, dilute solution for a number of hours. This allows the more soluble small crystals of BaSO 4 to dissolve and recrystallize on the larger crystals.

 

Table 3.2.5 Turbidimetric Method for Determination of Sulfate in Soils

Reagents	Barium chloride crystals (Grind analytical reagent grade barium chloride to pass a 1-mm sieve.)
	Ammonium acetate solution (0.5N) (Add dilute hydrochloric acid until the solution has a pH of 4.2.); Distilled water
Apparatus	Moisture can; Oven; 200-ml beaker; Burner and ring stand;
	Filtering flask; Buchner funnel, 90 mm; Vacuum source
	Filter paper, Whatman No. 40, 90 mm; pH meter
	Spectrophotometer and standard tubes (Bausch and Lomb Spectronic 20 or equivalent)
Procedures	1.  Select a representative sample of air-dried soil and weigh approximately 10 gm. to the nearest 0.01 gm. Determine the moisture content of the air-dried soil.
	2.  Add the ammonium acetate solution to the soil sample. (the ratio of soil to solution should be approximately 1:5 by weight).
	3.  Boil the soil sample for about 5 minutes.
	4.  Filter through Whatman No. 40 paper. If the extracting solution is not clear, filter again.
	5.  Take 10 ml of extracting solution (this may vary dependent upon the concentration of sulfate in the solution) and dilute with distilled water to about 40 ml. Add about 0.2 gm of barium chloride crystals and dilute to make the volume exactly equal to 50 ml. Stir for 1 minute.
	6.  Immediately after the stirring period has ended, pour a portion of the solution into the standard tube and insert the tube into the cell of the spectrophotomer. Measure the turbidity at 30-second intervals for 4 minutes. Maximum turbidity is usually obtained within 2 minutes and the readings remain constant thereafter for 3 to 10 minutes. Consider the turbidity to be the maximum reading obtained in the 4-minute interval.
	7.  Compare the turbidity reading with a standard curve and compute the sulfate concentration (as SO4) in the original extracting solution. (The standard curve is secured by carrying out the procedure with standard potassium sulfate solutions.)
	8.  Correction should be made for the apparent turbidity of the samples by running blanks in which no barium chloride is added.
Sample Calculations	Given: Weight of air-dried sample: 10.12 gms Moisture content: 9.36 % Weight of dry soil: 9.27 gms Total volume of extracting solution: 39.10 ml
	10 ml of extracting solution was diluted to 50 ml after addition of barium chloride (see Step 5 above). The solution produces a transmission reading of 81.
	Dilution rate = 50 ml/10 ml = 5
	From the standard curve (developed as described below), a transmission reading of 81 corresponds to 16.0 ppm (see figure below)
	Concentration of original extracting solution = 16.0 × 5 = 80 ppm
Development of Standard Curve	1.  Prepare sulfate solutions of 0, 4, 8, 12, 16, 20, 25, 30, 35, 40, 45, 50 ppm in separate test tubes. The sulfate solution is made from potassium sulfate salt dissolved in 0.5 N ammonium acetate (with pH adjusted to 4.2).
	2.  Continue Steps 5 and 6 in the procedure, described previously.
	3.  Draw the standard curve as shown below by plotting transmission readings for known concentrations of sulfate solutions.

 

Exception to the flexible base course: thickness may be reduced by one inch when the material is placed on solid rock.