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Publication Number:  FHWA-HRT-13-046    Date:  October 2013
Publication Number: FHWA-HRT-13-046
Date: October 2013

 

Federal Highway Administration Design Manual: Deep Mixing for Embankment and Foundation Support

APPENDIX A. LABORATORY PROCEDURE FOR MIXING, CURING,
AND STRENGTH TESTING OF TREATED SOIL SPECIMENS APPLICABLE
TO WET MIXING

This procedure was originally developed for use with relatively easily mixed soils such as sands, silty sands, clayey sands, soft or low plasticity silts, and soft clays. For stiff and high plasticity clays, it is recommended to cut the soil into 0.975-inch (25-mm) cubes prior to mixing.(135)

Equipment and Materials

Equipment and materials include the following:

Material Preparation and Storage Prior to Use

The procedure for material preparation and storage is as follows:

  1. For soils that are sensitive to open air exposure due to drying or oxidation reactions, place the samples in airtight containers as soon as possible after obtaining the soil. If samples are obtained in blocks suitable for storage in 5-gal (20-L) buckets, place the bulk sample inside a thick-gauge plastic bag inside of the bucket and pour a small amount of water between the bag and the bucket to create a humid atmosphere inside the bucket. Using a household vacuum, remove the excess air from the bag, seal the bag tightly, and place the lid on the bucket. If samples are retrieved in Shelby tubes, extrude the samples as soon as possible. Cut the samples into pieces, wrap each in a thin plastic film, and place the pieces into plastic storage bags. All samples should be kept sealed and stored in a moist room at or near 100 percent relative humidity and at 68 °F (20 °C) unless alternate curing conditions are stipulated for a specific project. For soils that are not sensitive to exposure to air, such careful handling and storage procedures may not be necessary.
  2. Estimate the amount of dry binder that will be required for the entire mixing program. Sift the binder to remove any lumps. Store in an airtight container until needed for mixing.

Soil Mixing,

The procedure for soil mixing is as follows:

  1. Determine the weight of moist soil necessary for creating a particular batch of soil-cement mixture (see appendix B). At least eight specimens should be prepared for each batch-two specimens to be tested at each of four curing periods. Thoroughly wet the inside of the mixing bowl used with the stand mixer and lightly pat dry with a paper towel. The inside of the mixing bowl should be lightly moistened but should show no visible water beads.
  2. Choose an attachment for the mixer (dough hook or beater style) that will produce the most thorough mixing considering the plasticity of the soil being mixed and the total amount of water in the mixture from the soil water and slurry water. Generally, a dough hook works well for mixing cohesive soils and a flat beater may work well for some non-plastic soils.
  3. Measure the required weight of moist soil to the nearest 0.0035 oz (0.1 g) and place it in the moistened mixing bowl. Reseal the soil samples. Record the actual weight of moist soil if different from the target weight.
  4. Place the mixing bowl onto the mixer and begin mixing at the lowest setting (approximate rotation of the mixing tool of 100 to 175 cycles/min and revolution of the mixing tool around the bowl of less than 100 cycles/min in the planetary mixing action). Mix for approximately 3 min. When necessary, use a spatula to remove soil from the beater and the sides of bowl and push the soil back towards the center of the bowl. Record the total mixing time. If the laboratory testing program requires that the soil water content be increased above its natural water content, the required additional water can be mixed into the soil at this time. Record the weight of water added.

Binder Slurry Mixing

The binder slurry mixing procedure is as follows:

  1. Determine the dry weight of binder and weight of slurry water slurry necessary for creating the batch of soil-cement mixture (see appendix B).
  2. While the soil is being mixed, measure the required weight of slurry mixing water and add it to the mixing container of the kitchen blender. Record the actual weight of slurry water if different from the target weight.
  3. Measure the required weight of dry binder and place it in the mixing container of the kitchen blender. Blend it for approximately 3 min, and record the total mixing time. The binder slurry should be mixed while the soil is being mixed. Record the actual weight of the binder if it is different from the target weight.

Soil-Binder Mixing

The soil-binder mixing procedure is as follows:

  1. After the binder slurry is mixed, turn off the blender and remove the pitcher from the base. Without stopping the soil mixing (do not turn the stand mixer off), slowly add the binder slurry to the mixing bowl. Use a rubber spatula to aid in transferring as much of the binder slurry as possible into the soil mixing bowl. Weigh the blender pitcher before and after transferring the slurry into the mixing bowl to determine the actual weight of binder slurry used in the soil-cement mixture and record this value. If the blades of the blender are such that they prevent complete, or near complete, transfer of the slurry binder into the mixture, it may be necessary to make excess binder slurry (above what is required to meet the design criteria) to ensure an adequate amount of binder slurry in the soil-cement mixture. This should be done so as to maintain the design water-to-binder ratio. The exact amount of slurry required for the mix design should then be added by weighing the pitcher before and after adding the slurry to the soil and continuing to add binder slurry only until the target weight of slurry is achieved.
  2. Mix the soil-binder mixture for approximately 10 min from the time the total amount of binder slurry is added to the mixture. Use a spatula to remove the mixture from the beater and sides of the bowl and push the mixture back towards the center, stopping mixing only when necessary. Record the total mixing time and mixing equipment used.

Placing the Mixture in the Molds

The procedure for placing the mixture in the molds is as follows:

  1. Appropriately label clean, dry molds (this can be done prior to beginning the mixing process or while the soil and binder are being mixed).
  2. Begin placing the mixture in the molds as soon as possible following soil-binder mixing. If the mixture is fluid, use a ladle or large spoon to stir the mixture by hand to keep the mixture from segregating, and immediately fill the ladle or spoon with a representative scoop of the mixture. Place the mixture from the ladle or spoon into a plastic mold. Exercise care to ensure that each lift placed in the mold is representative of the entire mixture. Allowing soil particles to settle toward the bottom of the mixing bowl during placement of the mixture in the plastic molds will result in samples with varied mixture properties (i.e., binder content, dosage rate, volume ratio, etc.), thus decreasing the usefulness of the test results.
  3. Fill each mold in approximately three lifts, rodding or tapping the sample after each lift as necessary to remove air bubbles and air pockets. For thicker lower water content mixtures, rodding may be necessary, while fluid mixtures with higher water contents may respond best to light tapping. Cease tapping if water begins to separate from the mixture. The objective is to completely fill the plastic molds without air voids while simultaneously minimizing segregation.
  4. Finish by screeding the top of the specimen flush with the top of the mold, using a straight edge to produce a flat surface. Cap the specimen immediately to prevent moisture loss.
  5. After all molds have been filled and capped, clean and dry the molds. Weigh each specimen individually in its mold. For fluid mixtures prone to segregation, specimens should not vary by more than 3 percent from the average weight of all samples. For thick mixtures that tend to trap air pockets, no specimen should weigh less than 95 percent of the weight of the heaviest specimen. Specimens that do not satisfy these tolerances should be discarded.
  6. Discard any mix that is not satisfactorily placed in a mold within 30 min of completing initial mixing. No remixing or other disturbance of the mixture should be allowed more than 30 min after completion of initial mixing.

Curing

Store the completely sealed specimens under controlled conditions at 95 to 100 percent relative humidity and at room temperature (68 to 77 °F (20 to 25 °C)) unless a different curing temperature is specified. If a humid room is not available, the sealed specimens can be stored under water. Specimens should be stored in the sealed cylinder molds under these controlled conditions for their specified curing periods. Often, samples are cured for 7, 14, 28, and 56 days.

Specimen Preparation and Testing

The procedure for specimen preparation and testing is as follows:

  1. After a specimen has reached its designated curing age, carefully remove the cap from the specimen to be tested. If bleed water has formed at the top the specimen, record the weight of bleed water as the difference in the weight of the specimen before and after pouring off the bleed water.
  2. With the cap removed, place the specimen in its mold on the sliding tray of the rock saw such that the lip of the mold rests inside the blade opening (not against the bottom of the tray). This should allow for the remaining portion of the specimen to lay flat against the sliding tray. If no bleeding has occurred, remove the upper end of the mold at a location just below the lip of the mold. If bleeding has occurred, saw the upper end of the mold at a location that will just penetrate the upper surface of the specimen and create an end that is perfectly planar and perpendicular to the height of the sample.
  3. Turn the sample around on the tray and use the saw to remove the bottom end of the mold from the specimen. The amount of sample removed during this process should be minimized as much as possible. Being careful not to penetrate the sample, use a utility knife to make a single lengthwise cut in the mold using an alignment guide and limited blade penetration. If the mold is too thick or tough for this, a grinding tool with a rotating cutting blade at a low setting may be used to make the cut. With care, the mold can be cut without making a significant penetration into the specimen. Peel the mold off of the sample. Soil-cement specimens are softer than standard concrete cylinders, and they may be damaged if extreme care is not used during extraction.
  4. Place the sample upright on a level surface and use a bidirectional level to ensure that adequately parallel ends have been achieved.
  5. Measure and record the specimen weight to the nearest 0.0035 oz (0.1 g). Use calipers or a Pi Tape® to measure and record the specimen height and diameter. Height and diameter measurements should be taken at a minimum of two locations on the specimen for each dimension.
  6. Conduct unconfined compressive strength tests as per ASTM D2166 at a strain rate of approximately 1 percent per min. Record the time and date of testing.(131)
  7. When reducing the data, apply the area correction based on axial strain, as described in ASTM D2166.(131) In addition, because the specimen heights will be less than twice the diameters as a result of trimming the specimen ends, a height correction should be applied. The height correction is described in ASTM C918, and the correction factors are provided in table 23.(136)

Table 23. Strength correction factors.

Length/Diameter

Factor

2.00

1.00

1.75

0.98

1.50

0.96

1.25

0.93

1.00

0.87

Values not specified in the table should be determined by interpolation.

8. Record the mode of failure and the stress and strain at failure. Retain the complete stress-strain diagram for determining modulus values, if necessary.

Data Reduction

The procedure for data reduction is as follows:

  1. After testing a minimum of two specimens at each prescribed curing time (typical curing periods are 7, 14, 28 and 56 days), plot all values of unconfined compressive strength versus curing time.
  2. Fit the logarithmic trend line qt = q0 + a ln(t) through the data, where qt = unconfined compressive strength at curing time t, and q0 and a = coefficients obtained by least squares regression.
  3. Determine the 28-day strength and the strengths at any other desired times from the trend line.
  4. Determine the binder factor, binder factor in-place, binder content, total water-to-binder ratio, and volume ratio of the batch based on the actual weights of materials used during mixing (as mixed) and consider the amount of water lost due to bleeding effects (as cured). If no bleed water occurs during curing, the as-mixed and as-cured values are the same. See appendix B for calculations.
  5. Report relevant values in a table with the trend line strengths. Table 24 shows an example.

Table 24. Batch mix proportions and trend-line strengths.


Batch Number

Mix Proportion Parameters

Trend Line Strengths

w:b

VR
(%)

a
(kg/m3)

ain-place
(kg/m3)

aw
(percent)

wT:b

7-Day
(kPa)

28-Day
(kPa)

LCS 1

1.00

24.5

186

150

11.3

3.04

604

1,280

1 kg/m3 = 0.062 lb/ft3
1 kPa = 0.145 psi

 

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