AASHTO Soil Testing: Procedures and Timeline for Determining Soil Properties, Exams of Designs and Groups

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Soil Tests

PARTICIPANT WORKBOOK

LABORATORY MANUAL

Introduction

The scope of soil tests covers everything from classification to compaction, density to drainage, consolidation to expansion, and liquid limit to plastic limit. Measurement of these factors plays a key role in determining how soil will behave when placed as the base foundations for roadways. The testing related to these properties shows expected performance of a soil over varying conditions.

Laboratory Safety

Personal Protective Equipment

All participants in the laboratory experience must wear the following safety equipment at all times:

• Safety glasses
• Safety shoes or shoe covers
• Other safety equipment may be necessary for certain tests

Heat

Ovens will be heated to temperatures of approximately 110 °C. Heat-resistant gloves must be work when working with hot asphalt samples and putting materials in or retrieving them from the ovens.

Ensuring Your Safety

For your safety, please follow all instructions provided by the laboratory experience instructors. Do not touch or handle equipment unless you have been given permission to do so.

Guidance on Precision Estimates

Each of the test methods described herein provide single-operator (repeatability) and multilaboratory (reproducibility) precision estimates. The single-operator precision provides an estimate of the expected variation of tests performed on the same material, by the same operator, using the same equipment in the same laboratory. The multilaboratory precision provides an estimate of the expected variation of two tests performed on the same material, by different operators, using different equipment in different laboratories. If the differences between properly performed tests exceeds these values, the testing practices and equipment should be investigated to determine the cause of the variation.

AASHTO R 58, Standard Practice for Dry Preparation of Disturbed Soil and Soil-

Aggregate Samples for Test

Background Information

The dry preparation test method describes preparation methods of soil and soil-aggregate samples as they are received from the field, prior to performing other testing on the samples, such as mechanical analysis (T 88) and physical tests (T 89, T 90, T 100).

Significance and Use

The purpose of the test is to prepare samples as received from the field for mechanical analysis and physical tests by reducing the sample to the appropriate test size. This method describes how to prepare samples, regardless of moisture content and composition. It is critical that the sample be prepared properly to ensure that the applicable test results will accurately reflect the true characteristics of the material.

Related Tests and Specifications

• AASHTO T 88, Standard Method of Test for Particle Size Analysis of Soils
• AASHTO T 89, Standard Method of Test for Determining the Liquid Limit of Soils
• AASHTO T 90, Standard Method of Test for Determining the Plastic Limit and Plasticity Index of Soils
• AASHTO T 100, Standard Method of Test for Specific Gravity of Soils
• AASHTO T 248, Reducing Samples of Aggregate to Testing Size

Timeline for Completion

Preparation time: 12 hours.

Samples must be air-dried before the preparation phase. This is typically done overnight so that preparation can begin the following day.

Sieving time: 30 minutes.

Samples are separated over sieves and portions are selected for use in further testing.

Total preparation time: 12.5 hours.

Apparatus

• Drying Apparatus (optional) – Capable of drying samples at a temperature not exceeding 60 °C (140 °F). Alternatively, samples may be air-dried.
• Sample Splitter or Riffle Sampler (optional) – Alternatively, splitting or quartering methods as described in AASHTO T 248 may be used to obtain a representative sample.
• Balance
• Sieves – The following sieves may be needed, depending on the specific tests for which the sample is being prepared: 4.75 mm (#4), 2.00 mm (#10), 0.425 mm (#40), and others.
• Pulverizing Apparatus – Either a mortar and rubber-covered pestle (Figure 1) or a mechanical device with a power-driven, rubber-covered muller. Other types of apparatus are satisfactory, provided the aggregations of soil particles are broken up without reducing the natural size of the individual grains.

Figure 1: Mortar and rubber covered pestle

Figure 2: Riffle sample splitter

Step 3 Break up the aggregations of soil particles with the pulverizing apparatus.

Step 4 Determine the portion of the dried sample selected for T 88 and the physical tests. This is the mass of the total sample uncorrected for hygroscopic moisture.

Step 5 Separate this portion into fractions by one of the following methods.

Method 1 Using 2.00 mm (#10) sieve – Separate the dried sample into two fractions using a 2.00-mm sieve. Grind the fraction retained on the sieve with the pulverizing apparatus, and separate the material again on the 2.00-mm sieve.

Note: If the sample contain brittle particles, such as flakes of mica, fragments of sea shells, etc., pulverize the material carefully and with just enough pressure to free the finer material that clings to the coarser particles.

Method 2 Using 4.75-mm and 2.00-mm (#4 and #10) sieves – Separate the dried sample into two fractions using the 4.75-mm sieve. Grind the fraction retained on the sieve with the pulverizing apparatus, and separate the material again on the 4.75-mm sieve. Thoroughly mix the fractions passing the 4.75-mm sieve and obtain a representative portion adequate for the desired tests using the sample splitter, or by splitting or quartering. Separate this portion on the 2.00-mm sieve and process it as described in step 5 a. Record the mass of the material retained on the 2.00-mm sieve for later use in the T 88 coarse sieve analysis calculations.

Step 6 Test Sample for T 88 and T 100:

1. Save the material retained on the 2.00-mm sieve from step 5 a, or the material retained on the 4.75-mm sieve after the second sieving from step 5 b, for use in the coarse sieve analysis.
2. Thoroughly mix the fractions passing the 2.00-mm sieve in both sieving operations and obtain representative portions adequate for the tests (see Sample Preparation) using the sample splitter, or by splitting and quartering.

Step 7 Test Sample for Physical Tests (T 89, T 90, Shrinkage Factors, Field Moisture Equivalent, Check and Referee tests).

Separate the remaining portion of the material passing the 2.00-mm sieve into two parts using a 0.425-mm sieve. Grind the fraction retained on the sieve with the pulverizing apparatus, and separate the material again on the 0.425-mm sieve. Regrind and re-sieve the material until only a small portion of material passes the 0.425-mm sieve. Thoroughly mix all the fractions passing the 0.425-mm sieve and set aside for use in performing the physical tests.

Common Errors

• The pestle is not rubber-covered.
• The sample is dried at 110 °C instead of 60 °C.
• The wrong sieve sizes are used.
• The individual sample masses are incorrect.

Calculations: 30 minutes

The percent of material passing each sieve is calculated and the results are graphed.

Total test time: 13 hours

Apparatus

• Sieves – Nest of sieves meeting the requirements of AASHTO M 92. Sieves come in many different shapes and sizes. Typically, sieves that are round with a diameter of 8 or 12 in., or larger square or rectangular sieves are used for this test.
• Balance – Capable of weighing the sample mass to 0.1%.
• Oven – Capable of drying 110 ± 5 °C (230 ± 9 °F).
• Mechanical Shaker (Optional) – Must provide a vertical or lateral and vertical motion to the sieve and provide sieving thoroughness within a reasonable time. Although the mechanical shaker is not necessary to complete this test, it is normally used for most sieving operations. There are three types of mechanical sieve shakers that are typically used for this test: Mary-Ann, Ro-Tap, and Screen Shaker. Other types of mechanical sieve shakers may be used, as long as they create motion of the sieves to cause the particles to bounce, tumble, or otherwise turn so as to present different orientations to the sieving surface. The requirement for sieving sufficiency must be met regardless of the type of mechanical shaker used.

Mechanical Shaker Sieving Sufficiency

If a mechanical sieve shaker is used, the sufficiency of the sieving action of the device must be verified. Sieving must be continued for a sufficient period so that, after completion of the sieving operation, not more than 0.5% by mass of the total sample passes any sieve during 1 minute of continuous hand sieving.

Sieving sufficiency of mechanical shakers should be verified regularly for the types of materials that are typically tested in the laboratory, and for each sieve size used. AASHTO R 18 requires that sieving sufficiency be verified at a minimum interval of 12 months. The sieving time used during routine testing should be adjusted to the minimum amount of time required to meet the 0.5% requirement.

Note: Sieving times over 10 minutes can degrade the material under test.

Example: A ¾-in. sieve is checked for sieving sufficiency. The total weight of the sample is 2074.2 grams.

The results indicate that 0.12% of the material passed the ¾-in. sieve during hand sieving. The percentage is less than 0.5%, so the check for sieving sufficiency indicates that the thoroughness of sieving is adequate.

Sample Preparation

The samples should be obtained from the field in accordance with AASHTO T 2 and reduced to the test size in accordance with AASHTO T 248. Samples should be dried to a constant mass in an oven maintained at 110 ± 5 °C (230 ± 9 °F). The test size is determined by the nominal maximum particle size of the aggregate. For fine aggregate, use a minimum of 300 g, and for coarse aggregate, or a mixture of coarse and fine aggregate, use the information in Table 3.

Table 3: Minimum Sample Mass for Coarse Aggregate Samples for T 27

Nominal Maximum Size Aggregate, mm (in.) Minimum Mass of Test Sample, kg (lb.) 9.5 (3/8) 1 (2) 12.5 (1/2) 2 (4) 19.0 (3/4) 5 (11) 25.0 (1) 10 (22) 37.5 (1½) 15 (33) 50.0 (2) 20 (44) 63.0 (2½) 35 (77) 75.0 (3) 60 (130) 90.0 (3½) 100 (220)

Procedure

Step 1 Assemble the proper sieves for the sieve nest from largest sieve on top to the smallest mesh size on the bottom. A pan should be placed underneath and a lid should be placed on top of the sieve stack. If the sample was washed in accordance with AASHTO T 11 prior to running this test, a 75-μm (#200) sieve must be used.

Table 4: Guidance on Sieve Overloading

Sieve Opening (mm) Mass (g) – 8 in. dia. Mass (g) – 12 in. dia. #4 4.75 338 469 ¼ in. 6.3 449 796 ⅜ in. 9.5 677 1, ½ in. 12.5 891 1, ¾ in. 19 1,354 3,

Note: Why avoid overloading sieves? If there is too much material on a given sieve, then each individual particle will not have proper access to the sieve screen, which affects the efficiency of the sieving process. In addition, an excess of material on the sieve may cause undue damage to the sieve mesh, especially on small-sized sieve openings.

If any of the sieves are found to be overloaded, the results of the sieve analysis are invalid and the sample should be retested. Overloading of the sieves can be prevented in a number of ways, including the following:

• Insert an additional sieve with opening sizes that are between the sieve that may be overloaded and the sieve immediately below it.
• Split the sample into two or more portions, sieving each portion individually and combining the results obtained before completing the calculations.
• Utilize sieves with a larger frame size.
• If the material is a mixture of fine and coarse material, the portion of the sample finer than the 4.75-mm (#4) sieve may be distributed among two or more sets of sieves to prevent overloading of individual sieves.
• If the material is a mixture of fine and coarse material, the portion of the sample finer than the 4.75-mm (#4) sieve can be reduced in size in accordance with Section 8.31.5 of AASHTO T 27.

Calculations

The results of the analysis can be calculated as cumulative percent passing, cumulative percent retained, or as a percentage passing based on individual size fractions. Results should be calculated to the nearest 0.1% based on the original dry mass of the sample. If the material was first washed in accordance with AASHTO T 11, include the mass of material finer than the 75- μm (#200) sieve by T 11 as the basis for calculating all percentages.

Step 1 Determine the total mass of the sample (W1). If AASHTO T 11 was performed on the material prior to performing the sieve analysis, W1 will be the initial mass recorded before running AASHTO T 11. If AASHTO T 11 was not performed in conjunction with the sieve analysis, then W1 will be the initial mass of the sample prior to sieving.

Step 2 The method used to calculate the percent of material retained on each sieve will depend upon whether or not the balance is tared between the addition of each size fraction or if the size fractions were weighed cumulatively.

If the balance was tared between the addition of each size fraction Determine the percentages as follows:

Determine the cumulative percent of material retained on each by adding the mass of material of all the larger sieve mesh sizes to the mass of material on sieve x, Sx. Determine the cumulative percent retained as a percentage of the original sample mass, W1:

If AASHTO T 11 was performed on the material prior to sieving, add the percent passing the 75- μm (#200) sieve from T 11 to the percent passing the 75-μm (#200) sieve from the sieve analysis for the total number reported.

If masses were determined cumulatively Determine the percent passing each sieve from the cumulative mass recorded for each sieve

size,

Total Mass after Sieving (D): 2041.1g

Calculations:

Total Minus #200 Material (Pan + C): 57.5 g

Mass Lost During Sieving (G = B - D): 0.4 g

Percent Lost During Sieving ((G/B) x 100): 0.02% (must be < 0.3%)

Note: Notice that in this example, 8-in. diameter sieves were used for testing, and the #8, 50, and 100 sieves were overloaded (i.e., more than 200 g was retained on each sieve). Therefore, these test results are invalid. In this case, the sample should be retested.

Reporting the Test Results

For each sieve, report one of the following, in accordance with the specification being followed: (1) the total percentage of material passing each sieve, (2) the total percentage of material retained on each sieve, or (3) the total percentage of material retained between consecutive sieves.

Percentages should be reported to the nearest whole number. If the percentage of material passing the 75-μm (#200) sieve is less than 10%, report this value to the nearest 0.1%.

The results of AASHTO T 27 are often plotted on a semi-logarithmic graph with the percent passing on the y-axis, and the sieve size plotted on the x-axis (log scale). For the example described previously, the plot would look as shown in Figure 3.

Figure 3: Graphical Plot of AASHTO T 27 Sieve Analysis

Interpreting and Utilizing the Test Results

The results of AASHTO T 27 are used in conjunction with AASHTO M 145 to classify soil and soil- aggregate mixtures. For more information on utilizing sieve analysis results of soil samples, please review the section of the manual on AASHTO M 145.

Common Errors

• The most commonly-observed errors made by technicians performing AASHTO T 27 are as follows:
• The time of sieving is longer than 10 minutes, which may lead to aggregate degradation.
• The sample mass used does not meet the minimum required mass according to the nominal maximum size of the aggregate used.
• More than 0.3% of the original sample mass is lost during the sieving operation.
• The balance is not capable of weighing the sample to 0.1% of the sample mass.
• Sieves are overloaded during the test procedure.