AASHTO T 215-2023
Permeability of Granular Soils (Constant Head)
- Standard No.
- AASHTO T 215-2023
- Release Date
- 2023
- Published By
- American Association of State Highway and Transportation Officials
- Latest
- AASHTO T 215-2023
- Introduction
AASHTO T 215-23 Technical Analysis
Standard T 215-23, published by the American Association of State Highway and Transportation Officials (AASHTO), is the core technical specification for determining the permeability of granular soils in geotechnical engineering. This standard, which underwent technical and editorial revisions in 2023 and is maintained by the Soils and Unconstrained Recycled Materials Technical Subcommittee 1a, provides key parameters for drainage design in highway infrastructure.
Scope and Limitations
This test method specifically determines the water permeability of cohesionless granular soils under steady-state conditions. It is based on the fundamental assumption of Darcy's law, which defines the permeability coefficient, k, as the ratio of the flow rate to the hydraulic gradient. The standard is specifically limited to disturbed granular soils containing no more than 10% of the particles passing a 75 μm sieve (No. 200 sieve).
Soil characteristics Applicable conditions Alternative methods Technical limitations Fine particle content ≤10% Fully applicable — Meets standard requirements Fine particle content 10-14% Liquid limit ≤25 and plasticity index ≤5 To be verified Need to ensure no consolidation effect Fine particle content >14% Not applicable ASTM D5084 Flexible Wall Penetrometer Risk of consolidation
Test Method Technology Comparison
The standard provides two test methods: Method A and Method B, each corresponding to a different permeameter configuration. Method A utilizes a traditional constant-head permeameter system, while Method B uses a Trautwein permeameter. The two methods differ significantly in their equipment construction and measurement principles.
Technical Features Method A Method B Applicable Scenarios Permeameter Type Rigid Wall Constant Head Permeameter Trautwein Permeameter Depending on laboratory equipment configuration Head Control Constant Pressure Water Tank + Piezometer Bubble Tube + Tailwater Container Method B is more suitable for field application Specimen Size Requirements Minimum diameter according to Table 1 76mm/101.6mm/152mm Selection based on maximum particle size Saturation Method Vacuum saturation from bottom to top Water saturation Both must reach 98-100% saturation
Key Equipment Technical Requirements
The core of the test equipment is the permeameter system, which includes the sample tube, porous disc, pressure measurement system, and constant head device. The standard sets strict requirements for equipment specifications, especially the sample tube diameter, which must be selected according to the maximum particle size as specified in Table 1.
Porous Disc Technical Requirements
The upper and lower ends of the sample must be covered with a porous disc made of silicon carbide, aluminum oxide, or a similar corrosion-resistant material. The porosity must be fine enough to prevent soil particle intrusion, and the permeability must be at least one order of magnitude higher than that of the soil being tested. Method A requires a top perforated disk equipped with a spring mechanism that applies a light pressure of 22-45N.
Temperature Measurement Accuracy
The standard specifies the use of a thermometer to measure water temperature. Liquid-in-glass thermometers, platinum resistance thermometers, or metal-sheathed thermistors are permitted, but the measurement accuracy must be within 0.5°C (1.0°F) and must be calibrated regularly using an ice point calibration.
Specimen Preparation and Compaction Control
Specimen preparation is critical to ensuring the accuracy of test results. The standard details a layered packing technique with a layer thickness approximately equal to the maximum particle size or 15mm, whichever is greater. Different filling methods are used for soils of different particle sizes:
Soils with a maximum particle size of 9.5mm or less
Use a funnel equipped with a cylindrical nozzle with a diameter of 13mm (No. 10 sieve) or 25mm (3/8-inch sieve) and a length greater than the full length of the sample tube or a minimum of 150mm. Use a slow spiral motion to evenly distribute the soil from the periphery toward the center.
Soils with a maximum particle size >9.5mm
Use a shovel to fill the sample tube by sliding it along the inner surface, tilting the soil from the periphery toward the center in a single motion to prevent particle separation. Compact as needed after each layer.
Relative Density Control Methods
The standard provides three compaction methods to achieve different relative densities: a vibratory tamper, a sliding weight tamper, or other suitable equipment. The compaction process must not cause individual soil particles to break, fracture, or deform.
Saturation and Test Procedure
Specimen saturation is the most critical step in the permeability test. The standard requires applying a minimum vacuum of 50 cm of mercury using a vacuum pump or tap aspirator for at least 15 minutes to remove air trapped in soil particles and pores. Saturation should be performed slowly from the bottom up under continuous vacuum, using degassed water or connate water with a low mineral content.
Determination of the Laminar Flow Region
The standard recommends repeated measurements under increasing hydraulic heads to accurately determine the laminar flow region and ensure that the flow rate is proportional to the hydraulic gradient. The initial test should be conducted under stable head conditions with no significant piezometer water level drift.
Calculation and Temperature Correction
The permeability coefficient k is calculated based on Darcy's law: k = QL/(Ath), where Q is the drainage volume, L is the piezometer spacing, A is the specimen cross-sectional area, t is the drainage time, and h is the piezometer head difference.
The standard requires that the measured permeability coefficient be corrected to the value at a standard temperature of 20°C using the viscosity ratio RT = 0.1702 + 0.9842/T - 2.290/T², where T is the average water temperature (°C) during the test.
Engineering Application and Quality Control
Granular soil permeability data is critical to road foundation design. Inadequately drained soils can lead to excess pore water pressure, which ultimately causes pumping distress under load. Pumping distress is the forced injection of waterborne soil from the subgrade beneath cement concrete pavement and is particularly common on high-volume roads subject to heavy axle loads.
Quality Control Requirements
The standard emphasizes that the quality of test results depends on the competence of the personnel performing the procedure and the performance, calibration, and maintenance of the equipment used. While organizations that comply with R 18 are generally considered competent and objective testers, compliance with R 18 alone does not guarantee reliable results.
Standard Implementation Recommendations
Based on the technical characteristics of the T 215-23 standard, the following implementation recommendations are proposed:
- Equipment selection and matching: Select method A or method B according to laboratory conditions and test frequency. The Trautwein permeameter of method B is more suitable for on-site rapid testing
- Sample representativeness: Ensure that no particle separation occurs during sample preparation, and check the uniformity of the sample after filling
- Saturation verification: After the test, measure the water content through T 265 to verify that the saturation reaches the range of 98-100%
- Temperature control: Strictly control the test water temperature and perform temperature correction in time to ensure data comparability
- Quality control system: Establish a complete equipment calibration and quality control procedure to meet the requirements of R 18 and R 61
Through strict implementation of T 215-23 standard, engineers can obtain reliable granular soil permeability parameters, providing a scientific basis for road drainage design, roadbed stability analysis and pavement performance prediction.
AASHTO T 215-2023 Referenced Document
- AASHTO M 231 Standard Specification for Weighing Devices Used in the Testing of Materials*， 2025-10-23 Update
- AASHTO R 18 Establishing and Implementing a Quality Management System for Construction Materials Testing Laboratories
- AASHTO R 61 Standard Practice for Establishing Requirements for Equipment Calibrations, Standardizations, and Checks*， 2025-10-23 Update
- AASHTO T 180 Standard Method of Test for Moisture–Density Relations of Soils Using a 4.54-kg (10-lb) Rammer and a 457-mm (18-in.) Drop*， 2025-01-01 Update
- AASHTO T 265 Standard Method of Test for Laboratory Determination of Moisture Content of Soils*， 2024-05-19 Update
- AASHTO T 88 Standard Method of Test for Particle Size Analysis of Soils*， 2024-05-19 Update
- AASHTO T 89 Standard Method of Test for Determining the Liquid Limit of Soils*， 2024-05-19 Update
- AASHTO T 90 Standard Method of Test for Determining the Plastic Limit and Plasticity Index of Soils*， 2024-05-19 Update
- AASHTO T 99 Standard Method of Test for Moisture–Density Relations of Soils Using a 2.5-kg (5.5-lb) Rammer and a 305-mm (12-in.) Drop*， 2025-01-01 Update
AASHTO T 215-2023 history
- 2023 AASHTO T 215-2023 Permeability of Granular Soils (Constant Head)
- 2024 AASHTO T 215-2022 Standard Method of Test for Permeability of Granular Soils (Constant Head)
- 2014 AASHTO T 215-2014 Standard Method of Test for Permeability of Granular Soils (Constant Head)
- 1970 AASHTO T 215-1970 Standard Method of Test for Permeability of Granular Soils (Constant Head)
