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D6391-11 – Standard Test Method Technical Guide
📐 Overview and Scope
ASTM D6391‑11 (Reapproved 2020) establishes a standard test method for field measurement of hydraulic conductivity using borehole infiltration. This method is designed for compacted fills and natural deposits, both above and below the water table, with a mean hydraulic conductivity ≤ 1 × 10⁻⁵ m/s (1 × 10⁻³ cm/s). The test employs a cased borehole technique to assess the coefficient of permeability of porous materials.
In isotropic conditions the method directly yields the hydraulic conductivity of the material. For anisotropic conditions a single stage provides an upper limit for vertical hydraulic conductivity, while a second stage offers a lower limit for horizontal conductivity. Determining the actual hydraulic conductivity in anisotropic settings requires further analysis by qualified personnel. All values are reported in SI units, with parentheses for reference. The standard emphasizes conformance to Practice D6026 for significant digits and rounding.
| 🟦 Condition | 📏 Hydraulic Conductivity (m/s) | 🎯 Test Method |
|---|---|---|
| Mean conductivity ≤ 1 × 10⁻⁵ | ≤ 1 × 10⁻⁵ | D6391‑11 (cased borehole) |
| Mean conductivity > 1 × 10⁻⁵ | > 1 × 10⁻⁵ | Ordinary borehole tests (e.g., USBR 7310) |
⚙️ Test Procedure and Key Concepts
A key distinction in this test method is between “saturated” (Ks) and “field‑saturated” (Kfs) hydraulic conductivity. During infiltration events or leaks, field‑saturated conditions develop, where entrapped air prevents full water movement. This entrapped air can reduce the measured hydraulic conductivity by as much as a factor of two compared to true saturated conditions. The method specifically develops the field‑saturated condition.
The test is most effective in materials with a degree of saturation of 70 % or more and relatively horizontal stratification or compaction planes. Use in other conditions is considered experimental.
| 🟦 State | 📐 Description | ⚡ Impact on Conductivity |
|---|---|---|
| Saturated (Ks) | True saturation without entrapped air | Baseline, rarely achieved |
| Field‑Saturated (Kfs) | Develops during infiltration; includes trapped air | Reduced by up to 50 % |
⚠️ Field‑saturated conditions are realistic for most vadose zone scenarios. Due to entrapped air, the hydraulic conductivity can be up to 50 % lower than in true saturated conditions, as noted in the standard.
📊 Data Interpretation and Requirements
The results from this test method are specific to the volume of soil permeated during the test. Extending these results to larger areas requires multiple tests and the judgment of qualified personnel. The number of tests depends on factors such as the size of the area, the uniformity of the material, and the variation in test data.
All observed and calculated values must conform to the significant digits and rounding guidelines of Practice D6026. The procedures for data collection and calculation are industry standard, but users may adjust significant digits based on their objectives, with engineering design analysis beyond this standard’s scope.
📊 The hydraulic conductivity measured pertains only to the soil volume tested. For site‑wide assessment, multiple tests and professional interpretation are essential.
❓ Frequently Asked Questions
🔍 What is the scope of ASTM D6391‑11?
The standard covers field measurement of hydraulic conductivity using a cased borehole technique for porous materials with mean hydraulic conductivity ≤ 1 × 10⁻⁵ m/s, applicable to compacted fills and natural deposits above or below the water table.
💡 How does the test address anisotropy?
In anisotropic conditions a single‑stage test yields an upper limit for vertical hydraulic conductivity, while a second stage provides a lower limit for horizontal conductivity. Actual values require further analysis by qualified personnel.
⚡ What is field‑saturated hydraulic conductivity?
Field‑saturated (Kfs) conditions occur during infiltration, accounting for entrapped air that reduces conductivity by up to 50 % compared to fully saturated (Ks) conditions. This method measures Kfs, which is more realistic for field scenarios.
📌 What materials are best suited for this test?
The method is designed for materials with a degree of saturation of at least 70 % and relatively horizontal stratification or compaction planes. Use in other conditions is experimental.