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D5855 – Standard Test Method Technical Guide
📚 Scope and Terminology
This practice, ASTM D5855/D5855M−20, provides an analytical procedure for determining transmissivity and storage coefficient of confined aquifers (leaky or nonleaky) using the constant drawdown method in flowing wells. The standard is under the jurisdiction of ASTM Committee D18 on Soil and Rock and is essential for groundwater investigations.
Key terms are defined in accordance with Terminology D653. The principal symbols used are:
| 🟦 Symbol | 📏 Parameter | 📐 Dimensions |
|---|---|---|
| T | Transmissivity | L² T⁻¹ |
| S | Storage Coefficient | Dimensionless |
⚙️ Test Procedure and Analysis
The analytical procedure is used in conjunction with the field test described in Practice D5786. During the test, a constant head is maintained in the control well, and the flow rate is recorded over time. This data is then analyzed using the solution provided in this standard to compute aquifer properties.
Limitations of this technique are primarily related to the correspondence between the field situation and the simplifying assumptions of the solution. All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D6026.
📊 Key Measured Properties
The transmissivity (T) and storage coefficient (S) are the key properties determined. Values may be reported in either SI units or inch-pound units, which must be regarded separately as standard. The following table shows typical unit systems:
| 📏 Parameter | ⚡ SI Unit | 🎯 Inch-Pound Unit |
|---|---|---|
| Transmissivity | m²/s | ft²/s |
| Storage Coefficient | Dimensionless | Dimensionless |
| Flow Rate | m³/s | ft³/s |
Important Limitation: The accuracy of results depends on how well the field conditions match the model assumptions. This practice should be used in conjunction with professional judgment, as not all aspects may be applicable in all circumstances.
Unit Consistency: The values in each unit system may not be exact equivalents; therefore each system shall be used independently. Combining values from the two systems may result in non-conformance with the standard.
❓ Frequently Asked Questions
🔍 What is the constant drawdown method? It is a test method where the water level in a flowing well is held constant while the flow rate is measured to determine aquifer properties such as transmissivity and storage coefficient.
💡 How is the storage coefficient interpreted? The storage coefficient is a dimensionless parameter that indicates the volume of water released from storage per unit decline in hydraulic head, per unit area of aquifer.
⚡ What are the key assumptions of this analytical solution? The solution assumes a confined, homogeneous, isotropic aquifer of infinite extent, with constant drawdown at the well and consistent well radius.
📌 Which ASTM standards are directly referenced? Key referenced standards include D653 (Terminology), D3740 (Quality Assurance), D4043 (Test Selection), D5786 (Field Procedure), and D6026 (Significant Digits).