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D2325-68 – Standard Test Method Technical Guide
📐 Test Overview and Scope
ASTM D2325-68 (Reapproved 2000) specifies the standard test method for determining capillary-moisture relationships in coarse- and medium-textured soils. This method applies to soil-moisture tensions ranging from 10 to 101 kPa (0.1 to 1 atm). It involves placing saturated soil samples on a saturated porous plate within a pressure chamber and applying controlled air pressure to achieve equilibrium moisture content at specific tensions.
This test is essential for understanding soil-water behavior but is not suitable for fine-textured soils, which require Test Method D3152 using a pressure-membrane apparatus.
⚙️ Apparatus and Test Procedure
The apparatus consists of a pressure container, typically a 15-L capacity pressure cooker, fitted with one or more porous ceramic plates. Each plate has an air entry value of 203 kPa (2 atm) and measures 280 mm in diameter and 6 mm in thickness. The bottom of each plate is sealed with a rubber membrane to maintain airtight conditions, and a brass spout provides drainage with specific dimensions as shown below.
| 🟦 Parameter | 📏 Value (Metric) | 📐 Value (Imperial) |
|---|---|---|
| Plate Diameter | 280 mm | 11 ¼ in |
| Plate Thickness | 6 mm | ¼ in |
| Air Entry Value | 203 kPa | 2 atm |
| Brass Spout Tube ID | 1.7 mm | 1/16 in |
| Brass Spout Unthreaded Length | 9.5 mm | 3/8 in |
| Brass Spout Threaded Length | 15.8 mm | 5/8 in |
During the test, saturated soil samples are placed on the plates, and air pressure is introduced into the chamber. The pressure difference causes water to flow from the soil through the plate and out the drain tube. Equilibrium is reached when water flow ceases, and the moisture content of the soil is then determined. Multiple tests at various tensions are performed to construct the complete capillary-moisture relationship curve.
✅ Key Point: Ensure the porous plates are fully saturated before use to maintain accurate tension control and reliable results.
⚠️ Safety Advisory: Users must establish appropriate safety practices when operating pressure chambers. Refer to regulatory requirements for safe handling of pressurized equipment.
❓ Frequently Asked Questions
🔍 What types of soils are suitable for this test method?
This test method is designed for coarse- and medium-textured soils. It is not suitable for fine-textured soils, which should be tested using Test Method D3152.
💡 What is the tension range applied in this test?
The test covers tensions between 10 and 101 kPa (0.1 to 1 atm) as specified in the standard.
⚡ What is the air entry value of the porous plates?
The porous ceramic plates have an air entry value of 203 kPa (2 atm), ensuring they can withstand the applied pressures.
📌 How is equilibrium determined in the test?
Equilibrium is achieved when water ceases to flow from the soil samples and porous plate, indicating that the soil moisture content is stable under the applied tension.