Soil Moisture Measurement
GB 7172-87
1. Scope of Application
This standard is designed to determine the moisture content in all types of soil, with the exception of gypsum soil and organic soil that contains more than 20% organic matter. It is widely used in geotechnical engineering, agricultural studies, and environmental assessments to ensure accurate moisture measurements for various soil samples.
2. Measurement Principle
The principle behind this method is based on the weight loss of a soil sample when it is dried at a constant temperature of 105 ± 2°C until it reaches a constant weight. This weight loss corresponds to the mass of the moisture present in the soil sample, providing an accurate measure of its water content.
3. Instruments and Equipment
3.1 Soil auger: For collecting undisturbed soil samples from the field.
3.2 Sieve: With a pore size of 1 mm, used to screen the soil sample for uniformity.
3.3 Aluminum boxes: Two sizes are required—small (diameter ~40 mm, height ~20 mm) and large (diameter ~55 mm, height ~28 mm). These are used to hold the soil samples during drying and weighing.
3.4 Analytical balance: Equipped with a sensitivity of 0.001 g for precise measurements.
3.5 Electric oven: A small, controlled-temperature oven set to 105 ± 2°C for drying the samples.
3.6 Desiccator: Filled with silica gel or anhydrous calcium chloride to keep the samples dry during cooling.
4. Sample Selection and Preparation
4.1 Air-dried soil samples: Select representative air-dried soil, crush it, pass it through a 1 mm sieve, and mix thoroughly before use.
4.2 Fresh soil samples: Collect fresh soil in the field using a soil auger. Remove the top layer of surface soil, take about 20 g from the middle of the core, place it into a pre-weighed aluminum box, seal it tightly, and transport it to the laboratory. Clean the box, weigh it immediately, and proceed with moisture analysis as soon as possible.
5. Measurement Procedure
5.1 Determination of Moisture in Air-Dried Soil Samples
Place the small aluminum box in the oven at 105°C for approximately 2 hours. Transfer it to a desiccator and cool to room temperature. Weigh the box accurately to 0.001 g. Add about 5 g of soil, spread it evenly, cover the box, and reweigh. Bake the sample at 105 ± 2°C for 6 hours, then cool again in the desiccator and weigh immediately. Perform two parallel determinations for air-dried samples.
5.2 Determination of Moisture in Fresh Soil Samples
Weigh the large aluminum box containing fresh soil to 0.01 g. Uncover, place it in the oven preheated to 105 ± 2°C, and bake for 12 hours. Cool in the desiccator and reweigh. Conduct three parallel tests for fresh soil samples.
Note: Once the sample has reached a constant weight after baking, it is considered fully dried.
6. Calculation of Results
6.1 Formula for Calculation
Moisture (on an analytical basis), % = [(m1 - m2) / (m1 - m0)] × 100 ....................................(1)
Moisture (on a dry basis), % = [(m1 - m2) / (m2 - m0)] × 100 ....................................(2)
Where:
m0 – Mass of the empty aluminum box, in grams
m1 – Mass of the aluminum box and soil before drying, in grams
m2 – Mass of the aluminum box and soil after drying, in grams
6.2 The results of parallel determinations should be expressed as the arithmetic mean, rounded to one decimal place.
6.3 The difference between parallel results must not exceed 0.2% for air-dried soils with less than 5% moisture, 0.3% for wet soils with 5–25% moisture, and 0.7% for soils with large particles (>10 mm) and high moisture content. This ensures accuracy and reliability in moisture measurement.
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