Principles and Common Problems of Soil Density Measurement by Circular Knife Method
In agricultural research, measuring soil density is a very important step. The measurement of soil density can not only accurately determine the basic information of the soil, but also provide important decisions for fertilization, irrigation and planting. There are many methods for measuring soil density, including ring knife method, irrigation method, and sand filling method. Among them, the circular knife method is simple to measure, requires less instruments, and has relatively accurate results, so it is often used in practical operations. This paper gives a detailed introduction to the principle steps, common problems and error analysis for measuring soil density with the ring knife method.
1 The principle and steps of measuring the density by the ring knife method
1.1 Principle of Density Measurement by Ring Knife Method
The ring knife method uses a ring knife of known mass and volume to cut the soil sample so that the volume of the soil sample is the same as the volume of the ring knife. The volume of the ring knife is the volume of the soil. After weighing, the quality of the ring knife is removed. To get the quality of the soil, then calculate the density of the soil.
1.2 steps
1) Determine the mass and volume of the ring knife. The inner diameter and height of the ring knife are measured with a caliper caliper, and the volume of the ring knife is calculated. Then, the ring knife is placed on the balance and the ring knife mass m1 is obtained.
2) Cut soil samples. Apply a thin layer of petrolatum to the inner wall of the ring knife, place the ring knife edge on the surface of the soil sample, use a soil trimmer to cut the soil sample into a soil column slightly larger than the ring knife, then gently press the ring knife downwards, press the edge Cut back until the soil sample is higher than the ring knife. Firstly cut the rest of the soil at the upper end so that the soil surface is flush with the edge of the ring knife, and then place it on the glass plate; then, cut off the residual soil at one end of the cutting edge so that it is flush with the cutting edge of the ring knife. If there is a small amount of exfoliation on both sides of the soil, it can be gently filled with the cut crushed soil.
3) Determine the quality of the ring knife and soil sample. Clean the outer wall of the knife and measure the mass of the ring knife and soil sample m2.
4) Calculation. The formula is:
Where: p is the soil density, g/cm3; m2 is the quality of the ring knife and the soil sample, g; m1 is the ring knife mass, g; v is the soil volume, cm3.
2 ring knife detection main factors
2.1 Operation process
Cutting soil samples is the key step in determining the quality of the ring cutter method. During the sampling of the ring knife, due to the force conditions such as force size and direction of force, the density of the soil samples changed, as well as the detection of the ring knife due to the self-gravity of the soil, operation of soil cutting, and test records. Influences make the test result biased.
2.2 representative of the sample
It is particularly pointed out here that when there are many soil samples with mixed layered soil and thickness, there will be insufficient representativeness of the sample taken and errors will occur. In particular, there are original soil embankments and transported soil at the construction site, and it is very easy for the two to mix or mix other miscellaneous materials during the construction process.
On the other hand, there is a great difference in the water content of soil samples in the same tool body, which makes the test representation insufficient. Under normal circumstances, 15-30 g is required for cohesive soils. For sand and gravel soils, due to the great difference in water content, it is stipulated that the soil is weighed at 50 g. However, due to the fact that some construction sites are far from the laboratory, soil samples are Improper protection during transportation and storage occasionally evaporates part of the water and affects the test results.
2.3 Test apparatus
At the scene, special aluminum boxes are required to hold the samples. Although each aluminum box is generally a constant weight aluminum box of 22.40g, it will wear out due to long-term use, and some of the impurities will be attached to the oxidized surface to change its quality. Weighing the use of electronic weighing apparatus due to prolonged use may also cause drift, especially in the event of wind weather will occur about 0.05g drift or even more; as well as such as the construction site will produce strong vibration will also make the electronic meter obviously Drift phenomenon.
2.4 Data revision
The data reduction practice actually refers to how to organize data and statistics.
From the perspective of the rules for data rounding, the following three aspects may be the cause of the error:
1) When the leftmost digit of the digit to be discarded is less than 5, it is discarded, that is, each reserved digit does not change.
2) The left-most digit of the number to be discarded is greater than 5 or 5, and the digits followed by not all zeros are further rounded, ie the last digit retained is incremented by 1.
3) The leftmost digit of the digit to be discarded is 5, and when there is no digit or all 0 on the right, if the last digit retained is odd (1, 3, 5, 7, 9), the next digit is even, which is even (2,4,6,8,0) are discarded.
For example, when the 1.050 repair interval is 0.1 or 10-1, the value is 1.0; when the 0.300 trim interval is 0.1 or 10-1, the value is 0.4; and when the 1.25 trim interval is 0.1 or 10-1, the value is 1.2.
3 Countermeasures for Influencing Factors
3.1 Operation process
In the ring knife test to detect the sampling process, uniform force and accurate direction of force are crucial. With the coordinated cooperation of both hands, when the top surface of the soil is smoothly connected with the cover, the hammering is obviously difficult, and this is the best condition. Avoid soil being artificially compacted.
3.2 representative of the sample
In the case of soil layers with large soil changes in the natural soil layer, double sampling is required to standardize the quantity and frequency of detection, and the scope of the sampling points will be expanded to minimize unfavorable risks. In the case of special soil containing water, take a double sample in the same body to test the moisture content, and take the average value as the final test result. When the soil sample is loaded into the aluminum box, it is quickly sealed and gently put into the storage box. , reduce moisture evaporation as much as possible.
3.3 Test Container
The long-term use of the aluminum box is worn out. Before the test test, the carried aluminum box is weighed one by one using a precision balance, and the aluminum box with the deviation greater than 0.05g is removed; the doors and windows are closed when the indoor sample is weighed, and the door and window of the test room are avoided. Flow of the wind; when dropping the test piece, wear gloves and use your fingers to rotate around the aluminum box for one to two weeks to remove all the residual dirt from the inner wall of the aluminum box and keep the aluminum box dry and tidy; the oxidized aluminum box is removed.
When weighing the sample on the spot, place the electronic measuring instrument on the surface of the wind, or make a closed tin box in advance. Weigh the sample on the ground with a leveling scale and ensure the stability of the weighing instrument.
3.4 Data revision
According to experience summaries: when data is collated, the problem is repaired, the measured data is included in the excel table, the next digit of the retained value is displayed in the last column, and the situation involving carry-and-trade is checked. Usually, the excel program automatically operates. 5 into, and according to the data revision requirements encountered five rounds of choice not fully rounded down. In the case of a 5-carriage trade-off, format conditions are set in excel. The left one is an odd number carry, and the left one is an even number non-carry not only improves work efficiency, but also improves work quality. People can only check the results.
4 Conclusion
After the above four points of analysis, the error in the ring knife test can be avoided and reduced during the work process. The factors influencing the detection of the ring knife method mainly include the operation process, the representativeness of the sample, the experimental apparatus, and the data revision and so on. They are the unity of the organic combination and are closely connected with each other. When the detection results are abnormal, do the representative adjustment of the soil sample, increase the detection point group, and use the probability method to reduce the error of the actual measurement value. Inspection instruments regularly check, check and check the status of the instrument before testing. Improve the accountability of operators, standardize operations, and regular data revision. Combined with many years of test data, the circular knife method density test is a better method for detecting the degree of compaction of embankments, and can reflect the actual compaction level of the embankment site in combination with the actual situation on the site.
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