Roadbed compaction testing method status and comparison test?

At present, the cause of highway has attracted general attention in countries all over the world, and the construction of high-grade highway in China is also booming. Roadbed compaction testing technology, as the key and basic technology in highway construction and management, is not only crucial for controlling the quality of the project, but also determines the scientificity of the road network maintenance decision-making, and directly affects the reasonableness of the distribution of maintenance funds. The quality of the project not only directly affects the interests of the construction unit or contractor, but also has a pivotal impact on the development of the transportation industry. The quality of the project is assessed by the test data, and whether the test data is accurate and reliable depends largely on whether the testing instruments and testing methods are advanced and reasonable. In this paper, on the basis of the evaluation of the current roadbed compaction testing method, through the roadbed compaction testing method comparison test, analyze the advantages and disadvantages of different testing methods, and analyze the problems of the current roadbed compaction standards.

I. Evaluation of current roadbed compaction testing methods

1.1 Sand filling method

Sand filling method is the most common method at present, and a lot of projects have listed sand filling method as the main method for on-site density determination. Its disadvantages are: the need to carry a larger amount of sand, and weighing more often, so it is slower testing. This method appears to be relatively simple, but the actual operation is often not good at mastering, and will cause large errors. In order to make the test results accurate, should pay attention to the following links:

(1) measure sand to rule. Measurement of sand if reused, must pay attention to drying, otherwise affect the measurement of sand density of loose square. Every time you change the measuring sand, you must determine the loose square density, the number of sand in the funnel should also be redone every time.

(2) the ground surface treatment should be flat, placing the substrate; in digging the pit when the test pit perimeter wall should be straight, to avoid the situation where the top is big and the bottom is small or the top is small and the bottom is big.

(3) sand filling test thickness should be the entire milling layer thickness, can not only take the upper part or take to the next milling layer.

1.2 Ring Knife Method

The ring knife method is the traditional method of measuring density in the field. Since the density of a compacted soil layer generally decreases from top to bottom, the density of the soil should be determined by the ring knife method in such a way that the measured density is representative of the average density of the entire compacted layer. However, this is more difficult in the actual test, only so that the ring knife taken by the soil is exactly the soil in the middle of the compacted layer, the results of the ring knife method and the results of the sand filling method may be roughly the same. In addition, the ring knife method is narrower, for the stabilized soil containing granular materials and loose materials can not be used.

1.3 Nucleon Densitometer

The nucleon densitometer method utilizes radioactive elements (usually yah rays and neutron rays) to measure the density and water content of the soil or pavement material. This type of instrument is characterized by fast measurement and requires few personnel. Its disadvantage is that the radioactive material is harmful to the human body, in addition to the need to hit the hole, in the process of hitting the hole is easy to make the hole wall near the structure of the soil was damaged, affecting the accuracy of the measurement, can be used for construction control, but need to be compared with the conventional method to verify its reliability.

1.4 Falling Hammer Spectrum Roadbed Compaction Rapid Determination Instrument (LY-1 Instrument)

Falling Hammer Spectrum Roadbed Compaction Rapid Determination Instrument is to use the impact of the falling hammer to make the soil body to produce rebound force, and the use of low-frequency measurements of the soil body response value of the roadbed can be obtained without measuring the water content of the compaction of the test instrument. Before the determination of this method, the calibration of the roadbed compaction curve is very important, and the selected soil type should be representative of the project.

Two, the existing roadbed compaction standard problem analysis

In the test section of the soil base on the four methods of compaction testing, the location of the measurement point corresponds to each other. The results are shown in Table 1.

Table 1 Summary of compaction results of soil base measured by different methods

The sand filling method is used as the standard method to determine the density in the field, and other methods are calibrated by regression with the sand filling method. The standard method of water content test is the drying method, the water content determined by the nuclear density meter shall be compared with the drying method, and the water content shall be converted to the water content of the drying method by the correlation equation. Its its statistical scatter plot is shown in Figure 1.

Figure 1 Comparison of soil compaction measured on site by different methods

From Figure 1, it can be seen that the compaction determined by the ring knife method and the falling hammer spectral roadbed compaction rapid measuring instrument correlates well with the sand filling method, while the compaction determined by the nucleon densitometer correlates poorly with the sand filling method. The determination of water content of ring knife method and sand filling method both use drying method, and both results are basically the same. The correlation between the water content determined by the nucleon densitometer and the sand filling method was poor.

From the above analysis, it can be seen that there are some differences in the results of the four methods. The sand filling method, as a standard method to determine the density in the field, can better reflect the compaction condition of the soil base because the depth of the hole is usually the thickness of the crushed layer, and the quantity of sand in the sand storage cylinder is constant each time it is applied, and the height of the sand falling and the falling speed of the sand are constant, so it has high precision and good accuracy of measurement. However, from the results, there are three compaction exceeding 100%, one is related to the determination of the maximum dry density, and the other is also related to the operation of the test, so it is necessary to strictly follow every detail of the test protocol to improve the accuracy, and at the same time should carry out the discussion of the compaction standard of the roadbed. The ring cutter volume is small, as long as the ring cutter is made to take soil just in the middle of the compacted layer, the ring cutter method determination results are comparable to the sand filling method. Nucleus

The results measured by the instrument are not stable, and its value is smaller than other methods, and the correlation between its compaction results and the sand filling method is poor, which may be mainly due to the process of drilling holes in the destruction of the soil structure near the wall of the holes, thus affecting the accuracy of the measurements; and the correlation between its water content results and the drying method is also poor, which is mainly due to the water content measured by the nucleus density meter is a certain depth and an area of the average water content within this area. This is mainly due to the fact that the water content measured by the nucleon density meter is the average water content in a certain depth and a certain area, which is a much larger range than that of the drying method, so only when the water content and compactness of the soil base are very uniform, the two measurements can be consistent. The results of the Falling Hammer Spectral Rapid Determination of Subgrade Compaction are stable. However, as can be seen from Figure 1, the results of the first three tests have good consistency, and the undulation of the compaction results is large, but the falling hammer spectrum type roadbed compaction rapid measuring instrument has some differences with the compaction trend of the first three methods, with less undulation.

So, through the comparative analysis, it is believed that the sand filling method as a standard method has its own reasonableness, but the test operation should be strictly required: the ring knife method has more influencing factors, and the soil in the middle of the crushed layer should be taken; the results of the nucleus meter are unstable, and it can only be used as a quick assessment for the construction: the results of the falling hammer spectral roadbed compaction rapid tester are more stable, but its applicability to the soil and its accuracy need to be further explored.

Three

Three, roadbed compaction testing comparison test

China's own construction of high-grade highways, some of which produced early damage, a significant portion of which is due to the causes of the roadbed, such as the uneven settlement of the roadbed resulting in localized subsidence of the pavement and longitudinal cracks and so on. These problems occur for many reasons, in addition to construction management reasons, the current compaction testing methods and standards are improper is also one of the important reasons.

China's current roadbed using the dry density ratio of compaction as a compaction standard, and the different layers of the roadbed provides a different compaction value. Engineering practice shows that the current testing methods and standards have the following problems:

(1) the specification provides for sand filling method as the standard method of field density determination, this method seems to be relatively simple, but the actual operation is often not good to grasp, and will cause a large error, the influence of human factors is very large, the determination of the results of the greater variability.

(2) the soil quality of the road section, the indoor compaction test cycle is longer, it is difficult to guide the construction in time, even if the test is done in advance, the maximum dry density of the selection of human factors is also very large;

(3) on the control of water content, in the process of compaction of the roadbed on the control of how to minimize the compaction of compaction to get the compaction of the specification, although in the "Technical Specification for Roadbed Construction" (JTB), the compaction of roadbeds is not the only method, but it is the only method. Technical Specification for Roadbed Construction" (JTJ034-93) stipulates that "fine-grained soil, sandy soil and gravelly soil, no matter what kind of compaction machinery is used, should be compacted within 2% of the optimum moisture content of the soil", but in the "Highway Engineering Quality Inspection and Evaluation Standards" (JTJ071-94) in the acceptance of the earthwork foundation does not have specific requirements for moisture content. Specific requirements for water content, without comprehensive consideration of the dry density and water content of the roadbed molding two factors. Roadbed molding water content and compactness of the roadbed to achieve or close to the stability of the compactness has a fairly important role, so only emphasize the compactness and dilute the roadbed molding water content on the stability of the impact is obviously not comprehensive;

(4) test proved that the fine-grained components of the soil content is small (such as sandy soil and plasticity index is not big on the sand heel, gravel soil), in the construction of the rolling, the density of its water content Insensitive, can relax the control of water content; fine-grained components of the soil (such as clayey soil, especially the plasticity index of the viscous), water infiltration so that the water content of the body increases, the volume of the volume of the expansion occurred significantly, and the expansion rate and the initial water content is related to the initial water content, the smaller the initial water content, the greater the rate of expansion, which caused the cracking of the soil body. Therefore, for the soil with many fine-grained components, it is not suitable to be compacted under the condition of small water content, and the current compaction method can not correctly reflect this characteristic of clayey soil in contact with water, and does not take into account the effect of water content on different soil qualities, which is obviously unreasonable;

(5) rock block material is applied in mountainous areas of the road in a large number of applications, and it is impossible to use the compaction degree to measure the compaction of the good or bad;

(6) for the high-fill road embankment

(6) For high-fill embankments, the higher the fill, the greater the autogravitational force on the lower soil, but the specification of the lower layer of compaction (90%) is lower than the upper layer (95%), which is inconsistent with its force and stability law.

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