1 preface compactness is one of the most important internal indexes of subgrade engineering quality. Only by fully compacting the subgrade can the strength and overall stability of the subgrade be guaranteed and the service life of the highway be guaranteed and extended. The on-site detection methods of subgrade compactness mainly include sand filling method, circular knife method, nuclear method and water bag method. According to the actual construction situation and the owner's requirements, the sand filling method is mainly used to detect the compactness of Shi Man expressway subgrade. Combined with engineering practice, some problems in subgrade compactness detection are briefly analyzed and discussed. 2 The basic principle of sand filling method The basic principle of sand filling method (standard method, but not suitable for compaction test of materials with large holes or pores such as rock-filled embankments) is to use clean and uniform sand with a particle size of 0.30~0.60mm or 0.25~0.50mm to freely fall into the test hole from a certain height, measure the volume of the test hole according to the principle that its bulk density remains unchanged (that is, replace the aggregate in the test hole with standard sand), and calculate the third sand filling according to the water content. When the maximum particle size of the sample is less than 15mm, and the thickness of the test layer is less than 150mm, a small sand filling pipe with φ 100mm should be used for testing. (2) When the maximum particle size of the sample is equal to or greater than 15mm, but not greater than 40mm, and the thickness of the measured layer is not greater than 150mm, but not greater than 200mm, the test is carried out with a large sand filling pipe with φ150 mm. (3) The maximum particle size of the aggregate reaches 40 mm ~ 60mm or greater than 600 mm. The diameter of sand filling tube and field trial hole is 200 mm ... φ 150mm sand filling tube is widely used in the construction site, its depth is 150mm, and the measured compaction degree is only 150mm. However, the thickness of the compaction layer is often around 200mm, and the compaction surface is generally relatively high, which is difficult to guarantee. Therefore, it is advisable to use φ200mm large sand filling pipe to detect aggregates with more gravels in mountain areas. 3.2 Influence of calibration accuracy of indoor sand measurement on compactness (1) Influence of sand surface height and total weight of sand in sand storage barrel on sand measurement density The height and quality of sand in barrel are clearly specified in Code for Highway Geotechnical Test (JTJ 05 1-93). The distance between the height of sand in the barrel and the top of the barrel is not more than 15mm, because the falling speed of sand with different sand surface heights is different, so the density of sand poured into the calibration tank is also different, which directly affects the density of measured sand. Therefore, the height of sand surface in sand storage barrel must be strictly controlled; In addition, the mass of sand in the cylinder is accurate to1g. This quality remains constant during each calibration and subsequent tests. Because when calibrating, as long as the total weight of sand is the same, that is, the self-weight of sand is the same, it is obvious that its falling speed can be kept consistent, thus improving the accuracy of sand measurement. Practice has proved that the height and weight of sand surface in sand storage barrel are consistent with indoor calibration, which greatly improves the accuracy of test data. (2) The influence of the depth of the calibration groove on the sand density. The test results show that when the depth of the calibration tank is reduced by 65438±0cm, the density of sand is reduced by about 65438 0.2%. It can be seen that different depths have great influence on sand density. Therefore, the depth of the field trial hole should be as consistent as possible with the depth of the indoor calibration groove. (3) The influence of sand grading composition on sand density measurement. Sand particles with different particle sizes have different gradations and densities, so standard sand (0.30~0.60mm or 0.25~0.50mm) must be used for each sand measurement, and the sand particles should be kept clean and dry. It can be seen from the above that the height of sand surface in sand storage barrel, the total weight of sand, the depth of calibration tank and the particle composition of sand all affect the density measurement of sand to some extent. Whether the calibration of sand density is accurate or not will also affect the detection accuracy of subgrade compactness. Therefore, before the subgrade compaction test, the calibration work can not be ignored and must be paid enough attention. 4 Matters needing attention in field test 4. 1 During field test, the height and weight of sand surface in sand storage barrel are consistent with those in indoor calibration. 4.2 Use the substrate as much as possible to ensure the test accuracy. 4.3 Try to make the detection surface smooth and flat. After the field test, it is necessary to check whether there is sand leakage between the bottom plate, pad and ground of sand filling pipe. If there is, take it out separately and weigh its quality. This part of the mass should be deducted when calculating the density. 4.4 The reclaimed sand must be screened, cleaned and dried before the next use, and left for enough time to make it consistent with the cleaning and drying conditions during calibration. 4.5 In-situ water content detection, compared with alcohol method (free surface of immersed aggregate, burning for three times), the result is less than 65438 0%, which proves feasible. However, it should be noted that the purity of the alcohol used must reach 95%, and the inferior alcohol will not be completely burned and become water, which will affect the test results. 4.6 The depth of test pit should be equal to the calibrated depth as far as possible, the pit wall should be straight, and the diameters of upper and lower openings should be equal to avoid big and small, or small and big. Posted in China Paper Download Center.