Gravel filling is the key in the construction control process. According to the characteristics of this material, corresponding quality control methods should be adopted in all construction links, such as receiving, discharging, paving, sprinkling, rolling and dry density detection, so that the filling quality can meet the design requirements.
Keywords: gravel; Embankment filling; quality management
I. Introduction
Gravel filling is the key in the process of construction control, and the completion of procedures such as yard review, mining and transportation, filling and rolling will directly affect the construction progress, project quality and economic benefits of the whole project.
Take a project as an example, the project is Grade III and the flood dike is Grade II. The flood control standard is designed according to the 50-year flood return period. Filling materials are obtained locally, and artificial lake gravel is excavated to fill the embankment. The upper section of the water front slope of the dike is 1: 5 and the lower section is 1: 3. When embankment is filled with riverbed gravel, the compaction standard is controlled by relative density (Dr), and the design requirement Dr is 0.73.
Second, the gravel filling requirements
The embankment is filled with gravel, and the design requires good gradation. The maximum particle size of gravel material shall not exceed 150mm, and the sand content (particle size
After field investigation and sampling, the relative density of gravel samples was tested according to SL237- 1999 Code for Geotechnical Tests. The test results showed that the maximum dry density was 2.1~ 2.18g/cm3, with an average of 2.143g. The minimum dry density is 1.76 ~ 1.83g/cm3, and the average is1.795g/cm3. See table 1. After calculation, the control value of dry density corresponding to Dr=0.73 is 2.037 g/cm3.
Third, the sand and gravel yard quality control
Quality control of gravel yard: ① The surface covering layer, tree roots, construction waste and other sundries of the yard must be removed; (2) The silt content of crushed stones should be less than 5%, and those with soil interlayer or clods should also be removed; (3) Gravels larger than 60mm in gravel materials should not be too concentrated, and should generally be controlled within 30%. The field is mainly based on "visual inspection", and representative samples can be taken for indoor test.
Mining method: The underground water level in the material yard area is high, and the material source is mostly underwater. The mining method adopts vertical mixed mining, which is in place at one time. This can also avoid the gravel (more than 60 mm) and sand (less than 2 mm) content in the gravel material being too small or too large. Only when the gravel material is well graded can it achieve better rolling effect.
Four, rolling test
Rolling is the most critical process to complete embankment filling, because only rolling can make the dry density or relative density of loose gravel meet the design requirements, and field rolling test should be carried out before construction.
Rolling test is an important content in dam filling construction, and its main purposes are: ① to verify the rationality of gravel design index (dry density or relative density); (2) Determine the corresponding rolling parameters to provide a basis for construction; ③ Select the type and model of compaction machinery; ④ Optimize the construction technology.
Through the rolling test, it is determined that YZ- 14B vibratory roller will be used in this project, and it will be rolled for 6 times. The paving thickness is 0.5m, the water content is controlled at around 8%, and the appropriate sprinkling amount is 30% of the filling volume. V. Filling and rolling
Embankment filling mainly includes unloading, paving, sprinkling and rolling.
(1) uninstall
According to the specification and design requirements, unloading can be carried out by moving in and moving back. However, due to the non-caking of gravel, it is difficult for ordinary vehicles to get in and out, which is limited by their own performance. Therefore, in the process of gravel filling, the method of back migration is often used. If particle separation occurs during unloading, gravel should be mixed evenly before paving.
(2) Paving the road
According to the width of the working face, paving with bulldozer can meet the requirements, and the paving thickness is 0.5m；; The paving edge line exceeds the design edge line 1m, so as to leave enough slope cutting margin for the embankment.
(3) Sprinkle water
Sprinkle water once before rolling, and then sprinkle water while rolling. The key to the selection of sprinkler equipment is to meet the engineering needs, because the total sprinkler volume is generally 30% of the volume of new crushed stone, which is quite different from the general soil material, so we should attach great importance to it in the construction process, otherwise it will restrict the quality and progress of the project; Second, the water adding equipment should be able to flexibly control the whole grinding surface, making the field operation simple and easy. During construction, the main pipe and movable branch pipes can be arranged along the filling surface.
(4) Rolling
In the process of rolling, the key is to control the driving speed and water addition of vibratory roller, and at the same time, sprinkle water while rolling. When there is open water on the rolling surface (basically saturated), the rolling effect is the best Vibration rolling adopts the method of staggering the distance of advance and retreat. The width of the rolling track is greater than 10cm, the walking direction is parallel to the dike axis, the walking speed is controlled within 2 km/h, and the working surface is paved in layers, with a thickness of 0.5m, and evenly rolled. The thickness of compacted layer after rolling is generally about 45cm. During the rolling process, personnel and bulldozers shall be equipped for leveling, and there shall be no ditches; During the rolling process, the roller shall be rolled in sections, and the working face of each section shall be 100 ~ 150 m. Signs shall be set for each section to prevent leakage, undervoltage and overvoltage, and the connecting positions of the upper and lower sections shall be staggered. The crossing areas of adjacent working faces are rolled and overlapped with each other, and the direction parallel to the dike axis is not less than 0.5m, and the direction perpendicular to the dike axis is not less than 3m.
Six, gravel filling quality control
According to SL260-98 "Code for Construction of Embankment Engineering", the quality of gravel filling is controlled, and the compaction index is controlled according to the dry density corresponding to the designed relative density value of 2.037 g/cm3. The sampling parts for quality inspection are evenly distributed and randomly arranged on the rolling surface; During self-inspection, the sampling quantity shall be controlled at 1 every150m3, and the sampling quantity shall be 1/3 of the self-inspection quantity.
According to SL275-200 1 Code for Field Test of Nuclear Moisture Density Meter, ZN-2000 nuclear moisture density meter is used for dry density measurement, which needs to be calibrated before testing, and the field detection adopts projection method. This method has the advantages of nondestructive, fast, flexible and simple operation. The specific operation steps are as follows: (1) Use the guide plate, drill pipe and heavy hammer to drill a measuring hole perpendicular to the detection surface, the depth of which is at least 50mm greater than the actual insertion depth of the instrument source rod, and the verticality of the measuring hole should ensure that one end of the instrument source rod will not tilt or tilt after being inserted into the predetermined detection depth. (2) Place the instrument gently at the measuring point, and slowly insert the instrument source rod into the measuring hole until the predetermined detection depth is 30cm on the premise that the source rod does not disturb the hole wall of the measuring hole. ③ Set the detection time as 15s. (4) Start the instrument to start the test, and record the test results after the test. It is worth noting that when there are large particles or cavities in the tested gravel, the test results will be greatly deviated. At this time, the instrument can be rotated by 90 for one test until it is rotated by 360 for four tests, and the arithmetic average of the four test results is taken as the test result at this time.
After testing, 5049 points were sampled in this project, with the minimum dry density 1.946 g/cm3, the maximum dry density of 2.250 g/cm3 and the average dry density of 2.087 g/cm3, of which 4995 were higher than the designed dry density of 2.037 g/cm3, and the qualified rate was 98.9%.
Seven. Concluding remarks
The quality control of gravel filling construction should pay attention to the following problems: ① The distribution amount of gravel should be controlled when there is open water on the rolling surface (basically saturated), which is completely different from ordinary soil materials. ② ZN-2000 nuclear moisture density meter is used to measure the moisture content and density of gravel packing, which has the advantages of nondestructive, rapid, timely and simple operation. It not only bought time for the construction unit, but also improved the work efficiency for the quality inspection department. This advantage is not possessed by conventional detection methods such as irrigation and sand filling. (3) In order to ensure that the compaction density of the gravel embankment slope in the design section meets the design requirements, when paving materials, sufficient cutting allowance must be reserved for the upstream and downstream embankment slopes. The width of this project is controlled at 1.0m to ensure that the filling density of gravel embankment after slope cutting meets the design requirements.
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