At present, the foundation of construction projects needs to compact the loose soil, which is inseparable from tamping. For small construction projects, tamping the foundation can be completed with a small electric rammer. For large construction projects, there are usually hard indicators for the foundation, which need to be tamped to a certain extent before it is qualified. In this way, there are some requirements for the rammer, Therefore, a dynamic compaction machine is needed.
The dynamic compaction method is the vigorous compaction method, also known as the dynamic consolidation method. The large crawler dynamic compactor is used to drop 8-30 tons of heavy hammer freely from the height of 6-30 meters, vigorously tamp the soil, quickly improve the bearing capacity and compression modulus of the foundation, form a relatively uniform and dense foundation, and change the pore distribution of the foundation soil within a certain range of the foundation.
Dynamic compaction method has been widely used in highway, railway, airport, nuclear power plant, large industrial zone, port reclamation and other foundation reinforcement projects. The utility model has the advantages of short construction period, good effect and low cost.
The dynamic compaction method is suitable for the treatment of gravel soil, sandy soil, silt and clay with low saturation, collapsible loess plain fill and miscellaneous fill. Before dynamic compaction construction, one or several test areas shall be selected on the representative site of the construction site for trial compaction or experimental construction. The number of test areas shall be determined according to the complexity of the construction site, construction scale and building type.
What are the advantages of dynamic compaction?
The construction process is simple; It is applicable to a wide range of soil, has remarkable reinforcement effect, can obtain high bearing capacity, and the general foundation strength can be increased by 2 ~ 5 times; The compressibility can be reduced by 2 ~ 10 times, and the reinforcement effect can reach 6 ~ 10m; High work efficiency and fast construction speed (general equipment can reinforce 5000 ~ 10000m foundation every month), which can shorten the construction period by half compared with backfilling and pile foundation; Save reinforcement raw materials; The construction cost is low and the investment is saved. Compared with soil replacement and backfilling, it can save investment compared with precast pile foundation reinforcement.
Analysis on three different reinforcement mechanisms of dynamic compaction method
Dynamic consolidation: when dynamic consolidation method is applied to treat fine-grained saturated clay, its reinforcement mechanism is dynamic consolidation theory. During dynamic compaction, the huge impact force produces a large stress wave in the soil, destroys the original structure of the soil, causes local liquefaction of the soil and produces many cracks, and the drainage channel makes the pore water escape smoothly. After the excess pore water pressure dissipates, the soil is consolidated. Due to the thixotropy of soft soil, the strength is restored.
Dynamic compaction: the consolidation of porous coarse-grained unsaturated clay by dynamic compaction is based on the mechanism of dynamic compaction, that is, impact dynamic load, which reduces the pores in the soil and makes the soil dense, so as to improve the strength of foundation soil. The compaction process of unsaturated soil is the process that the gas phase (air) in the soil is squeezed out. The compaction deformation is mainly caused by the relative displacement of soil particles.
Dynamic replacement: dynamic replacement can be divided into integral replacement and pile replacement. Integral replacement uses dynamic compaction to squeeze the gravel into the sludge, and its action mechanism is similar to that of soil replacement cushion. Pile replacement is to fill the gravel into the soil through dynamic compaction, and some gravel piles (or piers) are tamped into the soft soil at intervals to form pile (pier) gravel piles (piers). Its action mechanism is similar to the gravel piles formed by vibroflotation method, forming a composite foundation as a whole.
What are the correct construction steps of dynamic compaction?
Carefully investigate to ensure that there is nothing underground within the dynamic compaction site. Clear the surface soil within 2 ~ 3M outside the subgrade slope toe. After leveling, mark the position of one tamping point on the site, control the point position deviation within ± 20cm, and measure the site elevation.
Put the crane in place, align the rammer with the ramming point, and measure the top elevation of the hammer before ramming.
Lift the rammer to the predetermined height. After the rammer is decoupled and falls freely, put down the hook and measure the top elevation of the hammer. If the rammer is found to be skewed due to the inclination of the pit bottom, the pit bottom shall be leveled in time.
Repeat three operations to complete the tamping of one tamping point, i.e. 6 tamping for each tamping point. In addition to meeting the requirement that the average tamping settlement of the last two blows shall not be greater than 50mm, the total tamping settlement shall not be less than the total tamping settlement of trial tamping.
Change tamping points and repeat two and four operations until all tamping points are tamped once. Fill the tamping pit with bulldozer and measure the site elevation.
After the specified interval, complete 2, 3 and 4 times of tamping successively according to the above steps.
Select a hammer with a mass of 3T and a falling distance of 6m for 2 times of low-energy full compaction. Loosen the soil on the surface of the site and compact it. It is required to stagger the latter one with the previous one for half compaction; Measure the site elevation after tamping.
Lime soil sealing. Lay a layer of 0.20m thick 2:8 lime soil on the base ground. During construction, lime shall be sprinkled into the soil without watering for initial mixing. After mixing, the material shall be stuffy for 8 ~ 12h, and then watered for re mixing. After mixing, it shall be leveled and compacted; After the strength test is qualified, a layer of anti-seepage composite geomembrane shall be laid on the lime soil cushion.
Slope reinforcement. Two way fine woven geogrid shall be laid 0.60m below the subgrade surface. The vertical spacing of each layer of geogrid shall be 0.50m. The upper two layers shall be paved along the plane. The other layers shall be 2.50M deep into the embankment slope, and the distance between the grid and the slope line shall not be less than 0.10m.
During dynamic compaction construction, someone shall be specially responsible for the following monitoring work:
(1) Before tamping, the weight and falling distance of the rammer shall be checked to ensure that the click can meet the design requirements.
(2) Before each tamping, the setting out of tamping points shall be reviewed. After tamping, the position of tamping pit shall be checked, and any deviation found shall be corrected in time.
(3) Check the tamping times and tamping settlement of each tamping point as required.
(4) When the geological conditions are found to be inconsistent with the data provided by the design, it shall be studied and handled in conjunction with relevant departments in time.
(5) In case of any change to the workload or requirements formulated in the scheme during construction, it shall be reported to the superior competent technical department for approval before construction.
(6) During dynamic compaction construction, various parameters and construction conditions shall be recorded in detail on site in time.
(7) During dynamic compaction, there shall be no excessive uplift around the tamping pit. If there is any abnormality, it shall be reported to the supervising engineer in time, and the treatment method shall be made together with the design owner and other departments.
(8) The tamping pit shall be constructed and leveled on the same day to prevent rain and water.
(9) The dynamic compaction site after completion acceptance shall be put into use in time and shall not be left for a long time.
(10) The finished surface of dynamic compaction shall be prevented from rolling by heavy vehicles.
Dynamic compaction method can not only improve the strength of foundation soil and reduce its compressibility, but also change the ability of anti vibration liquefaction and clear the collapsibility of soil. Therefore, it is not only suitable for the treatment of gravel soil, sandy soil, silt, cohesive soil miscellaneous fill and plain fill foundation, but also often used for the treatment of liquefiable sand foundation and collapsible loess foundation.
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