1. 1 the origin of dynamic compaction method
Dynamic compaction method, also known as dynamic consolidation method, is a method in which a heavy hammer (generally 10~40t) falls freely from a height (10~40m) to tamp the foundation, so as to improve the strength of foundation soil and reduce its compressibility. 1957, the British Road Research Institute applied Proch Toff's compaction principle to the compaction of deep soil, but it was not until around 1970 that the dynamic compaction method was really applied to the reinforcement of deep soil on a large scale under the development and advocacy of French engineer luis mena. At first, the dynamic compaction method was only used to reinforce the sandy pebble layer with cone probe resistance 9s lower than 65438+1100mn/mz. With the improvement of construction machinery and technology, dynamic compaction can also be used to strengthen cohesive soil foundation.
1.2 characteristics of dynamic compaction technology
1.2. 1 is suitable for all kinds of soil layers: it can be used to reinforce all kinds of sandy soil, silt, general cohesive soil, loess and artificial fill, especially for reinforcing large pieces of gravel soil and miscellaneous fill composed of buildings, domestic garbage or industrial garbage that are difficult to be reinforced by general treatment methods, and can also be used to reinforce soft soil foundation with other technical measures.
1.2.2 has a wide range of applications: it can be applied to industrial and civil buildings, heavy structures, equipment foundations, airport runways, dams, road and railway subgrade, warehouses, storage yards, oil tanks, bridges, ports and docks, nuclear power plants, artificial islands, etc.
1.2.3 The reinforcement effect is remarkable: after dynamic compaction, the bearing capacity, compressive modulus, dry density, void ratio, compressibility, site uniformity, collapsibility and expansibility of foundation can be obviously improved, and vibration liquefaction can be prevented.
1.2.4 Effective reinforcement depth: 8000 kn·M high-energy dynamic compaction depth reaches 12m, multi-layer dynamic compaction depth reaches 24~54m, and general energy dynamic compaction depth reaches 6~8m.
1.2.5 Simple construction tools: crawler crane is the main dynamic compaction tool. When the lifting capacity is limited, facilities such as gantry can be supplemented.
1.2.6 Material saving: Generally, dynamic compaction is to apply energy to undisturbed soil without adding building materials, which greatly shortens the construction period.
1.2.7 Cost saving: Because materials are not needed in the dynamic compaction process, the cost of purchasing, transporting, manufacturing and driving of building materials is saved, and there is no other consumption except oil.
1.2.8 fast construction: as long as the technology is appropriate, especially for dynamic compaction of coarse-grained unsaturated soil, the period is shorter; However, rainy days have a serious impact.
1.3 scope of application of dynamic compaction method in strengthening building foundation
The invention has the advantages of simple construction tools, convenient construction, remarkable foundation reinforcement effect, wide application range, shortened construction period and reduced engineering cost. Generally speaking, dynamic compaction foundation is suitable for treating gravel, sandy soil, miscellaneous fill (excluding domestic garbage), low saturation silt, cohesive soil, collapsible loess and so on. However, the author believes that site supervision should refer to the geological survey report and master relevant technical indicators, such as soil particle composition, void ratio, liquid index, plasticity index, saturation and permeability coefficient. For miscellaneous fill containing construction waste, industrial waste and domestic waste, it is necessary to find out its distribution range, depth, organic matter content and whether it is corrosive to foundation. Because collapsible soil will produce additional settlement after soaking, its collapsibility coefficient should also be used as a control index. Whether to consider dynamic compaction foundation in the project is decided by the design, but the supervisor should remind the design to consider the above technical indicators comprehensively when necessary. For example, a municipal road project in Conghua, Guangzhou, chose dynamic compaction foundation in the design. However, after comparing the geological prospecting data, the supervisor reminded the design that some backfill soil contains a lot of wood chaff. After finding out its distribution range and buried depth, the design adopted corresponding technical treatment measures to ensure the construction quality of the project.
2. Mechanism of dynamic consolidation of foundation
At present, the opinions of experts and scholars on the mechanism of dynamic consolidation of foundation are not very consistent. However, for several types of soil often encountered in foundation treatment, the general view is that dynamic compaction exerts huge impact energy on foundation soil in a very short time (generally speaking, the impact energy is not less than 800 kn·m), and the loading time is about tens of milliseconds. For the soil layer with large water content, the loading time is about 100 milliseconds. This sudden release of huge energy will be converted into various waves and transmitted to the ground. The first wave that reaches the specified range is compression wave, which makes the soil compress or stretch, and can cause instantaneous pore water accumulation, thus greatly reducing the shear strength of foundation soil. According to theoretical calculation, this wave propagates with 7% vibration energy, followed by shear wave, which propagates with 26% vibration energy, and shear wave will lead to the destruction of soil structure. In addition, Rayleigh wave (surface wave) is transmitted with 67% vibration energy, which causes the ground uplift near the tamping point. Under the comprehensive action of these waves, the soil particles are rearranged and close to each other, and the gas in the pores is discharged, which makes the soil dense and improves its strength.
According to the above viewpoint, the process of improving the strength of foundation soil after dynamic compaction can be roughly divided into four stages: tamping energy conversion, accompanied by forced compression or vibration compaction (manifested by the discharge of water and gas in the soil and the increase of pore water pressure); Soil liquefaction or soil structure destruction (manifested as soil strength reduction or shear strength loss); Drainage consolidation and compaction (manifested by the change of permeability, the development of soil cracks and the improvement of soil strength); Thixotropic recovery is accompanied by consolidation and compaction (including part of free water becoming membrane water again, and the strength of soil continues to improve).
Three engineering examples
3. 1 project overview
According to the geological survey data, the Hunan section of G 106 national highway expansion project has miscellaneous fill ranging from 2 to 8 meters, which is mainly composed of gravel and cohesive soil, with peat shale blocks and construction waste. Gravel and cohesive soil are extremely uneven and not dense, and can be directly used as the foundation of high fill subgrade after reinforcement, and the fill height is 23 m. The design adopts dynamic compaction method to strengthen the fill layer. The designed tamping energy is 2250 kn·m, and the falling distance is 18m.
3.2 Dynamic compaction construction
3.2. 1 Before this dynamic compaction construction, first try to tamp the section K2325 ~ K2825. After the trial ramming area passes the inspection, the construction will be promoted on a large scale according to the construction parameters of the trial ramming area.
3.2.2 Adopt the construction scheme of "two-pass spot tamping and two-pass full tamping", and at the same time adopt "four-pass spot tamping and three-pass full tamping" to strengthen the treatment of road sections with complicated local geological conditions; For full tamping, 1 000 kn·m can be used, and the falling distance is 7~8m, and it is overlapped by 1/4.
3.2.3 During the construction, it is found that the soil quality of K2356 ~ K2402 section of the site is poor, and it still cannot meet the design requirements after reinforcement. Therefore, the foundation of K2356 ~ K2402 section is treated with soil replacement, and then the gravel soil is backfilled before dynamic compaction. At the same time, according to the design requirements, in case of rainy weather during construction, the requirements of dynamic compaction construction specifications shall be strictly followed.
3.3 Strengthening effect
The project was completed in June+10, 2005. The site was inspected by a qualified construction unit entrusted by the construction unit. The results show that the compactness of soil is obviously improved after dynamic compaction, and the bearing capacity and compactness meet the design requirements.
Four suggestions
4. 1 Do a good job in construction monitoring of dynamic compaction foundation. Because many technical parameters of dynamic compaction foundation need to be determined in the process of trial tamping and construction, it is particularly important to strengthen the trial tamping and construction monitoring of dynamic compaction foundation and grasp all technical parameters in time. After the start of dynamic compaction construction, the supervision engineer should conduct on-site inspection of the whole process, focusing on verifying the design parameters and feeding back the deviation to the design in time. The supervision engineer should pay attention to the following points in monitoring: mastering the design intention, auditing the qualification of the construction unit, carefully auditing the construction plan, on-site inspection records and safety control.
4.2 Before dynamic compaction construction, one or several test areas should be selected on the representative site of the construction site for trial compaction or experimental construction. The number of test areas should be determined according to the complexity of the construction site, the building scale and the building type. In the process of construction, technical parameters should be strictly observed. After the construction, do a good job in quality inspection, and do a good job in the later period and after asking households.
4.3 Comprehensive control of project quality should start from the following aspects: (1) Control the quality of all project contents of the construction project. (2) Control all contents of the quality objectives of the construction project. (3) Control all factors that affect the quality objectives of the construction project.
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