Dynamic compaction foundation treatment can effectively eliminate settlement, improve foundation bearing capacity and uniformity, and reduce post-construction deformation. Therefore, before the design of dynamic compaction, the main purpose and purpose of foundation treatment should be determined first, and then the method of dynamic compaction or dynamic compaction replacement should be determined according to the comprehensive requirements of all aspects. Appropriate hammer type, energy level, tamping point spacing, tamping times and technical intermission time should be selected according to specific engineering geological conditions and reinforcement depth. According to the prediction processing effect, whether the deformation index meets the upper design requirements is inversed.

(2) Design points of dynamic compaction method

1, rammer and construction machinery selection

The commonly used rammer has a diameter of 2.2~2.6m and a weight of 10~40T ~ 40t. The bottom of the rammer is flat and the hammer is cylindrical. Pear rammers are not allowed. The special-shaped hammer has a general diameter of 1. 1 ~ 1.3m, a flat bottom, a cylinder and a weight of 15 ~ 45t. The selection of rammer should meet the requirements of convenient transportation, high construction efficiency and flexible action. Ramming area is closely related to effective reinforcement depth. According to the research, at the energy level of10000 kn m m and the rammer radius of 1.25m, the maximum hammering stress is 37% and 76% higher than that at 1.5m and 1.75m respectively. In order to improve the effective reinforcement depth, the radius of rammer can be reduced appropriately. In high-energy dynamic compaction, the effective reinforcement depth and uniformity of the foundation with small distance of heavy hammer drop are better than those with large distance of light hammer drop.

2. Effective reinforcement depth of dynamic compaction method

The effective reinforcement depth of dynamic compaction method is not only an important parameter reflecting the effect of foundation treatment, but also an important basis for selecting foundation schemes. Generally, it is determined according to field trial tamping or local experience.

3. The unit tamping energy of dynamic compaction method

The unit tamping energy of dynamic compaction method should be comprehensively considered according to the foundation soil type, structure type, load size and required treatment depth, and determined by field trial tamping. Under the same conditions, the unit tamping energy of fine-grained soil is greater than that of coarse-grained soil. Generally, for fine-grained soil, it is1500kn m/m2 ~ 4000kn m/m2; For coarse-grained soil, take1000kn m/m2 ~ 3000kn m/m2.

4. Ramming times of dynamic compaction method

The tamping times of dynamic compaction method should be based on the principle of maximum compression of tamping pit and minimum uplift around tamping pit. In addition to the relationship curve between tamping times and tamping settlement obtained from field trial tamping, several conditions should also be met: a. The settlement of the last two blows is not more than 50mm, and the tamping energy is less than 100mm when it is large. B the ground around the rammed pit should not be excessively raised. C. Because the rammer pit is too deep, it is not difficult to hammer.

5, dynamic compaction tamping times

The number of times of dynamic compaction should be determined according to the nature of foundation soil. Generally, it can be used for 2 ~ 3 times, and finally it can be fully rammed with low energy 1 time. For the foundation with strong permeability composed of coarse particles, less tamping times may be required; On the contrary, the foundation with weak permeability composed of fine-grained soil needs more tamping times.

6. Dynamic compaction time interval

The interval of dynamic compaction means that there should be a certain interval between two tamping, which is beneficial to the disappearance of excess pore water pressure in soil. The interval time depends on the time when the excess pore water pressure disappears in the soil. In the absence of measured data, it can be determined according to the permeability of foundation soil. The interval of cohesive soil foundation with poor permeability should be no less than 3 ~ 4 weeks, and the foundation with good permeability can be continuously compacted.

7, dynamic compaction tamping point layout

Whether the layout of tamping points by dynamic compaction method is reasonable or not is directly related to the tamping effect and construction cost. Tamping points can be arranged in equilateral triangle, isosceles triangle or square according to the type of building structure. The spacing of tamping points in the first pass can be 5m ~ 9m, and the spacing of tamping points in subsequent passes can be the same as that in the first pass or reduced appropriately. For projects with deep treatment depth or high energy of one-click tamping, the spacing of first tamping points should be appropriately increased.

8, the scope of application of dynamic compaction method

The treatment range of dynamic compaction means that the treatment range of dynamic compaction should be larger than that of building foundation due to the stress diffusion of foundation. The width of each side beyond the outer edge of the foundation shall be 1/2 ~ 2/3 of the design treatment depth, and shall not be less than 3m.