The dynamic compaction method can be applied to gravel soil, sandy soil, silt and cohesive soil with low saturation, collapsible loess, miscellaneous fill and plain soil. It is widely used in airport runway, expressway, industrial and civil building foundation treatment construction. This paper introduces its working principle and construction technology.
1、 The principle of dynamic compaction is composed of three parts: solid and gas. Under the action of the heavy hammer, the gas in the soil is discharged, and the radial cracks appear around the tamping point, forming the infiltration channel of the pore water in the soft soil, which creates conditions for the dissipation of the excess hydrostatic pressure. A considerable amount of compaction energy is transformed into the compression deformation of the soil body. With the accumulation of the compaction energy, the strength of the soil body is improved.
In the specific construction, dynamic compaction parameters must be determined by trial compaction according to the actual situation on site.
. effective reinforcement depth. The effective reinforcement depth of dynamic compaction can be estimated by Menard empirical formula, where: wh - hammer weight × drop distance; K - is the reduction factor of influence depth; D - reinforcement depth.
It can also be determined according to the table: the energy of single click ramming is equal to the weight of hammer × drop distance, which is determined by the effective reinforcement depth in concrete construction. The actual construction shows that the reinforcement effect of heavy hammer with low drop distance is better than that of light hammer with high drop distance under the condition of the same single tamping energy. Therefore, the heavier rammer should be used when the lifting capacity is allowed.
Dynamic compaction energy: in case of strong energy, the air is discharged, the soil is compressed, and the pore water pressure changes. When the pore water pressure in the local foundation soil reaches the self weight pressure of the soil, the energy received by the soil is considered to be saturated, which is the dynamic compaction energy.
There are two ways to determine the tamping energy in practical construction
The first is to judge the appropriate base number by the pore water pressure measured by the pore water pressure gauge embedded in the foundation. When the pore water pressure measured by the last two or three blows is close, the optimal compaction energy can be determined;
The second is to determine the best ramming energy through the relationship curve between the number of tamping and the amount of tamping settlement. The difference between the two adjacent blows is ~ mm, and the surrounding of the tamping pit is uplifted.
In construction, the measurement of tamping settlement is simpler than the measurement of water pressure in soil. Generally, the best ramming energy is determined by the relation curve of tamping number and tamping settlement. Experienced engineers can also judge directly from the uplift degree of the ground around the tamping pit. .
Tamping times: the number of tamping times in China is generally ~ times, but it should be determined according to the nature of foundation soil. The permeability coefficient of foundation soil is low and the water content is high, so it needs to be increased again. The last time is full compaction with low energy.
Tamping interval. The interval time of tamping refers to the construction time interval of main and auxiliary tamping, which is mainly determined according to the dissipation of pore water pressure. Once the pore water pressure dissipates, auxiliary compaction can be carried out. It can be determined by the measurement of pore water pressure in the process of trial compaction. In case of lack of measured data, it can be determined according to the infiltration of foundation soil.
Layout and spacing of tamping points: tamping points can be arranged in square or plum shape. The distance between the first tamping points should be far, so that the deep soil can be reinforced, and then the auxiliary tamping points should be inserted in the middle. The spacing of tamping points is usually ~ M. considering the stress diffusion effect of the foundation, the range of tamping points should be larger than the scope of foundation H / (where h is the depth of reinforcement). When the soil quality is poor and the soft soil layer is thick, the spacing of tamping points should be appropriately increased and the tamping times should be increased to achieve the purpose of reinforcement. When the soft soil layer is thin and there is sand soil interlayer or soil with stone filling, the tamping distance can be appropriately reduced.