01-What roles does the cushion layer play in a composite foundation?

Aboutcushion layerThe roles of the cushion layer in a composite foundation are summarized in six points, for everyone's reference only.

This is an important condition for the cement-fly ash-gravel pile to form a composite foundation. When the foundation is subjected to vertical load, both the pile and the soil between piles will deform. The modulus of the pile is much greater than that of the soil, so the pile deforms less than the soil. Because a cushion layer of a certain thickness is set under the foundation, the pile can penetrate upward. Along with this change process, the cushion layer material is continuously replenished onto the soil between piles, thus ensuring that part of the load acts on the soil between piles through the cushion layer, realizing the joint action of pile and soil.

By changing the thickness of the cushion layer, the sharing of vertical load by the pile is adjusted. Generally, the thinner the cushion layer, the higher the percentage of the total load borne by the pile.Since the modulus of the CFG pile body is much greater than that of the soil between piles, the pile-soil stress ratio is generally relatively large.However, through the action of the cushion layer, the pile-soil stress ratio can be effectively reduced. This characteristic isCFG pilehas greater flexibility.This method adjusts the stress ratio of pile and soil, better exerts the bearing capacity of pile and soil, and thus achieves the purpose of reducing project cost.    

Since CFG piles are rigid piles, when no cushion layer is designed, the stress concentration of the pile on the foundation is very obvious. If the thickness of the cushion layer is 10-30 cm, the stress concentration of the pile on the foundation base slab is significantly reduced. After the thickness exceeds 30 cm, the foundation can be regarded as a natural foundation, and punching shear failure need not be considered at this time.

The thicker the cushion layer, the greater the percentage of the total load shared by the soil, and the smaller the percentage of the total load shared by the pile. The adjustment of the cushion layer thickness can affect the sharing of pile-soil load. According to this principle, in the application of CFG pile composite foundations, the non-uniformity of the foundation can be eliminated by adjusting the thickness of the cushion layer, so that the foundation achieves coordinated deformation.

In a composite foundation, if the top of the pile body (such as CFG piles and gravel piles) is directly connected to a rigid foundation, under load, the restraint of the foundation will cause relatively large shear force or bending moment at the pile top (especially when the stiffness difference between the pile body and the foundation is large), which can easily lead to fracture of the pile top.

The flexibility of the cushion layer can buffer this rigid contact and release the pile top through its own deformation.additional stressprotect the integrity of the pile body.

In pile body construction (such as cast-in-place piles and precast piles), the pile top elevation may have slight differences due to construction errors. The cushion layer can make up for the height difference of the pile tops through layered paving and compaction, ensuring that the foundation base is flat and avoiding additional stress on the foundation caused by uneven pile tops.

The cushion layer is the "link" in a composite foundation that realizes the coordinated work of "pile-soil-foundation". Its design (material, thickness, degree of compaction) needs to be comprehensively determined in combination with the pile type (flexible pile, rigid pile), superstructure load, and foundation conditions. For example, in composite foundations with rigid piles (such as CFG piles), the cushion layer needs to focus more on coordinating the forces on pile and soil; for flexible piles (such as gravel piles), the cushion layer is needed to improve the load diffusion capacity. A reasonably set cushion layer can significantly improve the bearing efficiency and deformation control effect of the composite foundation.