Gravel pile is a type of granular pile, mainly composed of crushed stone (cobble) without cohesive strength. It is formed by drilling a hole in soft ground using vibration, impact, or water jetting, then compacting crushed stone (cobble) into the hole to form a dense pile body of large-diameter gravel. The composite foundation reinforced by gravel (cobble) piles and the soil between piles is called a composite foundation reinforcement pile. According to the pile-forming process, it can be divided into two categories: vibroflotation (wet method) gravel piles and dry method gravel piles. The gravel pile made by the vibroflotation process with water jetting is called vibroflotation gravel pile or wet method gravel pile. The gravel piles made by various dry processes without water jetting (such as dry vibration, vibratory compaction, hammering, etc.) are collectively called dry method gravel piles. When the pile is made of gravel sand, coarse sand, medium sand, round gravel, angular gravel, cobble, crushed stone, etc., it is called a sand-gravel pile.

　　Vibroflotation was first developed in 1937 by the German company Keller, which designed and manufactured equipment with the prototype of a modern vibrator to densify sand and gravel foundations successfully. In the early 1960s, vibroflotation began to be used to reinforce cohesive soil foundations. Because the material used is crushed stone, it is called a stone column. China started applying vibroflotation in 1977.

　　The Role of Vibroflotation in Soft Soil Reinforcement


　　The so-called vibroflotation method is the abbreviation of vibratory water jetting method. It uses a crane to lift the vibrator, starts the submersible motor to drive the eccentric block, causing the vibrator to produce high-frequency vibration. At the same time, the water pump is started to jet high-pressure water. Under the combined action of vibration and jetting, the vibrator is sunk to the predetermined depth in the soil. After cleaning the hole, crushed stone is gradually filled into the hole from the ground. Each section of fill is compacted by vibration. After reaching the required density, the vibrator is lifted. This process is repeated, filling and compacting until the ground surface, thereby forming a large-diameter, very dense pile body in the foundation, achieving the effect of reinforcing soft soil.


　　1. Reinforcement Mechanism for Cohesive Soil　　

　　For cohesive soil foundations (especially saturated soft soil), due to the high clay content, strong inter-particle bonding, and low permeability, water in the soil is not easily drained under vibration. Therefore, the role of gravel piles is not to densify the foundation but to replace it. During construction, the vibrator, with its own weight, horizontal vibration force, and high-pressure water, turns the cohesive soil into mud slurry that is discharged out of the hole, forming a hole slightly larger than the vibrator diameter. Crushed stone is then poured into the hole and, under the lateral force of the vibrator, squeezed into the surrounding soil to form a pile body with a certain density and large diameter, which works together with the original clay to form a composite foundation.
Since the stiffness of the gravel pile is much greater than that of the surrounding cohesive soil, the stress in the foundation is redistributed according to the deformation modulus of the materials. Therefore, most of the load is borne by the gravel pile. The ratio of stress in the pile to that in the cohesive soil between piles is called the pile-soil stress ratio, generally 3 to 5. Although the bearing capacity of the cohesive soil itself is not improved, the bearing capacity of the entire foundation is increased. Gravel piles are sometimes also used to improve the sliding resistance of soil slopes. In this case, the pile acts like a general anti-slide pile, increasing the shear strength of the soil and forcing the sliding surface to move away from the slope surface and deeper.

　　2. Reinforcement Mechanism for Sandy Soil　　

　　In sandy soil, gravel piles mainly play a densification role, with the main purpose of improving the bearing capacity of the foundation soil, reducing deformation, and increasing liquefaction resistance. During the construction of gravel piles, saturated loose sand particles are rearranged and densified under strong high-frequency forced vibration. After a large amount of coarse aggregate is filled into the vibrated hole, it is squeezed into the surrounding soil by the strong horizontal vibration force. This forced densification increases the relative density of the sand, reduces the porosity, increases the dry unit weight and internal friction angle, improves the physical and mechanical properties of the soil, and greatly increases the bearing capacity of the foundation (generally by 2 to 5 times).

　　In addition, due to the significant increase in foundation width and the corresponding increase in relative density, the liquefaction resistance is also improved.