Geogrid, as one of the important geosynthetics, has been widely used in water conservancy, railway, highway, port and construction engineering because of its superior engineering performance. Bidirectional geogrid is a two-dimensional or even three-dimensional grid grid made of polyethylene, polypropylene and other high polymer by molding, thermoplastics and other processes, which makes the long chain molecules disordered orderly, thus improving the tensile strength and rigidity of the polymer, so it has the characteristics of high tensile strength, good durability, low elongation and strong adaptability. According to different structures, geogrid is mainly divided into one-way geogrid and two-way geogrid.

The two-way geogrid can form a very effective occlusion with the soil particles and restrict the lateral displacement of the soil particles. Generally, it is widely used in the subgrade engineering. The two-way geogrid can be buried in the soil as a reinforcement material, which can better homogenize the stress of the soft soil foundation, restrain the lateral deformation of the foundation, reduce the uneven settlement and improve the overall strength of the subgrade, Restrain the tension crack of embankment and shear failure of soil foundation.

There are few definitions of soft soil foundation in domestic highway code. Generally speaking, soft soil refers to natural water content (35% ~ 75%), void ratio (void ratio 1.0 ~ 1.9), high compressibility (compression coefficient 0.005 ~ 0.02), low strength (cohesion around 10KPA, fast shear internal friction angle 0 °~ 5 °) ）It is a kind of soft plastic cohesive soil with low bearing capacity (foundation bearing capacity is 20-100kpa) and sensitive structure. Soft soil can be divided into clay, muddy soil and peat soil. Adding two-way geogrid to the soft soil foundation can improve the overall rigidity of the embankment, the internal stability, and the uniformity and anti deformation ability of the foundation.

The two-way geogrid has certain rigidity and good ductility, which makes the load spread from the pavement, improves the bearing capacity of the foundation, and prevents the uneven settlement of the foundation. The tensile strength of geogrid is large, which can increase the stability of subgrade. Because the two-way geogrid can well adapt to the deformation of the foundation, the composite of the two-way geogrid and the soil will produce deformation under the load, making the two-way geogrid and the foundation soil bear the external load together. The friction between the mesh of the two-way geogrid and the upper and lower soil limit the lateral deformation of the soil and increase the shear strength of the soil. In order to ensure the safety of the pavement structure, it is necessary to reinforce the soft foundation. However, it is difficult to completely remove the soft soil layer in this section, because the silt layer is thick, the construction period is long and the construction is difficult. Through repeated analysis and comparison, the two-way geogrid is designed to reinforce the soft soil subgrade in this section, so as to improve the bearing capacity and reduce the uneven settlement.

The double-layer arrangement of two-way geogrid and gravel cushion is adopted in the design. The height of each layer of two-way geogrid is 0.1M, and the thickness of upper and lower gravel cushion is 0.3m. In the construction, the crushed stone cushion is first laid to form a construction platform, so that the two-way geogrid can play a role. First, lay the gravel cushion with certain thickness on the top of the soft foundation, and use the bulldozer to push it forward.

If such cushion is not set, the two-way geogrid will be directly laid on the top of the soft foundation, then the grid will be pressed into the soft foundation during the compaction of the filler, which will not only affect the compaction of the filler, but also lead to the failure of the two-way geogrid to play a corresponding role. As the filler in the grid is gravel, it can make the two-way geogrid layer form a horizontal drainage channel, accelerate the dissipation of pore water in the subgrade, accelerate the consolidation of the subgrade, and reduce the uneven settlement.

Two way geogrid performance requirements: the SWG50-4 type two-way geogrid is selected. The material is high-density polyethylene and anti-aging agent by extrusion molding. It has the characteristics of light weight, high strength, strong corrosion resistance and long service life. The height of the design grid is 0.1M, the welding distance is not less than 0.5m, and its longitudinal is not less than 0.5m. The transverse tensile strength is greater than 5.8knm, the yield elongation is less than 10%; the tensile modulus is greater than 650mpa, the maximum particle size of the filled gravel is not greater than 4.0cm, the particle size shall be uniform, and the mud content shall not be greater than 3%.