Study of Unconfined Compressive Experiment on the Geogrid Flexible Reinforced Compound

. In recent years, reinforced technique has been widely used in civil engineering technology. In order to deeply understand the behavior of the reinforced structure, this paper use the geogrid as the reinforced materials. The unconfined compressive tests were operated in the laboratory. The specimens were made by sharp sand which was classified according to the density at ρ=1.55g/cm3, ρ=1.75g/cm3, ρ=1.85g/cm3.Each specimen was reinforced with the layers of 0,1,2,3,4,5. Through analysis the experimental data, it’s founded that the density reinforced materials and the layers have an effect on the unconfined compressive strength. The specimen with bigger density and more reinforced layers has a better compressive strength. Different reinforced materials have different reinforced effect.

Nowadays, the use of reinforcement technology is becoming more and more extensive. In order to obtain a large number of systematic mechanical property test data of flexible reinforced soil complex, systematically sort out and analyze the test results, and extract the basic mechanical properties of flexible reinforced soil complex, this paper conducts an unconfined compressive test of flexible composite reinforced soil. The experiment mainly studies the influence of geotextile, the number of layers of reinforcement, and the density of soil on the compressive strength of soil. Provide some guidance for later practical projects.

Test instruments
There are test instruments required for unconfined pressure test: A Lu Qiangyi. The inner diameter is 152mm.One plastic round tube with a height of 189mm.And split into two halves, as shown in Figure  1(a)(b). The diameter is 150mm.There are several latex films with a height of 240mm, as shown in Figure 2.A pad with a diameter of 150mm and a thickness of 39mm.Several rubber bands, bench scales, shovels, stopwatches.

Test materials
The test sand used in this paper is pure sand, and the sand grains with particle size greater than 1.18mm in the sand are removed. The particle size distribution is shown in Figure 4 below. The geogrid adopts a two-way geogrid.

Test the reinforcement scheme
The test adopts indoor test to study the influence of the density of the sample, the reinforced material and the number of reinforced layers on the unconfined compressive strength. The height of the specimen is 150mm, and the number of layers reinforced is n=0, 1, 2, 3, 4, 5.

Preparation of test specimens
Before the test, the yellow sand for the test is baked in the oven for several hours, the moisture is dried, and then it is screened with a screening machine to remove the particle size greater than 1.18mm, and the particle size of the sand is measured. Tests of one density specimen at a time are performed. In accordance with the reinforcement scheme, the reinforcement is sequential. When preparing a specimen of this density, the mass of sand required for each density and the mass of sand required for each height of each layer under each reinforcement scheme are first calculated, and the specimen forming is the same except that the mass of the sand weighed in each layer is different. First of all, the latex film should be put on the bottom pad and hooped with a rubber band, and the plastic pipe should be hooped up with a rubber band on the cushion block, and the excess part of the latex film should be turned over on the plastic tube. There are rubber band hoops up to ensure fixation. Weigh the mass of each layer of sand and load it into a latex film with a plastic tube on the outside. Then the geotextile that has been cut in advance is laid with reinforced material. However, when the reinforcement is 0 layer, it is also best to add sand in layers. In this way, each layer is as uniform as possible. Start loading to keep the sand loose before tamping it to the height of each layer. After the test is formed, the outer plastic sleeve is removed, and then the unconfined compression test is carried out. The initial specimen is shown in Figure 3.

Unconfined compression test of geogrids
According to the data recorded in the test and the calculation results, a stress-strain (q-ε) relationship diagram was made. Q represents the compressive stress at the vertical pressure to which the specimen is subjected, ε represents the compressive strain over time. Take about ε=15% for the strain of sample failure. According to the relationship diagram, the change law of the strength of the flexible reinforcement complex at different densities and different number of reinforcement layers is analyzed. The geogrid is shown in Figure5, and the curve is shown in Figure 6 to Figure8.  (1) As the number of layers of the geogrid increases, the strength of the specimen increases layer by layer. In particular, the more layers are added, the greater the pressure value. Especially when the number of layers stiffened is n = 5, the pressure value of the three densities of the specimen at the time of failure strain increases by about 30 times compared with the pure sand. This shows that the number of layers reinforced by the geogrid has a certain influence on the strength of the specimen, which can enhance the unconfined compressive strength of the specimen, and the compressive strength of the unreinforced specimen is improved.
(2) At n=1, 2, the reinforcement effect of the geogrid is not so obvious, and the pressure does not increase very much in the process of increasing the strain, and the pressure value at the time of failure is about 4 times the pressure of the unadded geogrid. But it is still less than 0.5KN. However, when the number of layers of the geogrid is n=3, 4, 5, the pressure value increases significantly. When ρ=1.55g/cm 3 the sample is n=3, 4, 5 layers, the pressure value at the time of failure is 8, 17, 33 times without geogrid. When ρ=1.75g/cm 3 specimen is in n=3, 4, 5 layers, the pressure value at the time of failure is 6, 19, 33 times without geogrid. When ρ=1.85g/cm 3 the specimen is in n=3, 4, 5 layers, the pressure value at the time of failure is 10, 15, 30 times without geogrid. It shows that excessive density does not promote the increase of specimen strength. Therefore, it is necessary to choose the appropriate density according to the actual engineering situation.
(3) In the above 3 graphs, the curve of strain between ε=4%-8% is almost horizontal, and the pressure value does not change. I think it may be because each layer of the specimen is not very evenly pounded. The top layer is denser. So the pressure value of the sample increases with the increase of strain. But the lower layers may not be as dense as the top layer, and the sample is looser. So there is a period in the process of increasing strain that is that the pressure of the sample does not change much at ε=4%-8%.

Conclusion
Based on all the previous analyses, summarizing the number of layers of reinforcement, the difference of reinforcement materials and the influence of the density of the sample on the strength of the flexible complex, the author believes that the essence of the above phenomenon is the cause of the reinforcement mechanism. Now the more popular reinforcement mechanism is friction reinforcement mechanism and quasi-cohesion theory [1][2][3]. Because the geogrid is a geotextile with holes, the friction with the sand is reduced. If the density is very small and the sand body is very loose, such as ρ=1.55g/cm 3 , then the integrity of geogrid and sand formation is certainly not as good as geotextile and sand body formation. The higher the density of the specimen, the denser it is, and the reinforced material can form a whole, stability and integrity are good. Geogrid reinforcement is a good reinforcement material for flexible composite that improves the compressive strength of sand, and is a good flexible support structure.