Performance of Reinforced Subbase Materials by Geogrid as Abase Layer Under Weak Subgrade

. The current study examines how an unpaved road behaves when it is installed on top of a soft clay layer that's been strengthened at the subgrade/subbase interface with a single layer of geogrid type SS2 is used to reinforce a laboratory model, and cycling load is used to apply repeated loads to the model. Subbases with and without reinforcement over soft subgrade were built in a sizable test box. (1000 mm×1000 mm×1000 mm). The test results show that using a geogrid reinforcement layer enhances the number of cycles and stress distribution while decreasing displacement on the soft subgrade surface on reinforced models compared to similar unreinforced ones for the same rutting value (85 mm). The rate of increase in failure time is about (6.12, 3.7, and 1.7) for 150, 230, and 300 mm thickness, respectively. The rate of redaction in vertical stress is about (18, 12.5, and 7.1) % for 150 mm, 230 mm, and 300 mm thickness, respectively.

2. According to the result of CBR variation with water content shown in Figure 3, the soil was prepared at CBR (2%) in a manufactured box corresponding to a water content of (20%).

Subbase
Subbase Type B was collected from the AL-Niba'ee region north of Baghdad to be used in this study.Table 2 shows the results of both chemical and physical tests on the subbase material.Figure 4 and Table 3 show the gradation and sieve analysis of the subbase material according to the permitted limits of the Iraqi specification (R6).4: Grain size analysis curve of subbase.

Geogrid
The geogrid employed in this study was SS2, as shown in Figure 5, and was manufactured by QMOF Company (Quality Material of Oil Field) [16].The physical and mechanical parameters of the geogrid type SS2 are shown in Table 4.  kN/m 23.0

MODEL DESIGN AND MANUFACTURING
Figure 6 depicts an experimental setup designed and manufactured at a scale of about one-tenth that of the prototype.The test apparatus consists of a steel tank box (1.0 m x 1.0 m x 1.0 m).A cyclic load was applied using a 200 mm diameter circular steel plate under a hydraulic jack connected to a hydraulic system.In all tests, the frequency was set to 1 Hz.In all tests, the cyclic applied load was 20 kN, resulting in a pressure load of 552 kPa.It was a typical truck axle load simulation with a contact pressure of 552 kPa.A shaft encoder to measure the displacement in the middle of the model, displacement transducers (LVDT) to detect vertical displacements, and a programmable logic controller (PLC) to measure the loads during cyclic loadings At various points in the subgrade, a flexible force sensor and strain gauge are employed to detect the forces and strains, respectively.

TEST PREPARATIONS
The subgrade soil was dried, homogeneously mixed with a moisture content of 20%, and then placed in nylon bags for 24 hours.The subgrade was then placed in the box test in 6 layers, with each layer being compressed well to reach a height of 60 cm.Each layer was compacted well by steel tamping (10x20) cm.After it had been prepared, three strain gauges and three force sensors were installed on the center and sides of the subgrade.A steel beam fixed a four (LVDT) above the box to detect displacement while being loaded.The geogrid was positioned on top of the subgrade for the reinforced sections following the installation of the sensors, as shown in Figure 7.The base course thickness was adjusted based on the test sections (15 cm, 23 cm, and 30 cm).Repeated plate load tests were performed on the base in both its reinforced and unreinforced states.The LVDT was positioned on the sample and connected with a data logger and computer, is shown in Figure 8.

RESULT AND DISCUSSION
The failure criteria used in this study are 85 mm permanent deformation in the center of the base layer or when 1000 cycles are reached, whichever is more likely.This study carried out six tests, with and without reinforcement, three reinforced tests with geogrid, and three unreinforced.The interface between the subgrade and the subbase was reinforced.

Displacement -Time Relationship for the Model Tests
In unreinforced models, the vertical displacement of the unpaved layers is measured at the surface of the subbase layer.All unreinforced tests of three samples of 15 cm, 23 cm, and 30 cm thickness failed.A typical relationship between displacement and loading cycles is presented in Figure 9, showing that displacement increased with the increasing number of cycles, and this matches what he found [17].And this thing is selfevident since the base layer is stronger than the subgrade layer.For the reinforced models, the vertical displacement of the unpaved layers is measured at the surface of the subbase layer.From Figure 10, it is noticed that the model needs more cycles, i.e., a longer time when the thickness of the base layer is increased, In addition to the above, the models reinforced with the geogrid layer showed our improvement clearly, as the 150 mm subbase model lasted 80 seconds, while the reinforced model lasted approximately 600 seconds, while the unreinforced 230 mm subbase model lasted 170 seconds, while the reinforced model lasted 800 seconds, and the reinforced model 300 mm subbase lost 370 seconds, and the reinforced model gave a displacement of 65 mm for a period of 1000 seconds.[18] demonstrated how shear contact between the aggregate and the geogrid during the base's attempt to extend laterally is made possible by the presence of one or more geogrid layers at the base's bottom.The geogrid was put in tension by the shear load that was transferred from the base aggregate.The geogrid's comparatively high stiffness worked to prevent lateral tensile strain from developing in the base next to the geogrid.Geogrids can improve the performance of a pavement segment built on a low CBR subgrade, which matches with [17].The full-scale pavement test findings from the University of Illinois clearly demonstrated the performance advantages of employing geogrids, particularly in the decreased horizontal base course motions.One of the main goals of the geogrid reinforcement, which disperses load over a vast area, is to flatten the distorted shape of the subgrade, which is related to a more uniformly distributed stress on the subgrade surface.

Vertical Stress -Time Relationship for the Model Tests 5.2.1 Vertical Stress -Time Relationship for Unreinforced Model
From Figures 11 to 13, it can be shown that for unreinforced models, the vertical stress was reduced by increasing the base course layer thickness.This may be due to the degradation of the base course with repeated loading cycles.The vertical stress increases, which causes the stress distribution area to become smaller, which agrees with [16].Due to the drop in the base course's modulus and the stress distribution reduction angle, which matches [19].

Vertical Stress -Time Relationship for Reinforced Model
Figures 14 to 16 demonstrate that, compared to the unreinforced model, the vertical stresses decreased as the base thickness increased at around (18, 12.5, and 7.1%).This could be explained by the fact that the stress distribution angle has decreased due to the base course's degradation, which agrees with [20].The vertical stresses may have lowered because of the improved particle geogrid interlock, consistent with [21].Result from the creation of a stable composite between the base materials and geogrid.Because of the reorientation of the subbase particles over time, there is less interlocking at first and more over time, which is consistent with [16].

Strain-Time Relationship for the Model Tests 5.3.1 Strain-time relationship for unreinforced models
From Figures 17 to 19, It can be noted that the increase in the thickness of the base layer leads to a decrease in the strain at the interface and a decrease in the strain in the middle of the subgrade, but the strain at the edge is as little as possible, and this is self-evident because the strain is distributed, the greater the thickness of the base layer, the less stress reaches the base.

Strain-time relationship for reinforced models
The relationships between strain and time are shown in Figures 20 to 22.The strain value decreases when using geogrid between the base layer and subgrade and decreases as the base layer's thickness increases.This can be attributed to a fraction of geogrid carried lateral movement and tension component.The main uses of the geogrid reinforcement, which transports loads across a large area, agree with those of [20][21].

CONCLUSIONS
The following outcomes can be deduced from repeated load tests on test sections of unsurfaced pavement reinforced with and without geogrid: • Using geogrids can greatly enhance the performance of a pavement segment built on a low CBR subgrade.• Increasing the subbase thickness and utilizing geogrid reinforcement both increase the number of cycles in reinforced models, with the unreinforced model experiencing an increase in cycles with the highest percentage.Adding the geogrid layer at the intersection of the subbase and subgrade results in a more flattened deformed shape on the subgrade surface, resulting in a lower displacement value for the same subbase thickness.For all models, the displacement on the subgrade surface reduces as the subbase thickness increases, with the unreinforced model showing the biggest percentage decrease.

Figure 1 :
Figure 1: Grain size distribution curve of clay soil.

Figure 8 :
Figure 8: Sample during the test.

Figure 11 :
Figure 11: Vertical stress with time relationships for 150 mm subbase in the interface middle and edge.

Figure 12 :
Figure 12: Vertical stress with time relationships for 230 mm subbase in the interface, middle, and edge.

Figure 13 :
Figure 13: Vertical stress with time relationships for 300 cm subbase in the interface, middle, and edge.

Figure 14 :
Figure 14: Vertical stress with time relationships for 150 mm subbase in the interface, middle, and edge.

7 E3SFigure 15 :
Figure 15: Vertical stress with time relationships for 230 mm subbase in the interface, middle, and edge.

Figure 16 :
Figure 16: Vertical stress with time relationships for 300 mm subbase in the interface, middle, and edge.

Table 1 :
Physical and chemical properties of clay soil used in the study.

Table 2 :
Physical and chemical properties of the subbase material.

Table 4 :
Physical and mechanical properties of geogrid.