Investigation of Shear Strength of Subbase-Subgrade Interface with Geosynthetics Reinforcement Utilizing A Large-Scale Direct Shear Test

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INTRODUCTION
The application of geotechnical engineering, especially in the design and stability assessment of geosynthetic-reinforced soil structures, is of the utmost importance for the interaction involving soils and geosynthetics.In addition, rapid urbanization will inevitably lead to an increase in the need for soil stabilizing techniques so that transportation infrastructure can be built atop subgrade deposits with lower shear strength as well as high compressibility.Geogrids offer strength by laterally restricting the foundation or sub-base and enhancing the system bearing capacity, reducing shear strains on the weak subgrade.Moreover, geogrid confinement enhances vertical stress distribution over the subgrade, reducing vertical subgrade deformation.More than 70 direct shear tests were performed to assess a liner system's interface shear strength properties using project-specific materials under site conditions [1].Abu-Farsakh [2] examined the impact of dry density and moisture content on the cohesiveness of soil-geosynthetic interactions at thorough testing for direct shear.During direct shear testing, Liu [3] assessed the role of transverse ribs in the interaction between geogrid and the soil.For their soil-geogrid interfaces in direct shaer at large-scale device experiments, Liu [4] compared various setups of the shear box dimensions and concluded that a lower box of the same size as the upper box was the most appropriate.The primary benefits and drawbacks of each method were highlighted in [5].Comprehensive investigations on theoretical, experimental, and computational methods for analyzing the interaction between geosynthetics and soils are presented and discussed.Basudhar [6] conducted an experimental investigation on the relationship between woven geotextiles and soils utilizing the direct shear test and two geotextiles with various weave textures and proposed non-linear structure modeling to forecast the interfaces' behavior before and after the peak.
To use a modified large-scale direct shear LSD apparatus, the interface shear strength characteristics of the aggregate of interfaces in geogrid-reinforced building and demolition were studied [7].Various types of materials testing and other recent direct or interface shear test research studies can be found in the literature [8][9][10][11].Based on the abovementioned gaps, additional research is needed on the interfacing parameters between road-base materials and geosynthetics for road-base reinforcement and subgrade stabilization.This research used single-stage testing to conduct a series of direct shear tests on road-based materials without and with the addition of geosynthetic reinforcement.Soil stabilization is the chemical and/or mechanical reduction of compression, contraction, swelling limits, and permeability to increase soil strength and durability [12][13][14][15].

Subbase Material
The sub-base granular materials (SGM) Type B employed in this study were sourced from the AL-Nibaee area north of Baghdad.The physical and chemical properties of the subbase material are shown in Table 1.
Gradation and sieve analysis according to the permitted limits of the Iraqi specification of the subbase material (SCRB /R6, 2003) are exposed in Table 2 as well as in Figure 1. Figure 1: Grain size analysis of subbase.

Clay Soil
The clay soil was brought from the Tajiyat area north of Baghdad, Iraq.Physical tests were done on the soil, and it was assigned the soil classifications (CL) by the Unified of Soil Classification Systems USCS and according to ASTM (D2487-11) as well as (A-6) by AASHTO, ASTM (D3282-09).A grain size distribution and physical and chemical characteristics of the clay soil used in this study are illustrated in Figure 2 and Table 3.

Sand Soil
Physical testing was performed on sandy soil transported from the Iraqi city of Karbala.Soil type is categorized as (SP) soil, according to AASHTO and the Unified of Soil Classification Systems (USCS) according to ASTM (D2487-11), as well as (A-3) soil ASTM (D3282-09).Used sand grain size distribution is displayed in Figure 3; its physical characteristics are exposed in Table 4.

Geosynthetic Reinforcement
The following four geosynthetic product categories, which are frequently utilized in pavement engineering for base reinforcement as well as subgrade stabilization, were chosen: Rectangular apertures geogrid (G1) and biaxial geogrid BX1100, Rectangular apertures geogrid (G2), SS2, PP welded (Square apertures geogrid G3), and HT380PPI (woven geotextile GT), as shown in Figure 4 and Tables 5 to 8 illustrated the characteristics of geosynthetics used in this research.4: Geosynthetic reinforcement that was used in the study.

Testing of Equipment
The present study used a large-scale direct shear apparatus with variable normal stresses (25 kPa, 50 kPa, 75 kPa, and 100 kPa) manufactured by [16] 5) to maintain a constant shear area during the testing.Two horizontal arms were fastened to the machine frame to stop the upper of a shear box from moving.Guide rails were positioned at the base plate, where the low shear box was mounted, to allow for frictionless movement.The lower box was pushed down, and the sample was sheared using a horizontal hydraulic jack.Before each test, the internal walls of the box were covered with a lubricating oil cover to lessen friction at the sidewalls.
Moreover, the device contains a floating upper box separating the higher from the lower shear box.A robust steel plate was utilized as the loading plate, coupled to a loading lever, and the usual load was applied through it.A dead weight served as a counterbalance.A motor, control panel, and gear system powered by a hydraulic jack system sheared the test specimen while maintaining a controlled, consistent shear displacement rate.Two horizontal loadcells with a capacity of 50 kN and two linear varying differentiation transducers (LVDTs) with such a range of (50 mm) made up the measuring apparatus.The sample was sheared, and the vertical and lateral deformations were measured using LVDTs.A Data Acquisition System automated the measurements (DAQ).As shown in Figure 6, during interface testing, the reinforcement was positioned at the junction of the upper and bottom boxes and clamped firmly to the lower box.

Sample Preparation
The appropriate optimal moisture contents were added to the dry materials and carefully mixed to create the samples for the direct shear tests.By compacting the first soil preparation to the targeted unit of weight inside the shear box, the soil for the program's extensive direct shear testing is ready.Three layers are compressed for soil.The clay, as well as the subbase, are compacted with an electric vibrator and a conventional Proctor hammer, as well as the sandy soil is compacted by hand-striking steel of plate that was put on top of the soil until it reaches the desired unit weight as shown in Figure 7.The specimen was compressed in the shear box before being mounted in the apparatus to safeguard the sensitive electrical controllers.
The required mass of wet soil for each layer was determined, precisely measured, and then uniformly compacted to the required height for the required dry density.Before specimens were compacted in the shear box, the upper and lower boxes of the shear box were connected by keeping the two alignment pins in position to ensure that the top and bottom halves were correctly aligned.The geosynthetic specimen is set atop the lower shear box and secured to its front edge by two bolts and a steel clamping block.Four boxes were used for this testing, as illustrated in Figure 8. Normal pressures of (25, 50, 75 kPa, and 100 kPa) were applied to the sample before it was sheared, and it was given 5 minutes to consolidate [17].Once compression under each normal stress was complete, shearing was started under a constant rate equal to (1mm/min) until the shear of displacement reached 30 mm (10 % shear strain) [18], [3], [4], [19].Shear load, vertical displacement, and shear displacement were measured and recorded as the shearing process occurred.

TESTING PROGRAM
Direct shear measurement to simulate interface subgrade stabilization (40 tests).This series of tests was designed to assess the interface parameters for both road-base and sub-grade enhanced with geosynthetics, start comparing the performance of the four different kinds of geosynthetics implanted here between roadbase and subgrade with the case of an unreinforced road-base subgrade, and assess the impact of various soil types in the lower half of the shear box on the interaction shear stress.According to [ASTM D3080], direct shear testing on geotechnical materials (soil-soil) was performed.
According to [ASTM D5321], modified direct shear testing on soil-geosynthetic samples was performed.In order to conform with the [ASTM D5321] interfaces test method, the box size must be at least five times larger than the reinforcement's aperture.This ratio was determined to be 6.1 for (G1) geogrid reinforcements, 5 for (G2) geogrid reinforcements, and 5 for (G3) geogrid reinforcements.

FAILURE OF CRITERION
1-ASTM D3080 (2011) states that the specimen must be sheared to a horizontal displacement of at least 10% of the box's dimension, or 20 mm for a 200 mm-wide shear box.

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E3S Web of Conferences 427, 03007 (2023) https://doi.org/10.1051/e3sconf/202342703007ICGEE 2023 2-The test may be stopped early if the desired shear stresses have been attained, or it may continue until the displacement reaches 75 mm or some other value set by the user, both of which are in line with ASTM D5321M (2014).3-The shear displacements were limited to 10% shear strain and 30 mm shear displacement to avoid too much top cap warping and possibly wrong results [20].These criteria were used in this study.

Peak Shear Strength Envelopes and Shear Stress-Horizontal Displacement Curves
Interface shear stress tests with the inclusion of the four kinds of geosynthetics were performed at the specified unit of weight as well as optimum moisture of contents on the subbase-subgrades soil (clay and sand).For the clay-subbase interface, Figure 9 shows the difference between not.The content of these forms is initially included in the calculations to increase the attached figures for (shear stress-horizontal displacement) curves.We are satisfied with presenting the shear stress-horizontal displacement curves for each test at the applied normal stress of 50 kPa.For the four instances being considered, the interfacial shear stress curves showed a similar pattern of behavior.The figures show that increasing normal strength increases shear stresses for all cases.Furthermore, Figure 10 illustrates the shear stress for the sand-subbase interface.Figures 11 and 12 show the Mohr-Coulomb shear strength envelopes at peak.The shear strength parameters for each case are summarized in Tables 9 and 10.
In subbase-subgrade samples without reinforcement, the interface shear stresses rise with the horizontal displacement until they reach a peak and then oscillate after that.When aggregate particles are sheared, they rearrange themselves, and their levels of interlocking vary, which causes oscillations.However, the junction shear stress-horizontal movement curves of the subbase-subgrade example with geosynthetics do not exhibit a distinct peak even at a comparatively low normal stress of 50 kPa.Figures 11 and 12 show straight lines fitted through Mohr-Coulomb shear strength regions generated by the highest shear stresses.Straight lines fitted through the highest shear stresses produced Mohr-Coulomb shear-strength envelopes.The maximum shear strength range of subbase-subgrade examples with geogrids G1 is greater than that of an unreinforced subbase-subgrade sample due to the effects of particle-grid interlocking.Because of the smooth surface of the geotextile, Subbase-Gt-Subgrade has the lowest interface shear strength under several applied normal stresses.The three distinct geogrid types (G1, G2, and G3) used to support the subgrade, and road base have surprisingly similar shear stress curves because the three geogrids' opening area ratios are similar.• Subbase-Gt-Subgrade has the lowest interface shear strength under several applied normal stresses.In all cases, the value of (η) is less than unity.The values obtained are (0.90) for (subbase-clay) and (0.87) for (subbase-sand).• 6-When the effects of the four different types of geosynthetics assessed on contact shear strength are compared, geogrids frequently generate the most interaction with the shear stress than geotextiles.The soil-geotextile interaction is smaller than that of the soil-soil interface due to the smooth surface of the geotextiles, which greatly reduces interface shear stress.Therefore, extra consideration should be given to the geotextile-reinforced soils when sliding along contact is more likely to occur.• The three types of geogrids (G1, G2, and G3) used to support the subgrade and road base have similar behavior in the shear stress-horizontal displacement curves, and the reason for this is attributed to the convergence of these three types in the size of the openings as mentioned in the characteristics of these geogrids in Table 5 to Table 8.

Figure 2 :
Figure 2: Grain size distribution for clay soil.

Figure 3 :
Figure 3: The sand particle size distribution.
with some modifications.It comprises an upper square steel box measuring 200 * 200 * 100 mm and a lower rectangular steel box measuring 200 * 250 * 100 mm.The low box size was kept greater than the up box shown in Figure (

Figure 5 :
Figure 5: The large-scale direct shear device.

Figure 7 :
Figure 7: Compaction of sample: (a) used steel plate for uniformity, (b) used steel plate and plastic hummer.

Figure 8 :
Figure 8: Set of boxes used in the present study.

Figure 9 :
Figure 9: Modulation of shear stress of horizontal displacement for (sub-base over clay).

Figure 10 :
Figure 10: Modulation of shear stress of horizontal displacement for (sub-base over sand).

Table 1 :
Physical and chemical properties of subbase materials.

Table 2 :
Gradations of the subbase material.

Table 3 :
Physical and chemical properties of clay soil

Table 4 :
Properties of sandy soil.

Table 5 :
Physical properties of the PP geogrid (Tensar International Co.).

Table 9 :
Shear strength characteristics of (sub-base over clay).