The interface shear strength properties of geogrid reinforced recycled foamed glass (FG) were determined using a large-scale direct shear test (DST) apparatus. Triaxial geogrid was used as a geogrid reinforcement. The geogrid increases the confinement of FG particles during shear; consequently the geogrid reinforced FG exhibits smaller vertical displacement and dilatancy ratio than FG at the same normal stress. The failure envelope of geogrid reinforced FG, at peak and critical states, coincides and yields a unique linear line possibly attributed to the crushing of FG particles and the rearrangement of crushed FG after peak shear state. The interface shear strength coefficient
Lightweight fill materials are increasingly being used as construction materials in civil engineering applications. The prime purpose for lightweight fill materials is to reduce the weight of fills, thereby mitigating excessive settlements and bearing failures [
Reuse applications for industrial waste materials are increasingly being sought, inclusive of demolition wastes [
In recent years, there has been interest in the development of lightweight foamed materials with the usage of waste materials in engineering applications [
The focus of this research is to extend the research of FG usage as a lightweight fill material by evaluating the interface shear strength properties when reinforced with geogrids. Due to high tensile strength of geogrid, the overall shear strength of the geogrid reinforced FG would be improved. The geogrids are commonly used as extensible earth reinforcement for embankment and mechanically stabilized earth wall in Asia [
FG for this research was obtained from a supplier in Melbourne, Australia. The FG was manufactured by firstly grounding glass collected from the municipal waste stream. The recycled glass was fired with mineral additives in a furnace at temperatures up to 950 degrees Celsius. The recycled glass, comprising 98% ground recycled glass and 2% mineral additives, then foams and was removed from the furnace at which point it cooled down quickly forming low weight FG aggregates of up to 40 mm in size [
Close-up photo of FG.
Particle size distribution curves of FG are shown in Figure
Gradation curves for FG.
The CBR values of FG were 9–12%, which are within the local road authority specification requirements (typically a minimum of 2–5%) for a structural fill material in road embankments [
A commercially available triaxial geogrid used to reinforce FG in this study is made of polypropylene and has an aperture size of 46 × 46 × 46 mm and unit weight of 0.25 kg/m2. The triaxial geogrid has a higher tensile strength than biaxial geogrids and has in recent years become increasingly available commercially for various civil engineering applications. The mechanical properties of the triaxial geogrid used had an ultimate tensile strength of 32 kN/m and a failure strain of 18%, as determined from a tensile strength test undertaken using a 500 kN capacity universal testing machine, with geosynthetic grips.
A large-scale direct shear test (DST) apparatus, with shear boxes measuring 305 mm in length, 305 mm in width, and 204 mm in depth, was used to evaluate the interface shear strength response of FG with geogrid reinforcement. Control tests were undertaken on unreinforced FG aggregates to compare the effect of the geogrid reinforced FG with unreinforced FG. The tests were conducted as per ASTM [
The large-scale DST apparatus has two boxes: a fixed upper box and a moveable lower box. Initially, the lower and upper boxes were clamped when preparing samples for the tests. The samples were compacted in the shear box in three layers by using hand tamping with a plastic hammer to attain the maximum dry density of 290 kg/m3 obtained from the vibratory table method. The samples were submerged prior to the commencement of the consolidation stage, by filling the entire lower shear box and half of the upper shear box with water. The consolidation stage was for 12 hours with three normal stress levels of 10 kPa, 20 kPa, and 40 kPa. When the consolidation stage for the tests was completed, the connection between the lower and upper boxes was released, which provided an approximate 2 mm gap between the upper and lower boxes for friction minimization. The shearing stage of the test was next conducted under the same normal stress levels of 10 kPa, 20 kPa, and 40 kPa. A shear displacement rate of 0.025 mm/min was maintained throughout the shearing stage. The horizontal displacements, vertical displacements, and shear stresses were recorded. The tests were terminated once the horizontal shear displacement reached approximately 75 mm. The room temperature was maintained at
Shear box after completion of an interface shear test.
Figure
DST test results of FG.
FG was found to exhibit completely dilatant behavior in vertical displacement and horizontal displacement relationship for all normal stresses tested while exhibiting strain-hardening behavior in shear stress versus displacement relationship. This shear response is in contradiction to the typical shear response for typical coarse-grained geomaterial, where dilatant behavior is associated with strain-softening behavior and the peak shear strengths are attained at the maximum dilatancy ratio (slope of the relationship between vertical displacement and horizontal displacement). The increase in shear stress even after the maximum dilatancy ratio (strain hardening) is caused by the rearrangement of crushed particles (fine crushed particles are driven into the voids or pores). The shear response of FG is found to be similar to that of recycled glass cullets that has been used as aggregates in pavements and designated as having dilatancy associated strain-hardening response [
The vertical displacement versus horizontal displacement relationships are almost the same for low normal stresses of 10 and 20 kPa. The relationship for 40 kPa normal stress diverts from the relationships for the low normal stresses, indicating lower maximum dilatant displacement of 6 mm (half of the dilatant displacement for normal stresses of 10 and 20 kPa). The maximum dilatancy ratios (
The cohesion and friction angle based on the Mohr-Coulomb failure criterion at peak and critical (end of test) states are shown in Figure
Shear strength failure envelope for unreinforced FG.
Figure
DST test results of geogrid reinforced FG.
It is found that the peak interface shear stress of geogrid reinforced FG is slightly lower than the peak shear stress of FG, whereas the interface shear stress of geosynthetics reinforced backfill material is commonly significantly lower than the shear strength of backfill material. The high interface shear stress can be attributed to the geogrid having a large aperture size of greater than the average grain size of FG, about 2.6 times and having high tensile strength and stiffness. The geogrid prevents the movement of FG particles; hence the FG particles reorientate around each other, as the FG particles are unable to slide on the geogrid. Consequently, the interface shear strength is mainly contributed from the highly confined soil to soil interaction.
The effect of confinement is also observed by the stiffness of the interface shear stress versus displacement of the geogrid reinforced FG, which is higher than that of the shear stress versus displacement of FG (comparing Figures
The interface shear strength parameters for geogrid reinforced FG are determined based on the Mohr-Coulomb failure criterion and shown in Figure
Interface stress failure envelope for geogrid reinforced FG.
FG having low tensile strengths can fail by tensile stress due to the traffic load. When the geogrid reinforcement is introduced, the modes of failure of the composite material will be by either tension (or rupture) or slip failure. The rupture failure happens when tensile stress in the geogrid exceeds its tensile strength while the slip failure, which is movement of the FG on the geogrid reinforced FG, is controlled by the interface shear strength. The interface between geogrid and FG can be expressed as the interface shear strength coefficient [
Relationship between coefficient of soil-reinforcement friction
A large-scale DST apparatus was used to determine the interface shear strength properties of geogrid reinforced FG. Tests were undertaken on each of the respective FG aggregates when reinforced with triaxial geogrids. Tests were also undertaken on unreinforced FG aggregates for comparisons. The key conclusion is drawn as follows: Even though the FG is classified as coarse-grained material, the direct shear response of FG under drained condition is in contradiction to the typical shear response for typical coarse-grained geomaterial, where dilatant behavior is associated with strain-softening behavior and the peak shear strengths are attained at the maximum dilatancy ratio. This difference can be attributed to the breakage of FG particles during shear. The peak and critical failure envelopes are found to coincide and are represented by a linear line. With high strength parameters, FG can be used as lightweight backfill for MSE wall. The geogrid reinforced FG exhibits strain-hardening behavior in interface shear stress versus horizontal displacement relationship, which is similar to FG. The interface shear strength and interface shear stiffness increase while the dilatant displacement reduces with increasing normal stress. The interface shear strength of geogrid reinforced FG is slightly lower than the shear strength of FG. The high interface shear strength of geogrid reinforced FG can be attributed to the geogrid having a large aperture size of greater than the average grain size of FG of about 2.6 times and a high tensile strength. Consequently, the interface strength is contributed by highly confined soil to soil interaction. The geogrid increases the confinement and prevents shear and vertical displacement of FG. For the same applied shear stress, the shear displacement of the geogrid reinforced FG is smaller than FG. At the same shear displacement, the dilatant displacement of geogrid reinforced FG is significantly lower than that of FG, especially at high normal stress. Similar to the failure envelope of FG, the interface failure envelope at peak and critical states coincides and yields a unique set of interface shear strength parameters. The interface shear strength coefficient This research will enable FG, which has been recently assessed as suitable for lightweight backfill, to be used in combination with geogrid in a sustainable manner as a lightweight MSE wall. The geogrid carries the tensile force due to dead and live loads on the reinforced embankment and MSE wall while FG causes low bearing stress on foundation. This lightweight MSE wall is applicable particularly for soft clay deposits in coastal regions. The use of geogrid reinforced FG is thus significant from engineering, economical, and environmental perspectives.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The second and fourth authors are grateful for the financial support from the Thailand Research Fund under the TRF Senior Research Scholar program Grant no. RTA5680002, Suranaree University of Technology, and the Office of Higher Education Commission under NRU Project of Thailand. The fifth author is grateful for the financial support from National Natural Science Foundation of China (Grant nos. 51278100 and 41472258).