Unbound base layers deform under load and contribute to the accumulation of ruts. Therefore, this study was concerned with studying the effect of reinforcement on the behavior of unbound granular material that used in flexible pavement layers as a base course. Two main geothynthetic types were used in this study. These types were woven geotextile and geogrid. Two geogrid opening sizes were used (GR105 and GR420). The experimental work was designed to evaluate plastic and elastic deformation and the modulus of elasticity of reinforced limestone base course. This experimental work carried out utilizing the static plate loading test in a test-model which simulated the subgrade and base course of the flexible pavement. The effect of base thickness, geogrid depth, modulus of elasticity of base course and geogrid edges fixation on the deformation characteristics were studied. Furthermore, the effect of loading time on the accumulated deformation was investigated. Moreover, the effect of reinforcement on base thickness saving (BCR) and deformation reduction ratio (DRR) was studied. A great influence for reinforcement especially with geogrid (GR420) was observed in improving the deformation characteristics of base course.
Major pavement deteriorations, similar to those observed in some Egyptian roads, especially in Delta region, result basically from permanent deformation in base course or subgrade soil. This deformation causes alligator or map cracking, chuck holes, settlement and undulations. In recent years, geosynthetics have been proposed and used to improve the performance of paved roadways. The major functions of geosynthetic materials are separation, reinforcement, filtration, drainage and liquid barrier. In providing reinforcement, the geosynthetic material structurally strengthens the pavement section by changing the response of the pavement to loading. Studies to date have found that incorporation of geosynthetics in flexible pavement provides a degree of performance improvement. A few studies have tried to quantify the benefits of geosynthetic reinforcement, but no firm conclusions can be drawn due to differences of results. Thus, an important need exists to quantify the benefits derived from stabilizing flexible pavements with geosynthetics and the conditions necessary for successful geosynthetic stabilization if an adequate cost comparison is to be made.
Fannin and Sigurdsson (1996) [
The improvement in plastic surface deformation base course was investigated by Leng and Gabr (2002) [
In granular material layers, the mechanism of rut depth reduction through geosynthetic reinforcement may be explained the Lateral movements are prevented by aggregate confinement, leading to increase in bulk stress, and aggregate layer stiffness, along with decrease in vertical stress on top of subgrade and vertical compressive strain reduction in lower half of base and in the subgrade. Over the period of pavement construction, there are usually two feasible alternatives for ground improvement, namely, soil stabilization and geosynthetic reinforcement. At times, some of the contractors prefer to use geosynthetics to reinforce subgrade [
An experimental program was carried out to investigate the influence of geothynthetic as reinforcement for the granular base layer of a flexible pavement constructed on silty subgrade. Plate loading test was performed as a control test to evaluate the deformation characteristics and bearing capacity of reinforced and unreinforced base course.
A silty soil was used as subgrade. Crushed limestone was used as a base course. The grain size distributions as well as the grading limits according to AASHTO specifications for subgrade soil and base course are illustrated in
Two polyethylene geogrids (GR105 and GR420) with different opining size as shown in
Test | Subgrade soil | Base course |
---|---|---|
Natural moisture content, % Liquid Limit, % Plastic Limit, % Specific Gravity, gm/cm3 Loose density, gm/cm3 Maximum dry density, gm/cm3 Optimum moisture content, % AASHTO classification group Unified classification group | 7.0 54.0 40.0 2.68 1.33 1.665 16.0 A-7-5 MH | 1.30 19.0 13.6 2.65 1.70 2.12 7.13 A-2-4 GP |
Test | Subgrade soil | Base course |
---|---|---|
Cohesion (N/mm2) Internal friction(o) CBR (%) Unconfined comp. strength (N/mm2) Modulus of elasticity (N/mm2) | 0.067 19 8.8 0.165 4.88 | 0.055 23 97.0 - 45.0 |
Geosynthetic Type | Geogrid Type | Geotextile | |
---|---|---|---|
GR105 | GR420 | ||
Tensile strength (kN/m\) | 5.70 | 1.65 | 1.71 |
Elongation at Max. Load (%) | 50 | 40 | 34 |
Modulus of Elasticity (N/mm2) | 62.63 | 22.9 | 34.9 |
The test-mode consisted of a square iron box 0.5 m wide by 0.5 m long and 0.5 m depth. This box divided into two halves containing two layers, 0.25 m depth subgrade, and limestone base course with 10, 15 and 25 cm depths. The geosynthetic layer was placed at the interface between subgrade and base course and at different depths inside the base layer.
Initially, the subgrade soil was prepared by adding optimum moisture content and compacted in five layers. Then, the geosynthetic was incorporated in the aggregate at a specified location. After that, the base course material was prepared by adding the optimum moisture content (8%). Finally, the base course material was compacted in layers to obtain thickness of 10, 15 and 25 cm. At 25 cm base thickness for reinforced and unreinforced sections, four moisture contents were used (OMC−2%, OMC, OMC+1.5%, OMC+3%).
In this study, a contact pressure of 0.5 N/mm2 (70 Ib/in2) on asphalt surface layer was considered. BISAR-Linear elastic program was used to calculate the vertical stress at the surface of base course considering 5.0 cm asphalt wearing course and 5.0 cm asphalt binder coarse. The results indicated that vertical stress decreased to 0.35 N/mm2 on the top of the base course.
An initial static pressure of 0.0875 N/mm2 was applied on the steel plate by using the loading head, the deflection was allowed to reach a maximum (waiting time about 20 min.). As shown in
where:
E: modulus of elasticity (Mpa);
p: uniform applied pressure (Mpa);
a: radius of circular plate (mm);
w: deflection corresponding to the third load on the rigid plate (mm).
The plate loading test result for unreinforced 10 cm base course is shown in
The major objectives of this research were studying the influence of reinforcement, moisture contents of base course and geogrid fixation on elastic and plastic deformation. Moreover, the effect of base thickness, geogrid position and loading time on the deformation characteristics were investigated. For each base course thickness, reinforced sections (RS) and unreinforced sections (URS) were performed.
The amount of total deformation and the modulus of elasticity (E) values for each reinforcement case are shown in Tables 4-6 for each base thickness (h). The reinforcement depth (Dr) was investigated. Moreover, the reinforcement benefit ratio (RBR) was obtained as the reduction ratio in total deformation between the reinforced and unreinforced sections.
From
From the plate loading test results after the third loading cycle, elastic and plastic deformation could be calculated.
For all studied base thickness of unreinforced sections, the plastic deformation was found to be greater than the elastic deformation under the plate center. However, with increasing the distance from the plate center, the elastic deformation became greater than plastic deformation. For reinforced sections, it could be concluded that for base thickness less than 25 cm, the plastic deformation became greater than elastic deformation at all points. For base thickness of 25 cm, the plastic deformation became greater than the elastic deformation at all points at lower geogrid depth (Dr\h less than or equal to 0.2) however, at higher geogrid depth (Dr\h more than 0.2), the plastic deformation became greater than the elastic deformation under the plate center only.
The effect of loading time up to 48 hours on the accumulated deformation under a static load was performed for the URS and reinforced section (fixed BRS for 10 cm base course and DRS for 15 and 25 cm base course). A
Benefit Ratio (%) (RBR) | E (N/mm2) | Total Deformation (mm) | Reinforcement Case | Geogrid Type |
---|---|---|---|---|
42 | 0.92 | Unreinforced section | None | |
1.08 | 42.46 | 0.91 | 1-Bottom unfixed geotextile | Geotextile |
−110.8 −121.7 −65.2 | 19.9 18.94 25.42 | 1.94 2.04 1.52 | 1-Composite reinforcement 2-Bottom unfixed geogrid 3-Middle unfixed geogrid | GR105 |
4.34 7.6 −4.3 | 43.9 45.46 40.25 | 0.88 0.85 0.96 | 1-Bottom unfixed geogrid 2-Bottom fixed geogrid 3-Middle unfixed geogrid | GR420 |
Benefit Ratio (%) | E (N/mm2) | Total Deformation (mm) | Reinforcement Case | Geogrid Type |
---|---|---|---|---|
45.45 | 0.85 | Unreinforced Section (URS) | None | |
−4.7 | 43.41 | 0.89 | 1-Bottom Unfixed Geotextile | Geotextile |
3.53 12.94 4.47 6.82 7.65 21.18 | 47.12 52.21 47.58 48.21 49.22 57.67 | 0.82 0.74 0.812 0.792 0.785 0.67 | 1-Bottom Unfixed Geogrid 2-Bottom Fixed Geogrid 3-Composite Reinforcement 4-Middle Unfixed Geogrid 5-Middle Fixed Geogrid 6-Two Layers Reinforcemend | GR420 |
Benefit Ratio (%) | E (N/mm2) | Total Deformation (mm) | Reinforcement Case | Geogrid Type |
---|---|---|---|---|
47.7 | 0.81 | Unreinforced Section (URS) | None | |
−58 6.79 5.55 −4.94 2.47 14.8 16.3 | 30.19 51.18 50.5 45.46 48.9 56 57 | 1.28 0.755 0.765 0.85 0.79 0.69 0.678 | 1-Dr/h = 0.2 2-Dr/h = 0.4 3-Dr/h = 0.6 4-Dr/h = 0.8 5-Bottom Unfixed Geogrid (Dr/h = 1) 6-Bottom Fixed Geogrid (Dr/h = 1) 7-Two geogrid layers (Dr/h = 1 and 0.4) | GR420 |
great influence for reinforcement was observed where the accumulated deformation curves for reinforced sections were a semi constant or increased slightly with increasing the loading time especially at the end of the test period. Summary of the deformation progress under the plate center and at distances of 10 and 20 cm for 10 cm base course are represented in
Three base course thickness 10, 15 and 25 cm and additional unreinforced thickness of 40 cm were used. As shown in the previous results and in
From
Moisture content (%) | 6 | 8 (OMC) | 9.5 | 11 | ||||
---|---|---|---|---|---|---|---|---|
Section condition | URS | RS | URS | RS | URS | RS | URS | RS |
RBR (%) | --- | 23.5 | --- | 16.3 | --- | 12.5 | --- | 6.75 |
Using geogrid GR420 as reinforcement had the greatest effect on the reduction of base thickness (BCR) and the plastic deformation reduction ratio (DRR) where the base course thickness of 15 cm could be reduced to 10 cm (BCR = 33%, DRR = 14%) if reinforced with fixed bottom geogrid. Moreover, the unreinforced section of 25 cm base thickness could be reduced to 15 cm (BCR = 40%) if it is reinforced with fixed BRS or DRS to achieve DRR = 8.3% or 21.7% respectively. Furthermore, the unreinforced section of 40 cm base thickness could be reduced to15cm (BCR = 62.5%, DRR of 14.5%) if it is reinforced with DRS, and could be reduced to 25 cm (BCR = 37.5%) if reinforced with fixed BRS or DRS to achieve DRR of 5.7% or 12.7% respectively.
1) The geogrid GR420 was the optimal type to be used as reinforcement for base course where a great reduction of plastic deformation was obtained. Moreover, Fixation of the geogrid edges had a great effect on reduction of accumulative plastic deformation of base course.
2) The bottom reinforced section (BRS) was better than middle reinforced section (MRS). For 25 cm base course the optimal reinforcement depth ratio was obtained at (Dr/h = 0.4 to 0.6). By using geogrid GR420 the unreinforced section of 40 cm could be reduced to15cm (BCR = 62.5%, DRR of 14.5%) if it was reinforced with DRS.
3) For reinforced base course less than 25 cm thickness, the plastic deformation became greater than elastic deformation at all points. The same occurred for 25 cm thickness at lower geogrid depth (Dr\h less than or equal to 0.2). On another side, the accumulated deformation curves for reinforced sections were a semi constant with increasing the loading time up to 48 hours especially at the end of the test period.
4) With increasing moisture content, the reinforcement benefit ratio RBR decreased. The ratio of plastic deformation (PDR) for reinforced section was lower than it for unreinforced section for all moisture contents especially above OMC. Moreover, for all moisture contents, the plastic deformation was greater than elastic deformation under the plate center only. The same occurred at other points for 9.5% and 11% moisture content. While, the opposite occurred at 6% and 8% moisture content.
Ahmed Ebrahim Abu El-Maaty, (2016) Improving Rutting Resistance of Flexible Pavement Using Geosynthetics. Open Access Library Journal,03,1-11. doi: 10.4236/oalib.1102655