In recent years, applications of different types of solid waste in fiber-reinforced soil are developed to improve the strength of soil. This study presents an experimental investigation of mechanical properties of polypropylene fiber-reinforced fly ash-soil mixtures. A series of direct shear tests and unconfined compression tests were carried out. The effects of fly ash content and fiber content on compaction characteristics, shear strength, strength parameters, and unconfined compressive strength of the reinforced soil are investigated and discussed. Results reveal that when the fly ash content of the specimen exceeds 20% and the curing period exceeds 14 days, specimens become more brittle in the unconfined compression tests. It can be deduced that 30% fly ash and 1% fiber provide the optimum content, and the inclusion of fiber reinforcement has positive benefits on the mechanical properties of the reinforced soil to a certain extent.
Fly ash, a waste generated in the process of burning coal to produce energy in thermal power plants, has caused a lot of pollution to the environment. The large increase of fly ash production and its environmental protection treatment have become a global concern [
The application of the reinforced soil method in geotechnical engineering has a long history, including in construction, roadways, railways, dikes, stabilization of soil slope, and improvement of soft soil. Certain desired properties such as load bearing capacity, shear strength, and permeability of soil can be improved by various techniques. One of the most commonly used techniques is the use of geosynthetics to reinforce the soil. Latha and Varman [
Wu et al. [
Because of few available literatures on the shear properties of fiber-reinforced fly ash-soil mixtures, effects of fly ash and fiber on the strength characteristics of fine-grained soil are still not clear and further research is needed. Therefore, this paper focuses on the mechanical behavior of clay mixed with different contents of fly ash and fiber reinforcement. A series of direct shear tests and unconfined compression tests are carried out in this study. The influence of fly ash content and fiber content on the shear strength, strength parameters, and unconfined compressive strength of the fiber-reinforced fly ash-soil mixture is thoroughly investigated.
The clay was obtained from a road construction site in Hongshan District, Wuhan. The fly ash was collected from a power plant in the Wuhan City, and it belongs to Class C fly ash. The physical parameters of clay and fly ash were obtained by laboratory tests shown in Table
Physical parameters of clay and fly ash.
Characteristic | Specific gravity | Natural density (g/cm3) | Optimum water content (%) | Maximum dry density (g/cm3) | Liquid limit (%) | Plastic limit (%) |
---|---|---|---|---|---|---|
Clay | 2.68 | 1.35 | 27.8 | 1.42 | 34 | 17.8 |
Fly ash | 2.16 | 2.16 | 22 | 1.36 | 44 | NP |
Testing equipment and materials: (a) polypropylene fibers, (b) direct shear test apparatus, and (c) unconfined compression test apparatus.
Behavior parameters of fiber.
Fiber type | Tensile strength (MPa) | Elastic modulus (GPa) | Elongation at break (%) |
---|---|---|---|
Polypropylene fiber | 512 | 5.2 | 25 |
In the tests, the performance of a total of 20 stabilized soil mixes was investigated by varying the percentage of fly ash and polypropylene fibers. For each mix, two specimens were prepared. The soil was replaced by fly ash contents of 10%, 20%, 30%, 40%, and 50%, respectively, on dry weight basis. Further, four values of fibers (0%, 0.5%, 1.0%, and 1.5%) were considered for each of these mixes. The prepared mixtures were then stored in plastic bags for future use. The desired quantities of water were added to the mixtures as per the optimum water content and further mixed thoroughly to form a homogeneous mixture. The composite was used to estimate the direct shear test and unconfined compression test.
According to ASTM D698-12e2, the standard compaction test was carried out on fly ash-soil mixture to obtain the optimum water content and maximum dry density, and the content of fly ash varied from 0% to 50%. The direct shear test equipment is shown in Figure
The unconfined compression test equipment is shown in Figure
Figure
Standard compaction curves of fly ash-soil mixtures.
Variations of the optimum water content and maximum dry density of fly ash-soil mixtures.
Figure
Shear stress-strain curves of fly ash-soil mixture specimens with different fly ash contents.
The relationship between shear stress peak value and fly ash content under different normal stresses.
Shear stress-strain curves of reinforced fly ash-soil with different fiber contents and vertical normal stresses: (a)
The effect of fiber on the shear strength of fiber-reinforced fly ash-soil mixture can be evaluated by a strength ratio parameter
Table
Peak shear strength of reinforced specimens.
Fiber content (%) | Normal stress: 100 kPa | Normal stress: 200 kPa | Normal stress: 300 kPa |
---|---|---|---|
Peak stress (kPa) | Peak stress (kPa) | Peak stress (kPa) | |
0 | 90.8 | 179.9 | 266.4 |
0.5 | 129.5 | 198.8 | 309.2 |
1 | 187.5 | 246.1 | 373.7 |
1.5 | 156.2 | 232.9 | 342.8 |
Peak shear strength ratios versus normal stress.
The fiber content plays an important role in increasing the shear strength of the fiber-reinforced fly ash-soil mixture. It can be seen from Table
Shear strength parameters of fiber-reinforced fly ash-soil mixture.
Fiber content (%) | Cohesion (kPa) | Internal friction angle (°) |
---|---|---|
0 | 3.43 | 41.28 |
0.5 | 32.8 | 41.94 |
1 | 82.9 | 42.95 |
1.5 | 57.37 | 43.01 |
The reinforcement mechanism of the fiber-reinforced fly ash-soil mixture in the direct shear test has two main aspects: the one-dimensional tensile action of a single fiber and the three-dimensional tensile action of the fiber web. Before the fiber is pulled out, the surface of fiber is surrounded by a large number of soil particles. Since the elastic modulus of fiber is much higher than that of the soil, once fibers are loaded at the same time, the inconsistency of the deformation will inevitably lead to a mutual displacement tendency between the fiber and soil. The fiber is in tension, and the interface force between the fiber and soil particle is generated. Here, the value of interface force mainly depends on the interface friction and adhesion [
Different fiber inclusions may affect the main reinforcement pattern of fiber. For low fiber content, due to the large fiber spacing, an effective fiber web cannot be formed, so the contribution of the fiber to the
It can also be seen from Table
Figure
Relationship between deviator stress and axial strain with different fly ash contents under different curing periods.
The relationship between the peak unconfined compressive strength of the fly ash-soil mixture specimen and the curing period is shown in Figure
Relationship between UCS and curing period for different proportions of fly ash.
The changing amplitude of unconfined compressive strength (
The magnitude of the increase in the unconfined compressive strength value of the specimen is investigated, and the values are shown in Table
Effect of fly ash inclusion and curing period on
Curing period (days) | Different fly ash ratios | ||||
---|---|---|---|---|---|
10% | 20% | 30% | 40% | 50% | |
1 | 13.6 | 29.3 | 8.0 | 26.8 | 35.1 |
7 | −2.2 | 52.0 | 59.4 | 39.0 | 57.4 |
14 | 13.5 | 39.6 | 119.7 | 61.4 | 69.8 |
28 | −0.4 | 37.0 | 126.1 | 77.8 | 59.2 |
A common trend is observed that the unconfined compressive strength of fly ash-soil mixture specimen increases with the fly ash content and the curing period compared to pure soil, except for the 10% fly ash content, which may be caused by the uneven distribution of fly ash in the soil. For specimens with 30% fly ash content, the unconfined compressive strength values are greater than those of other fly ash contents at 7 days, 14 days, and 28 days, respectively, except for 1 day. Table
In order to better evaluate the effect of the curing period and fly ash content on the ductility of specimen, the brittleness index
The relationship between the brittleness index
Relationship between
The effect of fiber content (0.5%, 1%, and 1.5%) in fly ash-soil mixtures after 28 curing days on unconfined compressive strength is shown in Figure
Effect of fiber content on stress-strain curve of specimen after 28 curing days.
Adding fiber to the plain fly ash-soil can improve the unconfined compressive strength of fly ash-soil. This is mainly because the specimen can bear the partial tensile stress when shear deformation occurs under the action of the axial pressure, due to the presence of the fiber. This effect limits the deformation of the specimen, and the more the fiber content is, the greater the tensile stress that can be assumed, resulting that the unconfined compressive strength of fiber-reinforced fly ash-soil is higher than that of unreinforced fly ash-soil. With increasing the fiber content, the unconfined compressive strength of the specimen increases. After the initial cracks occur in the soil specimen, due to the existence of the fiber, the further development of the cracks can be delayed. Consequently, the axial pressure that the soil can withstand does not suddenly decrease, so the fiber-reinforced fly ash-soil exhibits fracture toughness, as shown in Figure
In this paper, the mechanical characteristics of fiber-reinforced fly ash-soil mixtures were studied by standard compaction tests, direct shear tests, and unconfined compression tests. The fly ash contents were 10% to 50% by dry mass of the soil, and the fiber contents were 0.5% to 1.5% by dry mass of the 30% fly ash-soil mixtures. Based on the experimental results, the following conclusions can be drawn: Compared to the unreinforced fly ash-soil mixture, the shear strength of fiber-reinforced fly ash-soil mixture increases significantly. However, the inclusion of fibers which is beyond 1% in the fly ash-soil mixture may result in a decrease in shear strength. For the reinforced fly ash-soil mixture specimens with different fiber contents, the internal friction angle and cohesion increases with increasing fiber contents. Compared with the unreinforced fly ash-soil mixture, the cohesion and internal friction angle of the fiber-reinforced fly ash-soil mixture when fiber contents vary from 0.5% to 1% are increased by 29.37 kPa to 79.47 kPa and 0.66° to 1.67°, which indicates that the fiber content has more obvious effect on the cohesion of soil. The unconfined compressive strength of fly ash-soil mixture is higher than that of plain soil under the same curing period, and there is an obvious tendency to increase at first and then decrease when the fly ash content exceeds 10%. Moreover, after the shear stress reaches the maximum value, the curve of the fly ash-soil is flatter than that of the plain soil and the specimen with 30% fly ash content shows more ductility. With increasing fly ash content and curing period, the brittleness index When the fiber content is 1%, the unconfined compressive strength of the specimen reaches a peak. But, the residual strength is still lower than the specimen with the 1.5% fiber content.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
The authors are thankful for the financial support given by the Hubei Provincial Science Foundation for Distinguished Young Scholars (No. 2018CFA063), National Natural Science Foundation of China (Nos. 51678224, 51778217, and 51708190), Hubei Central Special Fund for Local Science and Technology Development (No. 2018ZYYD005), and National Key R&D Program of China (No. 2016YFC0502208).