This study focuses on the performance of fibers, improving the resistance to liquefaction in loose sands, medium sands, and dense sands in Izmir, Turkey. A systematic testing schedule consisting of cyclic triaxial tests was held under stress-controlled and undrained conditions on saturated sand specimens with and without fiber reinforcements. The major parameters having effects on the dynamic behavior such as fiber content, fiber length, and relative density on the liquefaction behavior and the excess pore water pressure developments of specimens with and without fibers were investigated. If the fiber content or the fiber length was increased in the specimens, higher number of loading cycles was needed in order to experience the liquefaction of sands. The reinforcement effect in medium-dense specimens was found to be apparently distinctive compared to loose specimens. The curves of pore water pressures and shear strains were achieved for the fiber-reinforced sands. The boundaries of pore water pressure curves presented in the literature on the clean sands were utilized in comparison with the pore water pressure curves of fiber-reinforced sands of this study. As a conclusion, the results presented in this study are useful to develop insight into the behavior of clean and fiber-reinforced sands under seismic loading conditions. Based on the test results, it was found that the number of loading cycles had a strong impact on the excess pore pressure generation.
The liquefaction phenomenon in a layer of loose sand under dynamic circumstances occurs by the development of excess pore water pressure and decrement of average effective stress which corresponds to a complete loss of shear strength. Liquefaction may cause damages due to bearing capacity loss of strata, large settlements, tilting of structures, and lateral displacements. Condition of soil could be improved by reinforcement to eliminate the liquefaction hazard. Using reinforcement materials such as fibers in soil medium may provide an alternative to reduce the liquefaction potential. Compared to conventional improvement methods using reinforcement, fibers have some advantages like prevention of potential weak planes which mostly form parallel to plane oriented reinforcement and conservation of isotropic shear strength characteristics [
Wave propagation during the earthquakes originates undrained shear stresses in the soil medium, the particles of soil experience shear strains, and the pore water pressures are generated in the soil medium. Development of excess pore water pressure decreases the stiffness in response to an applied overburden pressure and triggers a vicious circle that causes larger shear strains and higher pore water pressures. At the final stage, the excess pore water pressure reaches a level of initial overburden pressure, and liquefaction is initiated. Since 1970s, analyzing and modelling of excess pore water pressures in soils under earthquake excitations gained interest among the researchers of geotechnical earthquake engineering. In this paper, along with the findings of this study, other literature is also reviewed and compared with the results provided by the experiments. A detailed testing program is followed by conducting experiments on clean sand specimens with varying conditions by using the cyclic triaxial compression testing device. The results of experimental sets are evaluated by stress-based methods, main parameters affecting the behavior and the uncertainties are considered during analyses, and practical solutions are compared with the existing models.
Over the last years, the effects of applying reinforcement materials to increase the shear strength of sands and the factors, including reinforcement type and reinforcement material, soil gradation, and reinforcement dispersion, have been studied only under static conditions by monotonic loadings [
The aim of this study is to identify the liquefaction resistance and the pore water pressure development of fiber-reinforced sand specimens by applying cyclic triaxial tests. The majority of the previous studies have explored the strength and deformation properties of fiber-reinforced soil under monotonic loading conditions; this study particularizes the effectiveness of fibers in the liquefaction resistance improvement of poorly graded sand through some series of dynamic testing. The influence of fibers on the dynamic behavior is investigated in reinforced sand specimens. The sets of experiments included specimens with 0%, 0.25%, 0.5%, and 1% polypropylene fiber contents. Furthermore, the effect of fiber length is investigated by using two different fibers with lengths of 6 mm and 12 mm. The relative density of the specimens was 30%, 50%, and 70%, representing the different stiffness states of the soil. The specimens were consolidated under a confining pressure of 100 kPa, and a cyclic loading frequency of 0.1 Hz was applied. The variations of pore water pressure ratios with number of loading cycles, with fiber content, and with fiber length under constant stress amplitudes are achieved and presented in this study.
A clean sand mass was obtained from an excavation site in the city center of Izmir, Turkey. The classification of the sand showed that it was poorly graded sand (SP) according to the Unified Soil Classification System. The effective size (
Grain-size distribution of the sand.
The monofilament polypropylene (PP) fiber materials used in this study were also produced in Turkey by a local company. The fibers were rectangular in cross section with a specific density of 0.91. The tensile strength of fibers was 400 MPa, and elastic modulus of fibers was 1000–2500 MPa. Fiber lengths were 6 and 12 mm (Figure
Materials used in this study. (a) 6 mm PP fibers. (b) 12 mm PP fibers.
A testing schedule was planned to investigate the effect of fiber reinforcement in sand specimens. The test cases were combinations of relative density of the sand, fiber length, and fiber ratio (Table
Test cases conducted in this study.
Relative density ( |
Fiber length (FL) (mm) | Fiber ratio (FR) (%) |
---|---|---|
30/50/70 | Without | |
6 | 0.25 | |
0.50 | ||
1.00 | ||
12 | 0.25 | |
0.50 | ||
1.00 |
The initial specimen diameter was 50 mm, and specimen height was 100 mm in the experiments. The so-called “undercompaction technique” of Ladd [
The stress-controlled cyclic triaxial tests were performed. The triaxial testing system includes a vertical pressure loading unit with air and water panel, a triaxial cell, a pneumatic sine loader, an electric measurement unit, including, pressure, displacement, and volume change transducers, strain amplifiers, and a dynamic data acquisition system.
The specimens were initially flooded with carbon dioxide; after this step, the specimen was flooded with deaired water, and back pressure was applied to saturate the specimens. Skempton (B) parameter defining the saturation was assured to vary between 0.96 and 1.00. The specimens were isotropically consolidated under 100 kPa of effective stress, and undrained cyclic loading was subsequently applied in a stress-controlled manner. In the liquefaction tests, the loading sequence applies a certain number of cycles necessary to reach a specified level of cyclic stress under a frequency of 0.1 Hz until the specimen develops a double-amplitude (DA) axial strain of 5%. During cyclic loading, continuous digital data were recorded for the following parameters: cyclic axial strain (
Figure
(a) Development of stress path. (b) Variation of stress path with cyclic axial strain. (c) Pore water pressure ratio with the number of cycles. (d) Cyclic axial strain with the number of cycles for a specimen of fiber-reinforced sand. (e) Cyclic deviatoric stress ratio with the number of cycles for a specimen of fiber-reinforced sand (
The main parameters of this study, namely, relative density, fiber length, and fiber ratio on the liquefaction resistance, are presented and discussed. It should be noted that all the outcomes of this study belongs to the experiments which were performed under 100 kPa effective confining pressure. The liquefaction criterion of the tests was to achieve the number of cycles when the specimen developed a double-amplitude (DA) axial strain of 5%.
The cyclic stress ratio is calculated as the cyclic strength normalized by the effective stress. In Figures
Variation of CSR with the number of cycles considering the effect of fiber length (a) FL = 6 mm and (b) FL = 12 mm (WO: without fibers, FR: fiber ratio,
Variation of CSR with the number of cycles considering the effect of fiber length (a) FL = 6 mm and (b) FL = 12 mm (WO: without fibers, FR: fiber ratio,
Variation of CSR with the number of cycles considering the effect of fiber length (a) FL = 6 mm and (b) FL = 12 mm (WO: without fibers, FR: fiber ratio,
The specimens with a fiber ratio of 0.5% and the ones without fibers showed a similar liquefaction resistance. The highest resistance was achieved in specimens that contain 1% of fibers. For specimens having a relative density of 50% and fiber length of 12 mm, liquefaction resistance of specimens that contain no fiber, 0.25%, and 0.5% showed a liquefaction resistance varying in a narrow band. The most remarkable improvement against liquefaction was obtained in specimens with 1% fiber content. For medium dense specimens, the effective length of fiber required to develop shear strength increased with the increase in fiber length at a constant fiber ratio. In this condition, the slippage taking place between individual fibers was reduced with fiber length increment, resulting in improved performance of fibers in soil.
The fiber ratios of specimens were chosen as 0.25%, 0.5%, and 1.0%. For all test cases, the liquefaction resistance was highest for specimens with
Variation of CSR with the number of cycles considering the effect of fiber ratio (a) FR = 0.25%, (b) FR = 0.50%, and (c) FR = 1% (
The cyclic stress ratios corresponding to 20 loading cycles at 5% double-amplitude axial strain are given in Figure
Cyclic stress ratio (CSR) values corresponding to
In liquefaction tests, the pore water pressure develops continuously and reaches the initially applied confining stress after a certain amount of loading cycles. Pore water pressure generation depends on the relative density of the soil and the existing cyclic stress ratio. In addition, shear strain of the soil is a dominating property that relates to the effect of number of loading cycles on the level of pore water pressure. Since four decades, an interest has been raised to evaluate the generation of excess pore water pressure of sands considering the above mentioned parameters, and some numerical models were proposed in literature. However, the behavior of reinforced soils is still not clear and requires further research.
In this section, the main parameters of this study, namely, relative density, fiber length, and fiber ratio on the pore water pressure generation curves, are presented and discussed. It should be noted that all the outcomes of this study belong to the experiments which were performed under 100 kPa effective confining pressure. The liquefaction criterion of the tests was to achieve the number of cycles when the specimen developed 5% double-amplitude axial strain.
In order to model the pore water pressure generation in fiber-reinforced specimens, two different stress-based models were considered, and
The model proposed by Seed et al. [
In this study, the model offered by Seed et al. [
All (a) alpha (
The relationship between the pore-water pressure ratio and number of cycles required to initiate liquefaction for the specimens having 30%, 50%, and 70% relative densities which were consolidated under 100 kPa of overburden pressure are given in Figure
(a) Pore-water pressure ratio (PWP) versus cycle ratio number (
Booker et al. [
Model parameters such as
All (a) alpha (
The same procedure explained in methodology of Seed et al. [
Pore-water pressure ratio (PWP) versus cycle ratio number (
A statistical evaluation of methodology of Seed et al. [
In this study, the finer passing through number 200 mesh was around 0.14%, therefore
All (a) alpha (
Pore-water pressure ratio (PWP) versus number of cycles ratio (
Pore-water pressure ratio (PWP) versus cycle ratio number (
The effect of number of loading cycles on the pore pressure level is basically a shearing strain function [
Variation of double-amplitude axial strain with number of loading cycles for reinforced sand. (a) Comparison of CSR values. (b) Comparison of fiber ratio.
Liquefaction occurred instantly in reinforced sand specimens with lower relative densities; because of this, double amplitude of axial strain follows a perpendicular path (Figure
A series of dynamic experiments was carried out by performing cyclic triaxial tests on poorly graded sand specimens. The sand was obtained in bulk form in an excavation site in Izmir-Turkey. The liquefaction and stress-strain behavior of the sands were investigated in laboratory triaxial tests performed on reconstituted specimens. The confining pressures were 100 kPa reflecting the actual overburden pressure in situ conditions. The frequency of testing was held at 0.1 Hz. Three different relative density values of the sand were considered: loose ( The fiber existence causes a major change in the liquefaction behavior and reduces the susceptibility of soil to liquefy. If the fiber ratio was increased, the number of cycles triggering liquefaction was also increased. Maximum improvement in resistance to liquefaction was for sand specimens reinforced with 1% fibers at CSR values increased with the increment of fiber length. This result is attributed to the development of a better mesh structure in the soil matrix as more grains can interrelate with a longer fiber. The liquefaction resistance of the poorly graded sand increases with an increase in relative density. In medium dense specimens ( In this study model parameter ( Seed et al. [ Statistical calculations depending on the fiber length, fiber ratio, and CSR values are compared with the statistical model offered by Polito et al. [ Fiber reinforcement could be an alternative in lowering or eliminating the lateral movement of the sands caused by liquefaction. Further research is planned to focus on lateral spreading of fiber-reinforced poorly graded sand.
The authors declare that there are no conflicts of interest regarding the publication of this paper.