Fiber reinforced sandy slopes under groundwater return flow

Authors: AKAY, ONUR - Okan Universitesi; OZER, TOLGA - Okan Universitesi; FOX, GAREY - North Carolina State University; Wilson, Glenn

Submitted to: Journal of Irrigation and Drainage Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/19/2017
Publication Date: 2/16/2018
Citation: Akay, O., Ozer, T., Fox, G.A., Wilson, G.V. 2018. Fiber reinforced sandy slopes under groundwater return flow. Journal of Irrigation and Drainage Engineering. 144(5):1-10. 10.1061/(ASCE)IR.1943-4774.0001300.
DOI: https://doi.org/10.1061/(ASCE)IR.1943-4774.0001300

Interpretive Summary: Seepage is a major cause of failure or slumping of natural hillslopes, river banks and engineered embankments such as levees and dams. To prevent failure, a new technology that involves mixing man-made fibers into the soil is becoming popular. The addition of such fibers has a favorable effect on the strength of sandy soils. In this study, laboratory experiments were conducted in sand-filled boxes on fiber reinforced soil with two different water levels (25 cm and 50 cm) in the water reservoir. Fiber reinforced sand was compacted in the box with a slope with dimensions of 55 cm height, 20 cm width, and 100 cm base length. The fiber content was selected to be 1% fiber per weight of dry sand based upon results of tests on fiber reinforced sand samples with varying fiber (12 mm long) contents from 0.1% to 1%. This study included slope stability calculations in order to estimate the overall factor of safety. The compression tests measured the increase in peak stress with increase in fiber content. The fiber reinforced sand in the experiments was stable against seepage conditions which would otherwise cause a shallow failure of the non-reinforced slope under 25 cm water pressure head. However, the soil with fiber showed a shallow failure under 50 cm water pressure head, whereas a large failure occurred for the non-reinforced soil as soon as the water level was raised above the top of the slope. Slope stability calculations showed the positive effect of including fiber in soil due to the increased strength which increased the factor of safety of slopes.

Technical Abstract: The instability of natural hillslopes, riverbanks and engineered embankments due to seepage has been a major concern. In an effort to prevent failures, tension resisting synthetic fibers may be an effective additive to increase the mechanical properties of engineered soils. In this study, triaxial compression tests measured the increase in peak deviatoric stress with increase in fiber content and length. In addition, laboratory lysimeter experiments (total of eight experiments) were conducted on sandy slopes reinforced with polypropylene (PP) fiber using two different fiber lengths (6 mm and 12 mm) and gravimetric fiber contents (0.3% and 1.0%) under two different constant piezometric head boundary conditions (25 cm and 50 cm) maintained in the water reservoir of the lysimeter. Fiber reinforced sand was compacted in the lysimeter to obtain a 45 degree slope with dimensions of 55 cm height, 20 cm width, and 100 cm base length. The only experiment that experienced seepage erosion under 25 cm water pressure head boundary condition was the sand slope reinforced with fiber length and content of 6 mm and 0.3%, respectively. The increase of the water pressure head boundary condition to 50 cm resulted in small-scale sapping of slopes reinforced with 0.3% fiber content independent of the fiber length. For slopes reinforced with 1.0% fiber content sapping did not occur, only erosion of sand particles due to seepage was observed. A slope stability analysis reflected the favorable effect of fiber inclusion as the increased effective cohesion increased the factor of safety of slopes.