Journal of Engineering Geology, Vol.9, No.2, Summer 2015

Impact of Fiber Reinforcement on Deformation
Characteristics of Cemented Sand-Gravel Mixtures
Dehghan A.,Hamidi A.;
School of Engineering, Kharazmi University, Tehran, Iran
Received: 3 Sep 2014 Revised 10 Nov 2014
Abstract
126873825500

Downloaded from jeg.khu.ac.ir at 11:34 IRST on Saturday October 28th 2017 [ DOI: 10.18869/acadpub.jeg.9.2.2729 ]

Downloaded from jeg.khu.ac.ir at 11:34 IRST on Saturday October 28th 2017 [ DOI: 10.18869/acadpub.jeg.9.2.2729 ]

This paper describes triaxial compression tests conducted to determine the effect of fiber inclusion on stiffness and deformation characteristics of sand-gravel mixtures. Tested soil was a mixture of Babolsar sand from the shores of the Caspian Sea and Karaj River gravel. Portland cement was used as the cementing agent and fibers 12mm in length and 0.023mm in diameter at 0%, 0.5% and 1.0% were added to the mixtures. Triaxial tests were performed on saturated samples in consolidated drained and undrained conditions at confining pressures of 100, 200 and 300 kPa. Deviatoric stress-axial strain, volumetric strain-axial strain, pore pressureaxial strain curves with deformation and stiffness characteristics were investigated. Tests results show that fiber addition increased peak and residual shear strength of the soil. Fiber addition resulted in an increase of the maximum positive and negative volumetric strains. In undrained condition, fiber inclusion caused increase in initial positive pore pressure and final suction. It has also been observed that fibers decreased initial tangent stiffness of the cemented sand-gravel mixture.
Keywords: Triaxial tests, reinforced soil, cement-fiber inclusion, stiffness, yield.
Introduction
126873825500

Downloaded from jeg.khu.ac.ir at 11:34 IRST on Saturday October 28th 2017 [ DOI: 10.18869/acadpub.jeg.9.2.2729 ]

Downloaded from jeg.khu.ac.ir at 11:34 IRST on Saturday October 28th 2017 [ DOI: 10.18869/acadpub.jeg.9.2.2729 ]

Different techniques like cementation, grouting and fiber reinforcement have been used to improve soil capability against deformation. Fiber reinforcement of the soil has been an interesting method of soil improvement for geotechnical engineers. New concept of soil reinforcement was introduced by the French engineer Vidal (1966). He demonstrated that the introduction of reinforcing metal elements in a soil mass increases the shear strength of the medium. Geosynthetics have been used from 1980 instead of metal as reinforcing elements extensively. According to the literature, the applied stress to the soil transfers to the reinforcing material by frictional mechanism. The resistance of reinforcing material against lateral deformation results in the increase in bearing capacity of the reinforced soil.
The behavior of cemented soil has been studied by a number of researchers (Clough et al. 1981; Coop and Atkinson 1993, Consoli 2000, Hamidi and Haeri 2008). Based on the results, cementation increased peak strength and initial stiffness of the soil and changes its behavior to a more brittle one. In order to reach to a more ductile behavior, some studies performed on the behavior of cement-fiber reinforced soils (Maher and Ho, 1993; Consoli et al, 2009, 2010, 2013; Park, 2009 and 2011; Dos santos, 2010; Heeralal and Prareen, 2011; Salah-ud-din, 2012; Festugato et al. 2013 and Hamidi and Hooresfand, 2013). These studies showed that fiber inclusion to the cemented soil results in an increased peak and residual strength and makes the brittle behavior of the soil softer compared to the unreinforced material. Park (2011) implied the increase in shear strength of fiber reinforced soil due to the increase in dilation by fibers reinforcement. Hamidi and Hooresfand (2013) investigated the effect of fiber reinforcement on triaxial shear behavior of cemented sand. The results showed an increase in friction angle and cohesion intercept and increased principal stress ratio by fiber addition. Influence of fiber inclusion decreased by increase in confining pressure.
126873825500

Downloaded from jeg.khu.ac.ir at 11:34 IRST on Saturday October 28th 2017 [ DOI: 10.18869/acadpub.jeg.9.2.2729 ]

Downloaded from jeg.khu.ac.ir at 11:34 IRST on Saturday October 28th 2017 [ DOI: 10.18869/acadpub.jeg.9.2.2729 ]

Limited studies have been carried out on the influence of fiber inclusion on the mechanical behavior of cemented sand-gravel mixtures. Conventional triaxial compression tests are used in present study to investigate peak and residual shear strengths, volume change, deformation characteristics and stiffness of the soil and the influence of different variables on the results is evaluated. Special consideration is paid to the effect of fiber content, confining pressure and drainage condition on deviatoric stress, stress ratio, dilation angle, tangent stiffness and shear modulus of the material.
Physical properties of tested soil
Clean sub-round to sub-angular sand from the shore of the Caspian Sea (Babolsar city) was used as the base material in the tests and it was sieved using a #30 sieve. Clean gravel from the river of Karaj, Iran was used. Uni-sized gravel particles between 0.5 inch and 0.375 inch sieves were added to the sandy soil. Gravel contents used in present study were 30% and 50%. Figure 1 shows gradation curves of the base soils and Table 1 lists its physical properties. All physical properties were determined according to the ASTM (1998) standard methods.
126873825500

Downloaded from jeg.khu.ac.ir at 11:34 IRST on Saturday October 28th 2017 [ DOI: 10.18869/acadpub.jeg.9.2.2729 ]

Downloaded from jeg.khu.ac.ir at 11:34 IRST on Saturday October 28th 2017 [ DOI: 10.18869/acadpub.jeg.9.2.2729 ]

Table1. Physical characteristics of the mixtures
Parameter Sand with 30% gravel Sand with 50% gravel
Soil name SP GP
Cu 3.37 57.13
Cc 0.78 0.037
emax 0.51 0.42
emin 0.37 0.30
Gs γd,min (kN/m3) γd,max (kN/m3) 2.709
17.60
19.40 2.69
18.58
20.29

Figure 1. Gradation curves of sand, gravel and sand-gravel mixtures
Polypropylene fiber with circular cross sections, 12mm in length and 0.023 mm in diameter with the aspect ratio of about 500, average tensile strength of 400 MPa were used as fibers. It was the same that used by Hamidi and Hooresfand (2013) which exhibited very good consistency with the cementing agent. Fiber contents of 0.0%, 0.5% and 1% by dry weight of the base soil were used in the experiments. Cementation agent was Portland cement (Type II) which was sieve using a #100 sieve and added in 3% content to the samples.

Sample preparation
126873825500

Downloaded from jeg.khu.ac.ir at 11:34 IRST on Saturday October 28th 2017 [ DOI: 10.18869/acadpub.jeg.9.2.2729 ]

Downloaded from jeg.khu.ac.ir at 11:34 IRST on Saturday October 28th 2017 [ DOI: 10.18869/acadpub.jeg.9.2.2729 ]

Samples used for triaxial testing were prepared by mixing sand, gravel, cement, water and polypropylene fibers using an electric mixer. During the mixing process, 8% distilled water was added to the fibers. Samples prepared using a split mold 100 mm in diameter and 200 mm in height. Samples were compacted in eight layers. Each layer was poured into the mold and compacted using a metal hammer. The tests were performed on samples in a relative density of 70%. Each sample was cured in a humid room at 25± 2°C for one day with mold and six days without mold. Table 2 shows the variables considered in sample preparation.
Testing program
After seven days for curing, the samples were set up in a triaxial cell. Saturation of samples was carried out in three stages. First, the sample was flushed with carbon dioxide (CO2) with 15 kPa pressure. After that the water was flushed from the bottom of the sample under a very low pressure of 20 kPa about 45 minutes. Finally, the sample was saturated using back pressure of 300 kPa. The saturation process was considered to be complete when a B value of 0.90 or more was reached. After consolidation, shear loading was applied at rates of 0.15 mm/min and 0.3 mm/min for drained and undrained tests, respectively.
126873825500

Downloaded from jeg.khu.ac.ir at 11:34 IRST on Saturday October 28th 2017 [ DOI: 10.18869/acadpub.jeg.9.2.2729 ]

Downloaded from jeg.khu.ac.ir at 11:34 IRST on Saturday October 28th 2017 [ DOI: 10.18869/acadpub.jeg.9.2.2729 ]

Table2. Description of variables in present study
Variable No. of levels Description of samples
Type of soil 2 Poorly graded gravel sandy that sand from the shores of Caspian Sea and gravel from Karaj river
Cementing 1 Portland cement (type II)
agent
Type of fiber 1 White monofilament polypropylene fibers
Gravel content 2 30.0% and 50.0% dry weight of base soil
Cement content 1 3.0% dry weight of base soil
Fiber content 3 0.0, 0.5% and 1% dry weight of sand-cement
Relative density 1 70%
Water content 1 8% weight of base soil
Sample size 1 100 mm diameter and 200 mm height, compacted in eight layers
Curing
condition 1 Cured for seven days in humid room

Analysis of the results
The present study used the results of triaxial tests on cemented gravel sandy (with gravel content 30% and 50%) and fiber content of 0.0%, 0.5% and 1.0% under confining pressures 100, 200 and 300 kPa in drained and undrained condition. The effect of fiber addition has been investigated on the behavior, deformation characteristics and stiffness of the reinforced material.
1. Effect of fiber reinforcement on shear strength behavior
126873825500

Downloaded from jeg.khu.ac.ir at 11:34 IRST on Saturday October 28th 2017 [ DOI: 10.18869/acadpub.jeg.9.2.2729 ]

Downloaded from jeg.khu.ac.ir at 11:34 IRST on Saturday October 28th 2017 [ DOI: 10.18869/acadpub.jeg.9.2.2729 ]

Figure 2 plots the variation of deviatoric stress-axial strain, volumetric strain-axial strain and dilation rate-axial strain in drained condition with different fiber contents. Stress-strain curves showed that maximum deviatoric stress and residual stress increased with increase in fiber content. It illustrated that fiber inclusions increased softening after failure point. Volumetric strain-axial strain curves show that contraction volumetric strain increased when fiber content increased. Volume of samples increased while axial strain increased. It has been shown that the rate of dilation increased when fiber content increased. This is agreement with the results of Chen 2006, Consoli 2004, 2009 and Salehuddin 2012 that reported an increase in maximum initial contraction and final dilation when fiber content increased. Rate of dilation

versus axial strain changes showed that the maximum rate of dilation decreased as fiber content increased. The maximum amounts have been indicated on stress-strain curves as points that take place after the failure. Leroueil and Vaughan 1990 showed that in uncemented soil, when dilation is due to the dense packing, failure and maximum rate of dilation are coincided but when the peak strength is controlled by cementation rather than density, the maximum rates of dilation take place after bond yield point. In fact in cemented samples, failure point happened after some bond breakage while some others stand against increase in volume. Fibers play the same role in fiber reinforced cemented soil. But for sandy soils, the maximum dilation rate and failure point are coincided. Data for other confining pressures (100 and 300 kPa) have shown similar trends, but are not shown here for the sake of conciseness

Figure

2



قیمت: تومان

دسته بندی : زمین شناسی

دیدگاهتان را بنویسید