Improving piping resistance using randomly distributed fibers

https://doi.org/10.1016/j.geotexmem.2013.12.005Get rights and content

Abstract

Piping is a problem that commonly occurs downstream of hydraulic structures under the influence of upward seepage. Piping is considered as the main mechanism of hydraulic structures failures. In this work an experimental program was set for determining the seepage velocity and piping resistance for unreinforced and randomly reinforced silty sand samples. Two types of fiber were used for preparing the reinforced samples. The experimental tests were carried out for different fiber contents (0.5, 0.75, 1.0 and 1.25%) and fiber lengths (5, 25 and 35 mm) under different hydraulic heads. Discharge velocity and seepage velocity of water flow through unreinforced and reinforced samples were calculated and compared with unreinforced sample. The results show that the inclusion of fibers reduced the seepage velocity, increased the piping resistance and increased the critical hydraulic gradient hence, considerably delaying the occurrence of piping. Furthermore, the amounts of increase in the piping resistance and hydraulic gradient are functions of percent and length of fibers.

Introduction

As the water flows through the soil there is a transfer of energy to the soil skeleton. This causes a seepage forces to act on the skeleton. When the flow of water is upwards and if the hydraulic gradient is high enough the resultant body force could be zero. The value of hydraulic gradient corresponding to zero body force is called the critical hydraulic gradient (ic). In this case the contact force between particles of soil will be zero and soil will have no strength. This leads to erosion of soil. This phenomenon is known as piping. The actual word ‘piping’ refers to the development of channels which develop at the downstream side of the structure where the flow lines converge and high seepage pressures occur (Sellmeijer, 1988). Ojha et al. (2003) stated that piping is a form of seepage erosion and involves the development of subsurface channels in which soil particles are transported through the porous medium.

Sherard et al. (1984) indicated that the piping of loose soils is a common problem in downstream of earth embankments under the influence of upward seepage. Foster et al. (2000) reported that about 1.5% of embankment failures are resulted from the occurrence of piping. Ubilla et al. (2008) reported that the main reason for the failure of the levees and flood walls protecting New Orleans, Louisiana and the surrounding area during Hurricane Katrina can be the occurrence of piping. Das and Viswanadham (2010) suggested that the piping failure is synonymous with sand boiling or quick sand condition. Some researchers such as Foster et al. (2000) and Ojha et al. (2003) indicated that piping erosion occurs in structures that are made up of loose soil with relatively high permeability. Hydraulic structures should be protected against piping using appropriate techniques (e.g., sheet piling, impervious clay blanket, filters, etc). Reinforcing soil with randomly distributed fibers can help improve the soil properties and protect against piping. Soil reinforcement is an effective technique for improving the mechanical behavior of soils. Reinforcement of soil achieved by either inclusion of strips, bars, grids etc within a soil mass in a preferred direction or mixing discrete fibers randomly with a soil mass. The use of randomly distributed fibers for soil reinforcement has many advantages. The mixing of fibers with granular soil is relatively easy. Also, another main advantage of randomly distributed fibers is the maintenance of strength isotropy and the absence of potential planes of weakness that can develop parallel to oriented reinforcement (Maher and Gray, 1990). The history of this technique goes back to more than 3000 years ago when Babylonians used a mixture of soil and straw as a construction material for improving the behavior of soil (Jha and Mandal, 1988). From 1970s investigators studied the mechanical behavior of this kind of soil reinforcement through conducting appropriate tests (e.g., Lee et al., 1973, Gray and Ohashi, 1983, Yetimoglu and Salbas, 2003, Yetimoglu et al., 2005, Consoli et al., 2007, Ahmad et al., 2010, Diambra et al., 2010 Lovisa et al., 2010, Edincliler and Ayhan, 2010, Tang et al., 2010, Falorca and Pinto, 2011, Plé and Lê, 2012, Ibrahim et al., 2012 and Li and Zornberg (2013)). However, randomly reinforced soils have recently attracted attention of researchers as a method of improving soil against piping.

Furumoto et al. (2002) were the first researchers who proposed to increase the resistance against piping by using randomly reinforced soil. Sivakumar Babu and Vasudevan (2008) showed that for a soil reinforced with coir fibers. Increase in fiber content and fiber length increases the critical hydraulic gradient. Das et al. (2009) showed the influence of polyester fibers on piping behavior of fly ash as a fill material. Das and Viswanadham (2010) used two types of fiber with diameters 30 and 32 μm to investigate their influence on the piping resistance of a silty sand soil. They concluded that inclusion of fibers increases the piping resistance of soil.

Section snippets

Aim of this study

A review of the literature shows that the study of application of randomly reinforced soil with fibers for improving piping resistance in hydraulic structures is very limited. Furthermore, the previous studies on the applications of fiber reinforced soil for controlling piping have mainly used fibers of small diameter (in the range of μm) which would be difficult to implement in practical applications. Therefore, it was decided in this work to examine the possibility of using two types of

Apparatus

There is no standard test procedure available to measure the piping resistance of soils (Das et al., 2009). Skempton and Brogan (1994) designed and fabricated an apparatus for studying the phenomenon of piping in sandy gravel material. After that researchers such as Furumoto et al., 2002, Sivakumar Babu and Vasudevan, 2008, Das et al., 2009 and Das and Viswanadham (2010) used an apparatus almost similar to the Skempton and Brogan's apparatus. In the present work, based on the considerations

Soil

The soil that was used in this experimental work was a silty sand. The gradation curve of this soil is shown in Fig. 2. Standard Proctor compaction tests were carried out on natural soil according to ASTM D 698-07. The physical and mechanical properties of the soil are summarized in Table 1.

Fibers

Two types of fiber were used as the reinforcing materials. They were manufactured from polyethylene and polyester in filament form as shown in Fig. 3 and hereafter they will be referred to as fibers No. 1

Test program and theoretical relationships

The piping tests were conducted on samples with 50 mm diameter and 100 mm length as follows:

After preparing the natural and reinforced soil samples the mould containing the sample was placed in the apparatus. The samples were saturated for 24 h under Δh = 0.0. The piping test was carried out by increasing the head of water in the reservoir (hydraulic head) at increments of 20 mm while the level of water above the sample was constant at 50 mm. The duration of each increment was nearly 10 min and

Results

A total of 25 tests were conducted in this work. To ensure greater accuracy each test was repeated two times. The test program included compaction and piping tests. Typical results of compaction tests for fibers No. 1 and 2 with lengths of 25 and 35 mm are shown in Tables 3 and 4. The results show that the optimum water content and maximum dry unit weight decreased with increasing the fiber content in comparison with natural soil. It is shown in Table 3 that the maximum dry unit weight and

Discussion

The results of compaction tests on the unreinforced soil and reinforced soils with different fiber contents (for the two types fiber) are presented in Tables 3 and 4. The results show that by increasing the percentage of fiber in the soil the maximum dry unit weight and optimum water content are both decreased. The decrease in the maximum dry unit weight and optimum water content can be explained by the fact that the replacement of soil particles with fibers (that have lower specific gravity

Conclusion

In this paper one-dimensional piping tests were conducted on samples of a silty sand reinforced with two different fibers with varying lengths and fiber inclusions. Based on the experimental observations and interpretation of the test results the following conclusions can be drawn:

  • -

    Reinforcing soil with fibers can reduce seepage velocity and increase the piping resistance of soil. This was observed for two fibers with different diameters and lengths and different fiber contents.

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    Increasing the

Acknowledgment

The authors thank the financial support from the Centre of Excellence of Evolution and Rehabilitation of Irrigation and Drainage Networks University of Tehran.

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