TENSILE STRENGTH OF FIBER REINFORCED SOIL

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OVERVIEW

• Introduction about fiber reinforcing of soil and material used.

• It’s advantage and application.

• Tensile and desiccation test for given soil and fiber properties.

• Result and discussion.

• Effect of fiber content, dry density and water content.

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OVERVIEW

• Relationship between tensile strength and interfacial shear strength.

• Conclusion.

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INTRODUCTION

• What is fiber reinforced soil?The standard fiber-reinforced soil is defined as a soil mass that

contains randomly distributed, discrete elements, i.e. fibers, which provide an improvement in the mechanical behavior of the soil

composite.

Source:

(http://newsroom.uts.edu.au)4

Why soil-reinforcing is essential?

• The primary purpose of reinforcing soil is:To improve stabilityTo increase bearing capacityTo reduce settlement To reduce lateral deformation

source: (http://www.authorstream.com) 5

APPLICATIONS

• It has several field applicationRetaining wallAbutment Embankment DamHill road

source:(http://www.authorstream.com) 6

source:(http://www.authorstream.com)7

MATERIAL AND ITS PROPERTIES

• This is case study done by Dr. Chaosheng Tang .

• The soil used in the experiment is collected from the Nanjing area of eastern China.

• Optimum water content in soil is 16.5% and dry density is 1700 kg/m3.

• The used polypropylene fiber length is 12 mm and diameter is 0.048 mm (as reinforcement).

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Soil Properties ValuesSpecific gravity 2.73   Consistency limits:   Liquid limit (%) 37 Plastic limit (%) 20 Plasticity Index (%) 17USCS Classification CLCompaction Study:   Optimum moisture content (%)

16.5

Max. dry density (kg/m3) 1700Grain Size Analysis   Sand (%) 2 Silt (%) 76 Clay (%) 22

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Fiber Properties Values

Specific gravity 0.91

Average diameter(mm) 0.048

Average length (mm) 12

Breaking tensile strength (MPa) 350

Modulus of elasticity (MPa) 3500

Fusion Point (0C) 165

Burning Point (0C) 590

Acid and alkali resistance Very good

Dispersibility Excellent

SPECIMEN PREPARATION

• Air dry the soil (w=4.16%) and pass through 2 mm sieve size.

• Add water to soil and finally mix the fibers manually.

• This way specimen of four different water content and five different fiber content were prepared.

(w = 14.5%, 16.5%, 18.5% and 20.5%)

(f = 0%, 0.05%, 0.1%, 0.15% and 0.2% by weight of dry soil)

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TENSILE TEST

• Prepare specimen using the 8-shaped compaction mould.

(80 mm long and 10 mm high) and required dry density of 1400,1500,1600,1700 kg/m3 is achieved.

• Take out the specimen from the compaction mould and place in the tensile mould.

• Connect the lower part of mould to the load plate of testing machine and upper to a load crossbeam through a load transducer (capacity 100N ).

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(Source: journal ascelibrary.org)13

(Source: journal ascelibrary.org)14

• Conduct the test at constant rate of 0.5 mm/min of vertical displacement.

• Note down the reading of load and corresponding displacement reading.

• The tensile strength is calculated using :

Where ; Tmax=max. tensile load

w=weight of lower half of the tension mould.

A=area of the neck .

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Observed and calculated data Test no. Fiber content

(f) %

Water content (w) (%)

Dry density (d) (Kg/m3)

Tensile Strength (t) (kPa)

S0 0 16.5 1700 47.15S1 0.05 16.5 1700 55.90S2 0.1 16.5 1700 62.30S3 0.15 16.5 1700 67.23S4 0.2 16.5 1700 78.11S5 0.1 14.5 1700 71.75S6 0.1 18.5 1700 53.45S7 0.1 20.5 1700 50.25S8 0.1 16.5 1400 14.90S9 0.1 16.5 1500 26.31S10 0.1 16.5 1600 45.10 16

Graph variation….

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Analysing graph….

• Tensile load is increasing continuously with displacement till peak value.

• Corresponding to 0% fibre content tensile load falls to zero abruptly.

• For fiber content S1 to S4 tensile load also decreased but residual tensile load was maintained.

• This indicate the effectiveness of fiber introduction in soil in improving the soil failure stability.

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DESICCATION TEST

• It is common form of soil tensile failure.

• Mix the dry soil powder with the water content double the liquid limit.

• Fibres are then mixed uniformly with the slurry.

• There should not be any visible clump of fibres.

• Fill the prepared soil mixture in the Oedometer ring.

• Then place it in laboratory in air at 300C.

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Crack pattern…

Unreinforced sample Reinforced sample

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Analysing sample…

• Reinforced sample has lesser number of crack.

• Depth and width of crack get reduced.

• It shows the effectiveness of fiber inclusion over unreinforced sample.

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EFFECT OF FIBER CONTENT

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• Graph is plotted “tensile strength vs fiber content” based on data in slide 17.

• Corresponding to S0 tensile strength is minimum and maximum for S4.

• Graph shows continuous increment of tensile strength with the fiber content.

• S0-brittle failure

• S1,S2,S3,S4-elastic/plastic extension of fiber.• Happens because of bridging effect.

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EFFECT OF DRY DENSITY

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• Graph is plotted “tensile strength vs dry density” as per data in slide 17.

• Tensile strength increases with the increase in dry density.

• Because higher density lead to Proper contact between soil particle Greater bonding forceIncrease in interfacial contact areaIncrease in pull-out resistance.

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EFFECT OF WATER CONTENT

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• Graph is plotted “tensile strength vs water content” as per data in slide 17

• Tensile strength decreases with increase in water content.

• Happens due to :Decrease in interfacial friction and cohesionDecrease in pull-out resistance

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Relationship between tensile strength and interfacial shear strength

• Tensile strength of soil composed of two parts = tensile strength of natural soil + contribution of fiber.

tf= 0 + f

f = f

Tensile strength increment per unit fiber content

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Where , d = fiber diameter

L = fiber length

k = constant

f = fiber content

= interfacial shear strength

Nf = no. of fiber crossing per unit area

Δf’ = tensile strength increment per unit fiber content

λ = coefficient related to fiber diameter, length, and content and the

interfacial shear strength

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Contribution of fiber reinforcement in crack reduction

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CONCLUSION

• The newly developed 8-shaped mould provides a better fixing of the specimen.

• Even very small amount of fiber significantly increases the tensile strength of soil.

• Tensile strength increases on increasing the dry density and fiber content but decreases with water content.

• Can be applied to many earthen structures, such as :

cover barriers, bottom liners of landfills, slopes, earth-filled dams and highway

and railway embankments.

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REFERENCES• Chauhan, M.S., Mittal, S., And Mohanty, B. (2008), “Performance Evaluation Of Silty Sand And

Subgrade Reinforced With Fly Ash And Fibre”, Geotextiles And Geomembranes, Vol. 26, Pp. 429-435.

• Consoli N. C., Da Fonseca, A. V., Cruz, R. C., And Silva, S. R. (2011a). “Voids/Cement Ratio Controlling Tensile Strength Of Cement-treated Soils.” J. Geotech. Geoenvion. Eng., 10.1061/ (Asce)gt.1943-5606.0000524,1126–1131.

• Tang, C.S., Shi, B., And Zhao, L.Z. (2010), “Interfacial Shear Strength Of Fiber Reinforced Soil”, Geotextiles And Geomembranes, Vol. 28, Pp. 54-62.

• Tang, C., Wang, D., Cui, Y., Shi, B., And Li, J. (2016). "Tensile Strength Of Fiber-reinforced Soil." J. Mater. Civ. Eng., 10.1061/(Asce)mt.1943-5533.0001546, 04016031.

(http://www.sciencedirect.com/science/article/pii/s1674775514000122)

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THANK YOU …

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