Lec 8 design examples

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LECTURE- 8 GEOTECHNICAL STRUCTURE DESIGN BY GEOTEXTILE & REINFORECED EARTH

Faculty: Prof. Samirsinh.P.ParmarDepartment of Civil EngineeringFaculty of Technology, Dharmasinh Desai University,NadiadMail Add: [email protected]

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Rankine active earth pressure

Where, Ka is the Rankine earth pressure coefficient γb is the unit weight of the granular backfill

Factor of safety against the geotextile rupture at any depth z may be expressed as

Where, σG is the allowable geotextile strength in kN/m, and Sv is the vertical spacing of the geotextile layersσa is active earth pressure.

where Φb is the angle of shearing resistance of the granular backfill

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The factor of safety against the geosynthetic pullout

Effective length, le,

The length, lr, of geotextile layer within the Rankine failure zone can be calculated as:

Where, Φr is the angle of shearing resistance of soil–geosynthetic interface and it is approximately equal to 2Φb/3.

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The total length of the geotextile layer at any depth z is

If the wraparound facing is to be provided, then the lap length can be determined using the following expression:

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ILLUSTRATIVE EXAMPLE Height of the retaining wall, H = 8 m Granular backfill Unit weight, γb =17 kN/m3

Angle of internal friction, Φb= 35 Allowable strength of geotextile, σG= 20 kN/m Factor of safety against geotextile rupture 1.5 Factor of safety against geotextile pullout 1.5 Calculate (i) the length of the geotextile layers, (ii) spacing of layers and (iii) lap lengths at depth z = 2 m, 4 m, and 8 m.

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SOLUTIONRankine earth pressure coefficient is

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At z =2 m,

• Lift Thickness / GT Layer Spacing =

• Total Length of GT

• Wrap Length of GT for GT Facing

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At z =4 m,




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At z =8 m,




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EMBANKMENTS

Overall slope stability failure

Lateral spreading

Embankment settlement

Failure mechanism of Embankments

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Overall bearing failure

Pullout failure

Failure mechanism of Embankments

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1. OVERALL SLOPE STABILITY FAILURE

Most common considered failure mechanism. Failure mechanism is characterized by a well-

defined failure surface cutting ... - the embankment fill, - the geosynthetic layer and - the soft foundation soil REASON: Tensile failure of the geosynthetic layer or Bond failure due to insufficient anchorage

of the geosynthetic extremity beyond the failure surface.

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ANALYSIS Conventional slope stability analysis with the

geosynthetic providing an additional stabilizing force T.

Geosynthetic provides additional resisting moment required to obtain the minimum required FoS.

It is conservative to assume ΔMg = Ty and to neglect any other possible effects on soil stresses

Where , ΔMg = resisting (or stabilizing) moment, T = tensile force R = radius of critical slip arc y = moment arm of the geosynthetic layer

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ANALYSIS The approach is conservative because...1. It neglects any possible geosynthetic reinforcement along

the alignment of the failure surface.2. Neglects any confining effect of the geosynthetic

Where , τe, τf are the shear strengths of embankment and

foundation materials. le, lf are the arc lengths within embankment and

foundation soil, respectively. We, Wf are the weights of soil masses within the

embankment and foundation soil, respectively. xe, xf are the moment arms of We and Wf, respectively, to

their centres of gravity

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OVERALL SLOPE STABILITY ANALYSIS.

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DESIGN PROCEDURE Trials are made to find the critical failure arc for which

the necessary force in the geosynthetic is maximum. Factor of safety is established and the maximum

necessary force is determined by searching the critical failure arc.

NOTE: A geosynthetic can resist creep if the working loads are kept well below the ultimate tensile strength of the geosynthetic.

The recommended working load should not exceed 25% of the ultimate load for polyethylene (PE)

geosynthetics, 40% of the ultimate load for polypropylene (PP)

geosynthetics and. 50% of the ultimate load for polyester (PET)

geosynthetics

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2. LATERAL SPREADING

Main Reason : Tension Crack in soil Mass. Horizontal pressure creates horizontal shear

stress at the base of embankment. If foundation soil not offers shear strength it will result in failure. If Cracks filled with water , hydraustatic

pressure will act.Foundation Soil

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BLOCK SLIDING ANALYSIS

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ANALYSISThe resultant active earth pressure

corresponding maximum tensile force

Where: γ is the unit weight of the embankment material;H is the embankment height; B is the embankment base widthτr is the resisting shear stressΦr is the soil–geosynthetic interface shear resistance angleFor no lateral spreading, one can get

or

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ANALYSIS In general practice : Consider a minimum safety factor of 1.5 with

respect to strength and A geosynthetic strain limited to 10%. The required geosynthetic strength Treq and modulus Ereq therefore are

lateral spreading failure mechanism becomes important only for steep embankment slopes on reasonably strong subgrades and very smooth geosynthetic surfaces.

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ILLUSTRATIVE EXAMPLE

A 4 m high and 10 m wide embankment is to be built on soft ground with a basal geotextile layer. Calculate the geotextile strength and modulus required in order to prevent block sliding on the geotextile. Assume that the embankment material has a unit weight of 18 kN/m3 and angle of shearing resistance of 35 and that the geotextile–soil interface angle of shearing resistance is two-thirds of that value.

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SOLUTION:

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3. EMBANKMENT SETTLEMENT

Main Reason: Consolidation of foundation soil. settlement can occur due to the expulsion of the

foundation soil laterally. This mechanism may occur for heavily

reinforced embankments on thin soft foundation soil layers.

Embankment settlement due to lateral expulsion of foundation soil.

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ANALYSIS The factor of safety against soil expulsion, Fe, can be estimated

from

The active and passive forces can be evaluated by earth pressure theories.

While, the forces at the base and top of the soil block can be estimated as a function of the undrained strength Su at the bottom of the foundation soil and adherence between the reinforcement layer and the surface of the foundation soil, respectively

where :Pp is the passive reaction force against block movement,RT is the force at the top of the soil block,RB is the force at the base of the soil block, and PA is the active thrust on the soil block

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DISCUSSION

Geosynthetic layer may reduce differential settlement of the embankment

The compressibility of the foundation soils is not altered by the geosynthetic.

The stress distribution may be somewhat different

Embankment settlement can result in excessive elongation of the geosynthetic

It is general practice to limit the total strain in the geosynthetic to 10% in order to minimize settlements within the embankment.

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DISCUSSION

Therefore, the modulus of the geosynthetic to be selected should be 10 Treq,

Where, Treq is based on the overall stability calculation.

For getting this benefit significantly from the geosynthetic layer, its edges must be folded back similar to ‘wraparound’ in retaining walls or anchored in trenches properly or weighted down by berms.

Prestressing the geosynthetic in field, if possible, along with the edge anchorage can further reduce both the total and the differential settlements within the embankment (Shukla and Chandra, 1996a)

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4. OVERALL BEARING FAILURE The bearing capacity of an embankment

foundation soil is essentially unaffected by the presence of a geosynthetic layer within or just below the embankment.

If the foundation soil cannot support the weight of the embankment, then the embankment cannot be built.

The overall bearing failure is usually analysed using classical soil mechanics bearing capacity methods.

Construction of an embankment higher than the estimated value would require using staged construction that allows the underlying soft soils time to consolidate and gain strength.

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5. PULLOUT FAILURE

Forces transferred to the geosynthetic layer to resist a deep-seated circular failure,

i.e. the overall stability failure must be transferred to the soil behind the slip zone.

The pullout capacity of a geosynthetic is a function of its embedment length behind the slip zone.

The minimum embedment length, L, can be calculated as follows:

Where: Ta is the force mobilized in the geosynthetic per unit length; ca is the adhesion of soil to geosynthetic; σv is the average vertical stress; and Φr is the shear resistance angle of soil–geosynthetic interface

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THE DESIGN PROCEDURE FOR EMBANKMENT WITH BASAL GEOSYNTHETIC LAYERS :

Step 1: Define geometrical dimensions of the embankment (embankment height, H; widthof crest, b; side slope, vertical to horizontal as 1:n)

Step 2: Define loading conditions (surcharge, traffic load, dynamic load).

If there is possibility of frost action, swelling and shrinkage, and erosion and scour, then loading caused by these processes must be considered in the design.

Step 3: Determine the engineering properties of the foundation soil

(shear strength parameters, consolidation parameters). Chemical and biological factors that may deteriorate

the geosynthetic must be determined.

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Step 4: Determine the engineering properties of embankment fill materials.

Step 5: Establish geosynthetic properties. Step 6: Check against the modes of failure.

THE DESIGN PROCEDURE FOR EMBANKMENT WITH BASAL GEOSYNTHETIC LAYERS : CONT….

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