lec 9 reinforced earth for shallow foundation & roads

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SPP / DDU/MTECH/GEOTECH

LEC-9 REINFORCED EARTH FOR SHALLOW FOUNDATION & ROADS

P R O F. S A MI R S I N H . P. PA R MA RD E PA RT M E N T O F C I V I L E N G IN E E R I N GD H A R MA S I N H D E S A I U N I V E R S I T Y, N A D I A DMA I L : S A M IR D D U @ G MA I L . CO M (S M A L L CA P S )

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R.E. Shallow foundations

The basic design approach for geosynthetic-reinforced foundation soils must consider their modes (or mechanisms) of failure.

1. Bearing capacity failure of soil above the uppermost geosynthetic layer

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Bearing Capacity failure

This kind of failure likely to occur if the depth of the uppermost layer of reinforcement (u) is greater than about 2/3 of the width of footing (B), (i.e. u/B 0.67).

if the reinforcement concentration in this layer is sufficiently large to form an effective lower boundary into which the shear zone will not penetrate.

This class of bearing capacity problems corresponds to the bearing capacity of a footing on the shallow soil bed overlying a strong rigid boundary.

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2. Pullout of geosynthetic layer

This type of failure is likely to occur for shallow and light reinforcement.

(i.e. u/B < 0.67 and number of reinforcement layers, N < 3).

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3. Breaking of geosynthetic layer

This type of failure is likely to occur with long, shallow, and heavy reinforcement.

(u/B < 0.67, N > 3 or 4). The reinforcement layers always break

approximately under the edge or towards the centre of the footing.

The uppermost layer is most likely to break first, followed by the next deep layer and so forth.

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4. Creep failure of geosynthetic layer

This failure may occur due to long-term settlement caused by sustained surface loads and subsequent geosynthetic stress relaxation.

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Geometrical parameters of the geosynthetic-reinforced foundation soil

The width of footing, BStrength of foundation soil, τs

The depth below footing of the first geosynthetic layer, u

The number of geosynthetic layers, N

The vertical spacing of the geosynthetic layers, h

The width of geosynthetic layers, b

The tensile strength of geosynthetic, σG.

The parameters, u, N, and h cannot be considered separately, as they are dependent on eachother.

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Some Comments

It has been reported that more than three geosynthetic layers are not beneficial.

The optimum size of the geosynthetic layer is about three times the width of the footing, B.

For beneficial effects, the geosynthetic layers should be laid within a depth equal to the width of footing.

The optimum vertical spacing of the geosynthetic reinforcement layers is between 0.2B and 0.4B.

For a single layer reinforced soil, the optimum embedment depth is approximately 0.3 times the footing width.

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Bearing Capacity Ratio (BCR)

Where,qu is the ultimate load-bearing capacity of the

unreinforced soil.q (R) is the load-bearing capacity of the

geosynthetic-reinforced soil at a settlement corresponding to the settlement su at the ultimate load-bearing capacity qu for the unreinforced soil

Bearing capacity ratio (BCR)

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Typical load-settlement curves for a soil with and without reinforcement

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(a) Geocell mattress configuration;

The mattress is placed upon the soft foundation soil of insufficient bearing capacity so as to withstand the weight of the superstructure.

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Why Geo Cell?

fabricated directly on the soft foundation soil (using uniaxial-polymer geogrids in a vertical

orientation connected to a biaxial base grid and then filled with granular material resulting in a structure usually 1 m deep.)

It forms stiff platform, provides working platform.

Below drainage blanket allows consolidation of sub soil and movement of ground water.

External anchorage not required for the self structure.

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(b) plan view of geocell mattress; (c) connectiondetails (after Bush et al., 1990)

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Load-bearing capacity analysis of geogrid mattress foundation (after Ochiai et al., 1994)

The ultimate bearing capacity qu without the use of the mattress may be given by Terzaghi’s equation as follows

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Bearing Capacity of Mattress

where γm is the unit weight

of the mattress, H is the thickness of

the mattress,Bm is width of

mattress.

The ultimate bearing capacity qm with the use of mattress may be given as follows:

The increase in the bearing capacity q due to the placementof the mattress can be given as follows:

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Mechanical foundation model (after Shukla and Chandra, 1994a)

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Some Discussion

This model allows the study of time-dependent settlement behaviour of the geosynthetic-reinforced granular fill–soft soil system.

The general assumptions are i. (i) the geosynthetic reinforcement is

linearly elastic, ii. (ii) rough enough to prevent slippage at the

soil interface and iii. (iii) has no shear resistance.

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Some Discussion

A perfectly-rigid plastic friction model is adopted to represent the behaviour of the soil–geosynthetic interface in shear.

The compressibility of the granular fill is represented by a layer of Winkler springs attached to the bottom of the Pasternak shear layer.

The saturated soft foundation soil is idealized by the Terzaghi’s spring-dashpot system.

The spring represents the soil skeleton and the dashpot simulates the dissipation of the excess pore water pressure.

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Some Discussion

The spring constant is assumed to have a constant value with depth of the foundation soil and also with time.

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Geogrid Layer Below Footing

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Shallow foundation on Reinforced Earth

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Geotextile below footing Profile

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Typical Data from BM Das (1999)

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Comparision for Different Gotextiles

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Comparative analysis for Bearing Pressure

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Comparative Analysis for Settlement

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Binquet and Lee Mechanism

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Questions ?