8- analysis and design of reinforced soil wall

16-10-2014

1

APPLICATION OF GEOTECHNICAL ENGINEERINGGEOTECHNICAL ENGINEERING

CE451A Dr. Rajesh Sathiyamoorthy, IIT Kanpur

Analysis and Design of Mechanically Stabilized Retaining Wall


Mechanically Stabilized Retaining Wall




16-10-2014

2


• More recently, soil reinforcement has been used in the construction and design of foundations, retaining walls, embankment slopes, and other structures. Such reinforcement is comparable to that of concrete structures.

• Reinforcement materials used to reinforce the backfill of retaining walls are generally referred to as mechanically stabilized retaining walls.

• Depending on the type of construction, the reinforcements may be galvanized metal strips, geotextiles, geogrids, or geocomposites.


p , g , g g , g p

• The beneficial effects of soil reinforcement derive from: • the soil’s increased tensile resistance (could be due to internal confinement

/ apparent cohesion / enhanced friction angle) and • the shear resistance developed from the friction at the soil‐reinforcement

interfaces.


• Wall is acted on by either the Rankine or Coulomb active earth wedge.

• Full‐scale tests have verified that the earth force developed from the active earth wedge at any depth z is carried by reinforcing strip tension.

• Strip tension is developed in the zone outside the active earth wedge from

Design Philosophy


Strip tension is developed in the zone outside the active earth wedge from the friction angle δ between reinforcement (strip/geosynthetic material) and soil and the vertical earth pressure γz on the reinforcement.

• With no lateral earth pressure left to be carried by the wall facings they can be quite thin and flexible with the principal functions of erosion control and appearance.


General concept of mechanically stabilised reinforced wall is

ΣTi = Pa cosδ (so the earth force against the wall/facing units is zero).



Backfill soil: • Backfill soil is usually specified to be free‐draining granular soil thus, the

effect of pore water development in cohesive soils, which, in turn, reduces the shear strength of the soil, is avoided.

• Recent research indicates that we can use cohesive soil if a porous geotextile is used for reinforcement to allow backfill drainage This allows

Factors considered while designing reinforced earth wall


geotextile is used for reinforcement to allow backfill drainage. This allows one to use the drained friction angle ϕ' to calculate friction between the soil and reinforcement.

• For cohesive materials, either use a narrow vertical back face zone of granular material or, alternatively, use strips of a permeable geotextile for vertical drainage.

16-10-2014

3


Surcharge: Surcharges are allowed on the backfill. These require analysis to ascertainwhether they are permanent (such as a roadway) or temporary and its location.For example:

• Temporary surcharges within the reinforcement zone will increase the lateral pressure which in turn increases the tension in the



lateral pressure, which in turn increases the tension in the reinforcements but does not contribute to reinforcement stability.

• Permanent surcharges within the reinforcement zone will increase the lateral pressure and tension in the reinforcements and will contribute additional vertical pressure for the reinforcement friction.

• Temporary or permanent surcharges outside the reinforcement zone contribute a lateral pressure, which tends to overturn the wall.


Surcharge:In most cases the lateral pressure from a backfill surcharge can be estimated using the Theory of Elasticity equation (Boussinesq equation) for vertical pressure, but it may be sufficiently

h ( ) h d


2: 1 method


accurate to use the 2 : 1 (2 on 1) method adjusted for plane strain condition.

Laba and KennedyBoussinesq equation for point load



Total vertical stress:



Trial Wedge: • Tests with experimental walls indicate that the Rankine wedge (of angle θ= 45° + ϕ/2) adequately defines the "soil wedge."

• This angle should be routinely checked using the trial wedge method for large backfill β angles.


A ti diti


Active condition: The wall should be sufficiently flexible that the active earth pressure wedge forms and any settlement/subsidence does not tear the facing unit from the reinforcement.

Zero stress on wall facing: It is usual to assume all the tension stresses are in the reinforcement outside the assumed soil wedge zone (typically the distance le)

16-10-2014

4


Corrosion: Corrosion may be a factor where metal reinforcements are used. It is common to increase the theoretical strip thickness somewhat to allow for possible corrosion within the design period, which may be on the order of 50 to 100 years.


S f t F t


Safety Factors: There will be two safety factors FS involved.

• One FS is used to reduce the ultimate strength of the reinforcements to a "design" value.

• The other FS is used to increase the computed length le required to allow for any uncertainty in the backfill properties and soil‐to‐reinforcement friction angle δ.


Analysis and Design of Mechanically Stabilised Retaining wall


Design unknowns: • Choosing appropriate tensile reinforcement, backfill and wall facing

• Vertical and horizontal spacing of reinforcement

• Length, width and thickness of reinforcement or tie


The general design procedure of any mechanically stabilized retaining wall can be divided into two parts:

1. Satisfying internal stability requirements2. Checking the external stability of the wall

Design procedure


• The internal stability checks involve determining tension and pullout resistance in the reinforcing elements and ascertaining the integrity of facing elements.

• The external stability checks include checks for overturning, sliding, and bearing capacity failure.


Internal stability

tie breaking or


Typical soil – reinforcement interaction mechanisms in a reinforced soil structure

gtie pullout??

16-10-2014

5

Additional passive resistance in geogrid


Internal stability


Typical values of tanδ


• The internal stability checks involve determining tension and pullout resistance in the reinforcing elements and ascertaining the integrity of facing elements.

• The reinforcement ties at each level, and thus the walls, could fail by either (a) tie breaking or (b) tie pullout.

• Reinforcing ties at any depth z will fail by pullout if the frictional resistance developed along the surfaces of the ties is less than the force to which the ties are being subjected

Internal stability


are being subjected.

The tie force per unit length of the wall developed at any depth z = T

The factor of safety against tie breaking FS(B)may be determined as:

Factor of safety against tie breaking (thickness of reinforcement)


Internal stability

• The maximum friction force that can be realized for a tie at depth z

Factor of safety against pullout (length of reinforcement)

• The effective length of the ties along which frictional resistance is developed may be conservatively taken as the length that extends beyond the limits of the Rankine active failure zone, which is the zone ABC. Line BC makes an angle of 45+ϕ1’/2 with horizontal.


can be realized for a tie at depth z

• The factor of safety against tie pullout at any depth z is

• Estimate le for a given factor of safety against tie pullout

• Total length of ties at any depth is


External stability


FS with respect to

16-10-2014

6


Steps involves in the design of MSRW (Metallic strip)

(2.5 to 3)









Steps involves in the design of MSRW (Geotextile)


16-10-2014

7


Steps involves in the design of MSRW (Geotextile)



Steps involves in the design of MSRW (Geogrid)


8- analysis and design of reinforced soil wall

Documents