REINFORCED EARTH SYSTEMEng. Giuseppe Lembo

VERTICAL REINFORCED EARTH SYSTEM WITH FRONT FACE MADE IN CONCRETE PANELS….A FAST DESCRIPTION

The system consists of a soil reinforced with linear high adherence reinforcements placed in the soil in successive layers

and connected to a flexible facing made of concrete panels.

The principle of operation of the system is based on the interface friction between linear reinforcements and the structural

filling soil. That means that the most important factor for the design is the structural filling soil and in particular the friction angle

value.

Lengths, dispositions and number of the reinforcements are

designed according to the earth pressure of the backfill soil and of

the in situ soil and according to the eventual overloads that insist

on the reinforcements and on the back of the reinforced structure:

the reinforcements are determined basing on the tensile stresses

transmitted to the structural soil for effect of the interface friction.

To adapt to the specific characteristics of each project, the system

offers the option of using either steel reinforcements than polymers

with different strengths for greater efficiency.

All are, high adherence and their characteristics permit the construction of structures, including very high and still highly

charged.

FRONT FACE

LEVELLING PAD

HIGH ADHERENCE REINFORCEMENTS

REINFORCED SOILMASS

REINFORCED EARTH SYSTEM ELEMENTS

EARTH RETAINING STRUCTURESRetaining walls are structures for the containment or stabilization of almost vertical (steeper than 70 degrees) or vertical

slopes and embankments.

British Standard BS8006:2010

French Standard AFNOR NFP 94-270

CODE WITH FORMULAS USED TO DESIGN REINFORCED EARTH SYSTEM

American Standard FHWA-2009

The philosophy followed in these documents is to design this type of structures against the occurrence of a LIMIT STATE .

For the purposes of reinforced soil design a limit state may be deemed to be reached when one of the following occurs:

a. Collapse or major damage;

b. Deformations in excess of acceptable limits;

c. Other forms of distress or minor damage that would render the structure unsightly, require unforeseen maintenance or

shorten he expected life of the structure.

The condition defined in a. is the ultimate limit state, and b. and c. are serviceability limit states; practice in reinforced soil

design is to design against the ultimate limit state and check for the serviceability limit state.

reduce the properties of reinforcement materials to obtain

DESIGN MATERIAL

STRENGTH

PARTIAL MATERIAL FACTORS

Amplify the actions (especially the one destabilizing) to create

DESIGN LOAD

PARTIAL LOAD

FACTORS

The philosophy followed in these documents is to design this type of structures against the occurrence of a LIMIT STATE .

By its nature, reinforced soil is a combination of structural and geotechnical engineering.

The evolution of limit state design in structural engineering has led to the definition of a number of PARTIAL LOAD

FACTORS, which are applied to loads in design combinations, and MATERIAL FACTORS , which are applied to the structural

components.

DESIGN STRENGTH should be equal to, or greater than , the design load.

EUROCODE 7

APPROACH 1

A1-M1-R1 A2-M2-R1 APPROACH 2

A1-M1-R2

OVERTURNING STABILITY CHECK EQU-M2

« Static equilibrium EQU is mainly relevant in structural design. In geotechnical design, EQU verification will be limited to

rare cases, such as a rigid foundation bearing on rock, and is, in principle, distinct from overall stability or buoyancy

problems. If any sharing resistance Td is included, it should be of minor importance.»

The most adverse loads likely to be applied to the structure should be considered in design. Load factor should be

applied to each component of load.

The value of these factors should depend on the Code applied.

The following descriptions of load cases identify the usual worst combination for the various criteria and they are in

accordance with BS8006:2010. It considers 3 static cases. All load cases should be checked for each layer of

reinforcements within each structure to ensure the most critical condition has been found and considered.

CASE A

This combination considers everything (stabilizing and destabilizing

actions) at its maximum values and, for this reason, it’s the case

that generate the maximum stress on reinforcement and maximum

pressure value in foundation.

This combination generally rules the internal stability check at

rupture and the external stability for bearing capacity analysis.

CASE B

This combination considers maximum values of destabilizing

actions and minimum values for own weight and stabilizing

loads.

This combination generally rules the internal stability check

for adherence and external stability at sliding.

CASE C

This combination considers only own weight and dead loads

without adding any amplifying factors.

This combination is used to evaluate settlement in foundation

because give the value of pressure discharged in foundation.

The design of reinforced soil walls and abutments should follow the principles involved in conventional earth retaining

structures, however, reinforced soil structures require additional consideration with regard to soil/reinforcement interaction.

For convenience analysis should usually be considered in two main parts covering external and internal stability.

It should be noted that external stability covers the basic stability of the reinforced soil structure as a unit, while internal

stability covers all areas relating to internal behaviour mechanisms, consideration of the stress within the structure,

arrangement and behaviour of the reinforcements and backfill properties.

There are two methods that may be used for the design of reinforced soil structures.

TIE BACK WEDGE METHOD

EXTENSIBLE reinforcements

COHERENT GRAVITY METHOD

INEXTENSIBLE reinforcements

a reinforcement is inextensible when it sustains the design loads at strains less than or equal to 1%, that Paraweb must be enter in this category.

The elongation at maximum load at failure for all grades of Paraweb strap covered by BBA Certificate is 12% ± 1%. But this is simply theory because in reality Paraweb reinforcement works far from the

nominal tensile strength. Considering all the reduction factors (creep, durability/environmental, extrapolation of data,

ramification of failure), the value assumed in calculation is closer to 65% of the nominal tensile strength.

HOW CAN I CONSIDER PARAWEB AS INEXETENSIBLE REINFORCEMENT?

Coherent Gravity Method – Basic Assumption

• The interaction coefficient soil/reinforcement (µ*) varies with the depth

• The structure is divided into 2 zones: active and resistant

• Lateral earth pressure coefficient is assumed to be Ko at the top of the wall, decreasing to Ka at a depth of 6 m belowthe wall

• Meyerhof’s approach is applied to the reinforced soil mass at each reinforcement level and the wall base

• This method applied to INEXTENSIBLE reinforcements

MAXIMUM TENSION LINE

If the total length of the reinforcement is limited to Laj then the transfer of load from soil to reinforcement in the active zone

would not prevent collapse of the active zone. To achive this, the reinforcement extends a length Lej into the resistance zone.

The tensile load in the reinforcement over the length Lej is not constant but decreases towards the free end where it becomes

zero.

Field observation (that are the base of specific Code) showed that for the Ultimate Limit State the coefficient of earth pressure

should be taken as K0 at the top of the wall reducing lineraly with depth to a value of Ka at a depth of 6,00m below the top of

the structure as set out below:

)()1(00

0 zzKz

zKK a+−=mzz 0,60 =≤

aKK =

where:

z is the depth measured from the upper level

of the mechanical height H

Ka=tan2[(π/4)-(φ/2)]

mzz 0,60 =>

Inextensible

Reinf.

Extensible

Reinf.

LOCAL STABILITY OF A LAYER OF REINFORCING ELEMENTS –MAXIMUM TENSION IN THE REINFORCEMENT

vjvjjj SKT σ=

VARIATION OF K VALUE DEPENDING ON THE DEPTH

Extensible

Reinf.

Inextensible

Reinf.

Extensible

Reinf.

VARIATION OF K VALUE DEPENDING ON THE DEPTH

TENSION AT FACING

jjconn TaT 0=−

85,00 =a

where:

Hz j 6.0≤if

)6.0(

)(15,010 HH

zHa j

−−

−= if Hz j 6.0>

The load in a reinforcement varies along its length and factors should be applied to determine tensile loads at various positions.

APPARENT FRICTION ANGLE COEFFICIENT µ*

Where: µ1* = ζ tan (φ)

µ0* = determined with pull-out experimental test on reinforcement

ADHERENCE CAPACITY OF THE REINFORCEMENT

( )jj vjefff zLbT σµ ⋅⋅⋅⋅= *2

CONCLUSIONS

• Fomulas and Concepts inserted in British Standard BS8006:2010 are applied in calculation;

• Check 3 different load combinations will guarantee the structures against the worst conditions;

• Maccaferri’s reinforcements are inextensible type;

• Ultimate Limit State is used in design accordingly to all Specific Code on reinforced earth structure

(BS8006:2010, AFNOR NFP94-270, FHWA2009)

• Coherent Gravity Method is applied;

• In Europe we use safety factors and load factors accordingly to Eurocode 7.

• we made calculation with Approach 1 and each sub-case (A1-M1-R1 and A2-M2-R1) will be

checked in the 3 load combinations of BS8006:2010.

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