GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE · 2017. 8. 4. · c) Connection strength For...

GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE

Prof. J. N. Mandal

Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: [email protected]

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay


Module - 6LECTURE - 28

Geosynthetics for reinforced soil retaining walls

Design factor of safety (FS) based on mode of failures

Ultimate limit state

Serviceability limit states

External stability under static loading

External stability under seismic loading

Internal stability

Recap of previous lecture…..


Alternatively, Le can be expressed as per FHWA (1998),

.F.C.2FS..S

L *rv

pullhve

F* = coefficient of pullout interaction between soil and geotextile.

α = scale correction factor.

F* and α for standard backfill materials except uniformsand (Coefficient of uniformity, Cu < 4).

Type of reinforcement Default F* Default αGeogrid 0.8 tanϕ 0.8

Geotextile 0.67 tanϕ 0.6

Factor of safety for pullout should be 1.5. Minimumembedment length (Le) is 1m.


Anchorage capacity of the reinforcement per unit width(P) is expressed as,

P = 2 Le. σv. Cr. F*. α

P ≥ FSpull . Tmax

Tmax = Sv. σh

For wrap-around face wall, geosynthetic overlap length(Lo) can be expressed as,

.F.C.4FS..S

L *rv

pullhv0 Minimum value of overlap (L0) is 1 m


c) Connection strength

For long-term creep and aging, consider a factor ofsafety = 1.5

5.1xRFxRFTT

CRD

sac

Ts = Seam strength as per ASTM D 4884 for seams

RFD = Durability reduction factor for chemicals, stresscracking, thermal oxidation and hydrolysis,

RFCR = Reduction factor for creep

i) Connection testing for geosynthetic rupture (Simac etal., 1993; Elias and Christopher, 1997):

Tac = maximum connection strength


ii) Connection testing for Pullout resistance ofgeosynthetic (Elias and Christopher, 1997):

The maximum connection strength can be written as,

5.1T

T pac

TP = maximum pullout load per unit width of thereinforcement or the pull out load at maximumdeformation of 20 mm, whichever occurs first


INTERNAL STABILITY CHECK FOR SEISMIC LOADING

The location of maximum tensile force line does notchange due to the rupture of the reinforcement or pull outforce during seismic loading.

Breakage failure

RFC.T)T( rallowablestaticmax

For dynamic case:RF75.0

C.T)T( rallowabledynamicmax

For static case:


Pull-out failure

p

rpullmax FSx75.0

C.PT

rve

*d

rpullmax C..Lx5.1x75.0

xFx2

5.1x75.0C.P

T

Fd* (dynamic) = 0.8 F* (static)

α = 0.6 (normally)

F* and α = Pullout factor (default value)

Due to seismic loading, the overall stiffness ofgeosynthetic will decrease. So, damping andamplification may increase.


Step 6: Serviceability Limit State - Internal stability

Internal wall deformation

Lateral displacement of wall face

Determine the settlement of reinforced soil wall

External wall settlement


COST CONSIDERATIONS

Koerner et al. (1998) compared the cost of geosyntheticreinforced soil walls with various other retaining walls.

The cost of geotextile is about 25 % of the wall’s totalcost (Bell et al.1983).

In India, the cost for geosynthetics reinforced soil wall isabout 30% - 50% less than that of conventional gravityretaining walls:

Geosynthetics and accessiroies = 35 %Panel/block facing and mould =32 %Labour =15 %Margin/profit = 18 %


0

100

200

300

400

500

600

700

800

0 2 4 6 8 10 12 14Height of wall (m)After Koerner et al., 1998

Gravity Walls

Crib / Bin Walls

MSE (Metals)

MSE (Geosynthetics)


The material cost depends upon the following elements:

Modular block or Panel facia

Gravels as drainage material

Geosynthetics material as reinforcement (Geogrid,

Geotextile, Geocomposite)

Leveling pad

Cost of design

Soil and Geosynthetic testing

Site preparation and site assistance

Under drain pipe

Placement of backfillProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

CONSTRUCTION PROCEDURE FOR PRECAST CONCRETE FACED WALLS

a) Foundation subgrade

Geogrid layers at the foundation Geocells as Foundation mattress


b) Leveling pad

Leveling pad (all dimensions are in mm)

The main objective of leveling pad is to erect the facingelements.


Plane cement concrete (P.C.C 1:2:4 lean concrete) isplaced beneath the leveling pad to minimize differentialsettlement.

Leveling pad should be constructed continuously up to20 m in length with a gap of 20 mm.

Curing for at least 24 hours prior to the placement ofthe blocks / panels.


c) Erection of facing elements

Sequence of erection of panels

Cast with M-35 gradeconcrete.

Erection of panel after curingfor 28 days.

The minimum thickness ofpanel is 140 mm.

The Panels are square,rectangular, hexagonal, cruciformand diamond in shape.


Panel dimension of precast faciaProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Face length = 200 mm-450 mm,

width = 200 mm-600 mm and

height = 150 mm - 200 mm.

weight of the block = 250 N to 500 N.

Precast concrete modular blocks (After Bathurst and Simac, 1994)


The facing batter is not less than 1 in 40 but 6 degreeis preferable.

Minimum M 25 grade concrete should be used.

The facing units should be connected with the help ofshear keys, shear pins or shear lips.

The spacing between reinforcement should not bemore than 600 mm.


Standard design of MSEW (After FHWA, 2009)


d) Placement of backfill and compaction

The backfill materials are compacted with a rollercompactor to achieve at least 95% modified Proctordensity. The lift thickness should not be more than 200 mm.

The light weight hand compactor not exceeding 75 kgshould be used to compact near to the facing elements.

Vibroplate compactor of maximum weight 1000 kgshould be used to compact within 1.5 m from the edge ofthe facing elements.

No heavy equipment should be kept near to the face ofthe walls.


e) Placement of reinforcement

After filling and compaction up to desired height,geogrid reinforcement is connected with the facingelements and placed horizontally.

The geogrid should be placed in the machinedirection perpendicular to the face of the wall.

It is very important that during the compaction, bothhorizontal and vertical alignment of facia panel or blockmust be observed regularly.


e) Drainage (DDD………)

Most of the reinforced soil structures fail (or collapse)due to the development of excess hydrostatic pressureat the back of the retaining walls.

Koerner and Soong (2001) documented that 20 outof 26 mechanically stabilized earth walls fail either byexcessive deformation or collapse.

Therefore, proper design of drainage system isneeded.


Drainage using a blanket with chimney drains (After Collin et al. 2002)


Drainage system and geomembrane barrier for reinforced soil wall (After FHWA, 2009)Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

f) Submission of materials and test reports bymanufacturer

The contractor should submit the following information tothe engineer,

Name of the manufacturer

Current full address

Polymer type for Geosynthetics

yarn/Fiber tie for Geosynthetic structure

Roll no of Geosynthetics, and

Certified test result.Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

g) Design critique

The mobilized strain in reinforced soil walls may berelatively high because of the extensibility ofreinforcement.

It is recommended to use critical state shearingresistance angle instead of peak friction angle of the soilfor design (Jewell, 1991).

At the end of construction, the serviceability straincriteria for long design life of reinforced soil wall,

- 1.0 % for walls, and

- 0.5 % for bridge abutmentsProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Most of the reinforced soil structures fail due to thelateral deformation at the top and bulging of the wall.

It is recommended to provide longer lengthreinforcements at the top, middle and bottom of thereinforced wall

For the remaining height of the wall, shorter lengthreinforcements can be used.


FAILURES OF STRUCTURES

The geosynthetic reinforced soil walls fail or collapse due toseveral reasons:

Excess hydrostatic pressure Excessive lateral deformation; Improper selection of backfill materials Local instability (distortion) of modular blocks or panels, Contractor’s lack of workmanships for quality control, Bearing capacity and deep-seated failure Global failure also causing formation of huge heave in frontof the facia elements No site-specific design and also Copycat and paste


Heavy compaction within 1 to 1.5 m of wall face

No outlet drainage provided for internal drainage

Geogrid tension failure

Geogrid connection failure

Geogrid slippage or block wall failure

Poor attention to facing connection,

poor inspection,

Improper compaction

poor quality control and assurance

Inadequate design of the backfill slope and the foundation


Codes and design Standards

Strengthened / reinforced soil and other fills, BSI (1995)

Mechanically stabilized earth walls and reinforced soil slopes: Design and construction guidelines. FHWA, 2001.

Design manual for segmental retaining walls, NCMA Design manual, 1996.

Segmental retaining walls – seismic design manual, Dr. R. J. Bathurst.


Please let us hear from you

Any question?


Prof. J. N. Mandal

Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: [email protected]


Date post:	16-Feb-2021
Category:	Documents
Upload:	others
View:	1 times
Download:	0 times

GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE · 2017. 8. 4. · c) Connection strength For...

Documents