Lecture 26 : Geosynthetics for Reinforced Soil Retaining Walls

GEOSYNTHETICS ENGINEERING: IN
THEORY AND PRACTICE
Prof. J. N. Mandal
Department of Civil Engineering, IIT Bombay,
Powai , Mumbai 400076, India.
Tel.022-25767328
email: [email protected]
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Module - 6
LECTURE - 26
Geosynthetics for reinforced soil retaining walls
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
OUTLINE
Part - I Mechanically stabilized reinforced soil retaining
walls with modular blocks or panel facings
 Introductions
 Geosynthetic reinforced soil wall system
 Different precast concrete modular blocks or panel
facings and connections
 Analysis and design procedures for geosynthetics
reinforced soil retaining wall
 Cost considerations
 Construction procedure for precast concrete faced walls
 Submission of material and test report by manufacturer
 Design critique
 Failures of structures
 Tolerances
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Part – II Geotextile or geogrid wrap-around-faced
mechanically stabilized earth (MSE) walls
 General
 Design of geotextile wrap-around-faced wall
 Wraparound face construction details
Part – III Gabion walls
 General
 Gravity gabion wall design
 Reinforced soil gabion wall design
 Feasibility Study on Fly Ash as a Backfill Material
 Geocell walls
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Part – I
Mechanically stabilized segmental reinforced soil
retaining wall
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Basic concepts
Soil mechanics
Interaction
Polymer properties
Applications
Soft soil applications
Unpaved roads
Embankments
(Short term reinforcement strength required)
Reinforced fill applications
Steep slops
Retaining walls
(Long term reinforcement strength required)
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Different types of conventional rigid retaining structures
made up of masonry and concrete are available to resist
the lateral pressures:
 Gravity retaining walls,
 Semi-gravity type retaining wall
 Cantilever retaining walls,
 Counter fort retaining walls and
 Bridge and abutment.
 Anchored Sheet Pile
 Soil Nailing
 Braced Excavation
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Gravity wall
Cantilever wall
Semi-gravity wall
Counter-fort wall
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Bridge and abutment
Soil Nailing
Anchored Sheet pile
Braced Excavation
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Inclusion of reinforcements in soil is not new. It has been
used since biblical age. The concept of reinforced earth
system is well established.
Vidal (1966)
(Lee et al., 1973)
Components parts and key dimensions of reinforced
earth wall
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
 The traditional concrete and masonry gravity walls or
cantilever retaining walls are almost obsolete due to higher
cost of construction.
 Reinforced soil wall is the best cost effective solution.
Metallic strips or geosynthetics can be used as
reinforcement. Geosynthetic is an emerging bona-fide
engineering construction material around the world.
 The mild steel degrades due to electro-chemical
corrosion whereas, the polymer materials suffer from creep
problem causing reduction in the ultimate tensile strength.
Therefore, adequate factor of safety should be
considered to meet the serviceability limits.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
There are many disadvantages of using metallic strips in
the mechanically stabilized reinforced earth wall,
 High Cost
 Long term susceptibility to corrosion. Protective coating
can reduce corrosion, but it is uncertain in the field due to
ground water or electric current.
 Sustainability depends on the correct choice of Backfill
material ( i.e. gradation, chemical properties etc.)
 It cannot be used with many indigenous materials.
 Back fill material cost is about 85% of the total cost of the
Reinforced Soil Wall.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Geosynthetic Reinforced Soil Wall System
Advantages:
 Polymer do not corrode
 Economical
 Used with many indigenous materials
 More deformable than the metal reinforcement
 Long term durability
 The geosynthetic is flexible
 Unskilled labour can place it
 Minimum excavation
 Good drainage
 Heavy equipment is not needed
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Superimposed (Tiered)
walls
Uneven reinforcement wall
(After FHWA-NHI-10-024,2009)
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
- Overall base width is
large
SM ≈ 0.5 H
- Overlapping of
reinforcement
L0 > 0.3 H
LR/H = LL/H ≈ 0.6
Back-to-back walls
(After FHWA-NHI-10-024,2009)
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Stable feature walls
(After FHWA-NHI-10-024,2009)
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Influence of surcharge for tiered walls
(After Simac, 1990)
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Water front structure
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
 In the past 40 years, a tremendous number of
geosynthetic reinforced soil walls have economically been
constructed around the world. The geosynthetics
reinforcements are placed horizontally in the retaining wall
backfill.
 Geosynthetics reinforced soil mass are basically gravity
structures resisting the earth pressure developed behind
the reinforced soil zone. The facia resists the mass of
reinforced soil, retained soil and the surcharge loads.
 Geosynthetics reinforced soil walls are flexible.
Therefore, it can tolerate larger settlements and earthquake
loading than the conventional retaining walls. The ground
improvement can also be avoided.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Components of geosynthetic reinforced soil walls
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Major components of reinforced soil system:
 Foundation soil
It is required to improve the foundation soil by introducing
reinforcement layers, geocells, prefabricated vertical band
drains or encased stone columns. Check the factor of safety
against bearing capacity failure.
 Reinforced soil
The reinforced soil is the combination of soil and the
horizontal layers of geotextiles or geogrids. It is preferable to
use CEG < 30 mm mol/ kg and molecular weight > 25,000
gm/mol for good quality PET resin.
 Backfill
The backfill soil is located behind the reinforced soil zone.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
 Drainage fill
Face drain behind the wall facia.
Blanket drain beneath the reinforced soil zone,
Back (chimney) drain behind the reinforced soil zone
To prevent build up of hydrostatic pressure. The drainage
outlet must be connected to the collection pipe.
 Polymeric geogrids or geotextiles
 Polymer geogrids and polyester strips, both flexible and
stiff, are usually used as horizontal layers.
Geocomposite reinforcement or hybrid reinforcement
Geotextiles (woven and nonwoven) are also used in
wrap-around faced mechanically stabilized earth walls.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
 Facia
The facings have aesthetic views and can be of any
shape and colours.
 Wrap-around facings
 Segmental precast concrete panels
 Full-height concrete panels
 Modular block wall
 Gabion facings
 Timber facing
 Welded wire meshes facing
 Gunny bag facing
 Brick facing
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Warp- around facing
 Vertical spacing of reinforcements = 0.3 m - 0.5 m
 It is required to protect the geotextile against ultraviolet
light, degradation, vandalism and damage due to fire. In
such case, shotcrete should be applied to the wall facing.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Segmental precast concrete panels
 HDPE geogrids are casted into the panels during
manufacturing process in the field. The main geogrid is
then connected to the HDPE geogrid (bodkin joint) about
30 cm away from the facing panel.
 The flexible polyester geogrid should not be casted due
to high alkalinity in presence of wet concrete.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Three types of Precast concrete face panels:
 Hexagonal shaped panel:
1.5 m height, 1.75 m width and 0.165 m thick
 Rectangular panel:
3.81 m long, 0.61 m height and 0.2 m thick
 T –shaped panel:
3.2 m area and 0.16 m thick
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Bodkin connection details
A rigid PVC pipe is used as bodkin. There should not be
any slack in the connection.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Full-height concrete panels
 The full height concrete panels are 12.5 cm - 30 cm thick,
240 cm - 300 cm wide and 750 cm high.
 Stiff polyethylene geogrids are casted into the panel
similar to segmental precast concrete panels.
 The minimum compressive strength of concrete at 28
days is 27.56 Mpa.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Modular concrete block wall (MCBW)
 Length = 200 mm - 600 mm
 Height = 100 mm - 200 mm
 Width = 200 mm - 0.6 m.
 The weight of dry casting MBW = 15 kg to 50 kg
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Gabion facing wall
The gabion is a kind of basket made up of galvanized
mild steel wire mesh and polymer geogrids filled with
rocks/stones.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Timber facing
Gunny bag facing
Welded wire mesh facing
Brick facing wall
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
DIFFERENT PRECAST CONCRETE MODULAR
BLOCKS OR PANEL FACINGS AND CONNECTIONS
Modular concrete blocks for segmental retaining walls
(After Bathurst and Simac, 1994)
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Shear Pin
Shear Key
Leading shear lip
Geogrids connected with modular blocks either mechanically
or by friction (After Simac et al. 1993)
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
(a) Frictional connection and (b) Mechanical connection
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Details of frictional connection between geogrid and
segmental panel
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Construction Details
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Wall Construction
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
General view on Wall During Construction
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Placing Facing Blocks
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Wall Ties Fixing False Facing
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
ANALYSIS AND DESIGN PROCEDURES FOR
GEOSYNTHETICS REINFORCED SOIL RETAINING WALL
Geosynthetic reinforced soil wall with inclined surcharge load
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Schematic view of segmental reinforced soil retaining wall
HM = Mechanical height, HF = Facing height,
HD = Design height
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Step 1: Physical characteristics of mechanically
stabilized soil walls.
 Wall geometry:
The height of wall = H,
The length of wall = L,
Wall face batter angle = ,
The wall requires a nominal batter of 3° to 10°
Slope angle of the soil surface = i,
 Loading:
Surcharge loads:
Live load = qL
Dead load = qD
Total surcharge (q) = qL + qD
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
 Type of facing
 Full-height concrete panels,
 Wrapped facings,
 Modular or Segmental concrete blocks.
 Gabion
 Vertical spacing of reinforcements (Sv)
 Wrapping:
Maximum spacing (Sv) is 0.5 m to 0.6 m for geotextile
(woven and non-woven) or geogrid wrapped face walls.
 Precast concrete face panels:
The spacing of the geogrid reinforcement may be kept
from 0.5 m to 1 m. However it is recommended to keep
the vertical spacing of reinforcement as 0.6 m - 0.8 m.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
 Modular block:
- For the modular concrete block of height 200 mm to 250
mm, the spacing of the reinforcement may be 200 mm,
400 mm, 500 mm, 600 mm, 750mm, 800 mm and 1 m.
- For segmental concrete block, if the spacing is more,
use secondary reinforcement.
Vertical spacing of the reinforcement depends on the
strength of the reinforcement, facing connection and types
of panels or blocks used for construction.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
 Establish preliminary wall dimensions
a) Minimum length of reinforcement
(FHWA –NHI-10-024, 2009)
Case
Static loading without or with
traffic surcharge
Sloping backfill surcharge
Seismic loading
Minimum L/H ratio
0.7
0.8
0.8 to 1.1
b) For walls founded on slopes, a minimum horizontal
bench of 1.2 m wide should be given in front of wall.
Minimum embedment depth should be 0.5 m.
Minimum 1 m embedment length is recommended
beyond Rankine failure wedge for pullout resistance.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Step 2: Evaluate engineering properties of the
foundation soil.
 Detailed soil exploration has to be carried out along
the alignment of the reinforced soil wall at every 25 m
interval.
 Evaluate grain size distribution, moisture content,
liquid limit, plastic limit, shrinkage limit and plasticity
index of soil.
 Calculate the shear strength and consolidation
parameters of foundation soil. Check the location of
ground water table.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Step 3: Evaluate reinforced fill and retained backfill soil.
 Check the grain size distribution and plasticity index.
 Plasticity index should not exceed 6 (AASHTO T-90)
 Coefficient of uniformity of reinforced fill ≥ 2.
 Organic content should be limited to 5 %.
 Determine optimum moisture content (OMC), maximum
dry density or relative density with the aid of standard
proctor test.
The minimum compaction of backfill soil should be 90%
of maximum proctor density.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
 Internal friction angle (Φr) of the soil in reinforced zone
can be determined from the drained direct shear test.
For retained backfill, the internal friction angle (Φb) can
be determined by drained triaxial compression test or
direct shear test.
Generally, angle of internal friction ≤ 34º.
 Coefficient of permeability should be ≥ 1 x 10-2 cm/sec
 No cohesion should be considered, i.e. fine silts and clay
should not be used for reinforced fill.
 Appropriate drainage system is required at the back,
base and front of reinforced soil retaining walls.
If the quality of backfill is poor, the adequate drainage
can not be achieved (Saidin, 2007).
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
 For polyester geosynthetic, pH value of soil should lie
between 3 and 9 (Elias and Christopher, 1997)
For polyethylene and polypropylene, pH of soil > 3
(AASHTO T-289-91).
 Minimum aperture size of geogrid > 3.5 times the
particle size of the backfill soil (Sarsby, 1985)
 In many cases, we use the minimum average roll
values (MARV) obtained from the manufacturer’s
certificate.
For good design, it is recommended to verify the test
results of geosynthetic materials from the third party.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Gradation of backfill soil for reinforced soil zone(Walls
and slopes)(After Koerner et al.1993,GSI/GRI)
Sieve Size
Number
# 4
10
40
Particle
Size
4.76 mm
2.0
0.42
Percent
Passing
100
90-100
0-60
100
0.15
0-5
200
0.075
0
Notes: FHWA adopts15% passing #200 sieve
NCMA adopts 35% passing #200 sieve
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
 Creep reduction factor for polymer
(FHWA –NHI-10-024, 2009)
Polymer type
Creep reduction factors
Polyester (PET)
2.5 to 1.6
Polypropylene (PP)
5.0 to 4.0
High Density
Polyethlene (HDPE)
5.0 to2.6
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Please let us hear from you
Any question?
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Prof. J. N. Mandal
Department of civil engineering, IIT Bombay,
Powai , Mumbai 400076, India.
Tel.022-25767328
email: [email protected]
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

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