GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE Prof. J. N. Mandal Department of Civil Engineering, IIT Bombay, Powai, Mumbai 400076, India. Tel.022-25767328 email: cejnm@civil.iitb.ac.in
Module - 6 LECTURE - 26 Geosynthetics for reinforced soil retaining walls
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
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
Part I Mechanically stabilized segmental reinforced soil retaining wall
Basic concepts Soil mechanics Interaction Polymer properties Applications Soft soil applications Reinforced fill applications Unpaved roads Embankments Steep slops Retaining walls (Short term reinforcement strength required) (Long term reinforcement strength required)
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
Gravity wall Semi-gravity wall Cantilever wall Counter-fort wall
Bridge and abutment Anchored Sheet pile Soil Nailing Braced Excavation
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
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.
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.
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
Superimposed (Tiered) walls Uneven reinforcement wall (After FHWA-NHI-10-024,2009)
- Overall base width is large S M 0.5 H Back-to-back walls (After FHWA-NHI-10-024,2009) - Overlapping of reinforcement L 0 > 0.3 H L R /H = L L /H 0.6
Stable feature walls (After FHWA-NHI-10-024,2009)
Influence of surcharge for tiered walls (After Simac, 1990)
Water front structure
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.
Components of geosynthetic reinforced soil walls
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.
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.
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
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.
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.
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
Bodkin connection details A rigid PVC pipe is used as bodkin. There should not be any slack in the connection.
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.
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
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.
Timber facing Welded wire mesh facing Gunny bag facing Brick facing wall
DIFFERENT PRECAST CONCRETE MODULAR BLOCKS OR PANEL FACINGS AND CONNECTIONS Modular concrete blocks for segmental retaining walls (After Bathurst and Simac, 1994)
Shear Pin Shear Key Leading shear lip Geogrids connected with modular blocks either mechanically or by friction (After Simac et al. 1993)
(a) Frictional connection and (b) Mechanical connection
Details of frictional connection between geogrid and segmental panel
Construction Details
Wall Construction
General view on Wall During Construction
Placing Facing Blocks
Wall Ties Fixing False Facing
ANALYSIS AND DESIGN PROCEDURES FOR GEOSYNTHETICS REINFORCED SOIL RETAINING WALL Geosynthetic reinforced soil wall with inclined surcharge load
Schematic view of segmental reinforced soil retaining wall H M = Mechanical height, H F = Facing height, H D = Design height
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 = q L Dead load = q D Total surcharge (q) = q L + q D
Type of facing Full-height concrete panels, Wrapped facings, Modular or Segmental concrete blocks. Gabion Vertical spacing of reinforcements (S v ) Wrapping: Maximum spacing (S v ) 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.
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.
Establish preliminary wall dimensions a) Minimum length of reinforcement (FHWA NHI-10-024, 2009) Case Minimum L/H ratio Static loading without or with traffic surcharge 0.7 Sloping backfill surcharge 0.8 Seismic loading 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.
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.
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.
Internal friction angle (Φ r ) of the soil in reinforced zone can be determined from the drained direct shear test. For retained backfill, the internal friction angle (Φ b ) 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).
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.
Gradation of backfill soil for reinforced soil zone(walls and slopes)(after Koerner et al.1993,gsi/gri) Sieve Size Number Particle Size Percent Passing # 4 4.76 mm 100 10 2.0 90-100 40 0.42 0-60 100 0.15 0-5 200 0.075 0 Notes: FHWA adopts15% passing #200 sieve NCMA adopts 35% passing #200 sieve
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
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Prof. J. N. Mandal Department of civil engineering, IIT Bombay, Powai, Mumbai 400076, India. Tel.022-25767328 email: cejnm@civil.iitb.ac.in