14-RETAINING WALLS



RETAINING WALLS

1. GENERAL

This part covers the quality control requirements and construction of retaining walls. The requirements related to site clearance, setting out of the works, earthworks including rock cutting and blasting operations, stone masonry, brick masonry, plain /reinforced cement concrete, reinforcement etc. shall be as per relevant sections of this Manual.

2. SOLID RETAINING WALL

  1. The solid retaining wall provides stability by virtue of its own weight and shall be built in stone masonry or plain cement concrete.
  2. Generally, solid retaining walls shall not be preferred where retaining heights exceeding 3 m.

3. RCC RETAINING WALL

  1. The RCC cantilever wall type retaining structure shall be preferred for heights 6 m to 7 m.
  2. The counterfort retaining wall consists of counterforts (Transverse stem supports) which are concealed within the retained earth (On the rear side of the wall) shall be preferred for heights above 7 m.
  3. The ‘buttress wall’ is similar to the ‘counterfort wall’, except that the transverse stem supports, called buttresses, are located in the front side. The buttresses interconnect the stem with the toe slab and not with the heel slab, as with counterforts.
  4. Backfilling of retaining wall shall be applied on heel slab first and then toe side filling shall be carried out.
  5. Inverted filter shall be provided behind retaining walls to drain off ground water table or rainwater seepage.
  6. Special precautions shall be taken to prevent any wedging action against structures and the slopes bounding the excavation for the structure shall be stepped or strutted to prevent such wedging action.
  7. Weep holes shall be provided in RR masonry walls at spacing of about 1.5 m centre-to-centre in either direction. The size of weep holes shall be 100 mm to 150 mm PVC (flexible) pipes and shall be sloped towards valley side to effectively drain the water from ground. The weep holes shall be provided above the low water level.
  8. In counterfort retaining walls, there shall be at least one drain in each pocket between adjacent counterforts.
  9. The back fill material shall consist of granular material of GW, GP, SW groups as per IS: 1498 and shall be free of clay and cement.
  10. The filter material shall be well packed to a thickness of not less than 600 mm with smaller size towards the soil and bigger size towards the wall and provided over the entire surface to the full height.
  11. The drainage layer shall be provided as per the specifications for the reinforced soil retaining wall.

4. REINFORCED SOIL RETAINING WALL

4.1 Reinforcing elements

  1. The reinforcing element shall be metallic in the form of strips (Aluminium alloy, copper, carbon steel, galvanised steel, stainless steel, ladder) or mats of metal (steel grids, woven and welded steel wire meshes) or synthetic (PET, HDPE, PVA, PP) reinforcement in the form of grid or strip or strap or combination of metallic or synthetic or any other proprietary material which may be approved by the Engineer or as shown in the Drawings.
  2. Aluminium alloy strip shall comply with BS: 1470 quality 5454 in the H24 condition.
  3. Copper strip shall comply with BS: 2870 quality C101 or C102 in ½ H condition and shall have 0.2% proof stress of not less than 180 N/mm2.
  4. Carbon steel strip shall comply with BS: EN10025 or IS: 2062 and have a section content of not more than 0.55%. The fabricated element shall be galvanised in accordance with IS: 4759 and IS: 2629 and the minimum zinc coating weight shall not be less than 1000 gm/m2.
  5. The steel strips with minimum bearing and shear strength of 490 MPa shall comply with the requirements of BS: EN 10025, Grade S 355 JR, or IS: 2062 grade Fe490, except the elongation (on base metal) for which minimum 22% is acceptable.
  6. The panel lugs shall be manufactured from hot rolled steel strips with the same steel quality and grades as specified above, except that the minimum zinc coating weight not less than 600 gm/m2.
  7. All permanent metallic connectors (exposed to soil), tie strips and lugs shall be hot dip galvanised. Nuts /bolts (fasteners) shall be galvanised as per requirements of IS: 1367-Part 3 and of grade 10.9.
  8. For all metallic components, where holes or penetrations are made through the reinforcing elements to accommodate connection such as bolts, pins, or other, the cross section thickness and/or width of metallic component shall be increased to account for section loss caused by the hole or penetration.
  9. Stainless steel strip shall comply with BS:1449 (Part 2) quality 315 S 31 or 3/6 S 33 except that the material shall be cold rolled to provide a 0.2% proof stress of not less than 400 MPa and the tensile strength shall not be less than 540 MPa.
  10. All metallic components buried in soil shall be of electrolytically compatible materials.
  11. High strength high tenacity geo-textile fabrics used as reinforcement in the construction of reinforced slopes or in the base of reinforced soil structure as reinforcement, geotextile fabric used for separation, filtration and/ or drainage shall satisfy all the requirements stipulated for geosynthetic reinforcing elements.
  12. The manufacturer of geogrids, geotextiles, geostrips, polymeric strips, polymeric ties or any other geosynthetic material, including any proprietary geo-synthetic material, for use as reinforcing element shall fulfil the following requirements:
  13. The manufacturer shall provide test reports from an independent laboratory with valid accreditation, for all the tests needed to establish all the reduction factors listed below:
  • Shall have ISO (ISO-9001) or CE Certification for manufacturing process and quality control.
  • The product shall have certification for use as soil reinforcing material from an agency accredited for certifying geo-synthetic reinforcement products.

RFCR - Reduction factor for creep.

RFID - Reduction factor for installation damage.

RFW - Reduction factor for weathering.

RFCH- Reduction factor for chemical/environmental effects.

fs - Factor for the extrapolation of data.

All the above factors shall be determined in accordance with the provisions of ISO/TR 20432-Guide to the determination of long strength of geosynthetics for soil reinforcement.

4.2 Sampling and testing

  1. Test for the ultimate tensile strength shall be carried out on a random sample basis for each grade of reinforcement as per ISO-10319. The test results shall be accompanied by stress-strain curves showing strength at 2% and 5% strain and strain/elongation at failure.
  2. Annual average daily temperatures (AADT)/design temperature of the project site shall be worked out and values of reduction factor for creep RFCR and for RFCH shall be provided as per procedures given in ISO/TR-20432.
  3. Tests shall be carried out to provide values of:
  4. Pullout coefficient as per ASTM D 6706 "Standard test method for measuring geosynthetic pullout resistance in soil" and
  5. Coefficient of interaction between reinforced fill soil and geogrids as per ASTM D 5321or IS: 13326: Part 1-1992 for all types of geogrids.
  6. One set of project specific tests shall be conducted at third party accredited laboratory or at a reputed institute.
  7. Each roll shall have at least one identification label with roll number and product type.

4.3 Earth fill

  1. The fill material in the reinforced soil zone shall have, drained or effective angle of friction not less than 30°, measured in accordance with IS: 2720 (Part 13), by conducting a drained direct shear test. In case the fill material has 25% or more particles of 4.75 mm or larger, drained shear test using large shear box may be conducted (IS: 2720 Part 39).
  2. The gradation of fill soil shall be as per table.

   Table:  Requirements for fill soil

Sieve size

Percentage  passing by weight

75 mm

100

425 micron

0-60

75 micron

Less than 15

 

Materials with more than 15% passing 75 micron sieve are acceptable if particles smaller than 15 microns are less than 10%, provided PI is less than 6 and angle of friction is not less than 30°.

  1. Properties of fill soil in the reinforced zone, unreinforced zone (or retained/back fill) soil and the foundation soil shall be determined accurately during the construction phase, as per quality assurance plans and directions of Engineer so as to ensure that these are the same as those considered in the design phase. The fill soil in the unreinforced zone shall conform to the requirements specified in the design.
  2. Where galvanised steel reinforcement is used, the fill material shall be free draining granular material and shall meet the requirements given in table.

 Table: Recommended limits of electrochemical properties for reinforced fills with steel reinforcement

Property  

Code

Specified value

Resistivity

AASHTO T-288

>3000 ohm-cm

pH

AASHTO T-289

>5 and <10

Chlorides

ASTM D 4327

<100 ppm

Sulphates

ASTM D 4327

<200 ppm

  1. Where geosynthetic reinforcement is used for reinforcing elements manufactured from polyester yarn, pH value of the fill material shall be between 3 and 9, and for reinforcing elements manufactured from PVA, PP and HDPE, the pH value shall be greater than 3.

4.4 Facia material

  1. The facing system shall be any one of the following:

i) Precast reinforced concrete panels.

ii) Precast concrete blocks and precast concrete hollow blocks.

iii) Gabion facing (See figure 4300-1).

iv) Wrap around facing using geosynthetics.

v) Metallic facing, prefabricated in different shapes including welded wire grid and woven steel wire mesh.

vi) Other proprietary and proven systems.

                                                                            

                                                               

 

 

 

 

 

 

Fig: Gabion boxes.

  1. Facing shall be sufficiently flexible to withstand any deformation of the fill and foundations.
  2. The facia units to be adopted in the project shall be shown in the Drawings and shall be approved by the Engineer.
  3. The minimum thickness of precast concrete panels shall be 180 mm including facing textures, logos and embellishments.
  4. Precast concrete blocks are dry cast and shall be manufactured from fully automatic block making machines.
  5. In case of hollow blocks, the hollow area shall not exceed 40% of the cross sectional area of the block. The outer side of the block shall have minimum thickness of 100 mm.
  6. The minimum grade of concrete for pre-cast reinforced concrete or plain concrete panels/blocks shall be M35.
  7. Facia panel systems shall have provision of both horizontal and vertical gaps to prevent concrete to concrete contact. The horizontal gap between the facing elements shall be maintained by provision of Ethylene Propylene Diene Monomer (EPDM) pad. Bedding material shall consist of either cement mortar or a durable gasket seating such as resin bonded cork, bitumen bonded cork or EPDM.
  8. The joints between the panels shall be covered from inside with non-woven geotextile strips glued to the facing element ensuring full coverage of joints. Synthetic glue shall be used for this purpose. The width of the geo-textile strip shall not be less than 100 mm.
  9. Where gabion facia is used, it shall conform to the provisions of BS 8006-1:2010 and EN 14475 and made of mechanically fabricated and selvedged double twisted hexagonal mesh. Wire used for the double twisted mesh shall be hot dip galvanized as per IS: 4826 heavily coated and soft type, with wire and mesh properties in accordance with EN-10223 with minimum Zn or Zn+ alloy coating as per EN-10244 and 0.5 mm thick PVC coating as per EN-10245 and ISO-527.
  10. Where geo-synthetics, including geo-grids are used as wrap around facia, these shall form a part of the reinforcing element. The wrap around shall have adequate length to resist pull out and the wrap around length shall be calculated on the basis of safety in pull out. Wrap around facia shall be protected by suitable means, against adverse effects of natural forces.
  11. Where steel sheet and steel grid are used for facing, the steel for steel sheet shall be as per BS: 1449-Part 1 and the steel grids shall conform to BS: 4482, BS: 4483 and BS: 4489.
  12. Where mechanically woven steel wire mesh in wrap around form is used for facing, the steel wire mesh (IS: 4826, IS: 280, IS: 13360, EN: 10218, EN: 10223, EN: 14475) shall be with mechanical selvedging and bottom panel shall continue as an integrated tail mesh.
  13. Where welded steel wire mesh units in wrap around form (EN: 10079, EN: 10080, and EN: ISO 1461, EN: 14475) are used as facing, the bottom panel shall continue as an integrated tail mesh.
  14. Connection between the facia panels and the reinforcing element shall be by using either nut or bolt, HDPE inserts with bodkin joint, hollow embedded devices, polymeric/steel strips/rods/pipes, fibre glass dowels or any other material shown in the Drawings. The connection between the panel and the reinforcement shall provide for 100% of the long term design strength of the reinforcing element in continuity.
  15. In case of modular block facia and other type of facia such as gabion facia, where the reinforcement is held by friction between the facia block and the reinforcement, the connection strength shall be determined as per ASTM D 6638.

4.5 Workmanship

  1. A strip footing, minimum 350 mm wide and 150 mm thick in M15 grade plain concrete, shall be provided at founding level to receive the facia or the bottom most reinforcement.
  2. The depth of embedment below the finished ground level at the foot of the wall shall not be less than 1000 mm. In case rock is met above founding level, the depth of embedment shall be adjusted as per ground conditions.
  3. The reinforcing elements shall be placed at right angles to the face of the wall or design axis, with greater cross-sectional dimension in the horizontal plane and the length shall be as shown in the Drawings. Reinforcing elements such as geogrids shall be stretched and held taut by driving nails or pegs at the farther end.
  4. It may be necessary to set facing unit at an additional batter than as provided in the Drawings since there is a tendency for initially positioned units of facia to lean outward as the fill material is placed and compacted. Care and caution shall be taken to accommodate this phenomenon. At the end of the  construction, the face may have a slight residual inward batter.
  5. Drainage bay shall be provided as shown in the Drawings. The width of the drainage bay shall be 600 mm behind the facing element. The drainage material shall conform to the specifications of the filter media as per MoRTH Specifications.
  6. The reinforcing elements shall be laid free from all kinks, damage and displacement during placing, spreading, levelling and compaction of the fill. The program of filling shall be such that no construction equipment moves directly on the reinforcement.
  7. All construction equipment having a mass exceeding 1500 kg shall be kept at least 2.0 m away from the face of slope or wall.
  8. In areas up to 2.0 m from the face of slope or wall, one of the following equipment shall be used:
  • Vibratory roller having a weight per metre width not exceeding 1300 kg with total weight not exceeding 1500 kg.
  • Vibratory plate compactor of maximum weight 1000 kg.
  • Vibro tamper having a weight not exceeding 75 kg.
  1. Before allowing the movement of vehicles over the reinforcement, a minimum compacted thickness of 150 mm shall be provided over the reinforcement and the speed of the vehicles shall be restricted to 10 km/hr.
  2. During construction of reinforced fill, the retained material beyond the reinforcement at the rear of the structure shall be maintained at the same level as reinforced fill.
  3. Fill shall not be placed on surface that contains mud, organic soil or area that have not met compaction requirement.
  4. The thickness of compacted layer shall not be more than 200 mm, compacted to 97% of maximum laboratory density measured as per IS: 2720 (Part 8).

4.6 Tolerances

  1. All elements of pre-cast RCC panels shall be manufactured within the tolerances specified in table.

Table: Tolerances for faces of retaining walls and abutments

Property

Tolerance

All dimensions

±5 mm.

Evenness of the front face

±5 mm over 1500 mm.

Difference between

lengths of two diagonals

5 mm max.

Thickness

+5 mm/-0 mm.

Location of plane of

structure

±50 mm - metallic reinforcement.

±75 mm - synthetic reinforcement.

Bulging (Vertical)

±20 mm in 4.5 m template (Metallic).

Bowing (Horizontal)

±30 mm in 4.5 m template (Synthetic).

Steps at joints

±10 mm.

  1. Dimensions of modular concrete blocks shall not differ more than ±2.5 mm for length and width and ±1.5 mm in height.
  2. Minimum vertical movement capacities required for facing systems to cope with vertical internal settlement of reinforced fill shall be as per table.

Table: Minimum Vertical movement capacities of facing systems

Structural form

Minimum Vertical Movement  Capacity of System

Discrete panels

Joint closure of 1 in 150 relative to  panel height.

Full height panels

Vertical movement capacity of connections  1 in 150 relative to panel height.

Semi-elliptical facings

Vertical distortion of 1 in 150 relative to  panel height.

Geotextile/ geogrid wrap-around facings

No specific limit except for appearance or

serviceability considerations.

5. CONSTRUCTION OF RETAINING WALL IN HILLY TERRAIN

  1. The construction of retaining wall in hilly terrain shall be based on IS: 14458 (Part 1 to 3).
  2. The construction of retaining/breast walls and gabion structures shall be conforming to the Drawings or as suggested by the Engineer.
  3. The foundation bed should be sloped towards the hillside.
  4. Resort to stepping up of the foundation bed of retaining walls in stable rocks.
  5. Ensure that the top level of the retaining wall matches the adjoining shoulder edge.
  6. Ensure that approved filter material is provided behind the wall before back filling. Also ensure that back filling is done only after the masonry work is approved by the Engineer.
  7. Do not construct retaining walls, which are having outer curve in plan, especially at hairpin bends.
  8. Do not allow construction of retaining walls until proper road geometrics has been achieved in hill cutting. Otherwise this may lead to excessive heights and quantities of retaining wall masonry, creating unnecessary cost escalation.
  9. If the retaining wall is made on rock slope, the foundation shall be stepped as shown in figure. In case of steep slopes (>35º), retaining walls with front face nearly vertical and back-face inclined shall be used as it will reduce the height of wall considerably.

                             

                         

 

 

a) Gravity wall    b) Reinforced soil wall  c) Stepping of foundation

Fig: Types of retaining wall in hilly terrain.

RELATED CODES:

1.IRC: 280:-Guidelines for design and construction of river training and control works for road bridges.

2.IS: 2720:-Methods of test for soils:

    (Part 8):-Determination of water content-dry density relation using heavy compaction.

3.IS: 4826:-Specification for hot-dipped galvanised coatings on round steel wires.

4.IS: 13326:-Method of test for the evaluation of interface friction between geo-synthetics and soil:

    (Part 1):-Modified direct shear technique.

5.IS: 14458:-Retaining wall for hill areas - guidelines.

    (Part 1):-Selection of type of wall.

    (Part 2):-Design of retaining/breast walls.

    (Part 3):-Construction of dry stone walls.

    (Part 4):-Construction of banded dry stone walls.

    (Part 5):-Construction of cement stone walls.

    (Part 6):-Construction of gabion walls.

    (Part 7):-Construction of RCC crib walls.

    (Part 8):-Construction of timber crib walls.

    (Part 9):-Design of RCC cantilever wall/buttressed wall & L-type walls.

    (Part 10):-Design and construction of reinforced earth retaining walls.

6.ASTM D5321  Standard test method for determining the coefficient of soil and geosynthetic or geosynthetic and geosynthetic  friction by the direct shear method.

7.ASTM D6638:-Standard method of test for determining connection strength between geo-synthetic reinforcement and segmental concrete units.

8.MoRTH:-Specifications for Road and Bridge works (5th revision)

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