CONSTRUCTION OF REINFORCED EARTH WALLS

 

 REINFORCED EARTH WALLS

 

This item of work deals with construction of Reinforced earth walls with approved materials as per the terms of contract agreement.

 

1.Generals.

All  elements  comprising  the  reinforced  soil/  earth  structure  such  as  the  facia  units,  the reinforcement with accessories/fittings and the select backfill material shall all be laid in conformity with the section 3100 of MORTH specifications and in compliance with the lines, grades, design and dimensions as per the approved drawings.

2. Materials.

Reinforcing Elements: It shall conforming to clause 3102 of MORTH specification.

Reinforced Soil /Earth: The reinforced earth shall be a selected granular fill having high frictional resistance, low compressibility and free draining and conforming to clause 3103 of MORTH specifications.

Facia Units: The fascia units, which help maintain a vertical face of the reinforced soil/ earth structure, control erosion of the fill and provide aesthetic appearance to the reinforced soil/ earth structure, shall be of the pre-cast reinforced concrete slabs/panels or pre-cast cement concrete blocks or any other proprietary material that may be approved by the engineer and/ or as indicated in the drawings and as per clause 3104 of MORTH Specification. The type and shape of the facia units to be finally adopted shall be subject to approval by the Engineer.

Concrete: The materials used shall adhere to the section 1000 of technical specifications. The concrete used shall be of approved design mix and shall confirm to section 1700 of technical specifications.

3.Drainage:

The drainage provision shall be strictly followed as per the approved working drawing and Technical Specification Clause 305. Drainage outlets at the bottom level joints of panels with provision of non-woven geo-textile as per IRC-SP-59 and backing shall be provided as per approved design and drawing along the fascia for drainage redundancy. The retained fill shall have the drainage way, 600 mm wide with well - graded crushed aggregate as per IS: 383 to allow free drainage of the reinforced fill along with the retained fill. The panel joints above 300 mm of existing ground level shall be filled with joint filler as per specifications.

4.Procedure.

Excavation:

1. Layout marking will be done as per approved working drawing and ground levels will be taken and recorded jointly
2. Excavation will be done conforming to the lines, grades, slopes and levels as shown on the approved working drawings, as per the specification clause 304.3.2
3. If required dewatering will be done as per the specification clause 304.3.3
4. All the excavated materials will be disposed such that the working space will not effect, as per the specification clause 301.3.11
5. Excavation in Hard Rock will be done as per specification clause 301.3.5

Preparation of foundation :

The foundation for RE wall shall be graded level for width equal to or exceeding the length of reinforcing geo-synthesics. will be done as per specification clause 301.3.4

• If there is any slips & slip outs in the excavation will be removed
• Filling below foundations up to require level and backfilling will be done with approved material in layers not exceeding 150mm compacted thickness. The compaction will be done with the help of suitable equipment such as mechanical tamper, plate vibrator etc. after necessary watering as per specification clause 301.3.7

Ground Improvement:

Where foundation soil is found to be unsuitable, either removal and replacement technique or ground improvement is required to be carried out, as required by the Engineer. The need for ground  improvement,  design  and  ground  improvement  methodology  shall  be  verified  and approved by the Engineer prior to construction.

 Suitable ground improvement technique shall be identified based on the results of subsoil exploration. Foundation preparatory works and foundation treatment/ improvement shall be treated as integral part of the reinforced soil structure and accordingly Contractor shall arrange for detailed sub-soil investigation works and employ his resources to design and construct the foundation/ ground improvement treatment, wherever necessary to satisfy the requirements of reinforced soil structure. The design check and validation for foundation treatment/ground improvement and the methodology shall be verified and approved by the Engineer prior to construction.

Leveling Concrete (PCC):

A leveling concrete pad shall be provided under the walls and seat beams (for- bridge abutments). Concrete shall have a minimum grade M-15.Maximum size of aggregates shall be 20mm and the pad shall be cured for at least 48 hours before placement of panels.

In case of facia panel, the foundation may be made through shaped, so that the first panel is placed in the trough.  After placing the panels, the gap in the trough, may be filled with mixture of sand and bitumen. 

Batter : 

The reinforced earth/ soil wall face may be provided with an initial inward batter specified by the system supplier or as given in the drawings.   Initial inward batter is required to counter the outer ward tilt that will occur as the wall height is built up and also due to construction process.  As a guideline the initial batter is generally in the range of 4⁰ to 6⁰.

Erection: 

All reinforcements shall be installed at the proper elevation and orientation as shown in the wall details of approved drawings or as directed by the Engineer. The reinforcement strips shall be placed normal to the face of the wall unless otherwise shown on the drawings.

 

 a) Pre-cast Concrete Facia Panels/ Slabs 

Pre-cast concrete slabs/ panels shall be placed vertically with the aid of a light crane. For erection, panels are handled by means of lifting devices set into the upper edge of the panels. Panels shall be placed in successive horizontal lifts in the sequence shown on the drawings as backfill placement proceeds. As fill materials is placed behind a panel, the panels shall be maintained in vertical positions by means of temporary wooden wedges placed in the joint at the junction of the two adjacent top rows of panels during construction. As construction proceeds and a fourth row is erected, the lowest row of wedges can be removed and so on.

External bracing may also be needed for the initial lift. However, bracings shall be placed in an area not more than 1.5m wide beyond the outer face of panels.

Vertical tolerances (plumb) and horizontal alignment tolerance shall not exceed 25mmwhen measured along a 3 m straight edge. The maximum allowable offset in any panel joint shall be 25mm.

 

 b) Cement Concrete Hollow Block units 

These  units  will  be  dry  stacked (without  mortar)  one  over  the  other  in  a  running  bond configuration, vertically. Adjacent units shall be connected by dowels, shear keys or shear pins to ensure positive connection with the reinforcing geo-synthetic. Tongue and groove type connections are not acceptable. Hollow area in the blocks shall be filled with aggregate of 10 to 18mm nominal size during constructions.

 

Dowels or pins used by the retaining wall supplier to interconnect block units shall consist of a non-degrading polymer or galvanized steel and be made for the use with the block unit supplied.

All materials shall be installed at the proper elevation and orientation as shown in the wall details on the construction plans or as directed by the Engineer. The drawings shall govern in any conflict between the two requirements.

Broken chipped, stained or otherwise damaged units shall not be placed in the wall unless they are repaired and the repair method and results are approved by the Engineer.

The spacing between primary reinforcing geo-synthetic shall not be more than 600mm or two block units, whichever is greater.

Overlap of the geo-grid in the design strength direction shall not be permitted. The design strength direction is that length of geo-grid reinforcement perpendicular to the wall face and shall consist of one continuous piece of material. Adjacent sections of geo-grid shall be placed in a manner to assure that the horizontal coverage shown on the plans is provided.

Geo-grid reinforcement should be installed under tension. A nominal tension shall be applied to the reinforcement and maintained by staples, stakes or hand tensioning until the reinforcement has been covered by at least 200mm of soil fill.

 

Joint Fillers: 

Fillers of vertical joints between panels shall be flexible open / close cell polyurethane foam strips or non-woven fabric strips (the latter used as joint cover instead of filter) as approved by Engineer. Bearing pads for horizontal joints between panels shall be made of elastomer with vulcanized EPDM.

Reinforced Earth Backfill Placement:

The reinforcing elements shall be laid free from all kinks, damage and displacement during deposition, spreading, leveling and compaction of the fill. The program of filling shall be such that no construction plant runs directly on the reinforcement. It shall been checked that the exposure of soil reinforcement to ultraviolet rays is minimal and should be covered with earth within one day of placement.

During construction of reinforced backfill, the retained material beyond the reinforcement at the rear of the structure shall be maintained at the same level as reinforced fill. The entire works related to compaction should be carried out generally in a direction parallel to the facing. Fill placement methods near the facing shall ensure that no voids exist below their enforcing elements.

Temporary formwork shall be used to support the construction as per specified details given in the drawing. The forms, scaffolding and props shall be sufficient in numbers to allow taking up of a sectoral construction schedule as specified in the design.

At the end of each day’s operations, the contractor shall shape the last level of backfill as to permit run-off rainwater away from the wall face.

Drainage Fill Placement:

Drainage fill shall be placed to the minimum finished thickness and widths shown on the construction plans or as modified by the Engineer. During placement and compaction of drainage material, care must be taken to ensure that there is no contamination with undesirable materials.

Vertical layers of drainage layer material shall be brought up at the same rate as the adjoining fill material.

Drainage collection pipes shall be installed to maintain gravity flow of water outside of the reinforced soil zone. The drainage collection pipe should discharge into a storm sewer, man hole or along a slope at an elevation lower than the lowest point of the pipe within the aggregate drain.

 

The main collection drainpipe just behind the precast facing, if used, shall be a minimum of 150mm in diameter. The secondary collection drain pipes should be sloped a minimum of two percent to provide gravity flow into the main collection drainpipe. Drainage laterals shall be spaced at a maximum 15 meters spacing along the wall face. The drainage collection pipe shall be a perforated or slotted, PVC or corrugated HDPE pipe. The pipe and drainage aggregate shall be wrapped with a geo-textile that will function as a filter. 

Cap Block Placement: 

The cap block and or top concrete block unit shall be bonded to the modular concrete block units below using cap adhesive.

3. Equipment/Machinery:

1.    Poclain / JCB

2.    Vibratory Roller

3.    Auto level with accessories

4.    Transit Mixers

5.    Needle Vibrators

6.    Cube Moulds