What is a Culvert?
The culvert is a small cross drainage structure having a span length of six meters or less between the inner face of the wall. It may be like a bridge designed to allow carrier or pedestrian traffic to cross over the stream providing adequate clearance from high flood level. Depending upon the requirement of the project like traffic volume, embankment height and high flood level, it may be a single cell or multicell box culvert having shapes including round, elliptical, flat-bottomed, open-bottomed, pear-shaped, and box-like structure.
Following are the different types of Culvert:
- RCC Box type culvert
- Hume pipe type culvert
- RCC slab type culvert
- Stone slab type culvert
- RCC arch type culvert
- Masonry arch type culvert
In this section, we know more about the construction methodology of RCC box culvert, which has a major role in providing cross drainage.
Complete Method Statement for Box Culvert Construction:
The construction sequence of box culvert mainly consists of the following activities-
- Raft Construction (Foundation)
- Wall Construction (Substructure)
- Deck Construction (Superstructure)
Construction of concrete culvert Raft: (STAGE -1):
Survey & Setting Out:
- Survey points shall be marked on either direction of the alignment by the total station from control points.
- A benchmark shall also be established at nearby site location where no disturbance will occur throughout the execution phase.
- The benchmark shall be checked at the frequent interval as and when required.
- The foundation shall be marked on the ground with the clearance of formwork space required.
All the Survey Instruments used shall be calibrated periodically as per the Calibration Plan. Survey Check Reports / other relevant data shall be maintained in relevant Inspection formats.
The open foundations are provided mainly for the cross drainage structures, hence required advance planning for dewatering before excavation to commence as per site conditions. If required water shall be diverted through other channels, or otherwise dewatering shall be done if required. At this location, depending upon the flow of water a pump of about 5 to 10 HP capacity may be sufficient to keep the foundation dewatered. Pumped out water shall be released at the farthest point to the maximum possible. The surface water source shall be checked and if possible, stopped from joining this stream or be diverted to some other stream.
Earthwork and Excavation for Raft:
The excavation shall be done with safe side slopes as per ground conditions and with sufficient margin for working on structural concrete (minimum 300mm). Excavation shall be carried out either by JCB or excavator, and excavated material shall be disposed of by using tractor trolleys or dumpers as directed. The pit level reached up to PCC bottom, shall be properly levelled and compacted before laying Plain Cement Concrete. The SBC at founding level shall be checked with design SBC as given in the drawing with the help of Plate Load Test for the individual culvert. In case actual SBC is less than the design SBC requirement, then granular filling up to thickness as specified in the drawing shall be done in layers and compacted to 95% of modified proctor density.
Levelling Course (PCC) for Cross Drainage Structure:
The excavated pit shall be manually dressed with sufficient working space around the raft to be constructed. Fix forms like ISMC 150 or wooden planks at the edge as formwork. P.C.C shall be supplied from the batching plant through transit mixers. It shall be unloaded through chute/s, and then it shall be manually spread & compacted in a uniform layer of levelling course thickness. After the initial set, the concrete surface shall be sprinkled with water to save it from cracking. Curing shall be done by keeping it moist with Hessian cloth or by a continuous spray of water. After allowing it to gain strength over 24 hours, it shall be released for raft activities.
Reinforcement Binding for Culvert Raft:
As per the approved construction drawings and bar bending schedule reinforcement of raft is pre-straightened /cut/bend in the rebar yard. Reinforcement bars shall be shifted to the location in advance. Marking for the raft shall be done on levelling course. Rebar shall be tied as per approved drawings and bar bending schedule. To hold the top rebar mesh of raft rebar chairs shall be provided at appropriate places to avoid the sagging of top mesh.
After completion of top reinforcement, erect the wall vertical bars with binders /spacers. Necessary pipe support with an inclined member shall be provided to the vertical bars temporarily from sides to avoid tilting of rods.
Formwork Fixing for Culvert Raft:
Concrete cover blocks of the same grade that of foundation concrete shall be tied to the reinforcement before fixing formwork. Steel formwork plates applied with de-moulding oil is fixed in position with the help of props & ties/spacers against each other to line & level as per formwork and staging drawing. The formwork shall be properly jointed with bolts and washer. To keep the formwork joints leak-proof, forms or masking tape shall be used in joints. During concreting, the formwork shall not loose its alignment. Foundation dimensions and reinforcement shall be as per the good for construction drawing. The inspection shall be carried on for formwork and reinforcement before pouring of concrete as per ITP and if any deviations found shall be corrected before pouring of concrete.
Box Culvert Raft Concreting:
Concrete shall be supplied from batching plant through transit mixers. Concrete shall be poured into the foundation using direct chute or concrete pump as per accessibility. Chutes shall be made using M.S. sheets / CGI sheets supported by structural steel members. Chutes shall be placed with an appropriate slope so that concrete free flows. In case of concreting using chute, the same shall be made wet by water before pouring concrete over it for concrete to free flow.
Concrete shall be laid in 300 mm to 450 mm thick layers and vibrated with diesel or electrically operated 60 mm needle vibrators. The layer thickness shall be selected such that the next layer is laid before the previous layer initially sets. The exposed top surface of the concrete shall be smooth finish using mason’s trowel. The wall portion shall be made with Shear keys (Construction Joint), as shown in the drawing. Cut piece Reinforcement rods of diameter higher than 25mm shall be embedded (depth of embedment not less than 50mm) along the centre line of the base raft to support inclined props to be used for 2nd stage formwork of wall before final setting of concrete. After the initial set, protect the concrete surface with a moist hessian cloth. After final set mortar bunds shall be made for ponding of water, the sand layer is laid in bunds to retain water for the proper curing. Within 24 hours, concrete will gain sufficient strength, and de-shuttering shall be done carefully.
Box Culvert Raft Formwork Removal:
The concrete shall be allowed to gain strength over 12 to 24 hours. Then forms shall be removed without damage to concrete. The forms, side supports and shoring, shall be safely removed, cleaned and stacked properly for future use. The exposed concrete side surfaces shall be inspected for blemishes. Blemishes if any shall be mended immediately. The pit shall be immediately backfilled with excavated material up to raft top level. This filling will help in curing the foundation if kept moist. Top of raft shall be provided with cement sand mortar bund and kept inundated with water for curing till curing period is over.
Box Culvert Wall Construction ( STAGE -2):
The construction joint between wall and deck shall be provided as per the following guidelines:
- For Box Culvert without Cushion – Construction Joint shall be 300mm below the end of corbel level.
- For Box Culvert with Cushion – Construction Joint shall be 300mm below the deck slab bottom level.
Maximum wall lift height in a single pour shall be restricted to 2.35m (Formwork height 2.4m with overlapping with previous layer 50mm). Incase wall lift height is greater than the above, and the intermediate lift shall be planned accordingly.
Box Culvert Wall Reinforcement:
Against the dowels left through the raft, balance reinforcement shall be tied for the predetermined lift to be concreted. Vertical rods shall be provided with required overlaps as per construction drawings. Efforts shall be made to minimize overlaps. Horizontals shall be tied in position. Concrete cover shall be provided by Concrete cover blocks tied with reinforcement as required.
Box Culvert Wall Formwork:
After completion of reinforcement work, Steel forms in panels shall be erected, aligned and fixed in position as required against the already tied reinforcement. The forms shall be placed over the seating channel fixed by tie bolts through left out holes in already concreted straight portion over haunch. The forms shall be side supported with the help of struts taking reaction from the channel placed against the embedded reinforcement in the already concreted raft. There shall be retrievable tie rods passing through HDPE conduit pipes holding forms from both faces of walls. The line and level shall be achieved as per approved drawings and then cleared the wall for concreting. The schematic wall formwork arrangement shown above is only to support the methodology, whereas the actual execution shall be done as per the final formwork /staging drawing issued by the engineering department.
Box Culvert Wall Concreting:
The concrete mixed from centralized batching plant/s shall be delivered through transit mixers. It shall be placed manually or through crane + bucket/Concrete pumps as per site condition. Concrete shall be laid in 350-400mm layer and vibrated with needle (electrical/petrol/diesel) vibrators. It shall be ensured that concreting is continuous and completed in a single pour to minimize the number of cold joints. The exposed area of the walls shall be green cut to receive next lift concrete. Shear keys (Construction Joint) shall be formed as per the construction drawing.
Formwork removal, inspection & curing:
After gaining strength over about one day, the side form panels shall be safely removed, cleaned and stacked for future use/lifted up and fixed for the next lift. The de-moulded concrete surfaces shall be inspected for blemishes if noticed shall be mended immediately as per approved methodology. The exposed concrete surfaces shall be covered with moist Hessian cloth & shall be maintained moist over the curing period.
Box Culvert Deck with Wall 2nd lift & Corbel Construction(STAGE -3):
The general construction methodology shall be comprised of followings –
a) Fabrication of shutter & reinforcement
c) Formwork & Reinforcement
Fabrication of Shutter & Reinforcement :
- Standard shutters shall be used, or required size shutters shall be fabricated at centralized fabrication yard as per drawing for G.A of Formwork & Staging. The shutters shall be transported to site by truck/trailer.
- Reinforcement shall be fabricated at the centralized yard as per BBS made in accordance with the latest “Good for Construction” Drawing and approved by Engineer and it shall be transported to site by truck/trailer.
- Store the fabricated reinforcements on wooden/concrete sleepers to avoid any contamination from soil/ground water.
Deck Slab Staging:
- After completion of the wall up to required height, staging for deck will start.
- Erect, vertical member in the form of props / H-Frame /MS Pipe/ Cup-lock pipe above the base slab of RCC Box in between the walls as per staging drawing. Brace the vertical members in both directions and diagonally.
- Erect the longitudinal girder and cross girders as per staging drawing.
Deck Slab Formwork & Reinforcement:
- Parallel with girder erection fix the balance wall inner side shutter and align by taking support from the staging erected as per staging and formwork drawing. Apply shuttering oil to the formwork before fixing.
- Fix soffit shuttering over cross girder. Check the soffit shuttering level and introduce necessary wooden packing below the shuttering joints for having provision for de shuttering and levelling.
- Apply shuttering oil over the soffit shuttering and start reinforcement work as per the GFC drawings.
- Fix the prefabricated reinforcement bars for deck as well as for balance wall portion of the wall as per GFC drawings. Reinforcement cover shall be maintained as per GFC Drawing by means of cover blocks placed at suitable intervals. The cover Block shall be of the same grade of concrete of deck.
- Fix the reinforcement bars for the crash barrier reinforcement as per GFC Drawing.
- Fix the side/end shutter of wall and deck after applying shuttering releasing agent.
- To keep the shutter joints leak-proof, forms or masking tape shall be used in joints.
- Support the side/end shutters by inclined support members, as shown in the formwork/staging drawing.
Box Culvert Deck Slab Concreting:
- After checking of staging, formwork and reinforcement, cast the wall and deck. Concrete shall be placed in a single continuous pour in such a sequence, which will avoid the formation of cold joints.
- Concrete shall be produced in the controlled batching plant as per approved Mix Design and shall be transported to site by Transit Mixer. The slump and temperature of concrete shall be checked at the site before pouring, and it should be as per the requirement of inspection and test plan of the casting of RCC Box/specification of the contract. Concrete shall be poured in the formwork by Static concrete Pump/placer boom without any disruption to avoid cold joints. Separate arrangement for concreting shall be kept ready for any major mechanical fault of concreting equipment.
- Needle vibrator is to be operated vertically in combination with the shutter vibrator. No vibrator to be run for more than 2 minutes at one place to prevent segregation. Do not allow any pocket formation or honey-comb in the concrete. Feel the flow of concrete by hammering the shutters by a wooden mallet.
- Deck concrete top-level shall be finished smoothly.
- Remove the end and side shutter after 24 hrs of concreting.
- Start curing of deck and wall by using wet Hessian cloth or by continuous water sprinkling arrangement to keep the girder wet up to 14 days.
- The soffit and staging shall be removed depending on the span length as per guidelines of relevant IS Code.