Rapid Construction of Dhansiri Bridge

    Dr. V K Yadav, VSM, CE, DDG (Bridges & Tunnels), Col RK Sharma, Jt Director, (Bridges), HQ. DGBR, New Delhi

    The army convoys originating from Rangapahar Army Cantonement (Dimapur, Nagaland) and proceeding towards NH-39, perforce had to pass through built up area of Dimapur town, causing avoidable inconvenience to the free and quick movement of the civil traffic. The HQ Eastern Command had projected the requirement of a free and direct route from Rangapahar Cantonement to NH-39. Keeping the important operational aspects in view, the improvement of the existing PWD road along Dimapur an Army bypass was included in the BRDB programme accordingly. River Dhansiri was a major water obstacle along this road alignment, with no crossing arrangement except through private boat/ferry services.

    River Dhansiri experiences heavy discharge during rainy season. In the vicinity of this bridge site a 72 m span permanent railway bridge constructed during the British period, had been washed away in the past due to heavy river discharge. A permanent bridge was a basic necessity to effect a permanent road crossing over this river. The proposal to construct a 120 m span high level major permanent bridge along the existing PWD alignment, was accordingly approved for construction. This work was to be executed by the 89 RCC of 15 BRTF in the Project Sewak. Some of the salient features and technical specifications for this bridge were as under :-
    1. Length of the bridge: Total length 120 m.
    2. Span arrangement : Three equal discontinuous spans of 40 m each.
    3. Type of superstructure : PSC box girder.
    4. Type of foundations for :Well foundation abutments/piers
    5. Overall depth of the: 2.700 m box girder
    6. Carriageway width : 7.8 m between inner faces of the kerbs.
    7. Design discharge : 2034
    8. Silt factor : 1.00
    9. Safe bearing capacity at founding level : 80 t/sqm
    10. Linear waterway : 120.00 m.
    11. Number of prestressing cables : 24 (m) Strand system : 12 T 13.
    12. Type of anchor : Usha Ismal/ CCL.
    13. Dia of HT wire : 12.7 mm (7 mm ply) area of 7 strands, each 12.5mm dia = 98.7 mm2.
    14. Concrete mix :-
      1. Well steining : M 35.
      2. Well cap: M 35.
      3. Substructure : M 35.
      4. Superstructure: M40.
    15. Type of cement used : OPC 33 Grade conforming to IS : 269 or IS : 12269 for OPC Grade 53.
    16. Type of bearing : Pot / Pot PTFE.
    17. Live load classification : Single lane IRC Class 70 R or two lanes of IRC Class ‘A’ whichever produces greater stresses.

    Sinking of Wells

    The general arrangement development GAD was evolved keeping all execution aspects in mind. The well caps for the two abutments were kept at a higher level in comparison to that of the pier wells, as dictated by the site profile. This arrangement reduced the quantum of excavation to a great extent. Both the pier wells were sited beyond the LWL line hence dewatering was avoided and work could proceed during normal fluctuations of the flowing river water.

    Rapid Construction of Dhansiri Bridge

    The construction agency, had the experience of constructing three major bridges viz. Dzulu, Lainey and Tizu bridges on Road Kohima-Meluri in Nagaland State, and as such had the feel of subsoil conditions obtaining in the state of Nagaland hence contractor could well anticipate the likely difficulties that could be encountered during well sinking. The agency could accordingly foreplan the requirement of resources in terms of plant/equipment, men and materials.

    The top soil strata with depth varying upto 12 m constituted mainly of clay mixed with dark grey colored sand/organic matter, and the next soil strata underneath this upto 30 m depth was of compacted sand. This soil strata was harder and could offer adequate resistance to the loads and forces. The sinking of wells in top layer was done by normal grabbing. As the cutting edge of the wells encountered the second strata, the sinking rate fell considerably. The sinking effort required here was much more than what was assumed based on the design data which showed properties of sand only. No water was encountered in any of the wells in this strata. The compact sand layer had to be cut with free falling heavy duty chisel, spade and crowbar. Many a times, water had to be poured in to the wells to soften the strata. The wells were found to be safe at higher levels, accordingly the founding levels for the piers P1 and P2 were revised upwards from earlier 92.00m to 97.37m and 96.75m respectively, refer Fig.- 4.

    Rapid Construction of Dhansiri Bridge

    Milestones of Work

    The work started at a rapid pace. Work commenced at site in September 2005 sinking of wells completed by December 2006, Substructure was completed by Feburary 2007, Pier-P1, Superstructure deck slab completed in February 2007, March 2007, April 2007.

    Construction Materials

    1732 HTM Cement, 1702 of sand, 3364 stone chips and 241.045MT Steel was consumed in the construction of this bridge.

    Constraints Encountered

    I General Constraints

    1. The area was highly infested with insurgency and precautions had to be taken during night working and during movements of manpower/vehicles to avoid any untoward incident.
    2. Abnormal delay was caused to vehicles carrying construction stores at various TCPs/forest check posts for checking, which caused delay in movement of stores.
    3. Local disturbances to site staff and laborers by local population.

    II Execution (Technical) Related Constraints

    1. During execution of well–sinking many problems were faced. Due to hard soil, laborers were unable to work regularly. Hener Laborers were replaced after 15/ 20 days.
    2. Due to excessive earth pressure surrounding the well, jetting system was provided to reduce side friction. Moreover, cantilever load was also provided about 100 mt.
    3. During execution of A2 well, two logs were observed below the cutting edge after a depth 10 m, chisels were used to cut these logs.

    Reasons for the Early Completion of Work

    The following reasons can be cited for early completion of the work:-
    Rapid Construction of Dhansiri Bridge
    1. The monsoon season in the North Eastern region is over by August end. The ground activities had been well planned and synchronised, hence there was no time lag between the date of acceptance of contract, placing of work order and the physical starting of the work.
    2. Requisite drawings and designs were submitted and approved expeditiously resulting in no time loss.
    3. Procurement of all constructional stores and materials were planned and procured in time. All equipment, plants, machineries and skilled labour were planned, arranged and inducted at site before the expected requirement, resulting in no time loss.
    4. Positive and objective interaction between the executing agency and the client.
    5. No exchange of unnecessary correspondence due to proper planning in advance.
    6. Dedicated and harmonious relationship between the client and the contractor.
    7. All payments were released by the T.F. with due alacrity. This is the first such experience as per the contractor in the last 40 years.
    8. Grace of God Almighty!
    Rapid Construction of Dhansiri Bridge

    Lessons Learnt

    Following points merit mention
    1. The contract was accepted just one month before the end of the monsoon season. This intervening one month was utilized for all preliminary works viz. planning, detailed design/drawing and fabrication of cutting edge etc. As such, all were geared up to start the work by November 2005. The acceptance of the contract in March 2005, would have meant a wastage of almost seven months till Oct 2005, without any physical work/activity at site. As such, the acceptance of contracts should be synchronized with the start of the working season.
    2. Well planned GAD is the main reason for the early completion of this Project. Delay usually occurs in foundations, which is the only activity beyond the predictable control or anticipation of human activity. The delays are mainly due to following reasons :-
      1. Natural–These are to be surmounted by requisite planning and efforts.
      2. Man made–These can be avoided through requisite foresight. Complete and dedicated guidance and immediate decisions by the department has culminated in its completion 11 months prior to the contracted time.

    NBMCW September 2007

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