146 HEGGADE, MEHTA & PRAKASH ON Fig. 3. Bore hole details along bridge alignment Fig. 4. Typical well foundation for <strong>Sutlej</strong>
due to differential earth pressure, s<strong>and</strong> blow <strong>and</strong> sudden drop, etc. it was ensured that the stresses at different levels <strong>of</strong> ste<strong>in</strong><strong>in</strong>g dur<strong>in</strong>g service conditions <strong>and</strong> construction stage were with<strong>in</strong> permissible limits. As the well foundations were to be plugged on soil, the grip for side earth resistance below the scour level was ensured to be <strong>of</strong> the maximum depth <strong>of</strong> scour below the design scour. As normally, well cannot be sunk to the precise verticality, the design catered for the cumulative moment effect <strong>of</strong> 1 <strong>in</strong> 80 tilts <strong>and</strong> 150 mm shift apart from account<strong>in</strong>g for other severe load comb<strong>in</strong>ation. The side earth resistance was calculated by Bombay Committee Method with the passive resistance factor <strong>of</strong> safeties <strong>of</strong> 2.0 <strong>and</strong> 1.6 for normal <strong>and</strong> seismic conditions respectively. The well cap was designed <strong>and</strong> detailed as uniformly thick plate for the external reactions <strong>and</strong> reaction components at the bottom <strong>of</strong> the pier with boundary condition as partially fixed at supports on well ste<strong>in</strong><strong>in</strong>g all around. The river bed level varied between RL 222 m to 225.50 m hav<strong>in</strong>g deep channel between the pies P3 <strong>and</strong> P10 with the low water level be<strong>in</strong>g at RL 223.723 m. Up to the deep channel i.e. P10, the service road was made up to A/R <strong>and</strong> wells up to P10 were started simultaneously. After the monsoon was over, on recession <strong>of</strong> floods, the service road was made on A/L side <strong>and</strong> with the help <strong>of</strong> the site made temporary bridge between P8 <strong>and</strong> P10; the wells were tackled <strong>in</strong> the channel. Initially, the well s<strong>in</strong>k<strong>in</strong>g was planned with four cranes. However, the s<strong>and</strong>wiched clayey strata necessitated overall eight cranes, as the s<strong>in</strong>k<strong>in</strong>g through the same was consum<strong>in</strong>g almost 7 to 15 days per metre depth. Most <strong>of</strong> the well foundations were constructed conventionally on l<strong>and</strong>, barr<strong>in</strong>g a couple <strong>in</strong> deep channels, which warranted s<strong>and</strong> isl<strong>and</strong>s. In the conventional construction (Photo 1.), the cutt<strong>in</strong>g edge fabricated <strong>of</strong> mild steel was laid on the ground level <strong>and</strong> curb with required re<strong>in</strong>forcements was concreted. The material <strong>in</strong>side was gradually scooped out with grabs to facilitate s<strong>in</strong>k<strong>in</strong>g under its own weight. As the s<strong>in</strong>k<strong>in</strong>g proceeded, the ste<strong>in</strong><strong>in</strong>g was built up <strong>in</strong> lifts, normally <strong>of</strong> around 2.5 m to further the s<strong>in</strong>k<strong>in</strong>g due to <strong>in</strong>crease <strong>in</strong> weight. Photo 1. Curb re<strong>in</strong>forcements & cutt<strong>in</strong>g edge DESIGN AND CONSTRUCTION OF PRE-TENSIONED SUTLEJ BRIDGE IN PUNJAB 147 S<strong>in</strong>ce the bed pr<strong>of</strong>ile was hav<strong>in</strong>g large variations, almost all wells were required to be sunk up to 5 m below the ground level (Photo 2.) <strong>and</strong> 3.0 m below the water level. The circular c<strong>of</strong>ferdam except a small flare to accommodate piers was cast up to water level. Photo 2. C<strong>of</strong>ferdam with flare to accomodate pier Had the well cap level been fixed at ground level or LWL, the job could have been completed three to four months earlier <strong>and</strong> substantial additional expenditure as a consequence <strong>of</strong> tak<strong>in</strong>g well cap below ground level could have been saved. As the c<strong>of</strong>ferdam was quite th<strong>in</strong> compared to ste<strong>in</strong><strong>in</strong>g, the non-availability <strong>of</strong> required weight hampered the s<strong>in</strong>k<strong>in</strong>g. This called for the creation <strong>of</strong> the sump below the found<strong>in</strong>g level to facilitate gradual s<strong>in</strong>k<strong>in</strong>g. At P16 location, the sump required was 3 m to enable last 1.80 m s<strong>in</strong>k<strong>in</strong>g. In the process the well jumped <strong>and</strong> sunk by 3.70 m at one go render<strong>in</strong>g the ste<strong>in</strong><strong>in</strong>g top almost 8 m (Photo 3.) below bed level. Photo 3. <strong>Construction</strong> <strong>of</strong> well cap below GL To raise the ste<strong>in</strong><strong>in</strong>g to the required level, the extensive shor<strong>in</strong>g, cont<strong>in</strong>uous dewater<strong>in</strong>g, protection with wire crated boulders, etc. had to be resorted to apart from stabilis<strong>in</strong>g the 6 m deep false walls by adequate structural brac<strong>in</strong>gs. To circumvent the creation <strong>of</strong> sump to s<strong>in</strong>k last 1.2 m depth at P3 location, the other measures such as air jett<strong>in</strong>g, water jett<strong>in</strong>g <strong>and</strong> Kent ledge on top <strong>of</strong> false wall were attempted. F<strong>in</strong>ally after 4 months, the comb<strong>in</strong>ed effect <strong>of</strong> 450 t Kent ledge, 5 m excavations outside the well <strong>and</strong> dewater<strong>in</strong>g yielded the well to the required depth.
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