24th February 2018
Kalabagh Bridge is a historic bridge located in the Punjab province of Pakistan, spanning over the Indus River near the town of Kalabagh. The bridge was constructed during the British colonial era in 1927-28 to connect the districts of Mianwali and Dera Ismail Khan.
The bridge was designed by Sir Alexander Rendel, a
British engineer who was also responsible for the construction of other notable
bridges in India, including the Lansdowne Bridge in Sukkur and the Godavari
Bridge in Rajahmundry.
At the time of its construction, the Kalabagh Bridge
was considered to be a major engineering feat, as it was one of the longest
bridges in the region, with a total length of 1,300 meters. The bridge
consisted of 14 spans, each measuring 91 meters in length, and was built using
reinforced concrete.
HISTORY FROM THE ARCHIVES
The
construction of a bridge over the Indus at or near Kalabagh was considered for
many years. So the first surveys were carried out in 1888 followed by an investigation from 1919 to 1924. Finally, in 1927 the project costing Rs. 40.36
lac was approved for construction. No provision for the roadway was made. The
bridge carried a single-line broad gauge railway.
The
bridge will connect the broad gauge (5-6″) system of the railway on the east of the
Indus with narrow gauge (2_6″) to the west. With the completion of the bridge, an alternative and direct route is available from Lahore to Waziristan. Commercially
and strategically the bridge will play an important role.
The
site of the bridge is about 1.25 miles below the gorge from which the Indus
emerges into the plains) the course of the river at this site is stable. The character of the river bed at this site is such that from the left (Mari) bank
half the width of the bed consists of an uppermost layer of fine sand covering
a layer of coarser and sharper sand with small pebbles. The uppermost layers of
sand disappear as the deep water channel is unapproached. Below this is a
compact stratum averaging 45″ thick of boulders set hard in the sand. The bed of
the deep water channel in the other half of the river consists of loose pebbles
and boulders above the compact boulder bed. An alternative proposal of a combined
weir for Thal canal head works and a railway bridge was earlier rejected mainly
because the site of the weir would be 4500′ long instead of 2500′ at the adopted
site.
The
Punjab irrigation records showed that an extraordinarily high flood occurred in
the year 1878, which was calculated as from 757,000 to 770,000 cusecs. For the
design of the bridge, the maximum flood was taken as 8 lac cusecs. But there
occurred in 1929 a flood of higher magnitude during the currency of work and
the help at the bridge site rose to 705.3′. It. Was estimated as 12,00,000 cusecs.
The
design was changed accordingly, adopting 12 lac cusecs as peak flood discharge.
Waterway provided initially consisted of 9 spans of 250′ clear (263′ center to
center of piers). After the flood of 1929, it was decided that the bridge
should be extended to cover the full width of the river between the Mari and Kalabagh banks. This entailed an extension of the bridge at the Mari end by 4
spans of 175’_4″ c/c. Girders of standard m.l. 1926 (for 22.5-ton axle loads
and a train of 2.3 tons per foot run behind the engine) are designed to carry a
single broad gauge line of railway. The live load is carried directly on an
open flooring of cross girders and stringers by n-type trough trusses with
curved top chords and eight sub-divided panels, the maximum depth of the truss
being 30-7.5″. The load is transmitted to the piers through knuckle bearings.
Temperature and elastic extension are provided for by roller bearings at one end
of each span, there being one pair of fixed and one pair of roller bearings on
each pier. Piers was initially designed to be made of concrete blocks but
later it was intended to construct mass concrete 1:2.5:5 to avoid
handling of blocks. Piers will rest on 2 feet-thick 1:2:4 reinforced concrete
base keyed to the top of wells. The maximum intensity of pressure at the base of
the pier is 9.5 tons per square foot.
The
wells were of twin octagonal type 38′-3″ long by 22′.1.5″ wide. The steining is
6 11.75″ thick, leaving two circular dredging holes each 8 2″ in diameter. The
depth of wells fixed were those considered probable safe depths. Deep water
wells were taken 40ft into the boulder stratum leaving 36 ft of the well and
pier exposed at high flood levels. As some of the wells had to be sunk in deep
water and as it was considered that pneumatic sinking would be necessary after
the lighter soil had been penetrated and the compact boulder stratum
encountered, all well
Curbs
took the form of caissons which would permit the attachment of air domes and shafts
for pneumatic work. Initially, ten wells were proposed to be sunk. But after the
addition of 4 spans, three more wells were added.
Position
at the end of the working season 1928-29 was that sinking of 5 wells was complete.
The wells were plugged, r.c pier footing and 5 6″ of pier masonry were built. The sinking of 4 more wells was in progress whereas work on the 10th well was not
commenced. The characteristic feature of the season's work was the realization
of the necessity for pneumatic sinking. It became evident to the contractor
that opens dredging in the compact boulder stratum gave very slow progress and
was in fact impossible below a certain level. Therefore by march, 1929, the
contractors had obtained a pneumatic sinking set. Precaution against scour was
taken by protecting wells with pitching. This was necessitated by a flood of 1929
which had caused the tilting of wells.
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