Engineers driving foundations for the New Galata Bridge in Istanbul witnessed an unusual effect which saw blow counts fall in the last few metres of penetration.
Despite being opened in 1992, the 'New' Galata Bridge across the Golden Horn channel in the centre of Istanbul is still not finished. Original cost of the bridge was set at £23M but additional work including increased protection against ship impact pushed this to £40M, and construction is still under way on a number of variation orders.
Greater Istanbul Municipality was forced to open the bridge early because the pontoon structure it was set to replace was severely damaged by fire.
The bridge, linking Eminönü on the southern bank to Karaköy on the north, is the fourth to be built on the site. Construction is being carried out by STFA in partnership with German firm Thyssen Engineering, with UK firm Mott MacDonald as consultant.
To blend with the historical surroundings of the Golden Horn, the 490m long structure has a low profile. In the centre, a hydraulically operated bascule bridge spans between two piled piers to allow shipping through the channel while on either side are double-decked approach bridges. The bridge carries four lanes of traffic in either direction on its top deck, while the lower level includes shops and restaurants.
Prior to construction site investigations revealed over 40m of loose fill on the Eminönü side and over 30m at Karaköy. The fill thins towards the middle of the Golden Horn and the underlying layer of sand, silt and clay is exposed, underlain by a cobbly gravel.
The entire site is underlain by a fractured sandstone.
The original plan was to build another pontoonstyle bridge but during the tender stage STFA proposed the alternative, cheaper design of a piled abutment bridge.
The bridge is supported by 114, 2m diameter tubular steel piles up to 80m long with a wall thickness of just 20mm. Piles were driven by a Delmag D100 diesel hammer from a pontoon in the channel through the soft clay and into cobbly gravel beneath. Fourteen piles were installed for each caisson to withstand ship impact and high intensity seismic loads.
While 92 of the piles were closed-end with conical shoes, where the underlying sandstone was closer to the surface on the Karaköy side 22 open ended piles were driven to rock head before a reverse circulation drill bored out 3m rock sockets and a reinforced concrete bottom plug was formed.
During driving an unusual relaxation effect occurred between the pile and the disturbed clay, explains Professor Ergun Togrol of Istanbul Technical University and STFA chairman. While driving resistance gradually increased after the first 20m, if driving was temporarily stopped, a much lower blow count was experienced when it restarted.
The problem was most evident in the deepest 5m to 10m of penetration, he says, where driving resistances increased steeply to virtual refusal of 400 to 600 blows per 250mm but after a small halt in driving fell back to just 100 to 200 blows over the same distance. An average of 10 re-drives had to be carried out to achieve the final set of 300 blows for 40mm of penetration.
Togrol relates the phenomenon to an observation by Ralph Peck. 'One explanation could be dilatancy of dense sands and silts, and the development of negative pore water pressure during the initial driving, 'he says.
Four of the piles were tested to vertical loads of up to 20,000kN and four more were tested to 250kN and 750kN horizontal loads to check ship impact protection.
A box girder bridge on temporary supports was built over each test pile and loaded with steel ingots.
Four hydraulic jacks on the head of the pile generated the reaction force of 20,000kN, 1. 5 times the working load. Settlements were measured by optical instruments and readings checked by gauges on an independent beam.
Settlements were generally around 1mm.
One of the major advantages of the piled option over the pontoon is that surface waters can now flow freely, helping reduce pollution in the Golden Horn.