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Foundations - Hole lot of drilling

Bridges - Two contractors are carrying out complex foundation strengthening for the Richmond-San Rafael Bridge in San Francisco Bay

When it opened in 1956, the 8.8km long cantilever and truss RichmondSan Rafael Bridge across the northern end of San Francisco Bay was one of the longest in the world, with a 327m main navigation span and 56.4m high shipping clearance.

It is a vital transport link carrying the Interstate Highway 580.

However it was built in a precarious spot, just 16km west of the notorious San Andreas fault and 6km east of the Hayward fault. Assessments on behalf of owner California Department of Transportation (Caltrans) in 1995 revealed the bridge's foundations would not be able to absorb the excessive horizontal movement generated by a major earthquake.

It therefore became one of five of the seven state-owned bay area toll bridges scheduled for a multi-billion dollar seismic retrofit programme. The design for reinforcing San Rafael was carried out by joint venture consultant Ben C Gerwick, DMJM+Harris and Jacobs Engineering.

Foundation strengthening called for 640 piles of varying sizes and depths to be added through and around the bridge's pier foundations. The bell-shaped concrete substructures consist of two or four concrete shafts interconnected by concrete spandrel beams and diaphragm walls, founded on concentric rings of raking steel H beams.

The new piles will take the shear forces transferred between them and the original foundations. Reducing the horizontal loading demand on the H piles should prevent them from buckling, while maintaining their capability to cater for overturning forces and axial loads.

North American foundation contractor Agra Foundations, part of the Amec group, was awarded the $40M subcontract.

It is installing high capacity micropiles as well as 3.8m diameter, 69m long rock socketed cast-in-drilled-hole (CIDH) reinforced concrete piles, incorporated with special pile caps to act as shear diaphragms to limit lateral movement of the piers.

For the demanding CIDH piles Agra teamed up with UK marine and offshore drilling contractor Seacore to design and build a Teredo T40-4 reverse circulation drill and specially shrouded full face drill bit.

This is believed to be the biggest pile top drill in the world, capable of drilling up to 6m diameter. For the San Rafael project the T40-4, together with its in-hole equipment, can drill at diameters from 2m to 4.4m.

Seacore designed and built the equipment at its headquarters in Cornwall, England and provided the crew and technical support team to drill the marine shafts. Agra, based in Seattle, Washington, supplied other support services, including the special closed loop drilling mud slurry system for environmental management of the spoil.

Positioning templates for the CIDH piles were provided by the precast concrete pile caps, placed centrally between the bell shaped foundations, anchored together with tremie concrete with preformed large diameter pilot guide holes to accommodate permanent 3.8m diameter steel casings.

Working from barges in up to 18m of water, the up to 60m long casings were driven by a hydraulic hammer through the pile caps and into the seabed overburden to found on the underlying bedrock with their top level with the pile cap.

A second, larger diameter, temporary conductor extension casing was then installed over the top extending above the water level, allowing for mucking out the spoil inside and thoroughly cleaning the interior wall.

Water was pumped from the two part casing and replaced with synthetic drilling slurry.

The T40-4 drill was placed on top of the temporary casing to drill the 3.35m diameter sockets 9m deep into the underlying rock.

During full face rotary drilling the spoil-slurry mixture was airlifted up the hollow drill string using reverse circulation and discharged into a barge with settling tanks and slurry cleaning system for recycling back to the shaft. This closed loop system was needed to meet the stringent environmental restrictions which prohibited any spoil being discharged into the bay.

Ground conditions were complex and included new bay mud overlying old bay mud and a Cretaceous/Jurassic Franciscan bedrock formation. This mixture of shale, greywacke, chert and serpentine was made even more complex by the extensive structural faulting and micro fissure fracturing from seismic activity.

Strength of the heavily fractured rock is about 20MPa.

To avoid disturbing the material surrounding the CIDH steel shell tip 'we designed and built a completely shrouded bottom hole assembly to match the depth of the socket', says Seacore divisional manager Jason Clark. 'This equipment resembled a vertical tunnel boring machine and, together with reverse circulation drilling and careful operation, aimed to minimise ground collapse during socket drilling.'

But despite this, two of the sockets caved in after drilling. To stabilise them Seacore and Agra had to drill incrementally larger diameters through the cave-in to produce an oversized hole.

The hole was then filled with a weak fibrous tremie concrete mix and redrilled to the original socket diameter.

In all the sockets, a reinforcement cage, complete with inspection and testing equipment, was lowered through the deck of the drilling platform, down the casing into the socket. An average 460m 3of concrete was tremied in to complete the pile.

The main $485M construction contract for the Richmond-San Rafael Bridge seismic strengthening was awarded in September 2000 to Tutor-Saliba, Koch and Tidewater Construction JV.

Work is due to finish in May 2005.

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