Thursday, 28 May 2015

GeoBusiness 2015

Today was the second and last day of The GeoBusiness Show in London. Billed as "The Geospatial Event", it was a chance for anyone and everyone associated with capturing, manipulating and presenting spatial data to meet and exchange new ideas as well as show off their wares.

The exhibition floor was reportedly 64% larger than last years inaugural show and this was evident by the additional stands situated at the entrance of the Business Centre in Angel.

GeoBusiness Show 2015
As ever, the Leica stand took centre stage. However, ironically, having spent the time and money attending the show, the in-joke was that Leica is about to announce a leap forward in technology and all of the Reps were sworn to secrecy before the official announcement on 1st June. Even its distributors, such as Opti-cal, had been instructed not to reveal any information!

Meanwhile, other brands such as Topcon were not so secretive and it was great to meet previous co-worker Peter Roberts, now Technical Support Manager, who enthusiastically demonstrated Topcons new instrumentation.

The show was ideal for company directors such as Steve Ashcroft of Paragon Surveys to discover new products that might be useful in carrying out surveys in ways not possible before now. The event was also a showcase for suppliers to gauge interest by demonstrating working prototypes of new gadgets and gizmos.

Personally, I did not have enough time to fully engage with this years event, but more than made up for this by meeting the legendary Ralph Perez, owner of New York Geomatics, whilst on a rare visit to London. Ralph is a great ambassador for the industry and it was an honour to meet up in person.

The future of surveying is assured and whilst technology is making the collection of data even easier and quicker, it is still driven by the surveyor and his imagination and skill.

Sunday, 17 May 2015

Bearing replacement (2)

The bearings that allow Hammersmith Flyover to expand and contract are situated underneath each of the 15 columns or piers rather than on the top - which not only means that the piers move as well, it also means that the bearings are in a 2m deep pit below the pavement and makes replacing them a very difficult operation.

Construction of a pier pit in 1960
To replace the bearings, it is therefore necessary to firstly lift the pier to remove the old ones. This is easier said than done. Hammersmith Flyover is over 50 years old and the piers are not designed to be lifted. There is a danger that the columns will crack, the pit floor will fail or that the pier will topple over setting off a domino effect and causing the whole bridge to tip over.

So, before any lifting could take place it was necessary to strengthen the base of the pit, by drilling steel rods into the concrete slab, stress more steel bars around and through the base of the pier to prevent it "exploding" apart and also to demolish then rebuilt the top of the pit wall (see below) to allow lateral jacks to be installed to prevent the pier toppling.  All of these operations have taken a year of preparation.

Pier pit wall strengthening

The new bearings are attached to the underside of the piers through large steel plates bolted and grouted to the underneath. As mentioned in an earlier blog, the carrier plates must be installed to a very small tolerance. The two plates have to parallel to less than 1mm. However, "if it can be measured, it can be improved". Using precise Leica theodolites and industrial grade corner cube reflectors, measurements of 0.2mm are achievable. Setting the carrier plates to a tighter tolerance than necessary, maximizes the amount of accuracy that may be lost due to fabrication tolerances.

Installation of the carrier plates
Supplementary jacks at each corner of the pier help to keep the pier level and steady whilst the precise work of setting the carrier plates is undertaken.

Working in tight areas means that Automatic Target Recognition (ATR) is critical. The distances measured are shorter than the minimum normal focusing range of approximately 1.5m. Similarly, ATR does not need ambient light and can be used in dark conditions.

The next step is to pour grout into the gap between the carrier plates and the pier base to ensure full load bearing...

Friday, 15 May 2015

Award winners

The Hammersmith Flyover Strengthening Project was awarded the “Special Award for re-engineering London” last night at the ICE London Civil Engineering Awards 2015 ceremony held at the Institution of Civil Engineers, One Great George Street, London.

The Award
Congratulations to the whole team for winning this award against such stiff competition.

Thursday, 14 May 2015

Bearing replacement

Hammersmith Flyover was built in the early 1960's and sits on top of 15 concrete columns or piers. As the 600m long concrete bridge expands and contracts with the changing temperature, then the piers also need to move. Until now, each pier has sat on two roller bearings, like large rolling pins that allow the bridge to "roll" backwards and forwards - up to 20mm a day between a cold night and warm day and 50-60mm between Summer and Winter.

One of the original bearings
However, over time, these bearings have become rusted and worn. As part of the re-stressing of the Flyover, the 15 pairs of bearings have gradually been replaced. This has meant that the bridge has had to be lifted up and a single temporary bearing installed whilst the old bearings are removed and the new bearings are installed.

Temporary bearing
The temporary bearing sits at the centre of each pier (between the old and new bearings) and is supported by 4 large hydraulic jacks.  To stop the bridge from falling sideways, there are also four jacks, one at each corner, holding the pier in position.

Lateral jack
The new bearings (working in pairs) have to be installed to an extremely tight tolerance to allow the piers (and the bridge) to move smoothly.  If the bearings were twisted or uneven, this would cause the bridge to jam or lean. However, the vibrations from the heavy traffic passing over the flyover can cause vibrations and effect the accuracy to which the new bearings are installed.  Therefore, the bridge has to be closed to vehicles - meaning that the work has to be carried out during the night when the flyover is less busy. 

To be continued...

Tuesday, 5 May 2015

Bridge stressing

After about 18 months of preparation (partial demolition, reconstruction, new bearings etc) the first long strands have been stressed on Hammersmith Flyover in West London.

The 622m bridge is constructed in 2 halves, each fixed at an abutment at the end ramps with a gap in the middle to take up any thermal expansion/contraction. Since its construction in the early 1960's the internal steel cables holding the bridge together have gradually deteriorated and became so corroded that the bridge was in danger of collapse.

New (short) cables are being installed to the outside (between precast concrete blisters) and long tendons are being stressed on the inside of the flyover to keep the precast segments from falling apart.

Long (internal) tendon jacking & external "blisters"

The initial jacking process will "take up some slack" and the process will be repeated again following a predefined sequence to prevent over stressing any given section..

Todays milestone sees the shorter 250m section of the bridge being shortened by approximately 20mm, about a third of the amount required for the Western half of the flyover. In total, the two bridge sections will be compressed by up to 130mm.

The replacement of the bearings underneath the 15 piers is almost complete with just 3 to go. The new bearings are more durable and have a longer travel to allow the shortening to take place.  A new expansion joint has also had to be designed to accommodate the bigger gap at the centre of the bridge.

Further details about the repairs can be found here: New Civil Engineer - February 2015