Flexible hydraulic system launches four different bridges

Engineers construct the new ring road around Teruel in Spain and developed with specialist Enerpac a flexible hydraulic launching system that can be adapted for uphill and downhill launching of all four bridges in the 16,6 km long new bypass of the motorway between Valencia and Zaragoza.

The new bypass will form part of the Valencia to Zaragoza motorway that is currently being built. For the most part, construction of this highway consists of upgrading the existing single-carriageway road to motorway standards, but at Teruel, where the road passes through the center of the city, a bypass was required. As individual structures the four new bridges that form part of the bypass being built round Teruel in northeast Spain, would not seem to be likely candidates for incremental launching. They are all modest in scale, the longest being just 325m, even if they are both formed of twin viaducts, one for each carriageway.

Although they are similar, they are not identical - the main differences being slope, cross fall and plan curvature - but even so, contractor Dragados has elected to build them using incremental launching, and has worked with Enerpac, the specialist in hydraulic integrated systems for civil engineering to develop a simple and flexible system that can be adapted for use on all four structures.

As Drace (Dragados Construccione Especiales) launching manager Carlos Polimon explains, the terrain around Teruel is somewhat hilly, prompting the need for so many structures - there are five bridges in total, although only four are being built by launching - not to mention culverts, over- and underbridges.

Up- and downhill bridge launching

The four launched bridges, each twin structures hence there are eight decks to launch in total - have been designed to be as similar as possible to enable the launching equipment to be moved from one to the other with the minimum possible fuss. They all have the same cross-section for the segments, and the distance between the piers, 52m, is constant for each structure.

But there are also differences. As Polimon points out, they may not seem very big differences at first glance, but when it comes to planning the casting and the launching, they prove to be quite important. Bridge number one is straight in plan, with a longitudinal slope of 4% and a cross fall of 2% from the inside edge to the outside edge on each deck. Bridge number two is curved in plan with a radius of 3200 meters, while three and four each curve with a radius of 5000 meters. On these three bridges, the longitudinal slope is identical at 1,3%, but whereas two of them will be launched against the slope, the third will be launched ‘downhill’. The cross fall on bridge number two is 2,7%, on both decks falling towards the centre of the curve, and similarly on three and four, although here the cross fall is reduced to 2%. So a system has been developed by Drace and specialist supplier Enerpac that can be used on all four structures, and the whole 'factory' - casting yard and launching equipment alike - will be relocated from one site to the next after completion of each structure. Two sets of equipment have been made, so that the two decks of each bridge can be built and launched at the same time. When it comes to construction of the two last bridges, which are almost adjacent to one another, they will be built one deck at a time because of space restrictions at the abutments.

Hydraulic pushing and braking system

It is space which is the main constraint preventing Dragados from simplifying the task by launching all bridges against the slope, as well as poor ground conditions at the abutments. Although Polimon says he has worked on previous schemes where downhill launching has been required, it has only been over a short distance and always at the start of an uphill launch. The requirement for a system that could cope with downhill launching as well as the standard uphill method led to the development of a more 'positive' system, which includes braking jacks as well as pushing jacks. Each of the segments is 26m long and the means by which the deck is connected to the launching system is very simple. Each segment is cast with temporary holes in the upper and lower parts of the box girder, through which the two specially-made steel uprights are inserted for the launching process. The lower part of the steel upright, which extends below the segment, is connected to the launching jacks via a number of 60 millimeters diameter, 6 meters long high strength steel bars connected in series. On a standard, uphill launch, the jacks - up to four 150 ton capacity, 600 mm stroke high-pressure jacks to deliver the required force - are placed at the abutments behind a specially designed 'bracing block'. Two provisional supports are provided between the abutment and the casting cell, which are topped by sliding plates, and on the top of the piers, steel covers are installed over the pot bearings, which are fixed into position for the launching procedure. Once the bridge is complete, the deck will be jacked up, these plates removed and the bearings released for normal service.

Before launching, the engineers calculate the total weight that must be moved - the appropriate number of segments plus the launching nose – and also calculate the friction that must be overcome. These values allow the engineers to determine how much load they need in the jacks in order to achieve the launch. Up to 600 ton is available from the four jacks on each launch system, but the team estimates they will only need about 440 ton at the most.

A maximum and minimum is set for each launch, and this is programmed into the PLC-control unit for the hydraulic system. The maximum is just as important as the minimum, even when launching uphill, because if too much load is required, it could be an indication that something is wrong. If the maximum value is exceeded, the system is programmed to automatically stop, prompting the engineers to check that everything is progressing to plan.

At the top of each pier is a pair of lateral guides that are used to keep the launch on the correct alignment. The effects of the friction from the supports, and the uneven stroke of the jacks can result in the deck moving anything up to 60mm off line. Carlos Polimon explains that the crew has found the system very easy to operate, and they are achieving one segment launch per week on each deck - or 52m of deck in total. Launching takes place every Monday, with concreting of the next segment taking place the following Wednesday and Friday. A launching speed of around 10m/hour is being achieved here, and this compares very favourably with previous projects where the best rate has been a launch of 4,5m/hour.

 A casting area is set up behind the abutments of each of the structures, and each area has a set of formwork for casting the 26m-long segments. One tower crane serves each set of formwork; one between the two would not have given sufficient flexibility. Even the formwork - from manufacturer Peri - has been designed so that it can be reused on all of the bridges, despite the differences in plan curvature between them. The formwork comes in three modules to make up the full length of the segment, with different connection elements that can take account of the curves. Operational efficiency comes from the fact that the striking process is automated - the outer formwork tips outwards on the hydraulic jacks that support it, and the inner formwork can be transported out of the segment on a small trolley that runs along a temporary rail on the centre-line of the segment. One additional complication for the casting process is that external post-tensioning is involved, requiring adaptation of the inner formwork.

Construction of the first bridges will serve as something of a warm-up for the deck that must be launched downhill. The site staff will have a chance to get used to all the other parts of the construction process before they have to cope with the altered launching procedure. For this part of the work, it was clear that a positive braking force was required, rather than just relying on friction to slow the unit down.

As a result, the jacks will be rearranged, with two remaining at the abutment, as for the standard launch procedure, and the other two being placed at the rear of the segment to be launched. These two will act as the braking jacks, and the steel rods will be fed all the way from the forward jacks through these jacks and back to transfer load jacks behind the braking jacks. The load transfer jacks are brought into play to keep the segment in place when the other jacks are fully extended and need to be released so that they can be retracted.

Nose recovery system

Beside the launching system Enerpac hydraulic equipment is also installed at the front of the launching nose. Two jacks with a capacity of 40 ton and a stroke of 400 mm correct the deflection of about 300 mm when the nose of the deck reaches the piers. This deflection is caused by the relatively long span of 52 meters between the piers.

System integration - ingenious solutions and practical skills

 The Teruel ring road project proofs to be an impressive example of the powerful combination with construction engineering and the high-pressure hydraulics specialist to come to ingenious solutions. Each of the four bridges with their twin decks have different characteristics such as slope, cross fall and curvatures, but hydraulic system integration is flexible enough to accommodate and overcome these differences.