A report has urged repairs to Dublin's Luas lines, writes Stephen Collins, Political Correspondent
Repair work on both of Dublin's Luas lines has been recommended in a draft report submitted last month to the contractor who built the lines.
The report was commissioned by AMB Joint Venture because of concerns about the safety and durability of the track in the sections that run through city streets.
The report was carried out by Prof Klaus Riessberger and Dr Albert Jorg at the Institute for Railway Engineering and Transport Economy at Graz University of Technology in Austria.
The experts concluded that sections of the track showed deficiencies with respect to the bonding between the concrete tray under the rail and the rail support system.
"The Edilon-track of the new Luas light rail system with individual supports in Dublin, Ireland, shows deficiencies with respect to bonding between concrete rail block and/or concrete tray and the intermediate elastic material Corkelast M.
"The debonding is gradually more serious with sharper curvatures of the alignment and manifests itself by opening of fissures and cracks which permit penetration of water into the load carrying system," it says.
The Corkelast material is responsible for the elasticity of the rail supporting system. It is made up of two components which are mixed in an automatic mixing unit.
The report points out that the Edilon embedded block system is well proven, and is used in the Madrid metro and Bilbao as well as in Dublin.
The preface says that an investigation of the causes of the damage was not the main aim of the study, and it does not draw firm conclusions as to why the bonding has deteriorated so quickly.
The authors of the report do come to conclusions about how the problem can be dealt with.
For a start they rule out rebonding by insertion of gluing material. This, they say, is not a reliable option.
"Protection of the surface of the Corkelast M material from water penetration is highly advisable.
"The seriousness of the deficiencies grows with reduction of radii. It is concluded that different methods of improvement should be applied to cater for those different conditions.
"In the very sharp curves the Edilon design is highly overstressed, particularly by the rotation of the rail under horizontal forces. This is a result of the independent bearing of the rails which are not ready to take the high spreading forces in the sharp curves and which in turn results in very high lateral forces in the rail blocks and over-stressing the concrete Corkelast interface."
A number of laboratory tests were conducted in Graz to establish reliable methods of repair. While they have still not been fully completed, a number of provisional recommendations are made.
The first is that the open surfaces of the Corkelast material, surrounding the rail blocks, should be protected against water penetration with an easy to apply permanent elastic coating.
The next recommendation is that overstressed Corkelast M fillings should be replaced with rubber "shoes". These are available and a prototype has been made by Semperit.
The report suggests that the sharp curves should be counteracted by an alteration of the load transmission. "Track panels, where the two rails are stiffly interconnected by cross-bars, transmit only the difference of the individual horizontal forces into the sub-base and additionally counteract rail rotation. To this end it is advised to strengthen the track in these critical sections by considerably stiff connections between the two rails."
The report says even before the lines were opened deficiencies had begun to show up.
"On straights and in wide curves the debonding mainly occurs on the long sides of the blocks; in tight curves debonding is noticed on the short sides. Also, in transitions between traditional track and other track systems debonding appears."
It adds that after installation a small number of blocks showed debonding but with frequent use by trams the number of affected blocks increased dramatically.
"So far no observation was made which indicates any reduction in operational safety. Nevertheless, the development of bonding deficiencies gives rise to foresee a growing uncertainty towards the reliability and proper functioning of the system, particularly in tight curves."
The report says that cracks will develop as a result of debonding, and these will allow water to flow into the gaps.
"Therefore it is strictly recommended to protect the working system from water in order to achieve the desired durability of the entire system."
While the report came to no definitive conclusions about the reasons for the debonding, one of the findings was that batches of non-fully cured Corkelast M material was used. "Most likely either the chemical composition was incorrect or the mixing of the two components imperfect."