DYWI ^® Drill Hollow Bar Micropiles Stabilize Collapsed Viaduct on one of Ireland’s Busiest Railway Lines

Recently, the main railway line between Dublin and Belfast suffered a major collapse on a viaduct that crosses the Broad Meadow Estuary at Malahide, just north of Dublin. Fortunately, the collapse happened just after a packed commuter train had crossed the old stone viaduct.

The 176m long viaduct was originally built in 1844 as a timber structure, but replaced in 1860 as a masonry pier structure with wrought iron deck sections.

In 1966, the deck sections were replaced with post-tensioned concrete sections. On August 21st 2009, one of the piers collapsed due to heavy scour at its underside, taking with it a 20m deck section. Major rainfalls had resulted in a significant increase in the volume of water flowing through the estuary. Erosion below one of the piers first caused a small breach to appear in the rock causeway. The breach rapidly enlarged itself in the proximity of the failed pier, causing scouring and undermining of the foundation.

The solution was to core through the original stone piers and then to insert 5-6m long, 168mm diameter casings into the rock armor. The silts beneath were immediately grouted in order to ensure that the borehole was sealed down to competent ground below the base of the causeway.

Afterwards, a total of 192 up to 18m long Type R51N DYWI ^® Drill Micropiles were inserted through the permanent casing and drilled into dense gravel to underpin the original stone piers. At the two piers immediately adjacent to the collapse, 14 Type T76N DYWI ^® Drill Hollow Bars in lengths of 22m were installed vertically from track level to depths of 22m in order to attain working loads of up to 600kN.

All 11 piers and the abutments at both ends of the viaduct were underpinned with Type R51N DYWI ^® Drill Micropiles, using 100mm diameter tungsten carbide drill bits. Simultaneous drilling and grouting was employed to ensure that the gravels in the proximity of the bore were fully permeated and any loose soils further consolidated.

The DYWI ^® Drill Hollow Bar proved to be the ideal solution for working in loose granular soils below the pier footings. The micropiles act solely in compression and were cast into a permanent 6m casing, which was cast into the cored socket of the pier.

The restricted headroom at the underside of the bridge decks necessitated specially modified drill rigs with short stroke drill booms.