Project Challenges and Background
The initial project proposal was to construct a pipe bridge, spanning 27m across Ruthven Water. The scheme was based on a 300mm diameter ductile iron pipe spanning approximately 15 meters between the two most critical supports; SP02 and SP03. It was noted that the columns of these supports - critical in supporting the pipe bridge - were positioned next to Ruthven Water, west and east riverbanks, respectively. To prevent erosion risk to the structures following construction, appropriate erosion control measures needed to be recommended.
A scour assessment report indicated a total of 3m scouring depth from the riverbed, affecting the two columns of SP02 and SP03, positioned within the banks of Ruthven Water. Piled foundations were recommended to transfer the loads of the pipe bridge to sufficient depth. Rip rap was also recommended for further protection of the pier against scouring.
What is scour? Scour is the term for the erosion of soil surrounding a bridge foundation. Bridge scour occurs when fast-moving water removes sediment from around the bridge foundation, leaving behind scour holes.
The most practical foundation option was to support the pipe bridge columns SP02 and SP03 with a group of micropiles. The group would consist of four, 10m long micropiles (from the underside of the pile cap) in a two by two symmetrical arrangement. The centre to centre spacing of the micropiles to be 1.0m and the micropiles raked 20° to the vertical, in the direction of the most critical horizontal loading (accidental) on the system. Due to possible scouring of up to 3m below the riverbed, the top 3.5m of the micropiles (from the underside of the pile cap) were to be further protected by a steel pipe/tube (i.e. outer casing). The steel tube was structurally assessed considering significant corrosion due to the river environment. The steel tube would also protect the grouting and hollow micropile steel bar from further scouring and corrosion in the top 3.5m below pile cap level.
Geological records show non-man-made or unfilled ground with Alluvium and River Terrace deposits as the superficial deposits, and Scone Sandstone as the bedrock underlying the site.