A6 Toome Bypass - Earthworks
Geology of the area
The underlying geology is Tertiary Basalt, but this is overlain by a thick accumulation of drift. These superficial deposits are divided into Glacial Boulder Clay, which forms the higher ground, and Pleistocene and Recent Deposits, consisting of peat, diatomite, alluvium and raised beach deposits, covering the floor of the Bann valley and adjacent valleys and wetlands.
- Boulder Clay: This is a stiff brown and grey clay with scattered basalt blocks.
- Raised Lacustrine: Alluvium and Raised Gravel Beach Deposits: These are the remains of a late-glacial shoreline and consist of silt, sand and peat with some bands of loose sand and gravel.
- Diatomite: A pale grey, fine-grained sedimentary deposit, made up of the skeletons of freshwater diatoms, thought to be deposited after the last Ice Age (7500-5000BP).
- Lacustrine Alluvium: The most recent deposits of Lough Neagh underlie the Bann floodplain and consist of silts, sands, loams, clays and peats.
- Peat: Peat, overlying lacustrine deposits is widespread on the eastern floodplain.
Site Investigation
59 'shell and auger' boreholes were sunk; these recorded the type and depth of the underlying strata and provided samples for laboratory analysis. 23 'cone penetrometer' tests were carried out; amongst other things this provided an indication of how groundwater travels horizontally in the soft grey silts in the floodplain.
Shell and auger borehole rig
Either side of the river there is very soft and highly compressible alluvial soils comprising typically 2.5 metres of peat overlying 3 metres of silt, further overlying clay soils are present in depths of between 14 metres and 20 metres. This soil sequence is underlain by about 2 metres of stiff glacial till, with sand and gravel present locally, and basalt bedrock.
Embankment construction
The road is mainly carried on low-level embankments, rising to about 5 metres high at the River Bann crossing. A short length of the road runs in a shallow cutting on the west side of the river.
With the exception of the material from the shallow cutting on the west side of the river, the majority of the fill is imported general and selected rock from the contractor's quarry at nearby Knockloughrim.
The majority of the approach embankment either side of the river is constructed using basal reinforcement, with band drains and stability berms. The band drains are installed at 1 metre spacing through the full depth of the soils where the embankment height exceeds 2.5 metres. The band drains help accelerate strength gain and settlement in the foundation soils and reduce construction time.
Installing Band Drains
Piles are used to support the embankments either side of the bridge for reasons of stability and settlement. A load transfer platform comprising selected imported fill reinforced with a single longitudinal layer of geotextile and upper and lower transverse geotextiles.
Driving piles in load transfer platform
Instrumentation and monitoring
Instrumentation arrays have been installed to monitor the stability and settlement of the embankments and includes hydraulic piezometers, magnetic probe extensometers, inclinometers and settlement markers and plates.
