Harbour Way is the final stage of the Port Talbot Peripheral Distributor Road; 4.8 km of dual carriageway plus side roads, five bridges and two new entrances into Tata steelworks. The road provides an access route from the M4 into Port Talbot and the Docks and provides a vital link into West Wales.
Construction used various strategies to maximise resource efficiency, minimise off-site disposal and to incorporate sustainable materials into the works. The road is built on embankment which required around 800,000 tonnes of material, the majority of which comprised secondary aggregates, including Blast Furnace Slag.
We were recently comissioned to assess the presence, and likely causes of, a series of surface irregularities and deformation along the eastern section of the route.
Signifcant road deformation was identified requiring ongoing maintenance and repair. The potential for compressible ground to be present is High. Underlying Tidal Flat deposits are probably low strength and based on local borehole logs and published local memoirs are likely to contain beds of peat and, hence, are potentially highly compressible. Such material could lead to significant settlement at the surface. The strength/drainage characteristics of Glaciofluvial Deposits are likely to contrast with those of the Tidal Flat deposits. Such a contrast in material strengths/properties (permeability) can lead to differing rates of soil settlement/consolidation. If not considered in the design of the highway embankment, differential settlement may occur.
There is no mapped Made or Artificial Ground associated with the road. From information obtained during our Desk Study, the embankment was constructed utilising both recycled natural and waste materials of which the following tonnages are documented:
- Processed demolition waste – 35,000m3
- Site won material – 84,000m3
- Blast Furnace slag (Bi-product from steel manufacture) – 220,000m3
- Remediation of contaminated ground – 9,000m3
Material at the surface comprises coarse gravels of clinker/slag type material. The presence of expansive slag and pyrite bearing strata is highly probable. Blast Furnace Slag is a waste product from iron and steel manufacture and varies chemically depending on the manufacturing process involved. There are three main types of slag:
- Blast Furnace Slag
- Basic Steel Slag
- Acid Steel Slag, along with further associated refractory products.
Some forms of slag, such as fresh blast furnace slag and acid steel slag, are generally volumetrically stable but, depending on their chemistry, some such as basic steel slags and weathered blast furnace slags can be extremely unstable when hydrated, which can lead to significant heave at the surface and damage to buildings and hard surfaces.
We identified that significant volumes of recycled materials were used to construct the embankment, including steel slag (two thirds); therefore the potential for a volumaetric instability (heave) hazard is considered high.
We concluded the construction of the embankment may have utilised unsuitable expansive slag. Correlation between recorded deformities and drainage suggest that the drainage system is also defective, resulting in water entering the embankment materials causing loss of strength over time. Given the location within a flood plain, the moisture content for any placed/engineered fill, including slag, may have altered as a result of seepage into the embankment, resulting in an exceedance of the intended design moisture content for that fill material, also causing swelling/expansion to occur.