Acid sulphate soils

Acid sulphate soils exist on a number of Pacific Highway projects.

These soils are naturally occurring and contain sulfides. This means, if these soils are exposed to air they are likely to oxidate and sulphuric acid will be formed.

These soils are a major environmental issue and a major risk to the environment.

They need special management and treatment.

On the left is acid sulphate soil. The image on the right shows limestone treatment.

Soft soils

Ground improvement is used for the treatment of soft soils. It involves a range of treatments to make soft ground sufficiently stable for the construction of roads and structures.

The area around the Ballina bypass contains deposits of land classified as "soft".

Soft soils generally occur in low lying areas or floodplains where the water table is close to the surface.

Without treatment, roads constructed on soft soils risk ongoing maintenance as a result of ground settlement, often resulting in poor road conditions and pavement damage.

For the Ballina bypass project, this challenge was overcome using innovative ground improvement solutions including wick drains, surcharging, vacuum consolidation, dry soil mixing and dynamic replacement.

These techniques ensure a stable platform for construction and long-term durability of the new road.

Wick drains

Wick drains remove water content from soft soils so settlement time of the existing soil is reduced from years to months. The process is like squeezing a sponge to remove moisture.

Pre-fabricated "wicks" are driven vertically into the ground in a grid pattern across the treatment area. The wicks are 100 millimetres wide and between five and 22 metres long and consist of fabric wrapped around fluted plastic drain material.

Fill material is then placed on top of the drains. The pressure created by the weight of the fill squeezes the water from the ground, pushing it up the wick drains to escape at ground level.

The water filters through a porous material (normally a layer of rock and sand) between layers of geotextile fabric and flows into drains on either side.

Wick drains are used at various high embankment locations on the Ballina bypass to accelerate settlement and avoid the need to constantly top up bridge approaches.

Vacuum consolidation

This method of treating soft soils is suitable for very deep soft soils. Drains are driven vertically into the soil in a similar way to wick drains. Large pumps are connected to the drains which remove the water and air trapped in the soft soils. This accelerates the settlement of the soft soils.

This method of soft soil treatment was used at the southern embankment of the Emigrant Creek bridge. The embankment at this location has settled over 6m.

Dry Soil Mixing (DSM) columns

Dry Soil Mixing columns improve the load-bearing capacity of the ground by mixing cement with the existing soil to improve its strength.

Installed in a grid pattern across the treatment area, they can be used under road embankments to reduce the amount of ground settlement.

Dynamic replacement

Dynamic replacement is an important method of soft soil treatment, particularly in the soft soil area of Ballina.

Dynamic replacement is typically used in soft soil areas where it is undesirable to remove soil or where traditional soil treatment methods are considered unsuitable.

A large weight is dropped in free-fall onto the ground compacting the soil and creating an indent where the weight was dropped. The indent is backfilled with coarse rock and the weight is again dropped to further compact the ground.

The rock effectively forms a column, improving the overall density of the soil block. The columns are constructed in a grid pattern across the treatment area.

Stone columns

Stone columns are used in a similar way to dynamic replacement in areas where there are deeper soft soils.

A hollow probe is vibrated into the soft ground which is then backfilled with course stone to form a stone column. This improves the capacity of the soil to carry highway loads with reduced settlement.

Low embankment strategy

This is an innovative way to deal with soft soils in the southern end of the Ballina bypass. In simple terms the road is designed to settle over an extended period of time during both its construction and its operation.

This method provides the best value for money to NSW taxpayers over the life of the project, with the minimum number of interventions to maintain the road and provides the best balance between minimising construction cost and impacts to the travelling public with post-construction maintenance.

Bulk earthworks have been constructed between Cumbalum and the Bruxner Highway. This is left to settle by 300 to 900 mm over next two to three years. The road will then be constructed to its finished height, opened to traffic and left to settle for a further 12 months. The final surface layer of asphalt will then be laid at a thickness of 50 to 100 mm.

The road will continue to settle at a reduced rate for the next 20 years. At about 5 to 10 year intervals a correction layer of asphalt will be laid where required at a thickness of 50 to 100 mm.

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