General:
Stony soils are problematic. The presence of rocks and stones interfere with foundations and it can be expensive to source fill materials and treat the foundation.
Sites in areas that are known to contain sand or sand lenses, silt or dispersive clays, or are susceptible to erosion may require specialised treatment to prevent piping failure through the foundation or containment structure, and prevent erosion along the toes of the embankments or in other areas where water flow may occur.
Silty or sandy soils:
Coastal aquaculture containment structures are required to hold saline/brackish water that may contain nutrients from fertiliser and the feeding of stock.
Therefore, the primary objective of the soil used to build the containment structures is to minimise the rate of seepage from the farm. Soils with a high silt or sand content are often too permeable, except when used for the embankment shell, armour or drains as outlined in Appendix 3. Containment structures should be lined if they are to be built in relatively permeable soils.
Expansive soils:
Although expansive soils have a very low permeability, they can shrink and swell with changes in moisture as experienced in growout ponds subjected to filling – drying cycles. This shrinking and swelling can cause large cracks to form in embankments and foundations. Where these soils demonstrate high dispersivity, tunnelling failure of the structures and embankments is also a risk.
Where possible expansive soils should not be used to build embankments particularly in zones where they can be subject to changes in moisture. High plasticity clays are often sticky and difficult to dry and till.
Structured soils, slickenslided soils:
These soils contain visible structuring, which becomes obvious in the soil fabric as cracks upon drying or parting planes upon disturbance. Many of these clay soils tend to form strong aggregates that can lead to high soil porosities. Some of these (slickenslides) may appear shiny when moist. This structure is associated with a history of movement or working of the soil, and is often seen in expansive soils. The structure is associated with high secondary permeability and significant weakening of the soil.
Dispersive soils:
These clayey soils generally possess a high percentage of exchangeable sodium, and have a tendency to go into solution and stay suspended within the water column.
Ponds affected by dispersive soils have high turbidity and therefore are subject to reduced light penetration and primary production. Dispersion, while often suppressed in coastal areas due to the high solute levels in seawater, can occur within containment structures during dry-out periods if dispersive soils are exposed to heavy rainfall.
When excavated, these soils can form tough and impermeable clods that require considerable force to be broken down. Dispersive soils can lead to wall failure due to erosion and tunnelling, and erosion of pond bottoms when ponds are drained.
Organic soils:
Soils with high concentrations of organic matter (generally greater than 20 per cent) should not be used in the embankments or foundations of aquaculture containment structures because they have the potential to:
- create high oxygen demands when decomposing
- be highly permeable
- be acid generating
- be difficult to compact
- have excessive settlement.
Laboratory tests will determine the level of organics present in the soils.
Alkaline soils:
Alkaline soils can harm culture production by causing the pond water pH to increase. The desirable range of soil as well as water pH for aquaculture is 6.5 to 8.5. pH. Greater than 9 is considered too alkaline for most aquatic animals. Alkaline soils are usually found in the driest parts (central and west) of the state, and are therefore generally not considered an issue in coastal areas where the majority of aquaculture farms are located.
However, alkaline soils may arise in coastal areas if lands have been exposed to certain types of industrial contamination. These soils should be avoided as they are likely to be also affected by other contaminants associated with industrial activity.
Acid sulfate soils:
Acid sulfate soils in pond construction can harm culture production by causing pond water pH to decrease. The desirable range of soil as well as water pH for aquaculture is 6.5 to 8.5. pH. Lower than 6 is considered too acidic for most aquatic animals, and acid sulfate soil leachate is commonly less than pH 4. Acid sulfate soils are usually found in low lying coastal areas, and are therefore considered an important issue for the majority of aquaculture farms. The topic is dealt with in Section 2.5
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