Soil Types vs Soil Classes

Soil Types vs Soil Classes – What’s the Difference and Why It Matters for Tank Installations

When planning a tank installation – whether it’s for stormwater detention, rainwater harvesting, or chemical storage – understanding the ground beneath your site is essential. The terms “soil type” and “soil class” are often used interchangeably, but in construction and engineering, they mean very different things.

Soil Types: Describing What the Soil Is

Soil type refers to the physical composition and texture of the soil — what it’s made of and how it behaves.
Common examples include:

• Clay – Fine particles, holds water well, can expand and contract

• Silt – Smooth texture, holds moisture, can be compact but erodes easily

• Sand – Coarse particles, drains quickly, low cohesion

• Gravel – Large particles, excellent drainage, stable under load

• Loam – A balanced mix of sand, silt, and clay

Knowing your soil type helps predict how it will react to excavation, drainage, and load-bearing. For example, clay-heavy soils may require extra foundation work to prevent shifting, while sandy soils may need compaction for stability

Soil Classes: Categorising for Engineering Purposes

Soil class is an engineering classification that groups soils by their behaviour under load and in seismic or wind conditions. In New Zealand, soil classification often refers to the NZS 1170.5 standard for seismic design or council geotechnical requirements.

Typical soil classes in this context include:

• Class A – Rock or very stiff material

• Class B – Very dense soil and soft rock

• Class C – Dense soil

• Class D – Stiff soil

• Class E – Soft soil

• Class F – Soils with special properties requiring site-specific investigation

The soil class impacts tank design and installation – including seismic restraint requirements, slab engineering, and anchoring systems.

At APD, we engineer our tanks to perform in Soil Classes A–D across seismic Zones 1–3, ensuring compliance with AS/NZS 4766:2020 and local council requirements. However, each site is unique — and understanding both type and class ensures:

• Correct slab design

• Proper seismic restraint specification

• Long-term tank stability and safety

• Compliance with building consent requirements

Our team works with geotechnical reports, council requirements, and site assessments to recommend the right tank, slab, and restraint system for your location. Whether you have clay-rich soil in a high seismic zone or sandy loam in a coastal setting, we can design a solution that’s safe, compliant, and built to last.