How Soil Tests Shape Your Biax Slab: Site Costs Explained

When you’re building a home in South East Queensland,

the thing you can’t see—the ground beneath your block—quietly influences everything you can see later: your slab design, your site preparation, the drainage plan, and the “site costs” line on your quote. At Homes by Markon (part of The Markon Group), we now use Biax Slab Foundation systems on every new home. The reason is simple: Biax offers consistent quality and excellent engineering flexibility, and your soil test determines the exact configuration we specify. Whether your report comes back Class M, H, or P under AS 2870, we tailor the Biax design so your structure suits your soil, not the other way around.

In this guide, you’ll learn what a soil test in QLD actually checks, what Class M/H/P means in practice, how Biax responds to each classification, and which site conditions push costs up (and how to keep them under control). We’ll also unpack piering, drainage, cut‑and‑fill earthworks, and how to read your engineering so you know where every dollar is going. If you’ve been Googling “slab types Brisbane” or comparing “site costs building Gold Coast,” this is the practical, engineering‑led view you’ve been looking for.

Visual idea: A simple diagram showing Biax rib grids overlayed on Class M, H, and P soil profiles, with notes on rib depth, edge beams, articulation joints, and optional pier locations.

Why Biax—And What Makes It Work So Well in Queensland

The Biax Slab Foundation is a slab‑on‑ground system that uses lightweight void formers to create a biaxial rib grid within your concrete slab. By concentrating concrete and steel where they add the most stiffness (ribs and edge beams), Biax delivers a robust, predictable platform without unnecessary bulk. That efficiency matters in Queensland, where seasonal moisture swings can cause reactive clays to expand and contract.

What you gain by standardising on Biax:

One proven system, many configurations: We vary rib depth, spacing, reinforcement, and edge beams to match your soil class, footing loads, and layout.
Predictable performance: The ribbed grid distributes loads and resists differential movement—key on H‑class clays.
Cleaner installation: Lightweight formers and straightforward reinforcement patterns improve site workflows and quality control.
Engineering transparency: Your soil test determines the exact Biax specification, so we can price against a real design rather than guesswork.

Bottom line: Choosing Biax for every home removes the “which slab system?” debate and focuses attention where it belongs—on your soil and site conditions.

Soil Test 101: The Data That Drives Your Slab

A geotechnical investigation (often simply called a soil test) is the first technical step. In Queensland, it typically includes:

Boreholes at key locations to identify layers (topsoil, fill, clays, sands), groundwater cues, and bearing capacity.
Assessment of soil reactivity (how much the soil will move as it gains or loses moisture).
A recommended AS 2870 site classification (e.g., M, H1/H2, or P) and notes on any special considerations (trees, slope, uncontrolled fill, or overland flow risks).

Why it matters:

Reactivity informs how stiff your Biax slab needs to be.
Bearing capacity influences edge beam sizing and, in some cases, piering.
Fill and variable layers help decide whether we improve the platform, replace material, or bypass weak zones with piers.
Moisture pathways guide drainage design, finished floor levels (FFL), and slab edge details.

From this report, our engineer specifies your Biax configuration: rib grid dimensions, reinforcement schedules, edge beam sizes, articulation/control joints, any drop edge beams, and whether piering is required at certain points.

AS 2870 Classes M, H, and P—What They Mean for Your Biax Design

You’ll commonly see three classes across Brisbane, Logan, Ipswich, Moreton Bay, and the Gold Coast. Here’s how each translates into design decisions for a Biax slab.

Class M (Moderately Reactive)

What it means: Soils will move a moderate amount with seasonal moisture changes—typical for many suburban sites.
Biax response: A standard rib grid with engineered depths and an appropriate reinforcement schedule, compliant edge beams, and sensible jointing. Good surface and subsurface drainage is still essential to keep moisture conditions steady.
Budget outlook: Often the “baseline” when access is decent and earthworks are minimal.

Class H (Highly Reactive: H1/H2)

What it means: Soils can move significantly—often high‑plasticity clays, tree influences, or uneven moisture regimes.
Biax response: Deeper ribs and upgraded edge beams, heavier reinforcement, and careful jointing around large openings. Selective piering may be specified at edges or point loads.
Budget outlook: Expect an uplift relative to Class M, primarily from slab upgrades and any piering. Good drainage and FFL planning are critical to long‑term performance.

Class P (Special Consideration Site)

What it means: Something about the site warrants extra care—uncontrolled fill, deep soft soils, steep slopes, abnormal moisture conditions, or risk factors such as nearby significant trees or overland flow.
Biax response: Case‑specific engineering that may include drop edge beams integrated into the slab, local or continuous piering to founding strata, fill replacement or improvement, and a robust drainage strategy.
Budget outlook: Variable. We provide clear allowances and triggers so you understand the range and what would change the scope.

Note on other classes:

Class A/S (often sandy profiles) can yield very efficient Biax designs, but councils may still require direct stormwater connection.
Class E (extremely reactive) exists in pockets; Biax can be engineered to suit, but with careful detailing and selective piering.

Piering with Biax: When It’s Needed and How We Plan For It

Even a stiff slab can benefit from anchoring into more stable material in certain conditions. Piering is not automatic—it’s specified by the structural engineer based on your soil test and the intended slab layout.

Common triggers:

Uncontrolled or variable fill, especially beneath slab edges or load paths
H‑class sites near large trees or where cut‑and‑fill transitions run under slab edges
P‑class conditions with variable founding depths across the footprint
Concentrated loads under columns or stacked structural elements

Types we use:

Screw piers (helical piles): Quicker install, minimal spoil, capacity verified by torque.
Bored concrete piers: Drilled to depth and founded in a specified stratum, then reinforced and poured—good where founding layers are well characterised.

Budget clarity:

We document an assumed number, diameter, and depth for piering, plus a clear per‑metre rate if deeper founding is required.
Founding criteria (torque values for screws, refusal or target depths for bored piers) are stated on the engineering, so there’s no ambiguity during install.

Drainage, Levels, and Stormwater: The Quiet Hero of Slab Performance

Moisture changes drive soil movement in reactive clays. Controlling water around your slab reduces the risk of differential movement and cracking.

Our drainage playbook:

Finished floor levels (FFL): We set the FFL to promote natural falls away from the slab perimeter. Even a small increase can make a big difference on a flat block.
Surface controls: Spoon drains, grated channels at garage thresholds, and surface pits to capture runoff before it reaches slab edges.
Subsurface drainage: Where drop edge beams or retaining conditions exist, we add ag drains and free‑draining backfill to relieve water pressure.
Lawful discharge: Councils across Brisbane and the Gold Coast have specific rules for stormwater. We design a compliant path to the lawful point of discharge early, so the site drains properly from day one.
Overland flow: If mapping shows flow paths, we coordinate levels, landscaping, and drainage so storm events don’t pond against the home.

Good drainage is not an optional extra—it’s an integral part of how your Biax slab maintains performance throughout the seasons.

Earthworks, Cut‑and‑Fill, and Retaining: Building the Right Platform

Your slab is only as good as the platform beneath it. Getting the earthworks right is one of the biggest levers you have to control risk and cost.

What we evaluate:

Platform strategy: Achieving a stable bench while minimising excessive export/import of material.
Cut/fill transitions: Avoiding abrupt transitions directly under critical slab edges; where unavoidable, we use drop edge beams or piering to bridge.
Drop edge beams vs external retaining: Integrating retaining into the slab edge can simplify the footprint and drainage. In other cases, external retaining is more cost‑effective—site‑specific engineering decides.
Controlled fill: When fill is required, we use engineered material compacted in layers with density testing. Uncontrolled fill is either replaced, improved, or bypassed with piers per the engineer’s advice.
Access and logistics: Tight infill sites may change the sequence of earthworks, piering, and pours; planning this early avoids delays and cost creep.

Budgeting Your Site Costs: An Engineering‑Led Breakdown

“Site costs” is the umbrella for everything ground‑related that protects your home’s structure. They’re separate from finishes and fixtures, and they vary from block to block based on soil, slope, and stormwater constraints. Standardising on Biax helps by keeping the foundation system constant while we tune the engineering to your soil test.

Typical inclusions:

Soil test QLD (geotech) and structural engineering for the Biax slab
Earthworks: cut‑and‑fill, platform preparation, and spoil management
Controlled fill import and compaction testing (as needed)
Biax slab specification: rib/edge beam sizes, reinforcement schedule, jointing details
Piering: quantities, diameters, assumed depths, and founding criteria (if specified)
Drainage: surface controls, perimeter subsurface drainage, pits and pipes, stormwater to lawful discharge
Temporary erosion and sediment control
Termite and moisture management layers per code

How to keep it predictable:

Test and survey early: Get the soil test and levels plan before final pricing.
Engineer first: Price against a real Biax design, not allowances pulled from thin air.
Define piering rules: Agree a realistic assumed depth and transparent rates for additional depth.
Lock down drainage: Confirm FFL and the stormwater strategy to avoid late changes.

If you’re comparing “site costs building Gold Coast” quotes, look for itemised lists that mirror the above. The more specific the engineering, the fewer surprises you’ll face during construction.

Reading Your Soil Report and Engineering—Without the Headache

Here’s how to skim the documents and grasp the essentials:

Site classification (AS 2870): Your headline—Class M, H (H1/H2), or P—with a short description of why.
Soil profile: Layer by layer, noting any fill, stiff clays, sands, and approximate depths.
Reactivity and moisture notes: Expected movement and any risks from seasonal wetting/drying or groundwater.
Bearing capacity: Often expressed in kPa; influences edge beams and pier sizing.
Recommendations and flags: Trees, overland flow, uncontrolled fill, slope considerations.
Biax slab schedule: Rib depth/spacing, edge beam sizes, reinforcement (top/bottom bars, mesh), articulation/control joint locations.
Piering (if any): Quantity, diameter, target depth or torque, and founding criteria.
Drainage: Surface and subsurface measures, plus the lawful point of discharge strategy.

If you’d like, we can sit down and translate the jargon line by line, connecting each engineering note to a practical outcome (and a cost line) so it all makes sense.

Brisbane vs Gold Coast: Local Nuances That Affect Design and Cost

Brisbane infill (tree‑affected clays)
Expect H‑class tendencies near mature trees. Biax designs typically deepen ribs and edge beams and may include selective piering, especially at edges and large openings. Overland flow overlays are common; drainage planning is non‑negotiable.
Gold Coast lowlands (sand profiles, pockets of higher water table)

Soils can fall into A or S classes, but many sites still require direct stormwater connection to a lawful point. Biax performs efficiently on sands; the key is preventing ponding near slab edges with considered levels and drainage.

Hinterland and slopes
Cut‑and‑fill platforms are the norm. Biax slabs may integrate drop edge beams for retained edges, and piering often bypasses fill to natural ground at critical lines. Access and staging are important cost drivers on steeper blocks.

These local realities are why “slab types Brisbane” searches return a wide spectrum of advice. By committing to Biax and letting the soil test dictate the configuration, we simplify the choice and focus on getting the engineering right for your particular block.

Planning Timeline: From “Block Found” to “Slab Down”

Initial Block Review

We look at planning overlays (flood, overland flow, bushfire), slope, access, services, and any nearby trees.

Site Intelligence

Commission the soil test and a levels/feature survey. If prior geotech exists (e.g., for a recent subdivision), we confirm whether a current site‑specific test is still required.

Concept Layout

We shape your floor plan with an eye on load paths, articulation joints, and simple, continuous slab geometry where possible.

Biax Engineering

The structural engineer sizes your Biax rib grid, edge beams, reinforcement, joints, and any piering or drop edge beams needed for your site class.

Site Costs and Documentation

You receive an itemised site cost breakdown tied to the engineering, including clear pier depth assumptions and the stormwater plan.

Approvals and Pre‑Construction

Final documentation for certification; we confirm levels, drainage, and any pre‑start earthworks sequencing.

Construction

Earthworks and platform preparation, piering (if required), form and steel for Biax, inspections, then the concrete pour.

At each step, decisions are linked back to your soil test and engineering so you always understand the “why,” not just the “what.”

Common Myths—Busted

“A stiffer slab means I’ll never get cracks.”
Any concrete can crack; the goal is to manage where and how. With Biax, we use joints, reinforcement, and detailing to control crack width and location while the slab rides out seasonal soil movement.

“If I’m on sand, I don’t need to worry about drainage.”
Sandy sites can still hold water depending on compaction and water table. Councils may require direct stormwater connection. Good drainage is always part of the plan.

“Piering guarantees zero movement.”
Piering anchors parts of the slab to stable material but does not eliminate all movement. It’s used strategically to reduce differential movement where the soil profile is weak or variable.

“Site costs are just a builder’s way to pad the price.”
Site costs are how we translate geotechnical reality into a safe structure. When they’re itemised and tied to engineering, they’re neither vague nor optional—they’re your home’s risk management plan.

Real‑World Scenarios in SEQ—And the Biax Response

Inner‑Brisbane H1 site near mature trees
Challenge: High‑plasticity clays with seasonal moisture variation and root influence.
Biax solution: Deeper ribs, upgraded edge beams, careful jointing at large openings, and selective screw piers along vulnerable edges.
Outcome: A stiff slab tuned to expected movement patterns, with clear pier depth allowances to keep the budget under control.

Gold Coast low‑lying block with sandy profile
Challenge: A/S class sands with localised shallow water table; strict lawful discharge requirements.
Biax solution: Efficient rib grid with attention to edge detailing; elevations and drainage arranged to prevent ponding; direct connection to lawful discharge.
Outcome: A cost‑effective slab with durable water management.

Hinterland slope, P‑class due to fill and gradient
Challenge: Mixed layers, uncontrolled fill pockets, and a platform that needs retaining.
Biax solution: Drop edge beams integrated along retained edges, bored piers to natural ground through fill sections, terraced landscape levels with robust drainage.
Outcome: Reduced external retaining, predictable performance, and a stable building platform.

A Quick Homeowner Checklist

Before you finalise a plan:
Get a soil test in QLD and a levels survey.
Ask for a preliminary Biax engineering concept for your site class.
Confirm assumed pier depths and rates per extra metre if piering is required.
Review the stormwater plan and FFL to ensure water runs away from the slab.
Check that site costs are itemised and directly linked to the engineering.

During design:
Keep slab geometry simple where practical; it reduces stress concentrations and costs.
Coordinate heavy loads (e.g., feature masonry) with rib directions and jointing.
If your site has trees, discuss their placement and impact on soil moisture.

Before construction:
Walk through the platform levels and drainage with your builder.
Confirm access logistics for earthworks, pier rigs, and concrete deliveries.
Make sure inspection points (steel, formwork, pier founding) are scheduled and documented.

FAQs

Do you only build with Biax now?
Yes. Homes by Markon has standardised on Biax Slab Foundation systems. The soil test sets the configuration—rib depth, spacing, reinforcement, edge beams, and any piering.

How much can site costs vary?
They depend on soil class, earthworks, piering, and drainage complexity. The fastest way to narrow the range is to engineer the Biax slab against your soil test and levels plan before final pricing.

Is piering always required on H‑class sites?
Not always. Many H‑class sites are handled with deeper ribs, edge beams, and reinforcement upgrades. Piering is used where the soil profile, loads, or cut/fill transitions justify it.

What if my report says P‑class?
P‑class means “take care,” not “can’t build.” We tailor the Biax design, consider drop edge beams, and use selective piering or platform improvement as directed by the engineer.

I keep seeing “slab types Brisbane”—does that still apply if you only use Biax?
Yes. “Slab types” is shorthand for performance requirements across various soils. We meet those requirements by adjusting the Biax configuration rather than switching systems.

Can drainage really make that much difference?
Absolutely. Consistent moisture around the slab is one of the best protections against excessive movement and cosmetic cracking. Getting FFL and stormwater right is essential.

About Us: Engineering First, Always

The Markon Group is a custom home builder. We also design and import custom‑built modular prefabricated buildings, applying the same engineering discipline across off‑site and on‑site projects. Homes by Markon focuses on site‑built custom homes throughout Brisbane, the Gold Coast, and surrounding regions. Standardising on Biax foundations allows us to deliver consistent quality, transparent pricing, and performance you can rely on—because your slab is tuned to your soil test, not a generic template.

Ready to Move Forward? Get a Site Assessment with Homes by Markon

If you own a block—or you’re comparing lots—you deserve clarity before you commit. We’ll arrange your soil test, complete a levels survey, and engineer the right Biax slab design for your site class. You’ll receive:

A soil test in QLD and an engineering‑led Biax configuration aligned to your classification (M, H, or P)
An itemised breakdown of site costs tied directly to the engineering, with clear piering assumptions
A simple visual showing how your Biax rib grid, edge beams, joints, and any piers respond to your soil and layout
A documented drainage and stormwater strategy to protect your slab long‑term

Avoid the guesswork and budget shocks. Get a site assessment with Homes by Markon and build with confidence—on foundations designed precisely for the ground beneath your home.

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