Soakage Testing for New Builds and Subdivisions in Auckland

New roofs, driveways, accessways and paved areas change how stormwater moves across a property. Instead of soaking into exposed ground, more rainfall runs off hard surfaces and enters the site’s drainage system.

For a new build or subdivision, this additional runoff needs to be considered during stormwater planning. Where disposal to ground is being investigated, soakage testing in Auckland can provide site-specific information about how readily water enters the soil or underlying rock.

The results do not constitute a complete stormwater design and do not guarantee that soakage will be suitable. They are generally provided to a civil or stormwater engineer, who assesses them alongside the proposed development layout, planning requirements, site constraints and other technical information.

What Is Soakage Testing?

Soakage testing, sometimes called infiltration testing, assesses how readily water can enter the ground at a particular location.

A test location is prepared, water is introduced using an appropriate method, and changes in water level are monitored over time. The observations are then used to calculate or report an infiltration or soakage rate.

This information helps the project team understand whether stormwater disposal to ground may be feasible and what assumptions may be appropriate for further design work.

Results are site-specific. Ground conditions can vary between neighbouring properties and even between different parts of the same development site. It is therefore risky to assume that a result obtained elsewhere will accurately represent the proposed building platform or stormwater device location.

Auckland Council’s Stormwater Soakage and Groundwater Recharge in the Auckland Region guideline, GD07 provides Auckland-specific guidance for investigating and designing soakage and groundwater recharge devices.

Why Soakage Testing Is Relevant to New Builds

A new dwelling or commercial building normally introduces roof surfaces and may also require new driveways, parking areas, patios or paths. These impermeable surfaces reduce the area through which rainfall can naturally enter the ground.

The resulting runoff must be addressed as part of the project’s drainage strategy.

New Zealand Building Code Clause E1 covers the disposal of rainwater from external surfaces and requires buildings and sitework to protect people and other property from the adverse effects of surface water.

MBIE also publishes Acceptable Solutions and Verification Methods that provide recognised compliance pathways in applicable situations.

This does not mean every new build requires soakage testing. The appropriate investigation depends on factors such as:

• The proposed development and increase in impermeable area

• Whether an approved stormwater connection is available

• Existing drainage arrangements

• Ground and groundwater conditions

• Applicable planning and consent requirements

• Advice from the project’s engineer, architect or designer

Where soakage is being considered, testing replaces broad assumptions with information obtained from the actual site.

Why Subdivision Projects Can Be More Complex

Soakage testing for subdivisions may need to account for a wider range of development variables than a single new dwelling.

A subdivision can include multiple proposed lots, future building platforms, shared or separate accessways and larger areas of paving.

There may also be limited space available for stormwater devices once buildings, retaining structures, vehicle manoeuvring areas, underground services and landscaping have been accommodated.

Changes to site levels can affect where runoff travels and where a stormwater device might be located. On larger sites, the soil or rock encountered at one end of the development may differ from the conditions found elsewhere.

For these reasons, the number and position of test locations should be selected with reference to the proposed development rather than simply choosing the easiest area to access.

The exact investigation scope should be agreed with the relevant designer or engineer and adjusted to suit the site.

Auckland’s Variable Ground Conditions

Auckland includes a range of soil, rock, groundwater and geological conditions. These variations influence how easily water can enter and move through the ground.

A soakage solution that performs satisfactorily on one property may not be appropriate for another. Conditions may also vary with depth, making the proposed position and form of a device relevant to the investigation.

GD07 was prepared specifically for Auckland’s geology, geography, climate, receiving environments and planning context.

Auckland Council’s broader Stormwater Management Devices in the Auckland Region guideline, GD01 also explains that regional technical specifications must reflect Auckland conditions.

This local variability is one reason stormwater soakage testing should be based on direct site observations rather than assumptions derived solely from maps or nearby developments.

What Happens During a Soakage Investigation?

The investigation process depends on the proposed development, available site information and expected ground conditions.

A typical scope may involve:

1. Reviewing the site plan, proposed development and available project information.

2. Selecting test locations that are relevant to possible stormwater device positions.

3. Drilling or otherwise preparing the test locations.

4. Recording the soil, rock and groundwater conditions encountered.

5. Introducing water using an appropriate infiltration testing method.

6. Monitoring changes in water level over time.

7. Calculating or reporting the observed soakage performance.

8. Supplying the findings to the project team or design engineer.

The required depth, number of locations and testing method should not be treated as fixed.

They may change according to the site, the proposed stormwater approach and the information required by the engineer or consenting authority.

Radiant Project Solutions’ ground investigation services can support this process by obtaining subsurface information before the stormwater design is finalised.

How the Results Support Stormwater Design

The purpose of infiltration testing in Auckland is to provide evidence that can be interpreted during stormwater design.

Depending on the project, the results may help a suitably qualified engineer or designer assess:

• Whether disposal to ground appears feasible

• Which design assumptions are appropriate

• Where a soakage device could potentially be positioned

• What type or capacity of device may warrant further consideration

• Whether additional testing is needed

• Whether another approved stormwater management approach should be investigated

The testing report is one input into a broader design process.

The engineer may also need to consider runoff from the proposed impermeable areas, groundwater conditions, setbacks, nearby structures, overland flow paths, site stability, maintenance and the applicable consent pathway.

Auckland Council’s Water Sensitive Design for Stormwater guideline encourages stormwater considerations to be integrated into land-use planning and development rather than addressed as an isolated component at the end of design.

What Happens If the Site Has Poor Soakage?

An unfavourable result does not automatically mean that the development cannot proceed.

It may indicate that:

• Further investigation is required

• Another part of the site should be assessed

• The proposed stormwater design needs revision

• Disposal to ground is not appropriate in the tested location

• An alternative approved discharge or management method should be considered

The appropriate response depends on the development, site constraints and professional advice.

A specific alternative system should not be selected without project-specific engineering assessment.

Variable results can also be important. They may show that the ground does not perform consistently across the site, allowing that uncertainty to be addressed before the stormwater design and consent documentation are completed.

When Should Testing Be Arranged?

Soakage testing is most useful when it is completed early enough to influence the site layout and stormwater design.

Testing after the building position, access and landscaping have been fixed may leave fewer practical locations for a device.

Late discovery of poor or variable ground conditions can also result in design changes, additional investigation and consent delays.

Early testing does not remove the need for engineering design or council assessment. It gives the project team better information before committing to a design based on unverified ground assumptions.

Other investigation methods may also be useful before construction.

For example, Ground Penetrating Radar for excavation planning can assist with locating underground features before drilling or excavation, although GPR does not measure infiltration rates.

Plan Stormwater Investigations Around the Actual Site

New builds and subdivisions introduce hard surfaces that can change stormwater runoff.

Where disposal to ground is being considered, soakage testing provides location-specific information about the soil or rock conditions and observed infiltration performance.

The results can help the project engineer assess whether soakage appears feasible, develop appropriate design assumptions and identify whether further investigation or another stormwater approach is required.

Testing requirements depend on the development, available stormwater connections, ground conditions, planning rules, consent pathway and professional advice.

Soakage testing should therefore be scoped around the actual project rather than treated as a standard requirement for every Auckland property.

Planning a new build or subdivision in Auckland?

Early ground investigation can provide useful site information before the stormwater design is finalised. Contact Radiant Project Solutions to discuss the available project information and an appropriate investigation scope.

Frequently Asked Questions

Is soakage testing required for every new build in Auckland?

No. Requirements depend on the proposed development, site conditions, drainage arrangements, applicable planning rules, consent pathway and advice from the project’s engineer or designer.

Testing may be requested where stormwater disposal to ground is being considered.

What does a soakage test measure?

It measures or observes how readily water enters the ground at a particular test location.

The reported infiltration or soakage rate can then be used as an input into stormwater design.

Is soakage testing the same as stormwater design?

No.

Testing provides site information. A suitably qualified engineer or designer generally uses the results alongside runoff calculations, site constraints, planning requirements and other technical information to develop the stormwater design.

Can results from a neighbouring property be used?

Nearby information may provide useful background, but it should not automatically be assumed to represent the subject site.

Soil, rock and groundwater conditions can vary over short distances.

What happens when the ground does not soak water away effectively?

The project team may consider further testing, a different location, a revised design or another approved stormwater management method.

The appropriate response requires project-specific engineering advice.