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QPOD™ And Good Ground

The QPOD™ is a CodeMarked & Branz appraised system when used in conjunction with Allied READY Superslab has been developed for ‘good ground’ situations in accordance with NZS3604 (and the definition from the New
Zealand Building Code B1) with custom bearing requirements as per Table 1 for the different building types A-E. Outside of this scope an engineer will need to design a customized slab design to suit your site, (See SED).

The New Zealand Building Code places the responsibility on the designer to confirm the soil conditions or to nominate a representative to undertake site verification of the of the soil conditions prior to construction. The conditions of the QPOD™ Allied Superslab require confirmation of “good ground” by the designer and local building authority prior to construction. The designer shall discuss with the local building authority and obtain their geotechnical reporting requirements for the building consent process. It is recommended the designer obtain a project specific geotechnical report prior to building consent to establish the site soil conditions to confirm the site is suitable for a CodeMark QPOD™ Allied Superslab or Engineering input is required.

“Good ground” is defined by NZS3604 and items 1-4:

1. Consistent bearing capacities of the building platform and underlying soils as per NZS3604 and per Table 1 for given building types A-E.
2. Stable platform, free from any instability or settlement from any scenario where vertical movement greater than 25mm over the 50-year design life is expected. Refer NZS3604 – site requirements for further information
3. Slightly expansive soils, with a maximum characteristic movement (Ys) of 20mm as per AS2870
4. Filled platform – Confirm Items 1-3 on the cut platform. Imported fill (well graded compacted hard fill or suitable fill material approved by a geotechnical engineer) less than 600mm deep placed as per NZS4431. Cut and fill batters to be completed in accordance with NZS3604 and figure 5 and 6.

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Outside “Good Ground”

The following are conditions excluded from the definition of “good ground” and are beyond the scope of the Codemark QPOD™ Superslab system and require SED Specific Engineered Design or ground improvement.

5. Expansive soils
Cohesive clays may be prone to drying out and shrinking or expanding and heaving with seasonal moisture changes. Clay or cohesive soil with liquid limits of more than 50% or linear shrinkage of more than 15% as per NZS4402 (test 2.2 & 2.6) shall be classified as expansive soil, prone to seasonal shrink swell movement. Any movement greater than 25mm is beyond the scope of NZS3604 and the definition of good ground. For example, class M (moderately expansive, Ys = 40mm) or class H1/H2 (Highly expansive, Ys = 60, 75mm). The soil class as per AS2870 shall be determined before building on any cohesive soils.

6. Topsoil and Peat
Construction on organic topsoil or buried organic material (peat). Peat and any organic material can be prone to decay and settlement over time and when loaded. Buried top soil is common in swampy areas or sites with uncertified fill such as landfills or re-contoured sites (Uncertified earthworks).

7. Building platform and bearing strength
Soft or weak soils with bearing strengths below 50-67 kPa allowable bearing as required for given building type as per table 1. Soil can be soft for a number of reasons however the most common reason is due to poor compaction and consolidation. Fluctuating ground water can also cause soils to lose bearing strengths. Consistent bearing capacity across the building platform is also very important. If there is significant variability of the bearing strength across the building platform this can result in differential settlement which needs to be considered with specific engineering design of the foundations.

8. Site instability – whether global or local.
o Construction within “close proximity” (1:2, vertical to horizontal) of any retaining walls, existing or proposed.
o Construction on sites within slopes steeper than 5 degrees and as defined by NZS3604. If there are any signs or historical instability a full investigation by a charted geotechnical engineer is required to determine the conditions and recommend foundation design parameters

9. Uncertified fill and bridging.
Construction on any uncertified fill of any depth is prohibited. Uncertified fill can be defined as disturbed soil which has not be placed and properly compacted and as such is prone to settle under self or imposed load. Construction near any buried services, detention tanks or soakage trenches also requires specific design. Construction above or near services or soakage trenches can often encounter uncertified trench back fill or ground that is unstable. Where public services are identified near the foundations, the local building authorities shall be consulted as there are often minimum design standards required to protect their assets. If piling and bridging is required, piling parameters shall be provided in the geotechnical report and will need to be designed by a structural engineer and are beyond the scope of this document.

10. Liquefaction
Construction on sites with liquefaction risks. Loose sand and silts soils (non-cohesive) in combination with high ground water. Sites with significant risk are usually located around river deltas or reclaimed land with extensive silt and or sand deposits. Most local councils have hazard risk maps to aid in identifying these sites as Liquefiable soils are excluded by the building code

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Improving the ground conditions to suit CodeMark QPOD™

Foundations constructed or proposed on sites with any of the above soil conditions (items 5-10) may still be accommodated by QPOD™ Allied Superslab however will require specific engineering design (SED) from a structural engineer. The structural engineer will most likely require design parameters covering the previously mentioned Parameters and these shall be addressed by a geotechnical engineer carrying out the investigation. A geotechnical engineer is able to certify ground improvements (cut and fill) to confirm the building platform achieves “good ground” and satisfies the requirements of the Superslab manual to allow the designer to proceed with a QPOD™ Allied Superslab. It is recommended the designer obtain a statement and inspection schedule from a charted geotechnical engineer confirming the conditions of the CodeMark are achieved by their subgrade design which shall be submitted to the building consent authority.

Explaining Technical Categories 1, 2 & 3.

Most will be familiar with the development of three new categories for residential foundation design in 2011 for repairing and rebuilding homes in Canterbury following the earthquakes of 2010 and 2011.
Engineering experts divided ground conditions into three new technical categories (TC1, TC2, and TC3).
The categories, and the areas they apply to, are based on ground conditions, including the susceptibility to liquefaction, and the extent of land and building damage caused by the earthquakes. These categories have since been used as a wider New Zealand reference and common terminology following the dispersal on engineers that were focused in the Canterbury re-build.

The foundation design categories
Technical Category 1 (TC1): are unlikely to experience significant land damage from liquefaction in future earthquakes. Standard concrete slabs and timber floors are acceptable for foundation repairs or rebuilds.
Technical Category 2 (TC2): minor to moderate land damage from liquefaction is possible in future significant earthquakes. Lightweight construction, for example corrugated iron not tiled roofs, or enhanced foundations such as more robust floor slabs that better tie the structure together will be required for foundation repairs or rebuilds.
Technical Category 3 (TC3): moderate to significant land damage from liquefaction is possible in future significant earthquakes. Foundation solutions should be based on site-specific geotechnical investigation and specific engineering foundation design where foundation repairs or rebuilds are needed. This might involve deep pile solutions.

A summary of the DBH guidance for repairing or rebuilding houses following the earthquakes in TC1 and TC2 categories is available on www.dbh.govt.nz/canterbury-earthquake-residential-building.

Homeowners can find out what technical category their property is in by visiting the CERA land information website: www.landcheck.org.nz. The new technical categories only apply to residential properties in the green zone with foundations that are required to be repaired or rebuilt due to earthquake damage or for future major renovations or new builds

Specific Engineered Designs for QPOD™ Foundations.

In New Zealand we inherit a diverse landscape and vast range of subsoil structures and in turn need to be adaptive in our approach to designing customized foundation solutions.
If you are building in a large subdivision project the chances are the developer has already significantly improved or prepared the ground for your build meaning a Codemark QPOD™ Superslab or minor variation of it will be your most economical choice.
Where you have chosen a specific site of a smaller subdivision the likelihood of requiring an SED slab is much higher.

QPOD™ Foundations are being built in New Zealand everyday in all diverse locations, the majority of the slabs QPOD™ produce for are SED situations.
We have included here examples of slab designs adopted for liquefaction and expansive soil situations.

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Download: QPOD Continuous with Stirrups Solution
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The additional strength can be achieved in various ways such as increasing the steel reinforcing, adding fiber reinforcing, increasing in-slab thickened ribs and other factors to achieve the necessary sheer strength determined by the engineer based on soil conditions of a site.
Many factors need to considered per site and the above examples can be used as a guide in your SED slab design.

For more information on SED and help finding an engineer for your project get in touch with us today or view our QPOD specialists map to find a professional near you.