FOUNDATIONS AND FLOORS
The primary difference between foundations designed for minimum standard homes and those designed to the Healthy Homes Design Guide requirements is that the foundations for Healthy Homes are all required to be insulated.
Refer to Walls & Panels for details.
Good Ground / TC1
Any of the foundation solutions outlined in NZS3604:2011 are suitable. If the ground is not prone to differential movement then the foundation is unlikely to settle.
Note that if in-slab heating is to be installed, either the heating pipe is installed on the polystyrene on the underside of the slab, or fixed to the reinforcing mesh. If fixed to the reinforcing mesh the slab must be at least 125mm thick.
From a structural point of view, it is better that the heating is fixed to the polystyrene, if that’s the insulation surface especially if the reinforcing has a structural purpose or is being used to resist cracking in a polished concrete floor.
Designing Foundations in Ground Affected by Seismic Events
Repairing and Rebuilding Houses Affected by the Canterbury Earthquakes, otherwise colloquially known as the MBIE Guidance is the only document available for design reference in New Zealand.
Note that care should be taken when using the solutions directly from the MBIE guidance as their design premise is based on the concept of “readily repairable”. Refer MBIE Table 8.1 for further information.
This approach is an alternative solution in the hierarchy of the Building Act and has no place in the design of High Performance Homes as it does not conform to the “no repair required” criteria above.
Adverse Ground Conditions
The following design principles should be used to design resilient foundations in adverse ground conditions:
a. Light-weight materials should be used for all roof and wall claddings. This is to reduce the inertial loading on foundations and the potential for settlement in future seismic events.
b. Stiffened and tied together foundation solutions are required to improve resistance to lateral stretch and ground deformation. A slip layer beneath shallow foundations or foundation slabs will improve the performance against lateral spreading (stretch) at the surface.
c. Regular structural plan shapes are preferable to more complex plan shapes.
A regular house plan is defined as meeting three basic criteria:
● A base plan shape that is essentially rectangular with an aspect ratio no greater than 2:1.
● One major projection (ie, greater than 2m out from the base shape) is permitted. (This might result in an ‘L’, ‘T’ or ‘V’ shape base plan). The ratio of the projected dimension divided by the length of the side in common with the base shape should be no greater than 1.
● Any number of minor projections (ie, 2m or less) are permitted off the base shape, or off the major projection. The ratio of the projected dimension divided by the length of the side in common should be no greater than 1.
d. Mixed foundation systems within the same structure are not recommended.
(eg, Type 1 timber floor house and attached concrete slab garage). Instead, separate the two structures with a seismic gap.
e. Minimising penetrations of the crust (the ground between the surface and the layer that is likely to liquefy) will reduce the likelihood of liquefaction ejection coming to the surface. This principle is followed particularly with the shallow surface solutions and for service trenches where possible. Liquefaction ejection results in soil loss and is a primary mechanism of ground deformation.
f. The location and accessibility of services needs to be taken into account.
It is preferable that new service connections and interfaces are appropriately flexible. Services should enter the building at a few well-defined and well-recorded locations, through connections that are as flexible as possible.
Should failure occur, this will be in well-defined locations outside the foundation system and services are then easy and quick to reconnect. Plumbing services in particular should be located near outside walls for access for reparability. Services located below floors must be properly restrained to move with the floor and minimise the risk of damage that is difficult to repair. Where slip layers are provided, services must not impede the ability of the foundation system to move laterally (this may require services to be fully enclosed within surface slabs, for example).
TC2
Concrete foundations
The MBIE Guidance document provides guidance for specific engineering design of concrete slab foundations in TC2 as follows:
a. Design Type C house foundations for the potential for differential settlement of the supporting ground that will allow a maximum unsupported length for the ground floor of 4m beneath sections of the floor and 2m at the extremes of the floor (ie, ends and outer corners).
b. Design to ensure that the floor does not hog or sag more than:
● 1 in 400 (ie, 5mm hog or sag at the centre of a 4m length) for the case of no support over 4m, and
● No more than 1 in 200 for the case of no support of a 2m cantilever at the extremes of the floor.
c. Appropriate provision should be made for flexible service entry to the home to accommodate the potential differential settlement of the foundation as indicated in the geotechnical report.
d. Designs should accommodate settlements up to 50mm at SLS and 100mm at ULS.
Four options for simple house plan shapes are provided for designing concrete slab foundations on grade; Options 1, 2, 3 & 4. By far the most common is Option 4 – Waffle slab.
Timber foundations
A one or two storey house with a light roof and light- or medium-weight wall cladding supported fully on a NZS 3604 shallow timber or concrete pile foundation is considered to be a valid option in TC2.
For timber floors with concrete perimeter foundations for one or two storey dwellings with light- or medium-weight cladding and roofing in TC2, should follow the details stated below. Reinforcing details should be as shown below.
Floor construction details in NZS 3604 are generally adequate, but in practice the jointing between members often falls short of what is required. This is particularly important where resistance to lateral spreading is required. The following should be noted:
● Pile to bearer connection: Ordinary pile connections in Figure 6.3 of NZS 3604. Braced pile connections in Figures 6.6 to 6.8. Anchor pile connection in Figure 6.9.
● Bearer to foundation wall connection: See Figure 6.17 of NZS 3604.
● Bearer butt end joints: See Figure 6.19 of NZS 3604.
● Joist butt end joints: See Figure 7.1 of NZS 3604.
Proprietary materials like Cross Laminated Timber (CLT) can be used to make stiffer floors that span further, therefore needing less piles. The fewer piles inside the perimeter foundation, the lower probability of localised liquefaction induced differential settlement occurring under a pile.
Shallow Foundations in TC3
The intention of this design guide is to look at only shallow foundations as there are all manner of piled slabs and ground improvement techniques that could be utilised if necessary. It is important to ask one question at this juncture, “Should you really be trying to build a high performance home on poor ground in the first place?”
Concrete foundations
Concrete slab foundations are not recommended in TC3 as they cannot sustain the same levels of deformation as timber foundations without exhibiting damage.
Careful services detailing is required for these foundations.
MBIE 15.4.8 provides design approaches and parameters for relevellable concrete surface structures. However, it is important to note that these parameters don’t necessarily meet the Healthy Homes cornerstone criteria of a resilient foundation design.
Timber foundations
The MBIE guidance provides for three surface structures with shallow foundations; Type 1, 2 & 3. These are further broken down to sub categories depending on the expected future performance of the ground conditions as outlined in the site specific geotechnical report.
The most common, resilient and flexible of these is the Type 2 Surface Structure. This has a concrete underslab of 150 or 300mm depth depending on future liquefaction expectations and a timber superstructure. Any number of options can be used for the floor structure including a concrete ring beam that can make the structure very stiff indeed. CLT flooring reduces the number of piles needed and proprietary products can make construction and future adjustment easier.
Expansive Soils
The Acceptable Solution B1/AS1 was released on the 28th November 2019. This document amends and adds clauses to NZS3604:2011. Additional depth and reinforcing have been added to external and internal footings for all wall cladding weights.
In expansive soils a concrete raft type foundation is likely to be desirable with the stiffness dependent on the reactivity of the soil. For that reason, one of the TC2 concrete foundations or TC3 proprietary solutions is likely to be suitable, with the depth and reinforcing dependent on the soil class determined within the geotechnical report.
The same design principles for designing foundations in ground affected by seismic events should apply as while expanding and contracting soils is generally a seasonal event as opposed to an infrequent earthquake, the effects are similar.
Another option is to found the foundation below the layer that is affect by the moisture fluctuation.
So potentially timber floor systems that are piled down below 1.5m, (if this is what is considered the sensitive depth from the site specific geotechnical investigation) are also an option.
Other Considerations
There is not much point having an undamaged foundation if the dwelling cannot be occupied due to services no longer working or it being under water, so it is worth ensuring the resilience requirements for key infrastructure are met.
Services
If lateral spread or differential movement of the ground occurs, there is potential for damage to services, so provision must be made in the design and installation of services to minimise the potential effects. This is particularly important when services such as sewer and water penetrate or are attached to concrete floor systems. Flexibility of service lines is the key to resilience. Services should remain within the concrete slab until exiting the perimeter and not underneath.
Drinking water
Water supply to a property is delivered via flexible MDPE pipe. When installed in a trench, they should be laid down in a snake pattern, which provides extra length should ground extensions occur. Where the pipe penetrates the foundation and the floor slab, a duct/sleeve 125mm greater in diameter than the pipe should be provided to allow the pipe to move independently. The sleeve may be filled with a compressible filler, which allows differential movement but which also provides limited access beneath the slab should a leakage issue arise.
Sewer pipes
Sewer pipes from the house to the sewer main are uPVC plastic, which has some limited flexibility. Waste pipes may pass through the floor of the home to serve plumbing fixtures such as baths, showers, basins, and soil pipes from toilets. These pipes will likely pass below the floor in Good Ground and TC1 foundations, and through the beams and ribs of the raft slab in TC2. If there is vertical or horizontal movement between the foundations and the ground, the expected failure plane is across the bottom of the beams or ribs.
Consideration should also be given to the provision of greater falls in sewer lines than the minimums required. This will make the continued operation of the system more viable should tilting of the ground occur during any future liquefaction event. Where the pipes pass through the slab a sleeve is recommended. Ideally, the duct should have a diameter 125mm greater than the service pipe. Otherwise, a flexible seal should be employed to allow some movement between the pipe and the floor.
Where sewer pipes are installed in a trench parallel to the foundation, the branch drains, such as those connecting to gully traps, should contain a flexible connection (rocker pipe) adjacent to the foundation but beyond the edge of the gravel raft. There should be no main collector drainage pipework installed within the gravel raft.
Plumbing codes require at least one gully trap on the perimeter of a house. Invariably, waste pipes pass through the foundation slab and discharge into the gully trap from above it. Sometimes the waste pipes enter via the side wall of the gully trap. It is recommended that the gully traps be encapsulated in concrete which is tied to the house foundation with hooped reinforcing bars, preventing differential movement, should there be ground spreading or settlement adjacent to the foundation.
Flooding
One of the first pre-purchase checks that should be completed is whether the site is subject to flooding in extreme events. The territorial authority is likely to provide a Building Code flood level and a District Plan flood level. This is the design flood level plus freeboard and the District Plan level will allow for climate change. Designing for resilience means ensuring that climate change is accounted for when choosing the floor level to avoid future flooding.
It’s also important to consider your neighbours within the flood plain. Building up the site to have level entry or a slab foundation will only increase the flooding levels on adjacent properties as the flood storage lost by land reclamation is pushed elsewhere in the catchment.
Proprietary Suppliers
Good Ground / TC1
● MAXRaft’s MAXSlab is a 250mm deep fully insulated slab. www.maxraft.co.nz
● Cupolex has a recycled plastic 200 or 260mm dome that has a very small footprint on the ground surface and can be insulated with the addition of a “thermal wrap” under the domes and perimeter insulation. https://cupolex.co.nz/
● RibRaft Xpod 215/1500 in combination with Hotedge. This is a similar Italian system to Cupolex. Note the Hotedge only provides partial edge insulation in that it doesn’t go the full height of the perimeter.
● Quickset provide perimeter insulation in the form of a permanent insulated formwork that can be used with a range of foundation products and options. https://quickset.co.nz/#home
TC2
● MAXRaft has a 320/400mm deep fully insulated slab.
● Cupolex has a 260mm recycled plastic dome that has a very small footprint on the ground surface and can be insulated with the addition of a “thermal wrap” under the domes along with perimeter insulation.
● RibRaft Xpod 215/750 in combination with Hotedge. This is a similar Italian system to Cupolex. Note the Hotedge only provides partial edge insulation in that it doesn’t go full height.
● Quickset provide perimeter insulation in the form of a permanent insulated formwork that can be used with a range of foundation products and options.
● ABI Piers make a “base isolation” or self-repositioning pile foundation system that may mitigate damage to lightweight buildings during large earthquakes. www.abipiers.com/demo
TC3
● The Armadillo Foundation System is one proprietary system that is being used in Christchurch in TC3 ground. It comes from the same stable as Xpod but has jacking pads around the perimeter of the foundation to allow it to be relevelled following a seismic event.
● Another proprietary system is Ribraft TC3. This has a concrete underslab with a waffle slab containing rattle gun operated jacking points that allow the house to be relevelled following a seismic event.
● TTT make a timber raft foundation system as an alternative to concrete.
● ABI Piers’ adjustable steel pier system can make construction and future adjustment easier.
Recommended Healthy Home guidelines for foundation design are set out in the table below:
*A floor level survey (FLS) is required to be carried out following the floor being laid. This will be part of the Home User Guide (HUG) and is important for quality control and for setting the baseline should the foundation be subject to a future seismic or settlement event.
FLOORING / Floor Coverings
Concrete slabs can have a cold feel, even when insulated to bare feet so floor coverings should be chosen in regards to the comfort you want to achieve. Carpet with a high quality underlay is one option. An alternative and cost effective way to keep your feet warm is to use cork plank flooring. The cork is a great insulator and the modern appearance of cork planks will suit most designs. Cork planks can also be installed in kitchen, laundry and bathrooms as long as these are not wet areas.
If installing underfloor heating, care is needed in choosing your underlay for carpet and laminate /timber floors as it could insulate the floor and reduce the effectiveness of the heating. For underfloor heating systems, polished concrete or tiles are the most effective solutions to maximise the warm feel effect.
Note that leaving some concrete exposed or installing tiles, near north facing doors, for example will enable some solar gain.
