Property:Development:Construction:23.Foundations
Summary
Concrete slab floors are typically slower than elevated wood floors or basements1) – so there is are some economic reasons to prefer a flat house pad, as opposed to hillsides that may need pilings and/or retaining. But there is also the reality that cables have to be known and laid early, as any change means the floor has to be broken up and relaid. Ouch.
Notes
Terms
* deep foundations:
pile foundations: a type of deep friction or bearing based foundation.- Used when:
- there is no firm strata to bear on (therefore use of friction piles)
- there is firm strata, but a depth that a strip or spread fooding is uneconomical.
- pumping of subsoil would be uneconomical.
- Types:
friction piles: transfer loads down via friction, using wood, steel or concrete columns.end bearing piles: transfer loads down, using wood, steel or concrete columns.combined friction/bearing piles: self-describing.micro-piles: used where driving is not feasible. A hole is bored, into which the micropile is grouted into place.screw/helical piles: sacrificial foundations made of steel that are screwed into the ground, without needing concrete, or causing much/any vibration to surrounding structures.batten/sheet piles: sheets of metal driven into the earth
pier foundations: a type of deep bearing foundation, but shallower than piles, using large diameter columns to transfer large loads to firm strata below. Generally shallow (as oppossed to pile foundations). Used when:- the firm strata is covered by decomposed rock, making driving difficult/uneconomical.
- stiff clay, making driving difficult/uneconomical.
* shallow foundations:
spread footing foundations: created under the frost level, under the perimeter walls and inner columns, with a wider bottom portion than the load-bearing foundation walls/columns they support (“spreading” the load over more area).mat-slab (or raft) foundations: Mat-slab foundations distribute heavy column and wall loads across the entire building area, to lower the contact pressure compared to conventional spread footings. Not common in residential, but appropriate if the soil is uniformly poor(excluding peat and organics).Slab on grade: a type of shallow foundation, where the concrete is directly poured onto graded soil, within a formed perimeter. Used where ground freezing and thawing is less of a concern and where there is no need for heat ducting underneath the floor. Although elevated slabs perform better, it's generally considered acceptable to use slab on grade over clay, and the best cost/benefit ratio for low-cost/tract housing, and generally avoided for better quality homes due to difficulties changing/adding/fixing things later.- Types:
monolithic slab on grade: when the perimeter wall/column footings and slab are poured at the same time.floating slab on grade: when the perimeter wall/column footing are poured first, and the slab second.
pad foundations:
* Insulated Concrete Forms (ICF)5): insulation + forming all in one. More expensive than traditional forming, but also provides insulation + vapour barrier + longer/stronger curing.
screed: the tool that is used to level and true a maerial like concrete.Screed coat: the material itself (eg: concrete) or a secondary thin, top layer of material (sand and cement, magnesite or calcium sulphate)) applied over underlying structural concrete or insulation, which has been flattened with a screed [tool], on top of which other finishing materials can be applied, or it can be left bare to achieve a raw effect.Gypcrete: a lightweight concrete for applying as a screed coat for leveling floors before carpet, etc. Lighter, but brittle. Note: not the same as light weight concrete.Light weight concrete: a lighter version off concrete (about 2/3rd the weight).
Recommendations
If the ground is level, and the work is undertaken by professional builders, the most economical solution is a poured slab foundation. Otherwise, consider pole based construction so that the work can be undertaken by the same builder as being used for the structure.
Options
- Concrete Slab:
- Advantages:
- Cheaper than wood platform or basements.
- Disadvantages:
- Have to (over) plan plumbing/electrical conduits early (there's no turning back) which is hard to do for the lifespan of the building (50+years).
- If the Damp Proof Channel (DPC) is compromised the concrete will bring up dampness and mold will set into the wood.
- Wood platform:
- Advantages:
- Can change plumbing/electrical needs more easily over the lifespan of the house (50+years).
- Disadvantages:
- Cost more than slabs
- Take longer than concrete slabs.
- Basements:
- Advantages:
- Disadvantages:
- More expensive than slabs.
- Discussions of pros and cons:
Considerations
- TBD
Specifications
If pouring a foundation, remember to consider: * Sandbox for draining tubs that can be post-finished (allows for location adjustment) 9) * Don't forget drainage of AC unit!10) * Kitchen:
- cold water supply for fridge (icecubes)
- hot/cold for sink
- sink drain
* Bathroom
- hot/cost for sink
- hot/cold for shower
- hot/cold for bath
- floor drain
- sink drain
- shower drain
- bath drain
Suppliers
- Cupolex in Wellington:
- Forms:
- https://nzfoundations.co.nz/ ← like their use of “ProBrace” (although not interested in ribrafts).
- MaxRaft is like ribraft, but has a thermal break on all sides. Which is more complete in terms of insulation (although back to more shipping, same as ribraft).
- WOW!:
Tools
DIY'ers should consider the following:
- TBD.
Testing
- Soil is tested to a depth of 2 m for strip or pile foundations or 600 mm below driven timber piles11).
- When searching for indications of “good ground” suitable for foundations, check12):
- foundations of adjacent buildings show no signs of settling or inadequate bearing strength
- no evidence of landslides in the vicinity
- no evidence of buried services
- no organic soil, peat or soft clay (soil is not form-able by hand, or smears on hand).
Materials
- Consider:
- Damp proof liquid application: http://supercoat.co.nz/products/waterproofing.html
Details
- IN NZ:
- Hardfill, Damp Proof Membrane (DPM), Hard insulation, reinforced 10mm concrete pad17).
- Good detail diagram
- Timber/thermal break might be sufficient, compared to external cladding source
Costs
- Slab on Grade is less expensive than crawl-space18) approach (both use a spread footing approach to bearing walls and columns).
- TBD.
- Make sure to understand that coordination of electrical and plumbing for kitchen, bathrooms and utility/laundry rooms has to be done up front in order to not have to make changes later.
Specifications
- Concrete on Grade:
- Insulate between ground and slap to keep out humidity and keep the thermal mass efficient.
- Consider Insulating 1m out from the slab: by grading away from the slab, putting down 10cm of extruded foam insulation, then putting down 20cm of soil.
Process
- Slab on grade:
- prerequisites are well-packed soil, proper drainage, insulation, vapour barrier, and a lot of mechanical infrastructure before pouring.
- laying out electrical early requires earlier coordination, but intelligently laying out electrical and plumbing early, on a flat surface, can decrease cost from drilling hundreds of holes through joists (but probably no gain versus pre-channelled SIP panels).
Foundations
Some common choices19) are:
- Pile options:
- Timber Piling:
- Advantages:
- Often the cheapest solution
- Considerations:
- Wood doesn't last as long as Screwpiles.
- Not suitable for shallow foundations.
- Screwpiles20)
- Bore piling:
- Considerations:
- Good for sand, etc. environments.
- Costly.
- Disadvantages:
- Costly and needs multiple skills (boring, steel, concrete pouring).
- Sheet piling:
- Considerations:
- Consider for holding up sand/etc.