Appendix B - Geotechnical Investigation Report

6.7   Floor Slabs
We understand that a timber frame floor slab is proposed and will be entirely suspended, including the
proposed later extension although the car port will comprise a concrete slab on ground.

Whilst slab-on-grade construction is theoretically feasible for the car port floor slab, it will be difficult to
complete high quality earthworks on a small site such as this, as it would require removing and re-compacting
any existing poorly compacted fill. Also, on-grade floor slabs in contact with varying thickness of foundation
soils would result in differential deflections, and potential cracking of the car port floor slab. We recommend
that the car port floor slab also be fully suspended over the soil profile between footings founded in bedrock.

For floor slabs suspended over soil subgrade areas, unless sacrificial formwork is used (such as ‘Bondek’) the
subgrade preparation would comprise the removal of any topsoil and/or any soil containing organics,
completion of and re-profiling of site surface levels and the nominal tracking of ‘formwork fill’ to the required
subgrade level. We forewarn that even this nominal subgrade preparation will be difficult on this sloping site
and the use of sacrificial formwork is strongly recommended. However, void formers will be required if the
slabs are in contact with the reactive soils. Design of the void formers should be to accommodate maximum
shrink swell movements of about 30mm to 40mm below the slab and beams between the piles so that the
swell pressures from the clays are not transferred to the slab.

The car port on-grade floor slab, if preferred, should be provided with joints capable of resisting shear forces
but not bending moments by providing dowels or keys. The on-grade car port floor slab should be designed
as ‘floating’ i.e. separated from all footings and walls in order to permit relative movement.

The future on grade car port slab will be supporting traffic loads and should be provided with at least a
100mm thick subbase layer of unbound or equivalent good quality crushed rock (e.g. ‘Type 2’ material in
accordance with the latest revision of TMR Technical Specification MRTS05, compacted in a single layer using
a large smooth drum roller to at least 98% of Standard Maximum Dry Density (SMDD). A bond breaker should
be provided between the concrete base and the sub-base. Moisture conditioning to within 2% of Standard
Optimum Moisture Content (SOMC) should be provided during placement so as to reduce the potential for
material breakdown. The subbase will provide more uniform slab support and protect areas of prepared clay
subgrade from deterioration. The subbase will also reduce the “pumping” of “fines” at joints.

Surface drains should be provided along the perimeter of the car port floor slab, with inverts not less than
0.2m below design subgrade level. The drains should be excavated with a longitudinal fall to appropriate
discharge points so as to reduce the risk of water ponding. The car port subgrade should be graded to
promote water flow to the drains.



6.8   Earthworks
For on-grade construction of the car port floor slab, we reiterate that it will be difficult to complete high
quality earthworks over such small site areas. Even if the earthworks are completed in accordance with the
following recommendations, it is likely there will be a significant variation in subgrade conditions, resulting

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