We are building a new house in Tasmania. I have had a small concern since we purchased the block of land because there was a natural drain running through it. I had a Geo-technical Engineer look at the land before I purchased it; he didn't do a full assessment but said it should be fine to build on. That was no issue with me because I'm not too afraid of extra slab costs, providing they are not exorbitant (i.e. more than $8000 extra, say).
So that's all fine and we decide to be owner builders. Obviously one of the things we need to do before we build is get the soil assessment done, which we did, and got the results yesterday. The soil classification came back as Class P, which to me sounds bad.
This leave me a few concerns that I hope someone can give me some assistance with;
- Is it going to cost us an exorbitant amount in extra slab costs?
- Is there extra risk of the house moving, cracking etc. due to the classification (even though it will be built to specifications)?
- Taking into consideration the above, are we taking too big of a risk being owner builders considering we would be liable for 10 years under Tasmanian laws?
- If the slab was built to specifications and the house moved, cracked etc, is the engineer liable?
Thanks for any assistance!
For reference I have OCR'd the soil assessment report below.
It is considered that the extent of the site investigation undertaken has been sufficient to correlate the
observed soil conditions with the known geology and published information for this area. Reference has
been made to the Mines Department of Tasmania Geological Maps as well as soil testing previously carried
out on nearby sites.
Localised and artificial variations in ground conditions (e.g. back-filled Wells, cellars, etc.) are Very difficult
to locate using bore-holes and test pits. In addition, natural soils can vary greatly over short distances.
Attention is therefore drawn to the importance of site inspections during the earthworks, in order that the true
site conditions are Verified.
It is also important to note that, in conjunction with the submission of building plans to the relevant
Municipal Council, the Building Code of Australia requires a complete drainage system to be installed (refer
also to AS2870 “Residential Slabs and Footings - Construction”). The location of plumbing trenches, the
specification of trench back-filling in the vicinity of the footing area and paving details should also be given.
The plan should also show details of surface grades on the uphill side of the building and the proposed
methods of supporting the face of permanent excavations more than 0.6 In deep (Reference AS 2870, Section
6.) It is preferable for these aspects to be carried out by the building designer.
Description of Site
The site is located on the flatter portion of a large variably sloping allotment in a rural residential building
estate.
The site has not been developed with structures, but many trees originally present in the vicinity of the
proposed residence had been removed.
Vegetation at the site consists of sparse native grasses, ferns and weeds. The site is sheltered by the
surrounding hills and tall forest.
The site appears to have been partially filled in the recent past without removing the existing topsoil.
Water was in pools near the centre of the allotment with moisture present throughout the soil profile.
Wet areas were visible emanating from the adjacent property indicating some sub-surface travel of the
moisture. Drainage appears poor and landscaping should take into account the necessity to remove excess
ground-water fiom the site.
Soil Assessment
The site contains a small amount of fill overlying the original topsoil material. The fill extends to a nominal
depth of 0.25m to 0.30 In generally. The topsoil consists of dark grey moist to wet silty sandy loam of low to
moderate plasticity with vegetable matter. The natural soil profile at the site consists of moist dark grey silty
clay transitioning to brown, grey and yellow moist clayey silt of moderate plasticity. The material was firm
in-situ at depth. The materials were moist in situ with no free water ingress into the test pits.
The overall profile may be classified as Class " P " ir1 accordance with AS 2870 “Residential Slabs and
Footings Code ~ Construction” due to the unconsolidated fll material on site and the moisture throughout
the soil profile
Land Stability
The site is considered to be stable, based on visual observations of the present conditions, reference to the
Tasmanian Geological maps and consideration of the proposed future development. If substantial changes in
the present landforms or drainage patterns occur within or within the vicinity of the site (e.g. significant
excavation or deposition of materials) a further assessment of the land stability should be undertaken.
Footing Construction Recommendations - General
Stripping of Site
Prior to construction, the topsoil zone containing roots and other organic matter must be stripped and
removed fiom the foundation area. The surface should then be subjected to rolling; in order to increase the
density of the upper layers.
Filling of Site
Surface filling at the site should be placed in layers and compacted to a maximum thickness of 150 mm to
achieve a density of at least 92 percent modified compaction. Non-engineered and unsuitable fill cannot be
used as a foundation material and footings must be founded on the brown, grey and yellow clayey silt of
the natural profile. Concrete floor slabs must be designed to suspend between the footing beams where the
depth of non-engineered fill exceeds 300 mm.
Allowable Bearing Pressures
The allowable bearing pressure under edge beams, strip and pad footings founded on the undisturbed
material noted above is 80kPa. Earthworks should be carried out in accordance with the requirements of
AS 3798 - 1996, Earthworks for Residential and Commercial Developments.
Articulation
Articulation of the superstructure implies that the masonry walls are separated into panels by full-height
control joints that allow some movement between adjacent panels. There should be an adequate number of
these joints in all internal and external masonry walls. The recommended positioning of control joints is
shown in Technical Note TN6l available from the Cement and Concrete Association of Australia, which
addresses some of the more detailed aspects of articulated construction.
Where articulation should be provided, the required locations have been shown on the slab and footing plans.
The articulation reduces the length of the overall wall panel and the locations shown should not be altered
without reference to this office. Articulation joints should extend through the full height of the wall panel.
Site Drainage and the Effect of Trees
The plans should show how stormwater is to be disposed of In particular, roof water should be carried well
clear of footings, and perimeter paving approximately 1 In wide should be graded at a slope of approximately
1 in 20 away from the edge of the building.
Ensure protection of the footing area fiom excessive wetting and drying as outlined in the “General Notes on
Footings”. Large trees and bushes should be planted a suitable distance from the structures. (Reference AS
2870 “Residential Slabs and Footings - Construction”)
Where trees have to be removed within 5oom of the slab or footings, the stumps and roots are to be
grubbed out to a depth of 6oomm below the finished surface and all disturbed ground back-filled with
compacted granular material. Placement shall be carried out in layers and compacted in accordance with
requirements for filling of the site.