I've recently completed a site investigation report. I've done this in preparation for a potential knockdown rebuild project.
As I'm not the brightest kid on the block nor an engineer, I was hoping that experts on this forum could help. I'm looking mainly looking to verify if the recommendations in the document are sound.
The current property has (and still is) giving us loads of problem due to highly reactive soil and potentially inadequately laid foundations. Should we proceed with this KDR, I do not want a repeat of this again.
With that said, I've included excerpts from the document. Thank you in advance for your time.
- Place three boreholes over the proposed building area.
- Conduct limited appropriate soil testing.
- Classify the site in accordance with AS 2870.2011 - Section 2 and Clause 2.2.2. 4.
- Provide footing recommendations as appropriate for this site and the proposed construction as outlined by the client at the date of this report.
Field Testing Method:
- Boreholes were constructed using a mechanical/hand auger.
- Representative insitu shear strength tests were conducted in any clay soils found at the site
- Over the likely area of the proposed building works the site fall is approximately 400mm.
Site Classification (in accordance with AS 2870 - 2011 - Section 2 and Clause 2.1.3)
- After considering the area geology, soil profile, the results of hand classification tests, the building superstructure and AS 2870 this site has been classified as CLASS P, due to the presence of buildings and trees and fill.
- Note that the above classifications may have to be reconsidered if any proposed earthworks result in either a site cut over 500 mm or site filling over 400 mm in depth.
- The use of a pier and beam system of footings is recommended in preference to stiffened footings.
- Where deepened footings are adopted due to the presence of abnormal moisture conditions the use of a pier and beam system is recommended.
- The piers must be founded at least 200 mm into the natural clay soil and at a minimum depth of 3000mm.
- Based on the measured shear strengths in the clay soil an allowable bearing pressure of at least 200 kPa will exist beneath these piers.
Concrete Floor System
- The following conventional CLASS H1 TYPE slab on ground is recommended
- Due to the reactive nature of the soil at this site, the connections of the building sewerage lines at the slab/soil interface must be designed to be movement tolerant. Also NO surface or soil moisture must pond against the slab edges. This normally requires the ground surface around the slab to be graded away from the building for the first 1.5 to 2.0 metres and a minimum slab freeboard of 230 mm.
- The edge beams (plus any major load bearing internal beams) ONLY must be founded on the natural clay soil indicated on the site investigation logs. Based on the measured shear strengths in the natural clay soil and after allowing for wetting up beneath the slab, an allowable bearing pressure of at least 100 kPa will exist beneath the slab beams founded on the natural clay.
- The slab as recommended above, is designed to act as a whole unit so that it can tolerate the differential soil movements that may occur in the basaltic clay that exists at this site. There will be no significant increase in slab performance by increasing the beam depths because of the solid brick sections of the building. In fact the slab's performance may be impaired by locally deepening the slab beams in these areas.
- As solid brick walls are more susceptible to cracking it is recommended that construction joints be placed in them to form straight sections of wall no greater than 3.0 metres in length.
- Up to 600 mm of granular levelling fill or 300 mm of site derived filling may be placed under the slab panels and/or non load bearing internal beams provided this filling is placed in accordance with clause 6.4.2 of AS 2870. Based on the likely condition of this levelling filling after having been placed in accordance with the above recommendations and after allowing for wetting up beneath the slab a conservative allowable bearing pressure of at least 50 kPa will exist beneath the slab panels and/or internal beams founded in this filling.
- Over the building area where the total depth of filling exceeds 600 mm the slab panels must be designed as fully suspended and supported by a grid of deepened beams founded through the levelling filling in accordance with the above edge beam recommendations.
- If shallow uncompacted surface filling is found to exist over parts of the site the internal beams and panels ONLY may be founded in this filling material, provided that all soil with significant organic matter is removed prior to slab construction. An allowable bearing pressure of 30 kPa may then be assumed in this shallow filling. If a higher bearing pressure is required the shallow fill must be compacted in accordance with clause 3.1.5 above
I can include pictures of the site logs later.