Earthworks Question

Hello everyone,

Quick question regarding earthworks on a single dwelling development site in Armadale. (clay :/)

The geotech advised removing 200mm from the highest point on the site, back filling to level with clean fill then 300mm build up of clean fill achieving an overall 500mm of clean-fill for the slab. The geotechnical engineer advised to save money i can just build up the 300mm for the perimeter of the slab? or do i need to do the whole site?

Extract from report:
"Please see below my response to the queries of earthworkers.

The site is overlain with Mixed SAND FILL (not clean SAND) up to 0.70m. Below the mixed fill is a reactive sandy CLAY. Class S footing is suitable for the proposed residence with 1000mm MIN clean SAND pad above the natural sandy CLAY materials. Sand pad is required to control/limit the ground surface movements and for drainage purposes. Below is the detailed sitework preparation. I suggest that you hire an experienced earthwork contractor to carry out the sitework.

·Establish a datum (highest point) on the building site.

·Based on the datum, remove all unsuitable materials up to 200mm within the building site (slab area). These are topsoil, loose fill, gravels, builders rubbles, roots and organic materials. However, the unsuitable materials can go up to 300 or more in some local areas, and these materials should also be removed. If the earthwork contractor has no experience in this kind of work, then I suggest that we should inspect the site if further materials are to be removed or not.

·Compact the excavated base surface by using plate compactor.

·Backfill with clean SAND up to final ground level. The final ground level should be300mm to 400mm above the datum or existing ground level. This means that the SAND pad is at least 1000mm above the natural sandy CLAY.

·If you decide not to raise the existing ground level by 300 to 400mm, then you request the structural engineer to upgrade the footing to Class M that can accommodate ground surface movement up to 30mm.

·Finally, the completed sitework should be inspected by and certified by our engineer."

I don't understand how this will work with no retaining walls?

If anyone has experience with these clay sites please share

Thank you

 

You can do the slab area but you won't just stop at the perimeter edge, you'll lose that 300-400mm by sloping down to existing. Depending on the the site will depend how you slope the ground around the slab.

 

Whats the slab area? Have you done some costs for the geotech recommendations? As opposed to just getting a slab engineered?

 

That is an interesting engineering report.

An M class slab design would likely be the easier option in my opinion.

 

I think the 300-400mm difference at the perimeter would be made up of the edge thickening (i.e. deeper than the rest of the slab to support the perimeter walls) and slab rebate that is typical to most brick veneer houses.