pls help with geotech report

Hi everyone,

just looking for some input to ease my mind while i wait for titles to be issued and also for builder's site cost to come back.

some info i have are the next door neighbor's site costs:

earthworks -PS 25k
engineers structural req for site classfication - PS 10k
retaining wall -8k

i got the geotech report today and below are some excerpts from it. to be honest this is my 1st ever project and i am still learning. all i know is Class A is suppose to be a good thing (especially if area is known to have lots of clay)

can anyone advise if based on the info below i would:
1. possibly less earthworks costs because class A ?
2. still need retaining wall if little slope.
3. any good news that you can see from the report and what it means?

The current land use of the lot is residential. The rear of the property is currently a grassed garden and also contains a small shed and some minor areas of hardstand. The site very gently slopes towards the north west but is almost flat in all directions. The site sits at approximately 9.5mAHD.

Field investigations took place on the 17/07/2014 with the hand augering of five holes to a depth of 2.5m or until refusal. The site has a typical soil profile consisting of sands to depths between 200-1500mm over clayey sands and sandy clays of the Guildford formation.

Groundwater was encountered in all excavations between 1300-1500mm below the surface. Using a constant head permeameter, the in-situ saturated field hydraulic conductivity, Ks, for the natural soil approximately 800mm below the surface was measured to be 26.2mm/hr.

For the reasons stated above and within this report the site classification is "Class A".

The minimum cover to reactive soils is to be 1000mm measured from the base of the slab ("Class A").

Thanks for your help and i am eagerly waiting to learn more.
How big is your block and how big is the footprint of your house ?

My rule of thumb is $15m2 for removing soil and vegetation
$25m3 for import / export of sand
Retaining is hard to quantify, type depth etc..


Class A usually means you have a site underlain by natural sand (in Perth) which is a non reactive material. The other classifications relate to clayey materials and range from S (slightly reactive) to E (extremely reactive). These materials shrink and swell with change in moisture between the seasons and this movement can cause damage to buildings if the foundations are not adequate. If you have a reactive clay site (like out near near the hills) you might have to remove the top 1m or so of soil and replace with sand or use more expensive pile foundations to prevent movement of your building. If your report says you are Class A then you shouldnt have to do these things and therefore I would expect your earthworks/foundation costs to be cheaper.

If your site is flat you should not need retaining walls, the geotech reports often just puts these comments in there generically in case you do need a wall installed.

The groundwater comments are useful when the engineer/builder is deciding how big your stormwater pit/soakwell needs to be.

Good Luck