Earthworks

Earthworks is the processing of ground matter to stabilise it sufficiently enough for your development.

Council requires what is known as Level One Earthworks Compaction supervised by a Geotechnical Engineer to ensure structural stability of the ground to accommodate a residential house slab. The Geotechnical engineer prepares a Report detailing the ground conditions, procedure & methodology of earthworks undertaken along with undertaking & preparing test results & certification to confirm your block has the right soil classification for Council’s approval for Plan Sealing & provision of title deeds.

The procedure is such that after existing house &/or shrubs demolition & removal has taken place, the site is stripped of all topsoil & organic matter which is stockpiled on site to the level of natural ground beneath. All roots, inorganic matter, rocks, rubbish, debris, construction & demolition waste etc are excavated & exported off site in trucks with a large excavator using a sifting bucket & manual extraction of unwanted material found in the ground. A 5ton excavator is sufficient for a small block up to about 450m2 after which a 13ton or preferably a 20ton excavator are far more efficient in undertaking this bulk earthworks.

90% of the time a large amount of matter under the ground needs to be exported as there are numerous & dense roots, ultra stubborn stumps of trees & vegetative matter to be extracted from the ground. Often whole septic tanks are found underground which the demolition contractors invariably miss exporting. And then there is the constant issue of rocks hidden beneath the surface.

Bedrock & shale can be found quite close to the surface especially in hilly areas. This slows down excavation significantly & though some rocks can be crushed by a roller for spreading across the site in areas of fill, a lot of it has to be exported as it cannot be re-used. Rocks can only be re-used in a crushed state & smaller rocks uncrushed can be no larger than a third of each layer of compaction with layer thicknesses stipulated under the AS 3798 Guidelines on Earthworks for Commercial and Residential Developments. Generally rocks beyond 150-300mm in size are unsuitable especially for earthworks within a metre of finished pad level.


The geotechnical engineer requires undertakes compaction tests confirming stability of ground to various levels of compaction specified by the Civil Engineer, generally 95% of standard compaction.

The Geotechnical Engineer starts with a Site Strip Inspection after the Civil Contractor has stripped the site of the above mentioned topsoil & exposed the natural ground beneath. In addition to the above, there may be what is known as “uncontrolled fill” where imported soil has been previously imported. All of this soil must be removed to expose the natural ground below. This must be compacted with a 6t roller & then re-mixed & spread with an excavator & compact again potentially with good quality imported fill in layers generally 300-400mm thick. There is a vibrator function on the roller which literally vibrates the ground beneath and minimises air locks, dissipates moisture & “soft spots” which are hard to compact, crushes the lighter, smaller, more fragile rocks enabling us to achieve the minimum compaction required.

As with the excavator’s bigger roller’s such as a 12 or 13ton Padfoot Roller mean significantly more efficient earthworks. Maximum layer thicknesses of fill can be compacted faster, significantly more rocks and particularly from shale found beneath a lot of Brisbane soils can be crushed & become excellent fill for compaction when mixed with other good quality material.

By the time pad levels are achieved, a lot of tests are taken confirming compaction results as per AS 3798, 1 test per 400mm layer for every 450m2 of lot. A final test is taken per block, possibly in a few different spots for larger blocks, to finalise the Level One Earthworks Report the Geotechnical Engineer compiles & then a shallow 100mm layer of topsoil is spread across the site.

Should tests fail, in certain spots, those locations are to be re-exposed & re-worked as per the discretion of the Geotechnical Engineer till satisfactory test results are achieved.

Some of the pictures & videos below show earthworks in action on some of our sites. It is a process that requires brilliant expertise & excellent logistical management for good results, something we master & love doing at Pembroke Construction.