GeoZone - Applied Geology & Geotechnics

Percolation Tests

Many property developments still rely on septic tanks and French drains (soakaways) for disposal of effluent. A septic tank collects the effluent from a house and 'processes the sewerage via a two chamber system and the activity of bacteria to break down the solid wastes. The processed water which exits from the septic tank is then directed into a gravel filled soakaway and is allowed to percolate into the surrounding soil substrate.

It is important therefore to determine whether a soil is suitable for the disposal of effluent via percolation methods. In addition a site must have sufficient surface area to process the anticipated volumes of treated sewerage water. To elaborate, any water percolating into the ground via the French drain is further processed and removed from the system via evaporation and transpiration i.e., the activity of plants. In this light then an area downslope of the French drain is required to process the water and this area needs to be vegetated and not developed in anyway over the lifespan of the sewerage disposal system.

Effluent entering the ground has the potential of becoming a nuisance to downslope neighbours. It can flow along the soil-bedrock interface and seep out of cut slopes further down hill or contaminate groundwater resources and boreholes. Clearly then, when siting, designing and building a septic tank system some precautions are necessary. The standards are set out in the National Home Builders Registration Council (NHBRC) manuals, and these must be strictly adhered to. GeoZone is well placed to conduct percolation tests, calculate percolation rates and determine the amount of effluent which may be introduced into the ground in a 24 hour period. We are also able to advise on the size of evapotranspiration areas and the design of septic tank systems.

GeoZone - Applied Geology & Geotechnics