This is a wall used to stabilize the moisture content of the soil under and around the foundation of your house.
A trench is dug 1 m deep around your house 1 to 2m away from your footings. This trench is designed as a barrier to stop the lateral movement of moisture through this barrier and so stabilizes the moisture content of the soil year around. This barrier also serves as a root barrier so inhibiting matric suction of moisture by any plant roots as well as slab upheaval from roots growing under. Roots can grow to 1.5 – 2 times in length compared to mature tree height.
The construction of the cut off wall can be made from a waterproof poly vinyl fabric as a stand alone or a concrete and polyvinyl sandwich construction.
It can eliminate problems of abnormal circumstances before,during or after construction such as
Preformed fabric allows for water stop and drain. Inserted into walls.
The value of a moisture cut off wall becomes very evident where you have clay reactive soils. High variance in volumetric change with moisture change.
In a sandy soil you don’t get as much volumetric change in particle with moisture variance but if there is excessive free water you will experience sand particle wash out. A moisture cut off wall handles this issue as well. Refer AS 2870/2011 for soil classification information.
The CSIRO has traditionally advised that house footing be protected from moisture fluctuations by abutting concrete paving against the homes sub walls to stop the ingress of water to soil supporting the foundations. This is good advice as surface water, directly from the rain or overflowing gutters is inhibited from penetration into soil due to ponding in proximity to the foundations. How ever this surface paving does not stop roots from growing under foundations or stop water flowing laterally through the soil. Depending on the location of your block this could be a virtually nil occurrence or an ongoing regular event.
Unfortunately, most home owners don’t wise up to these issues. In addition to requiring a moisture cut off wall, footings will need to be underpinned and wall cracks repaired, floors levelled etc.Preventative maintenance is much cheaper then correction after the walls are cracking.
1 mm permathene sheet 900mm deep x 100mm wide backfilled with blue metal $400 – $500 per Linear meter + any services location/ redirection.