Also known as polyurethane underpinning is the process of injecting a 2-part resin under the affected area of the slab, causing a rapid expansion which fills any empty spaces (or voids) in the soil. This strengthens the soil and creates a strong base which ensures that the property will no longer sink. Using this solid base, more resin is then injected on top causing the property to lift back into the original position and returning it to a near new state.

Traditional concrete underpinning vs polyurethane underpinning

Traditional underpinning services use concrete and steel to reinforce the area under the slab. To achieve this, excavation is required. This creates a big mess, potential damage to the site by the excavation equipment and can take a long time to complete.

Unlike the methods used by most underpinning Melbourne companies, underpinning using chemicals such as Polyurethane is a less-invasive method and achieves a stronger outcome. 16mm holes are drilled in the ground under the slab for the resin to be injected having no contact with the existing footings. This process is cleaner, quicker and can be done in a considerably shorter period of time.

• Founding in Reactive Clay Soil:

  Clay soils has the ability to absorb or dispel moisture. In a wet environment, clay will absorb moisture increasing its plasticity and expanding. This causes heave in footings and structures. In contrast, in hot and dry environments, it becomes hard, brittle and non-plastic shrinking in the process. During periods of prolonged drought, the clay shrinks to the extent that the structure supported by it subsides significantly. Any other cause for loss of moisture in clay will cause subsidence and settlement, this includes trees and vegetation, or the construction of a new structure which affects the impermeability of soil beside structure.

•  Flooding and water erosion:

  Poor site drainage can lead to water ponding against and seeping below a concrete structure, be it a house or a warehouse slab. The water weakens soil and washes away the silt/sands from silty and sandy clays. This leads to voids and weakening of soil causing subsidence. Cracked or leaking pipes (drainage or sewer) is another cause of poor site drainage. Therefore, we recommend regular checks of all pipes for any signs of leak.

• Undermining or due to adjoining footing construction:

  If the adjoining land to your property is vacant, and during slab preparation your footing is undermined, that is the adjoining cut is below the bottom of your footing, this may loosen soil and cause subsidence.

• Load exceeds concrete strength and soil bearing capacity:

  A 150mm thick concrete slab can typically support a weight of 860kg/m2. However, the soil supporting the slab can have less support weight/bearing capacity. Once a structure or another type of load, such as a heavy vehicle, overloads this limit, the concrete begins to weaken and cracks can begin to form and soil underneath starts to settle. This creates openings for moisture to enter; weakening the underlying soil. A crack that is 5mm wide has the potential to reduce the weight capacity by up to half of its initial strength.

• Poor Workmanship and inadequate founding:

  Waffle slab footing (edge and internal beams can be supported on up to 300mmof ‘controlled fill’. The controlled fill has a strict method of laying, and is required to be of clayey materials with 97% of moisture. We recommend that all edge ribs are trenched through this fill and founded into natural clay. However, when the ribs are founded on fill, if it hasn’t been prepared as per requirements set out inAS2870-Residential Slabs and Footings, then the fill will settle over time and the slab will subside and show signs of distress.g you peace of mind