The process of strengthening the foundation of an existing building or other structure is called underpinning. Usually this involves extending the foundation depth or distributing the load across a greater area. Underpinning is a repair project and in order to choose the correct method, it is necessary to understand the current situation and problems concerning the entire foundation. Underpinning can be carried out for a variety of reasons, the most common of which are:
- A change in soil conditions under the foundation
- Soil characteristics were not properly taken into account during the design phase
- The original foundation is no longer suitable/strong enough
- The construction/repair/excavation of neighbouring structures endangers the current foundations
- Additional floors are to be constructed underground
Typical methods of underpinning
Mass concrete underpinning
A traditional method of underpinning is soil replacement under the foundations by digging, followed by concrete pouring. The soil is often replaced in box-shaped parts in a predetermined sequence so that the bearing capacity of the current foundation is not compromised. The result is a foundation built underneath the existing one. This method can be used for relatively shallow foundations, and the current structure needs to have sufficient strength to span over the box-shaped underpinning pits.
A piled underpinning is generally used when firm soil layers are situated at substantial depths and the superficial soil is inadequate to bear the loads. Other influential factors favouring piling may be limited access to the construction site, environmental concerns regarding excavated soil or minimal structural movements during the underpinning process. The piles are usually steel-cased and can be filled with concrete. The piles may be divided into three categories by installation technique: driven piles, drilled piles and jacked piles. Piling methods require load transfer structures to transfer loads from the existing foundations to the piles.
The creation of a geometry of grouted soil by using a high velocity jet of cement grout (with or without water/air) is called jet grouting. Regarding underpinning, jet grouting is typically used to create columns of grouted soil underneath an existing foundation. The end result depends on soil characteristics, especially erodibility, but generally jet grouting is effective across a wide range of soil types. Load transfer structures are not needed, because in many cases the columns can be extended to the existing foundation. Because of the required drilling and high pressure used, jet grouting always causes a soil disturbance, and may not be viable for very sensitive foundations. The construction site must also be accessible to heavy equipment.
Over the years, geo-polymers have been used for a number of geotechnical applications, such as the stabilising and re-levelling of floors, compaction of granular soils or to increase the bearing capacity of foundation soils for existing structures. Geo-polymers are based on multi-component structural, expanding resins and they are environmentally neutral and water-resistant. The technique is based on chemical expansion pressure, instead of heavy pressurised hydraulic systems, which makes the application more controllable and safe. Comparing to traditional cementitious materials, geo-polymers have an effective and structurally strength curing process. They achieve 90% of the final compressive strength in less than 15 min. Most of the projects can be completed in days in cases where traditional ground repair methods would require weeks or even months to finish.
Unlike traditional underpinning techniques, geo-polymer solutions are excavation free, utilising small and light installation machinery, resulting in fast and non-disruptive solutions for commercial, industrial, residential and public buildings, as well as infrastructure and historic structures.
Geobear is specialized in geo-polymer injections. Please contact us for further information.