A hard foundation laid below ground level to support, strengthen, stabilize and renovate an existing Building or any other Structures is called Underpinning.
Methods And Construction Techniques
The Rehabilitation of an existing building, mostly 40 to 50 years old, motivated by a change of use or structural damage, which may be a consequence of insufficient Soil Bearing Capacity, may require an underpinning project.
This type of work requires skilled labour, not only constructors, but also in the planning stage, since there is not an universal solution applicable to all cases. In fact, the underpinning solution depends on many factors, among which are the mechanical properties of the support stratum of soil, the conservation conditions of the foundation elements and, above all, the restrictions imposed during this operation.
Micropiles are presented as a variant of deep foundations, and consist of piles of small diameter between 75mm and 350mm, cast in situ, vertical or executed with an angle. These elements, when compressed, transmit their forces to the ground primarily by lateral friction (floating piles), although there is a small contribution from the bearing resistance.. In general, the execution of micropiles is divided into the following stages.
- · Drilling to the specified depth.
- · Placement of the reinforcement.
- · Gravity fill injection of grout.
- · Pressure postgrouting injection, when applicable.
The use of this GEWI type systems results, firstly in the installation of a certain normal stress at the interface between the beam and the underpinned element. Moreover, the load transfer to the micropiles produces, according to the strut and tie method, tensions that can be absorbed at the expense of the resistance of these steel bars.
|MICROPILE/EXISTING ELEMENT CONNECTION SCHEME|
Jet grouting The physical process of jet grouting technique can be summarized in the following steps
- Soil fracture: the initial structure of the soil is broken and the soil particles or fragments are dispersed by the action of one or more horizontal jets.
- Mixing and partial replacement: a part of the particles or fragments of soil is replaced and the other part is mixed with the injected grout.
- Cementation: the remaining soil particles are bonded together as the grout sets and hardens, forming a single body.
This technique can be applied to both incoherent and cohesive soils, as a result of the conversion of the potential energy, obtained from pumping the grout, into kinetic energy.
Normally, this process needs to be designed or lead by a structural engineer for better results, but here are a few tips that will help you during the underpinning process.
The underpinning process must be started from the corners and the working inwards.
Underpinning must be made only on load-bearing walls.
- Do not underpin below non-load bearing walls.
- Start underpinning under a strip of footing. It is recommended to start with at least 3 feet long, two feet wide and two feet in depth.
- After the excavation has been completed, add concrete to the cavity. Concrete should be mixed using one part cement, three parts sand, and six parts aggregates.
- Remember to use formwork on the edges.
- Allowed concrete placed to set for at least two days.
- Use a rod bar ensuring that the cavity under the existing foundation is filled up.
- Ensure that the concrete is cured thoroughly before loading it.
- Once the concrete has gained sufficient strength, break off the projecting footing.
- Cut the concrete with the mass of concrete surface.
- Backfill and compact. If you are having problems achieving the required consolidation, use a hose to add water to the soil.