You’re probably feeling concerned if your building’s foundation has become unstable or needs strengthening. Foundation problems such as subsidence or settlement can be stressful, but the good news is that there’s a reliable and time-tested solution.
One of the simplest and most traditional methods for stabilising existing structures is the pit method of underpinning. This technique has been widely applied for decades, particularly for smaller buildings or where foundation movement is localised.
In this guide, we’ll explain what the pit method is, how it works, and the process of the pit method of underpinning.
What Is the Pit Method of Underpinning?
The pit method, among the various underpinning techniques, is also known as mass concrete underpinning or the traditional pit method. It is used to strengthen a building’s existing foundations. It is the most reliable solution, in which small sections (pits) are excavated beneath the foundation, and then each section is filled with concrete. Needles are used to support the load during curing.
The pit method of underpinning is a construction technique used to increase foundation depth and improve load-bearing capacity, stabilising the existing structure. It is a traditional form of underpinning used to extend the existing foundation to a deeper, more stable soil layer.
How Does the Pit Method of Underpinning Work?
The pit method of underpinning stabilises a building by transferring the load from weak soil to a deeper, stronger layer through a series of concrete pits. It involves creating vertical pits with gaps left between them under the existing foundation, then pouring concrete to increase strength and support the depth. It is an ideal technique for foundation repair in buildings with narrow foundations or where spot improvements are needed.
What is the Step-By-Step Process of the Pit Method of Underpinning?
The pit method involves excavation of small sections beneath the existing foundation, filling them with concrete, and repeating the process until the entire foundation is underpinned. Let’s take a look at the process in detail.
1. Site Assessment and Marking:
The first step in the pit method is site assessment, preparation, and marking. For this, the sections to be underpinned are identified, and small rectangular segments are marked out. For safety and stability, pit sequencing is alternated, not all sections at once.
2. Excavation of Pits:
The small sections that are excavated are called bays, pins, or pits. Only one bay or pit is excavated at a time, leaving the adjacent bays untouched until the first one has been completed, filled with concrete, and has gained enough strength. Pits are dug vertically beneath the foundation to the required depth, ensuring the wall remains supported, as only a small portion is disturbed.
3. Trimming and Cleaning:
After excavation, the sides and bottom of the pit are cleaned. Cleaning and trimming are carefully done by removing soil below the old foundation in the pit area.
4. Laying the New Foundation:
Once the pit is excavated and cleaned, a levelling course of concrete is poured to construct the new concrete footing to the desired depth. Proper compaction and curing of the concrete are important to ensure the foundation’s stability.
5. Curing and Load Transfer:
The concrete-filled pit is undergoing stability testing. To cure concrete adequately, it is usually allowed to rest for 3 to 7 days, depending on the required strength. The wall load is transferred gradually to the foundation naturally, without jacking.
6. Filling and Finishing:
When concrete is capable of bearing a load, the pit is backfilled with suitable material and compacted. Joints are sealed between new and old masonry if needed for finishing.
7. Repeat the Process for Remaining Pits:
After the settlement of the first bay that has fully cured and settled, start underpinning the alternate untouched bays in the same manner. In this way, uniform load transfer and wall stability are ensured throughout the process.
Conclusion:
The pit method of underpinning is a technique used to strengthen an existing foundation by excavating small sections, or pits. Pits are excavated beneath the soil and then filled with concrete to reach a stable soil layer. The pit method of underpinning is the best option for transferring the existing structure’s load onto a new, more stable concrete foundation or stratum below.
