1. Foundations for retaining/breast walls
Although retaining and breast walls are relatively small structures, they are no less demanding than larger ones because they have both a protective and an aesthetic function. A thorough analysis of the terrain and static calculations are vital before the foundations can be laid, i.e. before construction begins. This particularly applies to structures that are over 1.5 m in height.
Whether youre building a retaining wall or a breast wall, it is important to make sure that the steel reinforcements in the foundations and the wall itself are properly connected. At the same time, proper drainage is also important to avoid standing water between the wall and the ground. Finally, detailed thought needs to be given to all aspects of the structure and surroundings; in other words, having a precise plan is equally essential.
In essence, therefore, when planning and building the wall, it is necessary to take into account a number of factors, including the pressure of the soil on the retaining/breast wall, the dimensions of the foundations, the appropriate choice of reinforcement, and, finally, a high-quality concrete mix. However, in order for the wall to have aesthetic value and not to be some sort of environmental eyesore, it should be built in an appropriate way using suitable materials.
1.1. Foundations – preparation
The foundations for any wall, whether they are strip foundations or some other type, are even more important if you want to ensure its stability under pressure from soil. The first step is to dig a construction pit for the foundations and to cover the bottom with a suitable granular fill material to make the subbase. The subbase must be made of water-resistant material with a suitable grain size. Its thickness is on average between 10 and 20 cm, but more in some cases, depending on the dimensions of the structure.
After the subbase has been levelled and thoroughly compacted, concrete, reinforced if necessary, can be poured to form the foundations. The dimensions of the foundations depend on the dimensions of the wall that will stand on these footings.
1.2. Dimensions of the foundations for a retaining/breast wall
The width and depth of the wall foundations are determined according to measurements taken at site, but also in line with the recognised standards for laying strip foundations. The width should be 50-70 cm, while the depth depends on the level of the frost line, but is usually 80-100 cm. If the foundations are too narrow, the wall may settle/sink, while if too shallow, standing water will cause the structure to crack.
2. Formwork for the foundations of a retaining/breast wall
2.1. Why is formwork (shuttering) important?
Strip foundations are built using shuttering, i.e. laying wooden formwork. It is wholly inadvisable to lay concrete foundations without using formwork. However, some contractors simply avoid placing shuttering and instead pour concrete directly into the construction pit, which is certainly not recommended.
Obviously, shuttering is an additional cost and additional work which many would prefer to avoid, but without good formwork, the entire structure will be of poor quality and its longevity and safety will be severely compromised. Depending on the type of terrain and other factors, it is sometimes necessary to place steel reinforcement that allows the concrete to withstand greater compressive and tensile forces, thereby increasing the stability of the entire structure. The alternative to wooden formwork is metal, which can be reused several times, but it is more expensive and hence increases the overall cost.
2.2. Reinforcement in the formwork for the foundation of a retaining/breast wall
If necessary, before pouring the concrete, steel reinforcement is placed in the formwork, which increases the strength of the concrete against compressive and tensile forces, thus making the entire structure more stable and durable. Correct placement of steel reinforcement requires skill, knowledge, and experience, which is why the whole job should be left to professionals. If laid too slowly, the reinforcement may be exposed to atmospheric conditions and corrode.
3. Drainage in the foundations of a retaining/breast wall
The installation of drainpipes, or drainage for short, is a very important, but often neglected element of any such structure. Just like shuttering, it is a step that shouldnt be ignored if you want the retaining/breast wall you are building to last. When it rains, there is obviously an additional increase in soil pressure on the wall. Therefore, a retaining/breast wall designed for normal conditions should be slightly larger in size so that it can withstand even extreme loads. But thats not all! Also important are the drainpipes and weep holes at the bottom of the wall, which allow the water that accumulates behind the retaining wall or in front of the breast wall to run off. Weep holes must be placed at a certain distance.
The easiest way to ensure good drainage is to leave openings for water to drain away; however, when building a concrete wall, transverse plastic pipes are inserted into the formwork and then the concrete is poured over them. This shows the importance of shuttering and why it should not be skipped. In this way, water can flow unhindered from the back of the wall to the front, and at the same time significantly reduces the pressure of the water on the wall. Whatever the case, there must be a drainage layer along the entire length of the back of the wall, where water collects and then drains away through the weep holes. Just as drainage around a house is important, so too is drainage around a retaining/breast wall.
4. Price of retaining/breast walls
In addition to the cost of materials and labour, if youre building a wall that is over 1.5 m high, you will require a building permit. This, of course, increases costs. Also, the foundations for a higher retaining/breast wall will have to be larger to withstand the increased pressure of the soil. On top of all this, difficult or inaccessible terrain can also increase the price.