One kind of shallow foundation that is frequently used in construction, particularly for smaller buildings and extensions, is the rubble strip foundation. It is made up of a rubble masonry strip or bed made of bricks and stones arranged in mortar. This kind of foundation is intended to provide stability and avoid settlement by distributing the building’s weight over a larger area of soil.
The first step in constructing a rubble strip foundation is usually to dig a trench around the outside of the building’s footprint. The site’s soil conditions and the particular design requirements determine the trench’s width and depth. Typically, the trench is excavated to a depth that permits the laying of rubble masonry at a stable base and enough depth.
To guarantee a solid base, the excavated trench is leveled and compacted. Then, to give the rubble masonry more stability and to act as a leveling pad, a layer of concrete may be poured into the trench. This layer of concrete also aids in keeping moisture from entering the foundation from below.
The masonry made of rubble must then be placed into the trench. This usually entails filling the trench with a variety of sized and shaped stones and bricks, arranging them to form a level and sturdy foundation bed. The spaces between the stones and bricks are then filled in with mortar to create a sturdy and well-built structure.
The superstructure can be built after the rubble masonry is in place and the mortar has hardened. Because of its affordability and ease of use, this kind of foundation is preferred, especially in places where the soil is stable and strong enough to support the weight of the building without the need for deeper foundations.
- What it is?
- Advantages and disadvantages
- Application area
- Some tips on the device
- Installation stages and technologies
- Tools and materials
- Calculations
- Stone selection
- Preparatory work
- Sand-gravel pillow
- Preparation of a suitable solution
- Laying the first layer in the trench
- Laying out subsequent lines
- Waterproofing
- Video on the topic of the article
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What it is?
A rubble foundation differs from a typical strip foundation in that natural stone, also known as "rubble," is utilized as an adhesive force in place of reinforcement. It offers concrete a strong adherence, ensuring the strength of a monolithic structure.
A rubble strip foundation and a rubble concrete foundation are not the same thing. The first kind uses a 1:1 mortar to natural stone ratio. The ratio for the second kind is 4:1.
Advantages and disadvantages
Advantages:
- reliability and durability of the structure: the use of durable rubble allows the structure to stand for more than 150 years;
- high wear resistance: does not collapse under the influence of wind, natural mechanical influences, is not susceptible to ultraviolet radiation, temperature and other factors;
- water resistance: rock, unlike an ordinary monolith, which has a porous structure, has a higher density, therefore does not absorb water, and, therefore, does not collapse under the influence of moisture;
- aesthetic appearance: the finished base does not require additional finishing;
- savings: fairly cheap building material. Only transportation costs can make it more expensive if the deposit is located at a considerable distance from the construction site;
- the ability to carry out work independently without the involvement of specialized organizations and special equipment;
- environmental friendliness: natural cobblestone is completely safe for health.
Minuses:
- accurate calculations are required;
- time- and labor-intensive process.
Application area
This kind of subwall can only be used in areas with low groundwater and hard soils. This technology cannot be used if the soil is prone to seasonal heaving or if groundwater will come into contact with the base.
An analysis must be done in order to determine the soil’s composition and suitability for this kind of construction. These indicators will support computations and design as well.
Some tips on the device
Some skilled artisans are able to arrange the components in the shape of a trapezoid, with the wide base of the lower layers gradually transitioning into the defined width of the uppermost foundation. The frame is more stable thanks to this technology.
Finding smooth, identical copies of blocks can be difficult at times and is a costly pleasure.
That’s why different types of cobblestones are more often bought for construction, and you can later resize them to the exact size you want.
You can fill voids with less solution because of the boulders’ smooth surfaces. The structure is stronger when the fit is tighter.
Even though the rock has good moisture resistance, the walls of the recessed portion of the frame still need to be waterproofed using a mixture of roofing material and mastic.
Installation stages and technologies
The phases of rubble construction technology are the same as those of building any strip foundation:
- Design – calculation and drawing of the proposed subwall plan.
- Preparatory part – digging a trench, installing a sand and gravel cushion, waterproofing the underground part.
- Basic construction – laying out the first and subsequent rows.
Tools and materials
No specialized machinery is needed to build such a foundation, but one particular tool is needed to create:
- shovels – for digging a trench (in case of digging a whole pit, it is better to use an excavator and then finish the soil with a shovel);
- mallet – for laying out cobblestones;
- concrete mixer – for making concrete mix;
- a box for mortar at the place of laying;
- chutes – for lowering boxes and raw materials;
- stepladders – for descending into the pit.
Calculations
It is crucial to perform the calculations accurately when building this frame. This will, first of all, enable you to order the material in the necessary numbers. You must first compute the width, height, and total length of the support that is being erected in order to determine the correct quantity.
The minimal measurements for a strip rubble foundation are:
- width – 35 cm or more;
- trench depth – 70 cm;
- total height taking into account the depth – 1.6 m;
- length – measured according to the finished drawing.
The purported loads on the completed foundation, the makeup of the soils on which the construction is expected to occur, and the local climate should all be considered when determining these parameters.
The quality features of the constructed structure directly depend on the accuracy of the calculations, so you should speak with experts if you have any doubts.
The process for figuring out how much raw material is consumed:
- Based on the known parameters of length and width, it is not difficult to calculate the area of the upcoming structure.
- By dividing the area of the frame by the approximate diameter of the selected stone, we get the amount of material needed to lay out one row.
- By dividing, we find out the number of lines that need to be laid out.
- By multiplying the number of rows by the amount of raw materials needed for one row, we get the total volume of material for the complete laying of the planned building.
For illustration, suppose the foundation is 0.35 meters wide, 150 meters long, and 1.6 meters tall. The rubble stone measures 35 by 15 cm. Next:
- 0.35 m * 150 m = 52.5 m2 – the area of the base;
- 52.5 m2/0.35 m2 = 150 pcs – the amount of raw materials per line;
- 1.6 m/0.15 m = 11 rows;
- 150 * 11 = 1650 pieces – the amount of rock to build a complete structure.
If the size of the rock material used is roughly the same, this method is employed.
Tonnage calculation can be used if you intend to buy cobblestones for construction that are different sizes. It is known that there are 1.8 tons of rubble per cubic meter on average.
First, you must determine the total base volume using known design parameters. Simply multiply the length by the width and height to get the result (0.35x150x1.6=84 m3). Next, divide the volume of the skeleton that was obtained by the volume of rubble needed to construct one m3 (84 m3/1.8 t = 47 m3).
Stone selection
The selection of a high-quality sample directly affects the strength properties of the completed structure. You have to knock on a boulder to see if it’s suitable. Echo-producing resonant sound is a sign of strength. A deaf sound indicates that because its structure has already been compromised, it will soon be dispersed.
Among the rocks, you should try to find a suitable cobblestone. It might be:
- granite,
- dolomite,
- basalt,
- diorite,
- shell rock,
- sandstone,
- limestone and others.
To improve adhesion, the rock pieces should be large, ranging in size from 35 cm to half a meter, and "torn."
Preparatory work
The first step is to mark the location of the proposed building construction using the original drawing as a guide. You should then start working on the excavation and dig a trench.
The entire area of the future building must have a pit built over it if the house is intended to have a basement. For the purpose of creating and expanding the lower part of the base, there should be a margin of width in the trench.
The trench bottom mark should be at least 30 centimeters below the freezing point of the soil. Strength and resistance to abrupt temperature changes are increased by deepening the structure.
Formwork is necessary to stop soil from entering the solution if the soil in the trench has a tendency to crumble. This will stop any future breaches of the structure’s integrity.
Sand-gravel pillow
The blocks must never be placed on an unprepared surface. The excavated trench needs:
- The device of a sand-gravel pillow with a tamping with a thickness of 10 to 30 cm. For better tamping, sand must be moistened with water.
- Laying waterproofing from roofing material.
- Concrete preparation device from a B7 brand solution 10-15 cm thick.
Preparation of a suitable solution
It is best to knead the concrete with a concrete mixer because you will need a lot of it to build the base.
The following is the operational protocol:
- Dry mixtures of sand and cement are falling asleep according to the proportion of 3: 1;
- 5 parts of crushed stone are added;
- water is added with a small stream as it bursts.
Add each component to the solution and stir until a homogenous mass is achieved. The final mixture should have a "sour cream"-like consistency. Concrete mortar is the strongest material to use to fill the spaces between raw materials that have been laid.
In cold weather, tumble stone tends to remove heat from the mixture. It is preferable to complete building tasks during the warm season to keep the mixture from losing its adhesive qualities.
Laying the first layer in the trench
Good specimens with roughly the same shape are chosen for the initial layer’s layout. Their surface ought to be smoother as well. This will shield the roofing felt waterproofing layer underneath from harm.
Every block should be spaced out between 5-7 cm. Concrete needs to be used to fill in any remaining spaces.
Using a clean boulder is essential for construction and installation work. It is first cleaned to get rid of dirt, clay, and other contaminants before doing this. Additionally, it is moistened prior to laying to improve adhesion.
Laying out subsequent lines
It is best to start drawing subsequent lines only after the first layer has dried. When constructing with pebbles of varying sizes, it is preferable to place large specimens around the periphery and place smaller blocks in the center.
According to technology, laying out a brick structure is the same as laying out rubble stones. During laying, they are also adjusted with a mallet. The position of each individual object in a row, which alternates like bricks, follows the same rules.
It is preferable to choose lovely, level blocks for the outer belt when laying out above ground because additional base finishing is not anticipated for this kind of building.
Reinforced screed should be the final layer applied on top of the laid base to strengthen the structure and level it out for the installation of walls later on.
When the sub-wall has endured all weather variations and taken on its final position, the completed foundation should be allowed to dry thoroughly for a month before the walls are constructed.
Waterproofing
It is evident from the above that waterproofing of such a sub-base is only required below zero ground level and as an underlying layer. There is no need for extra waterproofing on the open top.
Video on the topic of the article
You will learn about the rubble stone strip foundation in the following video:
For buildings to be supported on soft or unstable soil, a straightforward but efficient technique in construction is the rubble strip foundation. The name comes from the fact that it is made up of a trench filled with compacted rubble stones that form a sturdy base. When soil conditions make traditional concrete foundations unsuitable, this kind of foundation comes in handy.
Excavation is usually the first step in the process of building a rubble strip foundation. Usually around the outside of the building, a trench is dug to the necessary depth and width. The stability of the soil and the weight of the structure it will support will determine the depth. After the trench is ready, a layer of crushed stone is carefully placed and compacted. This stone is frequently combined with sand or gravel.
Compaction is essential because it guarantees that the debris settles uniformly and gives the building above a stable foundation. The trench is filled to the necessary level by adding and compacting layers of rubble after the first layer is placed. By distributing the weight of the building uniformly across the foundation, this procedure helps to avoid uneven settling and future structural problems.
A rubble strip foundation’s affordability and relative ease of use in comparison to other foundation designs are two of its benefits. It can be built more quickly, particularly in places with difficult soil conditions where other foundation types might require extensive excavation or specialized equipment. To guarantee stability and the longevity of the foundation, it is imperative that the rubble used is of an appropriate size and quality.
In conclusion, rubble strip foundations are a workable option when soil stability is an issue, even though they might not be appropriate for every building project. These foundations are a useful choice for both building construction and renovation projects because they guarantee the structural integrity of buildings by offering a stable base made of compacted rubble.
A rubble strip foundation is a conventional but efficient building technique for supporting structures on squishy or uneven ground. For this kind of foundation, a trench is dug, and a bed of coarse stones or bricks—known as rubble—is laid out within. After the debris has been leveled, concrete is poured over it to create a strong foundation that equally distributes the weight of the building. In order to ensure stability and durability, careful excavation, precise leveling of the rubble layer, and proper concrete pouring are necessary when erecting a rubble strip foundation. This article examines the advantages of using a rubble strip foundation, as well as the steps involved in creating one and how to use it for construction projects.