What is a cushion under a strip foundation

A building’s foundation is essential to its stability and longevity during construction. One crucial element that’s frequently utilized in strip foundations is a cushion. By acting as a barrier between the soil below the foundation and the cushion, it helps to evenly distribute the weight of the building and avoid uneven settling.

A layer of material placed directly beneath the foundation itself serves as the equivalent of a cushion under a strip foundation. Its main objective is to offer a solid foundation that can support the weight of the structure above. This layer aids in reducing the effects of soil movement, such as expansion and contraction brought on by variations in moisture, which over time may jeopardize the integrity of the foundation.

The kind of soil, the climate where you live, and the particular needs of the structure can all affect the type of material used for the cushion. Sand, crushed stone, or even compacted gravel are examples of common materials. These materials were selected because they can offer a level, sturdy surface that can evenly distribute the weight of the foundation to the underlying soil.

A cushion under a strip foundation must be installed correctly, which requires careful planning and preparation. Make sure the ground below is level and adequately compacted before applying the cushion material. Once the foundation is in place, this preparation helps to create a stable base, reducing the chance of settlement or shifting.

What is a cushion under a strip foundation?

A pad is a horizontal, level surface made of compacted sand or sand-gravel bedding that is used to support a concrete strip.

It carries out a number of crucial tasks:

• Reference Line Alignment.
• Compensation for heaving loads created by soil layers.
• Replacement of problematic soils in direct contact with the belt with more stable and stable layers.
• Cutting off the concrete tape from moisture, preventing capillary absorption of water.

Because the filling layer is looser than the surrounding soil, moisture can be collected at the bottom of the trench and drained into a special well via a drainage pipe system.

Furthermore, heaving loads can be effectively compensated for by even a relatively thin layer of sand; this is particularly evident on shallow types of belt.

Simultaneously, soil layers are being replaced, which may trap ground moisture around the base of the concrete and result in absorption and material destruction when the water freezes.

Is she needed?

There are frequently arguments against the use of sand cushions, despite their significant and responsible role in the environment.

The following are the arguments made by those who oppose the sp Provation:

• No matter how thoroughly a layer of sand is thoroughly, some sediment will be present in any case. The thicker the pillow, the stronger the sediment, which creates a significant danger to the tape due to its uneven. Because of this, the thickness of the add-on is limited-it should not be more than a three-time tape width.
• A layer of sand, covered in a trench dug up in clay soil, will become a moisture battery. Clay will not release it from the recess, so a high -quality drainage system is needed.
• The pillow layer has to be taken into account when calculating the depth of the trench or pit, which increases the volume of earthwork and the amount of soil removed.

Experts who have worked in the field for many years all agree that the pillow’s primary purpose is to align the trench (or pit) bottom. Digging by itself can never produce a level, horizontal surface.

You can cut down on preparation time by using bedding because all other attempts in this direction end in a significant waste of time and unsatisfactory results. Since the hydrogeological situation at the site is examined first, the drainage situation is typically known in advance.

As a result, the pillow’s main job is to create a horizontal, level reference line at the appropriate height.

Even though they are very important and efficiently performed, the remaining uses of sand backfill can be regarded as extras.

Kinds

There are various ways to design a pillow beneath the ribbon:

• Sandy. It is a layer of sand of a certain thickness, maximally compacted and horizontal. Distributes the load from the weight of the belt and the building, ensures cutoff from groundwater, allows you to replace unsuitable soil with more stable and cheaper river sand. It is used primarily for relatively light buildings; such a pillow is not used for heavy and massive buildings.
• Gravel bed. A layer of gravel or crushed stone is used, which can significantly increase the weight of the building. In private construction, this type of pillow is used quite widely, since it allows the backfill layer to be compacted more densely and has a high load-bearing capacity for any home. Sand is also present in the gravel backfill, filling the cavities between larger elements, which makes the layer more dense. Typically 60% crushed stone and 40% sand are present.
• Concrete screed (footing). It is a layer of poured concrete of lower density. The most durable and reliable pillow option, providing horizontal position and high load-bearing capacity. Used for the heaviest buildings, costs significant amounts.

The weight of the future building, the properties of the soil, and other considerations determine which option is best. This is done when the project is being created, specifically when determining the foundation.

Conditions under which bedding is required

When building a shallow strip foundation, a pillow must be present. The base is forced out of the trench by loads created by soil layers beneath the tape. These are the efforts made by the pillow, which largely reimburses them.

An additional requirement for the existence of a cushion is the potential for precipitation or groundwater to enter the trench. At the bottom of the trench, where moisture enters drainage pipes and is released into designated wells or reservoirs, the backfill layer aids in accumulating runoff.

Remember that there is always a sand cushion present, even if it is only slightly thick.

By leveling the trench’s bottom, it eliminates the need for labor-intensive and sophisticated technological methods and gives you a horizontal, even plane.

What to choose, sand or crushed stone?

The weight of the building influences the choice of backfill material. The backfill fraction should be higher for heavier materials. Since the sand layer settles more noticeably, it is typically utilized for frame, wooden houses, cellular concrete buildings, etc.

There are no clay inclusions or outside (especially organic) contaminants in the pure river coarse sand that is used.

The layer of crushed stone is more elastic and capable of supporting weight. The pillow is more sturdy and dependable, and it is easier to use and compress. When building a house with multiple floors, backfill composed of crushed stone or gravel is utilized because it is a dense and substantial material.

In practical application, crushed stone is utilized in a ratio of 60% crushed stone to 40% sand, though the amount of sand may vary depending on the circumstances. Crushed stone is not utilized in its purest form.

In either case, an intermediate layer of sand about 15-20 cm thick is made before filling with crushed stone. In order to absorb heaving loads, a compensation layer must be created and the base leveled.

What thickness should be for different materials

External factors determine the recommended thickness of the backfill layer when building a strip foundation. The computation considers the building’s parameters (weight, dimensions) as well as the soil’s characteristics (composition, groundwater level, frost heaving). The parameters of the cushion are determined during the foundation calculation process; this is a complicated engineering task that should only be performed by qualified professionals.

The most typical layer thickness when using sand backfill is between 25 and 60 cm. The maximum layer thickness is three times the strip width, although this value is rarely reached.

The material is carefully compacted and laid out in layers of 20 cm each. The cushion is wet with water to improve the ramming’s density.

A crushed stone cushion may be made of various materials:

• Limestone.
• Gravel.
• Granite crushed stone.

The 20–40 mm fraction is the most widely used. Layer thickness minimum: 25 cm. positioned atop a 15-20 cm sand preparatory layer, which needs to be considered when determining the trench’s depth.

After the sand has been thoroughly moistened and compacted, crushed stone is laid, again with moistening and compaction.

Everything is done with the greatest care.

SNiP stipulates that the bedding layer’s density must be at least 1.6 t/m3, necessitating the use of construction equipment or labor-intensive hand tools like a tamper or roller.

A layer of material positioned beneath the foundation to equally distribute the weight of the building and reduce settlement is referred to as a cushion under a strip foundation in construction. This important component aids in preventing the structure from settling unevenly, which over time could cause structural problems. Usually composed of gravel or sand, the cushion enhances the stability and longevity of the foundation by offering a firm base. It plays a crucial part in maintaining the building’s levelness and structural integrity, particularly on locations with different soil types. Any building project seeking to build sturdy and dependable foundations must comprehend how to install this cushion correctly and select the appropriate materials.

Laying technology

Now let’s look at how to make a cushion for a strip foundation. You should consult the strip foundation for a large two-story piece-material (brick) cottage for more details.

It can use a gravel bed because of its size and weight.

The tasks listed below need to be finished:

Sand backfill

This layer, which is the lowest, offers a level, horizontal surface. It is horizontally leveled and filled in, with a thickness of 15 to 20 cm. Watering is used to thoroughly compact the soil until the maximum density is reached.

Walking on the backfill’s surface is a popular way to check the quality of sand compaction; if there are no traces after walking, the job was completed effectively. According to some sources, the sand backfill should be covered with a layer of rubble stone.

Sand fills in the spaces between big pieces and is harder to compact, which makes the recommendation dubious because it will cause the foundation to settle more.

Backfilling of crushed stone

Layers of the material are applied, with a minimum backfill thickness of 25 cm. The material is meticulously leveled horizontally, moistened, and compacted while being laid using a vibrating construction plate.

Sand helps to create a more dense layer in the crushed stone mass, which facilitates excellent compaction. The primary responsibility is to maintain constant control over the backfill layer’s horizontal surface. It is not acceptable to have a wavy layer with a noticeable slope if you do not check the condition and level.

Using a construction or laser level, the check is performed, and any distortions or irregularities found are corrected right away.

Leveling layer

A relatively thin (roughly 5 cm) layer of sand is refilled after the ASG layer has been laid and compacted. It creates a smooth and dense surface for the application of geotextiles by leveling the ground and effectively "sealing" the crushed stone layer’s surface.

This substance is required to stop water leaks during the main tape or concrete footing pours, if that is the plan. In order to ensure that the concrete is poured into the hollow and that the water does not seep into the ground, the material is placed on top of a layer of compacted leveling with an approach to the trench walls.

In case the reinforcement cage tears the waterproofing film, a layer of geotextile provides extra security against leaks while pouring the main tape.

Useful video

You will discover the correct way to construct a pillow beneath a strip foundation in this video:

A building’s stability and longevity are greatly enhanced by the cushion beneath a strip foundation. It helps to evenly distribute the load by acting as a protective layer between the soil below the foundation. Over time, uneven settling can cause structural damage, which the cushion helps prevent by dispersing and absorbing the weight of the structure.

The cushion, which is usually composed of gravel or sand, helps with drainage by letting water move away from the foundation. This lessens the possibility of water collecting around the foundation, which could erode the soil and jeopardize the structural integrity. When installed correctly, the cushion gives the load-bearing walls above it a stable base, which improves the foundation’s overall performance.

The cushion material’s thickness and compaction are carefully considered during construction. This guarantees that it can withstand the forces of the surrounding soil and sustain the weight of the building. When deciding on the requirements for the cushion beneath a strip foundation, engineers and builders take into account variables like the local climate, the type of soil, and building design.

Video on the topic

Foundation pillow. Start

How to properly make a pillow under a strip foundation

The whole truth about the danger of a sand cushion under the foundation, it tears up houses, engineers are silent.

STRIP FOUNDATION | How to avoid risks with strip foundations | SAND AND GRAVEL BED