Shallow strip foundation: design options, step-by-step construction instructions

Any structure must have a strong foundation, and because shallow strip foundations are so easy to build and so efficient, they are frequently used. These foundations are usually utilized in situations where the ground is suitable and the soil is strong enough to support the weight of the building. Smaller buildings and extensions can benefit from them because, in comparison to deeper foundations, they equally distribute the building’s weight over a larger area.

There are a few things to take into account when designing shallow strip foundations. The soil’s ability to support a given amount of weight, the building’s size and weight, and local building codes all come into play. Based on these variables, engineers determine the foundation’s width and depth to guarantee that the structure can be securely supported over time.

Shallow strip foundation construction requires a few crucial procedures. First, the ground must be dug out to the necessary width and depth, which is normally 300 mm deeper than the walls it will support. After that, the trench is compacted and leveled to give the concrete a solid foundation. The foundation is then created by pouring a layer of concrete into the trench. Depending on the soil conditions and design specifications, reinforcement may be added.

Concrete requires time to cure and strengthen after it is poured. During the several days that this curing process typically takes, it’s critical to keep the foundation from drying out too quickly or being exposed to adverse weather. Before more building work can start, the foundation must pass inspection after it has cured to make sure it satisfies construction standards.

All things considered, shallow strip foundations are a workable option for a lot of building projects since they give a solid base without the expense or complexity of deeper foundations. Builders can guarantee that these foundations sustain buildings efficiently for many years to come by adhering to appropriate design and construction practices, which will enhance the robustness and security of the structures they support.

Design Options	Step-by-Step Construction Instructions
1. Determine load-bearing capacity and soil type.	1. Excavate trench to required depth and width.
2. Choose depth and width of the foundation based on structural requirements.	2. Compact the trench bottom and add a layer of gravel.
3. Consider local climate and environmental factors.	3. Lay reinforcement bars and formwork if needed.
4. Select suitable concrete mix and strength.	4. Pour concrete into formwork and level the surface.
5. Ensure proper drainage and waterproofing.	5. Allow concrete to cure and protect from excessive moisture.

What is a shallow strip foundation

NZLF is a type of shallow strip foundation that is placed either on top of the ground or slightly below it (up to 10–20 cm). The technology used to create this kind of foundation base enables you to drastically cut down on the amount of time needed for installation and the amount of building materials used. This kind of foundation doesn’t require the help of professional builders; it can be created by you.

Small buildings composed of lightweight materials such as logs and aerated concrete blocks are intended to have a shallow foundation. It is mostly utilized in private residential construction for garages, utility buildings, and country homes.

Building a country cottage on this foundation with the intention of making it a permanent residence is not advised. If the soil does not differ in high supporting characteristics, it might not be able to support a heavy load. On rocky soils, shallow foundation bases can be set up for large brick buildings.

This article from "All about Concrete" discusses shallow strip foundations, going over different design options and offering simple construction guidelines. For structures on stable soil, shallow strip foundations are essential because they provide a simpler, more affordable solution than deeper ones. We walk you through every stage of the procedure, from the preliminary site preparation to the concrete pour and alignment check. This guide gives you the necessary knowledge, whether you’re a homeowner or a construction expert, to confidently take on shallow strip foundation projects, guaranteeing the stability and longevity of your structures.

Options for performing NZLF

Building an NZLF for a residential building in a country home or for outbuildings can be done in a number of ways.

Prefabricated strip foundation

The most common conditions for this kind of foundation are low-heaving sandy soils. We use FBS blocks as our material.

Prefabricated foundation installation is not advised on heaving soils because of uneven concrete block subsidence and pushing out during frost heaving.

Fundamental specifications for setting up a prefabricated foundation structure:

• bedding is made under the base;
• FBS are installed in the same plane;
• to connect individual slabs, steel embedded parts are used (welded together) or a cement-based mortar of grade M200/300;
• It is advisable to further strengthen such a foundation with a metal frame.

Prefabricated foundation structures have several advantages.

• ease of installation;
• minimal time spent on construction work;
• no need to install formwork, prepare and pour concrete;
• low cost of construction.

A construction crane must be hired because heavy concrete blocks cannot be installed manually. This is a drawback.

Monolithic

Pre-mounted formwork is situated beneath a solid concrete foundation. On heaving soils, such a foundation can be utilized to construct private residential buildings; however, backfill beneath the foundation and the installation of a functional drainage system are required.

Benefits of having a monolithic NZLF:

• the ability to build a foundation of any parameters;
• all construction work can be done independently without special construction equipment, the involvement of experienced workers;
• low cost.

Combined

One can create a shallow strip foundation by combining:

For small buildings, it is not recommended to use combined foundation structures.

Construction of a strip foundation

The building site is ready before the NZFL is installed:

• removing fertile soil;
• leveling the surface;
• marking the site according to the design documentation.

Backfilling a sand and gravel cushion

Without deepening, a sand and gravel cushion is created beneath the foundation base. To do this, a trench is dug beneath each of the building’s walls that is up to 15 cm deep and 1.5 times wider than the width of the reinforced concrete strip. Geotextile is used to cover the trench’s bottom. This is required to prevent the cushion from being exposed to subsurface currents and getting covered in silt.

There are two methods for filling the cushion:

Slag and expanded clay are two more non-metallic materials that can be used to arrange the cushion.

Installation of formwork

Plywood sheets or wood boards up to 5 cm thick are used in the formwork manufacturing process.

Fundamental specifications for the formwork structure:

• strength – must withstand the pressure of liquid concrete;
• height – 15 cm more than the height of the poured foundation tape;
• Flogging in a sand pillow by at least 5 cm.

The corners of the formwork design will bear a significant portion of the weight during the concreting process, so they need to be reinforced with wooden pegs driven into the ground.

Reinforcement

In order to provide the foundation with strength, it is reinforced with a steel frame, which is made using 12–14 mm diameter reinforcing rods. Round and rectangular shapes can be used in the reinforcing structure.

Method used in the production of the NZLF’s reinforcing structure:

• pre-baked metal wire Ø4-6 mm must be used to tie together the reinforcement bars located longitudinally (4-6 pcs.);
• Every 50-60 cm of the structure, reinforcement bars must be installed in a vertical position.

The completed reinforcing structure is positioned within the formwork on metal mushrooms that are positioned in the backfill and rise five to seven centimeters above the ground. Consequently, the ground and the formwork structure’s walls shouldn’t come into contact with the reinforcing frame.

Concreting

Concrete is applied in 20-cm layers, each of which is meticulously compacted. This is required for the concrete mass to shrink uniformly and for air bubbles to be removed from it.

In 28 to 30 days, the foundation will fully acquire its strength characteristics and be prepared for use. You must now keep a close eye on the concrete’s slow hardening process. It needs to be film-covered and moistened on a regular basis in order to achieve this.

Following this time, the building’s formwork structure is disassembled, the concrete strip is insulated, a blind area is built, and wall construction can start.

It is not advised to remove any weight from a shallow foundation base over the winter. The structure will rise as a result of winter soil heaving, and it may collapse as well as crack. The foundation needs to be loaded with at least 30% of the building’s estimated mass if winter construction is scheduled to occur.

Insulation and waterproofing of NZLF

It is necessary to cover a shallow foundation with insulation and waterproofing material.

Options for waterproofing the NZLF

— A unique mixture based on bitumen mastic, which deters water, is applied to the foundation.

The best method for completely waterproofing a strip shallow foundation. In addition to adding primer to the working mixture before pouring the foundation, a coating of waterproofing material is applied to the finished foundation’s surface.

Suggestions for base insulation materials:

• extruded polystyrene foam;
• penoplex.

These materials provide benefits for their own insulation boards:

• high heat retention coefficient;
• moisture resistance;
• not afraid of rodents.

The waterproofing is layered over them. A blind area is created and the building base is insulated at the same time.

Drainage for NZLF

The foundation base, which is directly on the earth’s surface, is unaffected by groundwater. However, the gravel-sand cushion can be washed away by high groundwater levels. Thus, setting up a drainage system is essential in these areas.

Polymer pipes are advised for the drainage of a summer home or a country cottage. The depth of the drainage pipes and the amount of groundwater will determine their diameter.

Standard specifications for a country home’s drainage system layout include:

• diameter of polymer pipes — 10-14 cm;
• pipes should be laid at a depth of 30-50 cm below the foundation pad;
• the drainage system should be laid at a distance of 2-3 m from the residential building along the entire perimeter.

Technology used to install drainage systems:

1. Before installing the pad under the concrete base, wells are dug at a distance of 2-3 m, their bottom is covered with fine gravel in a layer of 10-20 cm.
2. Drainage pipes are placed inside the pits. They must be wrapped in geotextile material to protect the drainage from silting.
3. The drainage system is installed under the future foundation and blind area. In this case, the pipes are laid at an angle of 5-10º from the center so that excess moisture can be removed by gravity.
4. After concreting the foundation, a trench is made half a meter from it to the depth of the already prepared wells 20 cm wide. A geotextile covering is laid on the bottom of the trench, small crushed stone is poured on top and a pipe is placed to drain water from the wells to the designated place. For this purpose, you can dig a separate drainage pit or ditch.

Planning and performing calculations for a strip type of foundation is the best time to design drainage. The building’s floors will be significantly warmer and the foundation will be dependable protected from groundwater’s destructive effects by a well-designed drainage system.

Blind area

NZLF is set up as a blind area to shield the concrete base from winter heaving and precipitation.

— 1–1.5 m by 3–8 cm.

Detailed guidelines for setting up a blind area in a shallow foundation strip:

1. A ditch is dug along the entire perimeter of the future structure 30 cm deeper than the foundation level of the building.
2. A formwork structure is made and installed in the ditch. The bottom is filled with crushed stone mixed with sand by 10 cm. The backfill layer is carefully tamped.
3. A waterproofing layer of roofing felt or geotextile, penoplex thermal insulation material is laid on top of the made cushion.
4. A steel reinforcement grid is laid on thermal insulation.
5. A concrete solution is poured into the trench, after the solidification of which it is finished with a decorative coating.

Every 2 m seams of deformation (up to 15 mm in width) must be equipped in the blind area to prevent cracking. And laids impregnated with bitumen are found in them. Whether or not the blind area is decorated, this needs to be done.

Water will be removed in an area designed for this purpose if a drainage system is installed beneath the concrete base. It is required to install a drainage tray at the end of the blind area if drainage was not supplied.

Anyone involved in construction projects, especially those involving structures like homes, small buildings, or extensions, must have a thorough understanding of the design and construction of shallow strip foundations. These foundations are preferred due to their affordability and ease of use, as they distribute the load-bearing capacity over a larger area close to the surface.

A shallow strip foundation’s planning must take several things into consideration. Design options are limited by the type of soil, load-bearing requirements, and local building codes. These foundations usually have the capacity to sustain the weight of the building while guaranteeing stability in the face of ground movement.

Excavation to the necessary depth—typically no deeper than the foundation’s width—starts the building process. After that, make sure the base is solid and level before adding concrete to the trench. For added strength, reinforcement bars may be added in accordance with structural engineer specifications.

Concrete requires several days to properly cure after it is poured. It’s critical to safeguard the foundation during this time against severe weather events or excessive moisture loss that can jeopardize its structural integrity. Backfilling around the foundation completes the process after it has cured and adds more lateral support.

When shallow strip foundations are installed correctly, they offer a sturdy base for buildings, guaranteeing their longevity and structural integrity. It is advised to perform routine maintenance and inspections to find problems early and stop possible harm. Both homeowners and builders can make sure their buildings endure over time by being aware of these fundamental ideas.

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