1. What is the depth of foundation burial? Basic classification of house foundations

​The foundation of a building is a part of the underlying structure of the building, it is linked to the load-bearing structure above such as columns, walls... The foundation is responsible for absorbing the load from the structure and transmitting it to disperse it down. background.

The depth of burying the foundation is the distance h from the bottom of the foundation to the natural ground, called the depth of burying the foundation. The contact surface between the bottom of the foundation and the foundation must be flat and horizontal (no slope). This surface is called the bottom of the foundation. So on what basis is the foundation depth calculated?

There are two main types of foundations commonly used in residential buildings, classified according to foundation burial depth: shallow foundations and deep foundations.

​What is a shallow foundation? These are types of foundations that are constructed on bare excavated holes, then filled with soil. The depth of burying the foundation is not too large, usually from 1.5 ÷ 3m. In many special cases, the depth of burying the foundation can be chosen from 5 ÷ 6m.
In fact, we can distinguish shallow foundations based on the ratio between foundation depth and foundation width (h/b). However, the quantitative ratio is not really clear. Based on what basis is the foundation burial depth calculated? The most accurate is based on the working aspect of the ground. When carrying a load, if the side friction of the surrounding soil with the foundation is not taken into account, it is a shallow foundation, otherwise it is a deep foundation.

Some common types of shallow foundations: Single foundation (single center foundation, single eccentric foundation, duck foot foundation), strip foundation (strip foundation under the wall, strip foundation under the column - one-way strip foundation or interference strip foundation), raft foundation.

​Deep foundation: These are types of foundations that do not require digging a foundation hole or only partially excavating and then using construction equipment to lower the foundation to the design depth. It is often used for projects with large loads. For deep foundations, on what basis is the foundation burying depth calculated?

Common types of deep foundations: pile foundations (driven, pressed), bored piles, caisson foundations, slightly pressed caissons...

Deep foundations are often used for projects that have a large load on the foundation (usually buildings higher than 8 floors) or projects that are subject to large horizontal loads and good soil layer located deep below. Deep foundations often use pile foundations. Depending on the material, it can be divided into the following types:

• Wooden pile
• Steel piles, reinforced concrete piles

Based on construction technology, reinforced concrete piles can be divided into precast piles (driven, pressed) and cast-in-place piles (bored piles).

2. Based on what basis is the depth of burying a house's foundation calculated?

- First, calculate the foundation depth based on the nature, characteristics, and structure of the project

Based on the nature of the project, we can calculate the load of the structure acting on the foundation. On what basis is the foundation depth calculated? For small residential buildings with less than 5-6 floors, we only need to use single foundations or strip foundations to save foundation construction costs. For 1- or 2-storey buildings with small areas, single foundations should be used, while for 3 to 5 floors, flat foundations should be used. Projects with an area larger than 300 square meters should use strip foundations to ensure a safe foundation. The foundation area depends on the project area.

On the contrary, if the project has a large area with 7 floors or more, it must use deep foundations (pressed piles) or bored piles to ensure it can withstand the load of the project. For large projects, on what basis is the foundation depth calculated? To determine the depth of burying the foundation based on the nature of the project, we need to be sure of the scale of the project we intend to build.

For some projects such as stilt houses or communal houses, the foundation will be treated with column bases driven deep into the ground rather than using conventional foundations. For projects placed on reinforced concrete ground, it is necessary to use the foundation options introduced above.

It is necessary to pay attention to whether the building has a basement or not, whether there is an underground communication system, whether there is a wine cellar or not...

- Second, calculate the foundation burial depth based on the geological conditions of the project:
The topographic and geological conditions of the project are the most important factors in deciding what type of foundation to use and how deep the foundation should be buried.

Terrain conditions are high mountains or flat terrain, or coastal terrain. Calculate foundation burial depth based on what natural basis? For projects in mountainous areas on sloping ground prone to landslides, choose deep foundations to ensure safety. However, if the project is located on a flat surface in mountainous areas, shallow foundations can still be used. For normal flat terrain, choose a shallow foundation. Coastal terrain should choose deep foundations.

Assuming the project is built on a slope, the foundation bottom principle must be horizontal.
For geological conditions; This is the basic deciding factor in choosing the right foundation type for your project. For small projects that only need to know whether it is normal garden soil, mainland soil or feralit soil, shallow foundations can be used because shallow foundations are applied to soils that are layers of clay (clay loam) in a state of Hard to hard rock with large enough thick rafts (usually 5 - 7m) distributed above. On what basis is the foundation depth calculated? There are many types of shallow foundations used for different soils, usually raft foundations are used for weaker soils.

As for muddy soils, sandy soils, and field soils, deep foundations must be used. It should be noted that even small 1-storey buildings built on weak soils as above must use deep foundations to avoid causing settlement phenomena after long-term use. Usually, with large-scale garden villa projects, we should hire a team of workers to survey the geology to be able to know the properties and characteristics of the soil layers, where the soil is weak and where the soil is hard because of the construction. Large areas usually have a large foundation area, from which a suitable foundation plan can be found.

Weak soil types have the following identification characteristics and properties:

Belongs to weak soil, usually clay mixed with a lot of organic matter, with small load bearing capacity (0.5 - 1kg/cm2). The soil has high compaction and subsidence (a > 0.1 cm2/kg). The void ratio e is large (e > 10). Great viscosity (B > 1). Deformation modulus is small (E < 50kg/cm2). Little cut resistance and little water absorption. Soil water content is high, water saturation G > 0.8, low density.

Main and common types of soft soil:

• Soft clay: Includes clay or sub-clay soils that are relatively compact, in a state of water saturation, and have low strength. For small 1-storey projects, slab foundations can also be used in the case of clay soil but on condition that the soil does not contain much organic matter.
• Mud soil: Types of soil formed in aquatic environments; Very fine grain composition, always saturated with water, very large void coefficient, very weak in terms of bearing capacity;
• Peat soil: A weak soil of organic origin; formed as a result of decomposition of organic matter in swamps; (organic content from 20 -80%);
• Flowing sand: Includes fine sand; loose grain structure, can be significantly compacted or thinned. This type of soil, when subjected to dynamic loads, changes to a flowing state called flowing sand;
• Basalt soil: is a weak soil with large porosity; Low dry density, high water absorption, easy to sink.

-  Third, calculate the foundation burial depth based on hydrological conditions in the area

What is the basis for calculating the depth of burying a house's foundation? Hydrological conditions are underground water sources; or ponds and lakes in the construction area or under the ground in the foundation area. For example, the depth of the water vein; The location of underground water sources or building a house too close to a pond or lake are both related to the problem; Choose the foundation plan and foundation burial depth. The foundation should be placed higher than the water table; to maintain the soil structure and not have to drain water during construction.

- Fourth, the depth of the foundation of the house or neighboring structure
The depth of the foundation should be set equal to the bottom of the adjacent building's foundation; Next to it is land and townhouses and tube houses. Only deeper placement is allowed; when ensuring that the soil structure is maintained below the foundation depth of the neighboring house. If the foundation is buried deeper, the adjacent house will easily tilt and sink on one side. In garden projects, it is not necessary to pay attention to this factor.

- Fifth, the influence of the value and characteristics of the tonnage
Based on what basis is the depth of foundation buried? When the load is large, the foundation burial depth should be increased; Foundation warehouses subject to large eccentric loads must bury the foundation at an appropriate depth; to ensure foundation stability.
- Sixth, the climate situation of the region
If the area where the project is located is prone to flooding or has a long rainy season, the foundation depth should be chosen at a safe level; as well as increase the quality of the foundation and compact it tightly.

3. How deep should a house foundation be dug?

• For shallow foundations: 0.5 to 3m, should not be less than 0.5m
• For deep foundations: depending on terrain and geological characteristics
• The depth of burying the foundation in all cases should not be less than 1/5 of the height of the building.
• The foundation burying depth must be placed in a layer of good soil with a minimum bearing capacity of 0.3m.
• For projects with small and medium loads, shallow foundations can be used; The depth of burying the foundation can be lowered to soil layer No. 2 or placed at position No. 1; There is an additional cushion layer, the cushion layer is placed in soil layer No. 2. Calculate the depth of foundation burial based on what? With this small tonnage project; (for example, building a model villa with 3 floors or less); Place the foundation at the hole location with minimum soil layer No. 1; Combine the cushion layer but at the same time check the bearing capacity of soil layer No. 2. If the foundation is placed at soil layer 3; Because the layer of good soil is so thin, it is necessary to dig deep; It's better to replace it with a cushion layer to have enough bearing depth; Foundation burial depth should be minimum.
• For large projects; Foundation burial depth can be placed at soil layer No. 2; If soil layer 2 has good bearing capacity; Or it's better to use pile foundations to lower the layer below.

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Hung Nghiep Phu Construction Investment Co., Ltd. with the mission of bringing the best solutions and services, and building a prosperous community with customers, Hung Nghiep Phu is gradually asserting its brand through the Sincere cooperation, with a leadership team with ethical capacity, creativity, dedication to the profession and strategic vision. Hung Nghiep Phu owns a team of skilled and highly qualified staff who will bring customers the best quality projects. 

Hung Nghiep Phu Construction Investment Company Limited - specializes in constructing civil and industrial projects. We look forward to accompanying customers in construction projects as well as legal procedures such as planning diagrams, applying for construction permits, completing processes, applying for fire prevention and fighting certificates,... If you have any questions or are in need of designing and perfecting the process, please contact us immediately for a free consultation!

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