CLASSIFICATION OF HOW TO CHOOSE THE BURY DEPTH OF A SHALLOW FOUNDATION STRUCTURE

1. Concept of shallow foundation. Advantages and disadvantages of shallow foundation?

1.1. Concept

Shallow foundations are foundations built on top of a bare pit, then filled back up, the depth of the foundation is about 2/3m, in special cases it can be up to 5m deep.

The basic difference between shallow foundations and deep foundations is that for shallow foundations, the friction on both sides of the foundation is ignored, while for deep foundations, the friction around the foundation must be taken into account (ignoring the work of the soil from the bottom of the foundation up, both in terms of friction and adhesion of that part of the soil to the side wall).

In addition to understanding what a shallow foundation is, its application is also very important. Shallow foundations are used for small and medium-sized projects (usually ≤ 5 floors). This type of foundation is very popular in Vietnam and is the "cheapest" type of foundation. This type of foundation takes advantage of the working capacity of the top soil layers. Therefore, the stability of load-bearing capacity (represented by the conventional load-bearing capacity index R0) and deformation (total deformation modulus E0) of these soil layers determines the stability of the structure.

1.2. Advantages of shallow foundations

• Simple construction, does not require much complex construction equipment. Shallow foundation construction can use manual labor to dig the foundation, in some cases with a large number of foundations, or a fairly large depth, machines can be used to increase productivity and reduce foundation construction time.
• Shallow foundations are widely used in small and medium-sized construction projects, the cost of foundation construction is much less than deep foundations.
• In the calculation process, the work of the soil from the bottom of the foundation up is ignored.

1.3. What are the disadvantages of shallow foundation structures?

Due to the small depth of the foundation, the stability of the shallow foundation against overturning and sliding is poor (withstanding moment and horizontal force). The upper soil layers do not have a large load-bearing capacity (except when the bedrock layer is located near the ground surface), so the load-bearing capacity of the soil is not high, so the foundation can only withstand the load in small projects. In cases where the surface water level is deep, the construction method is relatively complicated due to the need to increase the length of sheet piles and auxiliary works during construction.

2. What are the geological conditions for applying shallow foundation structures?

What are the geological conditions of the project to use the shallow foundation method?

In general, clay layers (clay loam) in a hard to hard plastic state with a large enough thickness (usually 5 → 7 m) distributed on the top can be used for shallow foundations.

The common foundation burial depth is from 0.5 m to 3 m, depending on many factors such as the thickness of the backfill layer, the depth of the underground water level, and the distribution of weak soil. The greater the foundation burial depth, the higher the bearing capacity of the foundation soil, but attention should be paid to the weak soil layers (mud or clay soil with a plastic, flowing state) distributed below it.

If there is weak soil located right under the good soil layer (quite common) and within the range of influence of subsidence stress (usually 5 → 10 m below the bottom of the foundation), limit the foundation burial depth to take advantage of the thickness of the good layer above. If the foundation depth is too large (high excavation and embankment costs, affecting neighboring structures during construction), it is necessary to consider other solutions such as bamboo piles, cajuput piles (if there is underground water) or pile driving solutions.

In the design calculation, the foundation size must be appropriate, usually 0.8m to 1.4m. Larger foundation sizes are often unrealistic, calculating the opening angle will be complicated and the house foundation cos (which almost no one cares about).

When the load bearing capacity problem is stable (ie the load of the structure transmitted down is less than the load bearing capacity of the ground), it is necessary to check whether the settlement of the foundation is guaranteed (ie the deformation problem)? The house usually has a settlement limit of Sgh ≤ 8 cm.

3. Classification of shallow foundation structures and structure of each type of foundation

3.1. Classification of shallow foundations based on load characteristics

Based on the load situation, shallow foundations are classified

• Footballs that bear the load at the center
• Footballs that bear the load at the center
• Footballs that bear the load at the center
• Footballs that bear the load at the center
• Footballs that often bear large horizontal forces (retaining walls, dams, etc.)
• Footballs that mainly bear vertical loads and small momenta.

3.2. Classification of shallow foundations based on the stiffness of the foundation

Absolutely stiff foundations: Foundations with very high stiffness (considered to be infinite) and very small deformation (considered to be nearly 0) belong to this type of foundation, including brick, stone, and concrete foundations.

Soft foundations: Foundations that can deform at the same level as the ground (large deformation, high bending resistance), reinforced concrete foundations with a long/short edge ratio > 8 times belong to the soft foundation category.

Finitely stiff foundations: Reinforced concrete foundations with a long/short edge ratio < 8 times. The calculation of each type of foundation is different, with soft foundations the calculation is complicated more.

3.3. Based on the manufacturing method, shallow foundations are classified

Based on the manufacturing method, people divide them into monolithic foundations and prefabricated foundations.

Monolithic foundations: Foundations are made of different materials, manufactured right at the construction site (cast-in-place foundations).

Prefabricated foundations: Foundations are made of many prefabricated prefabricated blocks assembled together during construction of the foundation of the project.

3.4. Classification by structure or working method of shallow foundations

3.4.1 Single foundation

Single foundations are the most common type of foundation, often used in civil construction works with medium and weak loads such as 1-storey, 2-storey and 3-storey houses. This type of foundation is very easy to construct and does not cost too much as other types of foundations.

​Structure of single foundations: Single foundations are made of a thick layer of reinforced concrete with a single column. For civil and industrial works in general, the foundation must be placed on a good layer of soil and at least 1m deep to avoid changes between the border areas of good and bad soil layers. The foundation should not be placed on the ground or on a newly built foundation to avoid damage from weather factors such as erosion, landslides or subsidence due to new soil. The foundation must be placed at a depth to avoid the expansion of the type of soil that swells when saturated with water.

​The single foundation assembled under the column can be made from one or more blocks, the weight of each block depends on the lifting capacity of the crane and the means of transport. The weight of a block is about 2.5 to 6 tons. In the monolithic concrete structure, the column steel is placed and anchored to the base plate of the single foundation. This connection detail structure is practically applied in most countries around the world.

3.4.2 Strip foundation

Strip foundation is a type of foundation with one dimension much longer than the other and the cross-sectional shape does not change along the length of the foundation.

Applicable conditions: when the foundation bears a large load, the ground is moderately bad, the foundation is under the columns.

In civil housing construction, this type of foundation is most commonly used because it sinks evenly and is easier to construct than a single foundation. Moreover, the cost is also relatively moderate. However, architects recommend that you should only use it when the soil layer width is <1.5 m to save costs, and if >1.5 m2, you should use raft foundations because the cost will be cheaper.

In addition, if you intend to build a house with more than 3 floors, you should use a strip foundation, from 2 floors or less, a raft foundation can be the best solution to save costs.

What is the structure of a strip foundation in relation to a shallow foundation?

A strip foundation includes a concrete lining layer, a continuous foundation plate connecting the foundation into a block, and a foundation beam.

• The concrete lining layer is 100mm thick.
• Common foundation plate size: (900-1200)x350 (mm).
• Common foundation beam size: 300x(500-700) (mm).
• Common foundation plate steel: Φ12a150.
• Common foundation beam steel: longitudinal steel 6Φ(18-22), stirrup steel Φ8a150,

In construction, strip foundations can be hard foundations, soft foundations or combined foundations.

3.4.3. Slab foundations, raft foundations

Slab foundations, raft foundations have large lengths and widths, culvert foundations, pumping stations, hydroelectric plants, water towers, etc.

Raft foundations: The load of the construction is very large, the soft and weak ground reduces pressure and distributes pressure more evenly. In places with weak ground, using raft foundations will be the safest method because this is a shallow foundation, with weak compressive resistance even if there is no water or due to the structural requirements of the construction.

When constructing a raft foundation, the foundation will be spread across the entire surface of the ground; the foundation columns can be in long strips, checkered or single. When constructing this type of foundation on weak ground; the weight of the foundation will be evenly distributed; causing the load of the construction to be distributed evenly on the ground, avoiding subsidence.

Raft foundation structure:

Raft foundation includes a layer of concrete lining the foundation; foundation slab spread under the entire structure, foundation beam.

• The concrete lining layer is 100mm thick.
• The height of the common foundation slab: 200mm.
• The size of the common foundation beam: 300×700(mm).
• The common foundation slab steel: 2 layers of Φ12a200 steel.
• The common foundation beam steel: 6Φ(20-22) longitudinal steel, Φ8a150 stirrup steel.

4. What are the conditions for choosing the burial depth of shallow foundation structures?

Choosing the depth of foundation burial depends on the following factors:

• Construction geological conditions, hydrogeology of the construction area

Geological and hydrogeology conditions of the construction area; are the factors that have the most influence on choosing the depth of foundation burial,;; in which the location of the bearing soil layer is the most important condition. Depending on the common diagrams in practice to choose the depth of foundation burial; and the appropriate types of foundation.

What are the hydrological conditions to determine the depth of shallow foundation burial? Regarding the hydrological conditions of the construction area, it is necessary to consider carefully; regarding the amplitude of fluctuations in the groundwater level, underground flows can cause sand flow... this is one of the factors that form the basis for choosing the foundation plan; depth of foundation burial, foundation construction methods... When the groundwater level is higher than the foundation base, due to the buoyancy effect of water; it will reduce the stress coefficient acting on the foundation; and limit the ability to resist sliding when bearing horizontal force. Therefore, in all cases, the foundation should be placed above the groundwater level.

• Influence of the load transmission properties of the structure

When the structure is subjected to heavy loads, the foundation needs to be placed deep to reduce the area of ​​the foundation base; and limit the possibility of uneven settlement and deformation of the ground.

When the structure is subjected to large horizontal loads and bending moments; the foundation must also be deep enough to ensure stability against sliding and overturning.

• Influence of the characteristics and requirements of the structure

The depth of the foundation also depends on the presence of structures; such as basements, roads, water pipes, etc. as well as neighboring structures that have been built. The bottom of the foundation must be placed at least 40cm deeper than the basement; and the top of the foundation must be below the basement floor. When the structure approaches underground traffic routes; the foundation base needs to be placed at least 20 - 40cm deeper than the above locations.

Influence of the method of constructing a shallow foundation structure.
The depth of the foundation is related to the foundation construction method. If the foundation depth is chosen appropriately, it can shorten the foundation construction time; and the construction method does not require complexity. Many foundation options can be proposed; the depth of the foundation is to choose the appropriate option. What is a shallow foundation is still a general issue; it is a general name including popular types of foundations; used for civil works; especially single foundations and strip foundations. Based on the nature of the project, geological characteristics; we can determine which type of foundation to use. Choosing the foundation is an issue related to the entire structure of the project; therefore, we must calculate the foundation structure as accurately as possible; determine the most suitable type of foundation.

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Hung Nghiep Phu Construction Investment Co., Ltd. with the mission of providing the best solutions and services, building a prosperous community with customers, Hung Nghiep Phu is gradually affirming its brand through sincere cooperation, with a leadership team with ethical capacity, creativity, high expertise and strategic vision. Hung Nghiep Phu owns a team of skilled, highly qualified employees who will bring customers the best quality technology.

Hung Nghiep Phu Construction Investment Co., Ltd. - specializes in constructing civil and industrial works. We look forward to accompanying customers in construction projects as well as continuous procedures such as planning diagrams, applying for construction permits, completing procedures, applying for fire prevention and fighting certificates, ... If you have any questions or are in need of design, completing procedures, please contact us immediately for free consultation!

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Source: Compiled from the Internet