Khái niệm phân loại cách chon độ sâu chôn móng nông

The foundation of a house is always the most important element in the structure of the building in general and the structure of civil houses in particular, but there are many types of foundations with different characteristics and structures, including shallow foundations. So what is shallow foundation? There are several types of shallow foundation and how to choose the depth of foundation, not everyone knows how to determine to choose the right type of foundation.

Shallow foundation structure concept. Advantages and disadvantages of shallow foundation?

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Shallow foundations are those that are built on top of a hole dug in the ceiling, then filled back, with a depth of less than 2÷3m, in special cases up to 5m deep.
The basic difference between shallow foundation and deep foundation is that for shallow foundation, when calculating, the friction on both sides of the foundation is ignored, and for deep foundation when calculating, the friction around the foundation must be taken into account (ignoring the working of the soil from the bottom). foundation and above both in terms of friction and adhesion force of that part of the soil to the side wall)
Besides 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 is a very popular nail type in Vietnam and is the most “cheap” nail type. This type of foundation takes advantage of the working ability of the top soil layers. Therefore, the stability of the bearing capacity (represented by the conventional load-bearing index R0) and the deformation (the modulus of total deformation E0) of these soil layers determines the stability of the structure.

Compared with deep foundations, shallow foundations have the following advantages:

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

What are the disadvantages of shallow foundations?

Due to the small depth of foundation, the stability of the foundation overturning and sliding is poor (withstanding moment and lateral forces). In the upper soil layers, the load capacity is not large (unless the bedrock layer is located close to the ground), so the ground bearing capacity is not high, so the foundation can only bear the load in small works. In case the surface water level is deep, the construction plan is relatively complicated due to the need to increase the length of sheet piles and auxiliary works during construction.

What are the geological conditions for applying shallow foundations?

What are the geological conditions of the project, using the shallow foundation option?
In general, layers of clay (clay) in the hard to hard plastic state with a large enough thickness (usually 5 → 7 m) distributed on top can be laid shallowly.
What are the geological conditions for applying shallow foundations? The common foundation burial depth is from 0.5m to 3m, depending on many factors such as the thickness of the backfill layer, the depth of underground water level, the distribution of soft soil. The greater the depth of foundation burial, the higher the load-bearing capacity of the foundation soil, but it is necessary to pay attention to the soft soil layers (mud or clay type soil with plastic and flowing state) distributed under it.
If there is soft soil located directly below the good soil layer (quite common) and within the influence of the settlement stress (usually 5 → 10 m below the foundation bottom), limit the depth of foundation burial to take advantage of the thickness of the foundation. good layer above. If the foundation burial depth is too large (high cost of excavation and backfilling, affecting neighboring works during construction), it is necessary to consider other solutions such as bamboo poles, melaleuca poles (if there is water in the ground) or other solutions. Pile.
How to determine shallow foundation size? In the design calculation, the footing size must have a suitable size, usually 0.8m to 1.4m. Larger foundation sizes are often impractical, calculating the opening angle will be complicated and cost the foundation (which almost no one cares about). When the load bearing problem is stable (ie the load of the building is smaller than the load capacity of the ground), it is necessary to check whether the settlement of the foundation is guaranteed (ie the deformation problem)? Ordinary houses have limited settlement Sgh ≤ 8 cm.

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Classification of shallow foundation and structure of each type of foundation

1. Classification of shallow foundations based on load characteristics

Based on the effect of the load, shallow foundations are classified

  • Foundation load bearing right center
  • Eccentric load bearing foundation
  • Foundations of tall buildings (water towers, chimneys, etc.)
  • Foundations often bear large horizontal forces (retaining walls, dams, etc.)
  • The foundation is mainly subjected to vertical loads, with small momen.

2. Classification based on nail hardness

  • Absolutely hard foundation: The foundation has a very high stiffness (regarded as infinity) and a very small deformation (regarded as close to 0) of this type with brick, stone, and concrete foundations.
  • Soft foundation: The foundation has the same deformability as the foundation soil (large deformation, high bending resistance), reinforced concrete foundation with long/short side ratio > 8 times belongs to the soft foundation type.
  • Finite hard foundation: Reinforced concrete foundation with long side/short side ratio < 8 times. The calculation of each type of foundation is different, with soft nails, the calculation is more complicated.

3. Based on how to make shallow foundation classification

Based on the manufacturing method, people are divided into block foundation and assembled foundation.

  • Solid foundation: The foundation is made of different materials, fabricated at the construction site (cast-in-place foundation).
  • Assembled foundation: The foundation is made of many prefabricated blocks put together when building the foundation.

4. According to structure or way of working, how to classify shallow foundation

a. Single foundation: Single foundation is the most common type of foundation, often used in civil constructions with medium and weak loads such as 1-storey houses, 2-storey houses and 3-storey houses. This type of foundation is very easy to construct and does not cost as much as other types of foundation.
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What is a shallow foundation that can be visualized through a single foundation construction work?

Single foundation structure: The single foundation is composed of a thick reinforced concrete layer with a single pillar. For civil and industrial works in general, the foundation bottom needs to be placed on a good layer of soil and has a depth of at least 1m to avoid the change between the boundary of good and bad soil. foundation on the surface or on the newly laid foundation to avoid the damage of weather factors such as erosion. landslides or subsidence due to new ground. The foundation must be placed at a depth to avoid swelling of the swelling soil when saturated with water.
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Structural drawing of a single foundation – a simple and common type of shallow foundation

Single foundation mounted under the column can be made from one or more blocks, the weight of each platform depends on the lifting capacity of the crane and the means of transport. The weight of a block falls between 2.5 and 6 tons. In a monolithic concrete structure, steel columns are placed and anchored to the base plate of a single foundation. This detailed link structure is practically applied equally in most countries around the world.
b. Tape nail: is a nail with size 1 much longer than the other and the cross-sectional shape does not change along the length of the nail.
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Ice foundation is a type of shallow foundation explaining what is shallow foundation

Conditions of application: when the foundation is subjected to a large load, the ground is of medium bad quality, the foundation is under rows of columns.
In civil construction, this type of foundation is most commonly used because it settles evenly and is easier to construct than a single foundation. Moreover, the price is also relatively affordable. However, architects recommend that you only use it when it has a soil width of <1.5 m to save costs, and > 1.5 m2, it is recommended to use raft foundations, the cost will be cheaper.
In addition, if you intend to build a house on 3 floors, you should use ice foundations, from 2 floors, rafting can be the best measure to save costs.
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Ice foundation construction works – a common type of shallow foundation

What is the ice foundation structure in relation to the shallow foundation?

  • The ice foundation consists of the concrete lining for the foundation, the continuous running foundation plate that connects the foundation into a block, the foundation beam.
  • The concrete lining is 100mm thick.
  • Common foundation plate size: (900-1200)x350 (mm).
  • Universal foundation beam size: 300x(500-700) (mm).
  • Common base plate steel: Φ12a150.
  • Common foundation beam steel: longitudinal steel 6Φ(18-22), belt steel Φ8a150.

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Construction of one-way ice foundation in civil works

In construction, ice foundation can be hard foundation, soft foundation or a combination foundation.
c. Ban foundation, raft foundation: large length and width, culvert foundation, pumping station, hydropower plant, water tower…
raft foundation: The construction load is very large, the soft ground can reduce the pressure and distribute the pressure more evenly. In places with soft ground, using raft foundation will be the safest method because this is a shallow foundation, weak compressive resistance even in the absence of water or due to the structural requirements of the building.
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Raft foundation works – one of the types of shallow foundations divided by structure

When the raft foundation is constructed, it will spread the foundation all over the ground surface, the foundation columns can be in long strips, checkered or single. When constructing this type of foundation on soft ground, the weight of the foundation will be evenly distributed, causing the work load to be evenly distributed on the ground, avoiding subsidence.
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Drawings of raft foundations in residential buildings

Structure of raft foundation:
The raft foundation consists of a layer of concrete lining the foundation, the foundation slab spreading under the entire building, the foundation beam.

  • The concrete lining is 100mm thick.
  • Universal footing height: 200mm.
  • Dimensions of common foundation beams: 300×700(mm).
  • Common base plate steel: 2 layers of steel Φ12a200.
  • Common foundation beam steel: longitudinal steel 6Φ(20-22), belt steel Φ8a150.

What are the conditions for choosing shallow foundation depth?

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How to choose a shallow foundation depth depends on many factors

The choice of foundation depth depends on the following factors:
– Construction geological conditions, hydrogeology of the construction area:
The geological and hydrogeological conditions in the construction area are the factors that have the most influence on the selection of the foundation depth, in which the location of the bearing soil layer is the most important condition. Depending on the popular schemes in practice to choose the appropriate foundation depth and 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 carefully consider the amplitude of fluctuations of the groundwater level, the underground flow can cause the phenomenon of sand flow…this is one of the factors underpinning the development of the project. the choice of foundation plan, foundation burial depth, foundation construction methods… When the groundwater level is higher than the foundation base, due to the floating head effect of water, it will reduce the stress ratio acting on the foundation and limit slip resistance when subjected to horizontal force. Therefore, in all cases should try to lay the foundation above the water table.
– Influence of the load transmission properties of the building
When the building is subjected to a large load, the foundation needs to be placed deep to reduce the area of ​​the foundation base and limit the possibility of settlement and uneven deformation of the foundation soil.
When the structure is subjected to large lateral loads and bending moments, the foundation must also have a sufficient depth to ensure stability in sliding and overturning.
– The influence of the characteristics and requirements of the works;
The depth of the foundation also depends on the presence of works such as basements, roads, water pipelines…as well as neighboring constructions already 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 work approaches the underground traffic, the foundation base should be placed at least 20 – 40cm deeper than the above positions.
– Effects of foundation construction methods.
The foundation depth is related to the foundation construction method. If the foundation depth is selected appropriately, the foundation construction time can be shortened and the construction method is not complicated. It is possible to propose many foundation options, foundation burial depth to choose the appropriate plan.
What is shallow foundation is still a general issue, it is a common name that includes common types of foundation used for civil works, especially single foundation and tape foundation. Based on the nature of the work and the geological characteristics, we can determine which type of foundation should be used. Foundation selection is a problem related to the entire structure of the building, so we must calculate the most accurate foundation structure, determine the most suitable type of foundation.

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