Solutions for building civil engineering foundations

For a construction project, foundation construction is extremely important, so choosing a reasonable civil construction foundation solution is also essential so that the project still ensures its load-bearing capacity. , ensuring construction progress without waste.

Solutions for building civil engineering foundations

Arguing for solutions to build civil engineering foundations is very important, from which we can propose a reasonable and complete type of work and survey volume. The argument is conducted on the basis of existing geological survey documents of the project in the previous stage and the expected project scale.

Our work is to demonstrate the solution to build appropriate foundations for civil works. Of course, the geological surveyor of the project is not the foundation designer, but must have certain knowledge about the foundation to be able to advise the designer on appropriate foundation solutions (if the project does not have any). special).

But not everyone realizes the important significance of this argument, especially students and less experienced geotechnical engineers. First, we must understand that geological survey work helps designers choose the most economical solution for building civil engineering foundations, but must ensure technical and safety standards.

That is, always choose the solution to build the foundation of civil works with the lowest cost and simplest construction. After calculating stability (according to bearing capacity and deformation), the foundation option that is guaranteed is selected. If stability is not guaranteed, plan Construction of civil engineering foundations Others will be chosen to ensure technical requirements, of course the cost will be more expensive. Cost level for the solution Construction of civil engineering foundations gradually increase (relatively compared) according to the following nail types:

Solutions for building civil engineering foundations
– Simple tape nails.
– The strip foundation has been reinforced with bamboo piles, cajuput piles, sand cushions, etc.
– Nails driving pile.
– Pressed pile foundation.
– Nails pile.

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Centrifugal pressing of concrete piles

Arguing for foundation solutions requires geological engineers to have certain knowledge about foundations, practical experience and logical thinking.
First, we must talk about the construction load transmitted to the foundation, there are two types: transmitted to the column (concentrated load, calculated on 1 column, usually calculated for the pile cap) and evenly spread load, calculated according to the length of the foundation (foundation ice). The transmitted load is related to the foundation plan.

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Build a solid foundation

1. Shallow foundation option:
Shallow foundations are used for small and medium-sized projects (usually ≤ 5 floors). This type of nail is very popular in Vietnam and is the "cheapest" type of nail. This type of foundation takes advantage of the workability of the top soil layers. Therefore, the stability of the 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 project.

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Shallow foundation

What are the geological conditions of the project so the shallow foundation method should be used? In general, layers of clay (mixed clay) in a hard to hard plastic state with a large enough thickness (usually 5 → 7 m) distributed on top can be used to place shallow foundations. The common foundation burial depth is from 0.5m to 3m, depending on many factors such as the thickness of the soil layer, the depth of underground water level, and the distribution of soft soil.

The greater the depth of burying the foundation, the higher the load-bearing capacity of the ground, but attention should be paid to the soft soil layers (mud or clay-type soil with a flexible, flowing state) distributed under it. If there is weak soil located right below the good soil layer (quite common) and within the influence range of stress causing settlement (usually 5 → 10 m below the bottom of the foundation), limit the depth of burying the foundation to take advantage of the thickness of the soil. good layer on top.

If the depth of foundation burial is too large (high excavation costs, affecting neighboring structures during construction), other solutions such as bamboo piles, melaleuca piles (if there is underground water) or other solutions need to be considered. Pile.

In calculating shallow foundation design, the foundation size must be appropriate, usually 0.8m to 1.4m. Larger foundation sizes are often impractical, calculating the opening angle will be complicated and the floor cos (which almost no one cares about). When the load-bearing capacity problem is stable (ie the transmitted load of the structure 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)? Normal houses have limited settlement Sgh ≤ 8 cm.
If one of the two problems of load bearing capacity or deformation is not satisfied, you must change to another foundation plan, which is the pile foundation.

2. Foundation options for pressed piles and driven piles (friction piles)

Used when the shallow foundation option is not technically satisfactory (unstable, highly deformed) or the cost of ground treatment in a shallow foundation is too expensive. It could be because the stratigraphy mainly consists of weak soil layers distributed above, good soil lies deep below, or the thickness of the upper layer of good soil is not large enough, the thickness is unstable, weak soil is distributed immediately. below with great thickness. Choosing a pile foundation solution must have a basis. When calculating a shallow foundation, the technique is not guaranteed.

When designing pile foundations, the following issues should be noted:
– Choose piles with dimensions, master steel, and belt steel suitable for reality such as 200×200, 250×250, 300×300, 350×350, 400×400,…
– Select pile depth must be consistent with reality, meaning it can be constructed normally. Usually the design pile's load bearing capacity (PTK) is chosen as the smallest value calculated based on material (PVL), laboratory test (Pđn) and field test (Pht - calculated according to CPT static penetration, standard penetration SPT). In order for piles to meet design requirements, it must ensure:

PVL > Pile pressure > (2÷3) x PTK

In there:

PVL: Pile bearing capacity according to material.
Pile pressure: Pressure on pile head.
PTK: Pile bearing capacity according to design.

Many THs deposit too deep compared to reality, leading to Pđn (or Pht) having an approximate or even greater value than PVL?! That is unreasonable because it is impossible to bring the pile to that depth with normal pressing or driving methods.

For example: PVL = 120 T, Pđn = 80 T ⇒ PTK= Pđn = 80 T (because it is smaller).
PVL = 120 T, Pđn = 180 T ⇒ PTK= PVL = 120 T (because smaller)

Unfortunately, these errors are very common.

In case the pile is too shallow, PĐn (or Pht) is much smaller than PVL, so it does not take advantage of the pile's working capacity, causing waste (having to increase the number of piles in the station while only increasing each pile by a few meters). is the increased load-carrying capacity). The direction of the pile is usually determined by the pile pressure or by the distance with the driven pile. This leads to PTK often fluctuating within a certain range as follows:

15 To 25T (200×200 pile)
20 To 35T (250×250 pile)
35 To 55T (300×300 pile)
50 To 70T (350×350 pile)

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Pressing piles of people's houses

Thus, with a determined pile size, PTK only reaches a certain value, leading to the design pile depth having to be appropriate (not placed anywhere). What about the number of piles in 1 station? When calculating, many THs use arithmetic rounding, which means rounding up if the odd number is greater than 0.5 (for example, 3.6 is rounded to 4 piles) and rounding down if the odd number is less than 0.5 (for example, 3.2 rounded into 3 piles). The case of rounding down is very dangerous because the remaining piles have to carry the excess load, easily causing instability.

In addition, pay attention to construction conditions and methods. In urban areas, the pile driving method cannot be used. In narrow areas, the counter-loading method cannot be used (the anchoring method must be used). If the house is built side by side, it cannot be pressed close to the neighboring house. In many cases, the pile does not reach the design depth due to the friction of the soil layers above being too large (leading to cases of pilot drilling)...

3. Bored pile foundation plan:

The bored pile foundation option is often used for high-rise buildings (usually over 10 floors). Bored piles are often more expensive than pressed piles, so no one wants to use them, except in cases where forced piles (or driven piles) do not meet technical requirements. The quality of pressed piles is usually more stable and easier to control than pile. For high-rise buildings built on weak geological foundations such as District 2 and District 7, bored piles are often used because they can bear a larger load.

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