In this article, I have explained in detail the methods of Deep Excavation.
Excavation is one of the initial and most fundamental issues of construction operations. Excavation types and methods depend on different factors such as the structure to be built, the area and environment where the building will be built, and the type of ground.
Undoubtedly, deep excavations are the riskiest excavation methods among all other excavation methods and require detailed analysis to determine the shape of the excavation.
Deep excavation methods can be listed as follows;
- Open Excavation (Free Tilt Excavation) Method
- Internally Shored Excavation Method
- Island Excavation Method
- Top-Down Excavation Method
- Anadolu Excavation Method
- Open-Close Excavation Method
- Well Type Wall Method
- Anchored Excavation Method
- Passive Anchored (ground nail) Excavation Method
- Prestressed Anchored (ground nail) Excavation Method
Let us examine the points to be considered in these methods listed above and the advantages and disadvantages in separate headings.
What are the Deep Excavation Methods?
1. Open Excavation (Free Tilt Excavation) Method
The open excavation method is used when the surrounding structures do not prevent open excavations and are more economical than supported excavations. The factors determining the stability of the excavation are generally the depth of the excavation, its slope and the groundwater condition.
The open excavation method , which is the most preferred excavation method , can be applied as inclined open excavation and cantilever shoring. In sloping open excavation application, there is no need for any shoring element or retaining structure for excavation . Excavation is done to the desired depth with a certain slope.
Gradual slope for the desired excavation depth, taking into account soil parameters (cohesion, slip angle)and landing with bridles This method; It is preferred for self-sustaining soils with high cohesion. The advantage of the open excavation method is that it can be done in a short time since no support elements are used. However, not using support elements necessitates the construction of a slope or a berm. For this reason, the amount of ground to be moved increases. It can be advantageous or disadvantageous in terms of cost, depending on the suitability of the field and the depth to be built. Although it was frequently preferred in the past years, it is not used much today due to the space limitation and the development of more practical methods. It is a method used in mines and clay pits, environmental waste storage areas and dam reservoir excavations.
As long as leakage forces are controlled in granular (cohesionless) soils, the instability does not go down from the excavation floor. In cohesive soils, on the other hand, instability can develop to include the material on the excavation base as well as the slopes. This type of instability, which is also defined as bottom swelling, is affected by the type of soil, its strength, depth of excavation, slope and berm geometry, groundwater conditions and construction method. Excavation depth, slope, crack system and groundwater are factors in rocks.
2. Internally Shored Excavation Method
Internal excavation excavation method is a method performed by supporting the horizontal soil pressure with horizontal shoring elements to be placed in front of the excavation mirror during excavation. It is an excavation-support system that has been increasingly used in our country due to the lack of construction areas recently.
Internal elixir excavation method It is a method made with elements such as shoring element, breast beam, strut and angle bracket. While the bracing beam transfers the lateral soil pressures to the horizontal element, it serves as a secondary beam by dividing the length of the main beam at regular intervals and minimizes the need for horizontal elements. The posts prevent the system from collapsing due to buckling by reducing the buckling length of the support element. Steel pipes are used as horizontal support elements and the assembly of these elements is made by welding or bolts. The biggest advantage of this method is that it can be applied to any width and depth of excavation. The disadvantage of the method is that the excavation works get harder and slower due to the horizontal and vertical elements in the excavation pit.
3. Island Excavation Method
The island excavation method can be thought of as a combination of many excavation methods such as internally supported excavation method and open excavation method . In order to apply this excavation method, there should be a larger excavation area than in other methods. The structure to be built must be designed in accordance with the formation of cold joints. Before the excavation area is completely excavated, the core part is excavated and the parts close to the vertical shoring elements are bevelled to act as a heel. After the core part of the building is excavated, the beveled parts are excavated and shoring elements are formed between the vertical shoring elements and the building. While the other parts of the building are being built, internal shoring is removed respectively.
It is a very advantageous method in terms of time during the construction phase. In this method; Compared to the internal excavation method, it requires less shoring elements, therefore less cost and labor. The building and shoring element connections, which are not made carefully, may cause problems during the installation and dismantling phase. In addition, the heel part, which is made without creating sufficient passive resistance, may cause excessive deformation and collapse, especially on soft floors.
Since the support elements are based on a certain width of the structure, both the length of the horizontal support elements decreases and their number decreases. In this way, the buckling length of the steel elements is reduced and buckling problems are prevented. The steel element, which has a high cost item, decreases in quantity and provides a more economical solution.
As it has many advantages; It is a method that can not be preferred in applications where excavation, shoring and superstructure works can be applied together and there is not enough space to create beveled steps in front of the shoring system.
4. Top-Down Excavation Method
In the top-down excavation method , the excavation progresses from the top of the land to the foundation. The excavation process is done gradually. First, a vertical shoring element (pile, diaphragm wall, etc.) is constructed. The vertical shoring elements made for lateral support also serve as the carrier element of the building. Beams and floors made for the building also serve as horizontal support elements. The support elements in this excavation are shown in the figure.
The advantage of this method is that the excavation works are carried out together and when the excavation process is completed, the part of the building below the ground level is completed. This process saves serious time. The most important of its disadvantages is that it is an expensive excavation support system.
5. Anolu Excavation Method
Anolu excavation method minimizes the deformation and lateral thrust that will occur by dividing large excavation areas into small pieces within certain limits. In the excavations where wide excavation pits will be formed in order to reduce the displacement that may occur in deep excavations; Instead of doing the entire excavation at once, it is done by dividing the excavation area into several parts and at different times. Thus, the dimensions of the excavation mirror and the displacements that will occur will be reduced. The excavation is shown below in slabs.
The dimensions of the long and short sides of the ano to be made in the Anolu excavation system should be chosen as close to each other as possible in order to create less deformations.
6. Open-Close Excavation Method
Cut and cover excavation method is used in the construction of long and narrow structures such as tunnels. Firstly, after descending to the required depth on the side facades with a piled system or reinforced concrete curtain, the upper part starts to open and the excavation process is carried out up to the excavation level. The ceiling part of the project is covered and then the upper part is filled up to the natural land elevation. One of the most important problems in this method is the groundwater level. If there is a high groundwater level affecting the project, it should be drained with appropriate methods.
It is frequently used in underground works, in the construction of network structures and, recently, in the station connection points on the surface in subway projects close to the surface. It should be applied at a height of approximately 10m in order to achieve a structure-cost balance. The application of cut and cover excavation method is shown below.
In terms of cost and workload on-off excavation method , conventional tunneling method is cheaper and easier than the method. The biggest disadvantages of this method are that existing buildings do not allow using this method in urban projects, environmental pollution, preventing traffic flow, and creating high noise.
7. Shaft Type Wall Method
The well-type wall method is the excavation method performed by supporting the well with horizontal supports after the excavation is made with sufficient width and depth to ensure the stability of the unsupported ground. Horizontal support elements; It generally consists of wooden molds and wedges and is supported by steel elements at certain points. The wooden molds used as horizontal support elements in the shaft type wall method are shown below.
After the excavation process is opened gradually up to the final excavation level, the wall construction starts from the bottom to the top. Shaft type reinforced concrete walls are constructed as 1.5–3.0m wide slabs. Tremie pipes are used in the concrete casting process. Wall construction is repeated up to the upper elevation of the shoring. Groundwater should be below the well depth. It is very difficult to apply on extremely loose grounds and gives good results on rock-like grounds such as sandstone and claystone.
8. Anchored Excavation Method
The vertical shoring elements made in the anchored excavation method and the vertical shoring elements in other excavation methods are almost the same. However, the horizontal support elements used in this method are anchorages. It is the main feature that distinguishes it from other excavation methods. In this method , resistance is provided against the ground behind the vertical shoring elements by anchoring the horizontal shoring element . In other excavation methods, this support is provided from the front of the shoring system, that is, from the excavated part. Below, demonstrations of the anchored excavation method are given.
One of the most important advantages of the anchored excavation system ; It is to give tensile strength to the ground by placing steel reinforcement or ropes in the ground that does not have tensile strength. The ground, on which steel material is placed, gains tensile strength and can withstand lateral thrusts up to a level. Another advantage in excavation methods made with anchor support elements is; Since the support elements are outside the excavation area, they provide a large area to work.
The most important disadvantage of the anchored excavation system ; the internal structure of the floor is not known completely. Facing with melting and deterioration caused by soft soil layers, pressurized groundwater and mineralogical structure in the ground depending on time in the anchorage drilling well; These
are the problems caused by the inability to fully control the internal structure of the floor . Encountering such situations causes great troubles with both the loss of time and the wear of the team and equipment. In addition, during the injection construction phase of the wells that are different from the expected, there may be cases where the production fails because the well cannot carry the desired design load during the stretching phase due to incomplete injection.
Anchor supported excavation method can be collected under two headings;
- Passive Anchored (ground nail) Excavation Method
- Prestressed Anchor Excavation Method
8.1. Passive Anchored (ground nail) Excavation Method
Passive anchor excavation method is the excavation method performed by activating the anchor element used during the excavation after the ground is completely activated without any preloading. This method works like reinforcement on a slope, strengthening the slope and making the slope work like a single piece.
Passive anchored excavation method is mostly preferred in rock or self-holding hard ground. It is applied in deep excavations with greater displacement flexibility and less depth. If it is planned to be applied on floors with high groundwater levels, weep holes should be left at appropriate intervals to drain the water. In addition, the ends of these holes and their surroundings should be wrapped with felt or similar material.
8.2. Prestressed Anchored (ground nail) Excavation Method
The function of the prestressed anchor in the prestressed anchor excavation method ; It is to transfer the lateral thrust acting on the vertical shoring elements to the ground outside the sliding circle and to carry the load formed due to the shear strength of the ground in this part and to ensure the safety of the system. The lateral load from the ground thrust that can be carried to the anchors can vary depending on the anchorage range and is between 200kN and 1000kN on average.
The advantages of the prestressed anchored excavation method are providing a wide working area, saving space since the excavation can be carried out perpendicular to the horizontal, being able to be manufactured in a short time and done with low cost .
The disadvantages of the method are; low anchorage capacity in soils with low bearing capacity and drilling is difficult in applications with high groundwater pressure.