The axes of activity and providing services of Istasazeh Engineering Construction Co. in Earth retention (Deep excavations stabilization) are generally as follows:
- Soil Nailing
- Ground Anchors (Tiebacks construction)
- Top Down Construction
- Strut of a Braced Wall System (bracing strut)
- Berlin wall – Soldier Pile wall
- Truss Retaining Structure
- Diaphragm walls – Slurry walls
- Sheet Pile
- Geosynthetic reinforced soil
- Monitoring of Excavation Walls
- Surveying
The axes of activity and providing services of Istasazeh Engineering In each of the above methods, according to the design requirements, a member can exist as a retaining wall. This member can be a diaphragm wall, concrete piles, jet grouting columns and deep mixing, metal retaining piles, sheet pile or similar members. Of course, in the nailing method, a layer of shotcrete is usually used as a cover.
Different retaining walls each have special advantages and limitations that depending on the projects, any one or a combination of them can be used.
1) Soil Nailing
Nailing the walls means in-site reinforcing the existing soil mass by installing nails at close distances on a sloping surface or in the excavation site vertically and by Top-Down construction. Nails are usually placed in boreholes drilled in the earth wall, which are filled with a cement slurry to prevent corrosion of the nails and proper transfer of forces between the soil and the nail. The Soil nailing method creates a stable reinforced cross-section that can hold the soil behind it
The method of soil nailing was first discussed in 1960. Soil nailing method is based on the method of bracing and screwing stones. This excavations stabilization system was first used for tunneling in Austria in 1960. In this tunnel, reinforcement, cement slurry injection and shotcrete were used for stabilization. In the following years, this method gradually spread around the world. In Iran, the use of the soil nailing method has developed significantly in recent years and is one of the most practical methods, especially in urban construction.
From the point of view of scientific principles in geotechnical engineering the soil nailing method is robust and exerts its effect through soil-nail interaction, resulting from deformation in the soil. Therefore, the use of soil nailing system in situations where the sensitivity to displacement due to excavation is high or in the vicinity of sensitive, old and worn structures or includes soft and medium soils in terms of stiffness and strength, is not recommended.
Stages of soil nailing installation
In the soil nailing method, the soil nails often act in tension, but in certain conditions, their flexural and shear performance are also considered. The effect of nail reinforcement to improve wall stability is achieved with the following two functions:
- Increasing the vertical force and thus increasing the shear strength of the slip surface in frictional soils
- Reduction of slip surface thrust in friction and cohesive soils
After the reinforcement nails are installed, a thin surface wall, usually containing shotcrete, along with a layer of lightweight steel mesh reinforcement, covers the surface of the gable or excavation wall. The goals of creating this wall are:
- Preventing surface erosion of soil
- Creating a more suitable finished surface for possible future constructions
- The more important purpose of constructing these walls is to increase the efficiency of the consolidated soil system, especially in the parts close to the excavation wall.
This concrete-steel layer also helps to better transfer the driving forces to the reinforcing elements. The final cover (Facing) can be designed and executed in the following 2 ways:
- Temporary stabilization
- Permanent stabilization
Soil nailing is a relatively new method for stabilizing deep excavations or earth retention
, which due to its economics and unique advantages, as a very suitable solution has many uses in various cases, including:
- Stabilization of slopes and trenches
- Increasing bearing capacity
- Limiting deformations by minimizing interference with the natural state of the earth
The use of soil nail technique has been widely used in the last two decades in most developed and developing countries to Earth retention as well as stabilize natural slopes. Some of the wide applications of this method of soil reinforcement include stabilization of natural slopes or trenches adjacent to roads, widening of roads, excavation in the vicinity of existing structures, stabilization and strengthening of old retaining structures, etc.
2) Ground Anchors (Tiebacks construction)
The principles of execution of the soil anchoring method have many similarities with the soil nailing method. The main difference is in the application of post-tension force to the reinforcing element. Execution stages are similar to the soil nailing method and include cross-sectional excavation, drilling of boreholes, installation of reinforcing nails, injection of cement slurry (in part of the length of the borehole), shotcrete and installation of nail and nut top plate. After a suitable period of time to process the cement slurry, by installing the base of the jack, tensile jack and dynamometer, the post-tension forces up to the desired amount of design is applied to the reinforcement nail and the surrounding slurry.
Stage 1: Excavation is done until the installation level of the first row of ground anchors.
Stage 2: Borehole drilling is done to perform anchoring.
Stage 3: Inhibitory placement and injection
Stage 4: After setting the slurry, stretching and excavation operations and concrete coating are performed.
In soil anchoring method, it is possible to apply for permanent coverage if needed. These coatings can also be of the type:
- Shotcrete
- Precast concrete
- Fine concrete in place
- Other newer technologies for executing patterned and environmentally friendly covers
In the soil anchoring method, there are various injection methods to provide resistance to the bond length.
3) Top Down Construction
In the Top Down construction method, instead of constructing a retaining structure, the main structure itself is executed from top to down and excavation is performed simultaneously. Top Down Construction method was used in the construction of metro stations in Europe to reduce time and operating costs and reduce disruption in urban traffic and people’s lives by making changes in the design and calculation of structures and having some special equipment and facilities. Due to its advantages in recent years in large projects have practically replaced the traditional construction method.
Construction, traditionally known as the Bottom-Up construction method, begins with excavation and then foundation, and ends with the construction of the main structure and the completion of the final roof.
Unlike this method, there is another method called top down Construction, which in recent years in large projects has practically replaced the traditional construction method. Unlike the traditional method, Top Down Construction method begins with the construction of round walls, columns and the final roof and ends with the completion of excavation and construction of the foundation of the structure. The most important advantages of top down Construction method can be mentioned to eliminating or minimizing the levels of formatting and eliminating or reducing temporary braces.
Stages of the top down construction method installation
In the top down construction method, first a diaphragm wall, steel pile or concrete pile is constructed around the perimeter. Then the columns of the structure are executed with a foundation of deep or semi-deep type and are executed from top to down of the main structure simultaneously with excavation, respectively. If the soil condition is good in terms of strength and groundwater conditions are favorable, the wall can be executed step by step simultaneously with the execution of the floors from top to down.
In top down Construction method, the whole structure or part of it can be executed from top to down. If the dimensions of the project are small plans, it is easily possible to execute the entire basement structure by the Top-Down construction method. In the case of the large area in the plan, the perimeter strip should be made from top to down method and the central area should be made from the bottom to top construction method. It is also possible to combine top down construction method with other Earth retention methods.
4) Bracing strut or Strut of a Braced Wall System
In the bracing strut method, braced excavationsare used to transfer soil pressure. The components of this system generally include:
- Wall which can be of Sheet Pile type, diaphragm wall, metal or concrete retaining piles, and so on
- Compressive members called Strut or Brace
- The interface members between the wall and the compressive members are called Wale.
In bracing strut method, the soil pressure is transferred to the wale through the wall and to the struts through the wale. As a result, the struts and their connection are designed based on the compressive force, the wales are designed based on the shear and flexural force and the wall is designed based on the combined forces.
In the soil braced excavations method, first, the perimeter retaining piles are executed as a temporary wall and then in each stage of the excavation, the wales are executed and the struts are connected to it.
In bracing strut method, 2D or 3D modeling can be used and the final limit states and operation can be controlled and structural members can be designed based on the obtained forces. Structural members are generally made of steel and are designed in accordance with the regulations of steel structures.
5) Berlin wall – Soldier Pile wall and Lagging System
One of the methods of Earth retention is the Soldier Pile wall method. Soldier Pile wall method is actually a combination of concrete or steel piles and shotcrete. For trapping steel and concrete piles, about 25% of their length is considered as the root of the pile. In-situ or precast concrete surfaces are used to prevent local spills between the piles.
In order to perform concrete surface in-situ, after excavation, a rebar steel mesh (welded wire mesh) is placed between the piles and then a concrete surface is created between the piles by shotcrete.
To stabilize deep excavations, a combination of a Berlin wall with Soil nailing or ground anchors is also used.
6) Truss Retaining Structure
In the truss retaining structure, which is one of the Earth retention methods of semi-deep excavations, the soil pressure on the wall of the excavation is transferred to the bottom of the excavation through horizontal and diagonal members.
Vertical members can be made of steel or concrete. The horizontal and diagonal members of the truss retaining structure are usually made of steel.
Truss retaining structures are suitable for excavations with a depth of less than 14 m and a width-to-depth ratio of more than five. In deeper excavations, it is difficult to provide a safe slope and limit deformations to the allowable level, and it is recommended to use alternative methods if possible.
In the design of generalities and arrangement of truss retaining structure members, to prevent the occurrence of progressive failure phenomenon, it is recommended to increase the degree of uncertainty of the structure as much as possible.
7) Diaphragm walls or slurry walls
Diaphragm wall or slurry wall is one of the methods of Earth retention. In Diaphragm wall method, a concrete wall is executed to protect the collapse of the excavation wall. Sometimes a combination of other Earth retention methods with diaphragm wall method is used to stabilize the excavation.
To execute this wall, special drilling devices called hydrofraise or grab are used. Bentonite slurry is used to prevent local collapse of the drilled wall. After drilling the diaphragm wall, the nail is done and finally the whole drilled part is concreted using a concrete tremie tube and a concrete wall is created. Tremie tube is a tube by which concrete is poured in the lower level of the excavation. Thus, during concreting, the higher specific gravity of concrete compared to bentonite, causes bentonite to rise and the above bentonite can be collected for reuse.
The thickness of the diaphragm walls varies between 0.6 to 1.1 m. The wall is made of panels with a depth proportional to the depth of the excavation. The width of the panels is between 2.5 to 6 m. The cross section of such walls is generally rectangular, but other shapes such as T and L can be used for specific purposes.
8) Sheet Pile
Sheet pile is an optimal and cost-effective method for concrete retaining walls. In the sheet pile method, concrete does not need to be set and excavation can be started after the sheet pile. Therefore, the project execution time is less than similar methods.
9) Geosynthetic reinforced soil
The word geosynthetic consists of two parts: Geo and Synthetic. The word geo is used where it refers to the earth, and synthetic is used where it is made of man-made material. Geosynthetic reinforced soil refers to products that solve geotechnical and soil problems. Some of the products that are called geosynthetic soil are:
- Geotextile
- Geogrid
- Geomembrane
- Geonet
- Geosynthetic clay liner (GCL)
- Geofoam
- Geocell
- Geocomposite
The polymeric nature of the mentioned products makes them very durable. Geosynthetics are used in different forms and different geotechnical, transportation, environmental and hydraulic applications in the following cases:
Roads, airports, railways, earthen dams, retaining walls, water storage pools, dams, canals, erosion control, sediment control, landfill bedding, landfill cover, mines, aquaculture and agriculture.
When geosynthetics are used in contact with soil, stone or any other engineering material, one or more functions of reinforcement, separation, filtration, drainage, dewatering and protection are always expected.
10) Monitoring of excavation walls
Due to the increasing development in the construction industry, the sensitivity of construction and maintenance of new structures needs more careful consideration. Because just as new structures are useful and instructive in human life, inaccuracies in their design, operation, and maintenance may cause irreparable damage. Therefore, in order to maintain accuracy, it is necessary to use special measurement methods and tools. Geomatic is one of the sciences that used for accurate measurements. In this science, various methods are used for accurate measurements, and microgeodesy is one of the most important methods available. Microgeodesy (small-scale geodesy) means determining the location of a small structure or area so that the effect of the earth’s sphericity is not discussed.
The position and shape of each structure change over time. Such as deformation of bridges due to the passage of heavy machinery, deformation of dams due to water pressure, subsidence in a vertical direction due to excavations and drainage of underground resources, as well as relocation of walls due to excavation and stabilization of the excavation.
Therefore, in short, it can be said that a set of measures that can be used by instruments to measure the behavior and study the movements of a large structure such as dams, bridges, oil tanks, structures adjacent to excavation, etc. with great accuracy and using observations and geodetic calculations are called microgeodesy.
Among the results of using instruments systems for monitoring and behavior measurement (Earth retention ) can be mentioned the following:
- Cost reduction: A proper and reliable instrument design in sensitive conditions such as the presence of highly compacted soils or other cases, significantly reduces the risk of hazards. Reducing the design risk increases the reliability coefficients in the execution phase. This will also reduce costs.
- Reducing safety factors: Without the necessary instruments and measurements, the selection of design parameters is based on assumptions that are conservative and as a result, design reliability coefficients and costs will be high.
- Changes in the design: If the designs are not responsive to the conditions, the information obtained from the instruments in the early phases of the project indicates the need to make changes in the design.
- Control of the process and method of executive operations: Instruments in operations such as soil stabilization and remediation and preloading, show information about the progress of operations. For example, the adequacy control of the number of drains used in preloading can be checked by instruments.
- Safety: Instruments warn of potential hazards before they occur, especially on slope stability issues, providing an opportunity to complete operations to eliminate the danger or leave.
In addition to the monitoring issue, quality control of retaining structural components during execution is also of particular importance. One of the control cases of retaining structure is quality control tests of concrete and nail.
The control tests and execution controls also include various components of the retaining structure such as nails and tensile braces. Types of tensile loading tests, loading and unloading cycles, creep testing and loading to the limit are such tests.
11) Surveying
Surveying engineering is one of the technical-university fields that deals with different methods of collecting ground information in order to prepare surveys for different users. Survey is one of the first and most necessary characteristics of any construction activity such as Eartn retention, ground improvement, road construction, power and water transmission lines, construction of tunnels and subways, etc.
Surveying has two phases:
- Measurement: Different devices and methods are used to obtain the information required in the second phase.
- Presentation of results: The results of the work are presented as follows:
- Analog (survey, longitudinal and transverse sections, and so on.)
- Digits (such as tables, digits models of the ground)
