Types of retaining walls based on the design
Retaining walls are structures that are used to hold or restrict soil behind it. They are used in the construction of basement walls in buildings and dams. Various materials can be utilized in the building of the retaining walls. They include blocks of concrete; concrete poured into the framework, treated timber, rocks or large boulders. The use of the above materials depends on the intended lifespan for the retaining walls and the required strength. This is because there are materials with a shorter lifespan as retaining walls while there are those with a longer lifespan.
The use of the walls is necessitated by the fact that there is growing scarcity of affordable and easy-to-build-upon land. As a result, contractors, engineers, and developers have come up with methods of using the undesirable land. The retaining walls enable the usage of previously unusable land such as and in areas with rough terrain or steep slopes. The retaining walls, therefore, allow for the reclamation of this land through flattening it making it fit for development.They also reduce the bulk excavation needed to achieve desired ground profiles. Besides the classification of the walls based on the materials used, the retaining walls are classified based on the structure, retaining methodology and anchorage or support that the walls have. Such classifications include the cantilever wall, gravity retaining walls, and piled walls. The application of the above systems is dependent on the soil and groundwater conditions, retained height, large deflections from the the differential settlement, desired wall geometry regarding the angle of the wall, and the desired lifespan.
a) Cantilever walls
They are those retaining walls that do not require external supports to enhance their strength. They can restrain the soil or the water by the internal strength as a result of the design. They are constructed using concrete. However, reinforced concrete walls have several drawbacks in their construction as cantilever walls. They include the high construction costs; they cannot be used in construction in areas experiencing wintry weather/ also, they are not beautiful, are fragile and develop cracks when subjected to forces caused by the settling of the restrained material exceed the strength of the concrete.
Reinforcement of gravity walls
b) Gravity walls
Gravity walls are those who can retain the soil or the water by the weight of the materials used to construct them. The walls mainly use soil stabilized with man-made materials. As a result, they are cheap to erect and can be constructed in typically every climatic condition. The walls also have a small degree of flexibility and are as a result able to withstand the forces caused by the differential settlement of the retained material. EnviroGrid is the technology/ manmade stabilization method used in this type of walls. It can also be used to create steepened slopes. These are inclinations that are erected in a manner much steeper that the expected slope of soil alone. The inclination angle is usually greater than 70°.In addition to the retaining of soil, Envirogrid or steepened slopes are also constructed when a rapid change in the gradient is required. Such walls work by confining soil or other materials allowing them to behave like a reinforced mass.
c) Pile walls
Pile walls are that whichare drilled into the ground from a point of higher elevation and excavations are done on one side of the walls after they have gained strength. The piles involve the use of primary and secondary pipes. The primary piles are installed first then secondary piles. There are various types of pile walls that include soldier, contiguous, secant, and sheet piling walls. Soldier pile walls are made of preconfigured elements mounted into pre-bored cavities or reinforced concrete cast-in-place piles. The piles are stationed at approximately 1.5 to 2.5 meters apart. In between the piles, concrete is sprayed to enhance the strength. They can either be of cantilever or gravity wall category (Vibropile).
Contiguous pile walls are non-waterproof walls constructed using the cast-in-place piling technique. In this type, the piles formed are closely spaced with a pile diameter of 40 to 90 centimeters and can either be cantilevered or anchored. The method of installation does not involve the driving of the piles into the earth, and this is thus advantageous in areas where vibrations and noise are undesirable. Secant pile walls are composed of interconnecting major and minor Continuous Flight Auger or Bored piles. As a result, this piling method offers superior waterproofing compared to contiguous piles. This type of piles is recommended when excavations are required in materials located beneath the water table to avoid the loss of soil between the pillars. Also, there are two types of secant piles. They are the hard/ soft and the hard/ hard piles. In both cases, principle unreinforced pillars are initially installed followed by the secondary reinforced piles.
soldier pile retaining wall
Sheet piling walls use interconnecting steel sheets and are available in a variety of segments to befit the application. They are used to offer support in trenches, marine walls support and for cofferdams. They are used for both temporary and permanent structures, a significant deviation from the other types of piles. The interlocking parts of the sheets are also known as clutches. They cause the sheets to form the endless barricade. They are installed using vibratory hammers depending on the conditions of the soil in the area and employ the use of guide frames during installation to enhance accuracy. The thickness of the sheets offers different degrees of stiffness, showing that thickness and stiffness are directly proportional.
Numerous contemplations should be made during the design of the walls. For example, gravity retaining walls should have sufficient weights and width or be supported. This is done to prevent the overturning of the walls due to the forced exerted by the restrained materials. The design considerations should be made to factor in outward and the in ward firmness. Outwardfirmnessis hindered by the failure in the global stability, basal sliding, tipping over and bearing capacity. Global stability is the solidity of the barricade, the soil behind and beneath the barricade. Thorough soil analysis is conducted to promote the global stability.
Basal sliding is the outward advancement of the lowermost parts of the retaining wall. It is caused by the horizontal forces exerted by the earth or water pressure. The forces that resist these forces are the friction between the soil under the bottommost layer and the first layer of the wall. If the resistive force is overcome, sliding occurs. To counter this, the designer may increase the basal length of the wall to increase the surface area of the area in contact. Also, fill material with significant roughness may be used to increase the friction. Overturning, on the other hand, is the tipping over of the walls about a common axis. The force causing the overturning is given by the product of the sum of each force causing the unstable nature and the moments. The stabilizing force is provided by the product of the weight of the barricade and the horizontal distance from the rotational axis to the center of the wall. Bearing capacity is the capability of the underpinning soil to withstand the wall’s weight. It is promoted by the increase in the basal area of the wall.
Internal stability is the capability of the different entities of the barricade to behave as one component. The designs should result in the construction of a harmonious unit in which the individual parts do not slide over or pull out.
Also, the gravity walls that are constructed from interlocking blocks are easy to assemble. The blocks have an inbuilt mechanism in them which helps them to interlock with each other. Such blocks are fairly easy to construct as a result. First, a foundation for the blocks is prepared through the digging of a trench along a predefined path. For a sloping trench, steps should be made to keep the blocks level. The dirt in the trench is the tamped down to keep the trench level, and it is then filled with about three inches of paver base that is spread and then tamped down. For the first row, the flanges are knocked off and the first block put in place beginning with the end with the lowest elevation in the case of sloping trenches. The level of the blocks should also be checked to provide a level wall.
The next step is placing the next blocks alongside the first block ensuring that they are all to the same degree. The next series of rows are then installed, and drainage laid behind the blocks through the use of landscape fabric. The blocks are adjoined using concrete adhesives to promote strength. This type of retaining walls is only suitable for the retaining of a small volume of earth.
There are different installation procedures for the various types of the retaining walls. They are outlined as follows. The building of the gravity wall EnviroGrid retaining wall involves the indication of the design and plan to be used in construction in the construction drawings. Site preparation is done through the clearing of vegetation and removal of debris. Initial excavations, earthworks or files are then done. The foundation soil is then prepared, and the footing section of the wall’s base expanded as specified. The footing section is then overfilled with the specified infill material and then leveled. The fill material is then compacted. Drainage systems are then installed as shown in the design papers. The EnviroGrid sections are connected, and the geosynthetic reinforcement later established.
As discussed earlier, most pile walls are installed using the vibratory hammer method. However, in the case of steel piles, care should be taken to prevent instances where the steel bends or breaks due to obstructions. This is caused by the fact that steel is sensitive to obstacles. This problem is overcome by predrilling. The installation of the pile walls involves various risks and challenges besides the bending of the steel piles. The second challenge is the vertical tolerance or the height to which the piles can reach above the ground. Various codes exist which give the required guidelines for the depth of the piles that could achieve certain heights. This issue has been addressed by the development of hammering and installation equipment. Vertical tolerance helps in providing the strength and water tightness of the wall.
All retaining walls experience the problem of movement during excavation and due to the settlement of the soil. This problem is, however, more significant for steel retaining walls due to their flexible nature. The movement of the walls is particularly problematic if there are adjacent buildings that may result in damages. Moreover, the vibrations caused when driving the piles into the earth could lead to damages to the buildings. If the pilings are made of CFA piles, considerations should be made in granular ground conditions. In such cases, uncontrolled horizontal soil excavations may occur if the penetration rates are not kept at an optimum constant rate. Besides, piled walls that are not cantilevered require the provision of horizontal anchorage to prevent unnecessary displacement. It is not advisable to do excavation works at the base of the retaining walls.
There are various advantages of using certain piles over others. When secant pile walls are used for deep excavations, they have the following benefits: better building alignment suppleness, better stiffness when equated to sheet piles. They have less noisy installation and can be installed in the difficult ground where there are large boulders. They however also have several disadvantages. For example, it is hard to achieve a certain vertical tolerance for deep piles; total waterproofing is not attainable particularly at the joints. They are more expensive to install (Vibropile).
Soldier piles, on the other hand, are advantageous due to their inexpensive installations and the ease and speed of construction. On the downside, the piles have several disadvantages including the fact that they are restricted to the temporary building, they cannot be used in areas with high underground water levels before water removal, and they are not stiff or resistive. Also, when used, it 's hard to control the basal movements of the walls, and they experience significant surface settlements (Vibropile).
Each type of retaining wall has its sets of advantages, disadvantages and risks and it is the work of the designer of the wall to conduct the due diligence to determine the best wall to put in place. There are walls whose internal components provide the required strength just like the cantilever beams in buildings. These are the cantilever retaining walls. The other classification is the gravity walls that provide their stiffness through the weight of the materials that make up the wall. The drawback to this particular wall is the fact that they can only be used for short heights. The other classification of the walls is the walls that require anchoring or support through the use of trusses or lateral anchors. The anchors are used to prevent the horizontal displacement of the walls.