Retaining walls are the structural components, which are designed with the purpose to retain the soil mass of two different ground levels. The retaining wall prevents the failure of the soil mass from sliding, overturning, shearing, and toppling.
Due to the loose mass of soil and gravity, the soil mass tends to move downward. And any vertical forces that are applied over such soil mass will make inclined resultant force due to the presence of friction between loose particles. And this way the lateral component of the force is developed in the soil.
The vertical forces are encountered and transfer to the ground due to the compressibility property of soil. But the lateral forces cause tension to the soil particles and tend to cause crack & displacement. Thus, the retaining wall is designed to resist the lateral forces that are experienced in soil.
The other causes of lateral force are the pore water pressure of soil. But the lateral forces are released instead of withstanding, with the help of the provision of weep holes in retaining walls.
There are different types of retaining walls described in this post. but let us read about retaining wall for further understanding about Types of retaining wall.
Cases of construction of retaining wall
The retaining wall is used to retain earth or other loose materials. These walls are commonly constructed in the following cases;
- In the construction of building basements.
- As wing wall or abutment in the bridge construction.
- In the construction of embankments.
The material which is retained by the retaining wall is called as backfill. The sloping backfill is called as “Inclined surcharge”. The term surcharge means the backfill above the level of the top of the wall.
Types of retaining wall
Following are the common types of retaining wall;
- Gravity retaining wall
- Semi gravity wall
- Cantilever retaining wall
- Counterfort retaining wall
- Sheet pile wall (Bulkheads)
- Anchored wall
1) Gravity Retaining Wall
The gravity retaining wall is based on the theory that the weight of the wall itself would gain enough resistance to counter the lateral pressure and moment due to it. So such walls have voluminous work to be performed.
They are not economical as they have a very large section for large height. They can be made up of compacted filling covered with a tough and impervious layer, or the masonry units such as stones, brick, or plain concrete only.
A gravity retaining wall is that retaining wall in which the weight of the retaining wall provides stability against the pressure exerted by the backfill. Gravity retaining walls are made up of massive stone masonry or plain concrete.
2) Semi gravity wall
The gravity wall-size sections are too large for higher depth. Thus, in order to reduce the size of such a section, a small amount of reinforcement is provided near the back face. Such walls are adopted where there is limited area or land.
3) Cantilever Retaining Walls
The reinforced concrete designed with thin wall stem and large base slab, constructed monolithically, are cantilever wall. The wall can be made up to 6m-8m.
It is the most common type of retaining wall which consists of a vertical wall called the stem, heel slab and the toe slab. As all the three components of this wall act as cantilevers, the wall is called as cantilever retaining wall. These walls can be L or inverted T shaped. The cantilever retaining walls are used up to a height of 6 m.
4) Counterfort Retaining Walls
When the backfill of greater height is to be retained and the required height of cantilever retaining wall exceeds 6m, then it becomes uneconomical to provide cantilever retaining wall.
In such cases, a counterfort retaining wall is to be provided. In these walls, counterforts are provided at some suitable interval along the length of the wall, on the backfill side. These counterforts are concealed in the backfill and tie the vertical stem and heel slab together.
The retaining wall containing thin inclined ribs to vertical stem slab are known are Counterfort retaining wall. Such ribs are spaced at regular interval and lies inside the backfilling of the retaining wall. These ties the vertical slab with base stems and prevents the prominent shear failure at intersection of base and stem. Also they prevents the bending of the wall.
Again, if such ribs are supported externally in front face of the retaining wall then such walls are called Buttress Wall.
Accumulation of rainwater in the backfill results in its saturation and thus a considerable increase in the earth’s pressure acting on the wall. This may result in unstable conditions. There are two ways of draining this rainwater:
- By providing weep holes in the wall.
- By providing perforated drain pipe with filter.
5) Sheet pile wall (Bulkheads)
These wall are long sheet of timber or steel or RCC, earth retaining structure built by continuous joining of them with interlocking arrangement. They are used for decreasing water permeability, seepage through soil and vive temporary support to soil mass. They are embedded on ground to gain passive pressure and thus to resist the active earth pressure above ground. There material doesn’t play important role in structural strength.
6) Anchored wall
The retaining wall provided with the anchorage elements like, tensional wires, Springs, cable attached to the fixed support in ground.
Purpose to build retaining wall
To prevent the soil mass failure by sliding, toppling and overturning, present at different elevation. Thus, these wall prevents the structure by stabilizing the soil over such areas.
To provide the artificial filling and raise the ground level of the surface. These wall provides support to infilling material and thus helps in their stabilization. These are generally adopted in construction of the road and maintain the level of the road surface.
Sometimes they can be used as river side training works as a guide bank of river. The soil may easily scour when they get contact with flowing river. So they provide hard layer against scouring of soil.
The retaining wall helps to increase the slope stability of naturally sloped hills. Moreover, it prevents the rolling of the soil mass and boulders if suitable freeboard is provided. Thus helps to mitigate the landslide of natural slopes
Steps of constructing retaining wall
The retaining design of retaining structure depends up on magnitude and line of action of lateral earth pressure. Moreover the type of retaining wall also depends on soil conditions, availability of space, purpose or retaining wall and economic factor. However the general procedure for the all types of retaining wall can be summarized as below.
Selection of retaining wall location
The area where retaining wall are constructed should be properly studied. The location where retaining wall are suitably adopted , only after first thoroughly analysis and evaluation of the soil test, soil behavior and suitability. This varies the soil excavation, backfilling, soil modifications of that area for retaining wall. Different soil test are also conducted in order to determine parameters required for design.
Determining the lateral earth pressure
The lateral earth pressure can be easily determine by any of approach of Rankine’s theory or Coulomb’s theory after finding all the parameters required for it by different soil test. The lateral pressure is most predominant factor for the construction of retaining walls. So,the more precisely the lateral pressure are determined, the retaining wall are more precisely able to be designed.
Surcharge pressure over the backfilled surface of wall.
The surcharge pressure is the additional vertical forces and loads that are applied over the retained soil. This plays very great role in the construction of retaining walls for the transportation purpose. The vehicular movement creates additional surcharge load. Thus, during design they also must be consider suitably in order to prevent premature failure of retaining wall.
Determination of geometry and type of wall
The geometry of the retaining walls is constructed on basis of the specifications and general thumb rule that has been adopted. From the information available, the elementary theoretical model of retaining wall is first depicted. Then the elementary section is tested for stability against different kinds of failure. And then such elementary Geometrical sections are then brought to a practical approach by suitable modifications and adaptation.
The suitability of different types of retaining walls is then analyzed to give equivalent strength retaining walls in reference to the elementary section. The type of retaining wall that is to be adopted depends upon the economic optimization and suitability of locations.
Moreover, the height of the retaining wall is one of the basic parameters for geometry which can be easily determined from site condition and which governs the other-dimensional assumption.
The basic thumb rule to determine the base of the gravity retaining wall is that the base at foundation always equal to half of the height of the wall that need to be constructed (if the height is greater than 1m) and its top width is generally maintained to 50cm- 60cm.
And thus the trapezoidal section (i.e. Section having sloping faces) is adopted for the retaining wall greater than 1m and the rectangular section and adopted for less than 1m. Also, the slope of the front face of the wall is limited to the slope of 1:10 (H: V).
Similarly, the equivalent RCC design of the cantilever wall can be designed after the evaluation of the elementary section. And the basic design procedures available and study of loads and strength of the materials.
Check against failure (Stability of wall)
After the design of the wall, the analysis of the wall is done in order to determine the stability of it. The design wall may dace four kinds of probable failure and thus needed to be checked for it. The check for stability is done against:
- No sliding
If the horizontal lateral forces are greater than the horizontal resisting force of the retaining wall then the wall undergoes in failure. This means for stability,
µRV > RH
where, RV and RH are vertical and horizontal components of resultant force.
For safe design ,
µ is coefficient of friction between base of wall and soil.
- No overturning
When the resultant forces are not maintained within the wall body. Then it creates the additional moment which can not be over the counter by the resisting forces of the wall. And thus failure occurs by overturning of the wall.
To be safe,
- No bearing capacity failure
Every point on the base of the wall is subjected to compression. And due to excessive pressure more than the allowable bearing capacity of the soil, the failure occurs in form of settlement and crushing of the bottom of the wall. The pressure distribution at the base is trapezoidal with a maximum at the toe. The intensity can be calculated by using the formula,
- Tensional crack
When there is high uplift pressure at ground strata due to the water table, then at the heel of the wall there is the development of the tensional cracks within the wall body. The general thumb rule to check such failure is the determination of eccentricity value (e).
e < B/6
;where B is width of wall.
This means that the resultant force must lie within B/3 (second of section of three equally divided width) for stability. Thus also known as one third thumb rule.
Construction of wall in site
After the stability check, the wall is ready to be get constructed over site. For this, the proper laying of the line of the layout is carried out. The soil mass is excavated and then with help of the thread and formwork, the proper designed shape or geometry of the wall is cast. And depending upon the type of wall and material used, they are constructed.
Stabilization of the back filling
The wall after construction is filled with backfilling in over excavated or exposed areas of the wall. The filling material must be suitably designed or tested for the durability of the wall. And well-compacted as per designed active pressure. Sometimes the wall front face is also filled with the filling material and compacted in order to create the passive earth pressure as required.
The proper filling and compaction of the wall are very important in the cantilever type of retaining wall.
Precaution during construction of retaining wall
- During the construction, the line of layout must be precisely set out by the help of instruments and equipment by the Engineer or under his/her supervision.
- The excavation should be carefully carried out. The installation of sheet piles or temporary support to soil must be provided observing conditions of the soil.
- The backfilling material should be specially designed if required with the desire compaction so that the angle of friction increases.
- The joint formation should be pre-planed for voluminous work with the help of the construction joint. If the work is interrupted, then the work should only be continued after observation of the feasibility of cold joints.
- For the lengthy retaining wall structure, the joints must be needed to be provided at an interval of length smaller than 10m.
- For the retaining wall with higher height, only work up to 900mm should be permissible at a time. At such conditions above the layer of the unfinished retaining wall should be left with the rough surface so that there is the proper interlocking of the new layer of construction over them while continuing work.
- The weep holes are needed to provided during construction at regular spacing and interval.
Plum concrete retaining walls
Generally, the gravity wall is constructed as retaining wall because of its economic suitability and easy method of construction. The generally use gravity walls are stone masonry wall, plain concrete wall, gabion wall, crib wall, etc.
The only use of plain concrete can be durable and stable but its economically way higher to achieve. Thus there is the provision of the use of plum concrete to build such walls. And such walls is generally known as plum concrete retaining wall.
The concept of plum concrete has been widely adopted all over India and the developing countries. And the method of plum concrete is most suitable for the retaining walls. These walls are very well adopted in the field of transportation and making banks of road.
The plum concrete retaining wall constitutes a big boulder size rock about 80mm- 150mm (for retaining wall) embedded over the concrete mass during construction.
The concrete mass of designated grade are spread about to thickness of 100mm – 150mm initially and over them, the Boulder rocks called plum stone are kept at well spacing. The spacing between two boulders and from the formwork must be minimum to 100mm.
The mass of concrete in such a wall is about 60 percent and the constitute of plum stone is about 40 percent of the whole structure, giving more optimization in the economy.
I hope this article on “Types of retaining wall” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Rajan Shrestha