Retaining Walls, Types, Design Insights, and Pressure Dynamics
Table of Contents
What is Retaining wall
A structure that holds or keeps soil behind it is called a retaining wall. Retaining walls can be made from a variety of materials, including treated wood, pebbles, boulders, concrete blocks, and poured concrete. While some are easier to use than others and have shorter lifespans, they can all hold onto dirt.
- Structure Constructed to retain earth or some other material on one or on both sides ( of different heights).
- Material may be earth, water, coal, grains, etc.
- Depending upon the type of material retained may be termed as earth retaining structures, water retaining structures, etc.
- However, the term retaining wall is generally used for earth-retaining structures
- Un-retained free-standing soil tends to slide and attain a natural slope (angle of slope with horizontal termed as the angle of repose)
- When soil is retained and prevented from attaining its slope it exerts horizontal thrust on the retaining structure.
- Try to slide or overturn the structure
- So, the proposed retaining wall should be stable against overturning and sliding
A retaining wall is a structure that serves to prevent a mass of matter, usually earth or rocks on a slope, from falling or collapsing.
Certain building operations include excavating portions of a mountain or removing dirt, which might leave gaps or extremely steep sides that could collapse. In these situations, retaining walls must be considered when planning the project.
When a desired change in ground elevation is achieved that is greater than the angle of repose of the soil, a retaining wall is a structure intended to withstand the lateral pressure of soil. Soil is supported laterally by retaining walls, which allow the soil to be kept at various levels on both sides. Retaining walls are structures designed by expert retaining wall contractors to restrain soil to a slope that it would not naturally keep to (typically a steep, near-vertical, or vertical slope). They are employed to bond soils between two elevations, frequently in terrain with unfavorable slopes or in places where the landscape must be drastically changed and constructed for more specialized uses, such as road overpasses or hillside farming.
Types of Retaining Walls
Depending on how the earth is retained, Retaining walls can be classified as
- Gravity Wall
- Cantilever Wall
- Counterfort Wall
- Buttress Wall
- Bridge Abutments
- Box Culverts
Gravity Walls
The retaining walls in which stability against overturning and sliding is provided primarily by the weight of the retaining wall.
Normally used for low heights (<4m)
Gravity retaining walls are usually lower in height and rely on their own weight and setback to hold the soil in place. For this kind of wall, Allan Block retaining walls are perfect since they lock into place when stacked to create a setback. The pressure from the soils behind the wall is lessened by this setback.
- Gravity retaining wall depends on its self-weight only to resist lateral earth pressure.
- Commonly, a gravity retaining wall is massive because it requires a significant gravity load to counteract soil pressure.
- When designing a retaining wall construction of this kind, sliding, overturning, and bearing forces must be taken into account.
- It can be built using masonry units, stone, and concrete, among other materials.
- It is economical for a height up to 4m.
- Gravity retaining walls also include bin retaining walls, gabions, and crib retaining walls.
Gravity walls may have a “batter” setback to increase stability by leaning back toward the retained soil. In order to withstand pressure from behind, gravity walls rely on their bulk—stone, concrete, or other heavy material. Segment concrete units (masonry units) or mortarless stone are common materials used to create short landscaping walls. Gravity walls that are dry-stacked have some flexibility and don’t need a strong foundation. More and more taller retaining walls are being constructed these days as composite gravity walls, which include geosynthetics like geocell cellular confinement earth retention or with precast facing; gabions, which are stacked steel wire baskets filled with rocks; and crib walls, which are log cabin-style cells filled with granular material and built from precast concrete or timber.
In order to prevent toppling and sliding, gravity retaining walls use the gravitational force of their own weight to oppose the lateral earth pressure from the soil behind them. They are the most basic and historically known kind of retaining wall.
Built of concrete, masonry, brick, blocks, or mass cast-in-situ concrete, these hard-wearing structures rely on their large weight to resist toppling and sliding caused by the lateral earth pressure from the soil behind them.
In order to withstand the increased lateral earth pressures at depth, gravity retaining walls are usually designed with slanted faces and a broader base. Because of this, this kind of retaining wall is simple to construct and appropriate for maintained heights of up to three meters.
Despite their advantages, gravity retaining walls are not suitable for retained height above 3m. Should the retaining structures be constructed even higher, they will likely occupy excessive space and may become too heavy for the underlying ground, which could result in bearing capacity failure. In the end, this can lead to the wall’s inability to hold soil.
Cantilever Wall
- The retaining walls in which stability against overturning and sliding is provided by utilizing the weight of the backfill (retained earth) itself.
- The most commonly used retaining wall
- Consists of a vertical wall and horizontal base slab
- A vertical wall is termed a Stem
- A portion of the base slab on which the backfill is resting is termed as Heel.
- A portion of the base slab on the front side i.e. side opposite to the backfill is termed as Toe.
- All three components i.e. stem, toe, and heel act as cantilever
- So, termed as Cantilever Retaining Walls
- Because of its shape, which resembles with inverted ‘T’ also called ‘T-shaped’ retaining walls.
- Based upon site constraints, may have to provide ‘L-shaped’ (with no heel) or ‘Reverse L-shaped’ (with no toe).
- Economical up to a height of 6-8m
Cantilever walls are built using reinforced concrete, with an L-shaped, or inverted T-shaped, foundation. The components of this type of retaining wall include a base slab, also known as a footing, that is positioned beneath the backfill. Cantilever walls can remain unhindered because the vertical load behind them is transferred onto the foundation, preventing it from collapsing due to lateral earth pressure from the same quantity of soil.
Furthermore, the weight of the soil (and consequently the vertical stress) in front of the wall helps a T-shaped foundation, giving the retaining structure even more stability. A “key” that protrudes from the ground in the base of some foundations is meant to stop sliding failure.
When compared to other retaining wall designs, cantilever walls have the significant advantage of taking up less area after construction and being appropriate for retained heights of up to 5 meters. However, unless temporary support is provided during development, these retaining walls are not well appropriate to sustain existing slopes.t. This is because the building does require room behind the wall.
An internal stem of mortared masonry or steel-reinforced cast-in-place concrete (often shaped like an inverted T) is used to construct cantilevered retaining walls. These walls transform horizontal pressures from behind the wall to vertical pressures on the ground below by cantilevering loads (like a beam) to a sizable, structural footing. To increase their ability to withstand heavy loads, cantilevered walls occasionally have a counterfort on the back or are buttressed on the front. Short-wing walls that are perpendicular to the wall’s main trend are called buttresses. Rigid concrete footings below the seasonal frost depth are necessary for these walls. A lot less material is used in this kind of wall than in a conventional gravity wall.
Counterfort Walls
- When the height of the wall is more, cantilever walls become uneconomical (>6-8m ) due to an increase in bending moments in the stem.
- To optimize, the stem and heel of the wall are connected with another wall perpendicular to the stem, termed a Counterfort.
- So, the wall is termed a ‘Counterfort Retaining Wall’
- Counterforts are provided at regular intervals along the length of the wall.
Counterfort walls are cantilever walls reinforced with monolithic counter forts with base and rear wall slabs. In order to lessen the bending and shearing loads, the counter-forts connect the wall slab and the base and function as tension stiffeners. Counterforts, positioned at intervals equal to or slightly greater than half of the height, are used to lessen the bending moments in very tall vertical walls. For high walls that are higher than 8 to 12 meters, counter forts are utilized.
Buttress Retaining Walls
- The stem and toe of the wall are connected with another transverse wall at regular intervals.
- The transverse wall is termed a ‘Buttress’. and the retaining wall is termed a’Buttress Retaining Wall’
- The advantage of providing this type of wall is that in the case of counterfort retaining walls counterforts are subjected to tension
- As concrete is weak in tension, so counterforts may not be an efficient choice.
- Whereas in Buttress Retaining walls, Buttresses are subjected to compression
On sloping terrain, a buttress retaining wall is an improved structural method for retaining soil. It consists of a vertical wall that is strengthened with buttresses—support pieces that extend from the wall’s face at an angle or at a right angle. This design serves to counteract the lateral ground pressure and strengthens the wall greatly. Compared to a simple retaining wall, the buttresses allow the wall to be thinner and more cost-effective by uniformly distributing the load. The buttress retaining wall, which is usually composed of concrete, is appropriate for locations with large loads and unfavorable soil conditions.
Buttress Retaining Walls are constructed with typical heights of 4’-25’ (1.22-7.62 m), base depths of 2.5’-15’ (.76-4.57 m), and wall thicknesses between 8”-24” (20.1-61 cm). Buttress Retaining Walls are often created with an angle of 2-6 degrees and buttress spacings between 1.5’-11’ (.46-3.35 m). Retaining wall lengths vary as needed.
Bridge Abutments
- The first and last support of the bridge is termed an abutment.
- Abutments are subjected to horizontal thrust due to the retained earth of the approach road in addition to vertical reaction from the first span of the bridge
- So, bridge abutments also fall in the category of retaining walls.
Box Culverts
- Box culverts are small-span bridges.
- Consists of RCC box
- Boxes can be single cells or multiple cells.
- Vertical walls retain earth on the approach road side, so act as retaining wall
Design Considerations Forces Acting on Retaining Walls
Two types of forces acts on retaining walls
Vertical Forces:- Self-weight of retaining wall, the weight of backfill, the weight of surcharge (if any)
Horizontal Forces:- Earth pressure due to backfill, Earth pressure due to surcharge
Earth Pressure can be Active Earth Pressure And Passive Earth Pressure
Active Earth Pressure
Pressure acts in the direction of movement of soil
Active Earth Pressure can be calculated using
Rankine’s Theory, Coulomb’s Theory
Rankine Theory:- Pressure at any depth ‘h’ from the top can be calculated using
p_a = wh\cos\eta\left(\frac{cos\eta- \sqrt{Cos^2\eta - cos^2\theta}}{cos\eta+ \sqrt{Cos^2\eta - cos^2\theta}}\right) \eta :- Angle of Surcharge
\theta :- Angle of internal friction
Where \eta = 0 backfill is horizontal
Passive Earth Pressure
Pressure acts in the direction opposite to the movement of soil, Earth Pressure at Rest – No movement
p_a = wh\cos\eta\left(\frac{cos\eta+ \sqrt{Cos^2\eta - cos^2\theta}}{cos\eta- \sqrt{Cos^2\eta - cos^2\theta}}\right)p_p = Kp\frac{wh^2}{2} act as h/3
Kp called coefficient of passive earth pressure p_a = \cos\eta\left(\frac{cos\eta+ \sqrt{Cos^2\eta - cos^2\theta}}{cos\eta- \sqrt{Cos^2\eta - cos^2\theta}}\right)
Hydrostatic or Water Pressure
- If water accumulates on the back of the retaining wall, it exerts hydrostatic pressure on the wall.
- However, the earth’s pressure on the wall decreases due to the accumulation of water soil becomes submerged and its weight decreases.
- So, we have to consider submerged weight for earth pressure calculation
- Total Earth at depth ‘h’ under soil-submerged conditions will be
- p_a = K_a w’h + w_w h
- w’ = Submerged weight of soil
- w_w = unit weight of water
- The hydrostatic pressure can be released by providing weep holes
- Normally 100mm dia weep holes are provided @ 1.5m c/c staggered
Design RCC RETAINING WALL (CANTILEVER TYPE) Excel Sheet
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