Segmental retaining wall is constructed of large number of collected dry masonry blocks. These masonry blocks are significantly economical and effective for segmental retaining wall construction and it is also broadly accepted.
There are various applications of segmental retaining walls such as: behind shopping centers, providing tiered grade changes for residential construction, and provide support for hillside along highways.
Masonry blocks for segmental retaining walls are available in different textures, sizes, and configurations.
A segmental retaining wall height of 12 m or more, which is greatly larger than the economical height of ordinary masonry or concrete wall, can be constructed if geogrids are employed to stabilize it.
Nevertheless, it is necessary to have at least seventy percent of wall height space behind the wall to install geogrid layers otherwise segmental wall cannot be used. It is estimated that annual construction of segmental retaining wall is about 18.6 million square meters.
Advantages of Segmental Retaining Wall
Segmental walls have following advantages:
- Setting pad of gravel is adequate as a foundation, this means footing is not necessary.
- Reinforcing, grouting, or mortar is not required because the wall is composed of dry stacked masonry block.
From the two advantages, it quite clear that segmental retaining wall construction is fast.
Types of Segmental Retaining Wall
There are two types of segmental retaining walls:
- Pure gravity segmental retaining wall
- Segmental retaining wall stabilized by geogrids
Pure gravity Segmental Wall
In this type, stability of retaining wall depends only on resistant moment of stacked block against overturning moment of lateral force. This leads to restrict the wall height to about 1.5 m. However, number of suppliers provides heavier masonry blocks to increase the height of the wall.
Segmental Retaining wall Stabilized by Geogrids
It is possible to increase retaining wall height by using layers of geogrid to reinforce soil at the wall backfill and create mass of reinforced soil. The mechanically stabilized soil or mass of reinforced soil is used to increase resistance moment against overturning action.
The geogrid layers are successfully used if it is fixed through joints in the wall and extend the other end beyond backfill soil failure line and vertical distance between two installed adjacent is about two or three blocks. Furthermore, the length of mechanically stabilized zone is about 70% of the wall height.
What is Segmental Blocks in Retaining Wall?
Segmental blocks are concrete blocks with compressive strength of 21 MPa or more and various sizes, colors, textures, and configurations are manufactured. Regarding sizes, blocks with 20 cm length, 45 cm width, and ranges of depths from 25 to 60 cm is common and broadly employed.
Moreover, there are blocks with larger and smaller sizes compared with common sizes. The larger blocks weight is around one to two tones and installed by small crane. The smaller sizes are used for non-engineered purposes and their weight is from 13.6 Kg to 50 Kg.
The design of the segmental block is in such a way that permits the constructed wall to have vertical batter with over 15 degree with vertical as shown in figure-1. Additionally, the manufactured blocks have necessary means, such as offset lips or pins between blocks, for controlling block offsets during block installation and consequently control the batter.
Typical segment blocks are provided with voids that are filled with backfill material. The angle between vertical and front face of the wall which is called batter angle is computed by the following equation:
Figure-1: Segment Block
Gravity Segmental Retaining Wall Design
Gravity segmental retaining wall stability depends on weight of the wall and overturning safety factor is 1.5. The wall height is limited to 1.5m.
The design steps for gravity segmental retaining walls are as follow:
- Select blocks with specified texture, size, and configuration.
- Specify embedment depth below grade is equal to either 30 cm or block depth and the wall height. The wall height includes below grade depth and the wall above the grade.
- Determine surcharge, backfill slope if applicable.
- Determine “hinge height” which is the height to which blocks can be stacked before tipping over. Larger height will cause overturning of the wall.
- Specify properties of both backfill and natural soil such as density and phi angles. The former should be well graded with phi angle of 34 degree.
Check for Lateral Soil Pressure on Segmental Retaining Wall
Coulomb equation which takes both friction angle at soil-wall interface and batter angle into consideration computes horizontal component of active pressure coefficient ( ). The friction angle is taken as 2/3 phi of backfill soil and the batter angle is calculated as per Equation-1.
: Angle of internal friction
: Angle of backfill slope
: Angle of friction between soil and wall (2/3 to 1/2 is assumed)
: Wall slope angle from horizontal (90o+ batter angle from vertical)
Check for Inter-Block Shear in Segmental Retaining Wall
The maximum value of interface shear is at the lowest joint. The shear resistance will be the weight of blocks above that compresses this joint (N-value) inserted into the supplier’s tested shear resistance equation.
Shear resistance at any depth “Z”
: Backfill soil density
D: dead load
L: live load
Check for Segmental Retaining Wall Against Sliding
The driving force is the shear at the wall base and resistance reaction is provided by friction coefficient between lowest block of the wall and setting pad or between setting pad and the soil below.
Safety factor against sliding is 1.5
N: weight above
: Friction angle of the base which is usually assumed to be 40o
Check for Overturning Moment and Resisting Moment
Overturning Moment at depth “Z”
N: weight of the stacked block
t: block depth
H: wall height
: wall batter angle
And safety factor for overturning is taken as 2.
Check for Soil Bearing Pressure
Meyerhof method is used to compute soil bearing pressure for segmental retaining wall. Rectangular pressure distribution is assumed under the base and vertical force spread over effective base uniformly. The effective base width is less than full base width by a distance equal to the two times the imposed load eccentricity on full base with.
Where B is the total bearing width.
Calculation of Soil Bearing Capacity
Ultimate soil bearing capacity is computed by Terzaghi’s equation and the term by which the cohesion is considered is removed from the equation because it assumed to be zero:
: In-situ soil density
d: bottom block embedment depth, m
Be: Effective bearing with, calculate as per Equation 9
: are non-dimensional coefficients
Table-1 provides values of for different soil friction