EARTH RETAINING STRUCTURES

Earth retaining structures are used to hold back soil and/or rock and maintain a difference in the elevation of the ground surface. They are commonly used to accommodate changes in grade however, the specific needs will vary depending on the project.?Retaining walls, which are the most common,?can range from small landscape stone walls to surround a garden to enormous soil-retaining projects along a highway. Others can help control erosion from hard rains or create a terraced yard to reduce maintenance.?

Types of Earth Retaining structures

In a broad sense, retaining structures can be classified according to their face inclination: if it is greater than 70 degrees, they are typically characterized as retaining walls, while slopes have face inclination flatter than 70 degrees.?There are several types of retaining structures, including gravity, sheet pile, cantilever, and anchored earth/ mechanically stabilized earth (reinforced earth) walls and slopes.

Gravity Retaining Walls?

Gravity retaining walls use their weight to resist earth pressures.?They are usually made from heavy materials such as concrete, rocks, or soil.?They are designed to provide resistance against various failure modes, including sliding, overturning, bearing and seismically induced failures. Due to the relatively low construction cost and time required to construct these walls, they are commonly used for supporting roadway and highway cuts.?For short landscaping walls, they are often made from mortarless stone or segment concrete units (masonry units).?Gabions are another common type of gravity wall.?They are used to provide erosion and scour protection?and?are typically made of wire mesh baskets that are filled with rocks and stacked on top of each other to form a retaining wall.?

Sheet Pile Walls

Sheet pile walls typically consist of steel sheet piles that are driven into the ground to support earth pressures.?Depending on the nature of construction and subsurface material, they can be installed by an impact hammer, vibratory equipment, or they can simply be pushed into the ground if ground conditions permit.?

Cantilever Walls

A cantilever retaining wall is one that consists of a wall that is connected to the foundation. A cantilevered wall holds back a significant amount of soil, so it must be well engineered.?Cantilever walls are typically made from a relatively thin stem, typically made of steel-reinforced, cast-in-place concrete.?Often they are shaped like an inverted T.?

Anchored Earth Structures

Anchored and reinforced earth structures (also commonly known as soil nail walls, or tie-back walls) are constructed by the assembly of facing units that are tied to rods or strips that are held in place by friction. Technically complex, this method is very useful where high loads are expected, or where the wall itself has to be slender and would otherwise be too weak.

Embankment Slopes

Where a right of way is available and the cost of a wall is high, an embankment slope is often considered a cost-effective alternative to a retaining wall.?The most common types of embankment slopes are engineered filled slopes and earth-reinforced slopes.?If an engineered fill slope cannot be safely constructed with a particular soil, reinforcement of the fill slope can be implemented as an alternative to retaining walls or to flatter unreinforced slopes.

In general, the cost of constructing a retaining structure is usually high compared with the cost of forming a new slope. Therefore, the need should be assessed carefully during preliminary design and an effort should be made to keep the retained height as low as possible.

A qualified engineer who is familiar with local conditions?should be consulted for the?choice?and design?of retaining wall type,?especially?where?unfavourable soil conditions occur or where piling is required under a retaining wall.??

During?the initial planning, there are several?factors that an engineer will consider?that will affect the material and type of wall?selected?build. Below we look at four of them.

Location:?When choosing a location for?the retaining structure,?it is important to?have a detailed understanding of property lines and both?above ground and underground utilities?including stormwater management systems and irrigation. Additional factors to consider about the location might include:

• If your wall is on a slope, where will you store additional infill that must be brought to the site?
• Excess soil. If you are cutting into a hillside, where will the excess soil be stored?
• Natural drainage patterns. Depending on the wall size, a retaining wall can impede on natural drainage patterns and have environmental consequences downstream.
• If your wall is along the property boundary, will the reinforcement system encroach the property line?
• Surcharge Loads. Will there be additional weight or vertical forces above the wall such as fencing, guardrails, driveways, parking lots, or swimming pools? Don’t forget about temporary construction equipment.

Soil:?The soil that creates the foundation, or base, needs to be examined to ensure it meets the strength required to support the wall.?The engineer will?determine the type, bearing capacity, stress parameters, and friction angle of the soil used for the foundation and reinforced zone along with the retained soil zone.

In general, the base soil needs to be firm, solid, and strong, and it should not be moist. Wet soils such as clay soil are also not recommended for infilling. They are already saturated with water, so additional moisture cannot make its way through to the drainage channels. Also, in areas where there is freezing, wet soil can expand and contract which will damage the wall. On the other hand, sandy soils allow for good drainage.?An engineer should provide technical expertise on?the on-site soils, expansive soils, poor chemical properties, groundwater conditions, and more.

Design:?To begin the design,?the engineer will calculate the corresponding wall heights, footprint sizes, slopes, and setback angle which are dependent on the site elevation and grade.?Gravity will cause the retained material to naturally move downslope, so?this should be counteracted within the design to minimize the amount of lateral earth pressure behind the wall, which, at maximum value, can ultimately overturn the wall.?The?wall height is dependent on soil and slope, setback, and size of the block.

• Wall Reinforcement:?If gravity alone won’t support your wall, there are reinforcement methods available that depend on the wall type, height, design, friction, angle, soil material, and more.?
• Drainage:?Water is the most common reason that retaining walls fail, it’s essential to make sure?the?wall has good drainage and that there will be no buildup of water behind the wall.?It is important to identify potential surface water sources and make sure the?drainage adjacent to the wall site has been accounted for.?Grading the site for drainage patterns and constructing?a drainage system behind the wall to minimize the amount of hydrostatic pressure the groundwater could create?would be paramount. An example drainage system could include backfilling with gravel, using drain pipes, and using “weep” holes to allow water to pass through the wall.?

Are you embarking on a construction project? Reach out to us at OraLabora to see how we can assist.