Retaining walls provide lateral support to vertical slopes of soil. They retain soil which would otherwise collapse into a more natural shape. The retained soil is sometimes referred to as backfill.
Retaining walls can be constructed of many different materials and with a variety of building techniques. This discussion will focus on rigid, monolithic, poured concrete walls as the structural material, but steel, timber, and reinforced soil are often used too.
This retaining wall discussion will focus on walls that are constructed from the bottom up and where a stable backslope exists (at least temporarily) prior to wall construction. With poured concrete retaining walls, backfill is placed between the wall and the slope after the wall is constructed.
This discussion will talk about the walls themselves, their design and some important construction considerations. It will not talk about groundwork or compaction, since these are entire topics on their own right. Designers and builders of any kind of retaining wall should be familiar with and follow the procedures and methods for soil preparation and backfill compaction methods dictated by the appropriate local building
codes.
Retaining wall design and wall type selection is driven by several factors.
These factors include:
- Cost
- Required wall height
- Ease and speed of construction
- Ground water conditions and soil characteristics.
Other factors can include skilled labor and material availability, building codes, site accessibility, aesthetics, local building practice, etc.
Ultimately, all retaining walls serve to hold back a vertical or near vertical face of soil that would, without adequate retention, cave, slump or slide to a more natural slope.
In most states, retaining wall designs taller than about four feet must be designed by or approved by a qualified, licensed professional engineer. Additionally, it is important to check with and adhere to local building codes prior to any construction, even when walls are shorter than four feet. Retaining walls are, and should always be viewed as load bearing members first, and aesthetic groundscapes second.
Once the lateral earth pressures are known, the wall is checked for stability. This includes checks for wall overturning, base sliding, and soil bearing capacity failures. After the wall is sized, each wall member is checked for adequate strength and steel reinforcing is determined.
One of the most common and telling failures of retaining walls is the inevitable tilting, cracking and bowing of brick, timber and concrete block retaining walls built by homeowners, well meaning builders, and landscapers. These "problems" truly are failures, since the wall has not performed the task it was built to do, and that is to hold back the soil.
The failures also clearly demonstrate the lack of knowledge or design that is required by a successful retaining wall design. By understanding how a wall works, and how it can fail, it is possible to engineer a retaining structure that will meet all foreseen environmental, structural and construction demands.
In most states, retaining wall designs taller than about four feet must be designed by or approved by a qualified, licensed professional engineer. Additionally, it is important to check with and adhere to local building codes prior to any construction, even when walls are shorter than four feet. Retaining walls are, and should always be viewed as load bearing members first, and aesthetic groundscapes second.
One area that can be commonly overlooked, or at least underestimated, is the necessity to drain the backfill of rainwater and/or groundwater. Hydrostatic pressure can cause or induce retaining wall failure, or at least damage.
Drainage of water as a result of rainfall or other wet conditions is very important to the stability of a retaining wall. Without proper drainage, the backfill can become saturated, which has the dual impact of increasing the pressure on the wall and lessening the resistance of the backfill material to sliding. Granular backfill material offers the benefits of good drainage, easy compaction, and increased sliding resistance.
Drainage systems usually utilize weepholes and drainage lines.
Weepholes actually penetrate the retaining wall and drain the area immediately behind the wall. Weepholes should have a minimum diameter so as to permit free drainage. For large walls, 4 inch weepholes are common. Adequate spacing between weepholes allows uniform drainage from behind the wall. Weepholes should always have some kind of filter material between the wall and the backfill to prevent fines migration, weephole clogging, and loss of backfill and caving.
Drainage lines are often perforated and wrapped in geotextile or buried in a granular filter bed, and serve to carry water to the weepholes from areas deeper within the backfill.
HOW TO INSTALL CONCRETE RETAINING WALLS
Prized for their strength and versatility, concrete retaining walls require precise construction methods. With this high level of technicality comes incredible versatility. Poured concrete retaining walls can be colored, textured, accented with embedded objects and much more. When installed properly, concrete offers much more room for customization than any other retaining wall material.
Here are the steps.
- Meet with clients and determine what components they want and how the space will function.
- Design the walls - determine shape, size and placement.
- Remove existing plants, top soil and other debris that may be in the way of construction, Ralston calls this process grubbing.
- Lay out and dig the footings.
- Build forms.
- Add rebar for reinforcement, Ralston places rebar every 16” on center.
- Pour footings and wall. If the wall exceeds four feet in height, footings should be poured separately.
- Allow concrete to cure.
- Create contraction joints every 4-6 feet.
- Remove forms and install waterproofing and drainage system.
- Finish the surface of the wall as desired.
- Install patios, steps and other hardscape elements.
Decorative options for concrete walls:
- Stamp the surface of the walls - Ralston uses texture skins
- Do a sponge finish while the concrete is wet - creates a stucco-like look
- Trowel in a Victorian swirl pattern - looks like old-fashioned plaster
- Apply an acid stain or color hardener
FOUR COMMON TYPES OF RIGID, MONOLITHIC CONCRETE RETAINING WALLS.
Cantilever Retaining Walls
Cantilever retaining walls are constructed of reinforced concrete. They consist of a relatively thin stem and a base slab. The base is also divided into two parts, the heel and toe. The heel is the part of the base under the backfill. The toe is the other part of the base.
- Use much less concrete than monolithic gravity walls, but require more design and careful construction.
- Generally economical up to about 25 ft. in height.
- Can be precast in a factory or formed on site.
Counterfort Retaining Walls
Counterfort retaining walls are similar to cantilever walls except they have thin vertical concrete webs at regular intervals along the backside of the wall. These webs are known ascounterforts.
Counterfort retaining walls:
- The counterforts tie the slab and base together, and the purpose of them is to reduce the shear forces and bending moments imposed on the wall by the soil. A secondary effect is to increase the weight of the wall from the added concrete.
- Can be precast or formed on site.
- Counterfort retaining walls are more economical than cantilever walls for heights above 25 ft.
Gravity Poured Concrete Retaining Walls
- Gravity retaining walls depend on their own weight and any soil resting on the concrete in resisting lateral earth forces.
- They are generally economical up to 10 feet in height for cast concrete structures.
- Usually are sufficiently massive to be unreinforced.
- Monolithic cast walls are generally formed on site.
GENERAL RETAINING WALL DESIGN
Retaining wall design and wall type selection is driven by several factors.
These factors include:
- Cost
- Required wall height
- Ease and speed of construction
- Ground water conditions and soil characteristics.
Other factors can include skilled labor and material availability, building codes, site accessibility, aesthetics, local building practice, etc. Ultimately, all retaining walls serve to hold back a vertical or near vertical face of soil that would, without adequate retention, cave, slump or slide to a more natural slope.
In most states, retaining wall designs taller than about four feet must be designed by or approved by a qualified, licensed professional engineer. Additionally, it is important to check with and adhere to local building codes prior to any construction, even when walls are shorter than four feet. Retaining walls are, and should always be viewed as load bearing members first, and aesthetic groundscapes second.
Once the lateral earth pressures are known, the wall is checked for stability. This includes checks for wall overturning, base sliding, and soil bearing capacity failures. After the wall is sized, each wall member is checked for adequate strength and steel reinforcing is determined.
One of the most common and telling failures of retaining walls is the inevitable tilting, cracking and bowing of brick, timber and concrete block retaining walls built by homeowners, well meaning builders, and landscapers. These "problems" truly are failures, since the wall has not performed the task it was built to do, and that is to hold back the soil.
The failures also clearly demonstrate the lack of knowledge or design that is required by a successful retaining wall design. By understanding how a wall works, and how it can fail, it is possible to engineer a retaining structure that will meet all foreseen environmental, structural and construction demands.
