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Retaining Walls

 

Here in Southwest Virginia, we are blessed with an abundance of rolling hills and mountains which create terrific beauty, interest, and mystique. However, erosion and runoff issues may be caused or exacerbated by hilly, rugged landscapes. Retaining walls are one method of supporting soil and eliminating erosion problems. They will also add value and curb appeal to your property.

 

How can I tell if my landscape needs a retaining wall?

Retaining walls are often a solution for dealing with erosion on sloping ground. Walls are also often used for terracing, as leaving soil exposed to the elements for extended time will usually result in the terrace disappearing and leaving a steeper slope, gradually eroding.

 

From what materials are retaining walls constructed?

Decades ago, retaining walls were primarily made from four materials:  wood, poured concrete, concrete-block, and natural stone.

Wood wall material is typically salvaged from railroad cross ties or treated lumber. Though wood is functional and plentiful, the effect of wood anti-rot treatments raise environmental concerns and the longevity of wood walls is short compared to other types.

Unlike wood walls, poured concrete walls lack an inherent aesthetic appeal, though this can be improved with the addition of veneers. The lateral strength of poured concrete walls is critically dependent upon proper construction, so the effectiveness of these walls can be illusionary.

Cinder-block or concrete block walls also lack an inherent aesthetic appeal but beautifying them with veneers is always an option. However, the lateral strength of cinder block is not reliable and so most cinder block walls fail over time.

Natural stone walls are very attractive and sturdy if constructed properly. The craftsmanship a stone wall requires makes them more expensive than most other retaining walls.

In today’s retaining wall market, a new material dominates: man-made segmental retaining wall blocks. In the last thirty years, the industry has excelled at developing these segmental retaining wall blocks, creating standards for material, design, and construction practices. Segmental retaining wall blocks are manufactured from concrete and come in a variety of shapes, sizes, locking mechanisms, and colors.

Companies that offer wall construction have a multitude of choices when deciding what kind of retaining wall is the best fit for a client. In some situations, any kind of retaining wall will work. However, the preferences of property owners and the unique characteristics of a landscape can dictate which materials are optimal. At times, size requirements and local government restrictions tend to make natural stone and poured concrete walls less accessible due to cost and difficulty of construction. Segmental block walls are perhaps the most flexible type of retaining wall in that they can be adapted to fit a variety of situations and locations that other walls may have difficulty conforming to.

 

How do retaining walls work?

Segmental retaining walls use gravity and friction amongst wall components to provide resistance; “retaining” soil and other landscape components in place. Individual wall blocks range from a few pounds apiece to several hundred pounds apiece. The larger of these require a machine to handle and set them in place. At times, reinforcement can be minimal or even not required due to the size and the friction created by these large blocks. Some are solid concrete and some are cored, meaning partially hollowed out to accept aggregate, to increase mass and to act as an additional locking mechanism between courses of block.  Most manufacturers design their blocks to be set back, or battered, from one course to the next. This, along with mass and a locking mechanism, dictates the walls resistance to pressure and movement. Add in reinforcement, and segmental retaining walls can be tooled to stabilize many feet of vertical slopes or excavations.

 

What do I need to do before constructing a retaining wall?

In most cases, you will need a permit from your local governing body to construct a retaining wall. The cost of the permit is usually a flat fee plus a percentage of the cost of the project. This would usually run somewhere north or south of 1%. In order to obtain the permit, you will usually need to submit a cross-sectional sketch and, if the wall exceeds six feet in height, you may need to provide a design by a licensed engineer. If the wall exceeds 10 feet in height, construction may even need to be monitored by a licensed engineer at the property owner’s expense.

What provisions are required when designing a retaining wall?

A provision for drainage, usually in the form of perforated pipe, is needed for most walls. Most walls are not built to do the job of a dam, so they require some means to keep hydro-static pressure from increasing. This is true for all walls regardless of the type of material of which they are constructed. Generally, a one-foot thick column of stone with a perforated pipe vented to daylight will suffice for drainage. Often, a slight swale, or contiguous depression immediately behind the wall, is necessary to divert surface water away from it.  Additionally, any erosion immediately in front of the wall must be eliminated to prevent destabilization of the footing.

The footing is a critical component of all walls. Footings for stone walls need to extend to just below the frost line and be slightly wider than the base of the wall.  Footings for poured walls also need to extend below the frost line, be reinforced with steel, and be of sufficient mass to conform to local codes. Footings for segmental block walls are normally made of aggregate and a minimum of six inches thick and two feet wide. The required depth for footings is directly related to the load behind the wall, the slope in front of the wall (if any), the type of soil where construction is taking place, and the overall height of the wall. Footing trenches may be lined with geo-textile fabric to prevent soil from eventually contaminating the footing and destabilizing it.

The back-filling of retaining walls requires consideration due to the type of material, the height of the wall, the eventual load, and the length of the backfill. Walls under three feet in height can usually be back-filled with only a draining stone, perforated pipe, and excavated soil. This method generally works for poured concrete and natural stone walls also. If constructing a taller segmental block wall, additional means of support in the reinforcement zone and proper compaction of the fill material will be necessary.  The following diagrams indicate profiles of a non-reinforced segmental retaining wall and a reinforced wall of over four feet in height.

 

 

Will constructing a retaining wall damage my landscape?

Minor damage to the immediate area around the construction site, including to the turf or ornamental beds, should be expected, but these damages can usually be mitigated easily.  Additionally, the right of way from the street or staging area to the site will incur some minor, easily repaired damage.

 

How are segmental retaining walls reinforced?

Geogrid is the most commonly used method of wall reinforcement.  Geogrid is a geo-synthetic material, somewhat reminiscent of a fishing net, strong in tension that will help bind soils together. Grid is installed with one end sandwiched between two courses of block and extending perpendicular to the wall to a depth of about 60% of the eventual height of the wall.  These layers of grid are usually from 16 to 24 inches apart.  The fill material is then installed and compacted in lifts.

There are two additional, less common methods available for reinforcing segmental block retaining walls. First, the construction team can use soil pins or nails that are attached to the back of the wall with steel rope to reinforce the wall. Second, reinforcement can be accomplished by installing flowable fill—a kind of permeable concrete—in the backfill area. The latter method usually reduces the amount of reinforced areas necessary, but geo-grids are still the primary method for reinforcing segmental retaining walls due to their efficiency, flexibility, and cost.

 

How much does a retaining wall cost?

There are a wide range of costs associated with retaining walls. Not all of these costs are directly related to the cost of the wall’s materials and construction. For example, costs accrue due to the distance that contractors need to travel and the distance that construction materials need to be moved from their source. There are also costs associated with access to the construction site for materials and equipment. In addition, the cost of construction may increase depending on the number of public and private utilities on or near the construction site and the wall’s proximity to nearby structures and impediments such as fences, trees, and bodies of water. Aspects of the construction design, such as whether excavated material can be used as backfill, charges for disposing materials, drainage issues that may be created by the wall placement, and the wall’s proximity to property lines may also increase overall costs. Further, costs may increase depending on how much reclamation of the surrounding landscape is required after construction.

With all wall types, the cost usually decreases as the size of the wall grows; as viewed on a per square foot basis. Though this may sound counterintuitive, it is influenced by economies of scale: a 1000-square foot wall will cost less to construct per foot of wall face than a 100-square foot wall if all other conditions are similar.

Generally, a poured concrete wall runs around $20 per square foot if in excess of 200-square feet. If a veneer is added on one or both sides, the cost may increase by approximately $15 per square foot. A cap of some sort is usually required to top off poured concrete walls and, depending on material, the cost of the cap can range from under $10 to near $50 per lineal foot.

Natural stone walls are the most labor-intensive walls to construct of all the types of walls mentioned here, typically requiring the expertise of experienced stone masons. Consequently, they are also the most expensive. These walls start at about $40 per square foot. If you want the grout to be invisible (what the industry refers to as “dry-set”), the cost rises to around $60 per square foot. Though more expensive, a well-built natural stone wall is aesthetically stunning and will last for decades.

With all elements considered, a segmental block retaining wall generally offers the best value when compared to concrete, stone, and wood walls. Another benefit is consistency: segmental blocks are consistent in size, shape, and color. Additionally, segmental walls are frost resistant and durable in most climates. If installing a taller wall (above six feet), you have the added security of working with a professional engineer who is obligated to warranty the stability of the wall provided construction is done properly. A professional engineer generally will charge from $1000-$1500 to design and certify a retaining wall. The cost per square foot of constructing a segmental retaining wall is between $25 and $50. The variables discussed in the earlier paragraphs under this section will determine if the wall falls in the low end or high end of that range.

 

What are “free-standing” walls?

Parapet walls, also known as free standing walls, are similar in looks to retaining walls and are most often used for patio seating or improving landscape aesthetics. Free standing walls generally have less demanding structural requirements than retaining walls designed to retain slopes or excavations. Many free standing walls include columns which improve their aesthetic appeal and provide an attractive and convenient mount for landscape lighting to be installed. Free standings walls are often built from segmental retaining block or natural stone. In some cases, a situation might call for a dual-purpose wall that is part retaining wall and part free standing wall. For example, if a client wants to build a patio on   sloping ground, the wall may begin as a retaining wall and transition into a free standing wall as it rises past the floor level.

Block style parapet walls can be constructed several ways; on a footing in the ground, on a concrete slab, or they can be used on a concrete paver, brick paver, or stone patio and set on the same base as the pavers. They can also be set by burying them to a proper depth. Parapet walls can be curved or straight and are normally capped with a short, slightly wider cap to finish them.

Poured concrete and natural stone free standing walls are both labor intensive to construct. It is almost like constructing two walls back-to-back to gain the desired effect. Both paved concrete and stone parapet walls need to be set on proper footing, either a concrete slab or compacted aggregate base. They are usually capped with a man-made product, though a cap can be fashioned from stone if preferable.

 

What do free standing walls cost? 

As with retaining walls, several variables affect the cost of a parapet wall. If the wall is made of segmental blocks, expect to pay between $20 and $50 per square foot. The cost of the wall will be affected by the quality of the segmental blocks and whether the wall is straight or curved. Poured concrete parapet walls are generally $50 to $70 per square foot since both sides of the wall need to be veneered and then the whole wall requires caps. Natural stone parapet walls, particularly when they are “dry set,” may be a bit more expensive. They start at $60 per square foot if the grout is visible and $80 if it’s dry set. All three walls normally require a cap.

 

What are the benefits of installing a retaining wall?  A free standing wall?

Retaining walls not only solve problems in the landscape, they also add character and interest to it. A completely new look for a landscape can be created by adding creatively designed, strategically placed, and attractively built retaining or free-standing walls.

 

Who are some experienced wall builders in the Roanoke Valley?

Retaining walls and parapet walls should be both functional and aesthetically pleasing. Competent engineering and careful construction are necessary to ensure walls can stand the test of time and nature. You should choose a contractor and/or designer that has the experience, creativity, and drive to help you accomplish your goals. Besides Roanoke Landscapes, here are some experienced builders in the area:

  • Sampson Construction of Roanoke, VA
  • Groundscapes of Rocky Mount, VA
  • Dirt Works of Fincastle, VA
  • Green Acres Landscaping of Elliston, VA
  • Varsity Landscapes of Roanoke, VA

 

What are some of the most common problems with retaining walls?

Most problems with retaining walls are linked to structural failure over time. If proper design and construction is followed, these failures rarely materialize.

 

Do retaining walls require maintenance?

Very little maintenance is required to keep properly constructed walls in good working form. One may need to inspect the drainage aspect to assure it is working properly. Depending on the location (e.g. under a tree) one might desire to clean a wall with a detergent to keep it looking fresh.

 

How long do retaining walls last?

If properly constructed and adequately designed, retaining and free-standing walls should last for generations.

Explore several different styles of segmental block retaining walls below:

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