Retaining Walls
What makes retaining walls so challenging is their strength, which is best illustrated by the incredible loads they carry, known as lateral earth pressure. This is created by more than just the weight of soil. Water behind the wall causes buildup of hydrostatic pressure which is the reason for most wall failures. Only through adequately designed drainage structures and waterproofing can the best designed wall survive over time.
Retaining walls are divided into two types divided by their height. Walls under four feet are designed and built by your contractor or per a landscape architect's drawing. Retaining walls over four feet require much larger footings due to the increased lateral earth pressure. It must be designed by an engineer who will specify the size and extent of footings and the amount of steel reinforcement required according to strict loading calculations.
Building permits often require these structural calculations due to the damage caused by retaining wall failures. This not only destroys your investment, it can cause significant damage to people and property, which puts the homeowner at serious liability. This is particularly important where slopes are unstable and the climate suffers periods of heavy rains that can lead to super saturated soils and catastrophic mudslides. California communities may require additional earthquake related design criteria.
Concrete block retaining wall systems are growing in popularity due to their improved aesthetics and single wall construction that requires less surface area to achieve optimum strength. Dry stone walls are a regional art form and those from colonial times can still be functional today.
The time tested veneer wall is the landscape architect's choice for high end landscaping. With its functional concrete masonry unit (CMU) core, there is no end to the options for swanky stone or brick veneer. This choice offers the designer unlimited options for making each retaining wall unique to a home site and allowing it to lend style to outdoor living spaces.
Always remember that however beautiful these walls may ultimately be, they are a powerful structure that must function as designed. This can only happen when they are composed of the proper materials and constructed by quality contractors. Get these things right and you’ll be able to count on your financial investment maintaining both beauty and integrity for many decades to come.
COST
With such variation in the method and materials of retaining wall construction, actual costs can be difficult to assess without a completed design. Fortunately, walls below 4' that do not require special engineering and are composed of a CMU block core with attractive veneer are relatively uniform in their cost of construction. In this application, which includes seat walls, it's the veneer itself that dictates the ultimate cost differential.
We've replaced a lot of walls where the original contractor failed to use gravel or used too little gravel behind the wall. In just two years those walls are covered with efflorescence (a moisture induced white mineral residue). We insist on being generous with drainage materials to make sure our walls last. Drain pipes are installed at the base of our foundations, behind our walls. Secondly, we apply a 6” thick layer of gravel over the drain pipe, and this may be even wider for more expansive walls.
This demonstrates how costs can often be lower due to failure to address the most important part of a retaining wall: drainage structures. Moisture may originate deep underground too, wicking up through the concrete footing. Moisture is the primary concern for us with all our masonry. Water always finds its way out. When a seat wall is a planter wall, it's always getting wet. To prevent wicking we use a rubber flashing at the base of the foundation that cuts moisture off from concrete footing so it doesn't suck up into stonework. No matter what kind of wall you build, however, there are some universal factors that can constitute hidden costs.
What Affects the Cost of Retaining Walls:
Location - Areas of extreme weather or earthquake risk may demand greater structural reinforcement and more extensive waterproofing.
Spoils - Larger walls or those with extensive engineered footings will require a great deal of excavation. In some cities, the removal of spoils can drive up costs considerably as does the demand for a great deal of fill behind a wall.
Material cost - Stone veneer that requires a stone mason to fit it adds higher labor costs to the bottom line.
Soils - Areas of expansive soils or bedrock can drive up excavation costs and foundation size.
Steps - Adding steps or pilasters to a retaining wall can increase the wall cost exponentially.
Access to building site - A site where masons and deliveries can be made to the location of the wall is less expensive than one where materials must be hand carried due to narrow side yards and limited backyards.
Fill material - Topsoil is brought onsite to fill the back side of a retaining wall against the slope, or where raised planters with retaining walls need to be filled. Limited access can require extensive labor compared to a simple dump and fill.
Step by Step Installation of a Retaining Wall System
Step 1 - Dig an anchor trench to the dept specified by the manufacturer to provide a gravel base that sets the first course of block at the designated level, which is usually a few inches below grade.
Step 2 - Lay and level the first course of block. This course is the most important and sets the base for the entire wall, so a string line for accuracy is vital to proper level.
Step 3 - Lay second course. At this point a variety of things may happen at the same time because drainage structures often here in order to gather water at the lowest point.
Step 4 - Install drainage structures. At the second course we will lay more gravel and set a corrugated perforated drain pipe spanning the entire length of the wall. We may also install the base of a waterproof membrane or filter fabric against the slope to further protect the wall from hydrostatic pressure.
Step 5 - Install structural backfill. Some kinds of block require a special porous structural backfill that's a lot like concrete which binds blocks. For other types, gravel may be added as courses are laid.
Step 6 - Apply geogrid with additional courses. A geogrid may be used to act as a deadman to help stabilize the wall. This flexible plastic grid is laid on top of a course and the corresponding gravel layer extending all the way back to the slope. The next layer of block and gravel is laid on top of the geogrid, and this may be repeated at designated intervals specified by engineer, manufacturer or landscape architect.
Step 7 - Apply capstone. When the wall reaches its designated height, a capstone will be attached to the last course of block to seal it against rain and runoff.
