Terracing is a landscape modification technique that transforms steep, continuous slopes into a series of level, stepped platforms. This process involves constructing retaining walls to hold back the soil, effectively leveling out the incline into manageable segments. The primary benefit of this approach is the substantial reduction of soil erosion, which occurs when water rapidly flows down a bare hill. By breaking the slope into horizontal benches, terracing slows down water runoff and allows for deeper infiltration into the soil, preventing the downhill transport of topsoil. This technique also converts previously unusable ground into stable, fertile areas suitable for planting gardens or creating flat recreational spaces.
Essential Pre-Construction Planning and Design
The success of a terracing project begins long before any earth is moved, requiring site assessment and design. Calculating the slope percentage is a foundational step, as this measurement dictates the necessary height and number of walls required to achieve level terraces. A slope is accurately measured by dividing the rise, the total height difference, by the run, the horizontal distance, and then multiplying that figure by 100.
Determining the ideal terrace width and wall height is a design decision balancing usability and structural integrity. Taller walls inherently require more robust engineering and material support to resist lateral earth pressure. Conversely, using shorter walls necessitates the construction of more terraces to cover the same elevation change across the slope.
Local governmental ordinances impose restrictions on wall height to ensure public safety and structural stability. It is common for jurisdictions to require engineering review and building permits for any retaining structure exceeding four feet tall. Ignoring these height restrictions can lead to stop-work orders and the mandated removal of the structure.
The choice of construction material—such as segmental concrete blocks, pressure-treated timbers, or natural stone—impacts both the aesthetic and the wall’s longevity. Concrete blocks are specifically engineered for interlocking stability and ease of stacking, while timber is often more cost-effective but has a shorter expected lifespan due to decay. Before any digging commences, the location of underground utility lines must be identified by contacting the appropriate regional service to prevent damage during excavation.
Preparing the Slope and Setting the Foundation
Physical preparation begins with clearing the slope of all vegetation, rocks, and organic debris down to the stable subsoil layer. Roots and other organic material must be removed because their eventual decomposition creates voids that compromise the structural integrity of the wall’s base. Any remaining soft or unstable soil must be excavated and replaced with stable material before proceeding.
Defining the location of the lowest, first retaining wall is achieved using stakes and taut string lines. These lines must be checked with a line level to ensure the trench excavation will result in a horizontal plane for the base course of blocks or stones. Establishing a level base is necessary for the wall’s long-term stability and appearance.
The trench for the wall’s footing must be excavated to a depth that places the base course below the local frost line to prevent shifting from freeze-thaw cycles. If freezing is not a concern, a minimum depth of six inches is required to ensure the wall is properly seated into the stable ground. This trench should be approximately twice the width of the wall material to allow for proper working space and drainage backfill.
Once the trench is excavated, the base material, typically crushed angular gravel known as aggregate, is added in lifts of no more than four inches. Each lift must be compacted using a plate compactor to create a stable, non-settling foundation. A properly prepared base layer prevents differential settling, which is a major cause of wall cracking and eventual failure.
Wall Construction and Backfilling
Wall construction starts with laying the first course of wall units directly onto the compacted gravel foundation. This base course is the most important for the wall’s structural stability and must be verified as level from side to side and front to back. Even a small deviation here will be magnified as subsequent courses are added.
Subsequent courses are stacked, with each unit slightly offset from the course below it, following a running bond pattern to distribute the load across the entire structure. Depending on the material, units are secured using polymer pins, integrated tongue-and-groove mechanisms, or high-strength masonry adhesive formulated for retaining walls. This interlocking action resists the tendency of the wall to shear or slide outward.
For walls exceeding three feet in height, the integration of geotechnical reinforcement, commonly called geogrid, becomes necessary to tie the wall face into the soil mass behind it. Geogrid is a high-tensile mesh laid horizontally between courses and extended backward into the backfill, creating a reinforced soil block that acts as a single, massive unit. This reinforcement significantly increases the wall’s ability to resist the lateral earth pressure exerted by the retained soil mass.
As the wall rises, the space behind it is filled with backfill in lifts that correspond with the wall courses. Proper backfilling involves placing the soil in lifts of six to eight inches and compacting it immediately, taking care not to exert excessive force that could push the newly built wall out of alignment. The compaction process is performed in lifts to prevent significant settling of the terrace surface later.
A slight backward tilt, or “batter,” should be incorporated into the wall design, between one and two inches for every foot of vertical height. This lean directs the weight of the wall structure back into the retained earth, thereby improving stability and counteracting the forward thrust caused by the weight of the slope. Correct batter is a passive defense mechanism against the pressure exerted by the retained soil.
Ensuring Stability and Drainage
Water management is important for the long-term longevity and function of any retaining wall system. Hydrostatic pressure, the force exerted by saturated soil and water, can easily exceed the wall’s designed structural capacity and lead to bulging, shifting, or failure. Preventing water accumulation behind the wall is necessary.
To mitigate this pressure, a drainage system must be installed immediately behind the wall face before the main soil backfill begins. This system involves placing a perforated drainpipe directly on the base layer behind the lowest wall course. The perforations must be positioned to collect water infiltrating from the terrace above and from the surrounding soil.
The space surrounding the drainpipe and extending outward from the wall should be filled with clean, coarse drainage gravel. This drainage layer ensures that water can move freely toward the pipe without carrying fine soil particles that could clog the system over time. The gravel layer acts as a filter and a conduit for water evacuation.
For longer walls, the drainpipe must be installed with a consistent slope, one-eighth inch per foot, toward an outlet point. This allows collected water to “daylight” and flow away from the structure and the hill, preventing the saturation of the retained soil. Alternatively, weep holes—small gaps left in the wall face—can be used to allow trapped water to escape directly. The surface of the completed terrace should be graded with a slight slope away from the wall to prevent standing water from soaking the soil immediately adjacent to the structure.