Terracing transforms steep slopes into a series of level, stepped platforms. This method effectively interrupts the downhill flow of water, which is the primary driver of soil erosion. Creating these flat steps significantly slows water runoff, allowing for maximum infiltration into the soil rather than stripping away valuable topsoil. This practice provides superior erosion control and converts difficult terrain into functional, cultivable garden or leisure space.
Essential Pre-Construction Planning
Before moving any earth, accurately assessing the slope’s grade is necessary to determine the structural dimensions of the future terraces. This measurement can be achieved by using a long string line stretched horizontally between two stakes and a standard carpenter’s level. The vertical drop measured over a precise horizontal distance provides the slope ratio, which directly influences the height and setback of each retaining wall.
The steepness of the slope dictates the total number of terraces required and the necessary height of each wall to achieve a consistently level platform depth. Calculating these dimensions allows for an accurate estimation of the necessary volume of backfill soil and the required linear footage of retaining wall material.
Consult local municipal building codes regarding the maximum allowable height for a retaining wall without professional engineering oversight. Walls exceeding four feet (1.2 meters) must often be designed by a licensed structural engineer due to the immense lateral pressures involved. Ignoring these regulations can lead to serious safety hazards. Proper planning ensures the structural design can withstand the hydrostatic and surcharge pressures exerted by the saturated soil mass.
Selecting Appropriate Retaining Wall Materials
The selection of the retaining wall material should balance the project budget, the desired aesthetic, and the required durability. Treated lumber and timbers offer a cost-effective option for lower walls, but their lifespan is limited due to decay, often lasting only 10 to 15 years.
Natural stone, when dry-stacked, provides a highly aesthetic and durable solution, though installation is physically demanding and requires careful selection for interlocking stability. Segmental retaining wall (SRW) blocks or concrete pavers are designed for easier installation, relying on engineered interlocking systems to handle significant lateral earth pressure.
Gabion baskets (wire cages filled with rock) are a highly permeable and robust option, effective in areas with high water flow or unstable subsoils. Gabion walls offer exceptional longevity and flexibility, tolerating minor ground movement better than rigid concrete structures.
Building the Terrace: Step-by-Step Construction
Construction begins with the excavation of the slope to create the level base for the lowest retaining wall. This trench must be dug below the local frost line, if applicable, and wide enough to accommodate the wall material and a substantial foundation. Establishing the lowest wall first provides a stable benchmark for subsequent terraces.
The trench is then filled with a compacted layer of crushed stone or gravel, typically six to eight inches deep, which acts as the foundation and facilitates initial subsurface drainage. This aggregate base distributes the wall’s weight evenly and prevents settling or shifting. The first course of wall material is laid directly onto this level foundation, ensuring it is perfectly level.
As the wall is constructed upwards, incorporate a slight backward slope, known as “batter,” where the wall leans slightly into the hillside. This intentional lean, often one inch for every foot of height, utilizes gravity to increase the wall’s resistance to the massive outward pressure exerted by saturated soil. Each subsequent course must maintain this consistent batter for maximum stability.
Effective drainage is the most critical component for long-term wall survival, as saturated soil pressure can be up to 60 percent greater than dry soil pressure. Directly behind the wall, install a layer of clean, coarse gravel or drainage rock to prevent fine soil particles from clogging the system. This gravel zone should extend at least 12 inches from the wall face and run the entire length of the structure.
Within this gravel backfill, a perforated drainpipe (French drain) is laid near the base of the wall with its perforations facing downward. This pipe collects hydrostatic water that seeps through the soil and directs it away from the wall face to a safe outlet point. In non-mortared walls, strategically placed weep holes also allow water to escape directly.
Once the drainage layer is complete, backfill the remaining space behind the wall with the excavated soil, creating the level platform. This soil must be added in lifts of no more than six to eight inches at a time and thoroughly compacted. Proper compaction minimizes future settling and reduces the likelihood of water channeling. This process is repeated sequentially up the slope until the desired number of terraces is completed.
Long-Term Stability and Maintenance
Once the structural terracing is complete, stabilizing the newly exposed soil surfaces is necessary to prevent surface erosion. Planting deep-rooted, non-invasive vegetation, such as perennial grasses or native shrubs, helps bind the topsoil and absorbs excess moisture. The root systems physically interlock with the soil, creating a living mesh that resists sheet erosion.
Ongoing maintenance involves regular inspection of the retaining walls for signs of shifting, bowing, or excessive cracking, which may indicate a drainage failure or structural overload. Small settlements are normal, but significant movement requires immediate attention to prevent structural failure.
Ensure that all drainage outlets, including perforated pipes and weep holes, remain completely clear of debris. A blocked drainage system allows water pressure to build up behind the wall, which is the most common cause of retaining wall collapse over time.