Plan Retaining Walls on a Slope Around the Whole Hillside

For dry-stacked segmental block walls, follow rising ground with level base sections that step uphill—not an inclined base course. Where the hillside rises behind the wall or falls away below it, the design must also address the stability of the surrounding soil—not just the wall face. Allan Block’s stepped-base instructions describe the construction method.

Before choosing blocks or booking excavation equipment, establish the maximum wall height, the space available behind it, the downhill ground conditions and a workable drainage outlet. Those constraints determine whether one wall, terraces or a different hillside layout is feasible.

Identify which way the ground slopes

“On a slope” describes three different conditions, often present together:

Ground condition	Planning consequence
Grade rises along the wall’s length	Lay out level foundation sections and their step-up locations.
Grade rises behind the wall	Include the backslope in the wall and reinforcement design; a level-backfill detail is not interchangeable.
Grade falls away in front of the wall	Check toe support, embedment, erosion and overall slope stability.

The Concrete Masonry & Hardscapes Association (CMHA) explains that a downhill toe slope reduces the soil’s resistance to movement, while an increasing backslope also reduces stability. Its design guidance also shows that backslope geometry affects required reinforcement length. CMHA global-stability guidance and SRW design guidance address these separately.

A stepped base solves the changing elevation along a wall. It does not, by itself, solve the other two conditions.

Lay out the hillside before selecting the wall system

Prepare a plan view, a profile along the wall and cross-sections through the steepest or most heavily loaded areas. Record:

• Existing and proposed grades at the wall face, behind it and downhill.
• Exposed wall height and buried wall depth, especially at the lowest section.
• Buildings, driveways, utilities, property boundaries and proposed upper walls.
• Seepage, drainage paths and the proposed discharge location.
• Excavation limits, reinforcement footprint, machine access and material-storage areas.

Do not estimate from average exposed height alone. A wall that looks low at one end can have a substantially taller section at the downhill end. Reinforced block systems also occupy ground behind the face: geogrid extends into compacted soil, and its required length depends on loads, soil and geometry. A narrow strip beside a boundary may not accommodate the selected system. See geogrid selection and installation for that part of the design.

Identify temporary loads as well as finished-site loads. Traffic and bulk material storage add surcharge, and another retaining wall can add permanent loading. Mark equipment routes and stockpile positions for the designer rather than treating the future terrace as an unrestricted staging area. CMHA explains these surcharge effects.

Confirm permit and engineering requirements with the local authority. CMHA recommends professional engineering for segmental walls exceeding 4 ft (1.21 m) in total design height where specific local requirements do not exist. That is not a safe-height exemption for hillside walls: its separate global-stability guidance identifies groundwater, tiered walls, weak soils and slopes of 3H:1V or steeper at the top or toe as important analysis conditions. Here, 3H:1V means three units horizontally for each unit vertically. Design recommendations and global-stability conditions.

Choose one wall or terraces as a complete system

Terraces create intermediate benches, but several short walls are not automatically independent structures. The upper wall can load the lower wall, and the whole arrangement can move with the hillside.

Allan Block’s residential guidance uses a terrace-spacing rule of at least twice the lower wall’s height, with the upper wall no taller than the lower wall, for treating the walls independently. It also identifies slopes, groundwater, surcharge and poor soils as conditions requiring engineering. That rule is a manufacturer-specific planning screen, not approval for an arbitrary hillside layout. Allan Block terrace guidance.

For example, a 3-ft lower wall gives a preliminary spacing of 6 ft under that rule. It does not establish that a 6-ft bench is sufficient for the reinforcement, drainage, access or stability of a particular project. CMHA states that global stability must be checked for all tiered walls. CMHA tiered-wall discussion.

Check what supports each upper wall, too. Excavating and backfilling the lower tier may leave disturbed soil beneath the upper foundation. Allan Block warns that poor compaction there leads to upper-wall settlement. Specify foundation preparation and fill compaction before construction, not after the upper trench is open.

Build level base sections that step uphill

For a segmental block wall, the slope-specific sequence is:

1. Set out the lowest foundation elevation first. Mark step locations and the required burial against proposed finished grade.
2. Excavate a level section into the hillside. Provide room for the specified base aggregate and buried units—not merely enough depth to hide the front edge.
3. Prepare and compact the foundation and aggregate base. Check each section for level before laying blocks.
4. Install the lowest base course and establish the next step. For Allan Block’s system, the base steps up by one block height; the next prepared base aligns with the top of the lower course.
5. Repeat uphill, maintaining burial at every step. Check course alignment, level and the specified wall batter as construction proceeds.

Allan Block’s commercial instructions require at least one full buried unit at every step. Its residential instructions also require compacting the next step-up area and keeping it level with the adjacent blocks. Use the selected system’s details and project drawings for actual base dimensions and embedment. Commercial step-up instructions and residential construction sequence.

A downhill toe may require greater embedment; burying one block is not a universal hillside design. CMHA relates embedment to wall height and toe slope, including protection against undermining by erosion. Embedment guidance.

This block-course method is not a footing design for reinforced concrete or mortared masonry. Those systems need their own foundation and step details.

Coordinate drainage across every elevation change

Plan surface runoff and subsurface drainage separately. Water arriving from uphill should not be left for the aggregate behind the wall to handle: CMHA describes the wall gravel and drain pipe as provisions for incidental water, not the primary site-drainage path. SRW drainage guidance.

Before backfilling, confirm on the drawings:

• How uphill runoff and water on each terrace are collected or diverted.
• Drain elevations, connections and outlets through the stepped layout.
• Where collected water discharges and how the discharge avoids eroding the toe.
• Required drainage aggregate, filters and any groundwater-control measures.

A level block base and a functioning drain are different requirements. Do not assume the pipe can simply mirror every foundation step; have the drainage route resolved before burying it. Coordinate temporary runoff controls with the permanent system and the site’s erosion-control plan.

Make excavation access part of the construction plan

The excavation needs room for foundation work, drainage, reinforcement and compaction—not just the block width. Establish machine working positions and material delivery routes before opening the slope, and do not undermine existing walls or foundations.

For U.S. construction work, OSHA requires cave-in protection for employees in excavations, except those entirely in stable rock or those less than 5 ft deep where a competent person finds no indication of a potential cave-in. Plan the protective system and its working footprint before excavation. OSHA protective-system requirements.

OSHA also requires utility-location work before excavation, protection where excavation endangers adjacent structures, and competent-person inspections before work, as needed throughout the shift and after rainstorms when employee exposure can reasonably be anticipated. Its 2-ft separation or retaining-device requirement addresses materials and equipment falling or rolling into excavations; it does not establish a universal safe machine setback for a hillside. OSHA excavation requirements.

Use inspection hold points before covering the foundation, drainage and reinforcement. If excavation reveals unexpected seepage, soft foundation soil or ground movement, stop that portion of the work and obtain revised direction. A neat stepped face cannot compensate for an unresolved hillside condition.