Segmental retaining walls in Seattle: a homeowner's guide

Gravity and reinforced soil mean different footprints

A gravity SRW needs enough weight and width for the ground, wall geometry and loads. Some configurations use deeper or multiple units. There is no single unreinforced height that works for every block and every site. A low wall beside a driveway can raise different questions from a low wall bordering a level garden.

A reinforced-soil SRW adds horizontal reinforcement, commonly geogrid: a manufactured grid buried in compacted fill and connected to the facing. The blocks, reinforcement and soil work together. Ask the designer to show its full extent; the completed face can occupy much less space than the construction and reinforced-soil zone behind it.

Sources: [1], [2]

Read the parts behind the visible blocks

A useful project section shows what supports the wall and what holds back the ground. The table translates CMHA's system components into questions an owner can ask. It is a document checklist, not a construction detail or a specification for your site.

Sources: [2], [1], [5]

The slope and nearby loads belong in the design discussion

Surcharge means an added load near the wall, such as a vehicle, driveway, building or another wall. Sloping ground above or below the wall also changes the design questions. Give the team the proposed use of the area, including future changes, rather than describing only the current block face.

Global stability means the stability of the wall and the surrounding ground together. A wall face can be intact while a larger mass of soil moves beneath or around it. CMHA identifies groundwater, slopes, tiered walls, loads and weak ground as relevant considerations. Ask who evaluates the broader slope and whether a project-specific geotechnical report is needed.

Sources: [1], [3], [5]

Check the whole work area against the property boundary

A face drawn inside your property does not prove that its reinforcement, excavation, drainage and equipment also fit. Request a plan and section showing the boundary, utilities, nearby foundations and proposed work area. Read the property-line guide when the wall is close to a neighbor or public land.

Small facing units can help with curves and handling, but they do not remove the need to excavate, deliver fill and dispose of soil. Establish access before comparing prices. A proposal that assumes unrestricted use of a neighbor's yard is a different scope from one constrained to your lot; ask how the needed permissions and design changes will be resolved.

Sources: [2], [1], [4]

Drainage needs a route beyond the wall

Gravel and a pipe behind the blocks do not settle all drainage questions. CMHA distinguishes incidental water at the wall from broader surface and subsurface drainage. Ask the team to show how water approaching the site is managed, how water at the wall is collected, and where it is discharged.

Seattle says connecting drainage behind a wall to a side sewer requires a side sewer permit. Have the bid identify the proposed outlet and associated approvals instead of leaving discharge to be decided after excavation. The drainage-outlet guide helps organize those questions and the maintenance record.

Sources: [1], [4], [5]

Seattle's permit test includes more than height

SDCI lists three conditions for avoiding a construction permit: the wall is four feet or lower measured from the bottom of the footing to the top, the parcel has no environmentally critical area, and the work will not damage adjoining property or structures during or after construction. All three conditions matter. The visible height is only part of the first condition.

An environmentally critical area, or ECA, can include a steep-slope or landslide-related area. SDCI says walls on ECA parcels must meet specific requirements or obtain an exemption, and walls are not eligible for small-project waivers. Start with the Seattle ECA address check and ECA project guide, then confirm the actual proposal with SDCI. Grading and right-of-way work can introduce additional requirements.

Sources: [4]

Compare bids for the complete arrangement

Issue the same current drawings, ground recommendations, material requirements and drainage scope to each bidder. Ask for separate identification of included work, exclusions, allowances and proposed substitutions. A cheaper block or shorter reinforcement layout is a proposed design change; have the responsible designer evaluate it before treating prices as comparable.

CMHA emphasizes accurate field information, specified materials and inspection. For a permitted Seattle project, SDCI's reviewer assigns any special-inspection items. Ask which stages must be observed before work is covered and who arranges them. Seattle says the owner selects and pays the special-inspection agency. Our special-inspection guide explains that process.

Sources: [2], [6]

Keep a record that explains the installed system

Before handover, ask for approved drawings and changes, material records, required inspection reports and drainage-maintenance information. Label the documents by date and revision so a future designer can understand what was actually specified and recorded. A photograph of the finished blocks cannot describe concealed reinforcement or foundation conditions.

If you later add a fence, change a driveway, excavate for utilities or build another wall nearby, bring that record to the responsible professionals. Those changes can affect loads or parts of the retaining system. Use the wall-system overview when discussing alternatives and the contractor-comparison guide when evaluating proposals.

Sources: [2], [1], [6]