Retaining Walls in Tacoma That Hold Through Western Washington's Worst.

A retaining wall holds soil at two different levels. People usually want one for three reasons: to create a flat area on a sloped yard, to stop a bank from sliding onto a driveway or patio, or to replace an old wall that is leaning or failing.

Sometimes a wall is not the right answer. A gentle slope can be regraded and planted. A short grade change at a patio edge can be a step or a seat wall. A wet, slumping bank may need drainage first, because a wall built on top of a water problem inherits it. The first site visit should answer whether a wall is needed and how tall it really has to be.

Most residential walls around Tacoma fall into four types. Each handles soil and water differently, and each has a height range where it makes sense.

The block itself is the simple part of a block retaining wall. The Concrete Masonry and Hardscapes Association (CMHA) describes segmental retaining walls as dry-stacked concrete units plus compacted soil, with or without geosynthetic reinforcement. It also says soils make up about 90 percent of the system.

A short wall acts as a gravity wall: its own weight and a slight backward lean hold the soil. A taller wall becomes a reinforced soil mass, with layers of geogrid running back into compacted fill so the block face and the soil behind it act as one block of earth. Where one type ends and the other begins depends on the block, the soil, and what is above the wall, and it comes from the manufacturer's design charts or an engineer, not a rule of thumb.

Water is what pushes walls over. Around Tacoma, NOAA's 1991-2020 normals for Sea-Tac show about 39 inches of precipitation a year, most of it from October through March. Soil behind a wall that fills with water every winter gets much heavier and pushes much harder than the same soil in August.

CMHA lists soils and water first among the factors that drive wall design and says they affect everything in it. Its best-practice guidance calls for a zone of free-draining gravel directly behind the block, a drain pipe sloped to drain and brought out to daylight at least every 50 feet, and compaction in lifts of 8 inches or less. It specifically does not recommend pea gravel, the round rock that never locks up.

Surface water matters too. A wall with a lawn or driveway draining toward its top is collecting water it then has to hold back. Swales or drains above the wall that carry water around it, not into it, are part of the design. A wall needs proper drainage whether or not it needs a permit.

Height is measured from the bottom of the footing to the top of the wall, not from the ground you can see. A wall that shows 3 1/2 feet above grade can be over 4 feet once the buried footing course is counted.

In the City of Tacoma, a wall is exempt from a building permit only if it meets all of these: 4 feet or less from the bottom of the footing to the top, flat ground on both the upper and lower sides, no surcharge from a driveway, building, or other structure such as a fence, and located on private property. A wall in the right-of-way needs a Street Occupancy Permit.

In unincorporated Pierce County, the height limits, surcharge rules, and engineering requirements are set by the County, so check with Pierce County Planning and Public Works before you design the wall. Cities such as Puyallup, Lakewood, University Place, and Gig Harbor set their own rules, so the address decides which ones apply.

Steep slopes can also be regulated as critical areas. The City of Tacoma, for example, asks owners to confirm with the City before removing trees in a critical area such as a steep slope. A wall project on a bluff or ravine lot should start with a call to the permit office.

Three situations move a wall from a landscape project to an engineered structure. The first is height. The second is a surcharge: anything above the wall that adds weight, such as a driveway, a parked car, a building, a fence, a deck, or a hillside that keeps rising behind the wall.

The third is tiering. Two short walls stacked close together can behave like one tall wall, because the upper wall's weight bears on the soil the lower wall is holding. Spacing the tiers far enough apart keeps them independent. How far depends on the wall heights and soil, so tiered walls are worth engineering even when each wall looks short.

CMHA's guidance notes that a slope above a wall adds load, while a slope below the wall reduces its stability. A wall at the top of a steep drop needs more attention than the same wall on flat ground.

Most wall failures are slow. Catching them early can mean a section rebuild instead of a full replacement.

Retaining wall bids vary for the same reason paver bids do: the important work is out of sight. Ask each company to put the hidden parts in writing.

We start with a free site visit and soil test, then a free 3D design showing the wall's height, length, curves, caps, and how it ties into the patio, driveway, or steps. The itemized proposal spells out excavation, footing, drainage, reinforcement, backfill, and haul-off.

Our own in-house crews build the wall, with no subcontractors. Wall crews work out of our Yelm office across Tacoma, Pierce County, and nearby Thurston County, under Washington contractor license NDBRIDB815M5.