When a project requires holding back earth, there are two fundamentally different approaches: build a structure in front of the soil (conventional retaining wall) or reinforce the existing soil in place (soil nail wall). Both work. Both are proven. But they are suited to different site conditions, different budgets, and different construction sequences. Choosing the wrong system can add months to the schedule and hundreds of thousands of dollars to the cost.

How a Conventional Retaining Wall Works

A conventional retaining wall is a structure built in front of (or within) an excavation to resist the lateral earth pressure of the retained soil. Common types include:

• Cantilever walls are reinforced concrete walls with a footing that extends under the retained soil. The weight of the soil on the heel of the footing resists overturning. These are the most common type for walls 4 to 20 feet tall.
• Gravity walls use their own mass to resist earth pressure. They can be concrete, stone, or gabion baskets. Practical for walls under 8-10 feet.
• MSE walls (mechanically stabilized earth) use layers of geogrid or steel strip reinforcement within compacted fill behind a wall face. The reinforced soil mass acts as a gravity block. MSE walls are cost-effective for heights above 10-15 feet and are the most common system for large retaining structures.

All conventional walls require excavation behind the planned wall location, construction of the wall, then backfill. This means the contractor must cut into the hillside or slope to create working room, build the wall, and fill behind it. On tight sites, the temporary excavation may extend onto adjacent property or require temporary shoring.

How a Soil Nail Wall Works

A soil nail wall reinforces the existing soil in place by drilling steel bars (soil nails) into the earth face and applying a shotcrete (sprayed concrete) facing. The construction sequence is top-down:

1. Excavate 3-5 feet of the face (one lift).
2. Drill holes into the exposed face at a regular grid pattern (typically 4 to 6 feet on center both horizontally and vertically).
3. Insert steel bars (typically #6 to #10 rebar, 15 to 30 feet long) and grout them in place.
4. Attach bearing plates and apply a 4-6 inch thick shotcrete facing over the exposed face, typically reinforced with welded wire fabric.
5. Repeat for the next lift down until the full wall height is reached.

The result is a reinforced soil mass that stands as a unit. The soil nails are passive reinforcement (they develop resistance as the soil tries to move) rather than active reinforcement (like ground anchors, which are pre-tensioned).

Side-by-Side Comparison

When to Choose a Soil Nail Wall

• The excavation is already being made. If the project involves a cut into a hillside (for a building pad, parking level, or road widening), a soil nail wall stabilizes the cut face as it is excavated. No temporary shoring is needed because the soil nail wall IS both the temporary and permanent support.
• There is no room to excavate behind the wall. Conventional walls require excavation beyond the back face of the wall for forming and backfilling. On sites bounded by adjacent buildings, property lines, or utilities, there may be no room for this. Soil nail walls do not require access behind the wall.
• Speed is critical. Soil nail wall construction is typically faster than conventional wall construction. A specialty soil nail contractor can install 400-600 SF of wall face per day. A conventional wall contractor installing forms, rebar, and concrete may produce 100-200 SF per day.
• The wall height exceeds 15-20 feet. Cantilever retaining walls become very expensive above 15 feet because the footing size and wall thickness grow rapidly. Soil nail walls scale more linearly with height and are often the most cost-effective system for walls 15-40 feet tall.

When to Choose a Conventional Retaining Wall

• The soil is not suitable for soil nails. Soil nailing requires soil that can stand unsupported for the short time between excavation and shotcreting of each lift (typically a few hours). Loose sands, soft clays, and soils below the water table may not stand long enough. These conditions require conventional walls or soldier pile/lagging systems.
• The wall face needs a finished appearance. Shotcrete facing has a rough texture. It can be sculpted or stained, but it will never look like a smooth formed concrete wall or a segmental block wall. For projects where the wall is a prominent architectural feature, conventional walls or MSE walls with precast panel faces provide better aesthetics.
• Soil nails would cross property lines. Soil nails extend 15-30 feet behind the wall face. If adjacent property owners will not grant an easement for the nails, a soil nail wall cannot be used. Conventional walls with footings that stay within the property line avoid this issue.
• The wall retains fill (not a cut). Soil nail walls work by reinforcing existing in-situ soil. If the wall retains new fill material (an embankment, a raised pad), soil nailing does not apply. MSE walls or conventional walls are used for fill retention.

Drainage Requirements

Both systems require drainage, but the details differ:

Conventional Retaining Walls

• Drainage aggregate (Class 2 permeable material or equivalent) behind the wall, minimum 12 inches wide.
• Perforated drain pipe (typically 4-inch or 6-inch SDR 35 or HDPE) at the base of the wall, sloped to drain to an outlet.
• Weep holes through the wall face at 6 to 10 feet on center (for cantilever walls).
• Waterproofing membrane on the back face if the wall is below grade adjacent to habitable space.

Soil Nail Walls

• Geocomposite drain strips (prefabricated drainage panels) installed between the shotcrete facing and the soil face. These channel groundwater seepage down to a collector drain at the base of the wall.
• Perforated collector pipe at the base of the wall.
• Weep holes through the shotcrete facing (PVC pipe stubs cast into the facing during shotcrete application).

Cost Drivers

The total installed cost depends on more than just the wall type:

On a typical 20-foot-tall, 100-foot-long wall (2,000 SF of face), the total installed cost difference between a cantilever retaining wall and a soil nail wall can range from $40,000 to $80,000 in favor of the soil nail wall, primarily due to the elimination of temporary shoring and reduced excavation. For shorter walls (under 10 feet), the cost difference narrows and the conventional wall may be cheaper due to the soil nail contractor's mobilization costs.

What the Civil Engineer Needs to Provide

Regardless of the wall type selected, the civil engineer's grading plans must show:

• Existing and proposed grades at the top and bottom of the wall.
• Wall alignment in plan view with stationing or reference dimensions.
• Cross-sections at key locations showing wall height, finished grades, drainage provisions, and utilities within the influence zone.
• Surface drainage design (swales, V-ditches, or storm drain inlets) above and below the wall to keep surface water away from the wall.
• Utility locations relative to the wall. Utilities within the reinforcement zone (soil nails or geogrid) create coordination conflicts that must be resolved during design, not during construction.

The structural design of the wall itself (whether conventional or soil nail) is typically performed by a structural engineer or the specialty wall contractor, based on geotechnical parameters provided by the geotechnical engineer. The civil engineer coordinates between all three parties and ensures the wall fits within the site grading, drainage, and utility plans.