Question 1

Why is excavation and base prep the most important part of a tall retaining wall?

Accepted Answer

The entire performance of a retaining wall system depends on what's beneath and behind it. Tall walls transmit massive loads to the base — vertical loads from block weight and horizontal loads from earth pressure — and if the base material is soft, uncompacted, or improperly sloped, the wall will settle unevenly, lose alignment, and eventually fail. Corners and ends are particularly vulnerable where improper base prep leads to differential settlement. This is why every tall wall we build starts with excavation to competent material before anything structural is placed.

Question 2

How deep should a retaining wall footing be on steep slopes?

Accepted Answer

On steep terrain, footing depth is determined by the engineering — not a standard rule. We excavate until we reach competent soil or bedrock meeting the bearing capacity requirements the engineer specifies for the specific wall height and load. In areas with good residual soils, 12–18 inches below the first block course may be sufficient. On sites with loose fill, active slope movement, or soft clay, we excavate 2–4 feet or more and may install a crushed-stone sub-base to achieve required bearing capacity.

Question 3

What does 'benching into the slope' mean for retaining walls?

Accepted Answer

Benching means cutting horizontal steps into the hillside as you excavate, so the wall base sits on a series of flat, firm platforms rather than on the angled face of the slope. This is critical for preventing the wall from sliding downhill as a unit under earth pressure. Each bench must be cut to competent soil, level, and wide enough for the wall base course plus drainage zone. Skipping benching and placing a wall directly on the sloped face is one of the most common base prep errors on failed walls.

Question 4

What base material should be under a tall retaining wall?

Accepted Answer

A tall retaining wall requires a compacted, angular crushed-stone leveling pad — typically #57 or a crusher-run aggregate specified by the engineer. This stone pad provides both bearing capacity and drainage at the base. Its thickness depends on wall height and base soil quality, typically ranging from 6 to 12+ inches for tall walls. The leveling pad must be compacted in lifts, not just dumped and graded. On soft or wet base soils, a geotextile fabric layer may be required first to prevent stone migration.

Question 5

How much compaction is required behind a tall retaining wall?

Accepted Answer

Engineered walls require controlled compaction in lifts — typically 6–8 inch compacted lifts of clean stone or specified structural fill, with mechanical compaction at each lift before the next is placed. Poorly compacted backfill consolidates after construction, creating voids behind the wall, reducing geogrid friction, and allowing the wall face to move over time. Compaction testing is part of our quality process on taller walls and is a required inspection item in many jurisdictions.

Question 6

Can you build a tall retaining wall on loose fill dirt?

Accepted Answer

Not safely without remediating the fill first. Loose fill hasn't been compacted to bearing capacity standards and can continue settling for years under wall load. Options include removing the fill and replacing it with properly compacted material in engineered lifts, treating the fill with compaction techniques if it's suitable material, or designing the wall foundation to bypass the fill and bear on competent soil below. The engineer specifies the right approach based on fill depth, type, and site conditions.

Question 7

What happens if you hit bedrock during retaining wall excavation?

Accepted Answer

Hitting bedrock during excavation is generally excellent news — bedrock provides ideal bearing capacity and prevents downhill sliding. However, bedrock must be properly shaped to receive the wall base. We use equipment to notch or step the bedrock surface into level benches so the wall base course sits flat on stable rock. A wall base placed on sloped bedrock without notching will slide as load increases. In some cases, epoxy-anchored pins into the bedrock provide additional resistance for the first block course.

Question 8

Why do tall retaining walls fail when base prep is done wrong?

Accepted Answer

When the base settles unevenly — because it was placed on weak soil, uncompacted fill, or an unbenched slope — the wall begins to shift as a unit. Corners drop, the wall face goes out of plumb, cap blocks separate, and horizontal cracks appear through the wall body. Over time the movement compounds because uneven settlement changes the load distribution and makes drainage less effective. This is why we document base conditions thoroughly: if the base is inadequate, the entire wall above it is compromised from day one.

Question 9

Do steep-slope retaining walls require engineering for excavation and base prep?

Accepted Answer

Yes — excavation and base preparation for any tall or steep-slope wall must be directed by engineering. The engineer specifies minimum footing depth, bench geometry, base stone type and thickness, compaction requirements, and geogrid placement starting elevation. Without these specifications, contractors are guessing at critical parameters that determine whether the wall moves or holds. Building department inspections at the footing stage exist specifically to verify base prep was done per the engineered drawings before block placement begins.

Question 10

How does ERWalls handle excavation on hard-access mountain or lake sites?

Accepted Answer

On hard-access mountain and lake sites, we use staged bench cuts with appropriately sized excavators, specialized rigging for steep approaches, and barge or crane delivery when road access doesn't support the equipment. We match excavation equipment to site constraints — smaller machines for tight lots, compact tracked excavators for steep access, remote controls where necessary. Getting base prep right is non-negotiable regardless of access difficulty; we've never compromised base prep because a site was hard to reach.

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RETAINING
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Excavation & Base Prep for Tall Walls