A retaining wall holds back soil, and soil is heavier than most people expect, especially when wet. The wall gets the attention, but the excavation and drainage behind it do most of the work in keeping it standing. This article covers the basics of excavating for a retaining wall in the Sacramento area and why drainage deserves as much planning as the wall.
What the excavation does
Before a wall goes up, ground has to be cut and prepared. Excavation typically includes:
- Cutting into the slope to make room for the wall and the backfill zone behind it
- Digging the footing or base trench to the depth and width in the plan
- Preparing the base, often compacted aggregate, so the wall sits on a stable, level foundation
- Creating working space so the wall can be built safely and backfilled properly
On existing slopes, the cut behind the wall can be larger than expected, since the excavation must be safe and leave room for drainage rock. That affects how close you can build to property lines, fences and structures.
Why drainage comes first
Water behind a wall adds pressure, called hydrostatic pressure, on top of the soil load. In clay, which is common in the Sacramento Valley, water drains slowly and soil swells when wet, increasing the load further. Most failures trace to water, not just the wall materials.
A typical drainage setup includes:
- Perforated drain pipe at the base of the wall, sloped to an outlet
- Clean drain rock around and above the pipe, and often in a zone behind the wall
- Filter fabric to keep soil from clogging the rock
- A reliable outlet, such as daylighting at the end of the wall or tying into an approved drainage path
- Surface grading above the wall so runoff does not pour over or behind it
The outlet is critical. A perfectly built drain with nowhere to discharge simply fills up. Rules about where drainage can go vary by city and county, so confirm with your jurisdiction. See How to plan a French drain for outlet planning, which applies to wall drains too.
Footings and soil
The base needs to bear on suitable soil. Soft, wet or organic ground is removed and replaced as the design requires. Footing depth, width and base material come from the plan, the wall type and the soils.
Local conditions can change this:
- Expansive clay moves with moisture and can push walls. Drainage and proper backfill reduce the effect.
- Hardpan and rock in parts of the valley and the foothills make excavation harder, but can also provide a solid bearing layer. An engineer can tell you which applies.
- Slopes in Folsom, El Dorado Hills and the foothills often involve bigger walls, tiered walls and more drainage.
- High groundwater in low-lying areas near rivers and the Delta side requires more careful drainage design.
See hardpan and rock excavation in the foothills for how rock affects scheduling.
Backfill matters
What goes behind the wall is as important as the wall. Native clay is generally a poor choice next to a wall, because it holds water and pushes hard. Free-draining aggregate, placed and compacted in layers as the plan specifies, is the typical approach. Heavy compaction equipment right against a wall can damage it, so the method is chosen carefully. Follow the engineer’s or manufacturer’s instructions for the wall system you use.
Permits and engineering
Permit requirements vary by city and county, so confirm with your jurisdiction. In general, taller walls, walls that support a slope, driveway or structure, and walls near property lines, easements or flood zones are more likely to need review or engineering. Your jurisdiction decides the thresholds, so ask rather than rely on rules of thumb. An engineer’s plan also helps the excavator know exactly what to build.
811 and utilities
California law requires contacting USA North 811 before digging, at least two working days ahead. Walls often sit near property lines, where utilities and easements run. See Call 811 before you dig in California. Also check for irrigation and private lines, and plan around tree roots.
Timing and weather
Excavating a slope in the wet season, roughly November to April, adds risk. Saturated cuts can slump, and erosion control becomes a bigger concern. If you can, schedule excavation in dry weather and complete drainage and backfill before the rains return. If a cut must sit open, protect it.
Common mistakes
- Skipping the drain pipe or giving it no outlet
- Backfilling with native clay
- Footing too shallow or on soft soil
- Ignoring surface runoff from above
- Building before confirming permits, property lines or utilities
- Compacting too aggressively against the wall
What to have ready for an estimate
- The wall location, length and approximate height
- Plans or engineering, if available
- Photos of the slope, access and surrounding structures
- Survey or property line information
- Notes on existing drainage, water problems and trees
- Your target schedule
Maintenance after the wall is built
Check the drain outlet each fall before the rains and keep it clear of leaves and debris. Watch for new cracks, leaning, bulging or water seeping through the wall, and look at the ground above the wall for settling. Keep irrigation off the slope above, since extra water adds load. Early attention to small signs is much cheaper than rebuilding a wall.
Next step
If you are planning a retaining wall, request a written estimate and send what you have. See retaining wall excavation for the typical scope. Final scope is confirmed per job and follows your plans and permits.
Quick answers
Why do retaining walls fail?
The most common cause is water. Soil behind a wall gets heavy when saturated and pushes harder, especially clay. Walls without drainage, with poor backfill or with footings that are too shallow or on poor soil are more likely to lean, crack or move. Drainage behind the wall is as important as the wall itself.
Do I need an engineer for a retaining wall?
Often, for taller walls, walls that carry a slope, driveway or structure, or walls in a flood or slope-sensitive area. Requirements vary by city and county, so confirm with your jurisdiction. Even when not required, an engineer can be worth it on difficult soil.
What kind of backfill goes behind a retaining wall?
Usually free-draining aggregate such as clean crushed rock next to the wall, rather than native clay, with drain pipe at the base. The design may call for filter fabric and specific compaction. Follow the plan or engineer, because the wrong backfill traps water and loads the wall.
This guide is general information for planning, not engineering, legal or permitting advice. Requirements vary by city, county and site. Confirm details with the agency or professionals involved.