When it comes to building a retaining wall that lasts, what goes underneath it matters just as much as the blocks, stone, or other materials you see above ground. The retaining wall base is the foundation of the entire structure. It supports the wall’s weight, keeps the first course level, and helps distribute pressure into the ground through heavy rain, freeze-thaw cycles, and changing soil conditions.

The right base material is only part of the equation. You also need the correct excavation depth, a firm subgrade, proper compaction, a drainage system, and enough wall embedment to keep the structure stable. For taller walls, reinforcement and professional engineering may be necessary.

These details matter particularly in the Chicago suburbs, where clay-heavy soils, seasonal moisture changes, and winter freezing can put additional stress on landscape structures.

So, what is the best rock base for a retaining wall? How deep should it be? And when is a simple gravel foundation no longer enough?

This guide explains how to choose the right aggregate, prepare the base, manage drainage, and recognize when a retaining wall needs professional design.

Why Your Retaining Wall Base Material Matters

Think of your retaining wall like a house. Even a beautifully built structure can develop serious problems if its foundation is unstable. The same principle applies to retaining walls, where uneven settlement, water pressure, and soil movement can cause blocks to shift, lean, or crack.

A properly prepared retaining wall base will:

• Distribute the wall’s weight across a stable foundation.
• Keep the first course of blocks level and correctly aligned.
• Reduce uneven settlement and movement.
• Help control moisture when combined with a suitable drainage system.
• Improve the wall’s resistance to seasonal ground movement.
• Support the wall system’s intended design and lifespan.

However, a gravel base cannot compensate for every site problem. Soft soil, uncontrolled water, excessive loads, steep slopes, and inadequate reinforcement can cause failure even when the base material is appropriate.

Before construction, identify the wall’s intended height, the soil beneath it, the slope above and below it, and anything that may add weight near the top. These factors determine whether a basic gravity wall is suitable or a more engineered system is required.

Best Base Materials for a Retaining Wall

Not all gravel performs the same way. The best choice depends on whether the material will support the wall, provide drainage behind it, or serve as general backfill.

3/4-Inch Minus Crushed Stone

Three-quarter-inch-minus crushed stone contains particles up to approximately 3/4 inch, including smaller particles known as fines. The combination of different particle sizes helps the material compact into a dense, stable layer.

Best for: Many small and medium segmental-block retaining walls when the wall manufacturer’s specifications permit it.

Why it works: Angular particles interlock, while smaller particles fill gaps between larger pieces. With suitable moisture and mechanical compaction, this can produce a firm, relatively uniform base.

What to watch: The amount and type of fines matter. Excessive fine material can reduce permeability, particularly when the material contains silty or clay-like particles. Confirm that the aggregate meets the wall system’s requirements rather than choosing it by name alone.

Crush and Run (ABC Stone)

Crush and run, sometimes called aggregate base course or ABC stone, is a graded mixture of crushed rock and fines. It is commonly used for road bases, driveways, and other applications that need a compacted surface.

Best for: Retaining wall foundations where the selected aggregate meets the manufacturer’s requirements for gradation, drainage, and compaction.

Why it works: Its range of particle sizes allows the aggregate to lock together and form a dense base.

What to watch: Product names and aggregate specifications vary by supplier. Not every crush-and-run blend is appropriate for every retaining wall. A dense, well-compacted base material should not be confused with the open, free-draining aggregate needed immediately behind the wall.

#57 or #78 Angular Stone

These are commonly used aggregate designations for particular stone gradations. The particles are generally angular, helping them resist movement, and the relatively low fines content of clean products allows water to pass through more freely.

Best for: Drainage zones behind retaining walls and other applications where free drainage is a priority. Use as the structural base only when the wall manufacturer’s specifications or engineered design allow it.

Why it works: Clean angular stone creates connected voids through which water can drain.

What to watch: Clean drainage stone and compactable base aggregate serve different purposes. Open-graded stone does not necessarily compact into the same dense, smooth foundation as a well-graded aggregate. Do not automatically substitute #57 stone for a specified compactable base.

Paver Base

Paver base products are typically graded crushed aggregates designed to support paving installations. Some may be suitable beneath small landscape walls, but the label alone does not establish suitability.

Best for: Small garden walls when the specific product and wall manufacturer’s instructions approve it.

Why it works: Appropriate graded products can be leveled and compacted into a stable foundation.

What to watch: Bagged products differ in composition, and a product marketed for pavers is not automatically suitable for a retaining wall. Check the product specification and wall installation instructions before use.

Class I–III Backfill Aggregate

Class I, II, and III are classifications used in some engineering and construction specifications. Their precise meaning depends on the applicable standard, jurisdiction, and project documents.

Best for: Engineered retaining walls where the plans specify a particular soil or aggregate classification.

Why it works: A defined specification helps the contractor select material with the required engineering and compaction properties.

What to watch: These classifications are not interchangeable across all standards. Follow the project specifications rather than assuming that any material sold under one of these labels is suitable.

How Deep Should a Compacted Retaining Wall Base Be?

The base needs to be thick enough to support the wall and establish a firm, level foundation. However, there is no single base depth that works for every wall height and soil condition.

For example, Allan Block’s commercial installation specifications specify a minimum 4-inch base for walls under 4 feet and a minimum 6-inch base for walls over 4 feet, subject to the wall design and installation requirements. Its residential instructions use different trench and embedment details for different applications.

The following table is a preliminary planning guide, not a construction specification.

Approximate wall height	Base-depth planning	Soil and site considerations
Under 2 feet	Some approved systems use a 4-inch compacted aggregate base	Firm, undisturbed soil and low loading make the simplest installations more feasible
2–4 feet	A 4-inch minimum may be permitted by some systems; the design may require more	Clay, wet ground, disturbed soil, slopes, and nearby loads may require additional preparation
Over 4 feet	Some manufacturer specifications use a 6-inch minimum base, but the full foundation must be designed for the site	Engineering, reinforcement, drainage design, and permit review become particularly important
Any height on soft, wet, filled, or sloping ground	Do not rely on a standard depth alone	The foundation may need excavation, replacement material, geotechnical assessment, or an engineered design

These figures refer to the compacted aggregate base layer, not the total depth of the excavation. The trench must also accommodate the required buried portion of the first course.

Base Depth vs. Wall Embedment: What’s the Difference?

These two measurements are often confused:

• Base depth is the thickness of the compacted aggregate beneath the first course of wall units.
• Embedment depth is the vertical depth of the wall’s bottom course below the finished ground level in front of the wall.
• Trench depth is the total excavation depth needed to accommodate the base, the buried wall units, and any additional design requirements.

A deeper trench does not automatically mean a thicker gravel base. For example, if a wall needs 6 inches of compacted aggregate and a specified portion of its first course buried below grade, the trench must accommodate both measurements.

The required embedment depends on the wall system, wall height, ground slope, erosion exposure, and design. Some Allan Block guidance uses a minimum buried-block depth and an additional allowance based on wall height, but the actual installation must follow the selected system’s instructions and approved plans.

How Soil Changes the Base Requirements

Firm, undisturbed granular soil: This generally offers better drainage and more predictable foundation performance, although it still needs to be checked for stability and bearing capacity.

Clay-heavy soil: Clay can hold water, soften when wet, and change volume as moisture conditions vary. The contractor may need to remove unsuitable material, improve the foundation, and pay particular attention to drainage.

Wet or saturated soil: Construction should not proceed on a soft, unstable trench bottom. Water must be managed, and unsuitable soil may need to be removed and replaced according to the design.

Previously disturbed or poorly compacted fill: This can settle unevenly beneath the wall. It may need to be excavated and replaced with suitable material compacted in controlled layers.

Organic soil or topsoil: Roots, vegetation, and decomposing organic material are not appropriate foundation materials. Remove them from the foundation area.

The Pea Gravel Mistake (Don’t Do This)

Pea gravel is often recommended for drainage because water can pass between its rounded stones. That does not make it an ideal structural base.

Rounded stones do not interlock in the same way as angular crushed aggregate. Under load, they can shift, making it difficult to establish the firm, level foundation required by many retaining wall systems.

For the base beneath a retaining wall, use a suitable compactable aggregate specified for the wall system not loose pea gravel.

Pea gravel may have uses in other landscape drainage applications, but the material immediately behind a retaining wall should be selected to suit the wall’s drainage design. Clean, angular drainage aggregate is commonly preferred because it provides connected voids for water flow and is compatible with many retaining wall drainage details.

The key distinction is that a structural base and a drainage zone perform different jobs. A material that drains exceptionally well may not be the best material for forming a compacted foundation.

How Much Gravel Behind a Retaining Wall?

The drainage zone behind the wall provides a route for water to move toward a drain outlet instead of building pressure against the wall.

Many segmental-block systems specify a minimum of 12 inches of suitable drainage aggregate behind the wall units. However, the actual width and depth depend on the wall system, drainage design, available space, soil conditions, and whether the wall uses reinforcement.

For an ordinary residential planning estimate, a 12-inch drainage zone is a useful starting point to discuss with your installer not a universal requirement for every retaining wall.

Choose Clean, Free-Draining Aggregate

Use clean, appropriately graded angular drainage stone that meets the wall manufacturer’s specifications. Avoid using soil-heavy or fines-rich material in a drainage zone designed to remain permeable.

The purpose is to provide a connected path for water to move downward without quickly clogging the spaces between the stones.

Install a Drain Pipe Where Required

A perforated drainage pipe is commonly installed near the bottom of the drainage zone, positioned according to the wall design. It needs an appropriate outlet, such as a permitted daylight discharge or an approved drainage connection.

A pipe that has no functioning outlet will not reliably relieve water pressure. The discharge point must also be arranged so it does not erode the ground, damage neighboring property, or direct water toward a building foundation.

Separate Drainage Aggregate From Fine Soil

A suitable geotextile filter may be required between the drainage stone and surrounding native soil. Its purpose is to reduce the migration of fine particles into the drainage zone while allowing water to pass through.

The correct filter depends on the soil and drainage design. Do not automatically wrap the entire wall or drainage zone in any available landscape fabric: some fabrics and installation arrangements can restrict drainage or clog under site conditions.

Keep Surface Water Away From the Wall

Drainage behind the wall will not solve every water problem if the ground above the wall directs runoff into it.

Where practical, shape the finished ground to direct surface water away from the wall, and address downspouts, irrigation runoff, and other concentrated water sources. The aim is to reduce the amount of water entering the retained soil in the first place.

Estimate the Amount of Drainage Stone

You can estimate the volume using the planned length, height, and drainage-zone width.

For a simple rectangular estimate:

Volume in cubic yards = length (ft) × drainage height (ft) × drainage width (ft) ÷ 27

For example, a 20-foot wall with a 3-foot drainage height and a 1-foot-wide drainage zone requires approximately:

20 × 3 × 1 ÷ 27 = 2.22 cubic yards

This is a theoretical volume before accounting for block geometry, variations in grade, pipe space, settlement, or waste. Measure the actual drainage zone and ask the supplier or contractor how much material to order. Do not apply a universal extra percentage without considering the aggregate and site.

Compaction: The Non-Negotiable Step

Even the right base material can perform poorly if the foundation is soft, uneven, or inadequately compacted.

Follow the wall manufacturer’s installation instructions and the approved design for compaction. Allan Block’s commercial specifications, for example, call for foundation preparation and base compaction to a stated level, including 95% of Standard Proctor maximum dry density in the specified applications. That requirement should not automatically be applied to every wall or every material without checking the relevant specification.

Step 1: Excavate the Foundation Trench

Mark the wall alignment and excavate to the required width and depth. Remove topsoil, roots, organic material, loose debris, and unsuitable soil.

The trench must be wide enough for the base and wall system. If the wall needs geogrid reinforcement, the excavation must also accommodate the designed reinforcement zone behind it.

Step 2: Inspect the Trench Bottom

Before placing aggregate, check whether the exposed soil is firm and stable. Soft spots, standing water, heavily disturbed ground, or unexpected fill should be addressed rather than covered up.

If the soil conditions differ from the design assumptions, pause and obtain guidance from the appropriate professional.

Step 3: Place the Approved Base Aggregate

Spread the specified compactable aggregate evenly across the trench. Place it in manageable lifts that suit the material and compaction equipment.

Avoid assuming that a single thick layer will compact uniformly. The correct lift thickness depends on the aggregate, equipment, and project requirements.

Step 4: Compact and Level the Base

Use appropriate mechanical compaction equipment and follow the specified procedure. Check the base for level along the wall and from front to back.

A plate compactor is commonly used for suitable small residential installations, but the equipment and compaction requirements may differ for larger walls.

Step 5: Set and Check the First Course

Place the first row of wall units on the prepared base, following the manufacturer’s orientation and alignment instructions. Check each unit for level and alignment, and make adjustments before stacking additional courses.

The first course is particularly important because errors at the bottom can carry upward through the entire wall.

Step 6: Maintain Compaction During Construction

As the wall rises, install the drainage aggregate and approved backfill in the specified sequence. Compact backfill in controlled lifts, using care around the wall units and any geogrid.

Excessive compaction pressure too close to an inadequately supported wall can displace blocks. Follow the wall system’s instructions for equipment, lift thickness, and setback from the wall face.

How to Organize Your Retaining Wall Project: Step by Step

A successful retaining wall is built in the right sequence. The following process outlines the main stages, but it does not replace the instructions for the selected wall system or approved engineering drawings.

Step 1 – Mark and Excavate

Establish the wall’s alignment, finished elevations, and excavation limits. Confirm property boundaries and underground utilities before digging. Remove unsuitable material and excavate for the required base, buried wall units, drainage system, and any reinforcement.

Step 2 – Prepare and Compact the Base

Inspect the trench bottom and address soft or unstable soil. Place the approved base aggregate, compact it to the specified requirements, and establish a consistent level surface.

Step 3 – Set the First Course of Blocks

Install the first course of retaining wall blocks according to the manufacturer’s instructions. Check alignment and level carefully before continuing.

Step 4 – Add Drainage Aggregate and Approved Backfill

Place the specified drainage stone immediately behind the wall. Use the approved backfill in the remaining areas, following the design requirements for each zone. Do not substitute clay-heavy soil in a reinforced zone when the design prohibits it.

Step 5 – Install the Drainage System

Install the perforated pipe, filter material, and outlet in the positions required by the wall design. Verify that the outlet is functional and that water can be discharged safely.

Step 6 – Install Geogrid if Required

If the design calls for geogrid, install the specified reinforcement at the required elevations and lengths. The grid must be properly oriented, extended into the reinforced soil mass, and covered with approved fill and compaction in the required sequence.

Do not add arbitrary pieces of geogrid or shorten specified lengths as a substitute for an engineered reinforcement layout.

Step 7 – Backfill and Finish the Landscape

Complete the remaining backfill and finish grading as specified. Direct surface water away from the wall where appropriate, and finish with the planned sod, planting beds, or other landscaping.

Before closing the work, check the wall alignment, drainage outlet, finished grades, and any inspection requirements.

Geogrid Guidance: When a Gravel Base Is Not Enough

A retaining wall’s stability depends on more than the strength of its base. As the wall becomes taller or supports additional loads, the soil behind it can exert greater pressure. In some designs, geogrid reinforcement helps the wall and the reinforced soil mass act together as a more stable structure.

Geogrid is a polymer reinforcement material installed in horizontal layers within the backfill at specified elevations and lengths.

When Is Geogrid Needed?

Geogrid may be required when:

• The wall exceeds the height or configuration permitted for an unreinforced gravity wall.
• The wall supports a driveway, parking area, building, fence, or other surcharge near its top.
• The ground behind or below the wall slopes significantly.
• The site has weak foundation soils or challenging drainage conditions.
• The wall manufacturer’s design requires reinforcement for the chosen height and layout.

There is no universal rule that geogrid must start at one specific wall height. Some wall systems offer unreinforced designs for certain lower walls, while others require reinforcement based on wall geometry and site conditions.

How Long Should Geogrid Be?

The required geogrid length depends on the wall height, soil properties, loads, slope conditions, wall system, and engineering calculations. It is not safe to select the length using a simple percentage of wall height without confirming the design.

The reinforcement must extend far enough into the soil to develop the required resistance. Its position, connection to the wall units, overlap, orientation, and installation sequence also matter.

What Soil Should Surround Geogrid?

Use the fill specified by the wall design. Heavy expansive clay and organic soils are generally unsuitable for many reinforced soil zones. Other soils may be acceptable if they meet the project’s requirements and are installed and compacted correctly.

Geogrid is not a replacement for drainage. Water management, soil suitability, reinforcement, and foundation preparation must work together.

Climate and Soil: Local Conditions Matter

For homeowners in Naperville, Hinsdale, Wheaton, and the wider Chicago suburbs, local soil and weather conditions should influence retaining wall design.

Clay-Heavy Soil

Clay-rich soil can retain moisture and may become soft when saturated. Changes in moisture can also cause certain clays to shrink or swell, contributing to movement and added pressure behind a wall.

The appropriate response is not simply to add more gravel beneath the blocks. The contractor must assess the foundation soil, select suitable backfill, and design drainage to manage water.

Freeze-Thaw Cycles

Water in and around the wall can freeze and thaw seasonally. When drainage is inadequate, moisture-related ground movement can contribute to displacement and settlement.

A suitable compacted base, correctly installed drainage aggregate, functioning outlets, and appropriate surface grading all help reduce avoidable water-related problems.

Heavy Rainfall and Runoff

Intense rainfall can saturate retained soil and increase lateral pressure against a wall. Downspouts, poorly graded lawns, and runoff from paved areas can make the problem worse.

Direct concentrated runoff away from the wall where possible. If the site has a high water table, persistent seepage, or limited drainage outlets, a more detailed drainage design may be needed.

Sloping Ground

Walls on slopes require additional attention because soil can move along potential failure surfaces beyond the immediate area behind the blocks. A wall may be unstable even if its individual blocks and base appear sound.

A slope above or below the wall is a reason to seek a site-specific assessment, especially for taller walls, constrained properties, or walls near buildings and paved surfaces.

Engineer and Permit Thresholds: When to Get Professional Help

The wall’s height is an important starting point, but it is not the only factor that determines whether a permit or engineered design is required.

Walls Over 4 Feet

A wall around or above 4 feet high should trigger a careful review of the design and local requirements. Many wall-system guidelines call for professional engineering or geotechnical input for taller walls, but the precise threshold and method of measuring height vary.

The relevant measurement may include the distance from the bottom of the footing or foundation to the top of the wall, rather than only the exposed height visible from the front.

Walls Supporting Additional Loads

Even a shorter wall may require engineering or a permit if it supports a driveway, vehicle parking, a building, a fence, or other surcharge. A pool, steep slope, nearby structure, or property-boundary constraint can also change the design requirements.

Local Permit Rules in the Chicago Suburbs

Requirements vary by municipality. For example, the City of Naperville’s residential building permit guide lists a permit requirement for landscape or retaining walls over 3 feet high. That is different from the 4-foot threshold used in some other jurisdictions.

Before excavation, contact your local building department to confirm the applicable permit, plan, engineering, zoning, and inspection requirements. Do not assume that a wall is exempt simply because it is under 4 feet tall.

Signs That You Need a Site-Specific Assessment

Arrange a professional assessment if the project involves:

• A wall above the applicable permit or engineering threshold.
• A driveway, building, or other significant load near the top.
• A steep slope above or below the wall.
• Soft, saturated, expansive, or previously disturbed soil.
• Persistent water seepage or no obvious safe drainage outlet.
• Multiple retaining walls that may interact with one another.
• A wall close to a property line, utility, or existing structure.

An engineer can evaluate the foundation, wall stability, reinforcement, soil conditions, and drainage requirements. A landscaping contractor can then coordinate the installation with the approved design and local requirements.

Retaining Wall Base Materials

Material	Best use	Compaction characteristics	Drainage characteristics
3/4-inch-minus crushed stone	Approved compacted bases	Generally good when properly graded and compacted	Depends on fines content
Crush and run / ABC stone	Compactable foundation aggregate where approved	Generally good	Usually lower permeability than clean stone
#57 or #78 clean angular stone	Drainage zones; base where specifically approved	Does not compact like well-graded base aggregate	Generally very good
Paver base	Small walls when the product is approved for the application	Product-dependent	Product-dependent
Specified engineered aggregate	Walls with project-specific requirements	Must meet the design specification	Depends on the selected gradation
Pea gravel	Limited drainage uses where specified	Poor choice for a typical compacted structural base	Water can pass between the rounded stones

The correct material is the one that meets the retaining wall system’s requirements for its intended location. A compacted foundation aggregate and clean drainage stone should not be treated as interchangeable products.

Don’t Let a Poor Base Ruin a Beautiful Wall

A retaining wall is a structural landscape feature, not just a stack of decorative blocks. Poor foundation preparation, inadequate drainage, unsuitable backfill, and missing reinforcement can all contribute to failure.

Choose an approved compactable aggregate for the base, follow the wall system’s embedment and compaction requirements, install a properly designed drainage zone, and use geogrid where the design calls for it.

For taller walls or difficult site conditions, obtain the required engineering and permits before construction begins. These steps help reduce avoidable repairs and protect the landscape around your home.

Ready to Build It Right the First Time?

At Ware Landscaping, we design and build retaining walls across Naperville, Hinsdale, Wheaton, and the greater Chicago suburbs. From excavation and foundation preparation to drainage coordination and final landscape installation, our team can help plan a wall that works with your property’s grades and conditions.

We also integrate drainage solutions into hardscape projects to help manage runoff and protect the surrounding landscape.

Request a Free Quote to discuss your retaining wall project.

From Structural Support to Outdoor Sanctuary

A great retaining wall does more than hold back soil. It can create usable space, define planting areas, manage changes in elevation, and help turn a challenging yard into an inviting outdoor living area.

As shown in our Complete Outdoor Makeovers, Ware Landscaping can coordinate the details that bring a landscape together, from precision excavation and retaining walls to lighting, planting, and sod installation.