Building from the Ground Up: Installation and Base Preparation for Interlocking Pavers

Part 2 of the “Designing with Strength” series for landscape architects and designers

Previous post in this series: Understanding Interlocking Pavers: Choosing the Right Unit for the Job

Note: This series of articles is intended to provide an introduction to the concepts behind paver design for designers. It is not intended as a 'How To' guide for installers or contractors.

While this is not an accurate illustration of EVERY technique, It will serve to show how the base is prepared in general terms. It's a layer cake.

Every successful pavement begins long before the first paver is set. Beneath the surface lies a quiet structure of layers—soil, aggregate, sand—each one playing a role in supporting the next. While the surface pattern catches the eye, it’s the unseen foundation that determines how that design performs through time, weather, and use.

As both a landscape architect and former ICPI-certified hardscape contractor, I’ve seen how often design intent is lost between the drawing and the base course. Understanding base preparation gives designers control over the outcome. It ensures the geometry they envision is supported by the science beneath it.

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The Role of the Base

Interlocking pavements work because of friction, confinement, and load transfer—not adhesive or mortar. The base and bedding layers allow each paver to “lock” with its neighbors under pressure. When these layers are stable, the surface remains tight and uniform. When they’re not, the pavement shifts, heaves, or settles unevenly.

The base is the bridge between design and earth. Its success depends on three things:

1. Assessment of native soils

2. Selection of proper base materials

3. Attention to compaction, drainage, and edge restraint

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1. Subgrade: Knowing What You’re Building On

Before any aggregate is placed, the designer should know what lies below. Subgrade conditions influence depth, drainage, and even material choice.

• Soil Type: Identify whether the subgrade is clay (poor drainage), silt (unstable when wet), or sand/gravel (excellent drainage and stability).

• Compaction: Minimum 95% Proctor density is a baseline for stability.

• Moisture Content: Overly dry or wet soils can compromise compaction; the goal is uniform density, not hardness.

• Geotextile Separation: A non-woven geotextile may be used over clay or silty soils to prevent base migration and maintain separation.

Design Tip: Soils analysis doesn’t have to be exhaustive. A simple field percolation or probe test can tell a designer much about infiltration and stability, especially for permeable systems.

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2. Base Course: The Structural Foundation

The base course bears the load and disperses stress across the subgrade. It’s typically composed of open-graded or dense-graded crushed stone, depending on the pavement type.

Dense-Graded Base (Traditional Pavers)

A mix of coarse and fine aggregates (¾" minus or similar) that locks tightly when compacted.

Advantages

• Excellent load distribution

• Readily available and cost-effective

• Provides a stable, non-draining base

Limitations

• Traps water; requires positive drainage or edge outlets

Depth Guidelines (Typical):

• Pedestrian: 4–6 inches

• Light Vehicular: 8–10 inches

• Heavy Vehicular: 12 inches +

Open-Graded Base (Permeable Systems)

Composed of clean, angular stone (typically ASTM #57 and #2), allowing water to infiltrate freely through voids.

Advantages

• Integral drainage—no separate storm system needed

• Lighter weight and reduced frost heave potential

• Environmentally responsive design

Limitations

• Requires controlled gradation and higher precision

• Not suited for high-speed traffic or heavy point loads

Design Tip: Always extend the base beyond the edge restraint by a distance equal to its depth. This small detail prevents lateral creep and preserves edge integrity.

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3. Bedding Layer: The Final Adjustment Zone

The bedding layer, typically 1 inch of clean, coarse sand, allows fine-tuning of elevation and uniform support beneath each paver.

• Material: Washed, angular sand such as ASTM C33; avoid stone dust or limestone screenings, which can trap moisture.

• Depth: Uniform 1 inch after compaction—never thicker.

• Screeding: Use rigid rails or pipes to maintain consistent grade; soft or uneven bedding is a leading cause of surface failure.

For Permeable Systems: Replace bedding sand with open-graded aggregates (ASTM #8 or #89) to maintain infiltration.

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4. Compaction and Testing

Compaction is where design intent becomes performance. A properly compacted base will feel solid underfoot even before pavers are set.

• Layer Compaction: Compact in 3- to 4-inch lifts using a vibratory plate compactor or roller.

• Edge Areas: Compact parallel to borders and walls to prevent settlement gaps.

• Testing: Field density tests or proof rolling can verify stability.

After the pavers are installed, a final compaction pass with a plate compactor (equipped with a protective mat) vibrates the pavers into the bedding layer and locks sand into the joints.

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5. Drainage Design: The Hidden Lifeline

Water is the base’s greatest adversary. Designers should plan drainage as carefully as they plan grading and planting.

• Surface Slope: Maintain a minimum 2% cross slope on traditional pavements.

• Subsurface Drainage: Incorporate perforated drainpipes in low areas or behind retaining edges.

• Permeable Systems: Ensure an underdrain or infiltration layer is included where native soils have limited permeability.

Design Tip: Even permeable pavements need overflow routes—water must have somewhere to go during extreme events.

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6. Edge Restraints and Integration

Edge restraints complete the system by locking the field into the base. Whether concrete haunching, plastic edging, or metal, they must sit directly on the compacted base—not the bedding sand—and be anchored firmly before jointing material is applied. See: Holding the Line: Edge Restraints and the Art of Durable Paver Design

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Designer’s Role in Construction Oversight

Designers rarely control the shovel, but they do control the specification. Clear details—section diagrams, material gradations, and compaction requirements—translate design into buildable intent.

When possible, visit the site during base preparation. Observing compaction, moisture control, and screeding practices can prevent post-construction callbacks and costly repairs.

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Common Field Failures and How to Prevent Them

Failure	Cause	Prevention
Edge settlement	Base too narrow or uncompacted	Extend and compact base beyond restraint
Surface rutting	Inadequate compaction	Compact in lifts and test density
Joint loss	Bedding sand migration	Use angular sand and stable joints
Heaving or rocking	Poor drainage or frost action	Design positive drainage and frost depth
Differential settling	Inconsistent subgrade	Proof roll and stabilize before base placement

Each of these problems starts below the surface. A designer who understands these mechanisms can specify proactively rather than reactively.

Conclusion: Structure Is Design

A well-built base is invisible when done right—and unmistakable when it’s not. The art of paving lies as much in the layers you never see as in the ones you do.

When designers understand base construction, they design with realism. They anticipate movement, drainage, and longevity. They bridge the gap between linework and labor.

A drawing might guide the pattern, but it’s the base that gives it life.

Previous Article: Building from the Ground Up: Installation and Base Preparation for Interlocking Pavers

Next in the Series: Holding the Line: Edge Restraints and the Art of Durable Paver Design

Reference:For technical specifications, refer to the Concrete Masonry & Hardscape Association (CMHA) and its Tech Spec 2: Construction of Interlocking Concrete Pavement Bases and Tech Spec 3: Edge Restraints for Interlocking Concrete Pavements.