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Driveways on the Steep

Bed preparation is the key step in any driveway or paving assembly. If done right, installed surfaces will be stable--even on a slope.

By Erik Skindrud, regional editor, and The Brick Industry Association,

A base of properly-graded crushed stone or gravel should be installed to a minimum thickness of four inches. Use by heavier vehicles requires a correspondingly thicker aggregate base. Photo: the Interlocking Concrete Pavement Inst.

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Brick and paver installations are only as good as the ground they sit on. Time and experimentation have shown that a certain profile (of sand and aggregate) produces stable, resilient and long-lived surfaces.

Pitfalls lurk under the ground, of course. There's a well-known beach city in Southern California (which will remain nameless here) that paid millions of dollars for a plaza overlooking its waterfront and pier. Installed about 20 years ago, the interlocking concrete paver surface is now marked by embarrassing sags and buckles.

In all likelihood, the crew that installed the pavers did nothing wrong. The problem likely results from insufficient soil work. Composed mostly of sand, any structure built over a beach needs significant work by soil engineers--who will likely need to bring in tons of soil to create a compacted foundation where none existed before.

This stone paver driveway in Santa Barbara, Calif. was attached to an existing concrete base with an acrylic adhesive mortar. Set on a flexible sand base, they would soon buckle and shift and require an expensive replacement, according to Santa Barbara mason Michal Crookston. Photo: Robert Andrew Fowler

Doing the Driveway

The same rule about soil quality applies to driveways. When the driveway has a slope of 10 degrees or more, soil analysis (and possible remediation) is critical.

Assuming the ground is stable, a contractor has two general options. Driveways and hardscapes built of natural stone and thinner brick lack rigidity and need to be placed on a rigid concrete base.

Interlocking concrete pavers (ICPs) have one chief advantage over their thinner cousins. When fit together, ICPs offer a combination of flexibility and rigidity that help them stay in place over a flexible (sand or asphalt) base. That combination adds up to a quicker and more economical installation, explained Mark Ketchum of the Pavestone Company.

"What pavers do is give you a lot of joints, or surface area where the pavers fit together," the Sacramento, Calif.-based expert said. "That combination gives you the strength of concrete with the flexibility of asphalt."

Depending on their thickness, bricks provide a certain degree of rigidity too.

To qualify as an ICP, a paver needs to stand up to pressures of 8,000 psi, Ketchum said.

It's different with stone pavers, like the Golden River quartzite stone contractor Michal Crookston installed on a steep driveway in Santa Barbara, Calif. Given the stone's relative thinness and brittleness, Crookston relied on an existing concrete driveway to provide a rigid base.

Using a flexible, ICP-type base simply wouldn't work.

"The weight of our pavers would slide down the slope without the concrete," Crookston explained. "It would be similar to what a root would look like if it bulged underneath--only you would see it bulge much quicker."

Bricks or pavers set in a herringbone pattern, as seen here, resist loads in all directions. This is the preferred pattern for slopes or surfaces with heavier vehicular loads--which apply horizontal stresses as vehicles turn and accelerate. Photo: pavestone co.

Proper Bed Preparation

The following is adapted from general information provided on the Brick Industry Association's web site. Contractors should consult the manufacturers of individual paving products for specific bed-prep instructions.

Since brick and concrete are derived from the earth, it's only natural that they're used as paving material. Brick and other pavers provide an aesthetically-pleasing, stable and durable surface. Paving assemblies are comprised of the brick surface along with a base to provide support. In any paving assembly, the base is of prime importance, for if it is improperly designed or constructed, the entire system is prone to failure.

A well-compacted subgrade is essential to ensuring a stable and long-lasting driveway. Portable compacting rollers like this are available at many rental outlets. Photo: the Interlocking Concrete Pavement Inst.

Paving Assemblies

Paving assemblies are classified by the type of paving surface and the type of base supporting the surface. The paving surface receives the traffic wear, protects the base and transfers loads to the base. The base and subbase (if required) provide structural support to the paving system by distributing the load to the subgrade. A subbase consisting of graded aggregates may be required when subgrade conditions are poor.

The Laguna Beach driveway rises at its astonishing angle without grout or a rigid base due to the use of interlocking concrete pavers. Interlocking pavers provide a combination of flexibility and rigidity that bricks on a sand base cannot match. Concrete pavement edging on either side of the pavers keeps the system strong and in place. Photo by Doreen Darnell

Types of Brick Paving Surfaces

The two types of brick paving surfaces are mortarless and mortared. Mortarless brick paving contains sand between the units which are laid on a variety of materials. Conversely, mortared brick paving consists of units with mortar between the units and always laid in a mortar setting bed.

Be careful when installing concrete driveways on steep slopes that are subjected to heavy traffic and possibly large trucks such as water delivery trucks. Concrete may not be the best choice, as you can see. The damage is often extensive. Photo: I.B.Clicken

Types of Bases

A flexible base consists of compacted crushed stone, gravel or coarse sand. Only mortarless brick paving is suitable for this type of base.

A semi-rigid base, on the other hand,amazing fake consists of asphalt. Only mortarless brick paving is suitable over this type of base.

A rigid base is defined as a reinforced or unreinforced concrete slab on grade. Mortarless or mortared brick paving may be placed over this type of base.

Screeding refers to the use of a straight-edged screed board to create a flat, even bed surface. On steep slopes, crews should start at the bottom and screed up. That way, sand at the bottom holds sand above securely in place. Photo: the Interlocking Concrete Pavement Inst.

Paving Assembly Examples

Many combinations of bases, setting beds and brick paving surfaces can be used. The paving assemblies included are suggested methods based on experience for the various types of traffic uses. Although not all potential paving assemblies are shown due to space limitation, the following are the most popular configurations.

Flexible Base Pavements

Only mortarless brick paving should be laid over a flexible base. Flexible bases include crushed stone, gravel or coarse sand. Applications for flexible bases range from residential patios to city streets. Flexible paving systems are typically the most economical to install since less labor and fewer materials are involved. The thickness of each layer in a flexible pavement depends upon the imposed loads and the properties of each layer. A pavement subjected to heavy vehicular traffic requires a thicker base than a pavement subjected to pedestrian traffic.

Design Considerations

The weight and amount of traffic often dictate which paving system to use. The brick paving assembly must be capable of supporting traffic loads plus its own weight. The appropriate thickness of the subbase, base and brick paving units must be considered to adequately distribute vertical traffic loads. Three general classifications of traffic are light, medium and heavy, and should be considered when determining the subbase, base and brick paver thicknesses. The classifications of traffic are defined as follows.

Light Traffic. Residential pedestrian traffic only, such as on patios and walkways.

Medium Traffic. Commercial pedestrian traffic, such as on city sidewalks, building entrances and shopping malls. Light vehicular traffic, such as on residential driveways, commercial entranceways and parking lots.

Heavy Traffic. Heavy vehicular traffic, such as streets, crosswalks, loading docks and roads. The term heavy refers to both axle loads and frequency of loading.

Heavy vehicular traffic on grade will generally require rigid, semi-rigid, or thick flexible bases. Medium and light traffic may be supported on any of these or on suspended diaphragms.

These pavers are being set on a flexible (sand and aggregate) base by hand in a herringbone pattern. The herringbone pattern provides additional rigidity in sloped or high-traffic areas. Contractors laying bricks or pavers on a slope should start at the low point and work up. Photos: the Interlocking Concrete Pavement Inst.

Load Factors

Vehicular traffic, pedestrian traffic and the weight of the paving assembly impose vertical loads upon the paving system. These loads are distributed to each pavement layer in a radiating manner. Each layer resists a proportion of the load depending on its strength and thickness. The most important aspect of designing the pavement to resist vertical loads is determining the appropriate thickness of the base.

Inadequate base thickness will result in premature failure of the paving system, while excessive thickness will result in increased costs. For light and medium traffic applications, the minimum required thickness of each base material will normally govern. Pavements subjected to vehicular traffic, other than residential driveways, generally require design by an engineer.

Base Thickness

The minimum thickness of an aggregate base depends primarily upon the strength of the subgrade. Typically, a flexible base of properly graded crushed stone or gravel should be a minimum thickness of 4 in. (100 mm). The minimum thickness of a reinforced concrete or an asphalt base in pedestrian and light vehicular traffic applications is 4 in. (100 mm), provided it bears on adequate subgrade. Concrete and asphalt bases usually require a subbase. Very heavy loading requires an increase in thickness for all types of bases.

Paving Surface Thickness

A mortarless brick paving surface can be supported on a flexible base, a semi-rigid base, a rigid base or a suspended diaphragm. Typically the base is designed to resist vertical loads independent of the brick paving surface. However, the brick paving surface does, in fact, contribute to the load-carrying capabilities of the pavement and may be considered in design if it is of sufficient thickness and constructed properly.

The suggested minimum thicknesses for mortarless brick paving are: light traffic, 1 1/2 in. (38 mm); medium traffic, 2 1/4 in. (57 mm); and heavy traffic, 2-5/8 in. (67 mm).

Horizontal Loads. In addition to vertical loads, vehicular traffic imparts horizontal forces to the paving assembly from braking, acceleration and turning actions of the wheels. Resistance to horizontal forces is provided by the bond pattern of the brick paving assembly, the pavement edging and the bond of the brick units to the base.

Mortarless brick paving resists horizontal forces by transferring these forces through brick units and sand filled joints to rigid edging by means of an interlocking bond pattern. The greatest resistance to horizontal forces is obtained when the direction of vehicular traffic flow is perpendicular to the long joints in the bond pattern. Therefore, continuous joints in running bond and other bond patterns should be laid perpendicular to the traffic flow. The herringbone bond pattern resists loads in all directions and should be used in heavy vehicular areas.

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February 17, 2020, 1:44 pm PDT

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