Loading Docks And Heavy Vehicle Slabs

A car park slab and a loading dock slab look the same and are not.

A truck applies a wheel load many times higher than a car, concentrated on a smaller contact area, and it applies it repeatedly at the same points.

What changes with heavy vehicles

Wheel load and axle configuration. The design case is not the vehicle weight, it is the load on the worst axle group and the spacing between wheels. A semi-trailer's rear axle group is the governing case on most commercial sites.

Repetition. A slab under repeated heavy loading fails in fatigue, not in a single overload. Twenty deliveries a day for fifteen years is the design condition.

Turning and braking. A truck manoeuvring applies horizontal forces into the surface as well as vertical ones, and those forces tear at the surface and at joints. The tightest turning point on a site is where the pavement fails first.

Point loads from trailer legs and jacks, which are extremely concentrated and frequently overlooked entirely.

Impact at the dock face, where a reversing trailer strikes the building.

Rigid or flexible

Two pavement families, and the choice is usually clear.

Rigid pavement, which is a reinforced or plain concrete slab. It spreads load widely, tolerates point loads, resists fuel and oil spill, and lasts a long time. Higher initial cost.

Flexible pavement, asphalt over a granular base. Cheaper to build, quicker, and easier to resurface. It deforms under sustained point loads and it is damaged by turning trucks and by fuel.

For a loading dock, a trailer parking area or anywhere trailer legs sit, rigid is the answer. For access roads and lightly trafficked yards, flexible frequently makes sense. Many sites use both, with concrete at the dock and asphalt on the approach.

The subgrade decides everything

A pavement is a structure sitting on soil, and the soil does more of the work than the concrete.

Which means the design depends on the subgrade strength measured on site, and on that strength being achieved in construction. Testing during earthworks is not optional, and a pavement designed for a stronger subgrade than was delivered will fail regardless of the slab above it.

Where the subgrade is poor, the answer is either a thicker pavement, a stabilised subbase, or [ground improvement](/blog/ground-improvement-techniques-explained). The first is usually the most expensive of the three.

Drainage matters as much as strength. Water in the subgrade softens it, and a pavement that ponds is a pavement with a short life. Falls, pit positions and the connection to the [stormwater system](/blog/stormwater-management-urban-developments) are part of the pavement design rather than separate from it.

Joints are where it fails

Almost every concrete pavement failure starts at a joint.

Joint spacing has to suit the slab thickness and the restraint, following the same logic as any [control joint](/blog/expansion-and-control-joints-in-concrete).

Load transfer across the joint, using dowels, so a wheel crossing the joint does not deflect one slab relative to the other. Without it, the edges spall and the joint progressively widens.

Sealing, to keep water and grit out.

Armoured joints at the heaviest trafficked lines, where the arris is protected with steel.

A pavement with correctly designed joints and adequate thickness lasts decades. The same pavement with joints at the wrong spacing and no load transfer fails within a few years at every joint.

The dock itself

The dock face and the leveller pit, which take impact and need to be detailed for it.

Bumpers, which are cheap and prevent expensive damage to the building.

The slab in front of the dock, which takes the worst of the turning and braking and frequently warrants a thicker section than the rest of the yard.

Falls away from the building, so water does not run into the dock or the building.

Getting the vehicle right

The whole design rests on which vehicle actually comes to the site, and it is worth establishing rather than assuming.

The swept path of the largest expected vehicle determines the geometry, and the geometry determines where the heavy turning happens. Both belong in the civil design, and getting them from the operator rather than from a standard drawing is what prevents a pavement designed for the wrong truck.

ACSES provides civil engineering and pavement design for commercial and industrial sites across Sydney. Get in touch.