7. Pavement Construction

7.1 Gravel Pit Operation

7.1.1 Stripping

The operation of a gravel pit begins with the removal of unusable material from over the acceptable gravel deposit. Stripping involves removal of vegetation (trees, shrubs, grass, etc.) and topsoil. Any inferior overlying material must also be removed to expose the surface of the usable gravel layer.

7.1.2 Winning

The process of extracting the gravel is referred to as winning the material. The method which is used to win the gravel depends on four factors:
• the depth of the deposit;
• whether different layers have to be mixed;
• the method of loading to be used; and
• the equipment available for winning.
If the deposit is soft and no mixing is required, winning and loading may be carried out as a single operation using a loader or shovel. Otherwise winning is carried out as a separate operation to loading.

Relatively Flat Deposits

If the pit is shallow the material can be won by a grader or dozer pushing the material up into windrows, and loading is then carried out directly from the windrows by front end loader. Deep deposits not requiring mixing should be won by a dozer pushing the material up into stockpiles. Deep deposits requiring mixing (i.e. the deposit consisting of two or more layers, where the various layer materials must be mixed together in certain proportions to obtain an acceptable product) can be worked in several ways, including spreading each material on a flat area (one on top of another, in the correct proportions) and mixing by windrowing the material from side to side with a grader or dozer.

Steep Deposits

Steep deposits should always be ripped and won in the downhill direction, using gravity to assist the equipment in its operation.

Hard Deposits

Hard deposits will require loosening of the material with drilling and blasting.

7.1.3 Loading

Loading can be carried out with shovels or excavators but probably the most common method is using a wheeled front end loader. Crawler track loaders are also appropriate in some circumstances.

7.1.4 Hauling

The usual method of hauling is by tip trucks.

7.2 Gravel Pavement Construction

7.2.1 Tipping and Spreading Materials

Wherever possible the materials should be spread in layers as they are tipped, rather than being tipped in heaps and then spread.

When two materials are to be mixed it is best to spread one material directly on top of the other. Tipping the fine material first can be an advantage as it can generally be sealed off if wet weather threatens, and it provides a far better surface for even tipping of the subsequent material.

7.2.2 Mixing

The action of a grader in rolling material around the curvature and along the blade is a fairly efficient mixing operation provided both mnaterials have been accurately spread. The most important aspects are to get the moisture evenly distributed and avoid segregation.

With the coarse material spread evenly over the fine, the grader commences mixing by taking a full depth bite with the blade set for mixing and at a slight angle to the subgrade. This should allow both materials to rise and flow along the blade and spil under it before reaching the end of the blade. The water truck follows the grader through and evenly wets this thin layer. The grader on its return trip has its blade full down mixing the material and water as the material flows around and along the blade, to be deposited in a windrow outside the end of the blade. This procedure is repeated until all material is shifted across the road.

Generally speaking the materials require a minimum of three passes to achieve a homogeneous mix with even moisture content.

7.2.3 Incorporation of Water

It is essential that suitable moisture for compaction be evenly distributed through the mix. The main reason any job has difficulty in obtaining the required compaction standard is insufficient attention to moisture content and distribution.

7.2.4 Spreading and Rough Shaping

Following thorough mixing of the material it is ready to be spread and shaped. The spreading is done with a grade. The greatest problem in constructing a pavement with a grader is segregation of the mix. If segregation has occurred it will be evident at the pavement edge.

During rough spreading the material must receive minimum rolling - just sufficient to carry the weight of the grader. Rollers must be kept away until final shaping has been completed or compaction planes will result.

7.2.5 Final Shaping

Many roads are built that may agree with the plan levels and specification shape requirment at the pegged cross sections, but the shape and levels in between are purely dependent on how much material was tipped on that particular section in the first place. A set of boning rods (a boning rod consists of two pieces of wood in a T shape about 1m high) in constant use with the final trim grader is the best method of constructing a road to the required shape. The amount of 'out of shape' between pegs which is not visible to the eye but which can be picked up and corrected, is surprising.

The grader blade is set by working a few metres longitudinally, and boning across the pavement between a centre line and edge peg, until the correct crossfall is obtained. The blade is set on this crossfall and this becomes the starting point for the final shaping of this section of pavement. The grader now works along the edge pegs on one side of the pavement, with longitudinal boning. The resulting rill of material is carried in about three passes up to the centre line butt pegs. The centre line is now trimmed, boning longitudinally. The trimming process is then repeated with additional cross and diagonal boning. The same procedure is adopted on the other side working from centre line to outer edge.

It is essential that all compaction equipment be kept away from the section until final shaping is completed.

7.2.6 Compaction

The best all round compaction unit for normal pavement construction is probably a self propelled static three wheel steel roller. Rollers should work from the edges to the centre line, overlapping a half roll at a time.

7.2.7 Pavement Finishing

The art of presenting a finished surface is an independent thing for each pavement material. Generally after about 5 or 6 passes of the roller an attempt to dress the pavement should be made. This is done by shaving a thin layer from the surface using a grader and cutting material to waste.

7.3 Quality Control During Pavement Construction

• ensuring the materials used are to specification (achieved by sampling and testing material samples);
• ensuring materials are adequately mixed (achieved by sampling and testing mixed materials);
• ensuring adequate moisture for compaction in the materials (achieved by field moisture testing);
• ensuring adequate compaction occurs (achieved by field density testing, usually using the sand replacement method or nuclear gauges); and
• ensuring line and levels are correct (achieved by checking using surveying).