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Buoyancy control may be achieved in three ways: (1) by a mechanical anchoring system, (2) by backfill, and (3) by a density anchoring system.

Mechanical Anchors

Mechanical anchors (Figure 7-33) are designed to maintain a minimum holddown force on the pipeline by utilizing the shear strength of the underlying soil. They consist of individual or pairs of anchors attached to the pipeline by holddown clamps or straps. A mechanical

anchor involves the use of a steel anchor rod with an auger at the lower tip. They are not commonly utilized for large-diameter pipelines.

Backfill

Backfill, using either native or borrowed select material for buoyancy control, relies on the mass of the backfill over the pipe to counteract the buoyancy forces. Native backfill may be considered if it consists of stable, ice-free soil that is capable of achieving a reasonable level of strength. If the native backfill is not adequate, select backfill can be used. Select backfill should be of coarse-grained, free-draining materials exhibiting sufficient shear strength when thawed or mixed with water. Although gravel is undoubtedly the best material, other materials such as a mixture of gravel, sand, clay, and silt can be used.

The stability of the ditch wall is another important factor to consider in this method of buoyancy control. Unstable ditch walls will not provide sufficient support for the select backfill to keep it over the pipe. This method should not be used if the ditch contains water during backfilling. The ditch must be dewatered prior to and during the placement of any backfill. The backfill can then be placed in a controlled manner.

This method can be used when a ditch is dry during construction but may be subjected to buoyant forces after backfilling is complete. Subsequent buoyant forces could be due to thawing, flooding, or a rising water table. Select backfill can then be used in place of weights in this situation. It can also be used in relatively impervious soils where the ditch excavation has penetrated below the water table.

Density Anchors

Density anchors are a system of weights added to the pipeline. The types of anchors are usually in the form of concrete and include swamp weights (saddle or set-on), river weights (bolt-on), and continuous concrete coating.

Figure 7-34. Mechanical anchor

Swamp weights (Figure 7-35) are shaped like an inverted ‘‘U’’ and are placed over the pipeline after the pipe is in the ditch. They are constructed by pouring concrete into prefabricated molds or forms and are reinforced with steel reinforcing bars or wire mesh.

They can be manufactured at central batch sites or at a specific field site. The choice of location for manufacture depends primarily on the availability and proximity of materials and on transportation costs. Rockshield or felt lining is attached to the inside of the weight to prevent damage to the pipe coating. Lifting hooks are built into each weight for handling.

Swamp weights are the most economical density anchor for pipelines primarily because they are the easiest to handle and install. They are usually used for small stream crossings, drainage courses, river floodplains, muskeg (bogs), and swamps.

River weights (Figure 7-36) are constructed in two halves and are designed to be clamped to the pipe. High tensile corrosion-resistant steel bolts are used to fasten the two halves together. The weights are manufactured by pouring concrete into prefabricated molds or forms and are reinforced with steel wire mesh/reinforcing bars. River weights can be manufactured at central batch sites or at a specific field site. The choice of location depends primarily on the availability and proximity of materials and transportation costs.

Lifting hooks are built into each weight for handling. As with swamp weights, the inside corners of the weight are chamfered and the inside of the weight is lined with rockshield or felt to prevent damage to the pipe coating. The ends are chamfered to prevent snagging in cases where the pipeline has to be pulled into place.

Wood lagging and slats, strapped to the pipe along its length, are normally used between the river weights to prevent movement of the weights, thereby maintaining the required spacing and providing mechanical protection for the pipeline. If used, they should be taken into consideration in the spacing calculations because of their buoyant effect. River weights are used primarily at minor river and creek crossings (double-sag crossings) where the construction technique consists of welding up the pipeline in a staging area, installing the weights, and then walking the pipeline into place. For large-diameter pipelines, river weights may be used in muskeg or swamp areas, or any other

Figure 7-35. Swamp weight

areas where swamp weights may have been proposed but cannot be used because the ditch water cannot be evacuated.

Continuous concrete coating (Figure 7-37) consists of a coating that completely surrounds the pipe. It provides excellent mechanical protection for the pipeline, particularly in fast-flowing water or in rocky areas. It is reinforced with steel wire mesh or reinforcing bars.

The concrete is usually applied by forming and pouring. The coating may be applied at a central batching plant or at a specific field site. The choice of location depends primarily on the availability of materials, transportation costs and portability of manufacturing equipment.

Some of the other methods used to apply the concrete to the pipe include guniting, where the concrete is manually sprayed onto the pipe; impingement, where the concrete is sprayed onto the pipe as it is rotated past; and extrusion, where the concrete is applied to the pipe as the pipe passes through a machine. At the ends, where the joints are to be welded, the pipe is left bare for some distance, with reinforcement protruding from the concrete.

After the sections are welded together, the reinforcement is extended across the bare areas and concrete is applied.

Concrete coating is used for crossing the main active channels of major rivers where the pipeline installation will be performed by pulling. It is also used at large lakes and possibly in large floodplains, swamps, or bogs where the ditch is expected to be filled with water and cannot be dewatered.