Clay Pipe Installation Handbook

Chapter 3, Trench Excavation

Line and Grade

In most cases, the engineer establishes line and grade and places grade stakes and/or other reference markers to show station, depth and trench cut. The contractor works from these markers to dig the trench at the specified location and depth.

Common methods for establishing alignment of pipe and grade of trench bottom, bedding materials and pipe invert are the laser and gradeliner. The laser device sends a straight beam of laser light from a unit which has been positioned for proper line and grade to a target with predetermined reference marks. (Figure 1)

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The "gradeliner" is a device that transfers the line and grade data from the survey markers to the location of the invert of the pipeline. The survey marker oflset (line) and the pipe invert elevation (grade) may be found on the plans, located at the survey marker or obtained from the survey data sheets. (Figure 2)

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Trench Width

The trench width at the top of the pipe should be held to a minimum and must not exceed the specified width as shown on the design drawings. Even a small increase in trench width causes a large increase in loading. For example, if a 2'0" trench width is increased only 6", the load increases by about 50%. (Figure 3)

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In deep cuts, a narrow step-trench or sub-trench may be excavated after a wider trench is used above the level of the top ot the pipe. A vee or modified vee trench may also be used. In all cases, the specified trench width at the top of the pipe must not be exceeded without approval. (Figure 4)

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Sheeting, Shoring and Movable Trench Boxes

Vertically cut trench walls must be supported by proper sheeting, shoring or suitably sized movable trench boxes. (Figure 5)

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Note: If a movable shield is used, it is imperative that pipe already laid be securely blocked or braced to prevent opening of pipe joints when the shield is advanced in the trench. ncpi006.GIF (18696 bytes)

Where solid timber sheeting is driven, it should not be removed after the pipe has been installed as this will disturb the side support conditions. However, the sheeting may be cut off at the level of the top of the pipe without damaging the line.

Trench Safety

Current Federal (OSHA), State and local regulations for minimum safety standards must be observed. (Figure 6)

Dewatering

Water must be removed from the trench before final grading of the bedding foundation. The trench must be kept dry during all phases of pipe installation.

The ground water table can be lowered with well points wherever soil conditions permit. They should be located at intervals dictated by soil properties and placed reasonably close to the trench walls. They should be sunk to a depth below the elevation of the trench bottom. Several well points can be joined together so that one pump can handle a group of points. (Figure 7)

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Migration of fine materials in or out of the pipe zone can result in loss of pipe support and must be prevented. This can be accomplished through the use of waterstops or geofabrics. (Figure 18).

NCPI - Clay Pipe Installation Handbook

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