PIPE LOADING

Formulas and charts for determining loads on an underground pipe are determined from theories developed by A. Marston, Iowa State University. Earth loads and live or transmitted loads must be considered when designing underground conduits. Data on live and dead loads can be obtained from many different handbooks available as well as from pipe manufacturer's guidelines.

Every condition of bury or loading does not have to be checked for failure. In most cases, pipe loading does not need to be checked. Most underground pipe design regulations require that pipe be designed for normal depths of bury with normally expected live loads. Certain conditions should be checked for pipe loading conditions. These conditions can be the following:

Depths of bury exceeding 10 feet Abnormal soil conditions

Unusually high live or transmitted loads Live loads for depths or buy less than 3 feet Earth Loads

The amount of earth loads that is transmitted to the pipe is dependent on many factors. The primary factors that determine earth loading are:

Depth of cover

Width of trench at top of pipe Rigid or flexible pipe

Type of construction (trench or embankment) Soil density and cohesion characteristics

Formulas and charts developed by A. Marston provide a means to closely calculate the earth loading for the variable factors listed above. This data can be found in many available handbooks. AWWA (American Water Works Association) C-101, is a good example, as well as many catalogs and handbooks published by pipe manufacturers. Full descriptions of the various construction conditions are also given.

In order to simplify the determination of earth loading, Attachment 03 may be used for approximate values. Approximate values from the table are satisfactory for the following reasons:

Depths of cover are usually less than 8 feet.

Practice 670 210 1210 Publication Date 20Sep95 Page 3 of 8 FLUOR DANIEL

LOADS ON UNDERGROUND PIPE

Civil Engineering

This copy is intended for use solely with Piping Design Layout Training.

For other purposes, refer to the original document available through Knowledge Online.

Earth load values for a depth of 8 feet are well below the 3 edge bearing strength of vitrified clay pipe or reinforced concrete pipe for any properly installed system.

Unless the width of the trench is specified and controlled during construction, calculating earth loads from Marston's formula would be impractical.

The unit weight of soil used in Attachment 03 is 120 pounds per cubic foot.

Pipes with 12 inch diameter and less are assumed to have a trench width 1 foot wider than the outside diameter of the pipe. Pipes from 12 inches to 36 inches are assumed to have a trench width 2 feet wider than the outside diameter of the pipe.

The table in Attachment 03 is intended to be used as a guide in determining earth loads for underground conduits with normal bury conditions. For unusual bury conditions such as large diameter pipes or deep pipes, the designer should consult the pipe manufacturer's catalog for design criteria.

Live Loads

It is usually not necessary to consider live loads except where they are exceedingly large or where they occur on conduits with very little cover. A few computations under various conditions will establish the relative importance of live loads in the designers mind.

Trucks or construction equipment moving over the ground surface above underground piping subject the piping to loads. A certain percentage of the total load, based on depth of cover and size of pipe, is transmitted to the pipe. If paving is involved, flexible pavement will transfer more load to the pipe; whereas, rigid pavement such as concrete will tend to bridge the pipe transmitting more load to the surrounding soil. For calculating transmitted loads, use the guidelines that follow and the Table in Attachment 04 which gives the percent of live load that is transmitted to the pipe for various depths of cover.

For piping under roads, depth of cover should be based on rough grade elevations for the road, since underground lines will be subject to truck traffic before any asphalt surface is applied.

For design purposes, use a wheel load of 32,000 pounds (1/2 axle load of 64,000 pounds). The wheel load may be on dual tires but is still considered 1 wheel. This load is the heaviest that would be expected from a large unladen truck crane. Heavier loads could be possible during equipment handling or lifting activities and this point should be reviewed with Construction Management. Generally, the pipe is protected with timber mats or omitted entirely during these operations.

The wheel load of 32,000 pounds recommended above is twice that of H-20 truck loading which is used as a basis for bridge and highway design.

Where loads are known to be less or greater, the calculations should be based on the actual figures. The minimum wheel load used for design purposes is 16,000 pounds which is normal H-20 loading.

When expecting heavy 1 time construction or equipment loads, the conduit could be installed after the loading has been imposed.

PIPE BEDDING

The pipe bedding determines the load factor or number to multiply the 3 edge bearing strength to determine the field supporting strength. The bedding is the contact between the

Practice 670 210 1210 Publication Date 20Sep95 Page 4 of 8 FLUOR DANIEL

LOADS ON UNDERGROUND PIPE

Civil Engineering

This copy is intended for use solely with Piping Design Layout Training.

For other purposes, refer to the original document available through Knowledge Online.

pipe and the foundation on which it rests. The soil on the sides of the pipe and above it is the backfill. The field supporting strength of a rigid pipe and, therefore, the load factor for a particular conduit, depend chiefly upon 2 characteristics of the installation as follows:

Width of the bedding of the pipe and the quality of the contact between the pipe and bedding as it affects the distribution of the vertical forces.

Magnitude of the lateral pressure acting against the sides of the pipe and the area of the pipe over which the lateral pressure acts.

Cohesion for trench conduits is assumed to be negligible because of the following:

Considerable time must elapse before effective cohesion between the backfill material and the sides of the trench can develop.

The assumption of no cohesion yields the maximum probable load on the conduit.

Pipe Bedding Classes

Pipe bedding generally falls into 4 classes. These 4 classes are described below.

Class A Load Factor 3.4

This method of bedding involves either a reinforced 2,000 psi concrete cradle or arch. The concrete will extend to the springline, which is halfway up the side of the pipe. The cross sectional area ratio of steel to concrete should be 0.4 percent. If no reinforcing is used then the load factor will be reduced to 2.4. If p = 1.0 percent for concrete arches, then the load factor can be increased to 4.8.

Class B Load Factor 1.9

This method of bedding involves well graded crushed stone carefully placed and shaped to the bottom of the pipe with a minimum thickness below the pipe of 4 inches. The bedding will extend up the haunches to the springline of the pipe with select material as initial backfill.

Class C Load Factor 1.5

This method of bedding involves carefully placed and compacted material with a wide range of gradation and possibly locally obtained. The bedding generally extends from 4 inches below the pipe up to 1/6 of the OD of the pipe. Select backfill will be used as initial backfill.

Class D Load Factor 1.1

This method of bedding involves little or no care when shaping the foundation surface to fit the lower part of the conduit exterior or to fill all spaces under and around the conduit with granular materials. Initial backfill will be of select material.

It can be seen by the bedding classes described above that the better quality bedding provides a higher load factor and, therefore, more load carrying capability.

The economy of the different types of bedding along with the potential live and dead loads must be taken into account when specifying bedding requirements.

In order to simplify the selection of an appropriate load factor, use a conservative value of 1.5 which is based on ordinary bedding. In special and unusual situations where a line is run excessively deep (over 10 feet), or for other reasons, it becomes necessary to specify a

Practice 670 210 1210 Publication Date 20Sep95 Page 5 of 8 FLUOR DANIEL

LOADS ON UNDERGROUND PIPE

Civil Engineering

This copy is intended for use solely with Piping Design Layout Training.

For other purposes, refer to the original document available through Knowledge Online.

particular bedding condition, then the appropriate design handbook listed in Attachment 05 should be consulted.