8. Drainage Considerations
8.7 Pipe Loading
8.7.1.1 Formulas and charts for determining loads on an underground pipe have been determined from theories developed by Anson Marston, Iowa State University.
8.7.1.2 For designing underground culverts, earth loads and live or transmitted loads should be considered. Data on live and dead loads can be obtained from many different handbooks available and from pipe manufacturer’s guidelines.
8.7.1.3 Every condition of bury or loading does not have to be considered for design. Typically, pipe loading should not need to be checked.
Underground pipe design regulations typically require that pipe be designed for typical depths of bury with typically expected live loads.
8.7.1.4 The following conditions should be checked for pipe loading:
a. Depths of bury greater than 10 feet b. Abnormal soil conditions
c. Unusually high live or transmitted loads d. Live loads for depths of bury less than 3 feet 8.7.2 Types of Bury
8.7.2.1 Types of bury are classified as that in cut (e.g., trench condition) or that in fill (e.g., embankment condition).
8.7.2.2 Combinations of both trench and embankment conditions can exist.
To simplify and provide conservatism, this guideline is limited to trench conditions and embankment conditions. Combination conditions are considered embankment conditions.
8.7.3 Earth (Dead) Loads
8.7.3.1 The amount of earth load that is transmitted to an underground pipe is dependent on many factors including the following:
a. Depth of cover
b. Width of trench at top of pipe
c. Rigid or flexible pipe
d. Trench or embankment construction condition e. Soil density and cohesion characteristics
8.7.3.2 Formulas and charts developed by Anson Marston provide a means to closely calculate the earth loading for the variable factors listed in Section 8.7.3.1. This data can be found in many available handbooks.
Full descriptions of the various construction conditions are also given in the handbooks.
8.7.3.3 To simplify the determination of earth loading, Table 11 may be used for approximate values. For the following reasons, values from the table should be satisfactory for design:
a. Depths of cover are typically less than 8 feet.
b. Earth load values for a depth of 8 feet should be well below the 3-edge bearing strength of vitrified clay pipe or reinforced concrete pipe for any properly installed system.
c. Unless the width of the trench is specified and controlled during construction, calculating earth loads from Anson Marston’s formulas is impractical.
d. The unit weight of soil used in Table 11 is 120 pcf.
e. Pipes 12 inches in diameter and less are assumed to have a trench width 1 foot greater than the outside diameter of the pipe. Pipes greater than 12 inches to 36 inches are assumed to have a trench width 2 feet greater than the outside diameter of the pipe.
Table 11 – Dead Load from Earth Cover on Underground Pipe Loads in Pounds per Linear Foot of Pipe
Depth of Cover, Feet
Nominal Pipe Diameter, Inches
4 6 8 10 12 15 18 21 24 27 30 33 36 2 180 240 290 340 390 450 500 560 610 700 750 820 875 3 270 370 460 550 630 750 860 950 1,040 1,120 1,200 1,300 1,400 4 370 520 650 780 920 1,080 1,230 1,400 1,520 1,630 1,750 1,850 2,000 5 470 660 830 1,000 1,160 1,420 1,610 1,810 2,010 2,200 2,340 2,500 2,630 6 570 800 1,000 1,200 1,430 1,710 2,000 2,230 2,500 2,700 2,950 3,180 3,350 7 670 950 1,130 1,420 1,700 2,050 2,400 2,700 3,050 3,300 3,570 3,900 4,100 8 780 1,080 1,370 1,620 1,960 2,400 2,780 3,200 3,550 3,900 4,200 4,500 4,800
8.7.3.4 Table 11 is intended to be used as a guide in determining earth loads for underground conduits with typical bury conditions. For unusual bury conditions (e.g., large diameter pipes or deep pipes), the pipe
manufacturer’s catalog should be consulted for design criteria.
8.7.4 Live Loads
8.7.4.1 Typically live loads do not need to be considered unless the loads are very large or occur on culverts with very little cover. A few calculations under various conditions should establish the relative importance of live loads.
8.7.4.2 Based on depth of cover and size of pipe, trucks or construction equipment moving over the ground surface above underground piping can transmit a percentage of the total load to the pipe. If the area above the piping is paved with flexible pavement, more load can be transmitted to the pipe; whereas, with rigid pavement (e.g., concrete) the load can bridge the pipe transmitting more load to the surrounding soil.
8.7.4.3 Table 12 shows the percent of live load that can be transmitted to the pipe for various depths of cover. For determining transmitted loads, Table 12 should be used with the following guidelines:
a. For piping under a roadway, depth of cover should be based on rough grade elevations for the roadway because underground piping can be subject to truck traffic before any paving is applied.
b. The minimum wheel load for design purposes is 16,000 pounds which is normal H-20 loading. If greater loads are expected, the calculations should be based on actual loads. The heaviest wheel load that would be expected from a large unladen truck crane is 32,000 pounds (1/2 axle load of 64,000 pounds). The wheel load may be on dual tires but is still considered one wheel. 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.
Table 12 – Percentage of Wheel Loads Transmitted to Underground Pipe Note: Each value is a percentage of wheel loads to apply to one linear foot of pipe.
Depth of Cover, Feet
Nominal Pipe Diameter, Inches
4 6 8 10 12 15 18 21 24 27 30 33 36 1 9.3 12.8 15 17.3 20 22.6 24.8 26.4 27.2 28 28.6 29 29.4 2 4.3 5.7 7 8.3 9.6 11.5 13.2 15 15.6 16.8 17.8 18.7 19.5 3 2 2.9 3.6 4.3 5.2 6.4 7.5 8.6 9.3 10.2 11.1 11.8 12.5 4 1.2 1.7 2.1 2.5 3.1 3.9 4.6 5.3 5.8 6.5 7.2 7.9 8.5 5 0.7 1.2 1.4 1.7 2.1 2.6 3.1 3.6 3.9 4.4 4.9 5.3 5.8 6 0.5 0.8 1 1.1 1.4 1.8 2.1 2.5 2.8 3.1 3.5 3.8 4.2 7 0.2 0.5 0.7 0.8 1 1.3 1.6 1.3 2.1 2.3 2.6 2.9 3.2 8 0.1 0.4 0.5 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.3
8.7.4.4 For flexible pavement, the live load transferred to below grade varies in function of the depth as shown in Figure 3. Cranes are designed so that the live load applied to the soil is 2,000 psf maximum. Live load from
shown in Figure 3. The load is based on a locomotive axial load of 80,000 lbs with 4 axles spaced at 5 feet.
Live Load Transferred to Buried Structures at Different Depths
0 2,000 4,000 6,000 8,000 10,000 12,000 14,000
0.0 2.0 4.0 6.0 8.0 10.0 12.0
Depth, ft
Load on the Structure, psf
AASHTO H-20 Cooper's E-80
Figure 3 – Live Load Transfer Below Flexible Pavements
8.7.4.5 For rigid pavement, the live load transferred to below grade is negligible at any depth.
8.7.5 Pipe Bedding
8.7.5.1 Pipe bedding is the contact area between a pipe and the foundation on which the pipe rests. The soil on the sides of and above a pipe is the backfill.
8.7.5.2 The 3-edge bearing strength for a pipe is multiplied by the load factor or number determined from the pipe bedding to calculate the field
supporting strength.
8.7.5.3 The field supporting strength of a rigid pipe and, therefore, the load factor for a particular culvert, depend on the following installation characteristics:
a. Width of pipe bedding and quality of the contact between the pipe and bedding as the contact affects the distribution of the vertical forces
b. Magnitude of lateral pressure acting against the sides of a pipe and the area of the pipe over which the lateral pressure acts
8.7.5.4 Cohesion between backfill material and the sides of a trench for pipe bedding in trenches should be assumed to be negligible because of the following:
a. Considerable time is required for effective cohesion.
b. Assuming that cohesion is negligible provides the maximum probable load on the pipe.
8.7.6 Factor of Safety
8.7.6.1 A factor of safety should be applied to the pipe loading designs to
account for unforeseen stresses which can be imposed on an underground pipe. A factor of safety cannot be computed by laws and equations, but depends entirely upon engineering judgment and experience.
8.7.6.2 Typically, the factor of safety should range from 1.0 to 1.5 depending on a variety of conditions or situations.
8.7.6.3 The following factors should be considered when choosing a factor of safety for underground pipe designs:
a. Culvert (e.g., non-pressure pipe) typically fails gradually. Pipe under pressure typically fails quickly once a crack develops.
b. Pipe bedding and backfill variable factors
c. Typically, rigid pipes should be designed with a higher factor of safety than flexible pipes because flexible pipes can deflect more before failure.