NCPI Toolbox - Trench Load

Project Details
Name:
Location:
Engineer:
Prepared by:
Manhole from:
To:
Project Notes:

Soil Details
Soil Type:
Soil Weight: (85-150 lbs./ft³)

Trench Details
Pipe Size:
Pipe Strength:
(minimum 3-edge bearing strength)
Trench Type:

Constant Width and Depth
Variable Width/Constant Depth
Constant Width/Variable Depth

Trench Width: (in.)
Trench Depth:
(To the nearest ½ foot over the top of the pipe (in ft.))

Live Load Data
Applicable at cover depths of 8 feet or less
Live Load Enable Live Load Calculations

Wheel Load: lbs.
(10,000-60,000 lbs.)

Impact Factor:

Standard	Axle Load	Wheel Load
AASHTO H-15 and HS-15	24,000 lbs.	12,000 lbs.
AASHTO H-20 and HS-20	32,000 lbs.	16,000 lbs.
AASHTO H-25 and HS-25	40,000 lbs.	20,000 lbs.

Load Factors
For each bedding class shown in ASTM C 12, a load factor is provided. This factor is the amount of structural enhancement the bedding will give as a multiplier of the specified 3-edge bearing strength of the pipe. The bearing strength is multiplied by the appropriate load factor to give the load bearing capacity of the installed pipe. (The load bearing capacity is also referred to as field supporting strength).

Example:

Class C Bedding = Load Factor of 1.5

8" Extra Strength VCP = 2200 Bearing Strength (the minimum per ASTM C700)

Load Factor x Minimum Pipe Bearing Strength = Field Supporting Strength

1.5 X 2200 = 3300 Load Bearing Capacity

Safety Factor
The safety factor is the result of dividing the load bearing capacity of the installed pipe by the calculated backfill load on the pipe. A design safety factor within the range of 1.0 to 1.5 is desirable.

Example:

3300 Load Bearing Capacity (from Load Factor example)

2540 Trench Load (a function of soil type, soil weight, trench depth, and width)

Field Supporting Strength / Backfill Load = Safety Factor

3300 / 2540 = 1.3 Safety Factor