By Erik Early, E.I.
| Monday, January 23, 2017 | 2108 Views |
WHICH VEHICLE IS OF GREATER CONCERN TO THE INSTALLED PIPE?
A highway truck weighing 72,000 lbs or a heavy-duty modular trailer weighing 600,000 lbs? If you guessed the highway truck, then you are correct. In this particular case, the reason is the increased number of axles and the contact area that the load is distributed upon. Due to the 600,000 lb vehicle having more axles and tires, the total weight will be divided into more contact areas, thus resulting in smaller pressures. This is why it is not always intuitive which vehicles could compromise the pipe.
One needs to pay special attention to heavy construction equipment as it can impose concentrated live loads that are far greater than the pipe is designed to carry. There are many factors that must be taken into consideration in regards to construction loading, but the most important aspect is the backfill protecting the pipe. Compacted, competent backfill is essential for both rigid and flexible pipe to provide the needed structural capacity for the pipe.
Another point of consideration would be the vehicle properties which must be known in order to determine structural capacity of the pipe, such as:
- Weight of the vehicle
- Number of axles
- Axle spacing
- Contact area of the tires or treads on the ground surface
These parameters allow the designer to see how the pressure is distributed through the backfill material and applied to the pipe. The objective is to avoid large pressures caused by smaller contact areas or by shallow covers. When the height of cover is increased, the construction load pressure will be dissipated over a larger area, allowing the pipe to feel less load. In the case of crane loading, the crane specifications will be needed along with the maximum pressure at maximum boom, and with the maximum load.
Assuming that a pipe is able to handle a construction load is a bad idea. Always ask Contech, or your engineer, if it is suitable to handle the load. Assuming so can lead to this……
Some of the situations or applications where construction loads pose the biggest challenges are:
- Tight urban corridors
- Crane loading
- Aircraft loading
- Detention systems (multiple barrels, fittings, headers)
So, what measures can be taken to prevent failure? The easy solution would be to reroute construction traffic when possible. However, the most practical solution is to add temporary cover over the pipe. The following image from AASHTO Table 26.6 is a helpful tool to show temporary minimum covers based on typical axle loads of construction vehicles. There are other pressure mitigation methods, such as increasing the gage of the pipe, creating a larger footprint for outriggers on a crane, or using flowable fill in situations where there are pipe-arches or multiple barrels.
It should be noted that dumping soil backfill over top of the pipe is not sufficient to provide the necessary structural component to carry the live load. Even though it is temporary, the material still must be compacted, competent material. Too much emphasis cannot be placed on the necessity of adequate compaction of backfill. Faulty compaction has led to more trouble with pipe installations than all other factors combined.