The significance of accurately identifying the causes of accidents, particularly using thorough analysis techniques like fault tree analysis, cannot be overstated. At Alpine Engineering & Design, we pride ourselves on our extensive experience and skill in serving as expert witnesses across various domains, including mechanical engineering, intellectual property, premises liability, product liability, risk management, and safety. Our track record spans decades, during which we have handled numerous cases. One notable project involved a belly dump trailer, a piece of equipment critical for transporting and discharging loose materials like sand or gravel.
A belly dump trailer, also known as a bottom dump trailer, features a funnel-shaped hopper with a clamshell gate at the bottom. This design facilitates the easy discharge of materials, simply open the clamshell gate and gravity causes the material to flow out of the hopper. No need to lift or tip the hopper like you might on an end dump. However, despite being equipped with several safety mechanisms intended to prevent accidental discharge during transit, these features can sometimes fail, leading to catastrophic outcomes.
In the immediate incident, a semi-truck was towing two belly dump trailers on a highway when the clamshell doors of the rear trailer suddenly opened, spilling the gravel it was carrying onto the road. The plaintiff encountered this spilled gravel shortly thereafter, resulting in a crash into the center divider and causing severe injuries.
Our engineers have many analytical tools to investigate mechanical failures. In this case, our engineers conducted a fault tree analysis (see Figure 1). In a fault tree analysis, one failure mode is chosen, in this case, that the belly dump doors open along the road. Then, all possible causes are listed and linked logically to the failure mode. The causes secondary to the primary causes are likewise identified, and so on, until all root causes have been identified. Then, the likelihood of each of these causes can be estimated, making it easier to assess the probability of any chain of events leading to the failure mode. For this reason, fault tree analysis is helpful in engineering investigations where the root cause of an accident is not obvious.
Figure 1. The Fault Tree analysis in the belly dump case. Orange shapes indicate a higher level cause or failure mode, yellow rectangles represent root causes, and red ellipses indicate operator actions. The blue rectangles describe root causes ruled out because of unlikelihood.
Our fault tree analysis for this case revealed two primary possibilities: a structural failure or an unintentional actuation of the valve. The possibility of a leak in the valve (not shown in Figure 1) was ruled out because witness reports indicated the entire load was discharged within ten to fifteen seconds, a scenario inconsistent with a steady leak and the design of the doors. Furthermore, a thorough engineering inspection revealed the structural system was fully intact and did not fail. This left an unintentional valve actuation as the likely culprit.
Consequently, our engineers considered the possible causes of an unintentional valve actuation. The spool valve employs a solenoid coupled with an air spring, a manual lever, and capacity for electrical operation. Therefore, it could have been actuated electrically or manually. First, they considered an electrical failure mode.
The electrical components included a solenoid to actuate the valve, a master switch, and individual solenoid switches for the front and rear trailers. For the belly gates to open, both the master switch and the solenoid switches must be engaged. Since all these switches are normally covered, accidental activation by the operator was deemed highly unlikely.
Our engineers then considered the possibility of an electrical short due to faulty or damaged wiring. A short in the solenoid wiring was deemed implausible because it was not run next to a 12-volt power source. However, it is possible that the master switch was left on or actuated unintentionally, necessitating only a short in the solenoid switch. However, our engineers determined that switches typically do not resume normal functioning after a short, and they found no evidence of a switch replacement. Therefore, the likelihood of an electrical failure was found to be very low.
Our engineers then analyzed the causes of an accidental manual actuation. The manual lever could be pushed and held in a position that allowed air to enter the cylinder and open the gates. The air spring would normally have prevented the lever from staying open and allowing the belly dump doors to open. However, the air spring could be manually deactivated to allow the lever to rest in any position without applying constant force. Furthermore, a leak in one of several seals could also deactivate the spring. If the spring was deactivated or leaked, flying debris, a mud flap, or something else could have knocked the valve lever into an open position, causing the belly doors to open.
Our engineers determined using this thorough fault tree analysis that the most likely cause of the belly dump doors opening was a failure in the air spring within the pneumatic valve of the rear trailer. While an electrical failure remained a plausible theory, it was less likely given the lack of subsequent malfunctions. Reaching this conclusion required a thorough understanding of electrical systems, pneumatic systems, mechanical systems, the industry of belly dump trailers, and human factors, all applied through a robust investigative framework. Our engineers’ expertise in these and other areas ensures that we provide the most accurate and comprehensive mechanical engineering expert witness reports possible, even when the root cause is not readily apparent.
In summary, this case underscores the critical importance of meticulously investigating the causes of accidents using robust analytical methods like fault tree analysis. Such investigations not only help in understanding what went wrong but also help prevent future incidents. Accurate identification of the root cause is essential for accountability, improvement of safety measures, and understanding what actually happened in liability cases. Our work at Alpine Engineering & Design exemplifies our commitment to these principles, leveraging our extensive experience and technical expertise to uncover the truth behind complex engineering failures.