Choosing the Correct Trench Box
This blog post will help guide you in determining the best trench box to purchase. Important factors to consider are the weight of the trench box, lifting capacity of your excavator, how deep you will be digging, and the amount of safe working room that is required for your job.
A common starting point for many contractors when choosing a trench box to purchase is to simply look for the same system or something similar as to what they had been renting. This presents a few challenges as each manufacture offers unique configurations in their designs. These designs and specifications are not always easy to directly compare from one manufacture to the next. Sometimes it really is like comparing apples to oranges. For example a shield with 4″ double sidewalls from our one of our competitors will likely be heavier than our 5″ double sidewalls Titan 5 Lite Series. This can lead to some initial confusion in comparing different boxes. However, it is quickly resolved when we choose a shield based on the proper requirements, height, length, width, weight, and depth requirement. Often this will lead to finding a more appropriately sized system at a better price.
Height and Depth Requirements
A good place to start is determining depth of the excavation and if any sloping will be used. This dictates the required depth rating along with determining if there will be a need for them to be stacked. The depth requirement has obvious safety and OSHA compliance implications, but is also a key factor in determining the cost of the system. This is one of the most important factors, paying attention to this can save the contractor a lot of money. A shield that is rated for 21 feet in class C soil will be heavier and more costly than a one that is rated for 8-16 feet. If the work will not require going deeper than the height of the box, than the 21 foot rating is not needed for an 8 foot box. Additionally, the excavator required to handle a lighter weight system will be will be smaller and more cost effective to operate than a larger machine.
For example: A trench shoring box rated for 24′ in class C soil is going to be heavier and cost more to build than one with a lower depth rating. For example our Titan 5 Lite 8×20 is a perfect choice for a contractor that is not going to be using this as a bottom box in a deep excavation. The Titan 5 Lite 8×20 has a depth rating of 12′ in class C soil. This gives is the flexibility to use the Titan 5 Lite 8×20 with a 4×20 stacker for a depth of 12′. Alternatively the Titan 5 Lite 8×20 could be used just as a single 8×20. While the Titan 5 Lite has a knife edge, it can also be flipped over and used as a top box as part of a multiple box system. This feature can be handy, if there ever was a need to go beyond its 12′ rating, just flip it over and use it on top of a box with a depth rating of 16′ or greater.
An additional consideration is the option to add a two foot leg kit. This allows the user to be two feet off the bottom of the trench and thus increases the working depth.
Next it is important to consider the total length of shielding required to both be safe and efficient. The common recommendation is two to four feet of shielding extending beyond the edge of the pipe or where anyone will be working. Using too short of a system can quickly become dangerous, while using a system that is longer than needed will increase the time and cost of opening the excavation.
Additionally it is important to consider the total working width needed will determine the required spreader size. The recommended width should be at least 12 inches wider than the diameter of pipe. Some applications and preferences might require greater than an increase of 12 inches from the diameter of the pipe. For example working with 6 inch pipe and adding 12 inches would require a 24 inch spreader. This might not be enough room for some crews to work efficiently. An alternative to a fixed width spreader bar system is utilizing spreader adjusters. We offer both 2 foot and 4 foot adjusters that adjust in 6 inch increments.
Another important consideration when choosing a trench shield is its weight and the lifting capacity of the excavator. The best place to start is to consult the manufacture specifications and load chart for your excavator. The load chart is a valuable resource as it indicates the lifting capacity at various positions. If a load chart is not available, a general rule of thumb would be the machine will lift 20% to 25% of the weight of the machine. You can read a good article titled When Excavators and Backhoes Become Cranes over at www.constructionequipment.com
A Few Examples Using This Rule of Thumb
A Cat 303 with a weight of 8,209 lbs should be able to lift between 1,641 lbs and 2,052 lbs.
Mid Size Excavators
A Cat 308 with a weight of 18,512 lbs should be able to lift between 3,702 lbs and 4,628 lbs.
Standard or Full Size Excavators
A Cat 311 with a weight of 28,660 lbs should be able to lift between 5,732 lbs and 7,165 lbs.
A Cat 318 with a weight of 42,340 lbs should be able to lift between 8,468 lbs and 10,585 lbs.
A Cat 320 with a weight of 49,600 lbs should be able to lift between 9,920 lbs and 12,400 lbs.
A Cat 308 with a weight of 69,950 lbs should be able to lift between 13,990 lbs and 17,487 lbs.
The above example of applying these ranges for determining the lifting capacity of an excavator can be helpful. However, it also demonstrates the wisdom of consulting the manufactures load chart especially for the larger excavators as the range can vary significantly.