According to the US Department of Energy, diesel fuel prices have risen 348% since 2002. Diesel fuel cost is problematic for at least six reasons:
- You cannot operate your heavy machinery without it.
- You have no control over the price.
- It is a significant expense.
- The cost directly affects your profitability. A $5,000-per-month increase in fuel costs represents a $5,000 reduction in your net income.
- Few construction contracts have provisions to help alleviate the impact of your fuel cost increases.
- Higher costs directly affect your cash flow. Many fuel suppliers, for example, require weekly payments, and being late is unacceptable.
One cure for the “Diesel Fuel Blues” is to use some form of trench shielding for most of your underground work. Trench shields will significantly reduce the amount of material that you have to handle, as well as the time required to complete a project.
Let’s analyze just the fuel costs needed to dig a trench on a hypothetical sewer project.
Should You Slope or Use Shields?
This hypothetical job calls for you to lay 1,000 feet of pipe at a depth of 14 feet in soil that is classified as Type C using the OSHA Standard (OSHA Construction Industry Regulations 29 CFR 1926, Subpart P-Excavations). You are trying to decide whether to slope the walls of the trench or use trench shields. So, you make the following assumptions:
- You plan to use a mid-sized excavator that burns 120 gallons of diesel fuel during a ten-hour day.
- Diesel fuel costs $4.50 per gallon, so you’ll spend $540 per day for fuel.
- The excavator can dig 1,800 cubic yards of dirt per day. That is based on the machine and operator being able to complete three cycles per minute, for 50 minutes each hour of work, with a 1.2 cubic-yard bucket, and a 10-hour workday. (The math: 3 cycles per minute x 50 minutes x 1.2 cubic yards x 10 hours = 1,800 cubic yards per day.)
- You will also use a small dozer to handle the excavated material. The dozer will operate about 50% of the time that the excavator is working. The dozer burns 25 gallons of fuel in a 10-hour day. At $4.50 per gallon, you’ll spend $113 each day for dozer fuel.
Let’s assume that with sloping, you will need a width of 3 feet at the bottom of the trench to lay your pipe. Since you are working in a Type C soil, the OSHA Standard requires that the walls are sloped at a 34-degree angle, or 1.5 (H) to 1 (V). As a result, the trench will be 45 feet wide at the top, and you will have to dig 12.444 cubic yards of material for every linear foot of the excavation. The total for the 1,000-foot sloped trench will be 12,444 cubic yards of material.
At a production rate of 1,800 yards of dirt per day for the excavator, it will take 6.9 days to dig this trench (12,444 cubic yards of dirt at 1,800 cubic yards per day = 6.9 days to excavate). Taking a look at Table 1, let’s calculate just your fuel costs.
The Trench Shield Option
With trench shields, you would dig a slightly wider trench to allow for the thickness of the shield walls and still provide a 3-foot-wide workspace inside the shield. However, unlike sloping, the walls of the trench can be vertical.
A 4-foot-wide trench, 14 feet deep and 1,000 feet long, will require digging just 2,074 cubic yards of material, or only 17% of the amount of material that you would excavate in the sloping example.
Using the same production rate of 1,800 cubic yards of dirt per day, it will take just 1.2 days to dig this trench (2,074 cubic yards of dirt at 1,800 yards per day = 1.2 days to excavate). Your fuel costs can be seen in Table 2.
Obviously, you would save $3,722 in fuel costs alone on this relatively small job. And, of course, we have not even considered other costs that will be drastically reduced because of the smaller size of the excavation and because the work will be completed in a little more than one day instead of nearly seven:
- Personnel
- Right-of-way requirements
- Barricading the job site
- Removal of surface encumbrances (streets, sidewalks, curbs and gutters, utility/electrical poles, fire hydrants, trees, etc.)
- Trucking, if the spoil has to be stored off site
- Relocation and/or support of underground utilities
- Backfilling and compaction
- Restoration of the surface
- Wear and tear on equipment
In the view looking down the excavation, as shown in Figure 1, if sloping was your choice to meet the OSHA Excavation Standard, then the entire volume inside the dotted line (both the dark and light areas) would have to be excavated, temporarily stored, replaced, and the surface restored. If your choice were to use a trench shield, only the dark area in the center would get the same treatment. For a trench of any length, that’s only 17% of the volume that would be required to be moved if you used the sloping option.
- By using the trench shield option, your crews will also be significantly more productive. It will take less time to complete the job, and they can move on to the next job a lot sooner. Said another way, compared to sloping the walls of the trench, the same crew using the same equipment will be able to do more work with trench shields…and generate more revenue.
